lcs 11: cognitive science visual pathway€¦ · if the tips of two lines coincide in an image,...

Pomona College

LCS 11: Cognitive ScienceVision 2

Jesse A. Harris

April 22, 2013

Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 1

Agenda

Visual pathway1. Neural retina2. Lateral geniculate nucleus3. Primary visual cortex

Construction illusory surfaces

Reading for Wednesday: HoUman, 1998: ch 4

Group presentations May 1, 6, and 8


Visual pathwayEye


Neural retina

I Image focuses on neural retinaI Neural retina composed of discrete photoreceptor cells

which are attuned to particular stimuli

Rods Responsive to movementCones Responsive to color and detail

I Early selectivity within eye itself


Rods and conesRetinal ganglion cells

Retinal ganglion cellsRespond to small receptive Veld, consist of cells with ON andOFF regions.

ON Excitation

OFF Inhibition

Some cells have ON center, others have OFF center, approxequal cells that compete with one another in terms of Vringrates.


Retinal ganglion cellsNystagmus

Fading imagesMany cells in retina are detectors of fading; attuned tochanges in stimulus (transient cells).

I Constant slight tremor (nystagmus) to reactivatestimulus on photoreceptors

I If eye is stabilized by paralyzing eye muscles with drugsor by moving images to account for movement, worldbecomes a dull grey.


Troxler fading

I Fixate on upper cross for a while: Blurry grey linesdisappear.

I Then Vxate on lower cross: complementary after imagesappear.


Troxler fading

I Fixate on central dot and doughnut will disappear,replaced by orange background.


I Left visual Veld inputs right V1I Right visual Veld inputs left V1I Crossing at the optic chiasm


Lateral geniculate nucleus

I Cells similar to retinal ganglion cells, with small receptiveVelds.

I No discrimination between more complex features ofimage, e.g., orientation or movement.

Retinotopic mapsNearby parts of image are represented in the nearby cells ofthe retina, LGN, and V1 areas of the brain. Physiologicalorganization selectively mirrors organization of objectperceived.


Primary visual cortex (V1)

Columnal organizationCells are ordered via columns(approx 1mm strips) that aresensitive to particular features.

Each cell has a preferredorientation which responds toinput from a particular receptiveVeld.

But columns are not evenlydistributed across V1!

Distortion in retinotopic maps

Cortical magniVcation eUectCells most dense around fovea - far greater amount of V1dedicated to processing information from with the fovea.Results in a distorted map, where there is about a 2000-foldmagniVcation for area within 1-2 degrees of visual angle.


Compression system

High resolution in important areas,with low resolution elsewhere.

If the brain were required todeal with all information fromthe complete visual Veld withthe same degree of acuity as thefovea, it would weight 10 tons.


Simple cells

I Cells in LGN don’t response to orientationI Cells in V1 do - if bar covers ON center, than minimal

inhibition, if it crosses inhibitory area enough, then theinhibition cancels out Vring.

Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 23 Jesse A. Harris: LCS 11: Cognitive Science, Vision 2 24


Complex and hyper complex cells

Complex cellsLarger receptive Velds, without clear division betweenexcitatory and inhibitory areas.

Hyper complex cellsResponsive to orientation, as well as length.


The miracle of vision!The visual system transforms a fragmented 2D image into acoherent 3D construct.

I Physiological organization should feed into rules forconstructing percepts

I We construct visual perception of everything we see,according to basic constraints or principles.

I Illustrate this best with subjective contours and surfaces,in which we create the perception of a surface which isnot present in the image.


Rules

Rule of generic viewConstruct only those visual worlds for which the image isstable (i.e., generic) view.

Rule 1Always interpret a straight line in an image as a straight linein 3D.


Rules


Rule 2If the tips of two lines coincide in an image, then alwaysinterpret them as coinciding in 3D.


Rules


Rule 2If the tips of two lines coincide in an image, then alwaysinterpret them as coinciding in 3D.


Kanizsa triangle


Non-accidental features (Rule 12)If two visual structures have a non-accidental relation, groupthem and assign them to a common origin.


Non-accidental features

Non-accidental features (Rule 12)If two visual structures have a non-accidental relation, groupthem and assign them to a common origin.


Convex curvesConvex cusps (Rule 11)Construct subjective Vgures that occlude only if there areconvex cusps.


Blivet Vgure, aka the devil’s fork


Blivet Vgure, aka the devil’s fork


lcs 11: cognitive science visual pathway€¦ · if the tips of two lines coincide in an image,...

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