Section 4 Flashcards
What does it mean for the three dimensional world to be Euclidean?
It respects a set of basic geometrical rules, such that size, shape, and relative coordinates of objects remain constant when viewed at different distances
What is Euclidian geometry?
Objects remain the same size and shape as they move around in space
What are the two main ways our visual system infers the third dimension?
- monocular clues
2. binocular cues
What are the two main monocular depth cues?
Pictorial and movement-based cues
List all the pictorial cues
- occlusion
- relative size
- familiar size
- relative height
- texture gradient
- linear perspective
- aerial perspective
- shadows
What is the difference between a non-metrical and metrical depth cue?
A non-metrical depth cue cannot tell you anything about the relative or absolute distance between two objects, whereas a metrical depth cue can
Which pictorial cue is an absolute metrical depth cue?
Familiar size
How does relative height work?
Objects that appear higher in the visual field tend to be perceived as being further away
When do we experience the greatest feeling of distance between us and an object?
When the object is parallel to the ground
How does linear perspective help us capture the third dimension?
By relying on the idea that objects become smaller and closer to each other as they recede in space
What is a vanishing point?
The apparent point at which parallel lines receding in depth converge
Which combination of pictorial cues generates linear perspective?
relative size and relative height
What is the ponzo illusion?
When two parallel lines extend into the distance, our visual system makes us think they are converging together. If you place an object at two different points along these converging lines, because the further lines look smaller and closer together it will make the object placed further look bigger
How do shadows infer the third dimension?
If the shadows of objects are at different distances in the visual field, then the objects will also look to be at different differences
How do objects appear if their shadows are aligned?
It will look like the objects differ in height but not distance
How does motion parallax work?
It triggers the perception of depth based on the perceived translational movement of objects
How does optic flow work?
Allows the perception of objects while looking at where you are headed to
What is the focus of expansion?
The fixation point at the horizon from which the world seems to expand or vanish as you go forward or backwards, respectively
What two muscle groups send sensory feedback to your brain as you adjust your focus point?
Ciliary muscles and extraocular muscles
What information do the ciliary muscles send to the brain?
informs on the level of accommodation required to focus the image on the retina
What information do the extraocular muscles send to the brain?
informs on the level of vergence
Nearing objects cause _____ (convergence/divergence) of the eyes to keep the fixated object at the center of both retinas, whereas fixating a far away object increases eye _______ (convergence/divergence)
Convergence, divergence
TRUE/FALSE
Unlike most pictorial cues, oculomotor cues provide us with unambiguous information about absolute depth
true
What is stereopsis?
The rich impression of depth that we get from seeing with both eyes
Describe an advantage and disadvantage of binocular summation
Advantage: optimizes our interactions with objects in front of us
Disadvantage: narrows down the extent of our visual field
What is binocular disparity?
The difference between the two retinal images of the same scene
What is the concept of corresponding retinal points?
States that points on the retina of each eye where the monocular retinal images of a single object are formed are at the same distance from the fovea of each eye
What is the Vieth-Muller circle?
A circle drawn based on the point of fixation, whereby points that fall on this circle will have corresponding retinal points and be seen as one image
What happens when an object does not fall on the Vieth-Muller circle?
It will have dissimilar points on the two retinas which will make you see double of the object
What is the horopter?
It is a semi circular delineation where points along this path have zero disparity between the two retinal images
What is Panum’s fusional area?
An area around the horopter where the retinal disparities are small enough for our brain to correct them, so images from each eye will still be seen as one
What is a stereothreshold?
The smallest perceptible distance between two objects before they merge into one
When does diplopia occur?
When objects lie outside of Panum’s fusional area
What happens as object move further away from the limits of Panum’s fusional area?
The greater the duplication of the objects until they start to blur with the background
What is crossed disparity and how does it occur?
It is when the position of an object in the visual field is opposite to the eye’s anatomical position (projects onto the temporal retinas) and arises when an object is located in front of the horopter
What is uncrossed disparity and how does it occur?
It is when an image will be projected on the nasal retinas and therefore will appear to be located on the same side of the visual field as the eyes anatomical position. it arises when an image is located behind the horopter
How does the visual cortex determine whether an object is near or far?
By computing the binocular coordinates of the retinal projection of objects relative to the fovea
How are binocular neurons organized?
Their organization follows a gaussian distribution where neurons that respond to zero disparity are the most numerous
What do we call binocular neurons that respond to crossed and uncrossed disparity?
tuned near and tuned far
Based on neuron abundance, why do we experience diplopia?
Neurons that respond to objects outside of Panum’s fusional area are less abundant
TRUE/FALSE
Each binocular neuron has its own specificity for the amount of disparity it prefers
TRUE
What are the two hypotheses for how our visual system knows an object seen by the left and right eye are the same?
Hypothesis 1: object recognition is a monocular process that is followed by the binocular integration of mages from which stereopsis emerges
Hypothesis 2: binocular integration happens automatically without the need for object recognition
What is a random dot stereogram?
The stimuli are composed of dots that are distributed in a random fashion. Part of the image is duplicated so that its superposition with a stereoscope gives rise to a 3D image that can only be perceived with stereopsis
Why can a random dot stereogram determine which hypothesis is true?
It can only be seen stereoscopically, which is a process that cannot be done with monocular vision - this proves hypothesis 2 is the correct pathway
What is binocular rivalry?
Occurs when the images seen from the left and right eye are non-corresponding
How does the brain deal with binocular rivalry?
If the images are different the brain will choose what it wants to perceive, whereas if they are of similar interest the perception of the two images with alternate in time over the course of minutes
What is akinetopsia?
A rare neurological disorder in which the affected individual has no perception of movement
What is apparent motion?
The illusory impression of smooth motion resulting from the rapid alteration of objects that appear in different locations in rapid succession
What are the two variables of apparent motion?
Speed of the moving objects and position
Describe how a motion detection circuit works
There are 5 neurons involved: A, B, D, X, and M. The receptive field of neuron A will encode movement and send this signal to neuron D which acts as a delay neuron. Once the receptive field of neuron B sends a signal, neuron D will release the signal from A such that both signals reach neuron X at the same time. If neuron X recognizes a temporal connections between D and B, it will fire and send a signal to neuron M which will register a change in position between A and B
Which type of neurons are likely involved in motion detection?
rods
Magnocellular ganglion cells are responsible for ______ (movement, object recognition), whereas parvocellular ganglion cells are responsible for ______ (movement, object recognition)
movement, object recognition
Where would neurons A and B be located?
In the retina, LGN, and V1
Where would neurons D and X be located?
Higher up in the dorsal pathway (V3 and V4)
Define the aperture problem
When a moving object is viewed through an aperture (or receptive field), the direction of motion of a local feature or part of an object may be ambiguous
How do you overcome the aperture problem?
Each receptive field has several direction of motions that would be compatible with the stimulation the receptor is receiving, so by combining them all there will be one direction that is the same for all which is the true global motion direction of the object
What is a motion after-effect?
The illusion of motion of a stationary object that occurs after prolonged exposure to a moving object
Describe the smooth pursuit eye movement
Voluntary eye movement in which the eyes move smoothly to follow a moving object
Describe the saccade eye movement
A type of eye movement made by both voluntary and involuntary motion, in which the eyes rapidly change fixation from one object or location to another
Describe the vergence eye movement
Both voluntary and involuntary in which the two eyes move in opposite directions
Describe the reflexive eye movement
Automatic and involuntary
Describe the microsaccade eye movement
An involuntary small jerklike movement that prevents visual fading, improves visibility of sharp objects
What are the six muscles that are attached to each eye?
Superior oblique, inferior oblique, superior rectus, inferior rectus, lateral rectus, and medial rectus
What is the role of the superior colliculus?
It is a structure in the midbrain that is important in initiating and guiding eye movements
What is the role of the frontal eye field?
It is a structure in the frontal lobe that is important in initiating and guiding eye movements, but rather than encoding the direction of the movement, it encodes the part of the visual field where you want to direct your gaze towards - it gives instructions to the superior colliculus
What is saccadic suppression?
The reduction of visual sensitivity that occurs when we make saccadic eye movements, which eliminates the smear from retinal image motion during an eye movement
What is a comparator?
An area of the visual system that receives one copy of the order issued by the motor system when the eyes move
Why does the SC and FEF send a copy of the motor command to the parietal lobe before initiating the eye movement?
It allows you to plan the movement you want to make and then compare what you are perceiving to the command you sent
What is inattentional blindness?
A failure to notice (or at least report) a stimulus that would be easily reportable if it were attended to
What is change blindness?
A failure to notice a change between two scenes
Describe selective attention
The form of attention involved when processing is restricted to a subset of the possible stimuli
What is divided attention?
Splitting attention between two different stimuli
What is sustained attention?
Continuously monitoring some stimulus
What is external attention?
Attending to stimuli in the world
What is internal attention?
Attending to one line of thought over another or selecting one response over the other
What is overt attention?
Directing a sense organ towards a stimulus
What is covert attention?
Attending to a stimulus without giving an outward sign you are doing so
What is the feature integration theory?
A limited set of basic features can be processed in parallel preattentively, but other properties, including the correct binding of features to objects, require attention
What is the preattentive stage?
The processing of a stimulus that occurs before selective attention is deployed to that stimulus
What is the spotlight model of attention?
Attention is restricted in space and moves from one point to the next, where areas within the spotlight receive extra processing
Describe the top-down (exogenous) control mechanism of attention
Attention is voluntarily directed towards stimuli relevant to current goals
Describe the bottom-up (exogenous) control mechanism of attention
Attention is involuntarily captured by things that stand out
What is a cue?
A stimulus that might indicate where (or what) a subsequent stimulus will be
What are the three types of cues?
Valid (correct information), invalid (incorrect information), or neutral (uninformative)
In the Posner cueing paradigm, how do valid and invalid cues affect your performance?
Valid cues make your retention time for responding to the target shorter, whereas invalid cues make your retention time longer
What is an exogenous/peripheral cue?
Cues that are located outside of where your attention is, and can be valid or invalid
What is an endogenous/symbolic cue?
This cue is located in or near the current location of attention, and can have an associated meaning such us a red dot means look left - can also be valid or invalid
What takes longer to process, a peripheral or symbolic cue? Why?
Symbolic cues take longer to process because you need to extract the meaning from the cue before you can direct your attention, whereas with peripheral cues it guides your attention right away (more of a bottom-up approach)
What is stimulus onset asynchrony (SOA)?
The time between the onset of one stimulus and the onset of another
For the same stimulus onset, which type of cue would benefit more from a valid cue?
Peripheral
What types of symbolic cues work best? Why?
Gaze and arrows because these cues automatically tell your brain what to do
What is rapid serial visual representation (RSVP)?
An experimental procedure in which stimuli appear in a stream at one location at a rapid rate
What can RSVP be used for?
To study the temporal dynamics of visual attention
What is an attentional blink?
The difficulty in perceiving and responding to the second of two target stimuli amid a RSVP stream of distracting stimuli
We experience attentional blinks when the targets are 200-500 ms apart, but how come we do not experience it when the targets are very close together?
We are processing both targets at the same time
What is a visual search?
Looking for a target in a display containing distracting elements
What is a set size of a visual search?
The number of items in a visual search display
What is a feature search? List two features of it
A feature search is a search for a target defined by a single attribute, such as a salient color or orientation
- Do not need attention to do it
- Response time will not change as a function of the set size
What is a conjunction search? List two features of it
Searching for a target defined by the presence of two or more attributes
- Need spotlight of attention to do it
- Reaction time is a function of set size
What is a serial self-terminating search?
A search from item to item, ending when your target is found
What is a guided search?
Attention is restricted to a subset of possible items based on information about the item’s basic features (e.g. color or shape)
What is a scene-based guidance search?
Information in our understanding of scenes helps us find specific objects in scenes
What are the two types of pathways for looking at objects in scenes which occur in parallel?
- Non-selective pathway
2. Selective pathway
Describe the non-selective pathway for looking at scenes
Contributes information about the distribution of features across a scene as well as information about the gist of the scene, and is fast
Describe the selective pathway for looking at scenes
Permits the recognition of one or a few objects at a time and requires a visual search
What is the bottleneck of attention?
A slow point in visual processing where you need to pay attention to several features of a scene
How does the non-selective pathway compute the gist of a scene so quickly?
It uses ensemble statistics and does not pass through the bottleneck of attention
What is spatial layout?
The description of the structure of a scene without reference t the identity of specific objects in the scene
Memory for objects and scenes is amazingly good, but how does the relatedness of the objects affect your ability to remember them?
The more different the objects are, the easier it is to remember them
The parietal lobe is important for the _____ (what/where) pathway
where
What does damage to the parietal lobe do?
Causes a visual defect such that one side of the world is not attended to
Why does a lesion in your right parietal lobe lead to neglect of the left visual field, but damage to the left parietal lobe does not lead to neglect of the right visual field?
The right parietal lobe pays attention to both the contralateral and ipsilateral visual field, whereas the left parietal lobe only pays attention to the right
What is the difference between neglect and a visual field defect?
In the case of neglect, you can still see both sides of your visual field but you do not realize you can’t process one side, whereas with a visual field defect there is one part of your visual field you cannot see and you know it
What is extinction?
The inability to perceive a stimulus to one side of the point of fixation in the presence of another stimulus, typically in a comparable position in the other visual field
Which side of the visual cortex has its activity enhanced by attention?
The contralateral side relative to where your attention is directed
