Somatosensory evoked potential
By combining SEP recordings at different levels of the somatosensory pathways, it is possible to assess the transmission of the afferent volley from the periphery up to the cortex.
SEPs are routinely used in neurology today to confirm and localize sensory abnormalities, to identify silent lesions and to monitor changes during surgical procedures.
Although the origins and mechanisms of far-field SEPs are still debated in the literature, correlations among abnormal waveforms, lesion site, and clinical observations are fairly well established.
The approach based on clinical correlations supports the idea of a single generator for each SEP component, which is suitable for responses reflecting the sequential activation fibers and synaptic relays of the somatosensory pathways.
This model fits better with the parallel activation and the feedback controls that characterize the processing of somatosensory inputs at the cortical level.
The median nerve pathway then joins the posterior columns, sending off collateral branches to synapse in the midcervical cord.
It has always been assumed that cortical SEPs peaking before 50 ms following stimulation of the upper limb are not significantly affected by cognitive processes.
However, Desmedt et al. (1983)[4] identified a P40 potential in response to target stimuli in an oddball task, suggesting that attention-related processes could affect early cortical SEPs.
[5] Testing with median nerve SEPs is used to identify the sensory and motor cortex during craniotomies and in monitoring surgery at the midcervical or upper cervical levels.
Over time, SEP testing and monitoring in surgery have become standard techniques widely used to reduce risk of postoperative neurologic problems for the patient.
Continuous SEP monitoring can warn a surgeon about potential spinal cord damage, which can prompt intervention before impairment becomes permanent.
Schubert et al. (2006)[6] used SEPs to investigate the differential processing of consciously perceived versus unperceived somatosensory stimuli.
The authors used an 'extinction' paradigm to examine the connection between activation of S1 and somatosensory awareness, and observed that early SEPs (P60, N80), generated in the contralateral S1, were independent of stimulus perception.
The authors concluded that plastic changes in somatosensory processing might be induced by performing physical exercises that require attention and skilled movements.
