The Human Visual The Human Visual SystemSystemThe Human Visual The Human Visual SystemSystem
Vonikakis Vasilios, Antonios Gasteratos Vonikakis Vasilios, Antonios Gasteratos
Democritus University of Thrace Democritus University of Thrace 2006 2006
Democritus University of Thrace Democritus University of Thrace 2006 2006
The Human Visual SystemThe Human Visual System
Biological background
RetinaRetina Visual CortexV1, V2…
Visual CortexV1, V2…
Optic nerveOptic nerve
light(ganglion cells)
Ανθρώπινο Οπτικό Σύστημα
The eyeThe eye
3 3 kinds of coneskinds of cones ( (long, medium, shortlong, medium, short) – ) – color vision color vision (only in bright light – photopic vision)(only in bright light – photopic vision)
RodsRods – – achromatic visionachromatic vision ( (in dim light – scotopic visionin dim light – scotopic vision))
The photoreceptorsThe photoreceptors
Only Only oneone layer of photoreceptors layer of photoreceptors
Varying distribution of Varying distribution of photoreceptors photoreceptors ((Only L and M Only L and M cones in the fovea, only rods in the cones in the fovea, only rods in the peripheryperiphery))
Different ratios of photoreceptors Different ratios of photoreceptors between individuals (generally between individuals (generally L>M>S)L>M>S)
Hexagonal distribution of Hexagonal distribution of photoreceptorsphotoreceptors
No refresh rate – parallel No refresh rate – parallel transmission of visual information transmission of visual information to the brainto the brain
Differences from a ccdDifferences from a ccd
What retina seesWhat retina sees
DayDay NightNight
OutputOutput
photoreceptorsphotoreceptors
Ganglion cellGanglion cell
Basic retinal circuitBasic retinal circuit
Ganglion cells are the Ganglion cells are the onlyonly output of from the retinaoutput of from the retina
Digital output with an FM Digital output with an FM modulation (spikes)modulation (spikes)
The number of The number of photoreceptors that a photoreceptors that a ganglion cell “sees” and the ganglion cell “sees” and the kind of the connectionkind of the connection
Ganglion cells have Ganglion cells have antagonistic center-surround antagonistic center-surround receptive fieldreceptive field
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Receptive fieldReceptive field
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Center-surround antagonismCenter-surround antagonism
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++ ----
++ ----
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nothingnothing
lightlightNo lightNo light lightlightNo lightNo light
inhibitioninhibition
excitationexcitation
Center-surround responsesCenter-surround responses
Ganglion cells are edge detectors – they Ganglion cells are edge detectors – they respond respond onlyonly to changes and to changes and notnot to to uniform areasuniform areas
By stimulating only the cells that detect By stimulating only the cells that detect differences, the HVS minimizes the differences, the HVS minimizes the number of active neuronsnumber of active neurons
Example: Instead of transmitting a Example: Instead of transmitting a sequence of long numbers e.g.sequence of long numbers e.g. 2003453, 2003453, 2003453, 2003455, 2003451 2003453, 2003455, 2003451 it transmits it transmits only their differencesonly their differences: 0, 0, +2, -2: 0, 0, +2, -2
Center-surround : factsCenter-surround : facts
White paper in dim light reflects less light (is darker) White paper in dim light reflects less light (is darker) than the black letters in bright lightthan the black letters in bright light
The absolute value of reflected light is The absolute value of reflected light is notnot important important
By responding only to differences, ganglion cells By responding only to differences, ganglion cells prevent the white paper from being perceived as prevent the white paper from being perceived as blackblack
Center-surround : advantageCenter-surround : advantage
Dim lightDim light Bright lightBright light
Kinds of Ganglion cellsKinds of Ganglion cells
Biological background
Photoreceptor mosaic
Bcenter - (R+G)surround
Rcenter - Gsurround
Gcenter - Rsurround
(R+G+B)center - (R+G+B)surround
Red-Green oponency
Blue-Yellow oponency
Achromatic opponency
Midget ganglion cellsMidget ganglion cells
Biological background
Midget ganglion cellsMidget ganglion cellsMidget ganglion multiplex 2 signalsMidget ganglion multiplex 2 signals
1.1. Red-Green chromatic opponencyRed-Green chromatic opponency
2.2. Achromatic high acuity (1 cone = 1 center of the Achromatic high acuity (1 cone = 1 center of the receptive field)receptive field)
Parasol ganglion cellsParasol ganglion cellsParasol ganglion cells are:Parasol ganglion cells are:
1.1. AchromaticAchromatic
2.2. Have 3 times greater receptive filedHave 3 times greater receptive filed
3.3. Respond better to movementRespond better to movement
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Bistratified ganglion cellsBistratified ganglion cells
Bistratified ganglion cells:Bistratified ganglion cells:
1.1. Carry the Blue – Yellow opponencyCarry the Blue – Yellow opponency
2.2. Have 3 times greater receptive filedHave 3 times greater receptive filed
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Retinal outputRetinal output
At least 8 independent and parallel mosaics of At least 8 independent and parallel mosaics of ganglion cells outputs scan the photoreceptors and ganglion cells outputs scan the photoreceptors and transmit different information to the visual cortextransmit different information to the visual cortex
The primary visual cortex V1The primary visual cortex V1
The visual cortex analyses the retinal output in 3 different and independent maps:
1. color 2.motion-depth3.orientation of edges
The primary visual cortex V1The primary visual cortex V1
The visual cortex analyses the retinal output in 3 different and independent maps:
1. color 2.motion-depth3.orientation of edges
Demultiplexing RG in cortexDemultiplexing RG in cortex
In every position of the In every position of the visual field 4 visual field 4 different midget different midget cells (from the 4 cells (from the 4 mosaics) are mosaics) are connected in connected in couplescouples
Chromatic opponency is Chromatic opponency is canceled (same colors to canceled (same colors to center and surround). Now center and surround). Now only sensitive only to only sensitive only to luminance incrementsluminance increments
Chromatic opponency is canceled Chromatic opponency is canceled (same colors to center and (same colors to center and surround). Now only sensitive surround). Now only sensitive only to only to luminance decrementsluminance decrements
Center-surround Center-surround antagonism is antagonism is canceled (they canceled (they are the same)are the same)
Center-surround Center-surround antagonism is antagonism is canceledcanceled
Cell types Cell types
For every position of the visual field there are 8 different cells that detect chromatic and achromatic signals in 2 different scales
Cell outputs Cell outputs original Red-Green opponency Blue-Yellow opponency
Achromatic (dark-light)
Double opponent cells Double opponent cells Are formed by combinations of simple center-surround cells
Are excited only by chromatic differences of a very specific color (color edges)
ResponsesResponses Double opponent cells respond only to very specific changes
between certain hues (color edges)
original
Elongated receptive fields (formed Elongated receptive fields (formed by combinations of center-by combinations of center-surround receptive fields)surround receptive fields)
~~12 12 different orientationsdifferent orientations ( (everyevery 1515°°))
Detect edges of particular Detect edges of particular orientations orientations onlyonly in a very specific in a very specific positionposition
Simple Orientation cellsSimple Orientation cells
Formed by combinations of Formed by combinations of simple orientation cellssimple orientation cells
Detect edges of particular Detect edges of particular orientation orientation anywhereanywhere in their in their receptive fieldreceptive field
Complex Orientation cellsComplex Orientation cells
Orientation cellsOrientation cells At every position of the visual field there are all
possible orientations of an edge
Every edge excites a particular orientation cell in a particular position of the visual cortex
HypercolumnsHypercolumns
For every position of the visual field, all cells are grouped into hyper columns
Every hypercolumn is a complete and independent feature detector for a very small part of the visual field
Every hypercolumn contains color cells, orientation cells, disparity cells, motion cells
HypercolumnsHypercolumns Competition exists between cells of the same
hypercolumn and between hypercolumns
Association field
Biological background
Orientation cells prefer to be connected with others that favor the smooth continuity of contours
Connection of orientation cellsConnection of orientation cells
Smooth combinations emerge from the group of orientation cells
This is the first step for contour perception
Salient contoursSalient contours
More complex cells code certain combinations of More complex cells code certain combinations of salient orientation cellssalient orientation cells
Contour integrationContour integration
All the features (contours, colors, texture, depth) are All the features (contours, colors, texture, depth) are being bind in one perceptionbeing bind in one perception
Binding is described by the Binding is described by the GestaltGestalt rules e.g. rules e.g. common common fate rulefate rule, , proximity ruleproximity rule, , similarity rulesimilarity rule etc.etc.
Feature bindingFeature binding
There is a tendency to spatially diffuse strong signals There is a tendency to spatially diffuse strong signals over the weak onesover the weak ones
This way, regions that do not have a strong feature This way, regions that do not have a strong feature ‘get’ one from a nearby region that has a strong one‘get’ one from a nearby region that has a strong one
Edges act like barriers that stop the diffusions of the Edges act like barriers that stop the diffusions of the signalssignals
There is filling-in forThere is filling-in for::• TextureTexture• ColorColor
• DisparityDisparity
Filling-in the featuresFilling-in the features
Filling-in illusionsFilling-in illusions
shapeshape colorcolor texturetexture motionmotion
Object spaceObject space
bindingbindingbindingbinding
Binding to one perceptBinding to one percept
What & Where streamWhat & Where stream
Cell(s) for every objectCell(s) for every object
Finally there is one cell (or one population of Finally there is one cell (or one population of cells) that respond cells) that respond onlyonly to a very specific to a very specific object object
Every perception of an object (either vision Every perception of an object (either vision triggered or mind triggered) activates these triggered or mind triggered) activates these cellscells
This ‘databank’ of cells is located at the This ‘databank’ of cells is located at the inferior temporal cortexinferior temporal cortex
Inferior temporal cortexInferior temporal cortexInferior temporal cortex has columnar organizationInferior temporal cortex has columnar organization
Many aspects of an object are stored in neighboring Many aspects of an object are stored in neighboring columnscolumns
Similar objects are stored in neighboring rowsSimilar objects are stored in neighboring rows
Inferior temporal cortexInferior temporal cortexEvery object is stored in the object space in many Every object is stored in the object space in many rotated versionsrotated versions
……but we are trained only to the versions we usually but we are trained only to the versions we usually see…see…
Attention modelsAttention models
Attention modelsAttention models
Thank you! Thank you!