Cortical neural evoked correlates of somatosensory stimulus detection in the rhesus monkey

Rhesus monkeys were trained to respond to constant-current electrical pulse stimuli to the hand which are known to elicit touch sensation in man. Simultaneously, recordings of somatosensory evoked potentials (SEPs) were made from postcentral gyrus of the performing monkeys. The prominent features of the SEP at most recording sites were a negative (N1) component peaking at 50-65 msec followed by a positive wave (P2) peaking at 105-130 msec. Primary evoked activity (P1) was minimal or absent at most sites at the intensities employed. Differences in N1 peak latency ranging from 4 to 9 msec were observed between the central member of a cluster of recording sites and those surrounding it. These differences are thought to reflect the propagation of evoked activity from some unidentified focus in postcentral gyrus to surrounding regions. N1 and P2 amplitude was found to decrease as a function of behavioral response latency at both the center and surrounding sites of the electrode clusters. The signal detection theoretical model, which provided the interpretative framework for neurophysiologic and psychophysical responses, suggested that N1 and P2 peak amplitude may reflect somatosensory information processing events necessary for psychophysical performance of the monkey. The propagation of evoked activity to different sites on postcentral gyrus could therefore signify the transmission of this sensory information to surrounding cortical regions. Since the psychophysical model is equally applicable to monkey or man, it is suggested that evidence presented here and in similar studies may be relevant to the question of the neural coding of conscious somatic sensory experiences of man.