3: motion perception

biological motion

where we distnguish animate from inanimate objects

how sensitive we are to motion like running, walking

inanimate objects also move and their motion carries information on the 3d shape of these objects

structure from motion

ability to determine the form of an object by its motion

need to see the dots motion in order to tell its volume or true shape

with a transparent rotating cylinder you can see both back and front moves from left to rice or vice versa . this is a bistable percept

what is a bistable percept

we see fist a cylinder in which rotation is first with front moving left to right and after a few seconds the percept jumps to reveal a cylinder rotating in the opposite direction

structure on motion 2

motion of the oject unmasks its volume/3D form

individual cells in MT have been recorded whereby their activity is higher when the monkey reports seeing front surface in preferred direction of the cell, than when he reports the front surface moving in the opposite direction- but the visual stimulus is the exact same

motion processing

V1

V2

MT/MST

processes complex motion: and where we make the concoius decidion as to which way an object is moving

MT direction columns

MT has a columnar architecture organised systemically with regard to the preferred direction of motion of neurons

activity of MT neurons reflect the conscious percepts of subjects- whether they see cylinder rotating clockwise or anticlockwise

random dot motion

newsome and colleagues trained monkeys to perform a difficult motion discimination task, judging the direciton of motion of noisy, not very coherent moving dot stimuli

monkes view dots and decide which direction the dots using joystick

MT neurons are selectove for motion coherence and direction

motion selectivity in MT

as coherence increases / number of dots moving in one place there is an increase in the direction of preferrred movement

electrical stimulation affects perception

electrically stimulated one column eg downwards column, and wondered if it changed the monkeys perceptio, making them halluncinate them seeing something else or more

if a MT column selective for downwards motion was electrically stimulated, the monkey would be more likely to respond that the stimulus moved downards

electrical stimulation changes perception

blue curve sows behavioural response of an unstimulated monkey

yellow: with electrical stimulation, perception is influenced a s when coherence is 0 75% responded a specific direction, even when it was not moving that way

more likely to see stimulus move in direction that was electrically stimulated

motion can break camouflage

hard to spot things behind camorflage when still

if the object moves behind the camouflage then easier to spot

visual guidance of locomotion

when you move about, this movement itself causes motino toappear on the retina. this self produced visual motion helps guide out action

helps you move safely and accurately through the environment eg refers to the use of visual information to choose a path, maintain direction and speed, avoid abstacles, whilst moving

optic flow

pattern of the optic flow could be used to tell where one is headed and to steer around obstacles

when moving forward, environent around eg trees look like it is coming towards you

accurate in determing heading from simulated flow fields

elderly subjects not as good as younger in estimating heading

what does optic flow patterns excite

excite cells in the MST

neurons in MST respond to expansions and contractions as well as other optic flow patterns

neurons in MST have large receptive fields

arrival time

knowing where we are going is one thing but estimating how long it takes to get there is arrval time

birds that dive underwater fold their wings streamlining their bodies just before hitting the water - they are aware of their arrival time

how do we know our arrival time

when we move towards an object eg a door or tree, its image on our retinas expand

if we approach an oobject at a steady rate then the rate of expansion on the retina specifies the time to collision

however the size of object on retina doesnt change unless you get really close to it

if travelling slowly, expansion of retinal size doesnt change much

if travelling faster, image size does expand quicker but same size regardless of how fast r slow

looming signal

suggested that this signal , which indicates imminent collsion, is important for breaking ( interacting with what we move to ) as it occurs no matter the rate, distance or object size

referred to as a looming signal as the object suddenly takes up most of our visual field

looming in toddlers

we respond to a looming stimulus and that this instinct is present in both a myriad of creatures and in newly bor infants form their first encounter with looming stimulus

motion processing

motion detection

biological motion

shape from motion

aperture problem

plaids

motion coherence

camouflage

• all real forms of motions

motion illusions

see motion where motion is not actaully present

apparent motion/Phi motion

apparent motion

a stimulus is flashed briefly in one location and then another, uusally identical stimulus, is flashed in another nearby location

instead of seeing the stimulus in one place, turn off and then appear again next to it, we see the location move from location 1 to location 2 in a smooth manner

what is ISI

apparent motion depends on the interval of time between the flashing of the 2 stimulus ( the inter-stimulus interval)

<30msec will see no motion

30-60msec partial movement

60-200msec will see smooth movement

the flicks

aparent motion is a type of stroboscopic motion; motion seen with a strobe light

a more famliar instance with a stroboscopic motion occurs in movies and on Tv

movies are composed of a series of individual static frames ( snapshots) presented rapidlt

stroboscopic motion

apparent motio created by rapidly flashing or intermittently illuminated images rather than continous movement

brain percieves movement even though nothing is actaully moving

a spinning fan under a strobe light can look like its standing still, moving slow or backwards

correspondence problem

stimuli used to generate apparent motion can sometimes suffer from a temporal correspondence problem

the brain needs to match objects between diferent points in time

stimuli are atched based on similarity in shap colour and to their nearest neighbor

temporal correspondance problem

sometimes the temporal match can be ambigious

wagon wheel illusion

based on a problem of temporal matching

appears to rotate slowly or even not at all when infact it is spinnig forward

visual system samples motion in discrete time steps rather than continuously

as speed increases harder to tell thr direction of where its spinning ; clockwise or counter clockwise

stroboscopic motion illusion

strobe light used to generate snapshots for brain to process

stimulus is created by being composed of multiple objects that are rotated at speed

light illuminates stimulus generating snapshots or frames

bain matches objects between shots and creates illusion of moving objects

adaptation effects of motion illusions

2 sine wave gratinngs

adaption to a high contrast moving grating

makes it harde to percieve a lower contrast grating

slows down the percieved speed of low contrast gratings

motion aftereffects

after prologed viewing. stationary objects such as rocks at the side of a waterfall appear to be moving in the opposite direction to the adapting stimulus. this aftereffect is immediate and short lived