Sensory-Motor Coupling

Overview

The sensory-motor coupling mechanism creates a real-time connection between the mouse's running behavior and the visual stimulus presented on the screen. This coupling allows the investigation of how neural responses are modulated by self-generated visual feedback and provides a foundation for studying predictive processing in the context of sensorimotor integration.

Coupling Mechanism

The coupling system transforms the mouse's wheel movement into corresponding changes in the visual gratings' spatial phase, creating the illusion that the mouse is navigating through a visual environment. This creates a direct sensorimotor loop where the animal's movement controls the visual feedback.

Hardware Setup

The sensory-motor coupling relies on a specific geometric arrangement:

  • Screen Distance: 15 cm from the mouse's eye
  • Mouse Position: 5.5 cm from the center of the running wheel
  • Encoder: AMT10 encoder with 8192 counts per revolution tracking wheel rotation
  • Stimulus: Vertical drifting gratings displayed on the screen

Mathematical Foundation

The conversion from wheel rotation to grating phase offset is accomplished through the following geometric transformation:

Phase Offset = 2 * Math.PI * WheelRadiusOverScreenRatio * WheelAngle / (Math.tan((1/SpatialFrequency) * Math.PI / 180))

Where:

  • WheelAngle (Item1): The encoder reading in degrees representing the wheel's rotational position
  • WheelRadiusOverScreenRatio (Item2): The ratio of 5.5/15 = 0.36, accounting for the geometric relationship between wheel radius and screen distance
  • SpatialFrequency (Item3): The spatial frequency of the gratings in cycles per degree (typically 0.04 cpd)

Geometric Analysis

The Coupling Formula Explained

The formula implements a geometric transformation that accounts for the relationship between wheel movement and visual field displacement:

  1. Linear Distance Calculation: 2 * Math.PI * WheelRadiusOverScreenRatio * WheelAngle calculates the linear distance the mouse has "moved" based on wheel rotation:
  2. WheelAngle is converted from degrees to a proportion of full rotation
  3. Multiplied by 2 * Math.PI to get radians of wheel rotation

  4. Scaled by WheelRadiusOverScreenRatio to account for the geometric relationship between wheel and screen

  5. Spatial Frequency Conversion: Math.tan((1/SpatialFrequency) * Math.PI / 180) converts the spatial frequency to the appropriate scaling factor:

  6. 1/SpatialFrequency gives the spatial period (degrees per cycle)

  7. Converted to radians and tangent calculated to account for visual field geometry

  8. Phase Calculation: The division gives the final phase offset that corresponds to the appropriate visual displacement for the given spatial frequency.

Physical Interpretation

The goal is to create a realistic spatial coupling where:

  • Forward wheel movement → Forward visual motion
  • Backward wheel movement → Backward visual motion
  • The amount of visual motion matches the mouse's perceived movement through space

The formula ensures that when the mouse runs forward by rotating the wheel, the visual pattern shifts by a distance that accurately represents the mouse's movement through a stationary visual environment. The spatial frequency scaling ensures that the visual displacement is appropriate for the specific grating pattern being displayed.

Implementation in Bonsai

Data Flow

  1. Encoder Input: The AMT10 encoder continuously reports wheel position in degrees
  2. Parameter Combination: The wheel angle is combined with the coupling ratio (0.36) and spatial frequency (0.04 cpd)
  3. Phase Calculation: The coupling formula calculates the appropriate phase offset
  4. Modulo Operation: The result is wrapped to 360° to maintain continuous phase cycling
  5. Visual Rendering: The calculated phase offset is applied to the vertical gratings

Configuration Parameters

The coupling system uses several configurable parameters:

  • WheelRadiusOverScreenRatio: 0.36 (ratio of 5.5cm/15cm)
  • MotorCouplingCPD: 0.04 cycles per degree (spatial frequency of the gratings)
  • CountsPerRevolution: 8192 (encoder resolution)

These parameters can be adjusted to modify the coupling strength or adapt to different experimental setups.

Script Location

The sensory-motor coupling implementation is found in:

Technical Considerations

  • Latency: The system operates at the display refresh rate (typically 60 Hz) ensuring minimal latency between movement and visual feedback
  • Resolution: The 8192 counts per revolution encoder provides high-resolution tracking of wheel movement
  • Continuous Operation: The phase calculation updates continuously, providing smooth visual motion that tracks wheel movement in real-time

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