7. Motion Perception

How do we detect motion?

Cells in V1 respond to lines and edges moving in particular directions

How does a motion after-effect occur?

when viewing a stationary object 'up' and 'down' detectors fire equally- prolonged viewing of downwards motion causes reduced firing of down detectors (adaptation)- viewing a stationary object post -adaption results in greater 'up' activation than 'down' hence we perceive upwards motion

What is local motion?

Motion of individual (local) elements

What is global motion?

Grouping together the motion of individuals to perceive a more complex pattern of motion

How should we process global motion?

Must pool information from multiple local motion detectors

What is the motion coherence paradigm?

Mixture of signal dots (moving in same trajectory) and noise dots (moving randomly)

What is the minimum coherence threshold for the motion coherence paradigm?

The minimum proportion of signal dots needed to detect coherent motion. It depends on the proportion of signal to noise dots

What is the minimum coherence threshold for humans?

10% (5% when highly practiced)

What do local motion detectors have?

small receptive fields

What do global motion detectors have?

large receptive fields

Where in the brain is motion processed?

Area MT (middle temporal area)

How do we know that MT is an important area for motion processing?

Single cell recordings- nearly all cells in area MT respond to motion (with a preferred direction)

What was Salzman et al's (1990) study on artificial stimulation?

Identified cells in monkey's MT that had the preferred direction of down. Artificial stimulation of cells led to motion judgments being biased towards preferred direction of down

What was Tootell et als (1995) fMRI study on motion after-effects?

When we adapt to motion and then view a stationary image, we perceive motion in the opposite direction. Shows that MT is active during period of after-effect, there is no direct stimulation but motion is still percieved

What was Newsome & Pare's (1988) lesion study on minimum coherence thresholds?

Introduced small lesion to monkeys MT- Coherence thresholds of 5% with no lesion to MT- Coherence threshold of 80% with lesion to MT

What is optic flow?

Patterns of retinal motion produced when we move

What is expansion created by?

Forward translation

What is contraction created by?

Backwards translation

What is horizontal (constant speed) created by?

Eye, head or body rotation. All objects move at the same speed across retina regardless of depth

What is horizontal (parallax) created by?

Lateral (sideways) translation. Closer objects move faster on the retina than further objects

What is roll created by?

Eye, head or body roll?

What are 3 complex motions?

1. Forward translation 2. Head rotation 3. Combined

What was Smith et al's (2006) optic flow fMRI study?

Measured response of 2 subcortical-regions (MT and MST) to 5 types of motion- The difference in response to optic flow compared to random motion is greater in MST than MT- MST is more specialised for processing optic flow motion than MT

What were the 5 types of motion that were measured in Smith et als study?

1. Complex 2. Expansion 3. Rotation (roll )4. Translation 5. Random

What is optic flow used for?

heading, driving and postural stability

What is optic flow used for (heading)?

Gibson proposed that we use optic flow to tell us where we are heading and to control locomotion

What is optic flow used for (driving)?

Land & Lee (1994) showed that we don't look at FOE when driving, we look at other parts of the scene

What is optic flow used for (postural stability)?

Balance relies on vestibular and proprioceptive information but visual information is also important. Optic flow gives us information on how our posture changed

What is the swinging room experiment (Lee & Aronson, 1974)?

Walls and ceilings move but the floor is fixed- 13-16 month old toddlers- Room swings towards toddler and creates pattern of expansion (as would be produced if you swayed forward- Toddlers compensates by swaying backwards

What were the results from the swinging room, experiment (Lee & Aronson, 1974)?

Toddlers:- 26% swayed- 23% staggered- 33% fell over- adults also found to sway

What does the swinging room experiment show about optic flow and balance?

Optic flow is an important source of information for balance and can override other sources of balance information

What is optic flow used for?

Perception of object motion during self-motion

What are 2 sources of retinal motion?

1. Self-motion 2. Object motion

What is the flow-parsing hypothesis?

Retinal motion due to self-motion is subtracted and the remaining motion is attributed to object motion

What was Warren & Rushton's (2009) study on flow parsing?

Optic flow field is influenced by the perceived trajectory of a moving object even when the flow was in a different part of the stimulus

What is biological motion?

The motion of another person's body creates a complex pattern of movement

How is biological motion demonstrated?

Pointlight walker stimuli- small lights placed on a person's joints

What can biological motion provide information about?

1. What someone is doing 2. Gender 3. Identity 4. Affect

What was Grossman & Blake's (2001) fMRI study on biological motion?

Ppts viewed biological motion versus scrambled motion.- Area STS (superior temporal sulcus) was more active for biological motion than scrambled motion

What was Grossman et al's (2005) TMS experiment on biological motion?

Ppts viewed biological versus scrambled motion and had to determine if it was biological or scrambled motion- Noise dots were added to make task more difficult- Applied TMS to areas STS and MT- TMS to STS created decrease in ability- TMS to MT had no effect on biological motion

What does Grossman et al's (2005) study tell us about biological motion?

Biological motion is a special type of complex motion processed in area STS