Computed EEG topography of response to visual and auditory stimuli

EEG activity was monitored in 15 normal subjects at rest and while they were given 10 sec continuous light or tone stimuli of different intensities. Topographical cortical changes were assessed using 16-channel EEG power estimate maps of the left hemisphere. Statistical analyses were performed using analyses of variance and t test, and the reactivity of the EEG, i.e., the percent change from the baseline state, in response to stimulation, was topographically mapped. Significant EEG reactivity was recorded in alpha, theta and delta bands without modification in the beta band. Stimulation-induced effects were maximum in the posterior part of the brain for both tone and light. The light effect was stronger than tone in the alpha band, but no differences in modality effect were recorded in the delta or theta band. Analysis of frequency bands within the alpha and theta band revealed a different pattern of reactivity with different regional involvements for tone and light. EEG reactivity occurred mainly in 7-10 Hz frequency bands with no significant change in 5, 6, 11, 12 and 13 Hz. Reactivity was maximal in the temporal area during tone for 7-9 Hz and in the occipital region during light for 8-10 Hz. The 8 and 9 Hz frequency bands showed increased blocking with an increase in stimulus intensity. Taken together, the results indicate that visual and auditory stimuli both produce an EEG amplitude decrease which is greatest over their respective primary sensory cortex.