Retrosplenial cortex damage impairs unimodal sensory preconditioning

Abstract

The retrosplenial cortex (RSC) is positioned at the interface between cortical sensory regions and the structures that compose the medial temporal lobe memory system. It has recently been suggested that 1 functional role of the RSC involves the formation of associations between cues in the environment (stimulus-stimulus [S-S] learning; Bucci & Robinson, 2014). This suggestion is based, in part, on the finding that lesions or temporary inactivation of the RSC impair sensory preconditioning. However, all prior studies examining the role of the RSC in sensory preconditioning have used cues from multiple modalities (both visual and auditory stimuli). The purpose of the present experiment was to determine whether the RSC contributes to unimodal sensory preconditioning. In the present study we found that both electrolytic and neurotoxic lesions of the RSC impaired sensory preconditioning with auditory cues. Together with previous experiments, these findings indicate that the RSC contributes to both multisensory and unimodal sensory integration, which suggests a general role for the RSC in linking sensory cues in the environment. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Figure 1.

Timeline of Sensory Preconditioning training. During the Preconditioning phase, two neutral auditory cues (A1, A2) were presented serially and a third auditory cue (A3) was presented alone. The preconditioned (A1) and unpaired cue (A3) were either white noise (WN) or a tone (counterbalanced). During the Conditioning phase, a clicker stimulus (A2) was presented after A1 and paired with food reward. During the Test phase, A1 and A3 were presented in intermixed trials. Inset in third panel illustrated expected Test results in intact rats that successfully learned S-S association between preconditioned cue and click; that is, the preconditioned cue should elicit more post-CS responding than the unpaired cue.

Figure 2.

Photomicrographs (A) of representative RSC neurotoxic and electrolytic lesions. Schematic drawings (B) of the extent of damage in RSC-lesioned rats at six levels along the rostro-caudal extent of the RSC (indicated in mm with respect to bregma). At each level, lesion drawings were stacked onto a single image. The degree of shading (gray through black) indicates the number of lesion cases that include that area (black indicates that all cases exhibited damage). M2 = secondary motor cortex; RSCd = restrosplenial dysgranular; RSCg = retrosplenial granular; V2 = secondary visual cortex.

Figure 3.

Average food cup behavior during the CS epoch and Post-CS epoch during the Conditioning phase. Data are mean ± S.E.M.

Figure 4.

Food cup behavior during the Post-CS epoch of the preconditioned cue and unpaired cue for all six test trials (left panel) and averaged across all test trials for each cue (right panel) for (A) Sham rats, (B) RSC-Electrolytic rats, and (C) RSC-NMDA rats. *p < 0.05.

Figure 5.

Discrimination ratios during the Post-CS epoch during the Test session. Control rats but not RSC-lesioned rats exhibited discrimination ratios significantly greater than 0.5. Dotted line indicates equal amounts of conditioned responding to both auditory cues (i.e., no sensory preconditioning). Data are means ± SEM, *p < 0.05.