Novel acoustic stimuli can alter locomotor speed to hippocampal theta relationship

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 chronic ketamine on hippocampal cross-frequency coupling: implications for schizophrenia pathophysiology

Disrupted neuronal oscillations have been identified as a potentially important biomarker for the perceptual and cognitive symptoms of schizophrenia. Emerging evidences suggest that interactions between different frequency bands, cross-frequency coupling (CFC), serve an important role in integrating sensory and cognitive information and may contribute to disease pathophysiology. In this study, we investigated the effects of 14-day consecutive administration of ketamine (30 mg/kg i.p.) vs. saline on alterations in amplitude and changes in the coupling of low-frequency (0-30 Hz) phase and high-frequency (30-115 Hz) amplitude in the CA1 hippocampus of Long Evans rats. Intracranial electrode recordings were conducted pre- and post-injection while the animals performed a foraging task on a four-arm rectangular maze. Permutation analysis of frequency band-specific change in amplitudes revealed between-group differences in theta (6-12 Hz) and slow gamma (25-50 Hz) but not fast gamma (65-100 Hz) bands at both slow and fast speeds. Chronic ketamine challenge resulted in decreased coupling (pre to post) at slow speeds but increased coupling at faster speeds, compared to either no or modest increased coupling in the saline group. These results demonstrate that chronic ketamine administration alters the interaction of low-frequency phase and high-frequency oscillations chronically and that such coupling varies as a function of locomotive speed. These findings provide evidence for the potential relevance of CFC to the pathophysiology of schizophrenia.

Balanced Noise-Evoked Excitation and Inhibition in Awake Mice CA3

The hippocampus is known as a neuronal structure involved in learning, memory and spatial navigation using multi-sensory cues. However, the basic features of its response to acoustic stimuli without any behavioral tasks (conditioning) remains poorly studied. Here, we investigated the CA3 response to auditory stimuli using in vivo loose-patch recordings in awake and anesthetized C57 mice. Different acoustic stimuli in addition to broadband noise such as click, FM sound and pure tone were applied to test the response of CA3 in awake animals. It was found that the wakefulness of the animal is important for the recorded neurons to respond. The CA3 neurons showed a stronger response to broadband noise rather than the other type of stimuli which suggested that auditory information arrived at CA3 via broadband pathways. Finally, we investigated the excitatory and inhibitory inputs to CA3 neurons by using in vivo whole-cell voltage-clamp techniques with the membrane potential holding at −70 and 0 mV, respectively. In awake animals, the excitatory and inhibitory inputs CA3 neurons receive induced by noise are balanced by showing stable intervals and proportional changes of their latencies and peak amplitudes as a function of the stimulation intensities.

Theta variation and spatiotemporal scaling along the septotemporal axis of the hippocampus

Hippocampal theta has been related to locomotor speed, attention, anxiety, sensorimotor integration and memory among other emergent phenomena. One difficulty in understanding the function of theta is that the hippocampus (HPC) modulates voluntary behavior at the same time that it processes sensory input. Both functions are correlated with characteristic changes in theta indices. The current review highlights a series of studies examining theta local field potential (LFP) signals across the septotemporal or longitudinal axis of the HPC. While the theta signal is coherent throughout the entirety of the HPC, the amplitude, but not the frequency, of theta varies significantly across its three-dimensional expanse. We suggest that the theta signal offers a rich vein of information about how distributed neuronal ensembles support emergent function. Further, we speculate that emergent function across the long axis varies with respect to spatiotemporal scale. Thus, septal HPC processes details of the proximal spatiotemporal environment while more temporal aspects process larger spaces and wider time-scales. The degree to which emergent functions are supported by the synchronization of theta across the septotemporal axis is an open question. Our working model is that theta synchrony serves to bind ensembles representing varying resolutions of spatiotemporal information at interdependent septotemporal areas of the HPC. Such synchrony and cooperative interactions along the septotemporal axis likely support memory formation and subsequent consolidation and retrieval.