Category-selective phase coding in the superior temporal sulcus.

Area of Science:

• Neuroscience
• Computational Neuroscience
• Visual Processing

Background:

• Object perception and categorization can be extremely rapid, sometimes preceding neural firing rate changes.
• Phase coding, where spike timing relative to neural oscillations encodes information, is a potential mechanism for rapid representation.
• The role of phase coding in object-selective neocortex for rapid categorization remains to be fully elucidated.

Purpose of the Study:

• To investigate whether object-selective regions of the neocortex exhibit phase coding for object categories.
• To determine if phase coding contributes to the rapid processing of visual stimuli, such as faces and objects.

Main Methods:

• Recorded local field potentials (LFP) and single unit activity from object-selective superior temporal sulcus in monkeys.
• Presented images of faces and objects to assess stimulus-evoked neural responses.
• Analyzed firing phases and rates across different frequency bands (e.g., <20 Hz, gamma, high-gamma).

Main Results:

• Single neurons demonstrated category-specific preferred firing phases that emerged with initial spiking responses.
• Phase differences were observed in low (<20 Hz), gamma, and high-gamma frequency ranges.
• Phase-specific firing remained category-specific even after controlling for stimulus-locked LFP activity, and firing rate-to-phase conversion varied by frequency.

Conclusions:

• Object-selective neocortex employs phase coding to represent object categories rapidly.
• Phase-of-firing effects, particularly in gamma frequencies, reflect more than just evoked LFP and firing rate responses.
• These findings support theoretical models of fast object processing and extend phase coding evidence to the neocortex.