motion lecture

• Motion


  Displacement/change of position over time (speed = distance/time)
• Motion (in terms of gratings)

  Speed = temporal frequency/spatial frequency
• Temporal frequency


  Speed of grating cycle (e.g. in a black-and-white temporal grating, the time it takes to show both black and white (one cycle) is the temporal frequency)
• Motion is not sensed directly
• We don't judge position + time separately and divide 1 by 2 (difference in position across time; whatever this looks like in the brain)
• If the above was the case, we could measure errors in the above to figure out individual (errors in) speed + motion perception, which would mean our motion perception would be bad and we couldn't do anything
• There must be another mechanism to judge motion separately
• Reichardt motion detectors

  • Simplest way to build neural detector using neural units/receptors/RFs
  • Detector should respond to a particular motion/direction but not for the antagonistic way (direction-selective)
  • Receptor/RF 1 sends input to delay unit ⇒ stimulus reaches receptor/RF 2 which directly sends input to motion-detecting-AND-unit ⇒ this and delayed input reach m-d-AND-u concurrently to make it fire (to compare and ensure)
  • You'd need many units with many delays and separations to represent different speeds
  • Opposite direction motion detectors could be combined for a composite unit with fewer neurons + detects motion at the same speed in different directions
• Reichardt detectors don't detect motion in 2 double flashes (simultaneous flashes, one on each receptor, repeated)
• Motion detector comparator


  Compares both AND signals to determine which one's bigger; that one's the perceived direction
• Equal AND inputs would lead comparator to conclude there's no actual motion, just flashing
• Stroboscopic (apparent) motion


  • Movies are not actually moving; you're seeing a sequence of images at different positions
  • This activates Reichardt detectors the same way as actual movement ⇒ movies look like they're moving
  • Delay between frames makes it jerky (standard is 24 fps)
  • Reichardt detectors don't care what happens between receptors; they just need the stimulus to be the same on the receptors
• Wagon wheel effect

  • In movies, wheels (or rotating stimuli) look like they spin backward or at the wrong rate for their actual speed or that they're not spinning at all
  • This can happen in real life if the light conditions aren't stable (e.g. flickering lights esp. in fluorescent lights)
  • In film, helicopter blades can look stationary bc same rate as camera
  • Needs intimate visual stimulation + periodic motion stimulus (aspects are identical; e.g. wheel spokes, rotor blades)
  • If a spoke was displaced 90 degs across frames, it looks stationary bc they're identical ⇒ no perceived motion in any direction
  • 45deg displacement would be bi-stable bc there's no way to tell direction (you would see flickering + see it going both ways)
  • Why wheel seem backwards: a major displacement one way will also look like a short displacement the other way. since there's no way to tell the aspects apart, the brain perceives the stimulus to move counter to the actual movement bc the motion system is biased to perceive slower motion
• Motion aftereffect/waterfall illusion

  • Adaptation to motion in one direction ⇒ continue seeing direction even in exposure to opposite direction
  • Adaptation makes it harder to see adapted stimulus (increased detection threshold + intensity perceived as lower (e.g. stimuli moves slower)) + increases sensitivity to other stimuli
  • After adaptation stimulus is removed, you see a biased percept in the opposite way
• Ratio model explanation of motion aftereffect

  • Stationary stimulus: the comparator sees that the upward and downward detectors have the same level of activity ⇒ perceives no motion
  • Downward moving stimulus: up detector fires at baseline bc uninterested, down detector fires frequently, comparator registers down motion ⇒ we see stimulus going down
  • In adaptation, as motion goes on, down detector activity reduces
  • Re:stationary (motion-stopped): up detector keeps firing at baseline, down detector fires less than baseline, up > down signals ⇒ comparator thinks there's upward motion
• Motion physiology

  • MAE seen in rabbit (and other lower vertebrates') retinas, which have motion selective units
  • Primates have motion-selective units in the magnocellular (large, respond to fast RF changes; in LGN) pathway; V1 is selective for direction-selective units
  • Lateral + backward connections seen in cortex
• Motion LGN

  • M1 - M2 (magno layers; sensitive to flickering stimuli + low spat. freq./large RFs)
  • P3 - P6 (parvo layers; sensitive to sustained stimuli + high spat. freq./small RFs)
  • K1 - K6 (konio layers; between the major layers; sensitive to medium stimuli + very low spat. freq./very large RFs)
  • No motion selective cells; just cells that detect properties of motion
• Motion V1

  • First stage of direction detection
  • Directionally tuned cells (fire more at one direction + baseline for null direction)
  • Above are direction filters
• Motion MT+/V5

  • Motion + direction selective cells
  • Large RFs
  • Preferred directions are represented in columns
  • These directions change systematically as you move around the cortex to reach a certain direction flipped 180 degs
  • Directional columns, of which there are two, along an axis of motion (up, right, diagonal) are located next to each other
  • 80s macaque exps. - direction tuning is normally distributed ⇒ electrical stimulation in a different direction makes monkey hallucinate that direction (right when down)
  • Optic flow→there are cells sensitive to expanding stimuli around fixation point (e.g. field reveals itself as you move forward)
  • Akinetopsia→MT lesions see deficits in perceiving smooth motion; they tend to see choppy movement/snapshots (can see change in size + position, just not motion)
• Inflow theory


  • Eye movement signal comes from the eye muscles
  • This signal is sent to a comparator which also looks at retinal movement to determine whether the object or the eye moved
• Outflow theory


  • Eye movement signal comes from brain which commands how much eye should move (intention signal vs actual movement)
  • Brain also sends copy of intention signal (efference copy/corollory discharge) + retinal image motion to comparator
• Inflow theory explains tracking and afterimages
• Outflow theory explains externally generated eye movements in the dark and with stabilised image
• why wheel seem backwards: a major displacement one way will also look like a short displacement the other way. since there's no way to tell the aspects apart, the brain perceives the stimulus to move counter to the actual movement bc the motion system is biased to perceive slower motion