Loss of Somatosensory-evoked Potentials and the Timing of Perception

Abstract

To determine if patients with brain lesions who have a unilateral loss of their primary somatosensory-evoked potential (SSEP) have altered temporal perception. Benjamin Libet postulated that the neural processing of stimuli to reach the conscious awareness takes 300 to 500 milliseconds and that accurate temporal perception of stimulus onset requires a retroactive computation. Although Libet proposed that the primary SSEP acts as a timing marker for this backward referral of perceived stimulus onset time, there has not been a systematic study of the necessity of the primary SSEP for perceptual timing. Participants were 10 healthy older adults and 10 stroke patients with hemisensory deficits. SSEPs were recorded from each hemisphere using median nerve stimulation. The participants' temporal perception of sensory stimuli was determined by asking them the temporal order of bilateral hand stimuli over varying interstimulus intervals. Patients with unilateral loss of SSEPs had a significantly greater mean delay in perception of stimuli from their contralesional arm than participants with intact bilateral SSEPs [mean delay (+/-standard deviation): 134 (+/-142) msec vs. 2.5 (+/-13) msec; P=0.03]. These results demonstrate that loss of SSEP is associated with a delay in perceptual awareness. This observation is consistent with the hypotheses that the SSEP acts as a marker for cortical events important for perceptual timing.