5- Spatial Recognition Flashcards
what is a cells receptive field
a cells receptive field is the area on the retina which, when stimulated by light elicits a change in the firing rate of the cell
the effect of the stimulus can either be excitatory or inhibitory
what happens in single cell recording
An electrode, inserted into a neuron (e.g. V1)
measures electrical activity
• Activity is that of a SINGLE neuron
how do receptive fields increase in complexity
photoreceptors ganglion cells LGN cells simple cells complex cells hypercomplex cells
what is V1
visual cortex
what do V1 cells respond to
V1 simple cells respond to oriented bars and
edges
what did hubel and wiesel find
Hubel and Wiesel – found oriented bar detectors
in V1 of cats using single cell recording
what is a grating
patterns of black and white stripes
what is the tilt after-effect
Adapt to a pattern of tilted lines - stare for a minute
Test - a pattern of visual lines
Vertical pattern looked tilted in opposite direction to adaption pattern
Percieved isnt as much of a tilt - See it as tilted
what is the explanation of the tilt after-effect
- Orientation tuned neurons respond best to preferred orientation but also respond to other similar orientations
Vertically aligned receptive field
Response function of cell
Respond to preferred orientation and similar
Neuron in v1 - vertically orientated - strongest response vertical stimuli in the receptive field
how many components in tilt after-effects
3
what is the perceived orientation determined by
Perceived orientation determined by distribution of responses across cells
E.g. 7 cells in v1 - each have different preferred orientation
E.g. cell 7 - receptive field horizontally orientated - prefers horizontally organised stimuli
Visual system looks at distribution of responses across all cells and looks to where the peak is
what happens in adaption
cell’s response decreases following prolonged activity
All cells responding - their response decreases over a period of time - fatigues
what are the 3 components of tilt after-effects
3 components:
1. Orientation tuned neurons respond best to preferred orientation but also respond to other similar orientations
- Perceived orientation determined by distribution of responses across cells
- Adaptation – cell’s response decreases following prolonged activity
what happens in the tilt after-effect
Before adaptation the line looks vertical
Start of adaptation tilted line looks tilted
During adaptation tilted line continues to look tilted, but cells’ responses decrease
After adaptation vertical line looks tilted due to asymmetrical response distribution
why does the tilt after-effect happen
Even though viewing vertical patten - distribution of responses has shifted
Peak moves
See a tilt in opposite direction
Expected after adapted
Caused a reduction in the response of 3,4
Cell 2 wasn’t adapted by adaptive pattern
Cell 2 responding similarly - the peak
what does the size of after-effect depend on
Size of after-effect depends on difference between adapt and test
Depends on difference between adapt and test
Adapt - reduced range of cells - causing cells 4,5 to adapt , 3 isnt adapted due to difference
When go back to viewing the trust the peak of distribution doesnt change
When big difference dont get big after effects
what curve is the after effect
sine wave curve
Peak around 10-20 degrees of adapting pattern
When adaptation gets further away to 45 degrees
Get less of an effect
what would happen when 45 degree tilt
45 degree tilt
Peak wouldn’t shift too much
Smaller after effect in clockwise direction
what happens in a 20 degree anti-clockwise tilt
Big after effect in opposite direction
Anti-clockwise tilt - clockwise after effect
20 degrees tilt
what is the peak effect
Peak around 10-20 degrees of adapting pattern
When adaptation gets further away to 45 degrees
Get less of an effect
what does the tilt after effect provide evidence for
Tilt after-effect provides evidence for orientation tuned cells in human visual system
Psychophysics
Measure what people perceive
Make inferences about visual processes
Orientation tuned cells in visual system
Cant do single cell recording in humans so tilt after effects eviden ce that operates in same way as cats and monkeys
Cats and monkeys have cells with different sized receptive fields at each orientation… is the same true of humans?
yes
Different cells in v1 have different orientations and different size of receptive field
Respond best to different sizes of line
Receptive fields sizes vary but all have a preferred orientation
is the after-effect opposite to what you adapted with
yes
what happens before adaptation
Before adaptation size perceived veridically
are cells tuned to different sizes and width of lines
Cells are all tuned to vertical lines but different sizes and width of lines 1- skinny lines 7- fat lines 3 - perfectly matches receptive field 2,4, weaker response 1,5,6,7 dont respond
wha happens in adaptation
adapt to fatter bars
During adaptation cells’ response decreases
After adaptation lines look thinner due to
asymmetrical response distribution
Perceive narrower bars than what actually there bc peak is on the thinner side
what does tilt after-effect provide evidence for
- Tilt after-effect provides evidence for orientation- tuned cells in human visual system
- Size after-effect provides evidence for size-tuned cells in human visual system
are size and orinetation fundamental parts of the viusal scene
Size and orientation are fundamental features of parts of the visual scene and the brain has cells tuned to these features
what do we mean by size
ambigous
“Serious scientists do not, we are told, refer to bars as fat or skinny. Rather they define the size of these bars in terms of the number of bars that would fit in a given distance, so
big bars actually have what is called a low spatial frequency
skinny bars have a high spatial frequency
… (This makes the topic much more boring and much less clear and therefore the people who use these terms are much more important. So obviously we’re going to talk about spatial frequency from now on).”
If a cell responds strongly to a given stimulus, how will the cell’s response change following prolonged exposure to that stimulus?
response will decrease
After prolonged exposure to an adapting bar rotated 45 degrees away from vertical (clockwise), a subsequently viewed bar which is physically vertical will appear:
approximately vertical
what is spatial frequency
number of bars per unit distance (usually cycles per degree)
in fat bars how mant cycles per degree
Far bars One cycle per degree Peak white Trough black One unit of distance on the retina
how many cycles per degree in skinny bars
2 cycles per degree
Fit more bars in same difference on retina
2 cycles per degree
what frequency are fat bars
Fat bars = low spatial frequency
what frequency are thin bars
Thin bars = high spatial frequency
how do you define size of a feature on retinal image
spatial frequency
do natural images contain information at many spatial frequencies
yes
Natural images contain information at many spatial frequencies
what are high spatial frequency
fine details
low spatial frequency
course information
how do we filter spatial frequency in images
Filter image to extract certain spatial frequencies
Filter out spatial frequencies
Picked out fine details - high spatial frequency carries out e.g. eyes nose mouth - thin lines - high spatial frequency
Low spatial frequency - shading - fatter bars - big features
what is high contrast
big difference between light and dark
wht is low contrast
Only varies from slightly darker from mid grey and lighter mid grey here
at high contrast what spatial frequency can you see
High contrast see all different spatial frequencies
at low contrast and low spatial frequency
cant see bars
and low contrast
cant see any bars
do we have equal spatial contrast sesnitivity
Not equally sensitive
Sensitivity - how low turn contrast and still be able to detect
As change spatial frequency of pattern - sensitivity varies
Low spatial frequency - low sensitivity
Mid - sensitive to intermediate spatial frequencies
High - lower sensitivity
when do we have greater sensitivity
We have greater sensitivity to intermediate spatial frequencies. They can be perceived at low contrasts
what are spatial frequency channels
Spatial frequency channels – collections of neurons tuned to the same spatial frequencies (i.e. respond to the same range of spatial frequencies)
what happens when spatial frequency too high
Spatial frequency too high - no longer have cells that can rspond - no receptive fields small enough - details become invisible to us bc aren’t sensitive
what happens when we recognise
Recognise - high spatial frequency information carry fine details allowing us to recognise her face - fine details in face
what happens when far away
Far away - high spatial frequency - invisible to us - no longer able to recognise far away - information we need is invisible to us
Spatial frequencies get bigger on retina - range with which you can perceive
Spatial frequency gets lower when closer
what happens in the periphery
receptive field sizes increase
This means contrast sensitivity varies with eccentricity - we can’t see high spatial frequencies in the periphery
fovea
Fovea - central party of retina - densely packed with cones - good visual acuity - good at picking up intermediate spatial frequencies and high sensitivities
periphery of retina
Periphery of retina - cells have bigger receptive fields- spatial frequencies sensitive to get lower - lose ability to detect high sensitivity
far periphery
Far periphery - only detect very low spatial frequencies
Receptive field gets bigger as move to periphery
spatial contrast sensitivity function
- Receptive field sizes increase in the periphery
• This means contrast sensitivity varies with eccentricity - we can’t see high spatial frequencies in the periphery
• Therefore acuity is poorer in the periphery
how is spatial frequency processed by the visual system
When images processed by visual system, information at differing spatial frequencies carried by separate channels
Different cells in primary visual cortex
Processing in channels that are all processing different spatial frequencies
retinal vs real size
Spatial frequency tells us about size on the retina – it does NOT indicate real size in the world since the projected size depends on distance
any line on the retina could have been produced by infinite different lines in the world
what does spatial frequency tell u
size on the retina
retinal vs real size
Bars of different widths at different distances can project to same SF on retina
what did Burbeck do
Can we perceive both retinal and real size?
• Stimuli 1 & 2 at different distances from participant
• Discrimination experiment - which stimulus has thinner bars?
• Discriminating retinal size – task very hard
• Discriminating real size – task easy
• Conclusion – conscious perception in terms of real size
what is size constancy
Size constancy: we perceive an object’s real size in the world regardless of distance
what is orientation constancy
we perceive an object’s orientation in the world regardless of the orientation on the retina
why is it hard to draw a 3D object
No conscious access to retinal image properties explains why drawing a 3D object or scene can be such a challenge
retina - 2D representation
what are images composed of
Images are composed of lines and edges of differing orientation, size and contrast.
what is the visual image processed by
The visual image is processedby neurons tuned to orientation and spatial frequency
