UC Berkeley

Objective: To show that cortical responses to conditioned stimuli (CS) include intermittently induced spatial patterns of amplitude modulation (AM) of beta-gamma oscillation called frames.

Methods: EEGs were recorded from 8x8 high-density arrays fixed on primary sensory cortices of rabbits trained to discriminate CS with reinforcement (CS+) from those without (CS-). EEG frames were located with a pragmatic information index, He. The spatial patterns of the first 3 frames on each of 37-40 trials were measured by the square of 64 analytic amplitudes from the Hilbert transform to give points in 64-space. The questions were asked: Did the frames from CS+ trials and CS- trials differ within each sequential group? Did the three frames differ from each other (form 3 clusters of points)?

Results: EEG frames that were identified by high He had AM patterns that could be classified with respect to CS+ and CS- well above chance levels. Two stages of correct frame classification occurred on each trial: 40-130 ms after CS onset with a gamma carrier frequency, and 450-550 ms with a beta carrier frequency. Peak power in the beta frames was double that in gamma frames, and mean pattern surface area of beta frames was nearly four-fold greater.

Conclusions: Under the impact of a CS on a sensory neocortex the background EEG activity reorganized in sequential frames of coordinated activity, first local and modality-specific, thereafter global.

Significance: The size, texture and duration of these AM patterns indicate that spatial patterns of human beta frames may be accessible with high-density scalp arrays for correlation with phenomenological reports by human subjects.