Hippocampal unit response during temporal single alternation of classical conditioning with rewarding brain stimulation in the rat

The hearing hippocampus

The hippocampus has a well-established role in spatial and episodic memory but a broader function has been proposed including aspects of perception and relational processing. Neural bases of sound analysis have been described in the pathway to auditory cortex, but wider networks supporting auditory cognition are still being established. We review what is known about the role of the hippocampus in processing auditory information, and how the hippocampus itself is shaped by sound. In examining imaging, recording, and lesion studies in species from rodents to humans, we uncover a hierarchy of hippocampal responses to sound including during passive exposure, active listening, and the learning of associations between sounds and other stimuli. We describe how the hippocampus' connectivity and computational architecture allow it to track and manipulate auditory information - whether in the form of speech, music, or environmental, emotional, or phantom sounds. Functional and structural correlates of auditory experience are also identified. The extent of auditory-hippocampal interactions is consistent with the view that the hippocampus makes broad contributions to perception and cognition, beyond spatial and episodic memory. More deeply understanding these interactions may unlock applications including entraining hippocampal rhythms to support cognition, and intervening in links between hearing loss and dementia.

Effects of rewarding electrical stimulation of lateral hypothalamus on classical conditioning of the nictitating membrane response

1. Adult New Zealand albino rabbits were prepared with chronic hypothalamic stimulating electrodes and hippocampal recording electrodes. 2. Rabbits were restrained and classically conditioned by a tone CS and an airpuff US either followed or preceded by a hypothalamic stimulation (HS). Control rabbits were conditioned without the HS. 3. It was found that HS following the CS facilitated both behavioral and hippocampal responses, while HS preceding the CS inhibited them. 4. Enhanced hippocampal learning-related unit firing to the CS may represent an early indication of conditioning before the behavioral activity produces any observable change.

Frequency facilitation in the hippocampal slices of conditioned rats

Electrophysiological parameters in the hippocampal slices of two experimental (prepared immediately (n = 27) and one week (n = 9) after conditioning) and two control (passive (n = 10) and active (n = 8)) groups of rats have been compared. The experimental rats were trained in a two-way avoidance chamber. The active control rats received the same number of conditioned and unconditioned stimuli without pairing. The threshold and amplitude of population spikes recorded from the CA1 pyramidal cell body layer to the stimulation of stratum radiatum and its modification in a train of 10 pulses (0.2 Hz) were measured. The mean values of the threshold intensity were not significantly different between any pair of these groups. The direction of the changes in the population spike amplitudes following pseudo-conditioning or conditioning was the same. However, the population spike amplitudes decreased more significantly in the slices from the conditioned rats. The increase in the frequency facilitation was specific for the slices of conditioned rats. The modifications in the mechanism of frequency facilitation in the reinforced pathways may represent an important mechanism for behavioural learning.

Sequential dependencies regulate sensory evoked responses of single units in the rat hippocampus

Unit activity from 3 major types of hippocampal cells were recorded from the CA1, CA3 and dentate granule cell layers of the hippocampus in awake freely moving rats. Units were classified as complex spike (ComSp), theta and dentate granule cells (G-cell) based on their spontaneous and stimulus-evoked firing characteristics. Single trial records for each cell type were collected during criterion performance of a two-tone discrimination task. Results showed that: tone-evoked discharges of theta- and G-cells, but not ComSp cells, were influenced by the reward status of the tone presented; the firing tendency of all cell types was significantly influenced by the preceding trial sequence; different types of preceding trial sequences including single and double alternation and runs of positive or negative trials significantly affected the firing tendency of all 3 cell types; the pattern of theta- and G-cell discharges differed with respect to latency to peak discharge and the duration of the discharge following long runs of similar trials. These effects were similar to previously described sequential influence on synaptically identified sensory evoked potentials in the dentate gyrus.