Sensory gating in the human hippocampal and rhinal regions: regional differences.

To further explore the roles of medial temporal structures in mediating sensory gating of incoming irrelevant or redundant auditory input, twenty-seven patients with intractable epilepsy with depth electrodes implanted in the medial temporal lobe for presurgery evaluation underwent evoked response recording to auditory paired-stimuli (S1-S2). Seventeen subjects were diagnosed with left medial temporal lobe epilepsy (MTLE) and 10 with right MTLE. Only data from the nonlesion side were included. Twenty-three records from rhinal and anterior hippocampal regions, and 21 from posterior hippocampal regions were included in the analysis. The rhinal region had two prominent components (a negativity peaking around 200 ms followed by a positivity peaking around 400 ms). Both the anterior and posterior hippocampal regions exhibited a dominant negative potential peaking around 400 ms. These components were all composed predominantly of slower frequencies. In contrast, a negativity in the posterior hippocampus at around 100 ms was composed of slow and fast frequencies. All components but the early rhinal negativity were attenuated by stimulus repetition. This is the first report documenting that different regions of the medial temporal area are differentially involved in the processing of auditory input, most likely reflecting separate steps of processing. The data support the need for further exploration of the contribution of these regions to sensory gating. This information helps to increase our understanding of this basic but important and complex function.