PROBING THE NATURE OF THE CNV

A widespread depolarization in the dendritic trees of cortical pyramid al neurons generates surface-negative potentials. In turn, such potent ials may indicate facilitatory processes while positive-going waves ma y result from a lowering in cortical excitability. Accordingly, we may expect the processing of ''probe'' stimuli presented during surface-p ositive waves, i.e., during phases of lesser excitability, to be inhib ited and probes presented during surface-negative waves to be facilita ted. This hypothesis was tested by presenting acoustic probe stimuli a t various points in time during a forewarned reaction time task. The w arning stimulus (WS) elicited a late positive complex followed by a ne gative slow potential shift (CNV). In 75% of the total of 120 trials a probe could be presented 1.5 sec prior to the WS interval, 0.5, 1, 1. 5 or 2 sec after the onset of the 3 sec visual WS, and 3 sec following the imperative signal (WS offset, requiring a fast button press respo nse), while 25% of the trials were without any probe. Only one probe o ccurred during a trial. The EEG was recorded along the midsagittal lin e; responses to the probes were evaluated by reaction time (RT) and pr obe-evoked potentials. RT to probes presented late in the anticipatory interval were speeded up and probe-evoked potentials were enhanced du ring this interval, in parallel to the development of the slow potenti al and the CNV in particular. Results suggest that probe stimuli prese nted during the development of the CNV were processed more intensely, thereby supporting the hypothesis that slow cortical potentials indica te the timing of excitability in cortical neuronal networks. Such a tu ning mechanism may serve as a basis for attentional regulation.