Auditory and Visual Gratings Elicit Distinct Gamma Responses

Subject- and electrode-averaged change in power compared with baseline (−500 to 0 ms), pooled across all stimulus conditions with two different reference schemes. Results are shown for the unipolar reference scheme (left) and bipolar (right). The top row in A and the first two in B and C are in response to visual stimuli. The bottom row in A and the last two in B and C are in response to the auditory stimuli. Electrodes used for averaging in A and B are highlighted with black dots in scalp maps shown in C. A, Time-frequency change in power. Dashed vertical lines (black) represent stimulus onset and offset. B, Power spectra [first and third row panels—red trace is the stimulus period (250–750 ms), and the green trace is the baseline period (−500 to 0 ms)] and change in power spectra versus frequency (second and fourth row panels, blue traces). The solid traces represent the mean across subjects, and thickness represents SEM. Dashed vertical lines represent narrowband gamma (20–66 Hz, blue) and broadband high-gamma (70–150 Hz, cyan). Colored squares at the bottom represent the significance of differences in means (paired t test—purple, p values between 0.01 and 0.05; pink, p < 0.01). C, Scalp maps of 64 unipolar electrodes (left) and 114 bipolar electrodes (right). The first and third rows show the change in power for frequencies 20–66 Hz. The second and fourth rows show the change in power for frequencies 70–150 Hz. The color bar represents the log power ratio in decibels.

Visual and auditory responses for individual subjects. Change in time-frequency power spectra for males (A) and females (B). The top row represents the results in response to visual stimuli, and the bottom represents the results in response to auditory stimuli. Subjects are ordered horizontally based on the decreasing high-gamma activity in 70–150 Hz for the auditory protocol, starting from the left. The response is averaged across selected occipital and temporal electrodes (highlighted in the inset of Fig. 1) for respective protocols. The color bar represents the log power ratio in decibels. Empty plots refer to subjects rejected from the analysis (refer to Materials and Methods, Artifact rejection for more details).

Auditory-induced gamma is not correlated with visually induced gamma. Scatter plots for change in power in response to visual stimuli versus auditory stimuli. A, For narrowband gamma power averaged across electrodes for the respective protocol (as shown in Fig. 1 inset). B, For broadband high-gamma power averaged across electrodes. C, For narrowband gamma power for electrode with maximum change in power. D, For broadband high-gamma power for electrode with maximum change in power. Pearson's correlation coefficient (r) and p value are mentioned at the bottom right in each panel. Note the difference in axis limits across the panels. E, Violin plots showing the distribution of change in power from baseline for different gamma frequency ranges (NB refers to narrowband gamma and BB refers to broadband high-gamma) during different tasks. The inset box plot indicates the median (black line), mean (magenta line), box (spanning the edges of the second and third quartile), and whiskers from box edge to the farthest point at 1.5 IQR (interquartile range) with any outliers plotted as distinct points. The significance of the unpaired t test (with unequal variance) is indicated at the top of the violin plots.

No stimulus selectivity was observed for either of the induced gamma(s). Change in time-frequency power spectrum from baseline for different stimulus conditions. A, In visual modality. B, In auditory modality. Power is averaged across selected electrodes for each modality. Dashed vertical lines (black) represent stimulus onset and offset. The color bar represents the log power ratio in decibels.