17- Motion Perception Part III (10.1.25)

motion perception

part III

10.1.25

START

cont'd, again

Motion & Changes in the Retinal Image

When an object changes position on the retinal image, it can be because A) the object moved or B) the eye moved

To know which object moved in the physical world, the movement of the eyes needs to be taken into account.

"Real Motion" Cells

Motion-responsive neurons in visual cortex (discovered in area V3A) Respond differently based on whether the movement of the retinal image was caused by the movement of the eyes or movement of the object

Anticipating Eye Movements

The brain sends information about an upcoming eye movement before the movement is actually executed, Allows the perceptual system to plan accordingly

Deriving Direction & Speed

To represent the direction and speed of motion, neurons need to do more than monitor from 2 receptive fields By building in delays in transmission time, the direction and speed of motion can be represented

Population Code for Motion

• Receptive fields in motion-responsive neurons exhibit tuning curves/functions
• Note the suppression relative to baseline for directions opposite the preferred
• The direction & position of motion is represented as a population code

Motion aftereffect

Motion Aftereffect

happens when a stationary object appears to be moving in the opposite direction to a moving object shown just beforehand

Sometimes called the waterfall effect because, if you stare at a waterfall awhile, and then look at a rock, the rock will appear to be moving

Motion Aftereffect

happens when a stationary object appears to be moving in the opposite direction to a moving object shown just beforehand

This video is designed to create a natural hallucination based on the motion aftereffect

Motion Aftereffect

WHY IT HAPPENS:

Motion Aftereffect

Why it happens:

Neurons representing a particular direction of motion in a particular area of space can become fatigued

This gives rise to the perceptiion of the opposite direction of motion

THE MT (V5) CORTEX

A MOTION-SENSITIVE BRAIN AREA

MT cortex is selectively responsive to motion

Note that the MT shows little response to object features However, the MT appears to be involved in motion perception

MT cortex is selectively responsive to motion

The receptive field of neurons in early stages of visual processing (e.g., V1) are often too small to know the movement of a whole object To represent the movement of larger objects, it is necessary to pool over multiple neurons