Spatial vision John Greenwood Department of Experimental Psychology - - PowerPoint PPT Presentation

Spatial vision

John Greenwood Department of Experimental Psychology

NEUR3045 Contact: john.greenwood@ucl.ac.uk

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Today

• What is spatial vision?
• Physiology of spatial vision
• The dimensions of spatial vision: Fourier analysis
• Orientation
• Adaptation and population coding
• Spatial frequency
• The contrast sensitivity function (CSF)
• Population coding for spatial frequency
• Foveal vs peripheral vision
• Acuity and crowding

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What is spatial vision?

• Our perception of the spatial

distribution of light across the visual field

• The building blocks of object

perception

• The early stages of visual

processing

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Retina LGN V1

Spatial vision: LGN

• Retinal ganglion cells and neurons in

the Lateral Geniculate Nucleus have
 centre-surround receptive fields

• Both on-centre and off-centre subtypes
• Can highlight regions of change
• (i.e. transitions from light to dark or 


dark to light)

• But this does not give selectivity to

the orientation of edges

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• On-centre

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Off-centre

Spatial vision: V1

• Hubel & Wiesel (1962) found
• rientation selectivity in the

primary visual cortex (V1)

• Cells respond to particular
• rientations of edges & lines
• Have a preferred orientation that

produces maximal spike/firing rate 
 (Schiller et al., 1976)

• Likely built from particular

combinations of centre-surround LGN neurons (Hubel & Wiesel, 1968)

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V1 LGN

The dimensions of spatial vision

• We can consider the ‘building blocks of spatial vision’ via

Fourier analysis

• Fourier (1822) showed that any signal can be

decomposed into a sum of sine waves at different frequencies, amplitudes and phases

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Sine wave amplitude

• Amplitude for a sine wave grating gives luminance contrast
• The difference between light and dark regions in the scene

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75% 50% 25% 0% 100%

50 100

Sine wave phase

• Phase determines the point at which variations occur in

space, e.g. the starting point of the cycle

• Represented in radians with a cyclical structure
• Determines the position of edges in the scene

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π/2 π 3π/2 2π

Sine wave orientation

• For two-dimensional images we also need to consider the
• rientation of the sine wave

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0° 45° 90° 135° 180°

• Orientation is certainly a key dimension for visual

processing and we’ll return to this shortly

Sine wave spatial frequency

• Spatial frequency determines the variations across space
• Reported as the number of cycles in a spatial region (peak to peak)
• Captures the fine vs. coarse detail in an image

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4 cyc/image 8 cyc/image 16 cyc/image 2 cyc/image 1 cyc/image Low High

Summing the components

• How do we make an image using sine waves?
• Sum all of the component sine waves together
• Easiest example: a square wave
• How do you get a square wave from sine wave components?

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From to ..

• Take a sine wave

with matched spatial frequency: the fundamental

• Add the odd

harmonics (increasing SF) with decreasing amplitude

12 F+3F+5F+7F Fundamental (F) F+3F F+3F+5F Fundamental (F) 3F 5F 7F

Components Sum

Summing the ideas

• Fourier analysis tells us that we can break the visual scene

down into component wave forms characterised by:

• Amplitude (contrast)
• Phase (position)
• Spatial frequency (size)
• Orientation (orientation…)
• How are these dimensions encoded in the visual system?
• Let’s look at two aspects:
• Orientation
• Spatial frequency

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∑ Original Spatial frequency (low to high) Orientation ∑

Filtering an image with filters similar to the receptive fields of V1 cells gives us orientation energy at a range of spatial frequencies

Local orientation

• How do we encode orientation across the visual field?
• One way to examine this: adaptation
• Gibson (1937): prolonged viewing of one orientation

reduces sensitivity to that orientation, and produces repulsion in the perceived tilt of dissimilar gratings

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Adapt Test

Orientation adaptation

• Adaptation reduces sensitivity to the adapting orientation
• e.g. higher contrast required for detection
• Can be attributed to reduced sensitivity of the underlying neurons

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Pre-adapt Post-adapt

S t i m u l u s S t i m u l u s

Local orientation

• Gibson (1937): prolonged viewing of one orientation


reduces sensitivity to that orientation, and produces
 repulsion in the perceived tilt of dissimilar gratings

• i.e. adaptation reduces sensitivity 


to the adapting orientation 
 (performance)

• It can also alter the perceived

• rientation (appearance), which


we call the tilt aftereffect

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Adapt Test

Tilt aftereffect

• Subsequent dissimilar orientations appear repulsed away
• Produces a shift in the peak response away from the adaptor
• Suggests population coding of orientation (Blakemore et al., 1971)

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T e s t

Pre-adapt

T e s t

Post-adapt

A d a p t

• r

shifted peak

Two key principles

• Adaptation
• Reduces neural responses to

continued stimulation and enhances responses to novel stimuli (redundancy reduction)

• Population coding
• Adapting to one orientation

influences the perception of others

• Our perception of orientation is

inferred from the population of neural responses, e.g. as the peak

• Allows a resolution higher than the

sensitivity of individual neurons

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Orientation in context

• Suppressive effects are also seen outside the local region
• Contrast surround effects especially apparent with matched
• rientation & spatial frequency (Chubb et al., 1989)

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Surround suppression

• Can again be attributed to reduced sensitivity of the

underlying neurons, via connections from adjacent neurons

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S t i m u l u s S t i m u l u s

Context effects: Tilt contrast

• With dissimilar orientations can also see a shift in

perceived orientation - similar to the effects of adaptation and the tilt aftereffect (Gibson, 1937)

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Tilt contrast

• Adjacent orientations appear repulsed from one another
• Also accounted for by shifts in population response, induced by

adjacent neurons (Blakemore et al., 1970)

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T e s t T e s t S u r r

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n d

Surround effects

• Likely mediated by intracortical connections in V1
• Around 90% or neurons in V1 are suppressed by the activity of

their neighbours (Jones et al, 2001)

• Suppression minimises the response to homogeneous

regions and highlights differences

• Another instance of redundancy reduction (across space rather

than time)

• Minimises metabolic costs of neural firing (Laughlin et al., 1998)

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Spatial frequency

• Fourier analysis also gives us a way to think about scale
• Images contain information at different spatial frequencies
• Which of these components is visible to an observer?
• With Fourier analysis we can take a broader view of the image

content that is visible to a given observer

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4 cyc/image 8 cyc/image 16 cyc/image 2 cyc/image 1 cyc/image

SF in natural scenes

• What does spatial

frequency mean for natural scenes?

• Low-pass filtering:

allow only the lowest SFs to be visible (broad blobby things)

• High-pass filtering:

allow only the highest SFs to be visible (edges & fine detail)

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A note on units

• How do we characterise these spatial variations?
• Cycles/image is OK for theoretical Fourier analyses
• But for visual perception, image size on the retina is affected

by both size and distance

• Need to measure retinal size
• Calculated as degrees of visual angle, where 


tan(α) = Height/Distance

• For SF gives cycles/degree

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α

Contrast sensitivity functions

• Campbell & Robson (1968):
• Measured contrast sensitivity at

a range of spatial frequencies

• Contrast sensitivity function

(CSF) peaks around 4 c/deg

• Sensitivity is not greatest for

uniform regions (low SF)!

• Sensitivity also drops for high

SFs - highest visible spatial frequency is our acuity cutoff

• Altogether defines our 


‘window of visibility’

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Highest 
 visible SF (acuity 
 cutoff)

low high

1 deg.

A depiction of the CSF

• We visualise

the CSF by plotting contrast against spatial frequency

• Note: peak in

the middle & the drop in visibility on either side

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Spatial frequency Contrast (amplitude) high low low high

What produces the CSF?

• Why do we show this pattern of sensitivity?
• Campbell & Robson (1968) hypothesised that the visual

system is composed of spatial frequency channels - each sensitive to a restricted range of SFs

• Blakemore & Campbell (1969) tested this using adaptation

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Adapt Test Test

• r

Multiple Channels Single Channel

CSF adaptation: predictions

• Adaptation reduces sensitivity to contrast
• But does it affect all SFs or just those of the adaptor?

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Adapting SF Adapting SF

Multiple SF channels

• Adaptation to a sine grating

with 7.1 cycles per degree

• Sensitivity is strongly reduced at

the adapted SF and nearby values

• No effect for SF values at the

extremes of the range

• Consistent with multiple

channels for spatial frequency

• Evidence that we separate the

visual scene into its Fourier components (at least for SF)

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Adapting SF

SF population coding

• We can also see adaptation effects and surround effects

for spatial frequency: common principles

• e.g. simultaneous contrast illusions for spatial frequency


and the Titchner / Ebbinghaus illusion for size

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Peripheral vision

• Spatial vision differs markedly between foveal (central) and

peripheral parts of the visual field

• Partly due to the decrease in resolution with increased

distance (eccentricity) from the fovea

• Can be attributed to the decrease in retinal cone density

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Peripheral acuity

• Peripheral acuity is worse than

foveal acuity

• e.g. a fixed letter size is harder to

read in the far periphery vs. near
 to the fovea

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E S A C U I T Y A T I O N F I X K N I F I C M A O C L A C I T R

Anstis (1974)

Peripheral acuity

• Peripheral acuity is worse than

foveal acuity

• e.g. a fixed letter size is harder to

read in the far periphery vs. near 
 to the fovea

• We can overcome this decrease

in resolution by increasing letter sizes with increasing eccentricity in the visual field (scaling)

• Is this all that differs in

peripheral vision?

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Anstis (1974)

E S A C U I T Y

A T I O N F I X

K N I F I C M AO

C L A C I T R

Crowding

• Impaired recognition of objects in clutter
• Not a limitation in acuity: affects objects that are otherwise

visible in isolation (Bouma, 1970)

• Strong in peripheral vision; weak/absent in foveal vision

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R

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K R S

F F F F

When does crowding occur?

• The presence of flankers (F) affect recognition within an

interference zone around the target (T)

• Interference zones increase in size with eccentricity 


(Toet & Levi, 1992)

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T

eccentricity

T F F F F F F F F F F F F

crowding no crowding

What does crowding do?

• Observers can’t report

the target orientation but can report the average orientation 
 (Parkes et al., 2001)

• Crowding involves the

pooling of target and flanker identities, 
 e.g. through averaging

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Why crowd?

• Crowding simplifies vision
• Peripheral vision is undersampled
• Pooling gives a ‘gist’ of the scene

(average orientation, colour, etc) even though detail can’t be seen

• Differs from foveal processes

(e.g. tilt contrast) where differences are emphasised

• Emphasising differences makes

more sense when the image is finely represented, as in the fovea

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Crowding in the brain

• Size of interference

zones matches receptive field sizes in area V2 (Freeman et al., 2011)

• fMRI shows crowding

alters activity in many brain regions but most strongly in area V4 (Anderson et al., 2012)

• Perhaps multiple stages

but likely beyond V1

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T F F

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F F T F F F F

Summary

• Fourier analysis gives us the dimensions of spatial vision
• Contrast, phase, orientation and spatial frequency
• Population coding accounts for orientation perception
• Seen with adaptation (e.g. the tilt aftereffect) and surround effects
• Our perception of spatial frequency can be described by

the Contrast Sensitivity Function (CSF)

• Similar evidence for population coding
• Spatial vision differs in the fovea and periphery
• Acuity declines and visual crowding increases to capture ‘gist’ 


at the expense of fine detail

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Reading

• Chapter 3 of Wolfe et al. Sensation & Perception gives a good
• verview of these ideas
• Further reading (if interested / confused):
• Surround suppression: 


Blakemore, Carpenter & Georgeson (1970) Lateral inhibition between

• rientation detectors in the human visual system. Nature.
• Spatial frequency:


Campbell, F.W., & Robson, J.G. (1968). Application of Fourier analysis to the visibility of gratings. Journal of Physiology, 197, 551-566.

• Crowding: 


Whitney & Levi (2011). Visual crowding: a fundamental limit on conscious perception and object recognition. Trends in Cognitive Sciences.

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