Propagation of neocortical inputs in the perirhinal cortex.

The perirhinal area is a rostrocaudally oriented strip of cortex in which lesions produce memory and perceptual impairments. It receives topographically organized transverse projections from associative neocortical areas and is endowed with intrinsic longitudinal connections that could distribute neocortical inputs in the rostrocaudal axis. In search of distinguishing network properties that might support perirhinal involvement in memory, we have performed whole-cell recordings in horizontal perirhinal slices with preserved transverse neocortical links and intrinsic longitudinal connections. Neocortical stimulation sites in rostrocaudal register with regular spiking perirhinal neurons elicited a sequence of excitatory and inhibitory synaptic potentials. In contrast, apparently pure excitatory responses were observed when the stimulating and recording sites were separated by >/=1 mm in the rostrocaudal axis. This suggested that adjacent and distant neocortical stimuli influence regular spiking perirhinal neurons by pathways that respectively form and do not form synapses with inhibitory interneurons. In keeping with this, presumed interneurons did not respond to distant neocortical stimuli. These results suggest that neocortical inputs recruit perirhinal inhibitory interneurons located at the same transverse level, limiting the depolarization of principal perirhinal cells. In contrast, distant neocortical inputs only evoke excitation because longitudinal perirhinal pathways do not engage inhibitory interneurons. This leads us to suggest that the perirhinal network is biased to favor Hebbian-like associative interactions between coincident and spatially distributed inputs.