Surround suppression supports second-order feature encoding by macaque V1 and V2 neurons.

Area of Science:

• Neuroscience
• Visual Perception
• Computational Neuroscience

Background:

• Single neurons in primate visual cortex (V1 and V2) exhibit selectivity to luminance-modulated stimuli.
• Responses are influenced by stimuli outside the classical receptive field (CRF), indicating contextual processing.
• This contextual processing is vital for signaling second-order visual features, defined by spatial relationships of contrast and orientation.

Purpose of the Study:

• To investigate the neural mechanisms underlying second-order form selectivity in primate visual cortex.
• To determine how the spatial organization of receptive fields contributes to second-order orientation selectivity.
• To model and validate the computational basis of second-order visual processing.

Main Methods:

• Measured single-unit responses in anesthetized macaques to contrast-modulated gratings (second-order forms).
• Developed a computational model using spatial Gaussians to represent receptive field subregions (CRF and surround).
• Validated model predictions using conventional surround suppression measurements and spike-triggered analysis.
• Conducted psychophysical adaptation experiments on human observers.

Main Results:

• Most neurons showed selective responses to the orientation of contrast modulation, indicating second-order orientation selectivity.
• Second-order selectivity was attributed to asymmetric spatial organization of the excitatory CRF and suppressive extraclassical surround.
• The model successfully predicted neural responses and explained second-order selectivity arising from CRF elongation and/or asymmetric surround suppression.
• Human psychophysical data mirrored the neural response patterns.

Conclusions:

• Primate cortical cells in V1 and V2 possess the capacity for both first-order (luminance-based) and second-order (contrast-based) form processing.
• Asymmetric receptive field organization, including CRF shape and surround suppression, is key to second-order selectivity.
• These findings suggest a fundamental dual role for cortical neurons in visual information processing and perception.