No Evidence for Entrainment: Endogenous Gamma Oscillations and Rhythmic Flicker Responses Coexist in Visual Cortex

Area of Science:

• Neuroscience
• Cognitive Neuroscience
• Neuroimaging

Background:

• Cortical gamma oscillations (30-100 Hz) are linked to neurocomputation but their functional role remains debated.
• Investigating endogenous gamma oscillations' entrainability by external stimuli could clarify their causal role in cognition.

Purpose of the Study:

• To determine if endogenous human gamma oscillations can be entrained by rhythmic photic stimulation (>50 Hz).
• To explore the interaction between visual flicker responses and endogenous gamma oscillations in the visual cortex.

Main Methods:

• Magnetoencephalography (MEG) combined with a high-frequency projector to deliver rhythmic photic stimulation.
• Systematic investigation of oscillatory responses to flicker stimulation with and without induced endogenous gamma oscillations (using moving gratings).
• Source reconstruction to identify the generators of flicker responses and gamma oscillations.

Main Results:

• Photic drive elicited robust visual cortex responses up to 80 Hz.
• No phase or frequency entrainment of endogenous gamma oscillations by the photic drive was observed.
• Flicker response magnitude decreased with increasing frequency, and source reconstruction indicated separate generators for flicker responses and gamma oscillations.

Conclusions:

• Cortical gamma oscillations are not entrained by rhythmic visual stimulation.
• The mechanism generating endogenous gamma oscillations appears resilient to external perturbation.
• Findings challenge the notion of entrainment and suggest distinct neural substrates for flicker responses and endogenous gamma oscillations.