Supra-modal role for frontal cortex in sensory gating

The objective of this study was to determine if there is a cortical source in the frontal cortex that plays a role in somatosensory gating. Ten normal controls (aged 32 ± 10 years) were monitored with MEG. Somatosensory evoked fields (SEF's) were measured during presentation of pressure pulses to the middle finger. The right hand was tested first then the left. Pulse #2 followed pulse #1 by 580 ms. The interpair interval between pulse-pairs was 8 s. 100 epochs were averaged with a pre-stimulus baseline of 100 ms. MR-FOCUSS was used to localize the cortical sources. Four SEF peak responses were observed in all subjects and localized to the somatosensory cortex, where sensory processing for the middle finger is known to occur. The response to the first stimulus was associated with the pressurized bladder touching the finger; a secondary response to stimulus #1 was observed 30 ms later, this was due to bladder deflating. This “On–Off” response occurred for each tap. Averaged MEG data results from all subjects showed activation arising from the first pulse at ∼ 42 ms. MR-FOCUSS showed the underlying source for this response had average activity amplitude of ∼ 834 nAm. This response was followed by a second peak at ∼ 74 ms with average amplitude of ∼ 641 nAm. The locations of cortical sources for the “on” response were found to be very focal in the somatosensory cortex on the post central gyrus in the location normally active for finger tapping responses. For the second pulse, responses occurred at ∼ 639 ms with average amplitude of ∼ 597 nAm, followed by “off” response at ∼ 677 ms with average amplitude of ∼ 519 nAm. The second response localized to an extended regions of the somatosensory cortex. Thus locations included post central gyrus and activation in the superior frontal gyrus (Brodmann's area 6). Elucidating the role of the frontal cortex in mediating sensory gating is crucial for the understanding of neuronal mechanisms underlying pre-attentive filtering of irrelevant information in individuals with normal and abnormal getting function.