Abnormal single-unit activity recorded in the somatosensory thalamus of a quadriplegic patient with central pain.

We have performed single unit analysis of the activity of cells located in the ventral nuclear group of thalamus in a patient with dysesthetic pain below the level of a clinically complete traumatic spinal cord transection at C5. Cells located in the parasagittal plane 14 mm lateral to the midline responded to tactile stimulation in small facial and intraoral receptive fields, which were characteristic of patients without somatosensory abnormality [30]. In this patient the 16 mm lateral parasagittal plane contained cells with receptive fields located on the occiput and neck instead of the upper extremity as would normally be expected. Cells with receptive fields on the neck and occiput had not previously been observed in recordings from single units (n = 531) responding to somatosensory stimulation [30]. Thus, on the basis of their location in a region of thalamus which normally represents parts of the body below the level of the spinal cord transection and their unusual receptive fields adjacent to these same parts of the body, we propose that the cells in the 16 mm lateral plane have lost their normal afferent input. Analysis of the autopower spectra of spike trains indicates that cells in the 16 mm lateral plane exhibited a higher mean firing rate and greater tendency to fire in bursts than cells in the 14 mm lateral plane (P less than 0.005). Finally, electrical stimulation at the recording sites in the 16 mm lateral plane evoked a burning sensation in the occiput, neck and upper extremity. These results suggest that regions of thalamus which have lost their normal somatosensory input contain neurons which exhibit abnormal spontaneous and evoked activity and that electrical stimulation of these regions can produce the sensation of burning dysesthesia.