10a: Motion Processing

STAGES IN MOTION PROCESSING

  • Stages in the motion pathway/s:

    • Local-motion extraction

    • Local-motion pooling

    • Global-motion pooling

    • Optic-flow processing

    • Biological-motion processing (potentially part of a third cortical pathway)

LOCAL-MOTION EXTRACTION

  • Definition: Need to detect change in location over time. need to detect motion

  • Key Model:

    • Elaborated-Reichardt Detector (ERD):

    • A model for extracting local motion which relies on detecting the timing and spatial offset of signals.

LUMINANCE-DEFINED MOTION

  • Conceptual Focus: Understanding how luminance variations can signify motion in visual processing.

MOTION EXTRACTION PROCESS

  • Visual Representation:

    • Incorporates aspects such as time delay (ΔT) and spatial location (X) in the analysis of motion.

ELABORATED-REICHARDT DETECTOR (ERD)

  • Stages in an ERD:

    • Involves spatially offset receptive fields.

    • Incorporates a temporal delay (ΔT) on the output of one of the receptive fields.

    • Summation cell that combines (multiplies) the output from the two cells.

RESPONSE OF AN ERD

  • Activation: The summation cell is only active when the two signals arrive simultaneously.

  • No Response Condition:

    • Occurs when signal * no-signal = No response (0)

    • Happens when the stimulus arrives first at the filter with the delay stage and the delay matches the time the stimulus takes to travel to the next filter.

    • Direction of Motion:

    • In the opposite direction, signals arrive at different times, leading to a lack of response.

  • Potential Impact of Additive Combination Cell:

    • If the combination cell was additive instead of multiplicative:

    • Would lose both motion and direction selectivity.

    • Would respond to:

      • A static bar in either of the receptive fields.

      • A bar moving in either direction.

SPEED TUNING OF AN ERD

  • Modifying Speed Tuning:

    • Can be altered by either of the following two methods to adjust for higher speeds:

      • Changing Temporal Delay:

      • The shorter the delay, the faster the optimal speed.

      • Altering Spatial Offset:

      • A greater spatial offset results in a faster optimal speed.

MOTION-ENERGY MODEL

  • Overview: Another model akin to the ERD but not necessary for detailed understanding.

  • Components:

    • Sensitivity

    • Spatial Filters

    • Temporal Filters

    • Position sensitivity.

LOCAL-MOTION PROCESSING

  • Primary Location: Motion extraction occurs in V1 complex cells.

  • Receptive Fields (RFs):

    • Complex cells exhibit small receptive fields, leading to specific motion detection constraints.

COMPLEX CELL EXAMPLE

  • Orientation and Motion Alignment:

    • A complex cell tuned to vertical orientation and motion to the right requires a change in the light pattern hitting the RF to fire.

  • each complex cell is tuned to a specific orientation and direction of motion

  • to activate the cell, the light must change in a way that matches both orientation and motion

EXTRACTING MOTION

  • Long Bar Dynamics:

    • If a bar is longer than the RF height:

    • It can move in any direction.

    • Detectable Motion:

      • The cell can detect motion components orthogonal to its orientation.

    • Undetectable Motion:

      • Motion parallel to its orientation does not alter the illumination on the RF.

MOTION COMPONENTS

  • Vector Nature of Motion:

    • Motion encompasses both magnitude (speed) and direction:

      • Represented by an arrow, where:

      • Speed is the length of the arrow

      • Direction is the orientation of the arrow.

  • Representation in Cartesian Coordinates:

    • Motion occurs simultaneously in both directional components, not sequentially.

EXTRACTING MOTION AND THE APERTURE PROBLEM

  • Orthogonal vs. Parallel Motion:

    • Motion orthogonal to the orientation of the bar creates a light pattern change on the RF.

    • Motion parallel does not alter the illumination.

🧩 Simple Analogy

Imagine a flashlight shining through a vertical slit:

•  If a vertical stick moves sideways (left/right), the light pattern through the slit changes — the sensor detects motion.

•  If the stick moves up/down, the light pattern stays the same — the sensor doesn’t detect anything.

  • Aperture Problem (local motin detectors having small RFs):

    • Limitation of local motion units due to localized RFs (apertures).

    • Results in the inability to detect components of motion aligned parallel to the cell's preferred orientation.

    • Requirement for Accurate Motion Extraction:

    • Pooling outputs from multiple local-motion cells is necessary to ascertain true object motion.

GLOBAL MOTION POOLING

  • Purpose and Process:

    • Combining local-motion signals to derive the actual motion of larger objects (spatially extended objects, greater than 2 degrees).

    • The pooling process occurs during the 'Global Motion' stage.

  • Model: Intersection-Of-Constraints (IOC) is a recognized model of global-motion pooling.

WHAT YOU NEED TO KNOW

  • Key concepts include:

    • The aperture problem: definition, causation, and implications for motion perception.

    • The importance of pooling outputs from multiple V1 local-motion cells for accurate object motion determination.

STIMULI USED TO INVESTIGATE GLOBAL MOTION (GM) POOLING

  • Diverse Stimuli: All stimuli used must enable pooling from many local-motion units to derive correct motion perception.

  • Notable Stimulus Types:

    • Global-motion stimulus

    • Global-Gabor stimulus

GLOBAL-MOTION STIMULUS

  • Description: Signal embedded in motion noise:

    • Signal Dots: Move in the same direction.

    • Noise Dots: Move in random directions.

    • Operational Complexity: Requires pooling of local-motion signals over both space and time for effective perception.

  • Threshold Measurement:

    • Calculates the number of signal dots needed to ascertain the direction of global motion.

    • The system exhibits high sensitivity with thresholds around 6%.

GLOBAL-GABOR STIMULI

  • Definition: A collection of Gabors exhibiting consistent global motion.

  • Research Reference: Mentioned work by Amano, Edwards, Badcock, & Nishida (2009) published in the Journal of Vision.

  • Gabor Definition: A function representing a sine wave within a Gaussian envelope.

LOCATION OF MOTION STAGES

  • Positions in Brain:

    • Local-motion extraction occurs primarily in V1 with complex cells.

    • Global-motion pooling is handled in V5 (MT).

ORGANISATION OF V5 (MT)

  • Structural Similarities to V1:

    • Maintains a columnar organization, with columns specifically tuned to various motion directions.

LINKING V5 TO GLOBAL-MOTION PROCESSING

  • Research Methods for Connection Establishment:

    • Lesion studies and clinical investigations,

    • Transcranial magnetic stimulation (TMS),

    • Brain activity imaging via fMRI,

    • Single-cell recordings targeting V5 cells tuned to the strength of global motion signals,

    • Cortical micro-stimulation potentially biases perceived motion in calibrated directions.

LOCAL-MOTION POOLING STAGE

  • Dynamics of Responses:

    • When oppositely moving dots are spatially unbalanced, responses are strong at both V1 and V5, leading to perceived motion transparency.

    • With local balance, V1 responses remain strong, yet V5 responses weaken, inhibiting transparency perception.

  • Significance of Local-Motion Pooling Stage:

    • Primarily serves to provide an accurate estimate of motion, akin to local pooling for orientation processing.