Lecture 5 - Spatial Vision Flashcards
orientation sensitivity in humans
V1 simple cells respond to oriented bars and edges.
- tilt after-effect (provides evidence)
Three components of the tilt-after effect:
- orientation tuned neurons respond best to preferred orientation BUT also other similar orientations - this is the RESPONSE FUNCTION OF A CELL
- perceived orientation determined by distributions of responses across cells. - if the stimulus doesn’t match the receptive field of any of the receptors - the greatest response with most similar receptive fields and greatest peak will be perspective tilt point.
- adaptation - cells response decrease following prolonged activity - cells fatigue if responding for a long time causing response to decrease
size of after-effect depends on difference between adapting and test pattern:
- if the difference is BIG: we would expect the adapting pattern to cause adaptation of relevant cells = SMALLER AFTER-EFFECT
- if the difference is SMALL (adapting and test pattern similar): we would expect a bigger impact on the distribution of response and therefore bigger tilting = LARGER AFTER EFFECT
- NO AFTER EFFECT when adapting at vertical and testing at vertical as it causes no symmetry
TILT-AFTER EFFECT: if adaptation is on anti-clockwise tilt..
- clockwise after effect
- if the tilt is greater this = smaller after-effect in clockwise direction
different cells in V1 have different orientations and….
different sizes - respond to different sizes of line
size after-effect
- similar to tilt-after effect
- cells all tuned to same orientation are tuned to different sizes
- cell that matches size gives biggest response - surrounding cells give also do (e.g. matches cell 3, 2 and 4 also respond)
size and spatial frequency:
- big bars have low spatial frequency
- skinny bars have high spatial frequency
what is spatial frequency:
number of bars per unit (usually cycles per degree)
one degree refers to visual eye field
natural images and spatial frequencies
high spatial frequency = fine details - e.g. outline of face
low spatial frequency = course information - e.g. shade information where features are
contrast?
- contrast refers to the difference in luminance from light to dark areas
- another way to define features of the retinal image
perceiving spatial frequencies:
- high spatial frequencies you can see individual lines
you cant at low spatial frequencies
- therefore we have greater sensitivity to intermediate spatial frequencies and these can be perceived at low contrasts
what are groups of neurons that respond to different frequencies termed as:
channels
- see faces closer up because spatial frequencies get lower and details get bigger
spatial contrast sensitivity function:
- receptive field sizes increase in periphery
- contrast sensitivity varies with eccentricity i.e. we cant see high spatial frequencies in periphery
- so acuity is poor in the periphery
retinal versus real size
- spatial frequency tells us about SIZE ON RETINA
- doesn’t indicate real size
- size will get bigger on retina when object closer - but we know real size hasn’t changed.
Burbeck (1987)
two stimulus (of vertical lines) at different distances from P
- asked which has thinner bars’
- Ps really struggled to discriminate retinal size
- found PS couldn’t make estimates of the size of the bars when referenced to real world
- but discriminating real size was easy even though size difference was similar
- we are aware of the sizing of the world but we don’t have conscious access to size on the retina.
