Propagation of neocortical inputs in the perirhinal cortex

The perirhinal area is a rostrocaudally oriented strip of cortex in which l esions produce memory and perceptual impairments. It receives topographical ly organized transverse projections from associative neocortical areas and is endowed with intrinsic longitudinal connections that could distribute ne ocortical inputs in the rostrocaudal axis. In search of distinguishing netw ork properties that might support perirhinal involvement in memory, we have performed whole-cell recordings in horizontal perirhinal slices with prese rved transverse neocortical links and intrinsic longitudinal connections. N eocortical stimulation sites in rostrocaudal register with regular spiking perirhinal neurons elicited a sequence of excitatory and inhibitory synapti c potentials. in contrast, apparently pure excitatory responses were observ ed when the stimulating and recording sites were separated by greater than or equal to1 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 i nterneurons. 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 contra st, distant neocortical inputs only evoke excitation because longitudinal p erirhinal pathways do not engage inhibitory interneurons. This leads us to suggest that the perirhinal network is biased to favor Hebbian-like associa tive interactions between coincident and spatially distributed inputs.