Functional dissociation of the prefrontal cortex and the hippocampus in timing behavior

Insular and prelimbic cortices control behavioral accuracy and precision in a temporal decision-making task in rats

The anterior insular cortex (AIC) comprises a region of sensory integration. It appears to detect salient events in order to guide goal-directed behavior, code tracking errors, and estimate the passage of time. Temporal processing in the AIC may be instantiated by the integration of representations of interoception. Projections between the AIC and the medial prefrontal cortex (mPFC) - found both in rats and humans - also suggest a possible role for these structures in the integration of autonomic responses during ongoing behavior. Few studies, however, have investigated the role of AIC and mPFC in decision-making and time estimation tasks. Moreover, their findings are not consistent, so the relationship between temporal decision-making and those areas remains unclear. The present study employed bilateral inactivations to explore the role of AIC and prelimbic cortex (PL) in rats during a temporal decision-making task. In this task, two levers are available simultaneously (but only one is active), one predicting reinforcement after a short, and the other after a long-fixed interval. Optimal performance requires a switch from the short to the long lever after the short-fixed interval elapsed and no reinforcement was delivered. Switch behavior from the short to the long lever was dependent on AIC and PL. During AIC inactivation, switch latencies became more variable, while during PL inactivation switch latencies became both more variable and less accurate. These findings point to a dissociation between AIC and PL in temporal decision-making, suggesting that the AIC is important for temporal precision, and PL is important for both temporal accuracy and precision.

The neural representation of time distributed across multiple brain regions differs between implicit and explicit time demands

Animals appear to possess an internal timer during action, based on the passage of time. However, the neural underpinnings of the perception of time, ranging from seconds to minutes, remain unclear. Herein, we considered the neural representation of time based on mounting evidence on the neural correlates of time perception. The passage of time in the brain is represented by two types of neural encoding as follows: (i) the modulation of firing rates in single neurons and (ii) the sequential activity in neural ensembles. Time-dependent neural activity reflects the relative time rather than the absolute time, similar to a clock. They emerge in multiple regions, including the hippocampus, medial and lateral entorhinal cortices, medial prefrontal cortex, and dorsal striatum. Moreover, they involve different brain regions, depending on an implicit or explicit event duration. Thus, the two types of internal timers distributed across multiple brain regions simultaneously engage in time perception, in response to implicit or explicit time demands.

The dorsal hippocampus' role in context-based timing in rodents

To act proactively, we must predict when future events will occur. Individuals generate temporal predictions using cues that indicate an event will happen after a certain duration elapses. Neural models of timing focus on how the brain represents these cue-duration associations. However, these models often overlook the fact that situational factors frequently modulate temporal expectations. For example, in realistic environments, the intervals associated with different cues will often covary due to a common underlying cause. According to the 'common cause hypothesis,' observers anticipate this covariance such that, when one cue's interval changes, temporal expectations for other cues shift in the same direction. Furthermore, as conditions will often differ across environments, the same cue can mean different things in different contexts. Therefore, updates to temporal expectations should be context-specific. Behavioral work supports these predictions, yet their underlying neural mechanisms are unclear. Here, we asked whether the dorsal hippocampus mediates context-based timing, given its broad role in context-conditioning. Specifically, we trained rats with either hippocampal or sham lesions that two cues predicted reward after either a short or long duration elapsed (e.g., tone-8 s/light-16 s). Then, we moved rats to a new context and extended the long cue's interval (e.g., light-32 s). This caused rats to respond later to the short cue, despite never being trained to do so. Importantly, when returned to the initial training context, sham rats shifted back toward both cues' original intervals. In contrast, lesion rats continued to respond at the long cue's newer interval. Surprisingly, they still showed contextual modulation for the short cue, responding earlier like shams. These data suggest the hippocampus only mediates context-based timing if a cue is explicitly paired and/or rewarded across distinct contexts. Furthermore, as lesions did not impact timing measures at baseline or acquisition for the long cue's new interval, our data suggests that the hippocampus only modulates timing when context is relevant.

Effect of metformin treatment on memory and hippocampal neurogenesis decline correlated with oxidative stress induced by methotrexate in rats

Metformin is currently used as a first-line drug to treat patients with type 2 diabetes. Previous studies have demonstrated that metformin has antioxidant properties and reduces neuroinflammation and hippocampal neuronal cell loss, which eventually improves memory. Methotrexate (MTX) is an antimetabolite chemotherapeutic agent reported to activate cognitive impairment found in many patients. Moreover, MTX negatively affects the spatial working memory, related to neurogenesis reduction in animal models. Therefore, the present study aimed to investigate the antioxidant effect of metformin on the reduction of memory and neurogenesis caused by MTX. Male Sprague-Dawley rats were divided into four groups: control, MTX, metformin, and MTX+metformin. MTX (75 mg/kg, i.v.) was administered on days 7 and 14. Rats were administered metformin (200 mg/kg, i.p.) for 14 days. Memory was determined using novel object location (NOL) and novel object recognition (NOR) tests. Furthermore, cell cycle arrest was quantified by p21 immunostaining. Levels of neuronal protein expression, scavenging enzymes activity, and malondialdehyde (MDA) level changes in the hippocampus and prefrontal cortex were investigated. Rats receiving only MTX showed memory impairment. Decreases in scavenging enzyme activity and BDNF, DCX, and Nrf2 protein expressions levels were detected in the MTX-treated rats. In addition, MTX significantly increased p21-positive cell numbers and MDA levels. However, these adverse MTX effects were counteracted by co-administration with metformin. These results demonstrate that metformin can improve memory impairments, increase BDNF, DCX and Nrf2 protein expressions and antioxidant capacities, and decrease MDA levels in MTX-treated rats.

Developmental exposure to the synthetic progestin, 17α-hydroxyprogesterone caproate, disrupts the mesocortical serotonin pathway and alters impulsive decision-making in rats

The synthetic progestin, 17α-hydroxyprogesterone caproate (17-OHPC), is administered to women at risk for preterm birth during a critical period of fetal development for mesocortical pathways. Yet, little information is available regarding the potential effects of 17-OHPC on the developing fetal brain. In rat models, the mesocortical serotonin pathway is sensitive to progestins. Progesterone receptor (PR) is expressed in layer 3 pyramidal neurons of medial prefrontal cortex (mPFC) and in serotonergic neurons of the dorsal raphe. The present study tested the hypothesis that exposure to 17-OHPC during development disrupts serotonergic innervation of the mPFC in adolescence and impairs behavior mediated by this pathway in adulthood. Administration of 17-OHPC from postnatal days 1-14 decreased the density of SERT-ir fibers within superficial and deep layers and decreased the density of synaptophysin-ir boutons in all layers of prelimbic mPFC at postnatal day 28. In addition, rats exposed to 17-OHPC during development were less likely to make impulsive choices in the Delay Discounting task, choosing the larger, delayed reward more often than controls at moderate delay times. Interestingly, 17-OHPC exposed rats were more likely to fail to make any choice (i.e., increased omissions) compared to controls at longer delays, suggesting disruptions in decision-making. These results suggest that further investigation is warranted in the clinical use of 17-OHPC to better inform a risk/benefit analysis of progestin use in pregnancy.

In the temporal organization of episodic memory, the hippocampus supports the experience of elapsed time

Space and time are both essential features of episodic memory, for which the hippocampus is critical (Howard & Eichenbaum, 2015). Spatial tasks have been used effectively to study the behavioral relevance of place cells. However, the behavioral paradigms utilized for the study of time cells have not used time duration as a variable that animals need to be aware of to solve the task. Therefore, the behavioral relevance of this cell firing is unclear. In order to directly study the role of the hippocampus in processing elapsed time, we created a novel time duration discrimination task. Rats learned to make a decision to turn left or right depending on the preceding tone duration (10 s, left turn; 20 s, right turn). Once the rats reached criterion performance of 90% correct on two out of three consecutive days, they received either an excitotoxic hippocampal lesion or a sham-lesion surgery. After recovery, rats were tested to determine hippocampal involvement in discriminating time duration. Rats with hippocampal lesions performed at chance level on their first testing day postlesion, and they were impaired relative to the sham-lesioned rats. Although the hippocampal-lesioned rats began discriminating at above chance level, their performance never returned to criterion even with 50 days of postoperative testing. Furthermore, while sham rats showed no difference in the number of errors they made on 10- versus 20-s delay trials, hippocampal lesion rats similarly improved their performance under the 10-s delay condition, but not under the 20-s delay condition. Results indicate that hippocampal lesions resulted in a selective impairment in discriminating elapsed time only during the longer delay trials. The implications of these results are discussed in relation to the limits of working-memory capacity and to the role of sustained hippocampal time cell activity in memory performance depending on the perceived relevance of the delay period.

The differential role of the dorsal hippocampus in initiating and terminating timed responses: A lesion study using the switch-timing task

This study investigated the role of the dorsal hippocampus (dHPC) in the temporal entrainment of behavior, while addressing limitations of previous evidence from peak procedure experiments. Rats were first trained on a switch-timing task in which food was obtained from one of two concurrently available levers; one lever was effective after 8 s and the other after 16  s. After performance stabilized, rats underwent either bilateral NMDA lesions of the dHPC or sham lesions. After recovery, switch-timing training resumed. In a subsequent condition, the switch-timing task was modified such that food was available after either 8 or 32 s. Although dHPC lesions had subtle and complex effects on when rats stopped seeking for food at the 8-s lever (departures), it more systematically reduced the time when rats started seeking for food at the 16-s and 32-s lever (switches). No systematic effect of dHPC lesions were observed on the coefficient of quartile variation (normalized dispersion) of latencies to switch. Within the context of the pacemaker-accumulator framework of interval timing, these findings suggest that partially or wholly independent mechanisms control the initiation and termination of timed responses, and that the dHPC is primarily involved in encoding the time to start responding.

The hippocampus contributes to temporal duration memory in the context of event sequences: A cross-species perspective

Although a large body of research has implicated the hippocampus in the processing of memory for temporal duration, there is an exigent degree of inconsistency across studies that obfuscates the precise contributions of this structure. To shed light on this issue, the present review article surveys both historical and recent cross-species evidence emanating from a wide variety of experimental paradigms, identifying areas of convergence and divergence. We suggest that while factors such as time-scale (e.g. the length of durations involved) and the nature of memory processing (e.g. prospective vs. retrospective memory) are very helpful in the interpretation of existing data, an additional important consideration is the context in which the duration information is experienced and processed, with the hippocampus being preferentially involved in memory for durations that are embedded within a sequence of events. We consider the mechanisms that may underpin temporal duration memory and how the same mechanisms may contribute to memory for other aspects of event sequences such as temporal order.

Inactivation of the Medial-Prefrontal Cortex Impairs Interval Timing Precision, but Not Timing Accuracy or Scalar Timing in a Peak-Interval Procedure in Rats

Motor sequence learning, planning and execution of goal-directed behaviors, and decision making rely on accurate time estimation and production of durations in the seconds-to-minutes range. The pathways involved in planning and execution of goal-directed behaviors include cortico-striato-thalamo-cortical circuitry modulated by dopaminergic inputs. A critical feature of interval timing is its scalar property, by which the precision of timing is proportional to the timed duration. We examined the role of medial prefrontal cortex (mPFC) in timing by evaluating the effect of its reversible inactivation on timing accuracy, timing precision and scalar timing. Rats were trained to time two durations in a peak-interval (PI) procedure. Reversible mPFC inactivation using GABA agonist muscimol resulted in decreased timing precision, with no effect on timing accuracy and scalar timing. These results are partly at odds with studies suggesting that ramping prefrontal activity is crucial to timing but closely match simulations with the Striatal Beat Frequency (SBF) model proposing that timing is coded by the coincidental activation of striatal neurons by cortical inputs. Computer simulations indicate that in SBF, gradual inactivation of cortical inputs results in a gradual decrease in timing precision with preservation of timing accuracy and scalar timing. Further studies are needed to differentiate between timing models based on coincidence detection and timing models based on ramping mPFC activity, and clarify whether mPFC is specifically involved in timing, or more generally involved in attention, working memory, or response selection/inhibition.

Differential Encoding of Time by Prefrontal and Striatal Network Dynamics

Telling time is fundamental to many forms of learning and behavior, including the anticipation of rewarding events. Although the neural mechanisms underlying timing remain unknown, computational models have proposed that the brain represents time in the dynamics of neural networks. Consistent with this hypothesis, changing patterns of neural activity dynamically in a number of brain areas-including the striatum and cortex-has been shown to encode elapsed time. To date, however, no studies have explicitly quantified and contrasted how well different areas encode time by recording large numbers of units simultaneously from more than one area. Here, we performed large-scale extracellular recordings in the striatum and orbitofrontal cortex of mice that learned the temporal relationship between a stimulus and a reward and reported their response with anticipatory licking. We used a machine-learning algorithm to quantify how well populations of neurons encoded elapsed time from stimulus onset. Both the striatal and cortical networks encoded time, but the striatal network outperformed the orbitofrontal cortex, a finding replicated both in simultaneously and nonsimultaneously recorded corticostriatal datasets. The striatal network was also more reliable in predicting when the animals would lick up to ∼1 s before the actual lick occurred. Our results are consistent with the hypothesis that temporal information is encoded in a widely distributed manner throughout multiple brain areas, but that the striatum may have a privileged role in timing because it has a more accurate "clock" as it integrates information across multiple cortical areas.

SIGNIFICANCE STATEMENT

The neural representation of time is thought to be distributed across multiple functionally specialized brain structures, including the striatum and cortex. However, until now, the neural code for time has not been compared quantitatively between these areas. Here, we performed large-scale recordings in the striatum and orbitofrontal cortex of mice trained on a stimulus-reward association task involving a delay period and used a machine-learning algorithm to quantify how well populations of simultaneously recorded neurons encoded elapsed time from stimulus onset. We found that, although both areas encoded time, the striatum consistently outperformed the orbitofrontal cortex. These results suggest that the striatum may refine the code for time by integrating information from multiple inputs.

A framework for understanding and advancing intertemporal choice research using rodent models

Intertemporal choices are common and consequential to private and public life. Thus, there is considerable interest in understanding the neural basis of intertemporal decision making. In this minireview, we briefly describe conceptual and psychological perspectives on intertemporal choice and then provide a comprehensive evaluation of the neural structures and signals that comprise the underlying cortico-limbic-striatal circuit. Even though great advances have been made, our understanding of the neurobiology of intertemporal choice is still in its infancy because of the complex and dynamic nature of this form of decision making. We close by briefly discussing recommendations for the future study of intertemporal choice research.

Ventral medial prefrontal cortex inactivation impairs impulse control but does not affect delay-discounting in rats

Maladaptive levels of impulsivity are found in several neuropsychiatric disorders, such as ADHD, addiction, aggression and schizophrenia. Intolerance to delay-of-gratification, or delay-discounting, and deficits in impulse control are dissociable forms of impulsivity top-down controlled by the prefrontal cortex, with the ventral medial prefrontal cortex (vmPFC) suggested to be critically involved. The present study used transient inactivation of the rats' vmPFC via bilateral microinfusion of the GABAA receptor agonist muscimol (0.05, 0.5 μg/0.3 μl) to analyse its relevance for impulse control in a 5-choice serial reaction time task (5-CSRTT) and delay-discounting in a Skinner box. Intra-vmPFC injection of low-dose muscimol impaired impulse control indicated by enhanced premature responding in the 5-CSRTT, while flattening the delay-dependent shift in the preference of the large reward in the delay-discounting task. Likewise, high-dose muscimol did not affect delay-discounting, though raising the rate of omissions. On the contrary, 5-CSRTT performance was characterised by deficits in impulse and attentional control. These data support the behavioural distinction of delay-discounting and impulse control on the level of the vmPFC in rats. Reversible inactivation with muscimol revealed an obvious implication of the vmPFC in the modulation of impulse control in the 5-CSRTT. By contrast, delay-discounting processes seem to be regulated by other neuronal pathways, with the vmPFC playing, if at all, a minor role.

Representation of interval timing by temporally scalable firing patterns in rat prefrontal cortex

Perception of time interval on the order of seconds is an essential component of cognition, but the underlying neural mechanism remains largely unknown. In rats trained to estimate time intervals, we found that many neurons in the medial prefrontal cortex (PFC) exhibited sustained spiking activity with diverse temporal profiles of firing-rate modulation during the time-estimation period. Interestingly, in tasks involving different intervals, each neuron exhibited firing-rate modulation with the same profile that was temporally scaled by a factor linearly proportional to the instructed intervals. The behavioral variability across trials within each task also correlated with the intertrial variability of the temporal scaling factor. Local cooling of the medial PFC, which affects neural circuit dynamics, significantly delayed behavioral responses. Thus, PFC neuronal activity contributes to time perception, and temporally scalable firing-rate modulation may reflect a general mechanism for neural representation of interval timing.

Serotonergic hallucinogens as translational models relevant to schizophrenia

One of the oldest models of schizophrenia is based on the effects of serotonergic hallucinogens such as mescaline, psilocybin, and (+)-lysergic acid diethylamide (LSD), which act through the serotonin 5-HT(2A) receptor. These compounds produce a 'model psychosis' in normal individuals that resembles at least some of the positive symptoms of schizophrenia. Based on these similarities, and because evidence has emerged that the serotonergic system plays a role in the pathogenesis of schizophrenia in some patients, animal models relevant to schizophrenia have been developed based on hallucinogen effects. Here we review the behavioural effects of hallucinogens in four of those models, the receptor and neurochemical mechanisms for the effects and their translational relevance. Despite the difficulty of modelling hallucinogen effects in nonverbal species, animal models of schizophrenia based on hallucinogens have yielded important insights into the linkage between 5-HT and schizophrenia and have helped to identify receptor targets and interactions that could be exploited in the development of new therapeutic agents.

Neural correlates of interval timing in rodent prefrontal cortex

Time interval estimation is involved in numerous behavioral processes, but its underlying neural mechanisms remain unclear. In particular, it has been controversial whether time is encoded on a linear or logarithmic scale. Based on our previous finding that inactivation of the medial prefrontal cortex (mPFC) profoundly impairs rat's ability to discriminate time intervals, we investigated how the mPFC processes temporal information by examining activity of mPFC neurons in rats performing a temporal bisection task. Many mPFC neurons conveyed temporal information based on monotonically changing activity profiles over time with negative accelerations, so that their activity profiles were better described by logarithmic than linear functions. Moreover, the precision of time-interval discrimination based on neural activity was lowered in proportion to the elapse of time, but without proportional increase in neural variability, which is well accounted for by logarithmic, but not by linear functions. As a population, mPFC neurons conveyed precise information about the elapse of time with their activity tightly correlated with the animal's choice of target. These results suggest that the mPFC might be part of an internal clock in charge of controlling interval-timing behavior, and that linearly changing neuronal activity on a logarithmic time scale might be one way of representing the elapse of time in the brain.

Fos expression in the orbital prefrontal cortex after exposure to the fixed-interval peak procedure

It has been proposed that cortico-striato-thalamo-cortical circuits that incorporate the prefrontal cortex and dorsal striatum regulate interval timing behaviour. The present experiment examined whether performance on the fixed-interval peak procedure (FIPP), an immediate timing schedule, would induce neuronal activity in cortical and striatal areas, as revealed by enhanced expression of the Fos protein, a marker for neuronal activation. Regional Fos expression was compared between rats trained on the FIPP and rats trained on a variable-interval (VI) schedule matched to the FIPP for overall response rate and reinforcer delivery. Response rate in the peak trials of the FIPP conformed to a temporally differentiated pattern, which was well described by a modified Gaussian function; in agreement with previous findings, the peak time occurred close to the time at which the reinforcer was delivered in the fixed-interval trials, and the Weber fraction was within the range of values reported previously. The density of Fos-positive neurones (counts mm⁻²) in the orbital prefrontal cortex (OPFC) was greater in rats exposed to the FIPP than in rats exposed to the VI schedule, suggesting a greater activation of this area during the performance of the former task. This is consistent with the results of previous studies that have implicated the OPFC in interval timing behaviour. However, there was no significant difference between the levels of Fos expression in the dorsal or ventral striatum of the rats trained under the two schedules.

Dissociable roles of the medial prefrontal cortex and nucleus accumbens core in goal-directed actions for differential reward magnitude

The medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) are 2 structures within a larger corticolimbic network mediating goal-directed actions, especially when the procurement of different goals is sensitive to impulsive tendencies. The present study investigated the role of these structures in goal-directed action for differential reward by training rats to respond for sucrose reward at a nosepoke operandum such that longer duration nosepokes (up to 2 s) resulted in correspondingly larger volumes of reward. After 16 weeks of training, neurotoxic lesions of either the mPFC or the NAc-core were performed, followed by reassessment of sustained response behavior. Lesions of mPFC increased choice impulsivity by shifting responding away from large rewards toward rewards of smaller sizes. The total volume of reward earned remained unchanged, thereby dissociating the lesion effects on response parameters from overall motivation for reward. In contrast, NAc-core lesions decreased the total amount of responding and total volume of reward earned without altering choice impulsivity across differing nosepoke durations and reward sizes. These results suggest that the mPFC mediates the ability to maintain behavioral responding over longer durations for larger magnitude rewards, while the NAc-core mediates the initiation of responding, perhaps by affecting motivational drive, independent of reward magnitude.

The relationship between transient zinc ion fluctuations and redox signaling in the pathways of secondary cellular injury: relevance to traumatic brain injury

A major obstacle that hampers the design of drug therapy for traumatic brain injury is the incomplete understanding of the biochemical pathways that lead to secondary cellular injury and contribute to cell death. One such pathway involves reactive species that generate potentially cytotoxic zinc ion fluctuations as a major executor of neuronal, and possibly glial, cell death. Whether zinc ions released during traumatic brain injury are toxic or protective is controversial but can be approached by investigating the exact concentrations of free zinc ions, the thresholds of compromised zinc buffering capacity, and the mechanism of cellular homeostatic control of zinc. Rapidly stretch-injured rat pheochromocytoma (PC12) cells express cellular zinc ion fluctuations that depend on the production of nitric oxide. Chelation of cellular zinc ions after rapid stretch injury, however, increases cellular reactive oxygen species. In a rat model of traumatic brain injury, parasagittal fluid percussion, analysis of the metal load of metallothionein was used as an indicator of changes in cellular zinc ion concentrations. The combined results from the cellular and in vivo investigations caution against interpreting zinc ion fluctuations in the early phase (24h) after injury as a primarily cytotoxic event.

Prefrontal cortical contribution to risk-based decision making

Damage to various regions of the prefrontal cortex (PFC) impairs decision making involving evaluations about risks and rewards. However, the specific contributions that different PFC subregions make to risk-based decision making are unclear. We investigated the effects of reversible inactivation of 4 subregions of the rat PFC (prelimbic medial PFC, orbitofrontal cortex [OFC], anterior cingulate, and insular cortex) on probabilistic (or risk) discounting. Rats were well trained to choose between either a "Small/Certain" lever that always delivered 1 food pellet, or another, "Large/Risky" lever, which delivered 4 pellets, but the probability of receiving reward decreased across 4 trial blocks (100%, 50%, 25%, and 12.5%). Infusions of gama-aminobutyric acid agonists muscimol/baclofen into the medial PFC increased risky choice. However, similar medial PFC inactivations decreased risky choice when the Large/Risky reward probability increased over a session. OFC inactivation increased response latencies in the latter trial blocks without affecting choice. Anterior cingulate or insular inactivations were without effect. The effects of prelimbic inactivations were not attributable to disruptions in response flexibility or judgments about the relative value of probabilistic rewards. Thus, the prelimbic, but not other PFC regions, plays a critical role in risk discounting, integrating information about changing reward probabilities to update value representations that facilitate efficient decision making.

Cortico-limbic-striatal circuits subserving different forms of cost-benefit decision making

Research on the neural basis that underlies decision making in humans has revealed that these processes are mediated by distributed neural networks that incorporate different regions of the frontal lobes, the amygdala, the ventral striatum, and the dopamine system. In the present article, we review recent studies in rodents investigating the contribution of these systems to different forms of cost-benefit decision making and focus on evaluations related to delays, effort, or risks associated with certain rewards. Anatomically distinct regions of the medial and orbital prefrontal cortex make dissociable contributions to different forms of decision making, although lesions of these regions can induce variable effects, depending on the type of tasks used to assess these functions. The basolateral amygdala and the nucleus accumbens play a more fundamental role in these evaluations, helping an organism overcome different costs to obtain better rewards. Dopamine activity biases behavior toward more costly yet larger rewards, although abnormal increases in dopamine transmission can exert opposing actions on different types of decision making. The fact that similar neural circuits are recruited to solve these types of problems in both humans and animals suggests that animal models of decision making will prove useful in elucidating the mechanisms mediating these processes.

Lesion of the ventral and intermediate hippocampus abolishes anticipatory activity in the medial prefrontal cortex of the rat

The medial prefrontal cortex (mPFC) of the rat receives a prominent input from the ventral two thirds of the hippocampus, a structure important for spatial awareness, working memory and motivation. We recently found [Hok V, Lenck-Santini PP, Roux S, Save E, Muller RU, Poucet B. Goal-related activity in hippocampal place cells. J Neurosci 2007;27:472-82.] that neurones in the dorsal hippocampus exhibit anticipatory firing prior to the release of a food pellet on an operant task. Here we look for similar activity in the mPFC on the same task and test whether this activity is dependent on the hippocampus. Rats were trained to navigate to a goal zone, wait for the release of a food pellet and then forage for the pellet while unit activity was recorded in the prelimbic and infralimbic areas of the mPFC. Two 16 min sessions were conducted per day, one session with the goal delimited by a cue disc, the second without the cue. In controls, a large proportion of mPFC neurones exhibited activity similar to that seen in the hippocampus while the animal was stationary at the goal. Over half exhibited the same activity regardless of goal location. Anticipatory activity was largely abolished in animals with bilateral lesions of the ventral and intermediate hippocampus, both in cued and uncued sessions. Even though lesioned animals continued to perform the task, they tended to leave the goal zone prematurely. We suggest that the anticipatory activity in the mPFC is dependent on similar activity in the hippocampus and that both structures have a role in either impulse control or reward expectation.

The effects of hippocampal system lesions on a novel temporal discrimination task for rats

A novel, appetitive, Pavlovian conditioning task was used to assess interval timing. Experiment 1 showed that normal rats could discriminate between tones of 1.5s and 0.5s duration, or between tones of 12.0 s and 3.0 s duration. Learning was demonstrated by a greater duration of magazine responding in the period before the delivery of a food reward and after cessation of the CS+ compared to the same time period after cessation of the CS-. Learning was, however, asymmetric as it was much quicker when the CS+ was the longer of the two durations (1.5s and 12.0 s, respectively). Experiment 2 assessed the impact of fornix lesions on the acquisition of one version of this task (CS+ 1.5s, CS- 0.5s). No evidence was found of a change in discrimination learning following surgery. Experiment 3 examined whether rats with either fornix or hippocampal lesions affected discriminations between 12.0 s and 3.0 s stimuli. Again, there was no evidence of a lesion-induced deficit. T-maze alternation training confirmed the effectiveness of these lesions. The results not only reveal that neither the fornix nor the hippocampus is necessary for distinguishing temporal intervals within the ranges tested but also showed how under some circumstances these lesions can leave trace conditioning intact.

Neural systems implicated in delayed and probabilistic reinforcement

This review considers the theoretical problems facing agents that must learn and choose on the basis of reward or reinforcement that is uncertain or delayed, in implicit or procedural (stimulus-response) representational systems and in explicit or declarative (action-outcome-value) representational systems. Individual differences in sensitivity to delays and uncertainty may contribute to impulsivity and risk taking. Learning and choice with delayed and uncertain reinforcement are related but in some cases dissociable processes. The contributions to delay and uncertainty discounting of neuromodulators including serotonin, dopamine, and noradrenaline, and of specific neural structures including the nucleus accumbens core, nucleus accumbens shell, orbitofrontal cortex, basolateral amygdala, anterior cingulate cortex, medial prefrontal (prelimbic/infralimbic) cortex, insula, subthalamic nucleus, and hippocampus are examined.

Dorsal hippocampus involvement in trace fear conditioning with long, but not short, trace intervals in mice

Placing a "trace" interval between a warning signal and an aversive shock makes consolidation of the memory for trace conditioning hippocampus dependent. To determine the trace at which memory consolidation requires the hippocampus, mice were trained with 0-s, 1-s, 3-s, or 20-s trace intervals and tested for freezing to context and tone. Posttraining dorsal hippocampus (DH) lesions decreased context conditioning regardless of trace interval. However, DH lesions attenuated only the 20-s trace tone freezing. Like eyeblink conditioning, the DH is necessary for trace fear conditioning only at long trace intervals, but the time scale for the effective interval in fear conditioning is about 40 times longer. Manipulations that alter trace fear conditioning with short trace intervals probably do not reflect altered DH function. Given this difference in time scale along with the use of posttraining DH lesions, hippocampus dependency of trace conditioning is not related to a bridging function or response timing.

Temporal information processing in ADHD: Findings to date and new methods

The ability to perceive and represent time is a fundamental but complex cognitive skill that allows us to perceive and organize sequences of events and actions, and to anticipate or predict when future events will occur. It is a multidimensional construct, and a variety of methods have been used to understand timing performance in ADHD samples, which makes it difficult to integrate findings across studies. While further replication is needed, growing evidence links ADHD to problems in several aspects of temporal information processing, including duration discrimination, duration reproduction, and finger tapping. Neuroimaging studies of ADHD have also implicated cerebellar, basal ganglia, and prefrontal regions of the brain, which are believed to subserve temporal information processing. This line of research implicates more basic cognitive mechanisms than previously linked with ADHD and challenges researchers to develop and utilize innovative, multidisciplinary, scientific methods to dissect the various components of temporal information processing. Recent advances in neuroimaging, such as magnetoencephalography in collaboration with structural magnetic resonance imaging, can discriminate temporal processing at the level of a millisecond. This approach can lay the groundwork to provide a more precise understanding of neural network activity during different aspects and stages of temporal information processing in ADHD.

Hippocampal lesions facilitate instrumental learning with delayed reinforcement but induce impulsive choice in rats

BACKGROUND

Animals must frequently act to influence the world even when the reinforcing outcomes of their actions are delayed. Learning with action-outcome delays is a complex problem, and little is known of the neural mechanisms that bridge such delays. When outcomes are delayed, they may be attributed to (or associated with) the action that caused them, or mistakenly attributed to other stimuli, such as the environmental context. Consequently, animals that are poor at forming context-outcome associations might learn action-outcome associations better with delayed reinforcement than normal animals. The hippocampus contributes to the representation of environmental context, being required for aspects of contextual conditioning. We therefore hypothesized that animals with hippocampal lesions would be better than normal animals at learning to act on the basis of delayed reinforcement. We tested the ability of hippocampal-lesioned rats to learn a free-operant instrumental response using delayed reinforcement, and what is potentially a related ability -- the ability to exhibit self-controlled choice, or to sacrifice an immediate, small reward in order to obtain a delayed but larger reward.

RESULTS

Rats with sham or excitotoxic hippocampal lesions acquired an instrumental response with different delays (0, 10, or 20 s) between the response and reinforcer delivery. These delays retarded learning in normal rats. Hippocampal-lesioned rats responded slightly less than sham-operated controls in the absence of delays, but they became better at learning (relative to shams) as the delays increased; delays impaired learning less in hippocampal-lesioned rats than in shams. In contrast, lesioned rats exhibited impulsive choice, preferring an immediate, small reward to a delayed, larger reward, even though they preferred the large reward when it was not delayed.

CONCLUSION

These results support the view that the hippocampus hinders action-outcome learning with delayed outcomes, perhaps because it promotes the formation of context-outcome associations instead. However, although lesioned rats were better at learning with delayed reinforcement, they were worse at choosing it, suggesting that self-controlled choice and learning with delayed reinforcement tax different psychological processes.

Double Dissociation between Serotonergic and Dopaminergic Modulation of Medial Prefrontal and Orbitofrontal Cortex during a Test of Impulsive Choice

Dysregulation of the prefrontal cortex (PFC) has been implicated in impulse control disorders, including attention deficit hyperactivity disorder. A growing body of evidence suggests that impulsivity is non-unitary in nature, and recent data indicate that the ventral and dorsal regions of the PFC are differentially involved in distinct aspects of impulsive behaviour, findings which may reflect differences in the monoaminergic regulation of these regions. In the current experiment, levels of dopamine, serotonin and their metabolites were measured in the medial PFC (n = 12) and orbitofrontal cortex (OFC) (n = 19) of rats using in vivo microdialysis during the delay-discounting model of impulsive choice, where impulsivity is defined as selection of small immediate over larger delayed rewards. Yoked groups were also dialysed to control for instrumental responding and reward delivery. Significant increases in 5-hydroxytryptamine efflux were observed in the mPFC, but not in the OFC, during task performance but not under yoked control conditions. In the OFC, 3,4-di-hydroxy-phenylocetic acid (DOPAC) levels increased in animals performing the task but not in yoked animals, whereas mPFC DOPAC levels increased in all subjects. These data suggest a double dissociation between serotonergic and dopaminergic modulation of impulsive decision-making within distinct areas of frontal cortex.

The cognitive neuroscience of creativity

This article outlines a framework of creativity based on functional neuroanatomy. Recent advances in the field of cognitive neuroscience have identified distinct brain circuits that are involved in specific higher brain functions. To date, these findings have not been applied to research on creativity. It is proposed that there are four basic types of creative insights, each mediated by a distinctive neural circuit. By definition, creative insights occur in consciousness. Given the view that the working memory buffer of the prefrontal cortex holds the content of consciousness, each of the four distinctive neural loops terminates there. When creativity is the result of deliberate control, as opposed to spontaneous generation, the prefrontal cortex also instigates the creative process. Both processing modes, deliberate and spontaneous, can guide neural computation in structures that contribute emotional content and in structures that provide cognitive analysis, yielding the four basic types of creativity. Supportive evidence from psychological, cognitive, and neuroscientific studies is presented and integrated in this article. The new theoretical framework systematizes the interaction between knowledge and creative thinking, and how the nature of this relationship changes as a function of domain and age. Implications for the arts and sciences are briefly discussed.

Medial prefrontal cortex and precision of temporal discrimination: a lesion, microinjection, and microdialysis study

In this paper, we examined the role of the medial prefrontal cortex in temporal discrimination in three experiments using rats. Experiment 1 attempted to dissociate the roles of the medial precentral (PrCm) area from the prelimbic and infralimbic (PL-IL) area in temporal discrimination using fixed-interval (FI) schedule. The gradient of response rate distribution became more moderate by a lesion of the PrCm, but not by a lesion of the PL-IL. In experiment 2, the efflux of acetylcholine (ACh) in the PrCm area during temporal discrimination tasks was compared to that during non-temporal discrimination tasks. ACh efflux was not different between these two tasks. In experiment 3, microinjection of the anticholinergic drug scopolamine (10 microg) into the PrCm area made the gradient of response rate distribution moderate. These findings suggest that reduced activity of the ACh system within the PrCm area impairs the precision of temporal discrimination, even though enhancement of this system is not indispensable for performing temporal discrimination.

Limbic Corticostriatal Systems and Delayed Reinforcement

Impulsive choice, one aspect of impulsivity, is characterized by an abnormally high preference for small, immediate rewards over larger delayed rewards, and can be a feature of adolescence, but also attention-deficit/hyperactivity disorder (ADHD), addiction, and other neuropsychiatric disorders. Both the serotonin and dopamine neuromodulator systems are implicated in impulsivity; manipulations of these systems affect animal models of impulsive choice, though these effects may depend on the receptor subtype and whether or not the reward is signaled. These systems project to limbic cortical and striatal structures shown to be abnormal in animal models of ADHD. Damage to the nucleus accumbens core (AcbC) causes rats to exhibit impulsive choice. These rats are also hyperactive, but are unimpaired in tests of visuospatial attention; they may therefore represent an animal model of the hyperactive-impulsive subtype of ADHD. Lesions to the anterior cingulate or medial prefrontal cortex, two afferents to the AcbC, do not induce impulsive choice, but lesions of the basolateral amygdala do, while lesions to the orbitofrontal cortex have had opposite effects in different tasks measuring impulsive choice. In theory, impulsive choice may emerge as a result of abnormal processing of the magnitude of rewards, or as a result of a deficit in the effects of delayed reinforcement. Recent evidence suggests that AcbC-lesioned rats perceive reward magnitude normally, but exhibit a selective deficit in learning instrumental responses using delayed reinforcement, suggesting that the AcbC is a reinforcement learning system that mediates the effects of delayed rewards.

Prefrontal executive and cognitive functions in rodents: neural and neurochemical substrates

The prefrontal cortex has been implicated in a variety of cognitive and executive processes, including working memory, decision-making, inhibitory response control, attentional set-shifting and the temporal integration of voluntary behaviour. This article reviews current progress in our understanding of the rodent prefrontal cortex, especially evidence for functional divergence of the anatomically distinct sub-regions of the rat prefrontal cortex. Recent findings suggest clear distinctions between the dorsal (precentral and anterior cingulate) and ventral (prelimbic, infralimbic and medial orbital) sub-divisions of the medial prefrontal cortex, and between the orbitofrontal cortex (ventral orbital, ventrolateral orbital, dorsal and ventral agranular cortices) and the adjacent medial wall of the prefrontal cortex. The dorso-medial prefrontal cortex is implicated in memory for motor responses, including response selection, and the temporal processing of information. Ventral regions of the medial prefrontal cortex are implicated in interrelated 'supervisory' attentional functions, including attention to stimulus features and task contingencies (or action-outcome rules), attentional set-shifting, and behavioural flexibility. The orbitofrontal cortex is implicated in lower-order discriminations, including reversal of stimulus-reward associations (reversal learning), and choice involving delayed reinforcement. It is anticipated that a greater understanding of the prefrontal cortex will come from using tasks that load specific cognitive and executive processes, in parallel with discovering new ways of manipulating the different sub-regions and neuromodulatory systems of the prefrontal cortex.

Effects of Medial Septal Lesions on Action-Outcome Associations in Rats Under Conditions of Delayed Reinforcement

In operant tasks, control rats maintain high response rates under positive contingencies, when the probability of reinforcement is greater following a response (contingent reinforcement) than during the absence of that response. However, as contingencies approach zero, response rates decrease. In this experiment, under immediate contingent reinforcement, rats with medial septal lesions reduced their response rates, just like controls, when contingencies were shifted from positive toward zero. However, the septal rats were less sensitive to this contingency shift, compared with controls, when there was a 5-s delay between lever presses and contingent reinforcements. This lesion effect appeared to be due to a failure of voluntary response memory, which impaired sensitivity to operant contingencies when there was a delay between action and outcome.

Event-related potentials in the frontal cortex, hippocampus, and cerebellum during a temporal discrimination task in rats

The purpose of this study was to evaluate the roles of the frontal cortex, hippocampus, and cerebellum in timing and time perception using event-related potentials (ERPs) in rats. Rats were trained to discriminate between 2-s and 8-s auditory signal durations using a choice procedure. A simple reaction-time task using only the 2-s signal served as the control condition. In both tasks, ERPs were computed at stimulus onset and offset. Auditory ERPs were recorded from the frontal cortex, hippocampus, and cerebellum. The stimulus onset ERPs consisted of P2, N2, and P3-like components. Significantly greater amplitudes of the P3-like components were observed during the temporal-discrimination task when compared to the simple reaction-time task. At the stimulus offset, P2, N2, and P3-like components were elicited as well as the stimulus onset. Only in the frontal cortex was the P3-like component that appeared at the stimulus offset larger for the temporal-discrimination task than for the simple reaction-time task. These results suggest that the frontal cortex, hippocampus, and cerebellum contribute to interval timing in the seconds range.

Excitotoxic lesions of the prelimbic-infralimbic areas of the rodent prefrontal cortex disrupt motor preparatory processes

The medial prefrontal cortex (mPFC) is involved in a variety of cognitive and emotional processes; in rodents its implication in motor planning is less known, however. We therefore investigated how the mPFC contributes to the information processes involved in the execution of a reaction time task in rats. Subjects were trained to rapidly release a lever at the onset of a cue light, which was presented after an unpredictable period of variable duration (500, 750, 1000 and 1250 ms). Excitotoxic lesions of the whole mPFC or two mPFC subregions [e.g. the dorsal anterior cingulate and the prelimbic-infralimbic (PL-IL) areas] were achieved by intracerebral infusions of ibotenic acid (9.4 micro g/ micro L) at different volumes. Extensive mPFC lesions produced increased premature responding and disrupted motor readiness, e.g. the distribution of preparatory patterns during the variable preparatory periods. The deficits lasted for 3 weeks and could be reinstated 2 months after the lesion by varying the duration of the preparatory periods to increase time uncertainty. Furthermore, lesions restricted to the PL-IL cortex areas reproduced all the deficits of mPFC lesions, whereas pregenual anterior cingulate cortex lesions had no effect. The results emphasize a critical role of the rat PL-IL region in motor preparatory processes. Hence, discrete lesions of this area reproduce some deficits such as impairment of time estimation and disinhibitory behaviours observed in humans with frontal hypoactivity.

The role of the avian hippocampus in orientation in space and time

We examined the effects of hippocampus (Hp) and area parahippocampalis (APH) lesions in pigeons on their ability to perform a battery of tasks including autoshaping, time discrimination, spatial memory, and pattern discrimination. Pigeons with Hp-APH lesions were impaired in the acquisition of an autoshaped response, emitting fewer pecks to the stimulus than control animals. Hp-APH pigeons were also impaired in the acquisition of a radial-arm maze analogue task, and those birds that did learn the task seemed to do so by adopting a stereotypic response strategy. In contrast, Hp-APH lesions had no effect on the ability of the birds to discriminate on the basis of either temporal or visual information. Overall, these studies complement our earlier studies and suggest that the avian hippocampus is important for the processing and retention of spatial rather than non-spatial information.

Impulsive choice induced in rats by lesions of the nucleus accumbens core

Impulsive choice is exemplified by choosing a small or poor reward that is available immediately, in preference to a larger but delayed reward. Impulsive choice contributes to drug addiction, attention-deficit/hyperactivity disorder, mania, and personality disorders, but its neuroanatomical basis is unclear. Here, we show that selective lesions of the nucleus accumbens core induce persistent impulsive choice in rats. In contrast, damage to two of its afferents, the anterior cingulate cortex and medial prefrontal cortex, had no effect on this capacity. Thus, dysfunction of the nucleus accumbens core may be a key element in the neuropathology of impulsivity.

Concurrent modulation of anxiety and memory

We have previously shown that the ventromedial prefrontal cortex (vmPFC) is involved in spontaneous working memory and anxiety-related behaviour in CD-1 mice. Specifically, pretrial microinjection of the kappa(1) agonist, U-69,593, in the infralimbic (IL) area of the vmPFC produced a robust anxiolytic behavioural profile in the elevated plus-maze and enhanced spontaneous working memory in the Y-maze. In the present study we sought to determine whether these effects were specific to IL kappa receptors. We hypothesized that microinjection of the kappa antagonist, norBNI, in the IL cortex would influence anxiety and spontaneous memory in an opposite direction to the effects produced by the kappa(1) agonist. In week 1, transfer-latency reference memory and anxiety were tested in the elevated plus-maze in two separate trials with an intertrial interval of 24 h. In week 2, spontaneous working memory was tested in the Y-maze followed immediately by defensive/withdrawal anxiety in the open field for one half of the animals in each group, and the other half was tested in reverse order. Pretreatment with one injection of vehicle, 1, 5 or 10 nmol/0.5 microl norBNI in the IL cortex dose-dependently reduced transfer-latencies and produced an anxiogenic behavioural profile in the first elevated plus-maze trial. Following a 24 h delay, transfer-latency reference memory was not influenced, but a robust anxiogenic behavioural profile was observed in the second no-injection anxiety trial in the elevated plus-maze relative to control animals. In week 2, the same groups of mice were again pretreated with one injection of the same doses of norBNI in the IL cortex and tested in the open field and Y-maze. NorBNI pretreatment was anxiogenic in the defensive/withdrawal anxiety test and disrupted spontaneous working memory regardless of testing order. The present results show the influence of kappa receptor modulation on anxiety induction and spontaneous working memory. These results also support the hypothesis that immediate memory processing may modulate the induction of anxiety-related behaviours.