Movement & Action In Vision

Question 1

What is Perceptual Segregation?

Answer 1

• Involves the separation of one object from another.
• The problem of figure-ground segregation is a key aspect of studying perceptual segregation.
• When observing a separate object, the figure is standing out from its background, which is known as the ground.
• For example, seeing a book on a desk as the figure against the desk’s surface as the ground.

Question 2

Properties of Figure & Ground

Answer 2

• The figure is more “thinglike” and memorable compared to the ground.
• The figure appears in front of the ground, creating a sense of depth.
• The border separating figure and ground is perceived as belonging to the figure, a concept known as border ownership.

Question 3

What is Corollary Discharge Theory?

Answer 3

• This theory takes eye movements into account.
• Explains motion perception by taking into account signals, which are generated by movement of a stimulus on the retina and by movement of the eyes.

Question 4

Corollary Discharge Theory

Signals From the Retina and the Eye Muscles

Answer 4

• An image displacement signal (IDS) occurs when an image moves across receptors in the retina, as when Jeremy walks across Maria’s field of view while she stares straight ahead.
• A motor signal (MS) occurs when a signal is sent from the brain to the eye muscles. This signal occurs when Maria moves her eyes to follow Jeremy as he walks across the room.
• A corollary discharge signal (CDS) is a copy of the motor signal that, instead of going to the eye muscles, is sent to a different place in the brain.
• For example, this is analogous to using the “cc” function when sending an email message. The email goes to the person it is addressed to, and a copy of the email is simultaneously sent to someone else at another address.

Question 5

Corollary Discharge Theory

Comparator

Answer 5

• The brain contains a structure or mechanism called the comparator that receives both the IDS and the CDS.
• If only one type of signal (IDS or CDS) reaches the comparator, it signals the brain that motion has occurred, resulting in perceived motion.
• When both IDS and CDS signals reach the comparator simultaneously, they cancel each other out, leading to motion not being perceived.

Question 6

Physiological Evidence for Corollary Discharge Theory

R.W.

Answer 6

• R.W., a 35-year-old man, experienced vertigo and perceived motion when moving his eyes or looking out of a moving car.
• A brain scan revealed lesions in the medial superior temporal (MST) area of his cortex, crucial for eye movement control.
• Behavioral testing showed that when R.W. moved his eyes, the stationary environment appeared to move at a velocity matching his eye movement.
• Due to damage in his brain, there was an image displacement signal (IDS) without a corollary discharge signal (CDS).
• Only the IDS reached the comparator, causing R.W. to perceive motion even when there was none.

Question 7

Physiological Evidence for Corollary Discharge Theory

Monkey Cortex

Answer 7

• Physiological evidence from monkey cortex neuron experiments supports the corollary discharge theory.
• The motion-sensitive neuron responds strongly when a moving bar sweeps across its receptive field during steady fixation.
• When the monkey’s eyes move across a stationary bar, the real-motion neuron doesn’t fire, despite the bar’s image sweeping across its receptive field.
• Real-motion neurons respond only to external stimulus movement indicating they may receive a corollary discharge-like signal.