Probing the functional brain state during P300-evocation

Slow cortical potentials may represent the excitability of cortical neuronal networks. We have suggested that surface-negative potentia Is, such as the CNV, indicate increased cortical excitability while positive-going waves, such as P300, are produced when excitability is lowered transiently (Rockstroh, Elbert, Canavan, Lutzenberger, & Birbaumer, 1989). If true, the processing of "probe" stimuli presented during surface-positive waves, i. e., by brain tissue during phases of lesser excitability, should be inhibited, eading to slower responses and reduced evoked potential amplitudes. This hypothesis was examined by presenting a total of 900 acoustic stimuli comprising 70% standard stimuli (1200 Hz, 55 dB) and 30% orget stimuli (700 Hz). Twenty subjects were instructed that their major task was to silently count the targets. On 46% of the trials, clicks were presented as probe stimuli in addition to the standard or target stimulus; a probe could follow the onset of the stimulus at delays of 260, 290, 320, 350, 380 or 410 msec. A fast button press was required to every probe. Half of the sample exhibited a clear"oddball-P300"; all subjects who failed to produce a P300 differentiation between targets and standards showed a frontally negative Slow Wave which was larger to targets than to standards. N 1/P2 amplitudes to probes were smaller whenever the probes were added to a target stimulus, in particular when target stimuli had elicited a P300. n subjects with oddball P300, motoric responses to probes were delayed at times when a target-evoked positive shift was present. Subjects who produced no oddball P300 showed no such slowing of reaction time. Results support the hypothesis that widespread positive waves indicate disfacilitation of cortical excitation. tens tens