Working memory and prefrontal cortex

Involvement of the rat prefrontal cortex in a delayed reinforcement operant task

To determine whether the rat medial prefrontal cortex (PFC) is involved in delayed reinforcement operant behavior, we studied the effects of transient inactivation of the medial PFC (or the hippocampus as a control) during a delayed reinforcement lever-press task. We demonstrated the involvement of the PFC in this task: PFC inactivation but not hippocampal inactivation significantly impaired performance. In a separate experiment, we also recorded the prefrontal multiple unit activities during the task to determine the roles of the PFC in detail. Neuronal activity decreased during the delay period, suggesting that this decrease plays a role in delayed reinforcement operant behavior.

The prefrontal cortex and flexible behavior

The prefrontal cortex in primates guides behavior by selecting relevant stimuli for the task at hand, mediated through excitatory bidirectional pathways with structures associated with sensory processing, memory, and emotions. The prefrontal cortex also has a key role in suppressing irrelevant stimuli through a mechanism that is not well understood. Recent findings indicate that prefrontal pathways interface with laminar-specific neurochemical classes of inhibitory neurons in sensory cortices, terminate extensively in the frontal and sensory sectors of the inhibitory thalamic reticular nucleus, and target the inhibitory intercalated masses of the amygdala. Circuit-based models suggest that prefrontal pathways can select relevant signals and efficiently suppress distractors, in processes that are disrupted in schizophrenia and in other disorders affecting prefrontal cortices.

Gamma-band power elevation of prefrontal local field potential after posterior dorsal hippocampus-prefrontal long-term potentiation induction in anesthetized rats

Previously, we reported an elevation in the power of the medial prefrontal cortex (PFC) local field potential spontaneous gamma-band (40-100 Hz) after long-term depression (LTD) but not long-term potentiation (LTP) in the rat ventral hippocampus CA1 (vCA1)-PFC pathway. In the present study, we analyzed the PFC local field potential before and after the induction of posterior dorsal hippocampus CA1 (pdCA1)-PFC LTP/LTD in vivo. In contrast with vCA1-PFC, the present study found that gamma-band power elevation was associated with pdCA1-PFC LTP but not LTD, although we observed a correlation between LTD and the gamma-band power change. The differences between vCA1- and pdCA1-PFC pathways might be related to differences in synaptic plasticity and behavioral functions. Since the neural connection of the hippocampus and PFC is believed to be involved in the function of the working memory, sustained gamma-band power elevation in the PFC might be related to this function.

Activity of primate orbitofrontal and dorsolateral prefrontal neurons: task-related activity during an oculomotor delayed-response task

The orbitofrontal cortex (OFC) has strong reciprocal connections to the dorsolateral prefrontal cortex (DLPFC), which is known to participate in spatial working memory processes. However, it is not known whether or not the OFC also participates in spatial working memory and whether the OFC and DLPFC contribute equally to this process. To address these issues, we collected single-neuron activity from both areas while a monkey performed an oculomotor delayed-response task, and compared the characteristics of task-related activities between the OFC and DLPFC. All of the task-related activities observed in the DLPFC were also observed in the OFC. However, the proportion and response characteristics of task-related activities were different between the two areas. While most delay-period activity observed in the DLPFC was directionally selective and showed tonic sustained activation, most delay-period activity observed in the OFC was omni-directional and showed gradually increasing activity. Reward-period activity was predominant among task-related activities in the OFC. The proportion of neurons showing reward-period activity was significantly higher in the OFC than in the DLPFC. These results suggest that, although both the OFC and DLPFC participate in spatial working memory processes, the OFC is related more to the expectation and the detection of reward delivery, while the DLPFC is related more to the temporary maintenance of spatial information and its processing.

Relationship between prefrontal task-related activity and information flow during spatial working memory performance

While monkeys performed spatial working memory tasks, cue- (C), delay- (D), and response-period (R) activities or their combinations (CD, CR, DR, CDR) were observed in prefrontal neurons. In the present study, we tried to understand information flow during spatial working memory performances and how each task-related neuron contributed to this process. We first characterized each neuron based on which task-related activity was exhibited and which information (cue location or saccade direction) each task-related activity represented, then classified these neurons into 9 groups (C, Dcue, Dsac, CDcue, DcueRcue, DsacRsac, DcueRsac, CDcueRcue and CDcueRsac). Preferred directions were similar between cue- and delay-period activities in CDcue, CDcueRcue, and CDcueRsac, indicating that the directional selectivity of delay-period activity is affected by the directional selectivity of cue-period activity, all of which represented visual information. Preferred directions were also similar between delay- and response-period activities in DcueRcue, CDcueRcue, and DsacRsac, indicating that the directional selectivity of delay-period activity affects the directional selectivity of response-period activity in these neurons. By the comparison of temporal profiles of delay-period activity among these groups, we found (1) cue-period activity could affect directional selectivity of delay-period activity of CDcue and CDcueRcue, (2) cue-period activity of C, CDcue, and CDcueRcue might contribute to the initiation and the maintenance of delay-period activity of CDcue, CDcueRcue, Dcue, and DcueRcue, and (3) saccade-related activity of DsacRsac could be affected by delay-period activity of Dsac and DsacRsac. These results suggest that the combination of task-related activities, the information represented by each activity, and the temporal profile of delay-period activity are important factors to consider information flow and processing and integration of the information in the prefrontal cortex during spatial working memory processes.

Prefrontal neural correlates of memory for sequences

The sequence of actions appropriate to solve a problem often needs to be discovered by trial and error and recalled in the future when faced with the same problem. Here, we show that when monkeys had to discover and then remember a sequence of decisions across trials, ensembles of prefrontal cortex neurons reflected the sequence of decisions the animal would make throughout the interval between trials. This signal could reflect either an explicit memory process or a sequence-planning process that begins far in advance of the actual sequence execution. This finding extended to error trials such that, when the neural activity during the intertrial interval specified the wrong sequence, the animal also attempted to execute an incorrect sequence. More specifically, we used a decoding analysis to predict the sequence the monkey was planning to execute at the end of the fore-period, just before sequence execution. When this analysis was applied to error trials, we were able to predict where in the sequence the error would occur, up to three movements into the future. This suggests that prefrontal neural activity can retain information about sequences between trials, and that regardless of whether information is remembered correctly or incorrectly, the prefrontal activity veridically reflects the animal's action plan.

Neural correlates of a default response in a delayed go/no-go task

Working memory, the ability to temporarily retain task-relevant information across a delay, is frequently investigated using delayed matching-to-sample (DMTS) or delayed Go/No-Go tasks (DGNG). In DMTS tasks, sample cues instruct the animal which type of response has to be executed at the end of a delay. Typically, performance decreases with increasing delay duration, indicating that working memory fades across a delay. However, no such performance decrease has been found when the sample cues exist of present vs. absent stimuli, suggesting that pigeons do not rely on working memory, but seem to respond by default in those trials. We trained 3 pigeons in a DGNG task and found a similar default response pattern: The diverging slopes of the retention functions on correct Go and No-Go trials suggested that pigeons by default omitted their response following No-Go stimuli, but actively retained task-relevant information across the delay for successful responses on Go trials. We conducted single-cell recordings in the avian nidopallium caudolaterale, a structure comparable to the mammalian prefrontal cortex. On Go trials, many neurons displayed sustained elevated activity during the delay preceding the response, replicating previous findings and suggesting that task-relevant information was neurally represented and maintained across the delay. However, the same units did not show enhanced delay activity preceding correct response suppressions in No-Go trials. This activation-inactivation pattern presumably constitutes a neural correlate of the default response strategy observed in the DGNG task.

Prefrontal cortex and working memory processes

Working memory is a mechanism for short-term active maintenance of information as well as for processing maintained information. The dorsolateral prefrontal cortex has been known to participate in working memory. The analysis of task-related dorsolateral prefrontal cortex activity while monkeys performed a variety of working memory tasks revealed that delay-period activity is a neural correlate of a mechanism for temporary active maintenance of information, because this activity persisted throughout the delay period, showed selectivity to a particular visual feature, and was related to correct behavioral performances. Information processing can be considered as a change of the information represented by a population of neural activities during the progress of the trial. Using population vectors calculated by a population of task-related dorsolateral prefrontal cortex activities, we demonstrated the temporal change of information represented by a population of dorsolateral prefrontal cortex activities during performances of spatial working memory tasks. Cross-correlation analysis using spike firings of simultaneously isolated pairs of neurons reveals widespread functional interactions among neighboring neurons, especially neurons having delay-period activity, and their dynamic modulation depending on the context of the trial. Functional interactions among neurons and their dynamic modulation could be a mechanism of information processing in the dorsolateral prefrontal cortex.

Perceptual, premotor and motor factors in the performance of a delayed-reaching task by subjects with unilateral spatial neglect

We used a computerized delayed-reaching task with a simple reaction time (RT) to investigate the visuo-motor and spatio-temporal performance of right brain-damaged (RBD) patients with unilateral spatial neglect (USN). Fifty-three RBD patients (22 with and 31 without USN) and 25 controls performed the tasks. We recorded the following data: the first RT (RT-1), which is thought to reflect the detection of the target position (the perceptual factor); the second RT (RT-2), which represents the initiation of reaching (the motor initiation aspect of premotor factors); the movement time (MT), which is hypothesized to reflect the "pure" motor component of the task. RBD patients with both USN and hemianopia demonstrated significantly longer RTs towards the left than towards the right for both the RT-1 and the RT-2. Among the RBD patients without hemianopia, the laterality index (left side/right side) of the RT-1 in those with USN was significantly greater than in those without USN or the controls. Among the three groups, there were no significant differences between the laterality indices of either the RT-2s or the MTs. These results suggest that the impairment of leftward movement in RBD patients with USN might be caused primarily by a perceptual impairment rather than an impairment in motor initiation, and is certainly not a "pure" motor impairment.

Prefrontal activity during serial probe reproduction task: encoding, mnemonic, and retrieval processes

To study the prefrontal neuronal mechanism for the encoding and mnemonic processing of multiple objects, the order of object presentation, and the retrieval of an object among objects in the working memory, we recorded neuronal activity from the lateral prefrontal cortex while two monkeys performed the serial probe reproduction task. In the task, two objects (C1 and C2) were presented sequentially interleaved with a delay (D1) period, and after the second delay (D2) period, a color cue was presented. Monkeys were trained to select one target object on the basis of the color stimulus. During the C1 and C2 periods, we found responses that depended on the order of presentation (order-selective response). During the D1 and/or D2 periods, two-thirds of the neurons with object-selective delay-period activity showed order-selective activity coding either C1 or C2. Neurons with larger response magnitudes during the C2 period showed order-selective delay-period activity during the D2 period. These order-selective responses during the C2 period could also contribute to order-selective delay-period activity, and order-selective delay-period activity during the D1 and D2 periods could play an essential role in storing information on both the object and the temporal order of presentation. During the color cue period, two-thirds of the neurons with responses showed target object selectivity (CT and T responses), although the target object was not presented during this period. The CT and T responses could play a critical role in the retrieval of an item among various items in the working memory.

Functionally relevant measures of spatial complexity in neuronal dendritic arbors

We introduce a set of scaling exponents for characterizing global 3D morphologic properties of mass distribution, branching and taper in neuronal dendritic arbors, capable of distinguishing functionally relevant changes in dendritic complexity that standard Sholl analysis and fractal analysis cannot. We demonstrate that the scaling exponent for mass distribution, d(M), comprises a sum of independent scaling exponents for branching, d(N), and taper, d(T). The accuracy of experimental measurements of the scaling exponents was verified using computer generated self-similar binary trees of known fractal dimension, and with prescribed amounts of branching and taper. The theory was applied to measuring 3D spatial complexity in the apical and basal dendritic trees of two functionally distinct types of macaque monkey neocortical pyramidal neurons: long corticocortical projection neurons from superior temporal cortex to area 46 of the prefrontal cortex (PFC), and local projection neurons within area 46 of the PFC. Two distinct scaling subregions (proximal and medial) were identified in both apical and basal trees of the two neuron types, and scaling exponents were fitted. A small but significant difference in mass scaling in the proximal region distinguished long from local projection neurons. Interestingly, both classes of neuron exhibited a homeostatic pattern of mass distribution across the two regions: despite large differences between proximal and medial regions in branching and tapering exponents, these effects were compensatory, resulting in a uniform, slow reduction of mass with distance from the soma, over both scaling regions of the apical and basal trees. Given a uniformly excitable membrane, the electrotonic properties of dendritic arbors depend entirely upon mass distribution, and its relative contributions from dendritic branching and taper. By capturing each of these complex morphologic properties in a single, globally descriptive parameter, the new 3D scaling exponents introduced in this study permit efficient morphometric characterization of complex dendritic arbors in the fewest possible parameters, that can be directly related to their electrotonic properties, and hence to neuronal function.

Neural mechanisms of spatial working memory: contributions of the dorsolateral prefrontal cortex and the thalamic mediodorsal nucleus

The dorsolateral prefrontal cortex (DLPFC) has been known to play an important role in working memory. Neurophysiological studies have revealed that delay period activity observed in the DLPFC is a neural correlate of the temporary storage mechanism for information and that this activity represents either retrospective or prospective information, although the majority represents retrospective information. However, the DLPFC is not the only brain area related to working memory. The analysis of neural activity in the thalamic mediodorsal (MD) nucleus reveals that the MD also participates in working memory. Although similar task-related activities were observed in the MD, the directional bias of these activities and the proportion of presaccadic activity are different between the MD and the DLPFC. These results indicate that, although the MD participates in working memory, the way it participates in this process is different between these two areas, in that the MD participates more in motor control aspects than the DLPFC does.

Neural representation of response category and motor parameters in monkey prefrontal cortex

Conditional motor behavior, in which the relationship between stimuli and responses changes arbitrarily, is an important component of cognitive motor function in primates. It is still unclear how cognitive processing for conditional motor control determines movement parameters to directly specify motor output. To address this issue, we studied the neuronal representation of motor variables relating to conditional motor control and also directly to the metrics of motor output in prefrontal cortex (PFC). Monkeys were required to generate a force that fell within one of two categories ("small" and "large"). We found that most PFC neurons were activated as a function of force category, suggesting a role in conditional motor control. At the same time, we found that activity in many PFC neurons varied continuously with the force that was eventually produced, suggesting they participated in specifying the metrics of movements as they were executed. The results suggest that the PFC neural population encodes both "what" motor response should be performed and "how" the selected movement should be realized immediately after the visual instruction.

Pharmacological antagonism of metabotropic glutamate receptor 1 regulates long-term potentiation and spatial reference memory in the dentate gyrus of freely moving rats viaN-methyl-d-aspartate and metabotropic glutamate receptor-dependent mechanisms

Group I metabotropic glutamate receptors (mGluRs) are critically required for multiple forms of hippocampal synaptic plasticity in vivo. The role of the receptor subtype mGluR1 in long-term potentiation (LTP) and learning is unclear. We examined the contribution of mGluR1 to hippocampal LTP and spatial learning using the selective antagonist (S)-(+)-alpha-amino-4carboxy-2-methylbenzene-acetic acid (LY367385). Male Wistar rats were chronically implanted with recording and stimulating electrodes to enable measurement of evoked potentials from medial perforant path-dentate gyrus granule cell synapses. An injection cannula was inserted into the ipsilateral cerebral ventricle to enable drug application. Experiments were begun 10 days after the implantation procedure. We induced a robust LTP which lasted over 25 h with a 200-Hz tetanization. Injections of LY367385 at all concentrations under investigation (4-32 nmol in a 5-microL injection volume) did not affect basal synaptic transmission. In contrast, we observed a dose-dependent impairment of LTP expression: LY367385 (4 nmol) had no effect on LTP induction, whereas 8 and 16 nmol LY367385 reduced both LTP induction and expression, suggestive of an interaction with N-methyl-d-aspartate receptors. We assessed the effects of daily LY367385 application (8 nmol) on performance in an eight-arm radial maze. LY367385-treated rats showed deficits in reference but not working memory performance compared with vehicle-treated controls. Rearing, grooming and locomotor activity were unaffected by LY367385. These data suggest an important role for mGluR1 in LTP and learning and highlight the specific significance of this mGluR subtype for reference memory.

Context-dependent representation of response-outcome in monkey prefrontal neurons

For behaviour to be purposeful, it is important to monitor the preceding behavioural context, particularly for factors regarding stimulus, response and outcome. The dorsolateral prefrontal cortex (DLPFC) appears to play a major role in such a context-dependent, flexible behavioural control system, and this area is likely to have a neuronal mechanism for such retrospective coding, which associates response-outcome with the information and/or neural systems that guided the response. To address this hypothesis, we recorded neuronal activity from the DLPFC of monkeys performing memory- and sensory-guided saccade tasks, each of which had two conditions with reward contingencies. We found that post-response activity of a subset of DLPFC neurons was modulated by three factors relating to earlier events: the direction of the immediately preceding response, its outcome (reward or non-reward) and the information type (memory or sensory) that guided the response. Such neuronal coding should play a role in associating response-outcome with information and/or neural systems used to guide behaviour - that is, 'retrospective monitoring' of behavioural context and/or neural systems used for guiding behaviour - thereby contributing to context-dependent, flexible control of behaviours.

Neuronal Activity Throughout the Primate Mediodorsal Nucleus of the Thalamus During Oculomotor Delayed-Responses. I. Cue-, Delay-, and Response-Period Activity

The thalamic mediodorsal nucleus (MD) has strong reciprocal connections with the dorsolateral prefrontal cortex (DLPFC), suggesting that the MD, like the DLPFC, participates in higher cognitive functions. To examine MD's participation in cognitive functions, we analyzed the characteristics of task-related activities sampled homogeneously from the MD while two monkeys performed a spatial working memory task using oculomotor responses. Of 141 task-related MD neurons, 26, 53, and 84% exhibited cue-, delay-, and response-period activity, respectively. Most of cue- and response-period activities showed phasic excitation, and most of delay-period activity showed tonic sustained activation. Among neurons with response-period activity, 74% exhibited presaccadic activity. Most cue-period, delay-period, and presaccadic activities were directional, whereas most postsaccadic activity was omni-directional. A significant contralateral bias in the best directions was present in cue-period and presaccadic activity. However, such bias was not present in delay-period activity, although most neurons had a best direction toward the contralateral visual field. We compared these characteristics with those observed in DLPFC neurons. Response-period activity was more frequently observed in the MD (84%) than in the DLPFC (56%). The directional selectivity and bias of task-related activities and the ratios of pre- and postsaccadic activities were different between MD and DLPFC. These results indicate that the MD participates in higher cognitive functions such as spatial working memory. However, the manner in which these two structures participate in these processes differs, in that the MD participates more in motor control aspects compared with the DLPFC.

Neuronal Activity Throughout the Primate Mediodorsal Nucleus of the Thalamus During Oculomotor Delayed-Responses. II. Activity Encoding Visual Versus Motor Signal

We collected single-neuron activity from the mediodorsal (MD) nucleus of the thalamus, examined the information that was represented by task-related activity during performance of a spatial working memory task, and compared the present results with those obtained in the dorsolateral prefrontal cortex (DLPFC). We used two oculomotor delayed-response (ODR) tasks. In the ordinary ODR task, monkeys were required to make a memory-guided saccade to the location where a visual cue had been presented 3 s previously, whereas in the rotatory ODR task, they were required to make a memory-guided saccade 90° clockwise from the cue direction. By comparing the best directions of the same task-related activity between the two tasks, we could determine whether this activity represented the cue location or the saccade direction. All cue-period activity represented the cue location. In contrast, 56% of delay-period activity represented the cue location and 41% represented the saccade direction. Almost all response-period activity represented the saccade direction. These results indicate that task-related MD activity represents either visual or motor information, suggesting that the MD participates in sensory-to-motor information processing. However, a greater proportion of delay- and response-period activities represented the saccade direction in the MD than in the DLPFC, indicating that more MD neurons participate in prospective information processing than DLPFC neurons. These results suggest that although functional interactions between the MD and DLPFC are crucial to cognitive functions such as working memory, there is a difference in how the MD and DLPFC participate in these functions.

[Methods for assessing cognitive function utilizing operant tasks in rats]

Most human behaviors (responses) are volitional, the frequency of which is changed based on stimulus presentations contingent upon the response, that is, operant behavior. It is considered that the findings on cognitive functions based on operant behaviors are more reliable in extrapolating the results to humans. Impairment of memory function, most notably the impairments of working memory and attention, is an important research focus to elucidate the mechanism underlying the core syndrome of Alzheimer's disease. Among various methods to measure working memory and attention, a delayed matching-to-sample paradigm utilizing operant chambers equipped with 3 levers and a choice reaction time paradigm have been proven to be very useful. Aside from these, adaptation to new environment is an important function for survival, and its impairment has been considered to be one of the factors inducing psychiatric disorders. Preclinical methods to measure the adaptation ability include a position reversal learning paradigm utilizing 2-lever operant chambers. Since the findings of studies on cognitive functions utilizing operant behaviors have been in good correlation with clinical findings, it would serve as a good strategy for elucidating the causes of such disorders as well as developing therapeutic agents.

Effects of ventral hippocampal long-term potentiation and depression on the gamma-band local field potential in anesthetized rats

We examined the effect of long-term potentiation or depression (LTP or LTD) on the local field potential, focusing on the gamma-band (40-100 Hz) power, in the ventral hippocampus CA1 of anesthetized rats. LTP and LTD induction in the CA3-CA1 pathway increased the CA1 spontaneous gamma-band power by around 40 and 80-100 Hz, respectively, while neither changed the evoked levels significantly. These results suggest that the ventral CA1 local field potential can maintain bidirectional plasticity in the steady state for the long term. Given the involvement of synaptic plasticity in learning and memory, the gamma-band power change associated with LTP/LTD may relate to ventral hippocampal functions. The LTP increased the spontaneous power at around 40 Hz of the gamma-band frequency in the ventral CA1, and the LTD did the same at 80-100 Hz. The biphasic increase may distribute the subsequent input appropriately to regulate the relevant synaptic history in the ventral CA1 and anatomically related structures in vivo.

Properties of delay-period neuronal activity in the primate prefrontal cortex during memory- and sensory-guided saccade tasks

The dorsolateral prefrontal cortex (DLPFC) is involved in visuospatial short-term (or working) memory. Its cellular basis has been widely examined using the delayed-response paradigm in nonhuman primates. Sustained delay-period activity in DLPFC neurons with directional difference (i.e. directional delay-period activity) has been thought to represent visuospatial short-term (or working) memory. However, little is known about the activity of these neurons during a delay period when the sensory input remains. To address this issue, we examined neuronal activity in the DLPFC while macaque monkeys performed a memory-guided saccade (MGS) task and a delayed visually guided saccade (VGS) task. The MGS task required a memory-guided saccade for a remembered target location. The VGS task had the same temporal sequence as the MGS task, but the sensory stimulus remained during the delay period. We found that most of the DLPFC neurons with directional delay-period activity showed sustained activation during the 'delay' period in the VGS task only ('V-neurons', 49%), or in both tasks ('MV-neurons', 46%). Neurons showing directional delay-period activity in the MGS task only ('M-neurons') were only 5% of the DLPFC neurons with directional delay-period activity. These findings indicate that most DLPFC neurons that are active during the delay period are also active when the sensory stimulus remains, suggesting that DLPFC neurons driven by mnemonic information are also driven by sensory input. Such sustained representation of information should have potential utility in flexible cognitive controls of behaviour.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Effect of tacrolimus (FK506) on ischemia-induced brain damage and memory dysfunction in rats

The behavioral and neurohistological protective effects of tacrolimus (FK506) were examined in rats subjected to 15-min global forebrain ischemia. Learning and memory performance were evaluated in an aversive, non-food-motivated, eight-arm radial maze. In one experiment, naive rats were rendered ischemic, and 15 days later they were tested for acquisition of a spatial task (postoperative training). In a complementary experiment, rats were trained for 8 days and then subjected to ischemia (preoperative training); 15 days later (on Day 24 of testing) they were retested for retention of cognition. FK506 (1.0 mg/kg) was given intravenously at the beginning of reperfusion, followed by doses applied intraperitoneally 6, 24, 48 and 72 h postischemia. Behavioral performance was expressed by latency to find the goal box, and number of errors. Ischemia did not affect acquisition performance. In contrast, retention of cognition was markedly impaired by ischemia, particularly working memory (P<.05-.001). This ischemia-induced, retrograde amnesia was significantly reduced by FK506 compared to vehicle alone on Day 24, as measured by latency and working memory errors (P<.025). A neuroprotective effect of FK506 was also seen on working memory, when postischemic performance was compared to that prior to ischemia (P>.05, Day 24 vs. Day 8, paired samples), in contrast to the significant, retrograde amnesia found in the ischemic, vehicle-treated group (P<.01). FK506 also significantly reduced the extent of hippocampal CA1 cell loss; however, this effect did not correlate with behavior. The present results suggest that the histological, neuroprotective effect of FK506 may be accompanied by a reduction in cognitive impairment, as assessed in a novel, non-food-motivated, eight-arm radial maze after transient, global, cerebral ischemia in rats.

Functional Frontoparietal Connectivity During Short-Term Memory as Revealed by High-Resolution EEG Coherence Analysis

In this electroencephalographic study, the authors modeled the functional connectivity between frontal and parietal areas during short-term memory (STM) processes by spectral coherence analysis and the directed transfer function, that is, for the estimation of coherence "direction." A no-STM task was used as a reference. STM was characterized by an increased frontoparietal electroencephalograph coherence at high frequencies (beta and gamma, 14-45 Hz). In the control task, parietal-to-frontal flow prevailed at those frequencies. However, the STM task showed a bidirectional frontoparietal flow at the gamma band. In conclusion, frontoparietal connectivity would optimize "representational" memory during STM. In this context, the frontal areas would increase their influence on parietal areas for memory retention.

Neuronal activities in the monkey primary and higher-order gustatory cortices during a taste discrimination delayed GO/NOGO task and after reversal

The correlation between different gustatory areas in the frontal operculum, orbitofrontal area, and insula and the representation of different aspects of cues during a salt-water discrimination delayed GO/NOGO task was studied in a Japanese monkey. Four groups were identified among 169 neurons responding to cues before/after task reversal. Group I (n=78) responded to the physicochemical nature of the cue, Group II (n=8) responded to both the physicochemical nature of the cue and the subsequent behavior, Group III (n=51) (three subgroups) produced discharges related to the subsequent behavior, and Group IV (n=32) produced non-differential responses probably related to attention. The primary gustatory areas (area G and the oral part of area 3) almost exclusively contained Group I neurons, whereas the so-called secondary gustatory areas (the PrCO and area 12) contained most of the Group III neurons. Group IIIc showed discharges accelerating to the LED onset, probably representing preparation for subsequent behavior, and the response differed between the PrCO and area 12. The PrCO also contained Group IV neurons. The primary gustatory areas process pure gustatory signals, whereas the PrCO and area 12 may be involved in gustatory perception, attention, or behavior.

BIMU 1 and RS 67333, two 5-HT4 receptor agonists, modulate spontaneous alternation deficits induced by scopolamine in the mouse

The present study was conducted to determine the effects of two potent 5-HT4 receptor agonists, BIMU 1 (1 (3-ethyl-2,3-dihydro-N-[endo-8-methyl-8-azabicyclo (3.2.1)-oct-3-yl]-2-oxo-1H) benzimidazole-1-carboxamide hydrochloride; 1, 3, 10 mg/kg, i.p.) and RS 67333 (1-(4-amino-5-chloro-2-methoxyphenyl)-3-(1-n-butyl-4-piperidinyl)-1-propanone; 0.25, 0.5, 1 mg/kg, i.p.) on the learning impairment induced by the muscarinic acetylcholine receptor antagonist, scopolamine (1 mg/kg) in mice. Working memory was examined by observing spontaneous alternation behavior in the Y-maze test. Both BIMU 1 (10 mg/kg) and RS 67333 (1 mg/kg) prevented the scopolamine-induced alternation deficits, whereas no effect could be evidenced on locomotor or emotional indices. The reversal actions of BIMU 1 and RS 67333 on this cognitive dysfunction were abolished by the selective 5-HT4 receptor antagonist GR 125487 (1-[2-[(methyl sulfonyl)-amino]-ethyl]-4-piperidinyl-methyl-5-fluoro-2-methoxy-1H-indole-3-carboxylate; 10 mg/kg, i.p.). When given alone at the same doses, none of the three serotonergic agents had any measurable effect. These results demonstrate the ability of 5-HT4 receptor agonists to reverse spontaneous working memory deficits and further confirm the therapeutic potential of such ligands in the treatment of cognitive alterations that associate short-term working memory disorders and cholinergic hypofunction.

Evoked prefrontal gamma oscillation by hippocampal train stimulation in anesthetized rats

We previously reported a difference in short-term synaptic plasticity between the rat posterior dorsal CA1 (pdCA1)-prefrontal cortex (PFC) and ventral CA1 (vCA1)-PFC pathways. Here, to determine the effects of hippocampal train stimulation on the local field potential in the medial PFC, we recorded the PFC field potential with brief 250-Hz train stimulation (1, 3, 5, 7, and 9 pulses) of pdCA1 or vCA1 in anesthetized rats. Analysis of the gamma-band (40-100 Hz) power revealed stimulation-evoked gamma oscillation in the pdCA1-PFC, but not in the vCA1-PFC. These results indicate that these pathways have different responses to train stimulation. The function of the pdCA1-PFC may differ from that of the vCA1-PFC, a well-known hippocampus-PFC pathway.

Dopaminergic modulation of visual-spatial working memory in Parkinson's disease

Visual-spatial working memory (WM) impairment is frequently associated with the early stage of Parkinson's disease (PD). The aim of this study was to evaluate the performance of a group of PD patients in visual-spatial and visual-object WM tasks and to investigate the effect of administering the dopaminergic agonist apomorphine (experiment 1) or the dopamine precursor L-dopa (experiment 2) on the performance of tests assessing these functions. To study WM processes, the PD patients and age-matched normal controls were given an n-back task paradigm. In both experiments, the PD patients were submitted to two evaluations: one after a 12-hour therapy washout and the other 15 min after a subcutaneous infusion of apomorphine (average 0.04 mg/kg) or 20/30 min after L-dopa intake (200 mg p.o.). The apomorphine infusion had a worsening effect on reaction times in both visual-spatial and visual-object WM tasks, but it did not influence performance accuracy. Instead, L-dopa administration had a ameliorative effect on accuracy and reaction times in both visual-spatial and visual-object tasks. These results highlight the role of dopamine in the modulation of the WM function in PD patients.

Involvement of the dorsolateral prefrontal cortex of monkeys in visuospatial target selection

To examine the involvement of the dorsolateral prefrontal cortex (PFC) in visuospatial target selection, we induced local, reversible inactivation with muscimol at various sites in the dorsolateral PFC of two rhesus monkeys while they performed oculomotor visual search (OVS) and oculomotor detection (OD) tasks. The OVS task required the subject to select a target stimulus from among distractors and to make a saccade to the target location (target selection was required for correct performance), whereas the OD task only required a saccade to the target (target selection was not required for correct performance). The local injection of muscimol (5 microg, 1 microl) into the dorsolateral PFC induced a specific deficit in the OVS task but not in the OD task. The deficit in the OVS task was characterized by the disordering of saccades for some (mostly a few) particular target locations as well as by prolongation of the time required for the visual search in most cases. The target locations affected by muscimol were biased to the contralateral visual field. Further, the OVS task with "pop-out" and "non-pop-out" conditions was similarly impaired by muscimol injection. These results suggest that the dorsolateral PFC plays a role in target selection in visual space to guide goal-directed motor acts and particular sites are involved in target selection for a particular visuospatial coordinate. Further, this function of the dorsolateral PFC appears to involve both top-down (active) and bottom-up (passive) target-selection/selective attention processes to control interfering information (distractors).

Audiospatial and visuospatial working memory in 6-13 year old school children

The neural processes subserving working memory, and brain structures underlying this system, continue to develop during childhood. We investigated the effects of age and gender on audiospatial and visuospatial working memory in a nonclinical sample of school-aged children using n-back tasks. The results showed that auditory and visual working memory performance improves with age, suggesting functional maturation of underlying cognitive processes and brain areas. The gender differences found in the performance of working memory tasks suggest a larger degree of immaturity in boys than girls at the age period of 6-10 yr. The differences observed between the mastering of auditory and visual working memory tasks may indicate that visual working memory reaches functional maturity earlier than the corresponding auditory system.

The role of prefrontal cortex in working-memory capacity, executive attention, and general fluid intelligence: an individual-differences perspective

We provide an "executive-attention" framework for organizing the cognitive neuroscience research on the constructs of working-memory capacity (WMC), general fluid intelligence, and prefrontal cortex (PFC) function. Rather than provide a novel theory of PFC function, we synthesize a wealth of single-cell, brain-imaging, and neuropsychological research through the lens of our theory of normal individual differences in WMC and attention control (Engle, Kane, & Tuholski, 1999; Engle, Tuholski, Laughlin, & Conway, 1999). Our critical review confirms the prevalent view that dorsolateral PFC circuitry is critical to executive-attention functions. Moreover, although the dorsolateral PFC is but one critical structure in a network of anterior and posterior "attention control" areas, it does have a unique executive-attention role in actively maintaining access to stimulus representations and goals in interference-rich contexts. Our review suggests the utility of an executive-attention framework for guiding future research on both PFC function and cognitive control.

Prefrontal delay-period activity is affected by visual cues presented outside the memory field

To examine interactions between primate prefrontal neurons having memory fields, we examined whether primate prefrontal delay-period activity produced by a visual cue presented inside the neuron's memory field is affected by an additional visual cue presented inside or outside the memory field in 39 prefrontal neurons. Delay-period activity was either diminished (43%) or enhanced (16%) when an additional cue was presented at a certain area in the visual field. Since functional interactions among neighboring prefrontal neurons were present, these modulation could be caused by the interactions between neurons having memory fields in different positions.

Effects of hippocampus-induced prefrontal long-term depression on gamma-band local field potential in anesthetized rats

To determine whether long-term depression (LTD) affects cortical gamma-band local field potential (40-100 Hz), we conducted a LTD induction experiment in the hippocampus-prefrontal cortex (PFC) pathway of an anesthetized rat. The LTD induction increased the spontaneous level of PFC gamma-band power of 70-100 Hz, which was not affected after the long-term potentiation (LTP) induction in our previous experiment. In addition, the LTD induction increased the evoked PFC gamma-band power at 900 ms after hippocampal test stimulation; this latency appeared to differ from that (500-700 ms) observed in our previous LTP experiment. The results indicate that the PFC field potential increases its gamma-band power following both LTP and LTD in the hippocampus-PFC pathway, which is involved in working memory. Particularly, the sustained increase by LTD may reflect a representation of working memory.

Neuronal activity representing visuospatial mnemonic processes associated with target selection in the monkey dorsolateral prefrontal cortex

To investigate how visuospatial mnemonic and target selection processes are represented in the dorsolateral prefrontal cortex (PFC), we studied neuronal attributes of the dorsolateral PFC while monkeys were performing oculomotor delayed visual search (ODVS) and oculomotor delayed-response (ODR) tasks. In the ODVS task, the subject made a memory-guided saccade to a remembered target location that had been presented along with distractors before a delay period; in the ODR task, the target was presented without any distractors. A total of 252 neurons in the dorsolateral PFC showed directional delay-period activity and were divided into two groups; neurons that showed directional delay-period activity predominantly in the ODVS task (n=112), and those that showed such activity similarly in both the ODVS and ODR tasks (n=140). These neuronal groups shared similar temporal properties (i.e. onset latency, peak time of delay-period activity) and spatial tuning. Our findings suggest that the dorsolateral PFC contains a particular visuospatial memory system for information selected by target selection (selective attention), and this attention-memory system (or 'memory system for special use') appears to be represented in the dorsolateral PFC, in parallel with a more 'general' memory system that is not specifically associated with target selection.

Prefrontal task-related activity representing visual cue location or saccade direction in spatial working memory tasks

To examine what kind of information task-related activity encodes during spatial working memory processes, we analyzed single-neuron activity in the prefrontal cortex while two monkeys performed two different oculomotor delayed-response (ODR) tasks. In the standard ODR task, monkeys were required to make a saccade to the cue location after a 3-s delay, whereas in the rotatory ODR (R-ODR) task, they were required to make a saccade 90 degrees clockwise from the cue location after the 3-s delay. By comparing the same task-related activities in these two tasks, we could determine whether such activities encoded the location of the visual cue or the direction of the saccade. One hundred twenty one neurons exhibited task-related activity in relation to at least one task event in both tasks. Among them, 41 neurons exhibited directional cue-period activity, most of which encoded the location of the visual cue. Among 56 neurons with directional delay-period activity, 86% encoded the location of the visual cue, whereas 13% encoded the direction of the saccade. Among 57 neurons with directional response-period activity, 58% encoded the direction of the saccade, whereas 35% encoded the location of the visual cue. Most neurons whose response-period activity encoded the location of the visual cue also exhibited directional delay-period activity that encoded the location of the visual cue as well. The best directions of these two activities were identical, and most of these response-period activities were postsaccadic. Therefore this postsaccadic activity can be considered a signal to terminate unnecessary delay-period activity. Population histograms encoding the location of the visual cue showed tonic sustained activation during the delay period. However, population histograms encoding the direction of the saccade showed a gradual increase in activation during the delay period. These results indicate that the transformation from visual input to motor output occurs in the dorsolateral prefrontal cortex. The analysis using population histograms suggests that this transformation occurs gradually during the delay period.

Prefrontal cortical representation of visuospatial working memory in monkeys examined by local inactivation with muscimol

In primates, dorsolateral areas of the prefrontal cortex (PFC) play a major role in visuospatial working memory. To examine the functional organization of the PFC for representing visuospatial working memory, we produced reversible local inactivation, with the local injection of muscimol (5 microg, 1 microl), at various sites (n = 100) in the dorsolateral PFC of monkeys and observed the behavioral consequences in an oculomotor delayed-response task that required memory-guided saccades for locations throughout both visual fields. At 82 sites, the local injection of muscimol induced deficits in memory-guided saccades to a few specific, usually contralateral, target locations that varied with the location of the injection site. Such deficits depended on the delay length, and longer delays were associated with larger deficits in memory-guided saccades. The injection sites and affected spatial locations of the target showed a gross topographical relationship. No deficits appeared for a control task in which the subject was required to make a visually guided saccade to a visible target. These findings suggest that a specific site in the dorsolateral PFC is responsible for the working memory process for a specific visuospatial coordinate to guide goal-directed behavior. Further, memoranda for specific visuospatial coordinates appear to be represented in a topographical memory map within the dorsolateral PFC to represent visuospatial working memory processes.

The effects of dopamine and its antagonists on directional delay-period activity of prefrontal neurons in monkeys during an oculomotor delayed-response task

To examine the role of dopamine receptors in the memory field of neurons for visuospatial working memory in the prefrontal cortex (PFC), dopamine and its antagonists (SCH23390 for the D1-antagonist and sulpiride for the D2-antagonist) were applied iontophoretically to neurons of the dorsolateral PFC in monkeys that performed an oculomotor delayed-response task. In this task, the subject made a memory-guided saccade to a remembered target location that had been cued by a visuospatial stimulus (right, up, left, or down; 15 degrees in eccentricity) prior to a 4-s delay period. We focused here on PFC neurons that showed directional delay-period activity; i.e., an increased activity during the delay period, the magnitude of which varied significantly with the target location. Iontophoretic application of SCH23390 (usually 50 nA) decreased or increased the activities of most of these neurons (n=48/62, 77%); most neurons showed a decrease (n=43/62, 69%). For the neurons affected by SCH23390, a directional index of directional delay-period activity was attenuated by SCH23390, whereas the preferred direction was not greatly affected. The decreasing effect of SCH23390 was dose-dependent; the extent of the decrease was less with a lower dose (20-nA current) than with the ordinary dose (50-nA current), although the effect of the lower dose of SCH23390 on delay-period activity was similar in nature to that of the ordinary dose of SCH23390. Furthermore, the application of dopamine itself augmented directional delay-period activity in most of the neurons tested (n=12/16, 75%). Sulpiride did not have any significant effects in most of the neurons tested (n=15/17). These results suggest that the activation of D1-dopamine receptors play a facilitating role in the memory field of PFC neurons for visuospatial working memory processes.

Neural interaction between the basal forebrain and functionally distinct prefrontal cortices in the rhesus monkey

The prefrontal cortex in rhesus monkeys is a heterogeneous region by structure, connections and function. Caudal medial and orbitofrontal cortices receive input from cortical and subcortical structures associated with emotions, autonomic function and long-term memory, while lateral prefrontal cortices are linked with structures associated with working memory. With the aid of neural tracers we investigated whether functionally distinct orbitofrontal, medial and lateral prefrontal cortices have specific or common connections with an ascending modulatory system, the basal forebrain. Ascending projections originated in the diagonal band and the basalis nuclei of the basal forebrain in regions demarcated by choline acetyltransferase. Although the origin of projections from the basal forebrain to lateral, medial and orbitofrontal cortices partially overlapped, projections showed a general topography. The posterior part of the nucleus basalis projected preferentially to lateral prefrontal areas while its rostrally adjacent sectors projected to medial and orbitofrontal cortices. The diagonal band nuclei projected to orbitofrontal and medial prefrontal areas. Cortical and subcortical structures that are interconnected appear to have a similar pattern of connections with the basal forebrain. In comparison to the ascending projections, the descending projections were specific, originating mostly in the posterior (limbic) component of medial and orbitofrontal cortices and terminating in the diagonal band nuclei and in the anterior part of the nucleus basalis. In addition, prefrontal limbic areas projected to two other systems of the basal forebrain, the ventral pallidum and the extended amygdala, delineated with the striatal-related markers dopamine, adenosine 3':5'-monophosphate regulated phosphoprotein of M(r) 32kDa, and the related phosphoprotein Inhibitor-1. These basal forebrain systems project to autonomic nuclei in the hypothalamus and brainstem. We interpret these results to indicate that lateral prefrontal areas, which have a role in working memory, receive input from, but do not issue feedback projections to the basal forebrain. In contrast, orbitofrontal and medial prefrontal areas, which have a role in emotions and long-term memory, have robust bidirectional connections with the basal forebrain. Moreover, orbitofrontal and medial prefrontal cortices target the ventral pallidum and the extended amygdala, through which high-order association areas may activate motor autonomic structures for the expression of emotions.

Synaptic targets of the intrinsic axon collaterals of supragranular pyramidal neurons in monkey prefrontal cortex

The principal axons of supragranular pyramidal neurons in the cerebral cortex travel through the white matter and terminate in other cortical areas, whereas their intrinsic axon collaterals course through the gray matter and form both local and long-distance connections within a cortical region. In the monkey prefrontal cortex (PFC), horizontally oriented, intrinsic axon collaterals from supragranular pyramidal neurons form a series of stripe-like clusters of axon terminals (Levitt et al. [1993] J Comp Neurol 338:360-376; Pucak et al. [1996] J Comp Neurol 376:614-630). The present study examined the synaptic targets of the intrinsic axon collaterals arising from supragranular pyramidal neurons within the same stripe (local projections). Approximately 50% of the within-stripe axon terminals in monkey PFC area 9 targeted dendritic spines. In contrast, for both the intrinsic axon collaterals that travel between stripes (long-range projections), and the axon terminals that project to other PFC areas (associational projections), over 92% of the postsynaptic structures were dendritic spines (Melchitzky et al. [1998] J Comp Neurol 390:211-224). The other 50% of the within-stripe terminals synapsed with dendritic shafts. Dual-labeling studies confirmed that these within-stripe terminals contacted gamma-aminobutyric acid-immunoreactive dendritic shafts, including the subpopulation that contains the calcium-binding protein parvalbumin. The functional significance of the differences in synaptic targets between local and long-range intrinsic axon collaterals was supported by whole-cell, patch clamp recordings in an in vitro slice preparation of monkey PFC. Specifically, the small amplitude responses observed in layer 3 pyramidal neurons during long-range, low-intensity stimulation were exclusively excitatory, whereas local stimulation also evoked di/polysynaptic inhibitory responses. These anatomic and electrophysiological findings suggest that intrinsic connections of the PFC differ from other cortical regions and that within the PFC, feedback (within-stripe) inhibition plays a greater role in regulating the activity of supragranular pyramidal neurons than does feedforward inhibition either between stripes or across regions.

Neuronal mechanisms of executive control by the prefrontal cortex

Executive function is considered to be a product of the coordinated operation of various processes to accomplish a particular goal in a flexible manner. The mechanism or system responsible for the coordinated operation of various processes is called executive control. Impairments caused by damage to the prefrontal cortex are often called dysexecutive syndromes. Therefore, the prefrontal cortex is considered to play a significant role in executive control. Prefrontal participation to executive control can be partly explained by working memory that includes mechanisms for temporary active storage of information and processing stored information. For the prefrontal cortex to exert executive control, neuronal mechanisms for temporary storage of information and dynamic and flexible interactions among them are necessary. In this article, we present the presence of dynamic and flexible changes in the strength of functional interaction and extensive functional interactions among temporal information-storage processes in the prefrontal cortex. In addition, recent imaging studies show dynamic changes in functional connectivity between the prefrontal cortex and other cortical and subcortical structures depending upon the characteristics or the temporal context of the task. These observations indicate that the examination of dynamic and flexible modulation in neuronal interaction among prefrontal neurons as well as between the prefrontal cortex and other cortical and subcortical areas is important for explaining how the prefrontal cortex exerts executive control.

Dopamine increases excitability of pyramidal neurons in primate prefrontal cortex

Dopaminergic modulation of neuronal networks in the dorsolateral prefrontal cortex (PFC) is believed to play an important role in information processing during working memory tasks in both humans and nonhuman primates. To understand the basic cellular mechanisms that underlie these actions of dopamine (DA), we have investigated the influence of DA on the cellular properties of layer 3 pyramidal cells in area 46 of the macaque monkey PFC. Intracellular voltage recordings were obtained with sharp and whole cell patch-clamp electrodes in a PFC brain-slice preparation. All of the recorded neurons in layer 3 (n = 86) exhibited regular spiking firing properties consistent with those of pyramidal neurons. We found that DA had no significant effects on resting membrane potential or input resistance of these cells. However DA, at concentrations as low as 0.5 microM, increased the excitability of PFC cells in response to depolarizing current steps injected at the soma. Enhanced excitability was associated with a hyperpolarizing shift in action potential threshold and a decreased first interspike interval. These effects required activation of D1-like but not D2-like receptors since they were inhibited by the D1 receptor antagonist SCH23390 (3 microM) but not significantly altered by the D2 antagonist sulpiride (2.5 microM). These results show, for the first time, that DA modulates the activity of layer 3 pyramidal neurons in area 46 of monkey dorsolateral PFC in vitro. Furthermore the results suggest that, by means of these effects alone, DA modulation would generally enhance the response of PFC pyramidal neurons to excitatory currents that reach the action potential initiation site.

The role of D1-dopamine receptors in working memory-guided movements mediated by frontal cortical areas

Like the striatum, the frontal motor cortices receive dopaminergic fibers from midbrain dopamine cells and contain high levels of dopamine receptors. Among frontal cortical areas, the dorsolateral PFC (PFd1) and the dorsal premotor cortex (PMd) have strong neural connections and play a major role for working memory-guided directional movements. To reveal the role of dopamine in this cognitive motor function, dopamine antagonists (SCH23390 for D1 receptors and sulpiride for D2 receptors) were applied locally or iontophoretically to the PFd1 and PMd in monkeys that performed delayed-response tasks with memory-guided directional movements. Applications of SCH23390, but not sulpiride, to these areas had significant effects at both the behavioral and neuronal levels. In the PFd1 and at the behavioral level, local injections of SCH23390 induced specific errors for memory-guided saccades, whereas it had no effects on visually guided saccades. In the PMd, local injections of SCH23390 induced directional errors and increased reaction time and movement time in memory-guided reaching movements. At the neuron level, iontophoretic applications of SCH23390 attenuated directional tuning of neurons of the PFd1 and PMd, which showed directional activities during the delay-and/or response-period(s). These findings suggest that the activation of D1-dopamine receptors in these frontal cortical areas plays a facilitating role in a series of neuronal processes of working memory-guided directional movements; the working memory process for guiding motor act in the PFd1 and preparation/control of directional manual movements in the PMd. In addition, our findings may provide insight into symptoms of schizophrenia and Parkinson's disease; the dysfunction of D1-dopamine receptors in the PMd1 and PMd may contribute to some symptoms, such as bradyphrenia and bradykinesia, in these disorders.

Neurocomputational models of working memory

During working memory tasks, the firing rates of single neurons recorded in behaving monkeys remain elevated without external cues. Modeling studies have explored different mechanisms that could underlie this selective persistent activity, including recurrent excitation within cell assemblies, synfire chains and single-cell bistability. The models show how sustained activity can be stable in the presence of noise and distractors, how different synaptic and voltage-gated conductances contribute to persistent activity, how neuromodulation could influence its robustness, how completely novel items could be maintained, and how continuous attractor states might be achieved. More work is needed to address the full repertoire of neural dynamics observed during working memory tasks.

A model of working memory: bridging the gap between electrophysiology and human brain imaging

Human neuroimaging methods such as positron emission tomography and functional magnetic resonance imaging have made possible the study of large-scale distributed networks in the behaving human brain. Although many imaging studies support and extend knowledge gained from other experimental modalities such as animal single-cell recordings, there have also been a substantial number of experiments that appear to contradict the animal studies. Part of the reason for this is that neuroimaging is an indirect measure of neuronal firing activity, and thus interpretation is difficult. Computational modeling can help to bridge the gap by providing a substrate for making explicit the assumptions and constraints provided from other sources such as anatomy, physiology and behavior. We describe a large-scale model of working memory that we have used to examine a number of issues relating to the interpretation of imaging data. The gating mechanism that regulates engagement and retention of short-term memory is revised to better reflect hypothesized underlying neuromodulatory mechanisms. It is shown that in addition to imparting better performance for the memory circuit, this mechanism also provides a better match to imaging data from working memory studies.

Prefrontocortical serotonin depletion results in plastic changes of prefrontocortical pyramidal neurons, underlying a greater efficiency of short-term memory

The prefrontal cortex activity is involved in organizing the short-term memory. Although the involvement of serotonin for an appropriate performance in learning and memory tests is well known, its role is still unclear; as is the cellular basis of short-term memory behavioral performance. Sprague-Dawley rats were stereotactically injected with 1 microg/microl of 5, 7-dihydroxitryptamine to cause a lesion to the dorsal raphe nucleus. Sham-operated or intact rats were also studied as control groups. Before surgery and 20 days post-operatively, each animal was placed in the Biel maze for five consecutive trials. In the pre-treatment test, all three groups decreased significantly the number of errors beginning with the fourth trial. The same occurred in the post-treatment test, except for the experimental group, whose animals committed less errors beginning with the second trial. After behavioral testing, the dorsomedial prefrontal cerebral cortex was dissected out, and the Golgi study of the third-layer pyramidal neurons revealed that the length of both the apical and the basilar dendrites was smaller than that of controls, and that the apical and oblique dendrites had a greater spine density. A major proportion of thin spines was also seen on the basilar and oblique dendrites, and more stubby spines were seen on the apical dendrite. Serotonin depletion in the prefrontal cerebral cortex resulted in cytoarchitectural alterations of the prefrontocortical pyramidal neurons, which may be underlying partially the greater efficiency observed in the short-term memory behavioral performance.

Trajectory estimation in schizophrenia

Two experiments were conducted to investigate the ability of schizophrenia patients to maintain internal representation over time and space. It has been hypothesized that the ability to guide behavior by internal representation, mediated by the dorsolateral prefrontal cortex (DLPFC), is impaired in schizophrenia [e.g. Goldman-Rakic, P.S., 1996. The functional parcellation of dorsolateral prefrontal cortex and the heterogeneous facets of schizophrenia. In: Matthysse, S., Levy, D.L. (Eds.), Psychopathology: Evolution of a New Science. Cambridge University Press, Cambridge]. In Experiment 1, subjects observed a target, which traveled behind an opaque wall during a part of its trajectory. The task was to accurately assess the speed of the target by predicting when the target would re-emerge on the other side of the wall. In Experiment 2, subjects were asked to estimate the spatial trajectory of an established target path when it was partially occluded from view by another object. Schizophrenia patients were impaired in estimating the speed of a moving target and in estimating the spatial trajectory, without showing deficits in the control tasks. These results suggest that schizophrenia patients may not be able to accurately maintain the internal representation of a target over time and space. Such deficits may have deleterious consequences in goal-directed behavior.

Intrinsic excitatory connections in the prefrontal cortex and the pathophysiology of schizophrenia

Working memory, a fundamental cognitive process that is disturbed in schizophrenia, appears to depend upon the sustained activity of specific populations of neurons in the prefrontal cortex. Understanding the neural mechanism(s) that may contribute to the sustained activity of these neurons represents a critical step in predicting the types of alterations in prefrontal circuitry that may be present in schizophrenia, and in determining how such alterations may contribute to the cognitive symptoms of this disorder. This article reviews recent findings which suggest that intrinsic horizontal connections among pyramidal neurons in layer 3 of the dorsolateral prefrontal cortex may provide a critical anatomical substrate for working memory processes, and that alterations in these connections may account for the observations of disturbed working memory, adolescence-related onset of clinical features, and certain pathological changes in the prefrontal cortex of subjects with schizophrenia.

Role of dopamine in learning and memory: implications for the treatment of cognitive dysfunction in patients with Parkinson's disease

Along with dementia, Parkinson's disease (PD) is associated with subtle but widespread cognitive impairment even in the absence of clinically apparent cognitive decline. Many of the deficits are reminiscent of those observed in patients with lesions of the prefrontal cortex, that is, failure in executive function that involves skills required for anticipation, planning, initiation and monitoring of goal-directed behaviours. This paper reviews the dopaminergic brain circuitry, and preclinical and clinical evidence supporting the regulation of prefrontal cortex activity by dopamine, and the role of dopamine in cognitive impairment in patients with PD. It addresses the need to integrate these facts and the findings of positive, neutral or detrimental frontal cognitive response to dopaminergic drugs in PD which should be viewed mainly in the context of methodological differences for subject selection. The cognitive effect of levodopa does not much depend on a neuropsychological specificity of the drug, the years of evolution of the disease or the severity of the motor signs. Instead, it may be a function of the level of dopamine depletion in different parts of the basal ganglia and prefrontal cortex. Consequently, dopaminergic agents may enhance cognitive functions in some patients and impair them in others. De novo patients tend to improve during the first year of treatment; stable responders to oral levodopa tend to show no changes; and wearing-off responders tend to deteriorate with acute levodopa challenge. Enhancement and impairment of cognitive function with dopaminergic treatment is incomplete and task-specific, suggesting the need to integrate the above dopamine facts with other neurotransmitter systems findings in PD. Meanwhile, such cognitive dissociation can be useful in refining the definition of the cognitive deficit in PD patients without dementia and emphasising the need to develop new and specific strategies for treatment.

Dopamine-mediated stabilization of delay-period activity in a network model of prefrontal cortex

The prefrontal cortex (PFC) is critically involved in working memory, which underlies memory-guided, goal-directed behavior. During working-memory tasks, PFC neurons exhibit sustained elevated activity, which may reflect the active holding of goal-related information or the preparation of forthcoming actions. Dopamine via the D1 receptor strongly modulates both this sustained (delay-period) activity and behavioral performance in working-memory tasks. However, the function of dopamine during delay-period activity and the underlying neural mechanisms are only poorly understood. Recently we proposed that dopamine might stabilize active neural representations in PFC circuits during tasks involving working memory and render them robust against interfering stimuli and noise. To further test this idea and to examine the dopamine-modulated ionic currents that could give rise to increased stability of neural representations, we developed a network model of the PFC consisting of multicompartment neurons equipped with Hodgkin-Huxley-like channel kinetics that could reproduce in vitro whole cell and in vivo recordings from PFC neurons. Dopaminergic effects on intrinsic ionic and synaptic conductances were implemented in the model based on in vitro data. Simulated dopamine strongly enhanced high, delay-type activity but not low, spontaneous activity in the model network. Furthermore the strength of an afferent stimulation needed to disrupt delay-type activity increased with the magnitude of the dopamine-induced shifts in network parameters, making the currently active representation much more stable. Stability could be increased by dopamine-induced enhancements of the persistent Na(+) and N-methyl-D-aspartate (NMDA) conductances. Stability also was enhanced by a reduction in AMPA conductances. The increase in GABA(A) conductances that occurs after stimulation of dopaminergic D1 receptors was necessary in this context to prevent uncontrolled, spontaneous switches into high-activity states (i.e., spontaneous activation of task-irrelevant representations). In conclusion, the dopamine-induced changes in the biophysical properties of intrinsic ionic and synaptic conductances conjointly acted to highly increase stability of activated representations in PFC networks and at the same time retain control over network behavior and thus preserve its ability to adequately respond to task-related stimuli. Predictions of the model can be tested in vivo by locally applying specific D1 receptor, NMDA, or GABA(A) antagonists while recording from PFC neurons in delayed reaction-type tasks with interfering stimuli.

Maternal separation and early social deprivation in Octodon degus: quantitative changes of nicotinamide adenine dinucleotide phosphate-diaphorase-reactive neurons in the prefrontal cortex and nucleus accumbens

The influence of postnatal socio-emotional deprivation on the development of nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase-reactive neurons in prefrontal cortical areas and in subdivisions of the nucleus accumbens was quantitatively investigated in the precocious rodent Octodon degus. Forty-five-days-old O. degus from two animal groups were compared: (i) degus which were repeatedly separated from their mothers during the first three postnatal weeks and after weaning reared in complete isolation; and (ii) degus which were reared under normal undisturbed social conditions. Socially-deprived animals displayed a significant decrease of NADPH-diaphorase-containing neurons in anterior cingulate cortex (85.5%), the same tendency was observed in the infralimbic, precentral medial and prelimbic prefrontal areas. Similarly, the core region of nucleus accumbens expressed reduced NADPH-diaphorase-reactive neuron numbers in deprived animals (70%), whereas the shell region remained unchanged. Since during normal postnatal development the number of NADPH-diaphorase-reactive neurons gradually decreases in all prefrontal cortical and accumbal regions, the observed deprivation-induced changes may reflect either an excessive reduction of NADPH-diaphorase-positive neurons or a down-regulation of the enzyme in neurons that normally express it. Since some NADPH-diaphorase-containing neurons in the prefrontal cortex have been shown to be GABAergic, it is tempting to speculate that a reduction of these inhibitory neurons in the anterior cingulate cortex may result in an enhanced excitatory output activity of disinhibited projection neurons in this cortical region, including those that project to the core region of the nucleus accumbens. Our results indicate a link between early adverse socio-emotional experience and the maturation of NADPH-reactive neurons and further studies are required to analyse the functional implication for this experience-induced brain pathology.