Visual Motion Perception Notes

Visual Motion Perception

8.1 Motion Aftereffects

  • Motion aftereffect (MAE): The illusion of motion of a stationary object after prolonged exposure to a moving object.
    • The existence of MAE implies an opponent process system, similar to color vision.
  • Interocular transfer: The transfer of an effect (such as adaptation) from one eye to the other.
    • MAE exhibits interocular transfer.
    • Therefore, MAE must occur in neurons that respond to both eyes.
      • Input from both eyes is combined in area V1, so MAE must be in V1 or later.
      • fMRI studies confirm that adaptation in the middle temporal area (MT) is responsible for MAE.

8.2 Computation of Visual Motion

  • How to build a motion detector:
    • Motion is a change in position over time.
    • Start with two adjacent receptors (registers change in position).
    • Incorporate a delay (accounts for change in time).
  • The motion detector circuit works, but it doesn’t cover a very big area. String several motion detector circuits together can cover a larger area
  • Apparent motion: The illusory impression of smooth motion resulting from the rapid alternation of objects that appear in different locations in rapid succession.
    • First demonstrated by Sigmund Exner in 1875.
    • Motion detector circuit does not need real motion in order to fire.
  • Correspondence problem (motion): The problem faced by the motion detection system of knowing which feature in frame 2 corresponds to which feature in frame 1.
  • Aperture: An opening that allows only a partial view of an object.
    • Aperture problem: The fact that when a moving object is viewed through an aperture (or a receptive field), the direction of motion of a local feature or part of an object may be ambiguous.
  • Motion within any single aperture (or receptive field) is ambiguous, the visual system correctly perceive the overall motion of objects by combined motion information from several local apertures (or receptive fields) to determine the global motion of the object.
    • There are several directions of motion within each aperture that are compatible with the stimulation the receptor is receiving.
    • Whichever possible motion direction is the same in all apertures is the true global motion direction of the object.
  • Global-motion detectors:
    • Lesions in magnocellular layers of LGN impair perception of large, rapidly moving objects.
    • Middle temporal area (MT) also plays an important role in motion perception.
      • The vast majority of neurons in MT are selective for motion in a particular direction.
  • Newsome and Pare (1988) study on motion perception in monkeys:
    • Monkeys were trained to respond to correlated dot motion displays.
    • The MT area of the monkeys was lesioned.
    • Result: Monkeys needed about ten times as many dots to correctly identify the direction of motion.
  • Disadvantages of using lesion studies to study motion:
    • Invasive
    • Lesions may be incomplete or may influence other structures.
  • Electrical stimulation of MT neurons:
    • Avoids problems of lesion studies
    • Biases motion detection in the direction the MT neuron normally responds to
  • First-order motion: The motion of an object that is defined by changes in luminance.
    • Luminance-defined object: An object that is delineated by differences in reflected light.
  • Second-order motion: The motion of an object that is defined by changes in contrast or texture, but not by luminance.
    • Texture-defined (contrast-defined) object: An object that is defined by changes in contrast or texture, but not by luminance.
  • Akinetopsia: A rare neurophysiological disorder in which the affected individual has no perception of motion.
    • Caused by disruptions to cortical area MT

8.3 Using Motion Information

  • How do we use motion information to navigate?
    • Optic array: The collection of light rays that interact with objects in the world in front of a viewer (Term coined by J. J. Gibson).
    • Optic flow: The changing angular position of points in a perspective image that we experience as we move through the world.
  • Focus of expansion (FOE): The point in the center of the horizon from which, when we are in motion, all points in the perspective image seem to emanate.
    • This is one aspect of optic flow and tells the observer which way they are heading.
  • Biological motion: The pattern of movement of all animals.
  • Avoiding imminent collision: How do we estimate the time to collision (TTC) of an approaching object?
    • Tau (τ)(\tau). Information in the optic flow that could signal TTC without the necessity of estimating either absolute distances or rates.
      • The ratio of the retinal image size at any moment to the rate at which the image is expanding is tau, and TTC is proportional to tau.
  • Motion-Induced Blindness (MIB): A moving surface can cause stationary objects to “disappear”.
    • No clear explanation.
    • Related to Troxler fading: an unchanging target in peripheral vision will slowly disappear while fixating a central target.

8.4 Eye Movements

  • Why do we perceive the pencil to be in motion in the first case, but perceive the dot to be stationary in the second case? Because in the second case there was an eye movement
  • Types of eye movements:
    • Smooth pursuit: Voluntary eye movement in which the eyes move smoothly to follow a moving object.
    • Saccade: A type of eye movement, made both voluntarily and involuntarily, in which the eyes rapidly change fixation from one object or location to another.
    • Vergence: A type of eye movement, both voluntary and involuntary, in which the two eyes move in opposite directions.
      • Convergent eye movements turn the eyes inward.
      • Divergent eye movements turn the eyes outward.
    • Reflexive: Automatic and involuntary eye movements.
  • Six muscles are attached to each eye and are arranged in three pairs.
    • Controlled by an extensive network of structures in the brain.
    • Superior colliculus: A structure in the midbrain that is important in initiating and guiding eye movements.
      • When this structure is electrically stimulated, eye movements result.
  • Saccadic suppression: The reduction of visual sensitivity that occurs when we make saccadic eye movements.
    • Saccadic suppression eliminates the smear from retinal image motion during an eye movement.
  • How do we discriminate motion across the retina that is due to eye movements vs. object movements?
    • Efference copy (or corollary discharge signal): When an eye movement is issued, the motor command is copied and sent to other areas of the sensory cortices.
    • Comparator: An area of the visual system that receives one copy of the order issued by the motor system when the eyes move (the other copy goes to the eye muscles).
      • The comparator can compensate for the image changes caused by the eye movement.
  • How else do we compensate for eye movements to preserve the stability of the visual world?
    • Dynamic remapping of receptive fields
      • A saccade is planned but not yet executed.
      • Some neurons in parietal cortex remap their receptive fields relative to the upcoming fixation location.
    • Saccade is executed.
      • Receptive fields are already processing information from the new location before the eye lands there.
      • Receptive fields of neurons in the frontal eye fields also transiently shift inward towards the new point of fixation.

8.5 Development of Motion Perception

  • Sensitivity to visual motion develops over time.
    • Infants have some reflexive eye movements at birth.
    • However, adult-like sensitivity to motion does not reach maturity until about 3 to 4 years of age.
    • Sensitivity to motion-defined form and biological motion takes even longer to develop.
  • Guess Who’s Coming to Dinner
    • Question: How does a stationary praying mantis spot its moving dinner?
    • Hypothesis: The African praying mantis stays stationary for long periods of time while waiting to ambush its fast-moving prey. The mantis’s motion detection system might be different from other insects.
    • Test: Researchers evaluated the African praying mantis’s optomotor response to drifting sinusoidal gratings. Researchers coded whether the mantis turned left or right in response to drifting gratings going in those directions.
    • Results: The contrast sensitivity of the mantis depended on both the spatial and temporal frequencies in the gratings, not on their speed.
    • Conclusions: Mantis sensitivity to motion differs from humans and bees but similar to hoverflies and hawkmoths in that it allows them to respond to stimuli at low and high velocities.