Vision Photoreceptor cells Rod & Cone cells Bipolar Cells Connect in between Ganglion Cells Go to the brain.

Vision

• Photoreceptor cells• Rod & Cone cells

• Bipolar Cells• Connect in between

• Ganglion Cells• Go to the brain

VisionLGN

V1

V1

LGNganglion

cells optic chiasm: where theganglion cells cross so theleft side of each eye goes

to the right side of the

brain, and vice versa.

cone cells:rod cells:

• periphery• movement• black and white

• fovea (center)• detail• color

(broad tuning)

500 nm light

cone firing

blue green red

The Eye




(broad tuning) cone firing

blue green red

The Eye

b




b

b

b

The Eye

bipolar & horizontal cells

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hh

hh-

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The Eye


• lateral inhibition

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Lateral Inhibition

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Lateral Inhibition

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Lateral Inhibition

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Lateral Inhibition

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Lateral Inhibition

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The Eye


• lateral inhibition• edge detection

ganglion cells

front view

Bipolar cells




The Eye



ganglion cells

receptive field

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The Eye



ganglion cells

receptive field

+-• center/surround

Center-Surround(Blob detector)

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time

light position

firi

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requ

ency

Center-Surround

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light position

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Center-Surround

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light position

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Center-Surround

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Center-Surround

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Center-SurroundHow’s it done?

Difference of Gaussians (Mexican hat)

light position

Distributed Visual Representation

• Different cells respond to different properties, such as bars of light at different orientations (i.e. the simple cells in V1).

• Different areas of the brain are dedicated to processing form and location information (i.e. the “what” and “where” systems, in the temporal and parietal lobes, respectively)

How does your brain put it together again?

•“red” neurons•“blue” neurons•“square” neurons•“circle” neurons•“upper-right”•“lower-left”

Binding Problem

How do we know which featuregoes with which object?


Binding Problem



Binding Problem


The Computations of Human Vision

Visual system must calculateObject color (Mostly known)Object shape

Begin with edges (Known)Find blobs (Known)How edges/blobs combine to form objects

(Mostly Unknown)Object movement (Mostly known)

The Computational System of VisionObject Motion

Marr’s levels:Computational

Determine motionRepresentation & Algorithm

Mostly known (but not by us!)Physical Implementation

Neurons in the eye and brain

Hierarchical processing

Low-level processing

High-level processing Examples

Biological motion

Object motion

Optic flow

Retinal motion

Hierarchical processing

Low-level processing/Low complexity

High-level processing/High complexity

• Biological motion

• Object motion

• Optic flow

• Retinal motion

…Superior Temporal Sulcus (STS)

Medial Superior Temporal area (MST)Middle Temporal Area (MT/V5)

Primary visual cortex (V1)Lateral Geniculate Nucleus (LGN)

Retina

Limits of motion perception

We can’t perceive motion that is either too FAST or too SLOW.

Verticalposition

time

Upward motion

Downward motion

Apparent Motion• “Broken” motion

• Stimulus flashing at different positions at different times

Very fast

Very slow

Beta Movement

PhiMovement

Optimal

Somewhat fast

Fli

cker

Rat

e

Phi Movement/Motion

• A “pure sense” of motion without seeing the intermediate steps

• No link-up/fill-in

Beta Movement/Motion

• Perceptually linking up the frames

• Smooth motion

• Motion picture technology

The Correspondence Problem

?

The Correspondence Problem

• How do we figure out which frame-2 dot should match with each of the frame-1 dots?

?

Frame 1Frame 2

The Wagon-wheel illusionPerceived directionReal direction

t1 t2

Slow CW Medium CW Fast CWSlow CW Ambiguous Slow CCW

Reality:Perception:

The Aperture Problem• Following from the correspondence problem• When the line’s motion is viewed through an

aperture, how do we figure out the “correct” motion?

The Aperture Problem• There are “infinitely many” possibilities.

or or ……

The Aperture Problem: a solution

• It’s not a problem:if the line has texture, or

if the line has endings, or

if the line is not straight, or…

…as long as there’s a UNIQUE POINT!

The Barberpole Illusion

• Perceived motion direction is parallel to the orientation of the rectangle

• Can be explained by “Unique-point heuristic”– Unique points are assumed to be on the long

edge

Per

ceiv

ed d

irect

ion

Optic Flow

Optic Flow

• Background scene flows as our we move

• We process motion signals at different locations to understand the optic flow pattern

• Optic flow is usefulfor inferring thedirection of self-motion

Optic Flow• Most studied flow patterns

Translational- object movement, eye movement,

etc. Rotational (or circular)

- head movement, eye movement, etc. Radial (expansion/contraction)

- motion in depth, self-motion, etc.

– They can represent most of the optic flows we see– Computationally, rotational and radial flows are

more complex than translational ones

Retina Motion vs REAL Motion

• Motion constancy– try tracking your moving finger!

• Retinal motion is combined with eye-movement to generate motion percepts

Motion in depth

• Retina is flat Motion signals are only 2D

• How can we know when something is moving towards/away from us?

• Try moving your finger towards your nose

Motion in depth

• The brain combines motion signals from the two eyes to infer motion in depth

Left eye Right eye

Far

Close

Right eye: leftward+) Left eye: rightward Approaching

Right eye: rightward+) Left eye: leftward

Receding

Biological Motion

• We are very sensitive to biological motion

• An analogy: Face in object perception

• Appears to require– extremely complex computations– a special motion processing mechanism

Motion Blindness• Patient LM

– Certain brain areas damaged through stroke– Almost all cognitive functions were intact

except for MOTION perception– She reported

• What she saw when pouring coffee into a cup: appears frozen like a glacier, does not perceive the fluid rising, often spills or overflows it

• “When I'm looking at the car first it seems far away, but then when I want to cross the road suddenly the car is very near”

– YouTube: http://www.youtube.com/watch?v=B47Js1MtT4wTitle: Akinetopsia (4:01)(a reproduced documentary for a class project)

http://www.youtube.com/watch?v=B47Js1MtT4w

Vision Photoreceptor cells Rod & Cone cells Bipolar Cells Connect in between Ganglion Cells Go to the brain.

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