Psych 121 Lecture 11

8/13/2019 Psych 121 Lecture 11

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Motion detection A computational problems:

Local motion signals are ambiguousHow to combine motion signals into one coherent whole

Computation of Visual Motion

Aperture: An opening that allows only a partial viewof an objectCorrespondence problem (motion): The problem faced by the motion detection system of knowing

which feature in frame 2 corresponds to which feature in frame 1Aperture problem: The fact that when a moving object is viewed through an aperture (or a receptive

field), the direction of motion of a local feature or part of an object may be ambiguous

The correspondence problem

The aperture problem

Motion information from several local apertures (or receptive fields) can be combined to determine theglobal motion of the object

There are several directions of motion within each aperture that are compatible with the stimulation thereceptor is receiving

Whichever possible motion direction is the same in all apertures is the true global motion direction of

the object

Building a global-motion detector


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Computation of Visual Motion We can say something about where global-motion detectors are:

Lesions in magnocellular layers of LGN impair perception of large, rapidly moving objects

Middle temporal lobe (MT) also plays an important role in motion perceptionThe vast majority of neurons in MT are selective for motion in a particular direction

Motion-sensitive areas in the brain (Part 1)

Computation of Visual Motion

Newsome and Pare (1988) conducted a study on motion perception in monkeysTrained monkeys to respond to correlated dot motion displaysThe MT area of the monkeys was lesioned


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Result: Monkeys needed about ten times as many dots to correctly identify direction of motion

The Newsome and Pare paradigm

Computation of Visual Motion Interocular transfer: The transfer of an effect (suchas adaptation) from one eye to another

MAE exhibits interocular transferTherefore, MAE must occur in neurons that respond to both eyesInput from both eyes is combined in area V1, so MAE must be in V1 or laterRecent studies with fMRI confirm that adaptation in MT is responsible for MAE

Motion Information for FormA. Motion alone can indicate the 3-D form of objects.B. Originally, this was called the kinetic depth effect.C. More generally, these abilities are calledstructure-from-motion (SFM)

-- referring to the extraction of object structure from information in moving displays

Structure From Motion

Moving dots are often used to study SFM. Whenstationary dots do not reveal the contours or surface information of objects.

Thus, they can be used to study the motion effectsindependent of surface properties.

Mathematical Analyses of SFM

Focus has been on Rigid Motion.Rigid motion is the geometric term describing motion of an object in space during which there are nochanges in the distances between any two points on the object.In other words, the object does not bend or deform during motion.

Mathematical Analyses of SFM

Focus on Rigid Motion


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Mathematical Analyses of SFM: Rigid Motion

The Problem of Determining SFM

The problem of SFM in rigid motion is for the observer to recover 3-D form from the changing 2-Dprojection of an object.A number of theorems have been proven showing that from a small number of points (3-5) moving in the2-D image, the 3-D structure of a rigid object can be recovered

The Problem of Determining SFM

Some have argued that the visual system will fit a rigid solution to motion displays whenever this is

possible.

Non-Rigid Motion

We are clearly able to see non-rigid shape.

Examples:

Your hand (jointed motion)Jellyfish (elastic motion)

Non-rigid motionMuch research has looked at point-light walker displays, in which perceivers see a person walking frommotion of only a few points of light. (Johansson)

This is also called biological motion.

Using Motion Information Biological motion: The pattern of movement of allAnimals

Biological Motion

Humans are very sensitiveeasily identify various actions

actor traits (gender, identity)Importantly:

- affective (emotional) information- communicative information


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Mathematical analysis of non-rigid motion is much harderWhat constraints are there?

What distinguishes unified, non-rigid motion from disconnected motion of dots?

These are unsolved problems!

Using Motion Information

Avoiding imminent collision: How do we estimate the time to contact (TTC) of an approaching object?Tau (): Information in the optic flow that could signal TTC without the necessity of estimating either

absolute distances or ratesThe ratio of the retinal image size at any moment to the rate at which the image is expanding is tau,

and TTC is proportional to tau: X/X

With enough training, it seems even possible to do it completely without any influence fromconsciousness: samurai halving a bullet

But less experienced observers (orless observant observers) do not react fast enough.

Using Motion Information How do we use motion information to navigate?

Optic array: The collection of light rays that interact with objects in the world in front of a viewerOptic flow: The changing angular position of points in a perspective image that we experience as we

move through the world

The optic flow field produced by movement forward in space

Using Motion Information Focus of expansion (FOE): The point in the centerof the horizon from which, when we are in motion,

all points in the perspective image seem to emanate

This is one aspect of optic flowThe focus of expansion tells the observer which way they are heading

Only so when no gaze shifts are made


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Information for Motion Perception

There are multiple sources of information or multiple situations that lead to perceived motion.1. RETINAL DISPLACEMENT is the changing position of an object's image on your eye.2. OPTICAL PURSUIT occurs when we track a moving object with our eye. The image stays on the

fovea, yet we perceive it as moving.

Eye Movements Why do we perceive the pencil to be in motion in the first case, but perceive the dot to be stationary inthe second case? After all, both items moved across our retinas to the left

Because in the second case there was an eye Movement

Eye Movements Types of eye movements:

Smooth pursuit: The eyes smoothly follow a moving targetSaccade: A rapid movement of the eyes that changes fixation from one object or location to another

Vergence: The two eyes move in opposite directions, as when both eyes turn towards the noseReflexive: Automatic and involuntary eye Movements


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Eye Movements

Saccadic suppression: The reduction of visual sensitivity that occurs when we make saccadic eyemovements

Saccadic suppression eliminates the smear from retinal image motion during an eye movement Self-motion correction

Efference copy

Eye Movements

How do we discriminate motion across the retina that is due to eye movements vs. object movements?Comparator: An area of the visual system that receives one copy of the order issued by the motor

system when the eyes move (the other copy goes to the eye muscles)The comparator can compensate for the image changes caused by the eye movement

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Psych 121 Lecture 11