Sensation and Perception Flashcards
What are our sense? (there are 10)
vision, hearing, touch, smell, taste
temperature, pain, balance, acceleration, body position
What is sensation?
Receiving information about the world via our senses
Uses sensory receptor cells which are sensitive to physical properties of the world
What are receptor cells? Give an example
Specialised neurones which respond to particular physical properties of environmental stimuli
e.g., in order to see, our eyes have receptor cells sensitive to light
What is perception? What type of processing does it involve?
Our experience of the world
It is a complex process involving both bottom-up and top-down processing
Out of sensation and perception, which is the start point and which is the end point?
Sensation = start point Perception = end point
Why is perception important?
- It is our only source of info about the world
- It underlies all our interactions with the environment
- It allows survival
True or false?
Perception is the starting point for all psychological processes e.g., cognition, social, MH, developmental/education
True
What are some practical applications of perception?
Understanding changes in ageing, disease and injury
Understanding demands of driving and interacting with technology
Use when designing artificial perceptual systems e.g., driverless cars
True or false?
In a perceptual system each system has its own function
True
What are the different types of perceptual systems and what do they deal with?
Vision - object recognition, navigation, motion perception
Audition - object recognition, localisation
Touch - object recognition, pain
Taste + Smell - chemical detection, nutrition and poison avoidance
What are the parts of the perceptual process?
Distal stimulus, proximal stimulus, receptor processes, neural processing, perception, recognition, action, knowledge
What is the distal stimulus?
A physical object is the environment
can be vision, audition, touch etc.
What is the proximal stimulus?
a representation of the distal stimulus
info about distal stimulus is received by sensory receptor cells
each sense receives info about DS via different type of environmental physical energy
What do receptor processes carry out?
Transduction
What is transduction? Give 2 examples
The transformation of environmental physical energy into electrical energy in NS
vision - receptors in retina transform light
audition - receptors in inner ear transform sounds waves
What happens in neural processing?
Electrical signals are transmitted from one neuron to the next
the signal is changed as neurons interact
What is perception in the Perceptual Process
The conscious sensory experience
What is recognition?
Placing an object in a category
What is visual agnosia? What does it highlight the difference between?
= inability to recognise objects
highlights distinction between recognition and perception as you can perceive the object but not recognise it
What is action?
movement e.g., eyes, head, body
What is knowledge do?
Can influence perception, recognition and action
What does knowledge use? Why is that important?
uses top down processing which is important for helping simplify the complex perceptual process
Is recognition always required? Give an example
No
e.g., reflexes - something flying towards you, dont need to recognise what it is to move out of the way
What are the 2 approaches to studying perception?
- Physiological approach
- Psychosocial approach
What is the physiological approach to studying perception?
- what’s going on in the brain
- study anatomy
- record brain activity e.g., single cell recording, imaging (fMRI, MEG, EEG, PET)
- micro stimulation - inserting 2 electrodes in 2 different areas, stimulate neurons in one and record neuron activity in another
- lesioning and TMS
What is the psychophysical approach to studying perception
study what is actually perceived and measure the relationship between stimulus and perception
uses carefully controlled experiments to test perceptual performance
highlights relationship between physical world and perceptual experience
What 2 thresholds does the psychophysical approach measure?
- absolute (detection) - smallest magnitude perceived
- difference (discrimination) - smallest difference perceived
How are absolute thresholds measured? Any limitations?
- adjustment - but different people have different criteria for saying “yes I see it”
- forced choice
Do absolute thresholds vary sense to sense?
yes
What percentage is taken to find correlating detection threshold and difference threshold?
75%
When absolute and difference threshold data is plotted, what shape is the graph?
S-shaped
Is the difference threshold a constant value?
no
it is related to the baseline e.g., adding a book to a bag of cotton wool vs. to a bag of bricks
difference as a proportion of baseline level is constant (Weber’s law) e.g., cotton wool and bricks both need to become 5% heavier to detect difference
How can difference threshold be measured?
Using forced choice
What part of the perceptual process is light a part of?
Distal Stimulus
What type of energy is light?
Electromagnetic
What do psychologists view light in terms of?
Colour and brightness
How does light let us “see”?
Light is reflected from object into eye were the image is focussed on the retina
transduction occurs and signals are sent to the brain
Which part of the perceptual process would the eye be a part of?
Proximal stimulus
Where are receptors in the eye located?
The retina
What is the general function of the eye?
To focus images on the retina
What is the functions of the iris and pupil?
have adjustable aperture to:
limit amount of light passing through
allow us to deal with a great range of light levels
What is the size of the pupil?
between ~2mm and ~9mm diameter
What is the role of the cornea and lens?
Focus light on the retina
What % of focusing power does the cornea and lens have?
Cornea = 80% Lens = 20% but can change shape due to action of ciliary muscles
What is accommodation (the lens)
Lens becomes fatter to focus on close objects
Lens becomes thinner to focus on far objects
What are 2 types of refractive errors?
Myopia = near sightedness Hyperopia = far sightedness
What part of the perceptual process would the retina be a part of?
Receptor processes
What is the retina?
The light sensitive layer at the back of the eye
Has different types of cells
What 6 types of cells are a part of the retina?
Optic nerve retinal ganglion cells amacrine cells bipolar cells horizontal cells photoreceptors
What are photoreceptors?
Light sensitive cells that carry out transduction
How is transduction carried out by photoreceptors?
Occurs by visual photopigments reacting to light which triggers electrical signals
True or false?
Photoreceptors are the layer furthest from incoming light?
True - must pass through blood vessels, cells and axons first
What are the 2 types of photoreceptors?
Rods and cones
Do rods and cones differ in terms of length and shape?
Yes
How do rods and cones differ in terms of number?
Rods ~ 120 million
Cones ~ 6 million
How do rods and cones differ in terms of sensitivity?
Rods = very sensitive, respond well in very dim light, most useful at night Cones = less sensitive, most useful in daylight
When does scotopic vision occur? Are rods or cones active here?
no moon, overcast
only rods active
When does photopic vision occur? Are rods or cones active here?
early twilight, store/office lighting, outdoors when sunny
only cones active
When does mesopic vision occur? Are rods or cones active here?
moonlight, early twilight
both rods and cones are active
What does bright light do to photorecpetors?
Bright light bleaches photopigments so photoreceptors stop responding
What happens when lighting goes from bright to dark?
Photoreceptors have to “recover” and regain sensitivity
What is dark adaptation?
The increase in eyes’ sensitivity in the dark
When does dark adaptation occur? How much better is the sensitivity?
After 20-30minutes in the dark the sensitivity of the eye is 100,000x greater than sensitivity in light
Do rods and cones adapt to changes in lighting (and therefore adjust their sensitivity) at the same rate?
No - adapt at different rates
rods become more sensitive as time in dark continues but this takes longer
Cones adapt more quickly and plateau quicker but are less sensitive
What are the 5 ways that rods and cones differ?
number sensitivity involvement in colour perception retinal distribution neural convergence and acuity
How do rods and cones differ in their involvement in colour perception?
Cones = responsible for colour vision Rods = produce monochromatic vision
What are the 3 types of cones that are sensitive to different wavelengths of light?
Red = long wavelengths Green = medium wavelengths Blue = short wavelengths
What is the 1 type of Rod that is sensitive to wavelengths?
Green = medium wavelengths
What is the Purkinje shift?
At night red looks darker than green
What is the retinal distribution of rods and cones?
Not evenly distributed across retina
Fovea = small central area of retina containing only cones
At what visual angle is the fovea?
0 degrees
True or false?
When looking directly at an object the image falls on the fovea
True
At what visual angle is the blind spot?
20 degrees
What is convergence?
one neuron receives signals from many other neurons
How does convergence differ between rods and cones?
120 rods send signals to 1 ganglion cell
6 cones send signals to 1 ganglion cell
True or false?
Neural convergence determines acuity
True
What is acuity?
The ability to detect fine details of a stimulus
How does acuity differ between rods and cones?
Rods have greater convergence meaning they have lower acuity
Cones have less convergence meaning they have higher acuity
What does high and low acuity mean?
High acuity = can detect fine details
Low acuity = can detect only course details
Where is acuity the highest?
At the fovea - decreases as you move away from it
How do we ensure that objects of interest are imaged on the fovea?
By eye movements
Does acuity decrease in low lighting conditions
Yes
Are there more ganglion cells or photoreceptors?
Photoreceptors
Where do ganglion cells condense raw information from?
Photoreceptors
What is the aim of ganglion cells?
To extract important information from retinal image
Why do ganglion cells respond to changes in pattern of light?
The changes carry the most important information
How do ganglion cells reduce (condense) the amount of information in a stimulus?
They find the contours and boundaries of an image and only give an excitatory or inhibitory response then
How can ganglion cell response to contours and boundaries of images explain why line drawings are so effective?
There are clear boundaries between light and dark for the ganglion cell to respond to
Explain how single cell recordings are used to measure a single ganglion cells’ action potential
Electrode is inserted into the GC and measures the electrical activity to get its baseline.
Experimenters try to find stimuli that changes the activity of that ganglion cell - can increase or decrease
What is a receptive field?
An area on the retina which, when stimulated by light, elicits a change in the firing rate of the cell
What are the 2 responses that a ganglion cell can have?
Excitatory = increase Inhibitory = decrease
Why are ganglion cells influenced by a region on the retina?
Convergence
What is lateral inhibition?
Inhibition that is transmitted across the retina by horizontal and amacrine cells (they are transmitting inhibition)
What is the structure of an on-centre off-surround antagonism?
inner receptive field = +
outer receptive field = -
What is the structure of an off-centre on-surround antagonism?
Inner receptive field = -
Outer receptive field = +
Do ganglion cells respond to changes in light falling within a receptive field? Why is this ideal?
Yes
Ideal for detecting spots of light and edges
Are ganglion cells able to detect orientation of light bars?
No - will give same response regardless
Is there a change in response from ganglion cells in the overall level of illumination?
No
Are photoreceptors part of the receptive field of just one ganglion cell?
No, they are a part of more than one
Receptive fields of neighbouring ganglion cells overlap
How can the Hermann Grid illusion be explained?
Can be explained by receptive fields:
2 on-centre cells centred on light regions of grid
When RF at intersection - more light falls on the surround (off region) so receives more inhibition and cell fires less
Less firing interpreted as less bright so we perceive a dark spot
What is the Hermann Grid illusion typically explained in terms of? Is this correct?
Centre-surround antagonism
Varying receptive field sizes
Probably correct in terms of essentials but there may be additional processes, not yet understood, playing a role too
Why do the grey illusion spots of the Hermann Grid disappear when we fixate on them?
RF size varies with eccentricity
Fovea has small RF compared to periphery
How can the Simultaneous contrast illusion be explained?
brighter outer square causes lots of inhibition around the edge of the inner square = cells fire less = appears darker
darker outer square causes little inhibition around edge of inner square = cells fire more = appears brighter
How do ganglion cell fibres leave the retina?
Along the optic nerve
What is the optic chiasm?
cross over point - some fibres cross over, some don’t
reorganising how information is processed
What does the optic nerve become beyond the optic chiasm?
the optic tract
What is the optic tract?
information is now separated by visual field rather than by eye
information from RVF represented by LH and vice versa
What does the optic tract feed into?
the Lateral Geniculate Nucleus (LGN)
What is the LGN?
bilateral structure (1 in LH and 1 in RH) each LGN receives input from left and right eyes but keeps these inputs separate
What type of RF does LGN have?
same as ganglion cells = centre-surround antagonism
- ideal for detecting spots of lights and edges
- not able to detect orientation of bars
What does V1 (primary visual cortex) receive its input from?
LGN
Where is V1 located?
at the back of the brain
How do you get a response from a V1 cell?
by using lines (instead of spots like for ganglion cells)
Do V1 cells prefer lines of a particular orientation?
Yes
What are the 4 types of organisation of V1 cells?
Retinotopic mapping
Cortical magnification
Orientation columns
Ocular dominance columns
What is retinotopic mapping?
Objects close together in visual scene are analysed by neighbouring parts of V1
What is cortical magnification?
amount of cortex devoted to representing each part of the retinal field is distorted
fovea represented by large area of cortex
What are orientation columns?
orientation preferences of V1 cells arranged in an ordered way
perpendicular to the surface, all cells have the same orientation preference (orientation column)
at an angle to the surface, the cells’ orientation preferences vary systematically
What 2 ways were orientation columns investigated?
- recording from an electrode penetrating the cortex perpendicular to the surface
- recording from an electrode penetrating the cortex at an angle to the surface
What are ocular dominance columns?
80% of cells in V1 are binocular (respond to input from both eyes)
Most binocular cells respond better to one eye than the other = ocular dominance
Cells with the same ocular dominance (same eye preference) are arranged in columns
Are cells in LGN monocular or binocular?
Monocular
What are the 3 types of cell in V1?
Simple cells
Complex Cells
Hypercomplex cells
What is the RF of simple cells?
Respond to orientated bars and edges
RF has excitatory and inhibitory regions that are elongated
Some have on-centre RFs and some have off-centre RFs but all have a preferred orientation
Some only have 1 excitatory and 1 inhibitory region
Are phase sensitive - response depends on position of bar within RF
What is orientation tuning?
Orientation tuned neurons respond best to their preferred orientation but also respond to other similar orientations
What is the RF of complex cells?
respond to orientated lines but no discrete on/off regions
Are phase insensitive - response doesn’t depend on bar position within RF
Respond to moving orientated bars and edges
Respond best to a particular direction of movement
What is the RF of hypercomplex cells?
Respond to bars of particular orientation and moving in a particular direction and are of a particular length
Are there more visual areas than V1? Give examples
over 30 visual areas beyond V1
These areas are specialised e.g., V3 = form, V4 = colour, V5 = motion
Are the visual areas interconnected?
Yes –> no simple separation of function
What is the ventral visual stream –> “what” pathway?
Travels ventrally to inferotemporal cortex
Important for recognising and discriminating objects
What is the dorsal visual stream –> “where/how” pathway?
Travels dorsally to posterior parietal cortex
Important for determining where an object is and how to act upon it
What is the monkey lesion study? What did it find about ventral and dorsal visual streams?
Task 1 - object discrimination (food always hidden under the triangular prism) –> lesion to inferotemporal cortex causes problems for objects but not landmark discrimination
Task 2 - landmark discrimination (food was always located near cylinder) –> lesion to posterior parietal cortex causes problems for landmark but not object discrimination
What neuropsychological evidence is thereabout ventral and dorsal visual streams?
visual form agnosia - damage to ventral pathway = human cannot identify objects despite knowing their features
Optic ataxia - damage to dorsal pathway = cannot reach to grasp objects but can recognise and describe them
these are opposite deficits
Are the dorsal and ventral visual streams separate?
no –> many connections between them
signals flow both upwards and backwards
What are feature detectors? Which cells does this refer to? Give an example
V1 cells are referred to as feature detectors
feature detectors = respond to particular features of an image
higher up in the visual system = more complex RF and the features they respond to become more specific
e.g., inferotemporal area responds to faces:
–> response to monkey + human faces –> lesser response to human face without eyes –> lesser response to cartoon face –> baseline response to a pattern
What is the order of the visual system?
photoreceptors ganglion cells LGN cells Simple cells Complex cells Hyper complex cells
What are 3 keys things to remember about the retina?
its curved
its constantly moving
its being updated 50 times a second
What are 2 approaches to explaining perception of objects?
Marr’s computational approach
Gestalt approach
What is Marr’s computational approach primarily concerned with?
The representation of edges, contours and other areas of contrast change?
Is Marr’s computational approach bottom-up or top down? Explain
Bottom-up approach
starts with input to perceptual system in form of retinal image and describes the stages in processing this image
each stage takes its input from the information from the previous stage and transforms it into a more complex description/representation
What is a computational theory?
What is the model trying to do?
What are the processes for?
What is the goal?
What is computational theory made up of?
Algorithmic level –> what process?
Mechanism level –> what mechanism is needed to implement the algorithm? e.g., neural, biological etc. (Marr thought this level was less important)
What is the importance of a computational approach to perception?
an algorithm/rule/system is more likely to be understood by understanding the problem that has to be solved, rather than examining the mechanism in which it is embodied
study the function not the form
How did Marr apply his computational model to perception?
retinal image
Grey level description - measure light intensity
primal sketch - representations of contrast change e.g., edges, blobs etc.
2 1/2 D sketch - representation of orientation, depth, colour relative to the observer
3D representation - representation of objects independent of observer
How does the 2 1/2 D sketch differ to the primal sketch and the 3D representation?
its a primal sketch combined with depth cues, colour and motion
not 3D because it is observer-orientated so there are unseen parts of the scene/ objects
How is a 3D representation different to a 2 1/2 D sketch?
2 1/2 D sketch is analysed for 3D volume primitives e.g., cylinders, cones, cubes etc.
this produces a 3D representation that is independent of the observer
= conscious experience of vision
What is the Gestalt approach to perception?
The whole is different than the sum of its parts
don’t see lines and figures, instead see forms and shapes
interested in how we group parts of a stimulus together and the way we separate figure from ground
Is the Gestalt approach top-down or bottom-up?
top-down
What is perceptual organisiation?
How we see a stable and organised world
we see objects according to all their elements taken together as a whole
What are the 9 Gestalt laws of perceptual organisation?
similarity good continuation proximity connectedness closure common fate familiarity invariance Pragnanz
What is similarity (Gestalt laws of perceptual organisation)?
similar things appear to be grouped together
groupings can occur due to shape, lightness, hue, orientation, size etc.
What is good continuation (Gestalt laws of perceptual organisation)?
points that, when connected, result in straight or smoothly curving lines are seen as belonging together and the lines tend to be seen in such a way as to follow to smoothest path
What is proximity (Gestalt laws of perceptual organisation)?
things that are near to one another appear to be grouped together
What is connectedness (Gestalt laws of perceptual organisation)?
things that are physically connected as perceived as a unit
What is closure (Gestalt laws of perceptual organisation)?
of several geometrically possible perceptual organisations, a closed figure will be preferred to an open figure
What is common fate (Gestalt laws of perceptual organisation)?
things that are moving in the same direction are grouped together
objects with the same orientation are grouped together
What is familiarity (Gestalt laws of perceptual organisation)?
things are more likely to form groups if the groups appear familiar/meaningful
What is invariance (Gestalt laws of perceptual organisation)?
an objects in the world can have different orientation, be distorted or be made from different mediums but you know that it is the same object
this is a problem for computers e.g., captcha tests
What is Pragnanz (Gestalt laws of perceptual organisation)?
meaning “good figure”
central law of Gestalt psychology
many of the laws are manifestations of Pragnanz
What is figure-ground segregation (Gestalt)
How we separate figure from ground
extreme example = visual illusions
normally in a visual scene, some objects (figure) seem more prominent and other aspects of the field receded into the background (ground)
infers a top-down process
What properties affect whether an area is seen as a figure or as ground (figure-ground segregation)
symmetry = figure
convexity = figure
area –> stimuli with comparatively smaller area = figure
orientation –> vertical and horizontal orientations = figure
meaning/ importance = figure (implies attention = top-down)
What are some problems with the Gestalt approach to perception?
- underplay the parallel processing and unconscious processing that the brain does
- explanation of how some of their laws worked was wrong
- their laws provide a description of how things work rather than an explanation
- their laws are ill-defined
- stating the obvious?
What are some positives of the Gestalt approach to perception?
laws appear generally correct
- precepts can be analysed into basic elements
- whole = different than sum of its parts
- context and experience affect perception
During perception we quickly form and test various hypotheses regarding perception based on what?
what we sense (sensory data)
what we know (knowledge stored in memory)
what we can infer (using thinking)
what we expect
What are the 4 depth cues?
oculomotor cues
pictorial cues (monocular cues)
motion-produced cues
binocular disparity
What are oculomotor cues?
cues that depend on our ability to sense the position of our eyes and tension in our eye muscles
feelings you experience are:
- convergence as your eye muscles cause you to look inward
- accommodation as the lens bulges to focus on a near object –> ciliary muscles tighten for close vision and lens becomes more rounded, relax for distant vision
Shape of lens and position of eyes are correlated with the distance of the object we are observing
When are oculomotor cues effective?
At distances closer than 5 - 10 feet
What are pictorial cues?
Cues that can be depicted in a still picture
Don’t require viewing with both eyes in order to work
Often better to view monocularly e.g., TV, photos, paintings
What are the 7 key elements of pictorial cues?
- overlap or interposition or occlusion
- relative size
- relative height
- atmospheric perspective
- familiar size
- linear perspective
- shading and shadow
What is overlap or intersection or occlusion?
1 object obscures part of another or overlaps it
What is relative size?
the retinal size of objects gets smaller as they get further away
An object can look the same size at different distances but retinal size image changes with retinal distance
increase distance = decrease retinal image size
decrease distance = increase retinal image size
What is size constancy (relative size, pictorial cues)
The fact that an object can look the same size regardless of changing retinal image size
What is Emmert’s law (relative size, pictorial cues)
- objects that generate retinal images of the same size will look different in physical size if they appear to be located at different distances
- perceived size of object increases as it perceived distance from observer increases
- an object of constant size = smaller retinal images as distance from observer increases
- if retinal images of 2 different objects at different distances are the same, the physical size of the object that’s further away must be larger than the one that is closer
What is relative height?
- as object gets further away they get nearer the horizon
if the objects are below eye height then the highest object is furthest away
if objects are above eye height then the lowest object is furthest away
What is atmospheric perspective?
distant objects appear less sharp because there’s more air and particles to look through
Objects also appear more blue as blue light is scatter more by the atmosphere
What is familiar size?
if you know what object you’re looking at, telling how far away it is becomes easier
What is linear perspective?
lines that are parallel in the scene converge as they get further away
What is shading and shadow?
the shadowing that results from depth within an object is a cue to depth
i.e., attached shadows and detached shadows
the meaning of shading is ambiguous but we assume light comes from above
texture becomes smaller/ finer as distance increases
What are motion-produced cues?
cues that depend on movement of the observer, or movement of objects in the environment
What are the 2 types of motion produced cues?
motion parallax
deleted and accretion
What is motion parallax?
an observer moves relative to a 3D scene:
nearby objects appear to move more rapidly and far objects more slowly
- used more by animal who don’t have much binocular overlap
What is deletion and accretion?
as one object moves in front of another, deletion occurs
e.g., front object covers more of back object
as one object moves away from another, accretion occurs
e.g., front object covers less of back object
What is binocular disparity?
a cue that depends on the fact that slightly different images of a scene are formed on each eye
due to corresponding retinal points
What are corresponding retinal points?
for every point on 1 retina, there is a corresponding point on the other
would be identical if 1 retina was moved over to superimpose the other
What are non-corresponding retinal points?
points that do not correspond between the retinas
creates disparity
When does the effectiveness of binocular disparity diminish?
Diminishes with distance
determined by distance of the 2 eyes
What is hyperstero?
Can give increased depth from disparity –> increasing binocular disparity can make the world seem miniature
What percentage of people show stereo-blind performance? Why?
2 - 5%
lack mechanism for processing disparities
True or False?
the visual scene can give us disparity information directly to generate a percept of depth
True
can do this using a random dot stereogram
but there are many ways to create a stereogram
What is colour good for?
- ) Scene Segmentation –> variations in colour often signal object boundaries
- ) Camouflage –> animals use this to disguise themselves by colour markings
- ) Perceptual Organisation –> our visual system uses colour to grasp elements in a scene
Does colour have an evolutionary aspect?
Yes for certain species
e.g., food identification -> ripe fruit, correct leaves etc.
What is colour?
visible light forms a narrow band of frequencies in the electromagnetic spectrum
within this band, different frequencies (wavelengths) have different hues ranging from red (long wavelength) to violet (short wavelength)
What range do we see between in metres?
0.00000390 - 0.00000750 metres
Different objects absorb and reflect different wavelengths of light.
What does this do?
this gives objects their colour
colour also depends on the light source
What does wavelength of light reflected off an object determine?
Which hue is seen
What is the perceived colour of an object determined by?
wavelength of light reflected off object
intensity of the reflected light (how bright it is)
the saturation of the colour (how much white light is mixed in with the pure hue)
What are the 3 properties of light and the psychological attribute associated with them? Give an example for each
wavelength –> hue (colour) i.e., difference between blue and red
intensity –> brightness (perceived intensity) i.e., difference between light and dark blue
spectral purity –> saturation (how much colour/white) i.e., difference between red and pink
What are the 2 theories of colour percption?
Trichromatic theory
Opponent process theory
What is trichromatic theory?
3 receptor types and their combined responses account for all colours
Blue sensitive cones –> respond to short wavelengths (s-cones)
Green sensitive cones –> respond to medium wavelength (m-cones)
Red sensitive cones –> respond to long wavelengths (l-cones)
colour you see is determined by the relative levels of activity in the 3 sorts of receptors
What evidence is there for the trichromatic theory?
- 3 primary colour combine to produce all possible colour
- 3 forms of dichromatism (colour blindness)
- a mixture of green and red light produces same perception of yellow colour as monochromatic yellow light (metamerism) –> we can’t tell the difference
What is opponent process theory?
found that when ppts were asked to pick out pure colours, they picked out red, green, blue (as predicted by trichromatic theory) but also yellow
3 processes are opponent in nature:
- red - green
- yellow - blue
- black - white
e.g., red- green receptor will signal either red or green but not both
What evidence is there for opponent process theory?
- non-existence of certain colours e.g., bluish-yellow
- colour confusions in colour blindness (red and green)
- complementary afterimages
- colour context effects
Is trichromacy or opponent processes the correct theory?
Both are correct:
- trichromacy t the level of the cones
- opponent processes at the level of LGN and cortical cells
What are the 2 types of colourblindness?
Anopias –> insensitive to L, M or S wavelengths of light (missing a type of cone)
Anomalies –> misalignment of L or M in trichromats (distribution or deficiency)
What are the 3 types of anopias?
Protanopia - L-cone pigment missing
Deuteranopia - M-cone pigment missing
Tritanopia - S-cone pigment missing
What are the 2 types of anomalies? (colour blindness)
Protanomaly –> L-cone pigment deficiency
Deuteranomaly –> m-cone pigment deficiency
How does colour blindness provide support for both theories of colour?
- whole fact of anopia points to 3 cone types (trichromatic theory)
- opponent process theory support by the fact that people who have trouble with red also have trouble with green etc.)
What are human tetrachromats?
rare
humans that have 4 pigment cone types
can detect variations in hue that normal humans cannot see
What is the evolutionary importance of perceiving motion and events?
movement = life
no animals lack the ability to perceive movement
What are 6 functions of motion?
- ) attract attention
- ) provides info about an objects 3D shape
- ) helps segregate figure from ground and perceptual organisation
- ) movement breaks camouflage
- ) enables us to actively interact with env.
- ) informs you of your heading and time to collision, your movement and other objects
What do random dot kinematograms suggest about motion detection?
Motion detection is direct
We cannot imagine a visual system matching point for point over time in RDK displays
What are the 5 ways to makes a spot of light move?
- ) Real movement
- ) Apparent movement
- ) Induced movement
- ) Autokinetic movement
- ) Movement after-effects
What is real movement?
light physically moves
When do we perceive movement?
When are eyes are stationary –> image moves across the retina which stimulates a series of receptors
What are movement detectors?
excitation and inhibition interact to create a cell that responds only to movement from right to left
For movement detectors, what happen when:
something moves in proper direction?
2 signals meet at same time giving a strong response
For movement detectors, what happen when:
something moves in wrong direction?
no response
For movement detectors, what happen when:
something goes at the wrong speed?
The timing is off –> no response
For movement detectors, how do you change direction?
Change the order of delay
For movement detectors, how do you get different speeds?
Change spacing of detectors –> bigger separation detects motion faster
Where have movement detectors been found? Do humans have anything similar?
found in insects and frogs
humans have cells in cortex that are sensitive to different orientations, speed and direction movement
What is the aperture problem?
output of all detectors must not be integrated at the same stage (medial temporal area)
When can movement detectors not explain movement perception?
- ) when there is no movement on the retina
2. ) when you perceive no movement when there is movement on the retina
What does the threshold for perceiving movement depend on (real movement)?
depends on the object and its surroundings
What is the perception of velocity affected by (real movement)?
affected by surroundings and size of both the moving object and framework through which it moves
What is Helmholtz’s outflow theory?
- If there is a difference between muscle movement command and movement of image across the retina then we perceive movement
- When keeping eyes still and object moves across we perceive movement
- When we look around the world, eye movement command and retinal image movement are equal so we perceive no movement
What evidence is there for Helmholtz’s outflow theory?
1 - afterimages move when we move our eyes
2 - the world moves when we passively wobble our eyes
3 - immobilizing eye-ball results in attempted eye-movement leading to apparent movement of world in opposite direction
What is apparent movement?
The illusions of movement between 2 lights by flashing 1 light on and off, waiting 40-200 ms then flashing the other light on and off
What is the perception of movement in a film due to?
a series of static images
In apparent movement, what happens if the delay between the 2 lights flashing is < 30 ms?
no movement, simultaneous
In apparent movement, what happens if the delay between the 2 lights flashing is 30 - 60 ms?
partial movement
In apparent movement, what happens if the delay between the 2 lights flashing is approx. 60 ms?
optimum movement
In apparent movement, what happens if the delay between the 2 lights flashing is 60 - 200 ms?
- beta movement = while movement appears to occur between 2 lights, it is difficult to perceive an object moving across the space between them
- phi movement = perceive an object between
In apparent movement, what happens if the delay between the 2 lights flashing is > 200 ms?
no movement, successive
Does the distance between the 2 lights also affect the perception of apparent movement?
Yes
As distance increases, either the time interval or the intensity of the flashes must be increases to maintain the same perception of movement
What is induced movement? Give an example
surround spot with another object and then move this object
e.g., you are sitting on a train and feel it move backwards but the train is actually stationary and it is the train next to you that is moving forwards
What is autokinetic movement? Give an example
when surround framework of room is not visible (e.g., room is dark), the small stationary light appears to move, usually in an erratic path
e.g., the Sherif autokinetic conformity study
What are movement after-effects? Give an example
If an observer first views a pattern moving in one direction, and then views the spot of light, the spot (and surroundings) appear to move in the opposite direction
E.g., the waterfall illusion:
- depends on movement of stripes across retina
- supports idea of movement detectors, which respond only to movement across the retina
What is the ratio hypothesis and who provided direct evidence to support it?
Sutherland –>
argued that motion after-effects arose from an imbalance in the ratio of activities from 2 sets of directionally-tuned receptors, each sensitive to the opposite directions of motion
Barlow and Hill –> provide direct evidence to support
What is an example of event perception?
creating structure from motion –> sensors attached to a moving body create structure of a body
What is it possible to determine when creating structure through motion?
gender of walker
approx. weight lifted
activity engaged in
emotion demonstrated during dance
What is motion induced blindness?
motion producing blindness to spots on a screen etc.
What is sound?
changes in pressure
What is sound in the perceptual process?
the env. stimuli
On a sound wave, what is compression, rarefaction and amplitude?
compression = near peak of wave rarefaction = near trough of wave amplitude = height of wave
What is frequency?
measured in Hertz (number of waves per second)
high frequency = shorter wavelength
low frequency = longer wavelength
What is the human hearing range?
What is infrasound?
What is ultrasound?
- 20 - 20,000 Hz
- < 30 Hz
- > 20,000 Hz
What is amplitude?
- loudness
high amplitude = loud
small amplitude = quiet
What is the decibel scale?
created to represent a wide range of sounds
it is logarithmic
What is a decibel?
an intensity value of typical auditory stimuli
What are complex waves?
natural sounds make more complex waves than artificial sounds
they can have the same wavelength and pitch but sound different
What is Fourier analysis?
- fundamental frequency is the wavelength of the longest component
- this determines the pitch of the sound
- the harmonics determine the timbre (sound quality)
What does the outer ear do?
channels sound
amplifies
protects
What does the eardrum do?
vibrates
What is the vestibular system for?
balance
What are the features of the vestibular system and their functions?
- ) semi-circular canal - filled with fluid, fluid moves when head is turned
- ) cupula - pushed by movement of fluid
- ) endolymph - fluid
- ) nerves - tells brain head has moved
What is the vestibular-ocular reflex?
tells brain head has moved so eyes move in the opposite direction
makes it easier to focus on an object
What is the illusion of turning?
when you feel like you are still moving after spinning it is because the fluid is still turning
Where are the ossicles found and what are they?
found in the middle ear
they are 3 small bones –> first is attached to eardrum and moves when eardrum vibrates, passes this on to second and third bones which eventually passes it on to the cochlear
Where is the cochlear found? What is its shape?
inner ear
snail like shape
Is there a way for use to hear without the ear?
yes
base conduction of sound bypasses the ear and vibrates on the skull.
This can help people with conductive hearing loss by placing based conductors (bony-like) on the skin
What is the basilar membrane?
part of cochlear
the shape of the wave travelling along the basilar membrane depends on its frequency etc.
different frequencies move different populations of cells on this membrane
What do hair cells do?
hair cells in the organ of corti (within cochlear) detect vibrations in the basilar membrane and transmit this information into the firing of the auditory nerve
Do hair cells change their firing rate?
Yes
hair cells change their firing rate when they are bent
Hair cells are tonotopic, what does this mean?
hair cells respond preferentially to a particular frequency
How far is tonotopic cortical organisation maintained?
maintained as far as the primary auditory cortex in the temporal lobe
neurons next to each other respond to neighbouring frequencies
How does auditory transduction occur?
- ) air pressure changes (kinetic)
- ) vibration of eardrum –> middle ear–> oval window (mechanical)
- ) cochlear fluid flows (kinetic)
- ) hair cells bend (mechanical)
- ) auditory nerve fires (neural)
What 3 factors determine auditory perception?
- ) pitch and loudness
- ) space perception
- ) auditory grouping or streaming
What is pitch and loudness in auditory perception?
pitch depends on frequency and loudness depend on amplitude but they are not independent.
more intense low frequency sounds are perceived as lower pitch meaning perception of loudness is affected by frequency
What is the equal loudness curve?
low frequency sounds need to be more intense to be perceived as equally loud
What is space perception in auditory perception?
- ) monaural space perception depends on
- loudness
- doppler effect - ) binaural space perception
- need to perceive direction
- interaural intensity differences –> delay in hearing sound in ear furthest from sound (can be up to 0.07 seconds)
- head movements –> can perceive vertical location of sound source
What is auditory grouping or streaming in auditory perception?
Audio is groups into streams by proximity in:
- space
- time
- frequency
What is the Shephard tones auditory illusion?
appearance of an ascending scale
mixture of tones = ambiguous pitch –> we interpret them as always being a higher pitch
Is the skin the largest sense organ of the body?
yes
What is the skins area and weight?
area = 1.8m^2 weight = 5kg
What are the 2 types of skin?
glabrous
hairy
Where is glabrous skin found?
palms of hands and feet
Where is hairy skin found?
everywhere except palms of hands and feet
What is the physiology of skin senses?
- ) stimulus contacts skin
- ) receptor in skin fires
- ) signal sent towards the brain fire the spinal cord
- ) signal reaches somatosensory cortex on opposite side of body
What senses are skin receptors sensitive to?
- touch (mechanical stimuli)
- pain
- body sense (proprioception)
- temp
What are the 4 main types of tactile skin receptors?
- Merkel (tactile) disc
- Meissner corpuscle
- Ruffini (organ) corpuscle
- Pacinian corpuscle
Which tactile skin receptors are near the skin surface?
- Merkel (tactile) disc
- Meissner corpuscle
Which tactile skin receptor is furthest from the skin surface?
Pacinian corpuscle
What do Merkel disc receptors detect?
fine details e.g., braille
What do Meissner corpuscle receptors detect?
flutter e.g., objects slipping through fingers
What do Ruffini corpuscle receptors detect?
Stretching e.g., due to picking something up
What do Pacinian corpuscle receptors detect?
Vibration, fine texture e.g., using a tool
Why do we have multiple receptor types?
many receptors allow us to detect many types of information
Can a single stimulus activate many different receptor systems?
Yes
What are the receptive fields of the skin?
Areas of skin that a particular cell receives information about?
Which have larger receptive fields: Pacinian corpuscles or Meissner’s corpuscles?
Pacinian corpuscles
What is the 2 point threshold?
- The smallest separation of 2 separate but adjacent points of stimulation on the skin that produce 2 distinct impressions of touch
- need to be stimulation 2 different receptors
What is the 2 point threshold of a fingertip and an arm?
fingertip = 2mm arm= 3.5 cm
Why do we have differing 2 point thresholds for different body parts?
Body is like an homunculus –> some areas are more sensitive than others
What are receptors like on a fingertip?
- more receptors, more dense
- “fovea” of skin
- acuity can change with experience e.g., Braille readers and musicians
What are the 2 types of touch?
active touch
passive touch
What is active touch?
active exploration of environment
What is passive touch?
body is stationary e.g., something brushing past you
What are the advantages of active touch?
- more parts of the body contact the object
- you can search for the most diagnostic parts of objects to feel
- kinaesthetic senses are also engage
What are the 2 types of cues used to perceive texture?
spatial cues
temporal cues
What are spatial cues (perceiving texture)?
bumps and grooves
when finger is stationary or moving
What are temporal cues (perceiving texture)?
only when finger is moving across surface
What impairs Pacinian corpuscle receptors?
adaption to high frequencies –> prolonged exposure to a high frequency vibration
Can Pacinian corpuscle receptors perceive texture via a tool?
yes
What is the double dissociation found for what and where processing in the skin?
- tactile agnosia –> cannot identify objects by touch but no problems with spatial processing
- tactile extinction –> but not problems in object recognition (if touch with both hands, only feel touch on side opposite the brain damage as this extinguishes the feeling in the other hand)
In healthy ppts, what brain regions are active when saying what the object was and where it was?
what = activity in primary and secondary somatosensory cortex where = activity in superior parietal areas
What are top down influences on touch?
- emotional effect - the same sensation may be pleasant or unpleasant
- expectation and surprise
What was Aristotle’s illusion?
- crossing fingers then touching an object = gives the sense that there are 2 objects
- idea = putting fingers in a strange position and then using outer edges of fingers leads brain to think there are 2 objects
What is the Cutaneous rabbit illusion?
stimulus = widely separated taps perception = evenly spaced jumps
What does the Cutaneous rabbit illusion affect?
- affects S1
- illusion led to activity in primary somatosensory cortex as if P2 had really been stimulated
What can’t you tickle yourself?
- can predict the consequences of our own action
- same touch rated as more ticklish when produced by experimenter rather than self (experiment using tickling device)
What was the old view of pain?
pain was due to overstimulation of any system
What is pain actually due to?
Nociceptors - receptors for pain being stimulated
What are the 2 types of pain?
A delta fibres
C fibres
What are A delta fibres?
fast pain (sharp) e.g., pin pricks, pinches, extreme temp
What are C fibres?
slow pain (dull) slower due to C fibres being unmyelinated
Can some stimuli activate both A delta fibres and C fibres?
yes
What can pain do?
- affect a person’s mental state e.g., battlefield analgesia
- occur in the absence of stimulation e.g., phantom limb pain
- be affected by attention
How can pain be reduced?
- non-painful tactile inputs
- top-down input
influences the degree to which painful info reaches the brain
What are phantom limbs? Why does this happen?
Perceiving limb that isn’t there e.g., after amputation
- many ppts feel arms/ hands when touched on face –> believed that brain had remapped
- new work suggests that the missing limb is still represented in the brain
What is proprioception?
- where your body is in space –> signals from muscles
- also can use other modalities e.g., vestibular system, tactile receptors and kinesthesis (movement of limbs in space)
What is the case study of IW?
- lost most proprioception, kinesthesis and touch
- learned, over 3 years, to compensate using only visual info
- unable to move if it is dark
- lost fat myelinated fibres –> retained only slow C fibres
What was the new receptor that was discovered in 2002?
CT receptor (C tactile)
case study - lost all other sense of touch
- could still feel pain, temp and enjoyed cuddles
What are taste and smell both?
chemosenses - they detect chemicals
What is the survival value of taste and smell?
- prevents ingestion of toxins = avoid danger
- may be stronger than necessary in morning sickness
What are the social effects if smells?
pheromones –> attraction
What are the core tastes?
sweet sour salty bitter umami
What do taste buds contain?
taste receptors that respond to each taste
Do we have taste maps (different areas more responsive to certain tastes)?
No
= old view but has been disproved
What causes sweet taste?
sugars and artificial sweeteners
What causes sour taste?
all acids
What causes bitter taste?
no unique chemical class
- quinine, caffeine, peptide, phenols
- children less keen
What causes salty taste?
salts
What causes umani taste?
- savoury tastes e.g., meats and broths
- monosodium glutamate, monophosphates
What are supertasters?
- have more papillae and taste buds
- can detect a tasteless substance called PROP
Is there a 6th sense?
- been suggested recently
- detect a starch flavour e.g., chips
- detect slow release energy
How many different types of molecule can we discriminate between in smell? Is there any limitation of this?
- 10,000
- limited by our memory for what smells indicate
- no satisfactory classification of odours
What are the 2 routes for smell?
- ) Orthonasal
2. ) Retronasal
What is the Orthonasal route to smell?
via inhalation
What is the retronasal route to smell?
during chewing and swallowing –> goes to back of nose via mouth
How does smell reach the brain?
stimuli –> olfactory membrane –> olfactory nerve
How many types of smell receptors are there?
350
Do receptors of similar types project to the same glomeralus?
yes
What did recent research suggest about how many molecules we can discriminate between in smell?
1 trillion
What are the top down effects of smell?
- attention
- effects of labelling
- effects of learning
How does attention affect smell?
- sniffing e.g., subtle smell
- automatic attention e.g., strong smell
How does effects of learning affect smell?
e.g., the same odour smells worse when labelled as body odour rather than cheese
How does the effects of learning affect smell?
e.g., wine tasters
What is the Proust effect?
- vivid memories are brought back by particular smells
Is there a close link between smell and the limbic system? Why?
yes
- limbic system associated with emotion and memory
What sense is eating?
- eating is multisensory
- taste and olfaction
What influences perception of eating?
- taste
- olfaction
- texture
- pain
- sound
- vision
How does texture effect eating?
- tongue represented in somatosensory cortex
- many foods e.g., mushrooms are disliked because of texture
How does pain affect eating?
- chilli acts as pain receptors in tongue Can be partly supressed by other tastes: - sweet and sour liquids do this best - bitter tastes are not effective - salty has intermediate effects
How does sound affect eating?
- food tastes crunchier and fresher when sound is amplified or high frequencies are increase
- foods are rated as less sweet and salty in presence of background noise –> may explain blandness of airplane food
How does vision affect eating?
- oenology students (wine tasting) were fooled by being given white wine with
- tastiness rating increased for art inspired dishes
What is a multisensory stimulus?
- several independent energies detectable by different senses at the same time
What are multisensory receptive fields? give 2 examples
- single neurone may respond to more than one modality
e. g., - orbitofrontal cortex = taste and smell
- posterior parietal cortex = touch, vision and audition
Why is multisensory integration useful?
- can allow detection of weak stimulus in another modality
- can make sense of an ambiguous stimulus in another modality
- can alter the quality of a stimulus in another modality
Give 2 examples of visual information influencing where in space we perceive a sound?
- puppet/puppeteer
- visual capture of sound allows us to follow what is happening in TV/ cinema
What is the McGurk effect?
- watch lips moving to make sound “ga-ga”
- hear sound “ba-ba”
- perceive sound “da-da”
- visual information is affecting the sound you hear
What is the rubber hand illusion?
- one’s own hand may feel as if it is in the location of a rubber hand
- critical to feel own hand being touched at the same time as the rubber one
What is kinaesthesia? What illusion is it linked with?
- the sense of movement
- the illusion of speed
- the NS turns down the “gain” on steady-state inputs
- e.g., initially 70pmh, 10 minutes later after steady 70pmh, it feels like 50mph
How can awareness of speed be increased?
- multisensory approach
- painted and raised lines to increase awareness of speed via vision and audition (and possibly touch)
What is synaesthesia?
- stimulation of a particular type always leads to another perceptual experience
e. g., seeing coloured letters, tasting shapes
What is the prevalence of synaesthesia?
approx 1 in 200 people
What are cross modal correspondences?
a tendency for a sensory feature in one modality to be matched with a sensory feature in another sensory modality
e.g., is a lemon fast or slow?
Can you be trained to experience synaesthesia?
- yes
- after 9 weeks of training –> ppts pass tests of genuine synaesthesia
- ppts describe vivid experiences
- also led to increase in IQ
What are visual illusions?
something that is not in accordance with reality
What are visual illusions useful for?
- useful for discovering laws of means and processes by which normal perception originates
What does the word illusion mean?
to mock
Are illusions considered a positive or negative phenomena?
negative
considered a way of showing how faulty our brain processing is but this may not be the case
What is the brain searching for in perception?
- the best interpretation of the data that is being presented
- sometimes the “perceptual hypothesis” is incorrect, resulting in an illusion
Why do optical illusions mock our trust in our sesnes?
- e.g., suggest the eye is not a passive camera but instead perception is the active process that takes place in the brain
When do illusions occur?
- when what we see doesn’t correspond to what is physically present in the world
What are the 4 classifications on visual illusions (Gregory)
- Distortions
- Ambiguous figure
- Paradoxical figures
- Fictions
What are 3 examples of distortion illusions?
- Muller-Lyer
- Ponzo
- Poggendorff
What are 2 examples of ambiguous figure illusions?
- necker cube
- rubin vase
What is an example of paradoxical figure illusions?
Pensrose impossible objects
What is an example of fiction illusions?
Kaniza triangle
What does the Muller-Lyer illusion look like?
red line with outward facing arrows
blue line with inward facing arrows
blue line appears longer but they are the same length
How did Gregory explain the Muller-Lyer illusion?
- basis of misapplied size constancy
- fins on the blue line make this look like part of the inside corner of a room
- fins on the red line make this look like part of the outside corner of a room
- insider corner tends to look further away thus size-distance scaling causes this line to look larger
What are 2 issues with the Muller-Lyer illusions?
- still found with 3D displays when obvious that spaces between fins are not at different depths
- not cross-cultural –> people who live in natural env. (i.e., less man-made rectangles) are less prone to the illusion
What is the Ponzo illusion?
- illusion of size
- 2 lines converge towards a vanishing point
- gives the impression that the line nearer the vanishing point is further away and thus appears larger
What happens when the Ponzo illusion is flipped upside down?
- when lines converge at the bottom, this makes little sense to the brain
- apparent depth is diminished so the 2 lines appear the same length
What is the set up of the Poggendorff illusion?
straight line that passes behind a rectangle
What is the explanation of the Poggendorff illusion?
- actual angle dilation
- our brains make small angles appear larger than they actually are (angle theory)
What is the Herring illusion similar to?
Wundt and Titchener (Ebbinghaus) illusions
What is the Herring illusion?
- straight lines in the illusions appear to bow out in the centre
What is the explanation of the Herring illusion?
- you interpret the radiating lines in terms of depth, seeing the central spot as being further away than the edges
- therefore you believe that the heavy black lines must also be further away in the centre
- because heavy black lines are the same thickness at the centre as at the edges but are further away, brain thinks they’re more widely spaced at the centre
What do distortion illusions make clear?
- make clear the complex size and depth calcs the brain is doing all the time
- “unfair” situation for the brain of 2D drawings which are or are interpreted as 3D representations
What is the necker cube illusion?
2D drawing of 3D cube
dot in a corner of cube
is the dot in the near or far corner?
What is the explanation of the necker cube?
- brain sees this 2D drawing and visualises it as a 3D cube
- but this drawing doesn’t give enough information for visual system to know which face of the cube is the front
- visual system has a hypothesis that the cube is at 1 orientation, then suddenly another hypothesis is favoured and the cube flips
What is the rubin vase illusion?
unsure whether you see a vase or 2 faces
What is the explanation of the rubin vase illusion?
- based off figure ground segregation but the answer is ambiguous
What is forced interpretation illusions?
Forced perspective is a technique which employs optical illusion to make an object appear farther away, closer, larger or smaller than it actually is. It manipulates human visual perception through the use of scaled objects and the correlation between them and the vantage point of the spectator or camera
uses top down processing
What do impossible shapes and paradoxical figures show us?
- show how our brains automatically try to make us see in 3D
- however, only work if looked at from a very particular angle
what are examples of paradoxical figures?
3D triangle
2 rectangles but 3 cylinders
elephant with too many legs
What is the Kanzia triangle?
- 3 red circle with parts missing create illusion of a white triangle
What is the explanation for the Kanzia triangle?
- subjective contours (Schumann)
- it is common to see near object blocking our view of more distant ones
- this is why we “see” shapes which seem to be blocking the view of the circles
- near objects are usually brighter than distant ones of the same colour so the illusory triangle appears brighter than the background
What are the 2 illusions that occur in Ames room?
- ) room appears cubic when viewed monocularly from a certain point but it is actually trapezoidal
- ) objects and people appear to grow and shrink as they travel from one corner of the room to the other –> people of equal sizes appear different
What is Ames Room an example of?
Forced perspective
What is the moon illusion?
when the moon is on the horizontal it appears much larger than when it is directly overhead
What is a possible explanation of the moon illusion?
Kaufman and Rock –> apparent distant theory
- see the sky as a flattened dome
- appears closer over our heads than near horizon –> looks larger because the horizon is further away
What is the Ouchi illusion?
- circular middle section appears separate from the rest of the figure
- circle appears the be at a different depth and even move
What is a possible explanation of the Ouchi illusion?
- caused by eye movements
- eyes always move even when fixated
- brain cancels out the movement
- in this illusion, the pattern is such that the compensation is not necessary/ has a different effect on different orientations
- therefore the brain interprets that it must be moving
What are illusions caused by peripheral drift? Why do they happen?
- illusions appear to move
- even when fixated, eyes make small movements
- compensated for at the fovea but not at the periphery
- normally not a problem but can, in certain cases, cause perception of movement
