Generalizable knowledge outweighs incidental details in prefrontal ensemble code over time

Prefrontal neuronal ensembles link prior knowledge with novel actions during flexible action selection

We make decisions based on currently perceivable information or an internal model of the environment. The medial prefrontal cortex (mPFC) and its interaction with the hippocampus have been implicated in the latter, model-based decision-making; however, the underlying computational properties remain incompletely understood. We have examined mPFC spiking and hippocampal oscillatory activity while rats flexibly select new actions using a known associative structure of environmental cues and outcomes. During action selection, the mPFC reinstates representations of the associative structure. These awake reactivation events are accompanied by synchronous firings among neurons coding the associative structure and those coding actions. Moreover, their functional coupling is strengthened upon the reactivation events leading to adaptive actions. In contrast, only cue-coding neurons improve functional coupling during hippocampal sharp wave ripples. Thus, the lack of direct experience disconnects the mPFC from the hippocampus to independently form self-organized neuronal ensemble dynamics linking prior knowledge with novel actions.

A long-term ketogenic diet in young and aged rats has dissociable effects on prelimbic cortex and CA3 ensemble activity

Introduction

Age-related cognitive decline has been linked to distinct patterns of cellular dysfunction in the prelimbic cortex (PL) and the CA3 subregion of the hippocampus. Because higher cognitive functions require both structures, selectively targeting a neurobiological change in one region, at the expense of the other, is not likely to restore normal behavior in older animals. One change with age that both the PL and CA3 share, however, is a reduced ability to utilize glucose, which can produce aberrant neural activity patterns.

Methods

The current study used a ketogenic diet (KD) intervention, which reduces the brain's reliance on glucose, and has been shown to improve cognition, as a metabolic treatment for restoring neural ensemble dynamics in aged rats. Expression of the immediate-early genes Arc and Homer1a were used to quantify the neural ensembles that were active in the home cage prior to behavior, during a working memory/biconditional association task, and a continuous spatial alternation task.

Results

Aged rats on the control diet had increased activity in CA3 and less ensemble overlap in PL between different task conditions than did the young animals. In the PL, the KD was associated with increased activation of neurons in the superficial cortical layers, establishing a clear link between dietary macronutrient content and frontal cortical activity. The KD did not lead to any significant changes in CA3 activity.

Discussion

These observations suggest that the availability of ketone bodies may permit the engagement of compensatory mechanisms in the frontal cortices that produce better cognitive outcomes.

Single voxel autocorrelation uncovers gradients of temporal dynamics in the hippocampus and entorhinal cortex during rest and navigation

During navigation, information at multiple scales needs to be integrated. Single-unit recordings in rodents suggest that gradients of temporal dynamics in the hippocampus and entorhinal cortex support this integration. In humans, gradients of representation are observed, such that granularity of information represented increases along the long axis of the hippocampus. The neural underpinnings of this gradient in humans, however, are still unknown. Current research is limited by coarse fMRI analysis techniques that obscure the activity of individual voxels, preventing investigation of how moment-to-moment changes in brain signal are organized and how they are related to behavior. Here, we measured the signal stability of single voxels over time to uncover previously unappreciated gradients of temporal dynamics in the hippocampus and entorhinal cortex. Using our novel, single voxel autocorrelation technique, we show a medial-lateral hippocampal gradient, as well as a continuous autocorrelation gradient along the anterolateral-posteromedial entorhinal extent. Importantly, we show that autocorrelation in the anterior-medial hippocampus was modulated by navigational difficulty, providing the first evidence that changes in signal stability in single voxels are relevant for behavior. This work opens the door for future research on how temporal gradients within these structures support the integration of information for goal-directed behavior.

A long-term ketogenic diet in young and aged rats has dissociable effects on prelimbic cortex and CA3 ensemble activity

Age-related cognitive decline has been linked to distinct patterns of cellular dysfunction in the prelimbic cortex (PL) and the CA3 subregion of the hippocampus. Because higher cognitive functions require both structures, selectively targeting a neurobiological change in one region, at the expense of the other, is not likely to restore normal behavior in older animals. One change with age that both the PL and CA3 share, however, is a reduced ability to utilize glucose, which can produce aberrant neural activity patterns. The current study used a ketogenic diet (KD) intervention, which reduces the brain’s reliance on glucose, and has been shown to improve cognition, as a metabolic treatment for restoring neural ensemble dynamics in aged rats. Expression of the immediate-early genes Arc and Homer 1a were used to quantify the neural ensembles that were active in the home cage prior to behavior, during a working memory/biconditional association task, and a continuous spatial alternation task. Aged rats on the control diet had increased activity in CA3 and less ensemble overlap in PL between different task conditions than did the young animals. In the PL, the KD was associated with increased activation of neurons in the superficial cortical layers. The KD did not lead to any significant changes in CA3 activity. These observations suggest that the KD does not restore neuron activation patterns in aged animals, but rather the availability of ketone bodies in the frontal cortices may permit the engagement of compensatory mechanisms that produce better cognitive outcomes.

Significance Statement

This study extends understanding of how a ketogenic diet (KD) intervention may improve cognitive function in older adults. Young and aged rats were given 3 months of a KD or a calorie-match control diet and then expression of the immediate-early genes Arc and Homer 1a were measured to examine neural ensemble dynamics during cognitive testing. The KD diet was associated with increased activation of neurons in the superficial layers of the PL, but there were no changes in CA3. These observations are significant because they suggest that compensatory mechanisms for improving cognition are engaged in the presence of elevated ketone bodies. This metabolic shift away from glycolysis can meet the energetic needs of the frontal cortices when glucose utilization is compromised.

Complementary task representations in hippocampus and prefrontal cortex for generalizing the structure of problems

Humans and other animals effortlessly generalize prior knowledge to solve novel problems, by abstracting common structure and mapping it onto new sensorimotor specifics. To investigate how the brain achieves this, in this study, we trained mice on a series of reversal learning problems that shared the same structure but had different physical implementations. Performance improved across problems, indicating transfer of knowledge. Neurons in medial prefrontal cortex (mPFC) maintained similar representations across problems despite their different sensorimotor correlates, whereas hippocampal (dCA1) representations were more strongly influenced by the specifics of each problem. This was true for both representations of the events that comprised each trial and those that integrated choices and outcomes over multiple trials to guide an animal’s decisions. These data suggest that prefrontal cortex and hippocampus play complementary roles in generalization of knowledge: PFC abstracts the common structure among related problems, and hippocampus maps this structure onto the specifics of the current situation.

Samborska et al. trained mice on a set of problems with the same structure but different physical layouts to study generalization. Neurons in prefrontal cortex generalized across problems, whereas those in hippocampus were more problem specific.

A dentate gyrus- CA3 inhibitory circuit promotes evolution of hippocampal-cortical ensembles during memory consolidation

Memories encoded in the dentate gyrus (DG) - CA3 circuit of the hippocampus are routed from CA1 to anterior cingulate cortex (ACC) for consolidation. Although CA1 parvalbumin inhibitory neurons (PV INs) orchestrate hippocampal-cortical communication, we know less about CA3 PV INs or DG - CA3 principal neuron - IN circuit mechanisms that contribute to evolution of hippocampal-cortical ensembles during memory consolidation. Using viral genetics to selectively mimic and boost an endogenous learning-dependent circuit mechanism, DG cell recruitment of CA3 PV INs and feed-forward inhibition (FFI) in CA3, in combination with longitudinal in vivo calcium imaging, we demonstrate that FFI facilitates formation and maintenance of context-associated neuronal ensembles in CA1. Increasing FFI in DG - CA3 promoted context specificity of neuronal ensembles in ACC over time and enhanced long-term contextual fear memory. In vivo LFP recordings in mice with increased FFI in DG - CA3 identified enhanced CA1 sharp-wave ripple - ACC spindle coupling as a potential network mechanism facilitating memory consolidation. Our findings illuminate how FFI in DG - CA3 dictates evolution of ensemble properties in CA1 and ACC during memory consolidation and suggest a teacher-like function for hippocampal CA1 in stabilization and re-organization of cortical representations.

Lateral entorhinal cortex supports the development of prefrontal network activity that bridges temporally discontiguous stimuli

The lateral entorhinal cortex (LEC) is an essential component of the brain circuitry supporting long-term memory by serving as an interface between the hippocampus and neocortex. Dysfunction of the LEC affects sensory coding in the hippocampus, leading to a view that the LEC provides the hippocampus with highly processed sensory information. It remains unclear, however, how the LEC modulates neural processing in the neocortical regions. To address this point, we pharmacologically inactivated the LEC of male rats during a temporal associative learning task and examined its impact on local network activity in one of the LEC's efferent targets, the prelimbic region of the medial prefrontal cortex (mPFC). Rats were exposed to two neutral stimuli, one of which was paired with an aversive eyelid shock over a short temporal delay. The LEC inhibition reduced the expression of anticipatory blinking responses to the reinforced stimuli without increasing responses to nonreinforced stimuli. In control rats, both the reinforced and nonreinforced stimuli evoked a short-lived, wide-band increase in the prelimbic network activity. With learning, the initial increase of gamma-band activity started to extend into the interval between the reinforced neutral stimulus and the eyelid shock. LEC inhibition attenuated the learning-induced sustained activity, without affecting the initial transient activity. These results suggest that the integrity of LEC is necessary for the formation of temporal stimulus associations and its neural correlates in the mPFC. Given the minimal effects on the innate network responses to sensory stimuli, the LEC appears not to be the main source of sensory inputs to the mPFC; rather it may provide a framework that shapes the mPFC network response to behaviorally relevant cues.

Entorhinal and ventromedial prefrontal cortices abstract and generalize the structure of reinforcement learning problems

Knowledge of the structure of a problem, such as relationships between stimuli, enables rapid learning and flexible inference. Humans and other animals can abstract this structural knowledge and generalize it to solve new problems. For example, in spatial reasoning, shortest-path inferences are immediate in new environments. Spatial structural transfer is mediated by cells in entorhinal and (in humans) medial prefrontal cortices, which maintain their co-activation structure across different environments and behavioral states. Here, using fMRI, we show that entorhinal and ventromedial prefrontal cortex (vmPFC) representations perform a much broader role in generalizing the structure of problems. We introduce a task-remapping paradigm, where subjects solve multiple reinforcement learning (RL) problems differing in structural or sensory properties. We show that, as with space, entorhinal representations are preserved across different RL problems only if task structure is preserved. In vmPFC and ventral striatum, representations of prediction error also depend on task structure.

Evolving schema representations in orbitofrontal ensembles during learning

How do we learn about what to learn about? Specifically, how do the neural elements in our brain generalize what has been learned in one situation to recognize the common structure of-and speed learning in-other, similar situations? We know this happens because we become better at solving new problems-learning and deploying schemas1-5-through experience. However, we have little insight into this process. Here we show that using prior knowledge to facilitate learning is accompanied by the evolution of a neural schema in the orbitofrontal cortex. Single units were recorded from rats deploying a schema to learn a succession of odour-sequence problems. With learning, orbitofrontal cortex ensembles converged onto a low-dimensional neural code across both problems and subjects; this neural code represented the common structure of the problems and its evolution accelerated across their learning. These results demonstrate the formation and use of a schema in a prefrontal brain region to support a complex cognitive operation. Our results not only reveal a role for the orbitofrontal cortex in learning but also have implications for using ensemble analyses to tap into complex cognitive functions.

The Tolman-Eichenbaum Machine: Unifying Space and Relational Memory through Generalization in the Hippocampal Formation

The hippocampal-entorhinal system is important for spatial and relational memory tasks. We formally link these domains, provide a mechanistic understanding of the hippocampal role in generalization, and offer unifying principles underlying many entorhinal and hippocampal cell types. We propose medial entorhinal cells form a basis describing structural knowledge, and hippocampal cells link this basis with sensory representations. Adopting these principles, we introduce the Tolman-Eichenbaum machine (TEM). After learning, TEM entorhinal cells display diverse properties resembling apparently bespoke spatial responses, such as grid, band, border, and object-vector cells. TEM hippocampal cells include place and landmark cells that remap between environments. Crucially, TEM also aligns with empirically recorded representations in complex non-spatial tasks. TEM also generates predictions that hippocampal remapping is not random as previously believed; rather, structural knowledge is preserved across environments. We confirm this structural transfer over remapping in simultaneously recorded place and grid cells.

Prefrontal Neural Ensembles Develop Selective Code for Stimulus Associations within Minutes of Novel Experiences

Prevailing theories posit that the hippocampus rapidly learns stimulus conjunctions during novel experiences, whereas the neocortex learns slowly through subsequent, off-line interaction with the hippocampus. Parallel evidence, however, shows that the medial prefrontal cortex (mPFC; a critical node of the neocortical network supporting long-term memory storage) undergoes rapid modifications of gene expression, synaptic structure, and physiology at the time of encoding. These observations, along with impaired learning with disrupted mPFC, suggest that mPFC neurons may exhibit rapid neural plasticity during novel experiences; however, direct empirical evidence is lacking. We extracellularly recorded action potentials of cells in the prelimbic region of the mPFC, while male rats received a sequence of stimulus presentations for the first time in life. Moment-to-moment tracking of neural ensemble firing patterns revealed that the prelimbic network activity exhibited an abrupt transition within 1 min after the first encounter of an aversive but not neutral stimulus. This network-level change was driven by ∼15% of neurons that immediately elevated their spontaneous firing rates (FRs) and developed firing responses to a neutral stimulus preceding the aversive stimulus within a few instances of their pairings. When a new sensory stimulus was paired with the same aversive stimulus, about half of these neurons generalized firing responses to the new stimulus association. Thus, prelimbic neurons are capable of rapidly forming ensemble codes for novel stimulus associations within minutes. This circuit property may enable the mPFC to rapidly detect and selectively encode the central content of novel experiences.SIGNIFICANCE STATEMENT During a new experience, a region of the brain, called the hippocampus, rapidly forms its memory and later instructs another region, called the neocortex, that stores its content. Consistent with this dominant view, cells in the neocortex gradually strengthen the selectivity for the memory content over weeks after novel experiences. However, we still do not know precisely when these cells begin to develop the selectivity. We found that neocortical cells were capable of forming the selectivity for ongoing events within a few minutes of new experiences. This finding provides support for an alternative view that the neocortex works with, but not follows, the hippocampus to form new memories.

Prefrontal-hippocampal interaction during the encoding of new memories

The hippocampus rapidly forms associations among ongoing events as they unfold and later instructs the gradual stabilisation of their memory traces in the neocortex. Although this two-stage model of memory consolidation has gained substantial empirical support, parallel evidence from rodent studies suggests that the neocortex, in particular the medial prefrontal cortex, might work in concert with the hippocampus during the encoding of new experiences. This opinion article first summarises findings from behavioural, electrophysiological, and molecular studies in rodents that uncovered immediate changes in synaptic connectivity and neural selectivity in the medial prefrontal cortex during and shortly after novel experiences. Based on these findings, I then propose a model positing that the medial prefrontal cortex and hippocampus might use different strategies to encode information during novel experiences, leading to the parallel formation of complementary memory traces in the two regions. The hippocampus captures moment-to-moment changes in incoming inputs with accurate spatial and temporal contexts, whereas the medial prefrontal cortex may sort the inputs based on their similarity and integrates them over time. These processes of pattern recognition and integration enable the medial prefrontal cortex to, in real time, capture the central content of novel experience and emit relevancy signal that helps to enhance the contrast between the relevant and incidental features of the experience. This hypothesis serves as a framework for future investigations on the potential top-down modulation that the medial prefrontal cortex may exert over the hippocampus to enable the selective, perhaps more intelligent encoding of new information.

Distributed representations of temporal stimulus associations across regular-firing and fast-spiking neurons in rat medial prefrontal cortex

The prefrontal cortex has been implicated in various cognitive processes, including working memory, executive control, decision making, and relational learning. One core computational requirement underlying all these processes is the integration of information across time. When rodents and rabbits associate two temporally discontiguous stimuli, some neurons in the medial prefrontal cortex (mPFC) change firing rates in response to the preceding stimulus and sustain the firing rate during the subsequent temporal interval. These firing patterns are thought to serve as a mechanism to buffer the previously presented stimuli and signal the upcoming stimuli; however, how these critical properties are distributed across different neuron types remains unknown. We investigated the firing selectivity of regular-firing, burst-firing, and fast-spiking neurons in the prelimbic region of the mPFC while rats associated two neutral conditioned stimuli (CS) with one aversive stimulus (US). Analyses of firing patterns of individual neurons and neuron ensembles revealed that regular-firing neurons maintained rich information about CS identity and CS-US contingency during intervals separating the CS and US. Moreover, they further strengthened the latter selectivity with repeated conditioning sessions over a month. The selectivity of burst-firing neurons for both stimulus features was weaker than that of regular-firing neurons, indicating the difference in task engagement between two subpopulations of putative excitatory neurons. In contrast, putative inhibitory, fast-spiking neurons showed a stronger selectivity for CS identity than for CS-US contingency, suggesting their potential role in sensory discrimination. These results reveal a fine-scaled functional organization in the prefrontal network supporting the formation of temporal stimulus associations.NEW & NOTEWORTHY To associate stimuli that occurred separately in time, the brain needs to bridge the temporal gap by maintaining what was presented and predicting what would follow. We show that in rat medial prefrontal cortex, the former function is associated with a subpopulation of putative inhibitory neurons, whereas the latter is supported by a subpopulation of putative excitatory neurons. Our results reveal a distinct contribution of these microcircuit components to neural representations of temporal stimulus associations.

From Conditioning to Emotion: Translating Animal Models of Learning to Human Psychopathology

Emotional responses are not static but change as a consequence of learning. Organisms adapt to emotional events and these adaptations influence the way we think, behave, and feel when we encounter similar situations in the future. Integrating recent work from rodent models and research on human psychopathology, this article lays out a model describing how affective events cause learning and can lead to anxiety and depression: affective events are linked to conditioned stimuli and contexts. Affective experiences entrain oscillatory synchrony across distributed neural circuits, including the prefrontal cortex, hippocampus, amygdala, and nucleus accumbens, which form associations that constitute the basis of emotional memories. Consolidation of these experiences appears to be supported by replay in the hippocampus—a process by which hippocampal firing patterns recreate the firing pattern that occurred previously. Generalization of learning occurs to never before experienced contexts when associations form across distinct but related conditioned stimuli. The process of generalization, which requires cortical structures, can cause memories to become abstracted. During abstraction, the latent, overlapping features of the learned associations remain and result in the formation of schemas. Schemas are adaptive because they facilitate the rapid processing of conditioned stimuli and prime behavioral, cognitive, and affective responses that are the manifestations of the accumulation of an individual’s conditioned experiences. However, schemas can be maladaptive when the generalization of aversive emotional responses are applied to stimuli and contexts in which affective reactions are unnecessary. I describe how this process can lead to not only mood and anxiety disorders but also psychotherapeutic treatment.

Coincident Pre- and Post-Synaptic Cortical Remodelling Disengages Episodic Memory from Its Original Context

The view that the neocortex is remotely recruited for long-term episodic memory recall is challenged by data showing that an intense transcriptional and synaptic activity is detected in this region immediately after training. By measuring markers of synaptic activity at recent and remote time points from contextual fear conditioning (CFC), we could show that pre-synaptic changes are selectively detected 1 day post-training when the memory is anchored to the training context. Differently, pre- and post-synaptic changes are detected 14 days post-training when the memory generalizes to other contexts. Confirming that coincident pre- and post-synaptic remodelling mediates the disengagement of memory from its original context, DREADDs-mediated enhancement of cortical neuron activity during CFC training anticipates expression of a schematic memory and observation of bilateral synaptic remodelling. Together, our data show that the plastic properties of cortical synapses vary over time and specialise in relation to the quality of memory.

Prefrontal Theta Oscillations Promote Selective Encoding of Behaviorally Relevant Events

The ability to capture the most relevant information from everyday experiences without constantly learning unimportant details is vital to survival and mental health. While decreased activity of the medial prefrontal cortex (mPFC) is associated with failed or inflexible encoding of relevant events in the hippocampus, mechanisms used by the mPFC to discern behavioral relevance of events are not clear. To address this question, we chemogenetically activated excitatory neurons in the mPFC of male rats and examined its impact on local network activity and differential associative learning dependent on the hippocampus. Rats were exposed to two neutral stimuli in two environments whose contingency with an aversive stimulus changed systematically across days. Over 2 weeks of differential and reversal learning, theta band activity began to ramp up toward the expected onset of the aversive stimulus, and this ramping activity tracked the subsequent shift of the set (stimulus modality to environment) predictive of the aversive stimulus. With chemogenetic mPFC activation, the ramping activity emerged within a few sessions of differential learning, which paralleled faster learning and stronger correlations between the ramping activity and conditioned responses. Chemogenetic mPFC activity, however, did not affect the adjustment of ramping activity or behavior during reversal learning or set-shifting, suggesting that the faster learning was not because of a general enhancement of attention, sensory, or motor processing. Thus, the dynamics of the mPFC network activation during events provide a relevance-signaling mechanism through which the mPFC exerts executive control over the encoding of those events in the hippocampus.

Irrelevance by inhibition: Learning, computation, and implications for schizophrenia

Symptoms of schizophrenia may arise from a failure of cortical circuits to filter-out irrelevant inputs. Schizophrenia has also been linked to disruptions in cortical inhibitory interneurons, consistent with the possibility that in the normally functioning brain, these cells are in some part responsible for determining which sensory inputs are relevant versus irrelevant. Here, we develop a neural network model that demonstrates how the cortex may learn to ignore irrelevant inputs through plasticity processes affecting inhibition. The model is based on the proposal that the amount of excitatory output from a cortical circuit encodes the expected magnitude of reward or punishment ("relevance"), which can be trained using a temporal difference learning mechanism acting on feedforward inputs to inhibitory interneurons. In the model, irrelevant and blocked stimuli drive lower levels of excitatory activity compared with novel and relevant stimuli, and this difference in activity levels is lost following disruptions to inhibitory units. When excitatory units are connected to a competitive-learning output layer with a threshold, the relevance code can be shown to "gate" both learning and behavioral responses to irrelevant stimuli. Accordingly, the combined network is capable of recapitulating published experimental data linking inhibition in frontal cortex with fear learning and expression. Finally, the model demonstrates how relevance learning can take place in parallel with other types of learning, through plasticity rules involving inhibitory and excitatory components, respectively. Altogether, this work offers a theory of how the cortex learns to selectively inhibit inputs, providing insight into how relevance-assignment problems may emerge in schizophrenia.

Dissociable effects of advanced age on prefrontal cortical and medial temporal lobe ensemble activity

The link between age-related cellular changes within brain regions and larger scale neuronal ensemble dynamics critical for cognition has not been fully elucidated. The present study measured neuron activity within medial prefrontal cortex (PFC), perirhinal cortex (PER), and hippocampal subregion CA1 of young and aged rats by labeling expression of the immediate-early gene Arc. The proportion of cells expressing Arc was quantified at baseline and after a behavior that requires these regions. In addition, PER and CA1 projection neurons to PFC were identified with retrograde labeling. Within CA1, no age-related differences in neuronal activity were observed in the entire neuron population or within CA1 pyramidal cells that project to PFC. Although behavior was comparable across age groups, behaviorally driven Arc expression was higher in the deep layers of both PER and PFC and lower in the superficial layers of these regions. Moreover, age-related changes in activity levels were most evident within PER cells that project to PFC. These data suggest that the PER-PFC circuit is particularly vulnerable in advanced age.

Specific hippocampal representations are linked to generalized cortical representations in memory

Memories link information about specific experiences to more general knowledge that is abstracted from and contextualizes those experiences. Hippocampal-cortical activity patterns representing features of past experience are reinstated during awake memory reactivation events, but whether representations of both specific and general features of experience are simultaneously reinstated remains unknown. We examined hippocampal and prefrontal cortical firing patterns during memory reactivation in rats performing a well-learned foraging task with multiple spatial paths. We found that specific hippocampal place representations are preferentially reactivated with the subset of prefrontal cortical task representations that generalize across different paths. Our results suggest that hippocampal-cortical networks maintain links between stored representations for specific and general features of experience, which could support abstraction and task guidance in mammals.

Neural representations of time-linked memory

Many cognitive processes, such as episodic memory and decision making, rely on the ability to form associations between two events that occur separately in time. The formation of such temporal associations depends on neural representations of three types of information: what has been presented (trace holding), what will follow (temporal expectation), and when the following event will occur (explicit timing). The present review seeks to link these representations with firing patterns of single neurons recorded while rodents and non-human primates associate stimuli, outcomes, and motor responses over time intervals. Across these studies, two distinct firing patterns were observed in the hippocampus, neocortex, and striatum: some neurons change firing rates during or shortly after the stimulus presentation and sustain the firing rate stably or sidlingly during the subsequent intervals (tonic firings). Other neurons transiently change firing rates during a specific moment within the time intervals (phasic firings), and as a group, they form a sequential firing pattern that covers the entire interval. Clever task designs used in some of these studies collectively provide evidence that both tonic and phasic firing responses represent trace holding, temporal expectation, and explicit timing. Subsequently, we applied machine-learning based classification approaches to the two firing patterns within the same dataset collected from rat medial prefrontal cortex during trace eyeblink conditioning. This quantitative analysis revealed that phasic-firing patterns showed greater selectivity for stimulus identity and temporal position than tonic-firing patterns. Our summary illuminates distributed neural representations of temporal association in the forebrain and generates several ideas for future investigations.

Phasic and tonic neuron ensemble codes for stimulus-environment conjunctions in the lateral entorhinal cortex

The lateral entorhinal cortex (LEC) is thought to bind sensory events with the environment where they took place. To compare the relative influence of transient events and temporally stable environmental stimuli on the firing of LEC cells, we recorded neuron spiking patterns in the region during blocks of a trace eyeblink conditioning paradigm performed in two environments and with different conditioning stimuli. Firing rates of some neurons were phasically selective for conditioned stimuli in a way that depended on which room the rat was in; nearly all neurons were tonically selective for environments in a way that depended on which stimuli had been presented in those environments. As rats moved from one environment to another, tonic neuron ensemble activity exhibited prospective information about the conditioned stimulus associated with the environment. Thus, the LEC formed phasic and tonic codes for event-environment associations, thereby accurately differentiating multiple experiences with overlapping features.

DOI:http://dx.doi.org/10.7554/eLife.28611.001

The context in which an event occurs plays a large role in how the brain understands and responds to the event. While a key part of context is where we are, contexts can also change within the same space: different meetings are held at different times and with different people in the same room, and a grassy field can be a place of intense competition or a place to relax and gaze at clouds. However, we have little understanding of how the brain sets up and maintains a sense of context.

A region of the brain called the lateral entorhinal cortex (LEC) responds to events as they happen, but may also maintain a record of past experiences, and helps us to learn new associations between events. To find out how LEC neurons might represent context, Pilkiw et al. measured the activity of individual LEC neurons in rats as they experienced different combinations of events and environments. In each trial, the rats were placed in one of two different rooms and exposed to one of two sensory cues (sound or light) six times, either alone or, to test learning, paired moments later with a mild stimulation to the eyelid. The gaps between the cues lasted from 20 to 40 seconds.

As expected, some LEC neurons responded to the sensory cues, and varied their responses to cues depending on whether or not they were paired with eyelid stimulation. What was much more striking is that almost all cells in the LEC behaved very differently in different contexts, both in response to the cues and also during the long gaps between the cues. This suggests that the LEC provides the brain with information about the circumstances of an event, and may be the reason we expect different events under different circumstances – even if we are in the same place.

We tend to underestimate how much we rely on context to remember events and to guide our behavior. Many disabling health conditions, including addiction, post-traumatic stress disorder and obsessive-compulsive disorder, are affected by context. For example, people who are trying to overcome drug addiction can often reduce their cravings by avoiding places and situations in which they previously used the drug in question. Understanding how the LEC represents context may therefore help us to develop treatments that target this brain region in order to alter harmful behaviors.

DOI:http://dx.doi.org/10.7554/eLife.28611.002