Motion Perception

• Why is Motion Perception important?

  • enables predators to distinguish prey and vice versa
  • animals lacking colour/spatial vision evolved to be very sensitive to motion
  • one of the first movement thresholds to have been measured was a small ball of light moving through a stationary background
• The Basis of Motion Perception:

  • Reichardt Detector = direction selective unit
  • Composed of 2 spatially separate zones - response from one which is temporally delayed to the other , but travel to same cell body
  • To account for all speeds and directions of motion, must account for:

  *Seperations of zones

  *Time Delays

  *Alignments

  • Coded for in Dorsal Stream (Where Pathway)

  
• What is the Aperture Problem?

  • Early visual areas (lower brain regions) have restricted views of the world = SMALL RECEPTIVE FIELDS
  • Cells can tell an edge's movement (b/c of the receptive field) but NOT its direction
  • This ambiguity is solved in higher brain regions using additional information collated from multiple spacially-separated receptive fields
• What is the Barber-Pole illusion?

  • Stripes on a barber-pole appear to moving up/down despite the fact that it is just rotating
  • Cells/Receptive Fields at centre = cannot tell which direction the stripes are moving (ambiguous)
  • Cells/Receptive Fields at edges = can only tell stripes are moving in vertical direction (unambiguous)
  • Cells at edges feed this information back to cells at centre = stripes are perceived to be moving vertically up/down

  ! This is caused by the Aperture Problem
• The Basis of Motion Perception (2):

  1)A stimulus moving left to right will first be detected in Zone 1

  2)Zone 1 has a delay

  3)During the time of this delay, stimulus will have moved to Zone 2 4)Zone 2 will have sent a signal to cell body immediately

  5)Both signals from Zone 1 and Zone 2 will have reached cell body together at the same time = MAXIMUM ACTIVATION OF CELL BODY

  • Coded for by cells in 4B layer of V1 = DIRECTIONALLY SELECTIVE
  • Cell fires rapidly when line is moving in a particular direction
  • Motion studies use sine wave gratings/plaids , random dots/noise, orientated bars
• Component and Pattern Motion:

  • Component = movement of individual sine wave gratings
  • Pattern = movement of a (plaid) pattern moving as a whole, even though it is generated by individual sine waves moving in 2 different directions

  *Component = cells in V1

  *Pattern = cells in MT/ higher visual areas (medial temporal areas of occipital lobe)

  ! Pattern cells in MT receive inputs from component cells, each who have a different preferred direction

  ! Pattern cells responsible for integrating multiple motion signals associated w/ motion of multi-limbed objects IRL
• Role of MT in Motion Discrimination Tasks:

  • MT consists of 2 columns, where each column is sensitive to a preferred direction of motion
  • Each pair in the column has opposite directions of motion

  How can Electrical Stimulation of MT change Motion Perception?

  • Experiment carried out in monkies
  • The stronger the coherence/correlation of stimulus, the stronger the response of individual neurones and groups of cells (directional columns)
  • If an MT column selective for downward motion was electrically  stimulated, the monkey would be more likely to respond that the  stimulus moved downward

  
• Biological Motion Perception:

  • Used to distinguish animate vs inanimate objects - probably by cells in MT
  • uses patterns of movement to distinguish a person by their movement, what activity they are carrying out and even their identity through their gait
  • Johansson examined the min amount of info that was needed to recognise biological motion (200millisecs , 4months in child)
  • Attached lights to joints
  • Person in black clothes in darkened room and videoed moving about.
  • Upon moving viewers immediately report seeing a person moving, undertaking actions , and even recognised gait
• Structure from Motion:

  • the ability to determine the form of an inanimate object by its motion

  *When stationary = hard to tell much ​about its form

  *Motion of the object can reveal this

  • Bistable Percepts = if object is ambiguous (cannot determine its front/back surface), direction of motion can change depending on what is perceived to be its front surface/back surface

  *why we see changes in direction

  • Individual cells in MT activity is higher when the front surface moves in the  preferred direction of the cell – moves in the direction of what you perceive to be front/back surface​
• What is Optic Flow?

  • The flow of the visual scene across the retina as we move but CANNOT BE USED TO DETERMINE ARRIVAL TIME

  *Focus = the centre of the flow field, defines our heading

  Role of MST in processing Optic Flow Patterns:

  • Neurones respond to expansions (moving towards focus)
  • Neurones respond to contractions (moving away from focus)
• What is Looming?

  • When approaching a stimulus, the size of the image on the retina rapidly increases just before reaching it

  *Inversely proportional relationship b/w image size on retina and distance

  *This rapid increase in size allows determination of arrival time = rate of change in retinal size is the same regardless of speed

  • We repsond to looming stimulus subconsciously
  • Looming is present in infants who haven't even encountered a looming stimulus yet

  
• The Temporal Correspondence Problem and the Wagon Wheel Effect:

  • Temporal Correspondence = (affects stimuli used for apparent motion) visual system needs to work out which parts of frame 1 match with which parts of frame 2
  • Wagon Wheel Effect = Nearest Neighbour Matching Rule, where moving spokes are matched with preceding image of the spokes, and after reaching a specific speed...

  *Appears to slow down

  *Appears stationary

  *Appears to rotate in opposite direction

  ! In subsequent time measurement, spokes will have rotated enough to appear just before locations in previous measurement
• What is Apparent Motion?

  • If a stimulus is flashed briefly in one location and then another nearby location, it can appear just be the first stimulus moving smoothly to the location of the other, instead of being 2 separate stimuli
  • Inter-stimulus Interval = time b/w the 2 flashing stimuli

  *< 30 msec​ = Simultaneous (no motion - both stimuli present)

  *30 - 60 msec = Partial Movement

  *60 - 200 msec = Apparent Movement (one stimuli moving left to right)

  * > 200 msec = Successive (no motion - stimuli present one after the other)

  • Is stroboscopic - early movies at 16 frames per second (flicks)
• The Temporal Correspondence Problem:

  • Stimuli used to generate apparent motion suffer from this
  • In general, stimuli are matched based on similarity in colour, shape, global grouping and to their nearest neighbour
  • WAGON WHEEL EFFECT = DUE TO NEAREST NEIGHBOUR
  • If 2 possibilities are equally likely, we suffer from bistable percept = we switch b/w 2 views