Week 4 - motion

• In humans motion is not processed in the LGN or retina and starts being processed in the V1
• Blindsight is caused when V1 is damaged however we are still able to sense motion due to the connection between the LGN and MT skipping out the blindsight in V1, V2 and 3
• MT = medial temporal
• MST = medial superior temporal
• V1 + 2

  • Small receptive fields
  • Cells respond to simple stimuli and linear motion
  • Not tuned to speed or specialised for motion
• V3

  • Large receptive fields
  • Specialised for motion of complex stimuli
• V5/ MT

  • Large receptive fields, responds to anything
  • Direction and speed tuned
  • Simple patterns of motion
  • Motion contrast cells
  • Respond to static images that imply motion
• MST

  • V large receptive fields
  • direction and speed tuned
  • Complex motion patterns (such as generated by locomotion)
  • Static images that imply motion
  • Responds to vestibular cues
• V6 responds to self induced motion
• Sherrington believed as the eyes move the info from the muscles is sent to somewhere else in the brain that can incorporate it together with the retinal input allowing us to interpret the movement accurately
• Helmoltz disagreed due to it taking too long to get info from the muscles so instead we incorporate the intention to move
• Helmholtz theory seems to be correct due

  
• We arent able to detect motion in two separate points simultaneously as it is received when the object comes into the receptive field
• Time delay can be used in order to cause motion in A to be delayed therfore causing both to be detected simultaneously
• the perceived direction of the signal is dependent on the balance of left and right signals
• object motion is perceived as relative to the motion of our body, head and eyes
• opponent motion detectors respond to the balance between motion cells preferring opposite directions
• An object can appear at position A at time 1 and B at 2 without appearing at all in between due to the speed at which it has moved
• if the delay between time 1 and 2 is too long we will see displacement not motion
• Wagon wheel illusion shows how different speeds can portray different speeds of a moving object with it going smooth, juddering, fast, slow and forward/ backward easily
• looming stimuli are things which may collide with a child and these are seen to be detected from a very early age by the child and even present from birth
• sensitivity to motion seems to develop around 10-12 weeks
• rudimentary visual flow (6-8 weeks)
• rapid improvements seen between 6-14 weeks
• motion blindness - akinetopsia - damage to MT - we can do motion tasks with some level of blindness, also shown in monkeys
• with akinetopsia the world is perceived as a series of photographs
• loosing v1 makes you functionally blind but you can still respond to some stimuli
• loosing v3 would impede your motion perception but not destroy it
• loosing MST/ V6 would inhibit navigation, not stop you seeing motion
• the further away something is the faster it must move for the same visual speed to be shown
• waterfall illusion - a motion after effect illusion where focusing on the cross then causes the budda to expand after
• perceived motion is encoded by a population of cells (neural code)
• for static stimuli all these cells respond at about the same low rate
• after prolonged adaptation to a given direction cells responsive to that direction will reduce their output therefore adapting
• motion in the periphery fades over time
• principle of univariance - cells cannot separate speed and contrast
• motion cells respond more strongly to high contrast stimuli and faster motion
• change in contrast can be confused with changes in perceived speed due to lower contrast causing lower perceived motion
• The larger the area that includes motion is, the more inhibited the neuron is
• snow blind illusion - blinds cause increase speed of snow due to the stripes causing our motion contrast cells to be more excited making us visualise it as quicker