Subjective referral of the timing for a conscious sensory experience: a functional role for the somatosensory specific projection system in man

Abstract

Subjective experience of a peripherally-induced sensation is found to appear without the substantial delay found for the experience of a cortically-induced sensation. To explain this finding, in relation to the putative delay of up to about 500 ms for achieving the "neuronal adequacy" required to elicit the peripherally-induced experience, a modified hypothesis is proposed: for a peripheral sensory input, (a) the primary evoked response of sensory cortex to the specific projection (lemniscal) input is associated with a process that can serve as a 'time-marker'; and (b), after delayed neuronal adequacy is achieved, there is a subjective referral of the sensory experience backwards in time so as to coincide with this initial 'time-marker'. A crucial prediction of the hypothesis was experimentally tested in human subjects using suitably implanted electrodes, and the results provide specific support for the proposal. In this, the test stimuli to medial lemniscus (LM) and to surface of somatosensory cortex (C) were arranged so that a minimum train duration of 200 ms or more was required to produce any conscious sensory experience in each case. Each such cerebral stimulus could be temporally coupled with a peripheral one (usually skin, S) that required relatively negligible stimulus duration to produce a sensation. The sensory experiences induced by LM stimuli were found to be subjectively timed as if there were no delay relative to those for S, that is, as if the subjective experience for LM was referred to the onset rather than to the end of the required stimulus duration of 200 ms or more. On the other hand, sensory experiences induced by the C stimuli, which did not excite specific projection afferents, appeared to be subjectively timed with a substantial delay relative to those for S, that is, as if the time of the subjective experience coincided roughly with the end of the minimum duration required by the C stimuli. The newly proposed functional role for the specific projection system in temporal referral would be additional to its known role in spatial referral and discrimination. A temporal discrepancy between corresponding mental and physical events, i.e., between the timing of a subjective sensory experience and the time at which the state of 'neuronal adequacy' for giving rise to this experience is achieved, would introduce a novel experimentally-based feature into the concept of psychophysiological parallelism in the mind-brain relationship.