Brain Stimulation in the Study of Neuronal Functions for Conscious Sensory Experiences

Some features of cerebral neuronal functions that are uniquely related to generation of conscious sensory responses, have been discovered. Included are two temporal factors: (1) Substantial delays, of up to about 0.5 sec, before achieving cerebral "neuronal adequacy" appear to be required for eliciting a sensory experience. This includes the demonstration that a cortical stimulus (C) can retroactively modify a skin (S)-induced sensation even when C stimulus begins up to 500 msec after S stimulus. (2) However, there appears to be a subjective referral of the experience back to the time of the cortical primary evoked response to S; subjectively the skin sensation would thus appear to have no delay. Stimuli that are inadequate for eliciting conscious sensory experience can nevertheless evoke considerable neuronal activity, including that represented in direct cortical responses or in primary (early) components of evoked potentials. Virtually all the qualities of somatic sensation (except pain) can be elicited by stimuli at post-central gyrus, when intensities of suitable trains of pulses are kept down to liminal levels. Some additional implications, for the manner in which mental and neural events are related, are discussed.