Models Of Hippocampal Function Research Articles

How the evolution of air breathing shaped hippocampal function.

To make maps from airborne odours requires dynamic respiratory patterns. I propose that this constraint explains the modulation of memory by nasal respiration in mammals, including murine rodents (e.g. laboratory mouse, laboratory rat) and humans. My prior theories of limbic system evolution offer a framework to understand why this occurs. The answer begins with the evolution of nasal respiration in Devonian lobe-finned fishes. This evolutionary innovation led to adaptive radiations in chemosensory systems, including the emergence of the vomeronasal system and a specialization of the main olfactory system for spatial orientation. As mammals continued to radiate into environments hostile to spatial olfaction (air, water), there was a loss of hippocampal structure and function in lineages that evolved sensory modalities adapted to these new environments. Hence the independent evolution of echolocation in bats and toothed whales was accompanied by a loss of hippocampal structure (whales) and an absence of hippocampal theta oscillations during navigation (bats). In conclusion, models of hippocampal function that are divorced from considerations of ecology and evolution fall short of explaining hippocampal diversity across mammals and even hippocampal function in humans.This article is part of the theme issue ‘Systems neuroscience through the lens of evolutionary theory’.

233. Anterior Hippocampal Hyperactivity Limits fMRI Activation During Scene Processing in Early Psychosis

Extant data provide strong evidence for hippocampal hyperactivity in schizophrenia. We have found that volume deficits in early psychosis patients are more prominent in the anterior hippocampus. The origin of functional abnormalities along the hippocampal long axis is unclear. Models of hippocampal function have highlighted a role for the anterior hippocampus in scene processing and construction. Here we test the hypothesis that anterior hippocampal hyperactivity impairs the recruitment of this region during a cognitive task.

T184. Anterior Hippocampal Hyperactivity Limits fMRI Activation During Scene Processing in Early Psychosis

Extant data provide strong evidence for hippocampal hyperactivity in schizophrenia. We have found that volume deficits in early psychosis patients are more prominent in the anterior hippocampus. The origin of functional abnormalities along the hippocampal long axis is unclear. Models of hippocampal function have highlighted a role for the anterior hippocampus in scene processing and construction. Here we test the hypothesis that anterior hippocampal hyperactivity impairs the recruitment of this region during a cognitive task.

Ventromedial prefrontal cortex drives hippocampal theta oscillations induced by mismatch computations

Detecting environmental change is fundamental for adaptive behavior in an uncertain world. Previous work indicates the hippocampus supports the generation of novelty signals via implementation of a match–mismatch detector that signals when an incoming sensory input violates expectations based on past experience. While existing work has emphasized the particular contribution of the hippocampus, here we ask which other brain structures also contribute to match–mismatch detection. Furthermore, we leverage the fine-grained temporal resolution of magnetoencephalography (MEG) to investigate whether mismatch computations are spectrally confined to the theta range, based on the prominence of this range of oscillations in models of hippocampal function. By recording MEG activity while human subjects perform a task that incorporates conditions of match–mismatch novelty we show that mismatch signals are confined to the theta band and are expressed in both the hippocampus and ventromedial prefrontal cortex (vmPFC). Effective connectivity analyses (dynamic causal modeling) show that the hippocampus and vmPFC work as a functional circuit during mismatch detection. Surprisingly, our results suggest that the vmPFC drives the hippocampus during the generation and processing of mismatch signals. Our findings provide new evidence that the hippocampal–vmPFC circuit is engaged during novelty processing, which has implications for emerging theories regarding the role of vmPFC in memory.

Hippocampal cellular and network activity in freely moving echolocating bats

The hippocampus is crucial for episodic and spatial memory. In freely moving rodents, hippocampal pyramidal neurons show spatially selective firing when the animal passes through a neuron's 'place-field', and theta-band oscillation is continuously present during locomotion. Here we report the first hippocampal recordings from echolocating bats, mammals phylogenetically distant from rodents, which showed place cells very similar to those of rodents. High-frequency 'ripple' oscillations were also rodent-like. Theta oscillation, however, differed from rodents in two important ways: (i) theta occurred when bats explored the environment without locomoting, using distal sensing through echolocation, and (ii) theta was not continuous, but occurred in short intermittent bouts. The intermittence of theta suggests that models of hippocampal function that rely on continuous theta may not apply to bats. Our data support the hypothesis that theta oscillation in the mammalian hippocampus is involved in sequence learning and hence, theta power should increase with sensory-input rate-which explains why theta power correlates with running speed in rodents and with echolocation call rate in bats.

The Role of Hippocampal Regions CA3 and CA1 in Matching Entorhinal Input With Retrieval of Associations Between Objects and Context: Theoretical Comment on Lee et al. (2005).

Models of hippocampal function have proposed different functions for hippocampal regions CA3 and CA1, commonly proposing that CA1 performs a match-mismatch comparison of memory retrieval with sensory input. The study by I. Lee, M. R. Hunsaker, and R. P. Kesner (2005) tested these models using selective lesions of hippocampal subregions (see record 2005-01705-014). Their data suggest that CA3 and the dentate gyrus play an important role in the process of detecting the mismatch when a familiar object is placed in a new spatial location. Lesions of the dentate gyrus and CA3 strongly reduce the enhanced exploration associated with displaced objects, beyond the reduction caused by CA1 lesions. This supports the importance of convergent input to CA3 as well as CA1. Along with recent electrophysiological data, this provides a framework for more specifically modeling the role of CA3 and CA1 in matching sensory input with context-dependent retrieval for memory-guided behavior in different tasks.

Muscimol Inactivation of the Dorsal Hippocampus Impairs Contextual Retrieval of Fear Memory

Some models of hippocampal function have suggested a role of the hippocampus in contextual memory retrieval. We have examined this hypothesis by assessing the impact of reversible inactivation of the dorsal hippocampus (DH) on the context-specific expression of latent inhibition, a decrement in conditional responding produced by preexposure to a to-be-conditional stimulus. In Experiment 1, rats received tone preexposure either in the context that would later be used for extinction testing (context A) or in a different context (context C); a third group of rats did not receive tone preexposure. All rats then received fear conditioning, which consisted of tone-footshock pairings, in a third distinct context (context B). The following day conditional fear to the tone was assessed in one of the preexposure contexts (context A) by measuring freezing during a tone extinction test. Rats preexposed and tested in the same context exhibited less freezing to the tone than either rats preexposed and tested in different contexts or non-preexposed rats. These results indicate that the expression of latent inhibition is context specific. In Experiment 2, DH inactivation eliminated the context-specific expression of latent inhibition. Compared with saline-infused rats, rats infused with muscimol into the DH exhibited low levels of tone freezing independent of whether they had received tone preexposure in the test context or in a different context. Experiment 3 revealed normal contextual discrimination in rats after DH inactivation. These results suggest the DH is required for contextual memory retrieval in a latent inhibition paradigm.

Neuron numbers in the presubiculum, parasubiculum, and entorhinal area of the rat. J. Comp. Neurol.385:83-94

W.H.A.M. Mulders, M.J. West, and L. Slomianka (1997) Neuron numbers in the presubiculum, parasubiculum, and entorhinal area of the rat. J. Comp. Neurol. 385:83–94. We have recently identified an error in our procedures used to estimate the number of neurons in the layers of the human entorhinal cortex, which resulted in an overestimation of cell numbers by a factor of 1.66 (for details see Erratum to West and Slomianka, 1998). The corrected human entorhinal cell numbers are similar to those reported by Gómez-Isla et al. (1996) and compatible with estimates presented by Krimer et al. (1997). This error does not affect the data presented in our paper concerning the rat entorhinal cortex or those parts of the discussion relating to quantitative relationship within the rat hippocampal region. It does however have a significant impact on our discussion of possible differences in quantitative relationships in humans and rats. We concluded in our paper that the ratios of the numbers of cells in interconnected cell layers were similar in rats and humans. Table 1, which presents these ratios based on the corrected cell numbers in the human entorhinal cortex, illustrates that this interpretation is no longer tenable. The corrected data indicate clear differences in the degree of divergence of projections from entorhinal to hippocampal cells layers and in the degree of convergence of hippocampal projections to entorhinal cells layers. Following one of the lines of argument in our paper, it is likely that the degree of convergence of projections from CA1 to layer V of the entorhinal cortex is least affected by these changes (instead of being the only example of a different degree of convergence or divergence between the two species). Consequently, models of hippocampal function that incorporate neuron numbers determined in the rat can not a priori be expected to be appropriate representations of hippocampal function in humans. Gómez-Isla, T., J.L. Price, D.W. McKeel, Jr., J.C. Morris, J.H. Growdon, and B.T. Hyman (1996) Profound loss of layer II entorhinal cortex neurons occurs in very mild Alzheimer's disease. J. Neurosci. 16:4491–4500. Krimer, L.S., M.M. Herman, R.C. Saunders, J.C. Boyd, T.M. Hyde, J.M. Carter, J.E. Kleinman, and D.R. Weinberger (1997) A qualitative and quantitative analysis of the entorhinal cortex in schizophrenia. Cereb. Cortex 7:732–739. West, M.J. and L. Slomainka (1998) Total number of neurons in the layers of the human entorhinal cortex. Hippocampus 8:69–82 and Corrigendum (1998) Hippocampus, 8:426.

Extending Models of Hippocampal Function in Animal Conditioning to Human Amnesia

Although most analyses of amnesia have focused on the loss of explicit declarative and episodic memories following hippocampal-region damage, considerable insights into amnesia can also be realised by studying hippocampal function in simple procedural, or habit-based, associative learning tasks. Although many simple forms of associative learning are unimpaired by hippocampal damage, more complex tasks which require sensitivity to unreinforced stimuli, configurations of multiple stimuli, or contextual information are impaired by hippocampal damage. In several recent papers we have developed a computational theory of hippocampal function which argues that this brain region plays a critical role in the formation of new stimulus representations during learning (Gluck & Myers, 1993, 1995; Myers & Gluck, 1996; Myers, Gluck, & Granger, 1995). We have applied this theory to a broad range of empirical data from studies of classical conditioning in both intact and hippocampal-lesioned animals, and the model correctly accounts for these data. The classical conditioning paradigm can be adapted for use in humans, and similar results for acquisition are obtained in both normal and hippocampaldamaged humans. More recently, we have begun to address an important set of category learning studies in both normals and hippocampal-damaged amnesics. This work integrates experimental studies of amnesic category learning (Knowlton, Squire, & Gluck, 1994) with theoretical accounts of associative learning, and builds on previously established behavioural correspondences between animal conditioning and human category learning (Gluck & Bower, 1988a). Our work to

Computational models of hippocampal function in memory.

Computational models of hippocampal function in memory.

Computational models of hippocampal function in memory

Computational models of hippocampal function in memory

Brain state and plasticity: an integration of the reciprocal interaction model of sleep cycle oscillation with attentional models of hippocampal function.

The present paper relates the reciprocal interaction model for sleep cycle oscillation (McCarley and Hobson, ref. 29) to an attentional model of hippocampal function (Schmajuk and Moore, ref. 44). We consider mechanisms by which the interaction between gigantocellular tegmental field (FTG) cells and locus coeruleus (LC) activity proposed by the sleep cycle model may differentially modulate the information processing carried out in the hippocampus as described by the attentional model. Our fundamental assumption is that learning about the relevancy of different stimuli is proportional to the level of LC activation. If the environment becomes unpredictable during waking, the FTG and LC are activated and the LC facilitates hippocampal learning about stimulus relevancy. In a predictable situation during waking, FTG cells discharge rarely because no novelty is detected, and LC neurons are moderately active. If the predictable situation lasts, LC cells also decrease their activity, and a sleep period might start. At sleep onset, LC inhibition decreases and FTG activity is low leading to slow sleep. As FTG activity increases and LC activity reaches its low point, REM sleep starts. Because LC activity is low during REM sleep, values of stimulus relevancy remain unchanged. Since during sleep the threshold for external stimuli is high, only internally generated novel stimuli (subjectively perceived as dream mentation) may activate the LC. LC renewed inhibitory influence on the FTG ends REM sleep.