Extraction of Individual EEG Gamma Frequencies from the Responses to Click-Based Chirp-Modulated Sounds

Abstract

Activity in the gamma range is related to many sensory and cognitive processes that are impaired in neuropsychiatric conditions. Therefore, individualized measures of gamma-band activity are considered to be potential markers that reflect the state of networks within the brain. Relatively little has been studied in respect of the individual gamma frequency (IGF) parameter. The methodology for determining the IGF is not well established. In the present work, we tested the extraction of IGFs from electroencephalogram (EEG) data in two datasets where subjects received auditory stimulation consisting of clicks with varying inter-click periods, covering a 30-60 Hz range: in 80 young subjects EEG was recorded with 64 gel-based electrodes; in 33 young subjects, EEG was recorded using three active dry electrodes. IGFs were extracted from either fifteen or three electrodes in frontocentral regions by estimating the individual-specific frequency that most consistently exhibited high phase locking during the stimulation. The method showed overall high reliability of extracted IGFs for all extraction approaches; however, averaging over channels resulted in somewhat higher reliability scores. This work demonstrates that the estimation of individual gamma frequency is possible using a limited number of both the gel and dry electrodes from responses to click-based chirp-modulated sounds.

Keywords: auditory steady-state response (ASSR); dry electrodes; individual gamma frequency (IGF).

Figure 1

(A) A schematic representation of the sound stimulus used in this study. (B) Electrode placement for 64- and 3-channel systems. Channels used for analysis are colored in green. (C) A schematic representation of time-window definition for the calculation of IGFs from PLI. The bold red line indicates the timing of the stimulation; the red dashed line denotes the edge of averaging window (+150 ms). a.u.—arbitrary units.

Figure 2

An example of IGF estimation on an average of 15 channels and averaged chirp-down and chirp-up parts in two subjects: the matrix of 100 trial iterations and 5 frequencies displaying the highest PLI response. The extracted IGF is marked in red.

Figure 3

Example of time-frequency plots of PLIs. (A) Time-frequency plots of PLIs for two subjects from 15 gel electrodes data. Topoplots at IGF were created separately for chirp-down, chirp-up, and the averaging of both parts. (B) Time-frequency plots of PLIs for two subjects from three dry electrodes data.