Rhinal sulcus

About: Rhinal sulcus is a research topic. Over the lifetime, 116 publications have been published within this topic receiving 14784 citations.

Preserved recognition memory for small sets, and impaired stimulus identification for large sets, following rhinal cortex ablations in monkeys.

Abstract: Seven cynomolgus monkeys (Macaca fascicularis) performed a series of tasks designed to assess their visual memory and their ability to identify visual stimuli. Preoperatively they were trained and tested in delayed and simultaneous matching-to-sample, both with a large stimulus set and with a small stimulus set; there were ∼500 million possible stimuli in the large set, which effectively means that stimuli were trial-unique with this set, while in the small set there were only four stimuli, which appeared repeatedly in every session of training with the small set. Three of the monkeys then had the cortex within and adjacent to the rhinal sulcus removed bilaterally, while the other four served as an unoperated control group. Postoperatively, the animals with ablation of the rhinal cortex showed severe impairment in delayed matching-to-sample with the large set. With the large set they were also impaired, however, in matching-to-sample with no delay between sample and test (0 s delay) and in simultaneous matching-to-sample, in which the sample and the two choice patterns were simultaneously present for inspection. The impairment in simultaneous matching-to-sample was particularly clear when the task was made more difficult by reducing the physical discriminability of the trial-unique stimuli. With the small set of four stimuli, the animals with rhinal cortex ablation were not significantly impaired in overall performance level in delayed matching-to-sample, though their level was on average below that of the normal control animals. The stimulus set was then further restricted, so that there were now only two stimuli used throughout; in this condition, the animals with rhinal cortex ablation performed delayed matching-to-sample without any suggestion of impairment, showing indistinguishable performance levels from those of the control animals over a range of forgetting intervals. Subsequently, the animals were trained in trial-unique non-matching-to-sample with 0 s delay, which required reversal of the matching-to-sample rule they had previously learned; animals with rhinal cortex ablation showed a clear impairment in this rule-reversal learning. The final experimental task was a concurrent discrimination learning task in which 20 pairs of stimuli were presented once per session; the animals with rhinal cortex ablation learned more slowly than the control animals on average, but the difference between the groups did not attain statistical significance. Overall, this pattern of deficits and of preserved abilities is clearly inconsistent with the idea that rhinal cortex ablation produces an impairment in all forms of visual recognition memory, and only in visual recognition memory. Instead, the present results indicate a general impairment in the capacity for knowledge about visual stimuli.

Preservation of topography in the connections between the subiculum, field CA1, and the entorhinal cortex in rats

Abstract: In order to examine whether the entorhinal-hippocampal-entorhinal circuit is reciprocal and topographic, the connections between the subiculum, the CA1 field, and the entorhinal cortex were studied with the carbocyanine dye (Dil), which moves in both retrograde and anterograde directions. We investigated the organization of reciprocal connections revealed by injections of Dil in the entorhinal cortex along the rhinal sulcus. Anterograde fluorescent labeling showed the same pattern reported in previous studies of the dorsal hippocampus. When the injection site of DiI extended into the deep layers (IV-VI) of the same cortical column, the anterograde labeling of the perforant path was accompanied by retrograde labeling of the subicular neurons and the CA1 neurons. The distribution of labeled cells overlapped the distribution of labeled fibers, and the distribution of labeled cells paralleled that of the labeled fibers in the CA1 field. DiI injection into the medial entorhinal cortex revealed fewer retrogradely labeled subicular neurons than injection into the lateral entorhinal cortex, whereas the number of labeled CA1 neurons was not dependent on the injection site. The number of labeled CA1 neurons was always several times greater than the number of subicular neurons. Thus, the amount of information conveyed by the CA1 projection might be higher than that conveyed by the subicular projection. These results indicate that the entorhinal cortex, CA1, and the subiculum are connected reciprocally and topographically. We believe that the framework of the major hippocampal circuit proposed in previous studies should be reconsidered. We propose that the CA1 projection, rather than the subicular projection, is the main projection that feeds back information from the hippocampus to the entorhinal cortex.