Subcortical neuronal correlates of component P300 in man

Multiunit activity together with event-related potentials (ERP) were recorded from subcortical sites (the globus pallidus, the ventro-lateral nucleus of the thalamus and adjacent areas) in parkinsonian patients bearing electrodes for diagnosis and therapy. The patients viewed brief flashes of a visual stimulus (digit 6 or 9) randomly interspersed at 5-10 sec intervals. The patient's task was either to count silently the number of digit 6 or to press a button in response to its presentation. In the first part of the test (128 trials) the probability of the relevant stimulus was 0.25 while in the second part (another 128 trials) it changed to 0.75. Peristimulus time histograms for impulse activity of each neuronal population, averaged ERPs as well as profiles of neuronal reactions for the whole set of electrodes were computed and statistically analyzed. This study revealed the existence of subcortical P300-like activity that depended upon stimulus probability in the same way as the cortical P300 recorded from scalp electrodes. Components of responses of neuronal populations correlated with the components of subcortical ERPs were separated. The data support the multiple origin of the P300. The subcortical P300-like activity is suggested to be an electrophysiological reflection of the program selection mechanism implementing the selection of the patient's response from the set of possible programs.