Clifford G. Kentros

Grid cells and cortical representation

One of the grand challenges in neuroscience is to comprehend neural computation in the association cortices, the parts of the cortex that have shown the largest expansion and differentiation during mammalian evolution and that are thought to contribute profoundly to the emergence of advanced cognition in humans. In this Review, we use grid cells in the medial entorhinal cortex as a gateway to understand network computation at a stage of cortical processing in which firing patterns are shaped not primarily by incoming sensory signals but to a large extent by the intrinsic properties of the local circuit.

Attention Enhances the Retrieval and Stability of Visuospatial and Olfactory Representations in the Dorsal Hippocampus

Attention enhances the encoding and retrieval of olfactory and visuospatial representations by modulating place field stability, firing rate, and neuronal synchronization of pyramidal cells in the hippocampus.

What is remembered? Role of attention on the encoding and retrieval of hippocampal representations

The hippocampus is critically involved in storing explicit memory such as memory for space. A defining feature of explicit memory storage is that it requires attention both for encoding and retrieval. Whereas, a great deal is now known about the mechanisms of storage, the mechanisms whereby attention modulates the encoding and retrieval of space and other hippocampus‐dependent memory representations are not known. In this review we discuss recent studies, including our own, which show on the cellular level that attention is critical for the stabilization of spatial and reward‐associated odour representations. Our findings support the view that in the hippocampus attention selects the reference frame for task‐relevant information. This mechanism is in part mediated by dopamine acting through D1/D5 receptors and involves an increase in neuronal synchronization in the gamma band frequency. We propose that synchronous activity leads to enhancements in synaptic strength that mediate the stabilization of hippocampal representations.

Transgenically Targeted Rabies Virus Demonstrates a Major Monosynaptic Projection from Hippocampal Area CA2 to Medial Entorhinal Layer II Neurons

The enormous potential of modern molecular neuroanatomical tools lies in their ability to determine the precise connectivity of the neuronal cell types comprising the innate circuitry of the brain. We used transgenically targeted viral tracing to identify the monosynaptic inputs to the projection neurons of layer II of medial entorhinal cortex (MEC-LII) in mice. These neurons are not only major inputs to the hippocampus, the structure most clearly implicated in learning and memory, they also are “grid cells.” Here we address the question of what kinds of inputs are specifically targeting these MEC-LII cells. Cell-specific infection of MEC-LII with recombinant rabies virus results in unambiguous labeling of monosynaptic inputs. Furthermore, ratios of labeled neurons in different regions are largely consistent between animals, suggesting that label reflects density of innervation. While the results mostly confirm prior anatomical work, they also reveal a novel major direct input to MEC-LII from hippocampal pyramidal neurons. Interestingly, the vast majority of these direct hippocampal inputs arise not from the major hippocampal subfields of CA1 and CA3, but from area CA2, a region that has historically been thought to merely be a transitional zone between CA3 and CA1. We confirmed this unexpected result using conventional tracing techniques in both rats and mice.

Neural Correlates of Long-Term Object Memory in the Mouse Anterior Cingulate Cortex

Damage to the hippocampal formation results in a profound temporally graded retrograde amnesia, implying that it is necessary for memory acquisition but not its long-term storage. It is therefore thought that memories are transferred from the hippocampus to the cortex for long-term storage in a process called systems consolidation (Dudai and Morris, 2000). Where in the cortex this occurs remains an open question. Recent work (Frankland et al., 2005; Vetere et al., 2011) suggests the anterior cingulate cortex (ACC) as a likely candidate area, but there is little direct electrophysiological evidence to support this claim. Previously, we demonstrated object-associated firing correlates in caudal ACC during tests of recognition memory and described evidence of neuronal responses to where an object had been following a brief delay. However, long-term memory requires evidence of more durable representations. Here we examined the activity of ACC neurons while testing for long-term memory of an absent object. Mice explored two objects in an arena and then were returned 6 h later with one of the objects removed. Mice continued to explore where the object had been, demonstrating memory for that object. Remarkably, some ACC neurons continued to respond where the object had been, while others developed new responses in the absent objects location. The incidence of absent-object responses by ACC neurons was greatly increased with increased familiarization to the objects, and such responses were still evident 1 month later. These data strongly suggest that the ACC contains neural correlates of consolidated object/place association memory.

Neural Correlates of Novel Object and Novel Location Recognition Behavior in the Mouse Anterior Cingulate Cortex

The anterior cingulate cortex (ACC) is a component of the limbic system implicated in a wide variety of functions spanning motor and sensory information processing, memory, attention, novelty detection, and comparisons of expectation versus outcome. It remains unclear how much of this functional diversity stems from differences in methodology or interpretation versus truly reflecting the range of processes in which the ACC is involved. In the present study, ACC neuronal activity was examined in freely behaving mice (C57BL6/J) under conditions allowing investigation of many of the cited functions in conditions free from externally applied rules: tests of novel object and novel location recognition memory. Behavioral activity and neuronal activity were recorded first in the open field, during the initial exposure and subsequent familiarization to two identical objects, and finally during the recognition memory tests. No discernible stable firing correlates of ACC neurons were found in the open field, but the addition of objects led to lasting changes in the firing patterns of many ACC neurons around one or both of the object locations. During the novel location test, some neurons followed the familiar object to its new location, others fired exclusively where the object had been, and yet others fired to both current and former object locations. Many of these same features were observed during tests of object recognition memory. However, the magnitude of the neuronal preference for the novel or the familiar object was markedly greater than that observed during either the tests of location recognition or novel object preferences in animals that did not exhibit the expected behavior. The present study reveals, for the first time, single-neuron correlates of object and location recognition behaviors in the rodent ACC and suggests that neurons of the ACC provide a distributed representation of all of the salient features of a task.

State-Dependent Alterations in Hippocampal Oscillations in Serotonin 1A Receptor-Deficient Mice

Mice lacking the serotonin 1A receptor (5-HT1AR) show increased levels of anxiety-related behavior across multiple tests and background strains. Tissue-specific rescue experiments, lesion studies, and neurophysiological findings all point toward the hippocampus as a potential mediator of the phenotype. Serotonin, acting through 5-HT1ARs, can suppress hippocampal theta-frequency oscillations, suggesting that theta oscillations might be increased in the knock-outs. To test this hypothesis, local field potential recordings were obtained from the hippocampus of awake, behaving knock-outs and wild-type littermates. The magnitude of theta oscillations was increased in the knock-outs, specifically in the anxiety-provoking elevated plus maze and not in a familiar environment or during rapid eye movement sleep. Theta power correlated with the fraction of time spent in the open arms, an anxiety-related behavioral variable. These results suggest a possible role for the hippocampus, and theta oscillations in particular, in the expression of anxiety in 5-HT1AR-deficient mice.

Transgenic Targeting of Recombinant Rabies Virus Reveals Monosynaptic Connectivity of Specific Neurons

Understanding how neural circuits work requires a detailed knowledge of cellular-level connectivity. Our current understanding of neural circuitry is limited by the constraints of existing tools for transsynaptic tracing. Some of the most intractable problems are a lack of cellular specificity of uptake, transport across multiple synaptic steps conflating direct and indirect inputs, and poor labeling of minor inputs. We used a novel combination of transgenic mouse technology and a recently developed tracing system based on rabies virus (Wickersham et al., 2007a,b) to overcome all three constraints. Because the virus requires transgene expression for both initial infection and subsequent retrograde transsynaptic infection, we created several lines of mice that express these genes in defined cell types using the tetracycline-dependent transactivator system (Mansuy and Bujard, 2000). Fluorescent labeling from viral replication is thereby restricted to defined neuronal cell types and their direct monosynaptic inputs. Because viral replication does not depend on transgene expression, it provides robust amplification of signal in presynaptic neurons regardless of input strength. We injected virus into transgenic crosses expressing the viral transgenes in specific cell types of the hippocampus formation to demonstrate cell-specific infection and monosynaptic retrograde transport of virus, which strongly labels even minor inputs. Such neuron-specific transgenic complementation of recombinant rabies virus holds great promise for obtaining cellular-resolution wiring diagrams of the mammalian CNS.

A stable hippocampal representation of a space requires its direct experience

In humans and other mammals, the hippocampus is critical for episodic memory, the autobiographical record of events, including where and when they happen. When one records from hippocampal pyramidal neurons in awake, behaving rodents, their most obvious firing correlate is the animals position within a particular environment, earning them the name “place cells.” When an animal explores a novel environment, its pyramidal neurons form their spatial receptive fields over a matter of minutes and are generally stable thereafter. This experience-dependent stabilization of place fields is therefore an attractive candidate neural correlate of the formation of hippocampal memory. However, precisely how the animals experience of a context translates into stable place fields remains largely unclear. For instance, we still do not know whether observation of a space is sufficient to generate a stable hippocampal representation of that space because the animal must physically visit a spot to demonstrate which cells fire there. We circumvented this problem by comparing the relative stability of place fields of directly experienced space from merely observed space following blockade of NMDA receptors, which preferentially destabilizes newly generated place fields. This allowed us to determine whether place cells stably represent parts of the environment the animal sees, but does not actually occupy. We found that the formation of stable place fields clearly requires direct experience with a space. This suggests that place cells are part of an autobiographical record of events and their spatial context, consistent with providing the “where” information in episodic memory.

Potential Anatomical Basis for Attentional Modulation of Hippocampal Neurons

Lesions of the hippocampus and related structures produce profound anterograde amnesia. The amnesia is specific to what has been called “explicit,”“declarative,” and “episodic” memory. These memories are frequently believed to be central to the human condition, requiring such advanced cognitive functions as attention and even consciousness. However, the hippocampus and associated structures are evolutionarily conserved, which argues that the memories of lower mammals should be qualitatively similar in nature. Just as attention and arousal are critical components of appropriate memory formation in humans, an emerging body of evidence suggests that these processes bear on the firing patterns of hippocampal neurons in rodents. Here the evidence favoring this hypothesis is discussed and then the potential anatomical basis for such modulation is considered.