MOTION PERCEPTION

MOTION PERCEPTION

Introduction to Motion Detection

  • Motion is fundamentally defined as a change in position over time.

  • Building a motion detector involves:

    • Using two adjacent receptors that are fixedly spaced apart.

    • Incorporating a delay to account for changes in time.

Neural Circuit for Motion Detection

  • Constructing a Neural Circuit for Rightward Motion:

    • A ladybug passes through the receptive fields of two neurons:

    • Neuron A: First neuron that registers the motion.

    • Neuron B: Follow-up neuron that also registers the motion.

    • A third cell, called the motion detection cell ‘M’, is responsible for integrating the outputs from Neurons A and B.

    • Cell D: Necessary for introducing a delay in Neuron A’s response to capture the temporal aspect of motion; it adapts quickly, stopping firing if the image stays fixed in A’s receptive field.

Expanding Motion Detection Area

  • The basic motion detector circuit has limitations in covering larger areas.

  • Question arises: Can multiple motion detector circuits be concatenated to cover a more extensive area?

Apparent Motion

  • Definition of Apparent Motion:

    • An illusionary perception of smooth motion that arises from the rapid alternation of objects appearing at different locations in quick succession.

    • Notably, the motion detector circuit can trigger without actual motion being present, demonstrating the brain’s capability to perceive motion based on visual cues.

Global-Motion Detectors in the Brain

  • Specific brain regions are essential for detecting global-motion:

    • Lesions in the LGN (Lateral Geniculate Nucleus) impair the perception of large, fast-moving objects.

    • The Middle Temporal area (MT) plays a critical role in the perception of motion.

    • Most neurons in MT express selectivity for motion in particular directions.

Study by Newsome and Pare (1988)

  • Conducted research on motor perception using monkeys:

    • Monkeys were trained to respond to correlated dot motion displays.

    • Results after lesioning the MT area of the monkeys indicated:

    • They required approximately ten times more dots to accurately identify the direction of motion compared to unlesioned monkeys.

The Newsome and Pare Paradigm

  • Neurons must integrate signals from several local motion detectors to discern correlated motion.

  • In the experimental stimuli, all dots are the same color, devoid of direction indicators.

  • Through training, monkeys could identify correlated motion successfully even when as few as 2-3% of dots were aligned in movement direction.

Motion Aftereffects (MAE)

  • Definition of Motion Aftereffect (MAE):

    • The illusion of motion that occurs when a stationary object appears to move after prolonged exposure to a moving object.

  • The existence of MAE indicates an opponent process system, similar to the mechanism of color vision:

    • Neurons are responsive to either upward or downward motions.

    • When observing downward motion continuously, the neurons responsive to this motion adapt, leading to the perceived motion of stationary objects once this stimulus is removed.

  • Examples: The waterfall illusion illustrates this phenomenon.

Interocular Transfer

  • Definition of Interocular Transfer:

    • Refers to the transfer of an effect, such as adaptation, from one eye to the other.

  • MAE showcases interocular transfer, implying:

    • Adaptation must occur at the neural level, where both eyes contribute their input.

    • Area V1 combines inputs from both eyes, therefore, MAE must occur either in V1 or in later stages of visual processing.

  • Recent fMRI studies confirm that adaptations in the MT area are responsible for MAE.

Practical Applications of Motion Information

  • Using Motion Information for Navigation:

    • Optic Flow: Describes how the changing angular position of points in a perspective image informs navigational capabilities as we traverse through the environment.

Key Concepts of Optic Flow

  • Focus of Expansion (FOE):

    • This is the point in the center of the horizon from which it appears that all points in the perspective image emanate while in motion.

    • FOE is crucial in indicating to the observer the direction of their movement.

  • Optic flow provides multiple cues about one’s motion state:

    • Absence of optic flow generally indicates the lack of movement.

    • An observable outflow (visual flow towards the periphery) signifies that the observer is approaching an object.

    • Conversely, an inflow (flow towards the center) indicates retreating from an object.