basic visual processes
DESCRIPTION
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 PresentationTRANSCRIPT
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
Anatomy of the eyeAnatomy of the eye
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
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
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.
Contrast and spatial frequencyContrast and spatial frequency
GratingsGratings
Square wave gratings Sine wave gratings
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
Spatial frequencySpatial frequency
Jean Fourier
Any complex waveform can berepresented as the sum of a seriesof sine waves of different frequencies and amplitudes
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
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
The retinaThe retinaReceptor subtypesReceptor subtypes
The beginning of vision-presenting a flash of light to a photoreceptor producesan electrical response
Disks in outer segmentscalled lamellae contain a photopigment
In darkness, there’s a continuous current in theouter segment caused bythe circulation of sodium.
In light, sodium circulationslows down and receptorshyperpolarize
TRANSDUCTION
Rhodopsin -- the magic photopigment
Through the wizardry of biochemistry, sodium channels close
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
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.
Laser interferometryLaser interferometry
-takes advantage of diffraction to produce very fine gratings behind eye’s optics
The receptor mosaicThe receptor mosaic
AliasingAliasing
-Nyquist frequency is the spatial frequency above which confusion can occur.
-demonstration of aliasing
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
The S-cone mosaicThe S-cone mosaic
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
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.
Spectacular Spectacular images of in images of in
vivo vivo receptorsreceptors
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
Photoreceptor arrayPhotoreceptor array
The images show how the photoreceptor array varies with retinal eccentricity
The duplex retinaThe duplex retina
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.
Midget and parasol RG cellsMidget and parasol RG cells
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
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
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.
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
Retinal ganglion cell receptive Retinal ganglion cell receptive fieldsfields
Linear summation of centre and surround responses
Steven Kuffler
Graphical representations of the Graphical representations of the receptive fieldreceptive field
1-d 2-d
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
CSF for a typical LGN cellCSF for a typical LGN cell
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.
Phase reversing gratingsPhase reversing gratings
-this is one common way to measure the temporal properties of rf’s
Notice how CSF varies depending Notice how CSF varies depending on both spatial and temporal on both spatial and temporal
frequencyfrequency
The DOG operatorThe DOG operator
-this is a very common and useful way of modelling the spatial and temporal structures of receptive fields
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.
The retinogeniculostriate pathwayThe retinogeniculostriate pathway
The structure of cortexThe structure of cortex
Cortex is laminar and connections are very precisely organized
Orientation selectivityOrientation selectivity
Hubel and Wiesel’ssimple hierarchical modelof visual cortical processing
Columnar organization of VI
Ocular dominanceOcular dominance
The hypercolumn
Optical imaging of ocular dominance columns
Optical imaging of orientation tuning
Correlation between optical imaging and electrophysiological results for orientation tuning
Margaret Wong-Riley andthe cytochrome oxidase story
•autoradiography and activity•cytochrome oxidase and activity•intrinsic variability in cyo
Cytochrome oxidase in monkey VI and VII
Colour coding in blobsColour coding in blobs
-non-oriented colour opponent cells
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
Directional Directional selectivityselectivity
Disparity sensitivityDisparity sensitivity
V2 and cytochrome oxidase stripes
Lennie’s viewLennie’s view
A portrait of the visual cortical systems based on simple considerations of area
““Specificity” of rf propertiesSpecificity” of rf properties
Contrasting viewsContrasting views