basic visual processes

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Basic Visual Basic Visual Processes Processes Anatomy and physiology of Anatomy and physiology of the visual pathway and a the visual pathway and a few other pertinent facts few other pertinent facts to get us started to get us started

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Basic Visual Processes. Anatomy and physiology of the visual pathway and a few other pertinent facts to get us started. Anatomy of the eye. Visual angles. Distances in eyes are always given as angles:. Thumb width at arm’s length= 1.5 degrees Fist width at arm’s length=8-10 degrees - PowerPoint PPT Presentation

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Page 1: Basic Visual Processes

Basic Visual ProcessesBasic Visual Processes

Anatomy and physiology of the Anatomy and physiology of the visual pathway and a few other visual pathway and a few other pertinent facts to get us startedpertinent facts to get us started

Page 2: Basic Visual Processes

Anatomy of the eyeAnatomy of the eye

Page 3: Basic Visual Processes

Visual anglesVisual anglesDistances in eyes are always given as Distances in eyes are always given as angles:angles:

Thumb width at arm’s length= 1.5 degreesFist width at arm’s length=8-10 degreesHand span at arm’s length=20 degreesOne degree at retina=0.3 mmVisual angle of one cone=0.0084 degrees = 0.5 minutes of arc

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Measuring the effects of opticsMeasuring the effects of optics

Line and point spread functionsLine and point spread functions If we assume linearity, then we can use these If we assume linearity, then we can use these

to predict what any stimulus presented to the to predict what any stimulus presented to the lens will produce on the retinalens will produce on the retina

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The point spread functionThe point spread function

-Measuring these involves a number of very tricky problems-essentially, what is involved is measuring the reflected light that leaves the eye as a pinpoint of light is presented to it.-remember that what we’re looking at here is what gets through the optics and not necessarily what is seen.

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Contrast and spatial frequencyContrast and spatial frequency

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GratingsGratings

Square wave gratings Sine wave gratings

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Varying contrast and frequencyVarying contrast and frequency

Contrast=Imax/Imin/2*Iav so can be changed by varying top or bottom of fraction

-variation in spatial frequency is usually given in cycles/degree

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Spatial frequencySpatial frequency

Jean Fourier

Any complex waveform can berepresented as the sum of a seriesof sine waves of different frequencies and amplitudes

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Measuring opticsMeasuring opticsModulation transfer functionModulation transfer function A different and handy way to represent A different and handy way to represent

spread functionsspread functions

-what is measured is the ability of the eye’s optics to transmit variation in contrast at a variety of different spatial frequencies

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Measuring opticsMeasuring optics

Chromatic aberrationChromatic aberrationBecause refraction by any medium Because refraction by any medium

depends on wavelength, the depends on wavelength, the optical quality of the eye varies optical quality of the eye varies with wavelength. Resolving with wavelength. Resolving power is lowest at short power is lowest at short wavelengthswavelengths

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The retinaThe retinaReceptor subtypesReceptor subtypes

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The beginning of vision-presenting a flash of light to a photoreceptor producesan electrical response

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Disks in outer segmentscalled lamellae contain a photopigment

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In darkness, there’s a continuous current in theouter segment caused bythe circulation of sodium.

In light, sodium circulationslows down and receptorshyperpolarize

TRANSDUCTION

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Rhodopsin -- the magic photopigment

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Through the wizardry of biochemistry, sodium channels close

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The sensitivity of visual The sensitivity of visual transductiontransduction

7 photons produce a perceptible 7 photons produce a perceptible response (Hecht, Schlaer and Pirenne)response (Hecht, Schlaer and Pirenne)

1 photon closes about 101 photon closes about 106 6 ion channelsion channels

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The receptor mosaicThe receptor mosaic

Spatial frequency limits of the receptor Spatial frequency limits of the receptor mosaicmosaic -using interferometry to get past optics-using interferometry to get past optics

-when light waves collide, the effects can be destructive or constructive. Thomas Young showed that you can set up conditions where these patterns of construction and destruction produce a fine grating. In fact, this was an early, powerful demonstration of the wave nature of light.

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Laser interferometryLaser interferometry

-takes advantage of diffraction to produce very fine gratings behind eye’s optics

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The receptor mosaicThe receptor mosaic

AliasingAliasing

-Nyquist frequency is the spatial frequency above which confusion can occur.

-demonstration of aliasing

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The receptor mosaicThe receptor mosaicCan measure the sampling frequency of the cones using aliasing. The prediction is that when the Nyquist frequency is exceeded, the apparent spatial frequency of gratings should begin to decrease because of aliasing.

The frequency at which this happens depends on the types of cones involved.

Long and Medium wavelength cones – 60 cycles/degShort wavelength cones – 3 cycles/deg

This corresponds to retinal spacing of 10 arc minutes for S-cones and 0.5 arc minutes for M and L cones

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The S-cone mosaicThe S-cone mosaic

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The S-cone mosaicThe S-cone mosaic

Spatial frequency of S-cones makes a nice Spatial frequency of S-cones makes a nice match with point spread functionmatch with point spread function

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Using adaptive optics to visual Using adaptive optics to visual living photoreceptorsliving photoreceptors

Using the same technology as in the Hubble telescope, you can compensate for optical imperfections using a deformable mirror.

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Spectacular Spectacular images of in images of in

vivo vivo receptorsreceptors

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Cone mosaic summaryCone mosaic summary

• It is possible to measure the optical properties of the eye separately from visual function per se using various forms of refractometry.•It is possible to measure some simple properties of the photoreceptor array using laser interferometry•It is also possible to visualize the cone mosaic directly using various staining techniques or, in vivo, using adaptive optics•All of these measurements suggest ultimately that the receptor array is a very good optimized match for the eye’s optics. In other words, the organization of receptors is as good as, but no better than, the eye’s optics•There are significant differences between L and M cones and S cones. Again, though, these differences match the way that the eye’s optics respond to light with different spectral properties

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Photoreceptor arrayPhotoreceptor array

The images show how the photoreceptor array varies with retinal eccentricity

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The duplex retinaThe duplex retina

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Retinal circuitryRetinal circuitryNote important differences in connections of rods and cones

Cones can often have fairly direct access to retinal ganglion cells via bipolar cells.

Rods most often synapse with rod bipolar cells which, in turn, synapse with amacrine cells

Note the midget and parasol retinal ganglion cells.

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Midget and parasol RG cellsMidget and parasol RG cells

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Dendritic field sizes of RG cellsDendritic field sizes of RG cells

Parasol dendritic fields are much larger than midget fields.

It takes many more midget cells to cover visual field than parasol cells

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Lateral geniculate nucleusLateral geniculate nucleus

About 90-95% of retinal ganglion cell axons land here.

Layers 1 & 2 are magnocellular and layers 3-6 are parvocellular

So-called intercalated layers (between 6 layers) may be a special koniocellular pathway, dedicated to S-cone transmission

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M and P pathway response to M and P pathway response to contrastcontrast

*Note that these show response to contrast and not illumination. This is a very important distinction – visual systems respond (non-linearly) over about 5-6 orders of magnitude for illumination but the range of illumination in a typical scene is rarely more than one order of magnitude. By and large, visual systems are linear for contrast.

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Behavioural effects of lesions to Behavioural effects of lesions to P or M layers of monkey LGNP or M layers of monkey LGN

Filled circles = magnocellular lesionOpen circles = parvocellular lesion

Spatial frequency effects of P Temporal frequency effects on bothbut M system very important for high frequencies

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Retinal ganglion cell receptive Retinal ganglion cell receptive fieldsfields

Linear summation of centre and surround responses

Steven Kuffler

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Graphical representations of the Graphical representations of the receptive fieldreceptive field

1-d 2-d

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Can measure CSF for single cellsCan measure CSF for single cells

-low frequency dropoff tells about size of RF-high frequency dropoff tells about size of centre of RF

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CSF for a typical LGN cellCSF for a typical LGN cell

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Adding the time dimensionAdding the time dimension

Notice that temporal properties of centre and surround stimulation differ a bitOne way of describing this is to say that such receptive fields are space-time inseparable. If they were separable, the spatial profile of the rf would differ over time only by a scalar value. There ARE rfs like this, but not in retina or LGN.

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Phase reversing gratingsPhase reversing gratings

-this is one common way to measure the temporal properties of rf’s

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Notice how CSF varies depending Notice how CSF varies depending on both spatial and temporal on both spatial and temporal

frequencyfrequency

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The DOG operatorThe DOG operator

-this is a very common and useful way of modelling the spatial and temporal structures of receptive fields

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Adaptation and contrast Adaptation and contrast normalizationnormalization

-Notice that the CSF does depend on intensity, but only really for low intensities. At higher intensities, contrast sensitivity changes very slowly.

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The retinogeniculostriate pathwayThe retinogeniculostriate pathway

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The structure of cortexThe structure of cortex

Cortex is laminar and connections are very precisely organized

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Orientation selectivityOrientation selectivity

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Hubel and Wiesel’ssimple hierarchical modelof visual cortical processing

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Columnar organization of VI

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Ocular dominanceOcular dominance

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The hypercolumn

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Optical imaging of ocular dominance columns

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Optical imaging of orientation tuning

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Correlation between optical imaging and electrophysiological results for orientation tuning

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Margaret Wong-Riley andthe cytochrome oxidase story

•autoradiography and activity•cytochrome oxidase and activity•intrinsic variability in cyo

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Cytochrome oxidase in monkey VI and VII

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Colour coding in blobsColour coding in blobs

-non-oriented colour opponent cells

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A somewhat opposing viewA somewhat opposing view

Blobs represent convergence of iso-orientation contours so orientation preference is less obvious. Also, because these regions would be active with all contours, one might expect overall higher cytochrome oxidase levels

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Directional Directional selectivityselectivity

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Disparity sensitivityDisparity sensitivity

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V2 and cytochrome oxidase stripes

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Lennie’s viewLennie’s view

A portrait of the visual cortical systems based on simple considerations of area

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““Specificity” of rf propertiesSpecificity” of rf properties

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Contrasting viewsContrasting views