Motion and perception

Primary visual cortex cells register contours or edges of visual objects in short lines of various orientation.

It ignores areas that are homogenous (no edges or other changes).

The object moves past the focus point.

Movement cue: change in position across the retina.

What if your eye is tracking the object? Then it stays on the fovea.

How do you know it's moving? The perceptual system takes into account motor commands to the eye, head and body.

Motion is always relative something:

• Retina
• Body
• Background
• Other objects

Movement across retina --> primary visual cortex (occipital lobe) --> cells there code velocity and direction.

Neurons in the cortex code direction of motion; that is, when they detect the presence of their preferred direction of motion, they increase in firing.

At baseline conditions, all the motion neurons fire a small amount. 

The brain compares the firing levels of all the motion neurons. If a difference in firing occurs, then motion is detected.

Expanding/contraction motion is coded in area MT in the occipital lobe; a motion-processing area.

Neurons fatigue after firing, and fire at lower than baseline levels.

Baseline activity in neurons that code the opposite motion is then interpreted as actual motion.

Motion neurons code linear directions and expansion/contraction directions.

Codes movement direction and velocity.

Has large receptive fields (e.g. can cover entire objects).

Can, therefore, code holistic motion, such as expansion/contraction.

Motion blindness = lesion in area MT.

If a patient cannot perceive motion, the world appears as a series of snapshots as if the world were always lit by a strobe light.