Cortical γ responses: searching high and low

Abstract

In this paper, a brief, preliminary attempt is made to frame a scientific debate about how functional responses at gamma frequencies in electrophysiological recordings (EEG, MEG, ECoG, and LFP) should be classified and interpreted. In general, are all gamma responses the same, or should they be divided into different classes according to criteria such as their spectral characteristics (frequency range and/or shape), their spatial-temporal patterns of occurrence, and/or their responsiveness under different task conditions? In particular, are the responses observed in intracranial EEG at a broad range of "high gamma" frequencies (~60-200Hz) different from gamma responses observed at lower frequencies (~30-80Hz), typically in narrower bands? And if they are different, how should they be interpreted? Does the broad spectral shape of high gamma responses arise from the summation of many different narrow-band oscillations, or does it reflect something completely different? If we are not sure, should we refer to high gamma activity as oscillations? A variety of theories have posited a mechanistic role for gamma activity in cortical function, often assuming narrow-band oscillations. These theories continue to influence the design of experiments and the interpretation of their results. Do these theories apply to all electrophysiological responses at gamma frequencies? Although no definitive answers to these questions are immediately anticipated, this paper will attempt to review the rationale for why they are worth asking and to point to some of the possible answers that have been proposed.

Figure 1

Conceptual schematic illustrating a possible mechanism explaining the broad range of frequencies involved in high gamma responses recorded with electrocorticography. An electrode array is positioned over a slab of cortex (scaling distorted for illustration purposes) with inverted cones “illuminating” cortical regions contributing the most to signals recorded at each electrode. Cylinders in cortex represent assemblies of neurons with strong functional connectivity, e.g. macrocolumns or groups of tightly interconnected columns. Black (and white) cylinders represent neuronal assemblies that are not currently engaged in task-related cortical processing. Different colored cylinders represent assemblies that are engaged in different aspects of cortical processing (e.g. processing different object features) and have different resonant frequencies at which synchronous neuronal firing and associated membrane potential oscillations occur. Membrane potential oscillations with different center frequencies collectively contribute to signals recorded by ECoG macroelectrodes and their summation produces a broadband shape to high gamma responses in the power spectrum. This would require more oscillations than can be illustrated here unless their bandwidths at half power were quite wide (low Q). Unrelated to the mechanisms discussed in the text, red-highlighted circular area indicates a region of cortex with event-related desynchronization (ERD, power suppression) in alpha/beta frequencies, reflecting a thalamocortical gating mechanism in a larger zone of cortex that permits or facilitates cortical processing related to the current task. Reproduced with permission from Elsevier (Crone and Hao, 2002).