Figure 6.

Delta entrainment to attended auditory stimuli in intracranial recordings. A, Averaged ERP waveforms related to attended auditory stimuli recorded intracranially just above primary auditory cortex in a macaque monkey. While the orange trace is the average of recordings above A1 regions whose BF corresponded to the frequency of the attended tones (n = 10), the green trace is the average of concurrent recordings from a second electrode positioned above an area that was not tuned to the frequency of attended tones (non-BF). The rhythmic delta range baseline fluctuation is in opposite phase above non-BF compared with BF regions even recorded only 2 mm apart. Black tick marks on the x-axis denote the times when stimuli were presented. B, Gamma amplitude traces on top illustrate that there is a stimulus structure-related fluctuation of gamma activity that is also opposite in sign. MUA traces on the bottom show a similar trend. Note that the amplitude of the baseline MUA modulation is very small, since it reflects subthreshold excitability changes, as opposed to the MUA responses to tones reflecting suprathreshold activation and consequent firing of the neuronal ensemble. C, The first boxplot quantifies the gamma amplitude modulation: the difference of prestimulus and interstimulus gamma amplitudes is positive above BF regions signaling increasing gamma oscillatory activity in the immediate prestimulus time interval. As opposed to this, it is negative above non-BF regions, similar to human surface data. The second boxplot displays the amplitude of intracortical MUA modulation (prestimulus − interstimulus MUA), which shows the same trend. D, The distribution of mean delta phase across human subjects (top) and monkey recording sites (bottom) at the time attended auditory stimuli are presented. In controls, mean delta phases are pooled around the negative peak, while in patients the distribution of mean delta phases appears more random, even across subjects with significant delta phase bias (Rayleigh test, p < 0.05, 13 of 40 subjects; only these mean phases are displayed). In monkey intracranial recordings, mean delta phases are pooled around the positive peak above A1 sites that are tuned to the attended frequency (i.e., BF). In contrast, mean delta phases are pooled around the negative peak above A1 sites tuned to different frequencies (i.e., non-BF), similar to the mean delta phase of control subjects recorded on the scalp.