Lecture 12 - detecting motion 2

Question 1

Speed selectivity

Answer 1

Preferred speed with which MT cells respond to being investigated:

• Recorded by looking at how many cells in MT active during range of motion
• Deg/s = unit of angular speed
• Linear speed depends on viewing distance
• More active for speeds that take place within 30 degrees/s
• MT cells selective for speed at 32 digs/s

Question 2

Waterfall illusion: motion after effect

Answer 2

• When stare at water fall and then shift gaze to static rock start to see motion in opposite direction to waterfall
• Perceived motion encoded by neural code = population of cells
• After prolonged adaptation to given direction of cells - new direction will reduce output
• When look at motion in direction = motion detecting cells detecting downward motion will be more excited than cells detecting upward motion
• Over long period of time if image doesn’t change = to save energy get inhibitory mechanisms that dampen down amount of activity that is signalling preferred direction = when look at stationary image after moving object, still have baseline activity on cells detecting motion = see an upward after effect

Question 3

Perception of speed over time

Answer 3

• Speed judgement more accurate before adaptation to perceived direction
• Evidence for inhibitory mechanism occurred
• Cells opposite direction more active
• Perception of speed changes over time as judgement become more impaired

Question 4

Perception of speed over space

Answer 4

• Ability to perceive speed strongly effected by where appears on fovea or peripheral vision
• This is because more distracted at periphery
• Judgements of speed more accurate in centre when directly looking at it
• If looking to periphery of vision = ability to detect speed decreases
• Gets worse over time due to inhibition

Question 5

Implications of perception of speed over time and space

Answer 5

• Speed judgements are more accurate before adaption than afterwards
• Motion in the periphery fades over time
• Driving at 30mph feels slow after exiting the motorway where you adapt to driving at 70mph

Question 6

Principle of invariance

Answer 6

• Output from single neurone as only capable of detecting single type of stimuli
• Cells cannot separate speed and contrast (or frequency and intensity)
• Motion cells respond more strongly to high contrast stimuli and faster motion
• -Difficult to differentiate 2 stimuli with same output
• Low contrast, high speed = hard to differentiate how fast object moving

Question 7

Contrast and speed

Answer 7

• Ability to detect stimuli at different levels of contrast:
• -> How good people are at making speed judgements based on level of contrast
• -> When there’s a high contrast stimulus, peoples judgement of speed more accurate
• Motion cells in area V1 respond to contrast and speed
• Abides by principle of Univariance

Question 8

Motion contrast in MT

Answer 8

• As size of stimulus increased in width, so does output from motion sensitive cell
• Amount of output increases as width increases, provides evidence for motion sensitive cell = which doesn’t have centre surround property
• Get decrease in amount of AP generated by motion sensitive cells as stimulus width increases which provides evidence for centre surround feature
• As stimulus gets bigger, starts to be processed by surround portion of field = which is inhibitory and reduces output from cell

Question 9

Induced motion and vection

Answer 9

• Vection = sense of moving when in fact world is moving
• Common when motion is slight and acceleration smooth as vestibular cues don’t help
• Vection builds up slowly in a way that compliments the decay of vestibular sensation of movement

Question 10

Illusory motion

Answer 10

• Sensation of motion in static stimulus
• E.g. motion after-effect
• MT cells active when viewing Enigma image = = stipes over-excite orientation cells in V1, V2 or V3 and these cells are also motion sensitive so pass a motion signal to MT
• Different to apparent motion

Question 11

Aperture problem

Answer 11

• Problem brain needs to solve to be able to work out which direction motion is taking place in
• Aperture = viewing moving stimulus through small window so only see on edge of stimulus = will see motion at right angles to age
• Motion cells in V1 have small receptive fields and only respond to short edge = can only signal motion at right angles to preferred orientation
• Cells in MT must integrate these signals to derive to true motion to solve aperture problem
• But MT cells can be fooled

Question 12

Pinna illusions and aperture problem

Answer 12

• Diamond patterns good stimuli for V2 cells as orientated edges
• Rock back and fourth V1 cells signal motion orthogonal to the tilt of each diamond
• Aperture problem distorts true direction of motion for every diamond
• Distortion produced combination of rotation and explanation in MT
• But brain know you are rocking back and fourth so discounts the expansion/contraction signal
• Just see rotation

Question 13

Biological motion

Answer 13

-Can convey social cues such as gender, happiness
-Is processed in Superior temporal sulcus:
–> Close to MT
–> Links with range of sequential and social tasks:
 Theory of mind
 Eye gaze
 Attractiveness
 Speech
 Emotional information