WAVE FORM AND NEURAL MECHANISM OF THE DECISION P350 ELICITED WITHOUT PRE-STIMULUS CNV OR READINESS POTENTIAL IN RANDOM SEQUENCES OF NEAR-THRESHOLD AUDITORY CLICKS AND FINGER STIMULI

In 17 successful experiments on normal human adults random sequences of equiprobable acoustic clicks or electrical stimuli to the index finger were delivered at intervals varying at random from 1 to 15 sec. The stimuli were of low intensity and provided a feasible, but exacting task. No motor activity was involved. In alternate runs either the clicks or the finger stimuli were designated as targets to be identified and mentally counted by the subject. The accuracy of the counts was checked after each run. Scalp recorded brain potentials were stored on FM tape, edited for removal of sections with artifacts and averaged. Large P350 components were elicited by the targets, but not by identical stimuli when they were non-targets. The reciprocal experimental design provided consistent controls. No pre-stimulus negative shift of the CNV type was observed in spite of the use of long time constants (8 sec). The background EEG did not affect the results. The P350 to targets was maximal in the centro-parietal region and of equal amplitude on the left and right hemispheres. Because there was little overlap from other ERP components, the wave form of P350 could be analyzed, namely the latency to onset, the time from onset to peak and the total duration. The primary components of ERPs to finger stimuli were not modified by the cognitive task, which excludes centrifugal gating of corticipetal input as a mechanism. The ERPs to targets presented an increased N120. The latter was symmetrically distributed over the central region for target clicks, but was larger in the contralateral parietal region for target finger stimuli. The neural basis of CNV, P350 and N120 is discussed in detail. It is suggested that the mesencephalic reticular formation (MRF) exerts a diffuse bilateral facilitatory neuromodulation of cortical circuits which can be transiently adjusted (to produce CNV) by MRF control from prefrontal granular cortex. P350 is a post-decision event with bilateral distribution and results from a phasic inhibition of MRF at the closure of a cognitive processing epoch. P350 and CNV are dissociable: they are distinct events that involve roughly coextensive brain generators, but result from the operation of different prefrontal cortex controls on the MRF output to the telencephalon, in cognitive behavior. N120 indexes modality-specific focal processors engaged for stimulus identification before a decision is reached. N120 may involve the more specific thalamic reticular system that is also controlled by prefrontal cortex. © 1979.