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

Vision LGN V1 LGN ganglion cells optic chiasm: where the ganglion cells cross so the left 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 bluegreenred The Eye

cone cells:rod cells: periphery movement black and white fovea (center) detail color (broad tuning)cone firing bluegreenred The Eye

b cone cells:rod cells: periphery movement black and white fovea (center) detail color b b b The Eye bipolar & horizontal cells h h h h h h - - -

cone cells:rod cells: periphery movement black and white fovea (center) detail color The Eye bipolar & horizontal cells lateral inhibition

Lateral Inhibition

Lateral Inhibition

Lateral Inhibition

cone cells:rod cells: periphery movement black and white fovea (center) detail color The Eye bipolar & horizontal cells lateral inhibition edge detection ganglion cells front view Bipolar cells

cone cells:rod cells: periphery movement black and white fovea (center) detail color The Eye bipolar & horizontal cells lateral inhibition edge detection ganglion cells receptive field + -

cone cells:rod cells: periphery movement black and white fovea (center) detail color The Eye bipolar & horizontal cells lateral inhibition edge detection ganglion cells receptive field + - center/surround

Center-Surround (Blob detector) + - time light position firing frequency

Center-Surround + - time light position firing frequency

Center-Surround + - time light position firing frequency

Center-Surround + - time light position firing frequency

Center-Surround + - time light position firing frequency

Center-Surround + - time light position firing frequency

Center-Surround + - time light position firing frequency

Center-Surround + - time light position firing frequency

Center-Surround + - time light position firing frequency

Center-Surround How’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 feature goes with which object?

“red” neurons “blue” neurons “square” neurons “circle” neurons “upper-right” “lower-left” Binding Problem How do we know which feature goes with which object?

“red” neurons “blue” neurons “square” neurons “circle” neurons “upper-right” “lower-left” Binding Problem How do we know which feature goes with which object?

The Computations of Human Vision Visual system must calculate Object 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 Vision Object Motion Marr’s levels: Computational Determine motion Representation & 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. Vertical position time Upward motion Downward motion

Apparent Motion “Broken” motion Stimulus flashing at different positions at different times Very fast Very slow Beta Movement Phi Movement Optimal Somewhat fast Flicker Rate

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 ?

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

The Wagon-wheel illusion Perceived direction Real direction t1t1 t2t2 Slow CWMedium CWFast CW Slow CWAmbiguousSlow 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 ……

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 Perceived direction

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 useful for inferring the direction 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 eyeRight 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: Title: Akinetopsia (4:01) (a reproduced documentary for a class project)