3a - motion perception

why is motion perception important

important to understand that there are objects moving around us and us moving

how do we detect motion

cells in the V1 respond to lines and edges moving in a particular direction

what is motion after effect

When viewing a stationary object 'up' and 'down', detectors fire equally.

• Prolonged viewing of downward motion causes reduced firing of down detectors (adaptation)

• Viewing a stationary object post-adaptation results in greater 'up' activation than 'down' hence we perceive upward motion

what is local motion

motion of individual (local) elements

what is global motion

we can group the motion of many individual elements to percieve a complex pattern of glocal motion

how do we process global motion

we need to pool information from multiple motion detectors

what is motion coherence

motion coherence threshold is the minimum proportion of signal dots needed to detect coherent motion.

• in humans this is about 10%, 5% when highly practised

• depends on the proportion of signal to noise dots

whats the difference between local and global motion detectors?

• local - small receptive fields

• global - large receptive fields

which area of the brain is responsible for processing motion

Area MT (middle temporal)

how do we know MT is an important area for motion processing

nearly all cells in area MT respond to motion - and they have a preferred direction

what is artificial stimulation (Salzman et al 1990)

• salzman et al 1990 indentified cells in monkey area MT that all had the same preferred direction

• artificial stimulation of cells led to motion judgements being biased towards preferred direction (i.e. down)

what did tootell et al. (1995) find in fMRI imaging

when we adapt to motion, then view stationary test, we experience motion in the opposite direction.

(motion after effect)

lesion study on mt

Newsome and Pare (1988) introduced small lesion to monkey MT, undamaged coherence threshold was 5%, damaged was 80%

what is optic flow

patterns of retinal motion produced when we move

when is expansion in optic flow

created by forward translation (focus of expansion is the middle point you are moving towards)

what is contraction in optic flow

created by backwards translation

what is horizontal optic flow (constant speed)

created by eye, head or body rotation. all objects move at same speed across retina regardless of depth

what is horizontal optic flow (parallax)

created by lateral translation, closer objects move faster on the retina that further away objects

what is a roll optic flow

created by eye, head or body flow

what are complex motions optic flow

multiple directions, e.g. forward translation and head rotation

what did Duffy and Wurtz (1991) , & smithfind regarding gobal optic flow

found neurons that responded preferentially to global optic flow patterns - MT and MST

• smith found that response from optic flow rather than random motion was greater in MST than MT

what is optic flow used for

• Gibson - optic flow tells us where we are heading and to control locomotion

  • but land and lee note this isnt the only info we use and when we are driving we look at other parts of the scene

• postural stability - balance relies on vestibular and proprioceptive information but visual information is also important

outline swinging room experiment Lee & Aronson

moving walls and ceiling but floors fixed, simulates optic flow that would be experienced by someone swaying

• 13-16 month old toddler pps, toddlers compensated by moving backwards - 26% swayed, 23% staggered and 33% fell over

• adults also swayed, optic flow is an important source of information for balance

what are the 2 sources of object motion during self motion (warren and rushton)

self motion and object motion

what is flow parsing

• retinal motion due to self-motion subtracted

• remaining motion attributed to object motion

warren and rushton (2009) showed that an optic flow field influenced the percieved 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 persons’ body creates a complex pattern of movement.

• we appear to be particularly adept at percieving biological motion

• point-light walker stimuli shows that we can percieve gender, identity and affect through movement

outline Grossman and Blakes fMRI study on biological vs scrambled motion

• Area STS (superior temporal sulcus) more active for biological motion compared to scrampled

outline Grossman et al’s TMS experiment on biological motion

TMS to STS caused significant decrease in ability to distinguish bioloigcal motion from scrambled

• tms to MT had no effect on biological motion perception

• SO - biological motion is a special type of complex motion processed in the STS