Quadrature subunits in directionally selective simple cells: counterphase and drifting grating responses

Area of Science:

• Neuroscience
• Computational Neuroscience
• Visual System Physiology

Background:

• Directional selectivity (DS) in simple cells of the cat's striate cortex is crucial for visual processing.
• Previous models often used a single-subunit linear-nonlinear (LN) structure, failing to explain observed phase skews in cell responses.
• Space-time inseparable receptive fields (RFs) are characteristic of these simple cells.

Purpose of the Study:

• To investigate the neural basis of directional selectivity (DS) in cat visual cortex simple cells.
• To determine if a single-subunit model can account for observed response characteristics, specifically rotational phase skew.
• To validate a two-subunit model for DS simple cells using a different stimulus and analysis approach.

Main Methods:

• Employed counterphase sinusoidal stimuli and analyzed output signals.
• Utilized a two-branch (two-subunit) computational model with soft-threshold nonlinearities.
• Compared model predictions with experimental measurements of simple cells, focusing on polar phase plots.

Main Results:

• A single-subunit model cannot generate the observed rotational phase skew in polar plots.
• The two-subunit model, with subunits in spatiotemporal phase quadrature, successfully accounts for the phase skew.
• This supports a two-subunit structure as essential for DS in simple cells.

Conclusions:

• The presence of at least two nonlinear subunits is obligatory for directional selectivity (DS) in visual cortical cells.
• These subunits enhance responses to preferred-direction motion and may help overcome neuronal threshold limitations.
• The findings support a more complex neural architecture for motion detection than previously modeled.