Lecture 8- motion perception

Sherrington's proposal

proposed that we monitor the movements of our eye muscles + by comparing retinal image motion w/ the eye muscle movement we can determine whether objects in the world have moved signals come from the eye muscles

what are the 4 reasons motion perception is so important?

1. detection -- objects generally are easier to detect when moving 2. segregation of figure-ground -- movement helps to separate object from background 3. identification of structure -- motion can help define the shape of an object that's ambiguous when stationary 4. guidance of action -- determining direction of travel (heading) -- avoiding/catching approaching objects

what does motion reveal?

1. human form 2. age 3. gender 4. a person's identity is some cases 5. action 6. 2 ppl interacting (dancing) 7. emotion (happiness, sadness etc)

what are the 2 ways of detecting movement?

1. retinal movement system -- detects movement across the retina 2. eye-head movement system -- detects movement of the eyes in the head

what 2 scientists looked for answers as to why the world stays still when our eyes move?

Sherrington and Helmholtz

what do we need to tell when retinal motion is up, down, left, or right?

a motion detector

what is the motion after effect?

a stationary object would excited both up + down motion detectors equally -- there would be no diff + the object would be perceived as stationary the waterfalls downward motion should cause the down detector to fire a lot (up detector not as much) -- the prolonged stimulation of the down detector causes it to adapt to the stimulus + won't fire as much a stationary object now will excite the upward detector much more than the fatigued downward detector -- motion is then perceived as upwards rather than downward (opposite direction that the actual direction of movement) objects in motion never change position so detectors of motion + detectors of position are independent of each other

akinetopsic (motion blindess) is caused by?

damage to area MT the ability to judge direction + speed of moving objects impaired sees the world in snapshots, not continuous

waterfall illusion

experience that if you stare at a waterfall for a minute or so and then turn your eyes to nearby cliffs, the cliffs appear to flow upward also called movement after effect

microstimulation of area MT reveals...

found a group of cells in area MT that have the same preferred direction of motion (ex: downward) -- then placed a stimulating electrode near them -- when the cells were stimulated the monkey was more likely to report downward motion -- stimulating those cells you could fool the monkey into 'seeing' motion that wasn't actually being shown to it

lesions of area MT reveal...

if a dot pattern is presented to a monkey w a damaged area MT it needs 100% correlation of the dots (all need to move in the same direction) -- it's motion threshold increases after damage

wagon wheel effect

illusion that a spoked wheel is moving backward rather than forward -- disappears if the spokes are colored slow-moving wheel -- mvmt is slow enough that we're able to match spoke 1 in frame 1 to spoke 1 in frame 2 -- displacement is small + you see continuous forward motion fast-moving wheel -- mvmt is so fast that the distance spoke 1 travels bw frames 1 + 2 is much larger -- we interpret spoke 1 from frame 1 as rotating backwards -- the most obvious direction of motion for the brain to pick up on is backwards, since this direction suggests the minimal difference between the two frames

efferent copy

internal copy of a motor command as perceived by the eyes if the efferent copy + retinal motion signal are diff then motion is sensed

how do we know area MT is related to motion perception?

lesion of area MT + stimulation of area MT causes change to motion perception

retinal motion

motion on the retina caused by relative movement bw the observer + the environment when the viewer fixates on a stationary object + a moving object moves across the retina (field of view) when the viewer tracks a moving object, the moving object becomes stationary on the retina -- the stationary background moves in the opposite direction on the retina

where does this motion detector action occur?

occurs in the primary visual cortex -- all cells in V5 have directional selectivity primates = neither ganglion or LGN cells can tell direction in other animals directional selectivity is found in retinal cells

what is area MT (V5)?

part of the dorsal stream that contains motion-selective cells

biological motion

pattern of movement of living beings (humans and animals) Johansson point-light motion exp -- have someone dress in all black against a black background -- attach some point lights to them -- although we can only see the movement of a few dots the person's actions are easily recognizable -- can tell a lot just from the few lights

Helmholtz's proposal

proposed that rather than comparing image motion on the retina w eye muscle movement, the comparison should be made w the signal from the brain that tells the eye muscle to move take a efferent copy of the signal to move our eyes + compare this w any retinal image motion signals come from the brain

superior temporal sulcus STS

the most superior sulcus in the temporal lobe, situated just below the superior temporal gyrus has neurons that detect biological motion requires consistency bw body views + directions for movement

kinetic depth effect

the movement of a 3D object reveals its shape -- ex: motion makes a circle appear as a sphere movement reveals the 3D shape of objects can also break camoflauge

what is apparent motion?

the perception of movement as a result of frames (signals) appearing in rapid succession of each other -- interval between frames needs to be of the same order as the delay in the detectors used to sample the motion direction selective cells can't differentiate bw apparent + continuous motion -- ex of a metamer (physically diff but perceived the same)

Reichardt detector (delay-and-compare detector)

to detect some light moving, we could have a receptive field at A + one at B -- both receptors must fire within a certain time period to detect motion -- this type of detector would also respond to B --> A + A --> B movement the receptor must be asymmetrical (introduce a time delay) -- moving from A to B will excite A before B -- if a time delay is placed on the output signal from A, it'll arrive @ the same time -- movement from B to A will excite detector B first + sometime later A will fire but the firing of A will be delayed + A and B won't be active @ the same time -- this allows us to be sure that movement from A --> B excited the detector + not B --> A models also include a later stage which compares the output of the left and right in an antagonistic manner (such as subtracting one from the other) which means that anything which excites them equally will not produce any signal after this subtraction -- fact that we can tell rightward movement from leftward movement suggests that we do this but not where we do this

how are motion thresholds measured in the MT?

want participants to discriminate upward + downward motion w dot patterns + percentage correlations 3 factors: 1. 0% = all dots are random -- discriminating is DUE TO CHANCE bc there's no info abt upward/downward motion 2. 50% = half the dots are either up/down + the other half are random 3. 100% = all dots are either up or down

tracking

when the eye follows a moving object but it remains still on the retina