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7Motion Perception

7 Motion Perception

• computation of visual motion

• eye movements

• using motion information

• akinotopsia

• http://www.sinauer.com/wolfe/chap7/mottypesF.htm

7 Computation of Visual Motion

• How would you build a neural motion detector?

– (Reichart 1950’s)

• Step1

– two adjacent receptors

– small distance apart

• Step 2

– intermediary delay neuron

7 A Neural Circuit for Detection of Rightward Motion (Part 1)

7 A Neural Circuit for Detection of Rightward Motion (Part 2) 7 Computation of Visual Motion (cont’d)

• Apparent motion:

– illusory impression of smooth motion resultingfrom the rapid alternation of objects that appear indifferent locations in rapid succession

– Demo 1 - web activity

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7 Computation of Visual Motion (cont’d)

• Apparent motion demonstrated first by

– Sigmund Exner (1875)

• neural circuit does not need real motion inorder to fire

• What is required

– discrete time locked samples of images

– change between two images

» That is small enough to “correspond”

7 Still Images from an Animation

7 Computation of Visual Motion (cont’d)

• Related problems with neural detection

– correspondence problem (motion):

• problem faced by the motion detection system ofknowing which feature in frame 2 corresponds to aparticular feature in frame 1

– spokes on a wheel demo

– aperture problem:

• when a moving object is viewed through an aperture(receptive field), the direction of motion of a local featureor part of the object may be ambiguous

– web activity demo

7 Global Motion Detector

7 Computation of Visual Motion (cont’d)

• Neural solution to aperature problem

– global-motion detectors:

• lesions in magnocellular layers of LGN impairperception of large, rapidly moving objects

– middle temporal lobe:

• plays important role in motion perception

• MT neurons selective for direction of motion

7 The Medial Temporal Lobe

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7 Computation of Visual Motion (cont’d)

• Neurophsyiological experiment (Newsome and Pare, 1988)

– trained monkeys to respond to correlated dotmotion displays (demo)

– lesion MT areas of monkeys

– results:

• monkeys needed about 10 times as many dotsto correctly identify direction of motion

7 The Newsome and Pare Paradigm

7 Computation of Visual Motion (cont’d)

• disadvantages of lesion studies :

– invasive

– lesions may be incomplete or may influence otherstructures

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• Saltzman, et al. (1990)

– trained monkeys on the correlated motion task

– found MT neurons sensitive to motion direction

– ambiguous cases

• stimulated neurons

• biased the direction response

7 Computation of Visual Motion (cont’d)

• Motion aftereffect:

– illusion of a stationary object that occurs afterprolonged exposure to a moving object

• existence of this effect implies an opponent-process system, like that of color vision

• interocular transfer:

– transfer of an effect (e.g., adaptation) fromone eye to the other

7 Computation of Visual Motion (cont’d)

• What does interocular transfer tell us about the locusof the MAE in the visual system?

– result of activities of neurons in a part of the visualsystem where information collected from two eyesis combined (i.e., beyond LGN)

– input from both eyes is combined in area V1

– recent studies (fMRI) Huk et al. (2001):

• locate site of motion aftereffects more precisely

• MT

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7 Computation of Visual Motion (cont’d)

• first-order motion:

– motion of an object defined by luminance change

• second order motion (web demo):

– motion of an object that is defined by changes incontrast or texture, but not by luminance

• demo

– double dissociation

• 1st and 2nd order motions (Vaina et al. 96; 98)

7 Second-Order Motion (Part 1)

7 Second-Order Motion (Part 2) 7 Eye Movements

• Eye movements:

– smooth pursuit:

• eyes move smoothly to follow moving object

– saccade:

• rapid movement of eyes that change fixationfrom one object or location to another

– superior colliculus:

• structure in midbrain that plays important role ininitiating and guiding eye movements

7 Smooth Pursuit 7 Eye Movements (cont’d)

• Why do we perceive the pencil to be in motion in thefirst case, but perceive the dot to be stationary in thesecond case?

– because in one case there is an eye movement

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7 Eye Movements (cont’d)

• 6 muscles attached to each eye

– arranged in three pairs:

• controlled network of structures in the brain

– when stimulated with electrical signals, eyemovements can be observed

7 Physiology of the Eye (Part 1)

7 Physiology of the Eye (Part 2) 7 Eye Movements (cont’d)

• vergence eye movements:

– two eyes move in opposite directions, donedeliberately

• saccades:

– two eyes move in the same direction

• voluntary eye movements

7 Eye Movements (cont’d)

• saccadic suppression:

– reduction of visual sensitivity during a saccadiceye movement;

– eliminates blur from retinal image motion during aneye movement

7 Eye Movements (cont’d)

• motor system solves “problem” of why an object inmotion may appear stationary by sending out twocopies of each order to move eyes

– one copy goes to eye muscles

– another (“efference copy”) goes to an area ofvisual system that has been dubbed “comparator”

• comparator can then compensate for imagechanged caused by eye movement, inhibitingany attempts by other parts of the visualsystem to interpret changes as object motion

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7 The Comparator 7 Using Motion Information

• How do we use motion information to navigate?

– optic array:

• describes collection of light rays that interactwith objects of the world in front of viewer

– optic flow:

• changing angular position of points inperspective image that you experience as youmove through the world

– e.g., “radial expansion”

7 Optic Flow 7 Using Motion Information (cont’d)

• Are humans actually able to make use of optic flowinformation?

– Computer-generated displays of moving dots andlines to stimulate optic flow information

http://www.reading.ac.uk/arl/clips/demo_of_displays.htm#opticflow

7 Using Motion Information (cont’d)

• Biological motion:

– pattern of movement of living things (i.e., humans,animals)

• Demo 1

• Demo 2

7 Biological Motion

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7 Using Motion Information (cont’d)

• Avoiding imminent collision: how do we estimate thetime to collision (TTC) of an approaching object?

– Information source:

• Tau (τ);

– relies on information in retinal image

» track visual angle subtended byapproaching object over time

– Not clear if we actually make use of tau

7 The Man Who Couldn’t See Motion

• Akinetopsia:

– rare neuropsychological disorder in which theaffected individual has no perception of motion

7 The Man Who Couldn’t See Motion (cont’d)

• A 47-year-old man

– sees streams of multiple, frozen images trailing inthe wake of moving objects.

• when motion ceases, the images collapsed into eachother.

• When nothing is moving and he held perfectly still, hisvision was entirely normal.

– e.g., while out for an evening stroll, he saw a packof identical dogs lined up behind his WestHighland terrier.

7 The Man Who Couldn’t See Motion (cont’d)

• Akinetopsia

– Neurologicical deficit

• caused by disruptions to cortical area of MT