Lateral asymmetries and thalamic components in far-field somatosensory evoked potentials.

To investigate the neural generators of the subcortical somatosensory evoked potential (SEP) waves we studied the far-field SEPs in 20 normal adults. We stimulated the median nerve at the wrist and used a 40k gain, 25 msec sweep and a 150-3000 Hz bandpass. We recorded SEPs simultaneously over C3' and C4' using a non-cephalic clavicle reference. The following series of six positive waves were found reliably in all subjects: P9, P11, P13, P14, P16 and P17. The P16 and P17 probably arise from the thalamus and/or thalamocortical projections. Recent evidence suggests that the thalamus is not a closed field and thus one should be able to find corresponding waveforms in far-field recordings. We believe that this is a function of the bandpass used and with the above paradigm these components can be reliably recorded. We found significant ipsilateral and contralateral amplitude asymmetries beginning with the negative deflection after P14 and including P16 and P17. The amplitude was greater over the contralateral hemisphere. This suggested that both P13 and P14 are generated prior to decussation of the afferent fibres in the medial lemniscus. Bilateral recording allowed detection of this asymmetry which has not been previously reported as a means of determining the electrophysiological correlates of lateralization.