Perception WK 7 - Perceiving motion

Motion detection

Delay and compare

Measure images at one place and time and then later at another place and time

Velocity = Distance/Time

In a motion detector, the left signal is delayed so that both signals arrive at the comparator at the same time so that it can detect motion

Our perception depends on the output of these motion detectors (therefore making the design and implementation of these detectors important)

Apparent motion

The perception of movement between frame 1 and frame 2

We might seem smooth movement when they are just two snapshots in time

“Static” motion illusion

We might see motion when it might not exist → movies (frames), illusions

This static illusion works by micro-saccades (short movements of the eye), as stationary vision likely causes an image to fade

The correspondence problem

When apparent motion between two frames is more difficult when there are multiple objects - we don’t know exactly which object went where

Nearest neighbour matching

Solution to the correspondence problem

Matches the objects to the nearest one possible, even if this is not actually the case

Common fate

A gestalt grouping rule

The tendency to group things together if they are moving in the same direction e.g. a flock of birds

Aperture problem

A gap in an area is so small that it affects the perception of the object’s movement of direction

E.g. barberpole illusion

Barberpole illusion

Pole goes horizontally with the stripes originally being diagonal

However, because of the small aperture (visual space), it makes the direction and edge ambiguous and makes it look like the pole/stripes are going upwards

Also happens if you change the shape of the aperture (egdes)

Optic flow

The pattern of retinal motions that we see if we move towards or away from an object

Gives useful information about speed and direction of “heading”
e.g. first example = Focus of expansion

Corollary discharge

When the brain sends signals to the eye muscles to move the eyes around, it also sends a copy (efference copy) to the visual processing area

This tells the brain that we are about to move our eyes

If the 2 signals match, this suggests we have moved our eyes ourselves

However if they don’t match, it suggests the external environment has impacted our perception

Area V5/MT

Area containing neurons that detect motion + direction

A study involving a monkey and dots going different directions detected particular V5 activity in response to certain directions more than others

This area is made up of component motion detectors with smaller receptive fields

• MSTd → detects self motion, optic flow (rotation etc)

• MSTv → detects other objects and trajectories

• FST (Fundal Superior Temporal area) → Detecting action in 3D objects rather than 2D

Area V5/MT in humans

Humans have hMT+ in our visual area

Areas V3 (dynamic form) and posterior Superior Temporal Sulcus (pSTS) are important for interactions between form and motion pathways

Recent experiments on inputs from vestibular system (self-motion, balance etc) to motion sensitive cortical areas

The study of biological motion

Experiments connect lights to parts of the body - observers can detect ‘human motion’

These lights can express identity, emotion and gender

• Different movements and arrangements can convery how a person ‘knocks’ on a door

When the dots are scrambled, it is a lot harder to see the motion

Area sensitive to biological motion

Posterior Superior Temporal Sulcus

Biological motion in Autism

Even though both non-autistic and autistic participants could detect motion equally well, non-autistic group had access to different brain pathways compared to autistic

The autistic group tended to see it as a series of snapshots compared to motion directly in non-autistic participants

• Due to larger receptive fields in ganglion cells, complex cells and V5, leading to less fine detail of movement