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.
size constancy
we perceive objects real size in the world regardless of distance
Orientation consatncy
- we perceive an objects orientation in the world regardless of orientation on the retina
