Lecture 8: Perception spatial vision and visual pathways Flashcards
Square wave grating
consists of alternating light and dark bars with sharp, square-like edges, where the luminance (or light intensity) changes abruptly between the bars
Sine wave grating
consisting of alternating light and dark bars with a luminance profile that smoothly undulates,
Spatial frequency
the level of clarity and image
the larger the spatial frequency, the clearer the image will be
What does a small amount of oscillations you have for the same unit of space mean
the longer the distance between the peaks is going to be
Long wavelength in relation to spatial frequency
low spatial frequency
short wavelength in relation to spatial frequency
high spatial frequency
contrast
the difference of the darkest and brightest regions against the average backgrounds
High amplitude at the peaks and troughs in relation to contrast
High contrast
Low amplitude at the peaks and troughs in relation to contrast
lower contrast
Phase
the position of the wave at any point in space e.g., can start of bright then go dark, or start of dark then bright. Grating translated.
degrees
The peaks of the waves are at different points in that continuum known as a phase shift
Key facts about ganglion cells in relation to spatial frequency
Every ganglion cell will have it’s own receptive field size
In centre of visual field the receptive fields are smaller, in the periphery the receptive fields are larger
Depending on the size of their receptive fields each ganglion cell is going to respond most to a different range of frequencies
What does spatial frequency depend on
receptive field size
In relation to spatial frequency when would a cell fire most
When the the spatial frequency matches it’s RF size
What does it mean to say that Ganglion cells are phase selective
Their firing is going to depend on how exactly the light stimulus is positioned on the receptive field
Likes the perfect amount of light in the centre and surround
Why sine waves
- All images can be broken down into or built up from sinusoidal components (Fourier analysis and synthesis)
- The visual system may be processing images in this way
- Sine waves are useful and convenient stimuli for studying the visual system
What are composite gratings
stimuli that combine two or more gratings with different spatial frequencies and/or orientations, which are then perceived as a single, unified pattern
What would be seen in a high frequency picture
fine details e.g., sharp edges, different textures
What would be seen in a low frequency picture
coarse form, gradual changes in intensity or colour
Where is the highest visual acuity
centre of vision as has the highest density of cones
Visual acuity definition
the sharpness or clarity of vision, specifically the ability to discern fine details and shapes at a given distance.
How high spatial frequency affects visual acuity
makes visual acuity harder to perceive
What does the size of the visual angle mean for the image projection on the retina
the smaller the the visual angle the smaller the image on the retina regardless of how big the image might be in metres
what does it mean to say that visual acuity is constrained by cone spacing at the fovea
the sharpness of vision in the central part of the eye, the fovea, is limited by the distance between the cone photoreceptor cells
What does smaller cone spacing mean in terms of visual acuity
higher resolution or better acuity, cones need to be densely packed
What does 20/10 or 6/3 vision mean
good vision at 20 feet (6 metres) you can read what the average person reads at 10 feet (3 metres)away
What does 20/40 or 6/12 vision mean
not so good: at 20 ft (6 metres) you can read what the average person reads 40 ft (12 metres) away
Contrast sensitivity
how much contrast is needed for you to detect the pattern against the grey background
Relationship between contrast sensitivity and contrast needed to detect a pattern
contrast sensitivity is the reciprocal;
Michelson contrast
(Maximum luminance - Minimum luminance)/(Max + Min)
Relationship between contrast sensitivity and spatial frequency
contrast sensitivity depends on spatial frequency
Contrast sensitivity function
The ability to percieve sharp and clear outlines of very small objects, It is also defined as the ability to identify minute differences in the shadings and patterns. CS helps detect objects without a clear outline and distinguish them from their background contrast.
Contrast sensitivity: high frequency cut off
photoreceptor spacing the point where a perceptual system (like vision or hearing) loses its ability to detect or discriminate details or changes in stimuli above a certain frequency
Contrast sensitivity: low frequency cut off
lateral inhibition. The capacity of excited neurons to reduce the activity of their neighbours, so only the neurons that are the most stimulated and least inhibited respond.
when low spatial frequency has matured by
33 weeks
What happens to the nasal part of the optic nerve at the optic chiasm
crosses over and goes to the opposite area of the brain
LGN key facts
Part of thalamus
Left and right LGN - monocular input
6 Layers
4. 1-2: magnocellular layers (M-cells)
5. 3-6: parvocellular centre-surround RFs
6. Concententric centre-surround RFs
7. Feedforward and feedback connections
8. Relays input to cortex, brings input from two eyes into register
Magnocellular
large, fast moving objects rapid, transient response
parvocellular
details of stationary objects slow, sustained response
konicellular
s cone pathway
what would happen if you lost functionality in left visual cortex
wont see anything from your right visual field
Topographic map
in register in each eye
map like representation of visual space
adjacent parts of the visual field are represented by adjacent neurons
Primary Visual cortex or V1 striate cortex:
- 200 million neurons (100xLGN)
- Six layers (input LGN layer 4)
- Topographical mapping
- Cortical magnification
- Neurons turned to orientation, spatial frequency and direction
- Binocular neurons (response to both eyes)
- Ocular dominance (preferred for one eye’s input)
- Simple, complex and hypercomplex cells
- Columnar architecture
topographical mapping in V1
parts of the image near the centre of your gaze (4,5,6) are magnified so receive more cortical tissue, cortical magnification.
Cortical magnification in v1
Much more cortex is devoted to the centre of gaze than to the periphery.
Central 10deg = 50% of V1
1 deg at fovea = 20 mm of cortex
1 deg in periphery, 1- deg away = 1.5 mm of cortex
receptive fields of neurons in V1
v1 neurons care about edges, edge detectors not spots of light.
Also spatial frequency tuned, direction-of-motion tuned
Simple cells
Excitatory region well-defined: responds only if bar is properly positioned in receptive field.
phase sensitive - where is the bar positioned in the receptive field
complex cells
Excitatory region less defined; will respond if bar is positioned anywhere in receptive field
phase insensitive
Hypercomplex (end-stopped) cells
responds only to bars of particular lengths
what does it mean to say v1 neurons are organised into columns
- Neurons with similar orientation tuning form a column - all orientations within 0.5mm
- Neurons with the same eye preference form a column - 0.5 mm per eye
Dorsal stream
vision for action
ventral stream
vision for perception
specialized visual areas higher in the visual pathway
Parahippocampal place area (PPA): places or scenes
* Extrastriata body area (EBA): body parts
* Fusiform face area (FFA): faces or expert objects
* Modules of object specialization
properties of extrastriate visual neuron receptive fields
Larger: respond to larger regions of the visual field than ganglion cells and V1 cells
* More complex: respond to complex forms; not just dots and bars
* Binocular: respond to input from both eye
V4 neurons
respond to complex configurations of figures
How neuronal response changes from retina to extrastriate cortex
becomes more complex
the final representation of an object in the brain is built up from simple features such as spots and edges that are grouped into more complex forms
Adaptation: neural response to a vertical test stimulus after adaptation
Neurons previously most excited by the adaptor now
respond less to the neutral (vertical) test stimulus
* The peak firing rate is shifted to the left (-10 deg)
Adaptation: psychologists electrode
neural response inferred from behaviour
Previous studies (cat, monkey) involved recording of responses of single neurons.
Not easily done in humans.
* Adaptation provides an indirect measure of neural responses
Interocular transfer of adaptation
clue to location of neurons in the visual pathway responsible for the effect
If the effect transfers from one eye to the other, the site of adaptation must reside in binocular neurons in visual cortex
Spatial frequency adaptation
Adapt to a certain spatial frequency (e.g., 7 cycles/degree)
Sensitivity decreases for that spatial frequency, but not
others (i.e., there is a dip in the CSF only around 7 cpd)
Spatial frequency channels revealed by adaptation
SF channel: a set of neurons tuned to a limited range of spatial frequencies
The contrast sensitivity function looks continuous but contains several independent channels
Spatial frequency channels
Visual patterns are analysed by neurons with different receptive field sizes, corresponding to a
specific range of spatial frequencies, i.e., spatial frequency channels
* The perception of any scene or image depends on the activity of several channels
