The profile of the recovery cycle in human primary and secondary somatosensory cortex: a magnetoencephalography study

Objective: To estimate the lifetime of sensory memory in human primary (SI) and secondary (SII) somatosensory cortex with a view to furthering our understanding of the roles played by these cortices in the processing of tactile information. Methods: Somatosensory evoked fields (SEFs) were recorded following trains of 5 electrical pulses applied to the right median nerve at the wrist using a whole-head 80 channel magnetoencephalography (MEG) system. Recordings were acquired for trains of pulses with differing interstimulus intervals (ISIs) occurring at 100, 200, 300, 400 and 500 ms. The profile of SEF intensities for the different ISIs provided an estimate of the recovery cycle of evoked neuronal activity, and the time constant of the exponential curve fitted to the recovery cycle was calculated to obtain a putative measure of the lifetime of somatic sensory memory in SI and SII. Results: The estimated time constants were 0.11±0.06 s (mean±SD) in SI and 0.82±0.34 s in SII. The mean time constant in SII was significantly longer than that in SI (Studentʹs paired t test: P=0.021; analysis of variance: F1,3=19.7, P=0.021). Conclusions: These data indicate that the lifetime of somatic sensory memory is of longer duration in higher order cortical areas than in primary sensory cortex in the somatosensory information processing system.