Prefrontal cortical unit activity and delayed alternation performance in monkeys

Prefrontal cortex as the site of estrogen's effect on cognition

The hippocampus has long been presumed the primary site of action of estrogens on cognition; and explicit memory is considered the cognitive function most vulnerable to menopausal loss of estrogen. We hypothesize instead that the prefrontal cortex and its neural circuitry are prime mediators of estrogen's role in cognition. We also propose that previously reported menopausal cognitive decline, presumed to be hippocampally mediated, may be secondary to executive dysfunction. We used a cross sectional design to compare the performance of nine menopausal women on hormone replacement therapy (HRT) and 10 menopausal women with no prior exposure to HRT on a battery of neuropsychological tests. The battery was comprised primarily of tests of memory and executive functioning. Executive functioning is mediated by the frontal lobes and encompasses working memory, directed attention, the inhibition of inappropriate responses, cognitive set switching, and behavioral monitoring. Unlike most previous studies, we used a memory measure that yields multiple scores reflecting various problem-solving strategies and error types, thus isolating spared and impaired cognitive processes. Results yielded both qualitative and quantitative evidence for disruption of cognitive processes subserved by the frontal lobes rather than the hippocampus: 1) despite intact free recall on a list-learning task (CVLT), untreated menopausal women were relatively impaired in correctly recognizing words previously learned and distinguishing them from items not on the list (discriminability), 2) untreated women also had difficulty inhibiting inappropriate responses in the form of perseverative errors, and 3) the non-HRT group consistently performed worse on the N-back test of working memory. The prefrontal cortex is critical for intact working memory and estrogen enhances performance on working memory tasks. In conclusion, this study provides preliminary evidence for executive dysfunction in untreated menopausal women as women with HRT outperformed women without HRT on tests requiring directed attention, inhibition of inappropriate responses, and cognitive set switching.

Effects of neuromodulation in a cortical network model of object working memory dominated by recurrent inhibition

Experimental evidence suggests that the maintenance of an item in working memory is achieved through persistent activity in selective neural assemblies of the cortex. To understand the mechanisms underlying this phenomenon, it is essential to investigate how persistent activity is affected by external inputs or neuromodulation. We have addressed these questions using a recurrent network model of object working memory. Recurrence is dominated by inhibition, although persistent activity is generated through recurrent excitation in small subsets of excitatory neurons. Our main findings are as follows. (1) Because of the strong feedback inhibition, persistent activity shows an inverted U shape as a function of increased external drive to the network. (2) A transient external excitation can switch off a network from a selective persistent state to its spontaneous state. (3) The maintenance of the sample stimulus in working memory is not affected by intervening stimuli (distractors) during the delay period, provided the stimulation intensity is not large. On the other hand, if stimulation intensity is large enough, distractors disrupt sample-related persistent activity, and the network is able to maintain a memory only of the last shown stimulus. (4) A concerted modulation of GABA(A) and NMDA conductances leads to a decrease of spontaneous activity but an increase of persistent activity; the enhanced signal-to-noise ratio is shown to increase the resistance of the network to distractors. (5) Two mechanisms are identified that produce an inverted U shaped dependence of persistent activity on modulation. The present study therefore points to several mechanisms that enhance the signal-to-noise ratio in working memory states. These mechanisms could be implemented in the prefrontal cortex by dopaminergic projections from the midbrain.

Electrophysiological signs of reduced prefrontal response control in schizophrenic patients

The prefrontal cortex is considered as a brain region important in the etiopathogenesis of schizophrenic disorders. Based on converging results from different research areas, the prefrontal cortex is regarded as the anatomical and functional representation of response control under physiological conditions. In previous studies, a robust electrophysiological marker for the investigation of response control in healthy control subjects was validated. This parameter was termed NoGo anteriorisation and consists of a more anterior peak of the event-related potentials during the inhibition of a prepared motor response (NoGo condition within the Continuous Performance Test) than during its execution (Go condition). The present study investigated these brain electrical correlates of response control in 19 schizophrenic patients and 19 age- and sex-matched healthy subjects. Compared to control subjects, the event-related potentials in schizophrenic patients were located more anterior in the Go condition and, as a trend, more posterior in the NoGo condition. The NoGo anteriorisation was strongly reduced in the schizophrenic group. On a qualitative level, the NoGo anteriorisation was present in all control subjects, but not in eight of the 19 patients. The results were interpreted as an indication of a disturbed prefrontal response control in schizophrenic patients. Further studies will clarify whether this method may be useful as a global test of hypofrontality in different groups of chronic schizophrenias, or as a quantifiable measure of an affected response control system, especially in catatonic subgroups.

Coding specificity in cortical microcircuits: a multiple-electrode analysis of primate prefrontal cortex

Neurons with directional specificities are active in the prefrontal cortex (PFC) during tasks that require spatial working memory. Although the coordination of neuronal activity in PFC is thought to be maintained by a network of recurrent connections, direct physiological evidence regarding such networks is sparse. To gain insight into the functional organization of the working memory system in vivo, we recorded simultaneously from multiple neurons spaced 0.2-1 mm apart in monkeys performing an oculomotor delayed response task. We used cross-correlation analysis and characterized the effective connectivity between neurons in relation to their spatial and temporal response properties. The majority of narrow (<5 msec) cross-correlation peaks indicated common input and were most often observed between pairs of neurons within 0.3 mm of each other. Neurons recorded at these distances represented the full range of spatial locations, suggesting that the entire visual hemifield is represented in modules of corresponding dimensions. Nearby neurons could be activated in any epoch of the behavioral task (stimulus presentation, delay, response). The incidence and strength of cross-correlation, however, was highest among cells sharing similar spatial tuning and similar temporal profiles of activation across task epochs. The dependence of correlated discharge on the functional properties of neurons was observed both when we analyzed firing from the task period as well as from baseline fixation. Our results suggest that the coding specificity of individual neurons extends to the local circuits of which they are part.

Interactions between frontal cortex and basal ganglia in working memory: a computational model

The frontal cortex and the basal ganglia interact via a relatively well understood and elaborate system of interconnections. In the context of motor function, these interconnections can be understood as disinhibiting, or "releasing the brakes," on frontal motor action plans: The basal ganglia detect appropriate contexts for performing motor actions and enable the frontal cortex to execute such actions at the appropriate time. We build on this idea in the domain of working memory through the use of computational neural network models of this circuit. In our model, the frontal cortex exhibits robust active maintenance, whereas the basal ganglia contribute a selective, dynamic gating function that enables frontal memory representations to be rapidly updated in a task-relevant manner. We apply the model to a novel version of the continuous performance task that requires subroutine-like selective working memory updating and compare and contrast our model with other existing models and theories of frontal-cortex-basal-ganglia interactions.

The sensory nature of mnemonic representation in the primate prefrontal cortex

A long-standing issue concerning the function of the primate dorsolateral prefrontal cortex is whether the activity of prefrontal neurons reflects the perceived sensory attributes of a remembered stimulus, or the decision to execute a motor response. To distinguish between these possibilities, we recorded neuronal activity from monkeys trained to make a saccade toward the brighter of two memoranda, under conditions of varied luminance. Our results indicated that during the delay period when sensory information was no longer available, neuronal discharge was modulated by the luminance of the stimulus appearing in the receptive field, and was directly correlated with psychophysical performance in the task. The findings suggest that although prefrontal cortex codes for a diversity of representations, including the decision for an impending response, a population of neurons maintains the dimensional attributes of remembered stimuli throughout the delay period, which allows for flexibility in the outcome of a mnemonic process.

Computational approaches to the architecture and operations of the prefrontal cortical circuit for working memory

1. This article reviews recent progress in the computational studies towards the architecture and operations of the prefrontal cortical circuit, which are keys to understand the mechanisms of working memory processing. 2. The recurrent excitatory connections form closed-loop circuits, which contribute to the sustainment of delay-period activity. These connections subserve the cortical amplification of the activity. 3. Recent experimental studies (Wilson et al. 1994; Rao et al. 1999, 2000) suggested that at least two architectonically distinct types of intracortical inhibition, isodirectional and cross-directional inhibition, play significant roles in the formation of memory fields. 4. Computer simulations of a prefrontal cortical circuit model (Tanaka 1999, 2000a) showed that the isodirectional inhibition in the model regulated the amplitude of memory fields (i.e., the maximum firing rate) while the cross-directional inhibition contributed to the sharpening of the memory fields or the tuning curves. 5. The above characteristics enable the prefrontal cortical circuit to control memory fields, which would be necessary to general working memory processing. It would also be interesting to know whether different subtypes of the interneurons have distinct roles. 6. Another important issue is how neuromodulators contribute to working memory processing. Recent computer simulations by Durstewitz et al. (1999, 2000) showed that stronger dopamine action required stronger intervening input to destroy working memory, suggesting that dopamine contributes to the stabilization of working memory representation. 7. Further elucidation of these issues based on more detailed anatomical data of the cortical circuitry would make the architecture and operations of the prefrontal cortical circuit be more clearly described.

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.

Connections underlying the synthesis of cognition, memory, and emotion in primate prefrontal cortices

Distinct domains of the prefrontal cortex in primates have a set of connections suggesting that they have different roles in cognition, memory, and emotion. Caudal lateral prefrontal areas (areas 8 and 46) receive projections from cortices representing early stages in visual or auditory processing, and from intraparietal and posterior cingulate areas associated with oculomotor guidance and attentional processes. Cortical input to areas 46 and 8 is complemented by projections from the thalamic multiform and parvicellular sectors of the mediodorsal nucleus associated with oculomotor functions and working memory. In contrast, caudal orbitofrontal areas receive diverse input from cortices representing late stages of processing within every unimodal sensory cortical system. In addition, orbitofrontal and caudal medial (limbic) prefrontal cortices receive robust projections from the amygdala, associated with emotional memory, and from medial temporal and thalamic structures associated with long-term memory. Prefrontal cortices are linked with motor control structures related to their specific roles in central executive functions. Caudal lateral prefrontal areas project to brainstem oculomotor structures, and are connected with premotor cortices effecting head, limb and body movements. In contrast, medial prefrontal and orbitofrontal limbic cortices project to hypothalamic visceromotor centers for the expression of emotions. Lateral, orbitofrontal, and medial prefrontal cortices are robustly interconnected, suggesting that they participate in concert in central executive functions. Prefrontal limbic cortices issue widespread projections through their deep layers and terminate in the upper layers of lateral (eulaminate) cortices, suggesting a predominant role in feedback communication. In contrast, when lateral prefrontal cortices communicate with limbic areas they issue projections from their upper layers and their axons terminate in the deep layers, suggesting a role in feedforward communication. Through their widespread connections, prefrontal limbic cortices may exercise a tonic influence on lateral prefrontal cortices, inextricably linking areas associated with cognitive and emotional processes. The integration of cognitive, mnemonic and emotional processes is likely to be disrupted in psychiatric and neurodegenerative diseases which preferentially affect limbic cortices and consequently disconnect major feedback pathways to the neuraxis.

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.

Roles of intracortical inhibition in the formation of spatially tuned delay-period activity of prefrontal cortical neurons: computational study

1. This paper proposes a computational circuit model of the prefrontal cortex, with which computer simulations of the delay-period activity of the prefrontal cortical neurons are made. 2. The aim of this study is to address the question as to how the two types of local inhibition (the parallel inhibition and the anti-parallel inhibition), which were suggested previously and are assumed in this model, contribute to the formation of spatially tuned delay-period activity. 3. The results suggest that the parallel inhibition regulates the level of the delay-period activity, while the anti-parallel inhibition contributes to the sharpening of the activity profile in the delay period. 4. This study found a new prediction that the weakened parallel inhibition causes stronger anti-parallel inhibition of the inactive pyramidal cells due to the disinhibition of the active pyramidal cells. 5. These results suggest the important roles of the intracortical inhibition in the formation and maintenance of spatial working memory.

Neuronal activity in the primate prefrontal cortex in the process of motor selection based on two behavioral rules

This study examined neuronal activity in the prefrontal cortex (PF) involved in the process of motor selection in accordance with two behavioral rules. We trained two monkeys to select a target based on the integration of memorized and current sensory information. Initially, a sample cue (triangle or circle) appeared at one of three locations (top, left, or right) for 1 s. After a 3-s delay, one of two types of choice cue appeared. The first type asked the monkeys to reach for a target by matching the location (location-matching task). The second type asked the monkeys to reach for a target by matching the shape (shape-matching task). The choice cue for location matching consisted of either three circles or three triangles, and the choice cue for shape matching consisted of a circle and a triangle. When the color of the choice cue changed from red to green 1.5 s later (GO signal), the monkeys touched the correct object to obtain a reward. We found cue-, delay-, choice-, and movement-related neuronal activity in the lateral prefrontal cortex. During the sample cue presentation and delay periods, we found selective neuronal activity for the location or shape of the sample cue. Shape-selective neurons were located more anteriorly in the ventral bank of the principal sulcus and inferior convexity area, whereas location-selective neurons were more posteriorly. After the choice cue appeared, we found three main types of neuronal activity in the critical period when the subject selected the future target: 1) activity reflecting past sensory information (the location or shape of the sample cue presented 3 s earlier), 2) activity selective for the configuration of the current choice cue, and 3) activity reflecting the properties (location or shape) of the future target. During the motor-response period, we found neuronal activity selective for the location or shape of the reaching target. When muscimol was microinjected into the ventral bank of principal sulcus and inferior convexity area, the performance of both tasks was impaired. Furthermore, we found that the wealth of neuronal activity in the PF that seemed to play a role in motor selection was rarely seen in the primary motor cortex.

Composition and topographic organization of signals sent from the frontal eye field to the superior colliculus

The frontal eye field (FEF) and superior colliculus (SC) contribute to saccadic eye movement generation, and much of the FEF's oculomotor influence may be mediated through the SC. The present study examined the composition and topographic organization of signals flowing from FEF to SC by recording from FEF neurons that were antidromically activated from rostral or caudal SC. The first and most general result was that, in a sample of 88 corticotectal neurons, the types of signals relayed from FEF to SC were highly diverse, reflecting the general population of signals within FEF rather than any specific subset of signals. Second, many neurons projecting from FEF to SC carried signals thought to reflect cognitive operations, namely tonic discharges during the delay period of a delayed-saccade task (delay signals), elevated discharges during the gap period of a gap task (gap increase signals), or both. Third, FEF neurons discharging during fixation were found to project to the SC, although they did not project preferentially to rostral SC, where similar fixation neurons are found. Neurons that did project preferentially to the rostral SC were those with foveal visual responses and those pausing during the gap period of the gap task. Many of the latter neurons also had foveal visual responses, presaccadic pauses in activity, and postsaccadic increases in activity. These two types of rostral-projecting neurons therefore may contribute to the activity of rostral SC fixation neurons. Fourth, conduction velocity was used as an indicator of cell size to correct for sampling bias. The outcome of this correction procedure suggested that among the most prevalent neurons in the FEF corticotectal population are those carrying putative cognitive-related signals, i.e., delay and gap increase signals, and among the least prevalent are those carrying presaccadic burst discharges but lacking peripheral visual responses. Fifth, corticotectal neurons carrying various signals were biased topographically across the FEF. Neurons with peripheral visual responses but lacking presaccadic burst discharges were biased laterally, neurons with presaccadic burst discharges but lacking peripheral visual responses were biased medially, and neurons carrying delay or gap increase signals were biased dorsally. Finally, corticotectal neurons were distributed within the FEF as a function of their visual or movement field eccentricity and projected to the SC such that eccentricity maps in both structures were closely aligned. We conclude that the FEF most likely influences the activity of SC neurons continuously from the start of fixation, through visual analysis and cognitive manipulations, until a saccade is generated and fixation begins anew. Furthermore, the projection from FEF to SC is highly topographically organized in terms of function at both its source and its termination.

Basal ganglia and cerebellar loops: motor and cognitive circuits

The traditional view that the basal ganglia and cerebellum are simply involved in the control of movement has been challenged in recent years. One of the pivotal reasons for this reappraisal has been new information about basal ganglia and cerebellar connections with the cerebral cortex. In essence, recent anatomical studies have revealed that these connections are organized into discrete circuits or 'loops'. Rather than serving as a means for widespread cortical areas to gain access to the motor system, these loops reciprocally interconnect a large and diverse set of cerebral cortical areas with the basal ganglia and cerebellum. The properties of neurons within the basal ganglia or cerebellar components of these circuits resembles the properties of neurons within the cortical areas subserved by these loops. For example, neuronal activity within basal ganglia and cerebellar loops with motor areas of the cerebral cortex is highly correlated with parameters of movement, while neuronal activity within basal ganglia and cerebellar loops with areas of the prefrontal cortex is more related to aspects of cognitive function. Thus, individual loops appear to be involved in distinct behavioral functions. Studies of basal ganglia and cerebellar pathology support this conclusion. Damage to the basal ganglia or cerebellar components of circuits with motor areas of cortex leads to motor symptoms, whereas damage of the subcortical components of circuits with non-motor areas of cortex causes higher-order deficits. In this report, we review some of the new anatomical, physiological and behavioral findings that have contributed to a reappraisal of function concerning the basal ganglia and cerebellar loops with the cerebral cortex.

Basal ganglia output and cognition: evidence from anatomical, behavioral, and clinical studies

The traditional view that the basal ganglia are simply involved in the control of movement has been challenged in recent years. Three lines of evidence indicate that the basal ganglia also are involved in nonmotor operations. First, the results of anatomical studies clearly indicate that the basal ganglia participate in multiple circuits or 'loops' with cognitive areas of the cerebral cortex. Second, the activity of neurons within selected portions of the basal ganglia is more related to cognitive or sensory operations than to motor functions. Finally, in some instances basal ganglia lesions cause primarily cognitive or sensory disturbances without gross motor impairments. In this report, we briefly review some of these data and present a new anatomical framework for understanding the basal ganglia contributions to nonmotor function.

Synaptic basis of cortical persistent activity: the importance of NMDA receptors to working memory

Delay-period activity of prefrontal cortical cells, the neural hallmark of working memory, is generally assumed to be sustained by reverberating synaptic excitation in the prefrontal cortical circuit. Previous model studies of working memory emphasized the high efficacy of recurrent synapses, but did not investigate the role of temporal synaptic dynamics. In this theoretical work, I show that biophysical properties of cortical synaptic transmission are important to the generation and stabilization of a network persistent state. This is especially the case when negative feedback mechanisms (such as spike-frequency adaptation, feedback shunting inhibition, and short-term depression of recurrent excitatory synapses) are included so that the neural firing rates are controlled within a physiological range (10-50 Hz), in spite of the exuberant recurrent excitation. Moreover, it is found that, to achieve a stable persistent state, recurrent excitatory synapses must be dominated by a slow component. If neuronal firings are asynchronous, the synaptic decay time constant needs to be comparable to that of the negative feedback; whereas in the case of partially synchronous dynamics, it needs to be comparable to a typical interspike interval (or oscillation period). Slow synaptic current kinetics also leads to the saturation of synaptic drive at high firing frequencies that contributes to rate control in a persistent state. For these reasons the slow NMDA receptor-mediated synaptic transmission is likely required for sustaining persistent network activity at low firing rates. This result suggests a critical role of the NMDA receptor channels in normal working memory function of the prefrontal cortex.

Spatial working memory assessment by a visual-manual delayed response task: a controlled study in schizophrenia

'Working memory' dysfunction has been proposed as a central cognitive feature in schizophrenia. To further explore this issue we developed a computerized easy and fast to administer test using the standard keyboard as visual-manual subject-computer interface along a delayed-response paradigm. The test has been administered to 25 patients who met the DSM-III-R criteria for schizophrenia and 25 healthy control subjects matched as possible for sex. The data confirm the visuo-spatial 'working memory' dysfunction in schizophrenic patients. The test maintains the discriminative capacity of similar previously devised tasks with the advantages of being usable on almost every standard computer and shorter and more acceptable for severely disabled patients also. The test can be considered an useful tool to study the 'working memory' impairment in the cognitive deficit of schizophrenia.

Architecture and dynamics of the primate prefrontal cortical circuit for spatial working memory

In the experimental protocol of working memory tasks using a monkey as a subject, tuned activity of the prefrontal cortical neurons that is sustained during the delay period is a neuronal substrate of the working memory. This study addresses the question as to how this tuned activity is formed and maintained in the prefrontal cortex by means of computer simulations of the dynamics of a model prefrontal cortical circuit. The model assumes that pyramidal cells receive two types of intracortical inhibition, "parallel" and "anti-parallel", in accordance with recent experimental findings. The parallel and anti-parallel refer to the relationship between the preferred directions of presynaptic interneurons and postsynaptic pyramidal cells. The following three factors are suggested to be crucial for the formation and maintenance of spatial working memory: cortical amplification of the activity due to excitatory closed-loop circuitry, suppression of excessive excitation by the parallel inhibition, and sharpening of the activity profile by the anti-parallel inhibition.

Medial prefrontal cortices are unified by common connections with superior temporal cortices and distinguished by input from memory-related areas in the rhesus monkey

Medial prefrontal cortices in primates have been associated with emotion, memory, and complex cognitive processes. Here we investigated whether the pattern of cortical connections could indicate whether the medial prefrontal cortex constitutes a homogeneous region, or if it can be parceled into distinct sectors. Projections from medial temporal memory-related cortices subdivided medial cortices into different sectors, by targeting preferentially caudal medial areas (area 24, caudal 32 and 25), to a lesser extent rostral medial areas (rostral area 32, areas 14 and 10), and sparsely area 9. Area 9 was distinguished by its strong connections with premotor cortices. Projections from unimodal sensory cortices reached preferentially specific medial cortices, including a projection from visual cortices to area 32/24, from somatosensory cortices to area 9, and from olfactory cortices to area 14. Medial cortices were robustly interconnected, suggesting that local circuits are important in the neural processing in this region. Medial prefrontal cortices were unified by bidirectional connections with superior temporal cortices, including auditory areas. Auditory pathways may have a role in the specialization of medial prefrontal cortices in species-specific communication in non-human primates and language functions in humans.

Cognition and control in schizophrenia: a computational model of dopamine and prefrontal function

Behavioral deficits suffered by patients with schizophrenia in a wide array of cognitive domains can be conceptualized as failures of cognitive control, due to an impaired ability to internally represent, maintain, and update context information. A theory is described that postulates a single neurobiological mechanism for these disturbances, involving dysfunctional interactions between the dopamine neurotransmitter system and the prefrontal cortex. Specifically, it is hypothesized that in schizophrenia, there is increased noise in the activity of the dopamine system, leading to abnormal "gating" of information into prefrontal cortex. The theory is implemented as an explicit connectionist computational model that incorporates the roles of both dopamine and prefrontal cortex in cognitive control. A simulation is presented of behavioral performance in a version of the Continuous Performance Test specifically adapted to measure critical aspects of cognitive control function. Schizophrenia patients exhibit clear behavioral deficits on this task that reflect impairments in both the maintenance and updating of context information. The simulation results suggest that the model can successfully account for these impairments in terms of abnormal dopamine activity. This theory provides a potential point of contact between research on the neurobiological and psychological aspects of schizophrenia, by illustrating how a particular physiological disturbance might lead to precise and quantifiable consequences for behavior.

A neural network model with dopamine-like reinforcement signal that learns a spatial delayed response task

This study investigated how the simulated response of dopamine neurons to reward-related stimuli could be used as reinforcement signal for learning a spatial delayed response task. Spatial delayed response tasks assess the functions of frontal cortex and basal ganglia in short-term memory, movement preparation and expectation of environmental events. In these tasks, a stimulus appears for a short period at a particular location, and after a delay the subject moves to the location indicated. Dopamine neurons are activated by unpredicted rewards and reward-predicting stimuli, are not influenced by fully predicted rewards, and are depressed by omitted rewards. Thus, they appear to report an error in the prediction of reward, which is the crucial reinforcement term in formal learning theories. Theoretical studies on reinforcement learning have shown that signals similar to dopamine responses can be used as effective teaching signals for learning. A neural network model implementing the temporal difference algorithm was trained to perform a simulated spatial delayed response task. The reinforcement signal was modeled according to the basic characteristics of dopamine responses to novel stimuli, primary rewards and reward-predicting stimuli. A Critic component analogous to dopamine neurons computed a temporal error in the prediction of reinforcement and emitted this signal to an Actor component which mediated the behavioral output. The spatial delayed response task was learned via two subtasks introducing spatial choices and temporal delays, in the same manner as monkeys in the laboratory. In all three tasks, the reinforcement signal of the Critic developed in a similar manner to the responses of natural dopamine neurons in comparable learning situations, and the learning curves of the Actor replicated the progress of learning observed in the animals. Several manipulations demonstrated further the efficacy of the particular characteristics of the dopamine-like reinforcement signal. Omission of reward induced a phasic reduction of the reinforcement signal at the time of the reward and led to extinction of learned actions. A reinforcement signal without prediction error resulted in impaired learning because of perseverative errors. Loss of learned behavior was seen with sustained reductions of the reinforcement signal, a situation in general comparable to the loss of dopamine innervation in Parkinsonian patients and experimentally lesioned animals. The striking similarities in teaching signals and learning behavior between the computational and biological results suggest that dopamine-like reward responses may serve as effective teaching signals for learning behavioral tasks that are typical for primate cognitive behavior, such as spatial delayed responding.

Association of storage and processing functions in the dorsolateral prefrontal cortex of the nonhuman primate

The prominent role of the prefrontal cortex (PFC) in working memory (WM) is widely acknowledged both in nonhuman primates and in humans. However, less agreement exists on the issue of functional segregation within different subregions of the PFC with regard to the domains of spatial and nonspatial processing or involvement in simpler versus more complex aspects of WM, e.g., maintenance versus processing function. To address these issues, six monkeys were trained to perform four WM tasks that differed with respect to domain (spatial vs nonspatial) and level of WM demand (recall of one vs three items). The delayed response format was used to assess simple one-item memory, whereas self-ordering tasks were used to require the monkey to maintain and organize three items of information within WM. After training, the monkeys received bilateral PFC lesions in one of two different areas, Walker's areas 9 and 8B (dorsomedial convexity; n = 3) or areas 46 and 8A (dorsolateral cortex, n = 3) and then tested postoperatively on all tasks. Monkeys with lesions of the dorsomedial convexity were not impaired either on spatial or nonspatial WM tasks, whether the task required simple storage or sequential processing. By contrast, lesions of the dorsolateral cortex produced a significant and persistent impairment in both simple and complex spatial WM but no impairment in the two nonspatial WM tasks. These results support a functional segregation within the dorsolateral prefrontal cortex for WM: the dorsolateral prefrontal cortex (area 46/8A) is selectively involved in spatial WM, whereas the dorsomedial convexity (area 9/8B) is not critically engaged in either spatial or nonspatial working memory. Furthermore, the specific involvement of area 46/8A in spatial sequencing as well as in single-item storage WM tasks supports, in the nonhuman primate, an areal dissociation based on domain rather than on processing demand.

Reactivation of hippocampal cell assemblies: effects of behavioral state, experience, and EEG dynamics

During slow wave sleep (SWS), traces of neuronal activity patterns from preceding behavior can be observed in rat hippocampus and neocortex. The spontaneous reactivation of these patterns is manifested as the reinstatement of the distribution of pairwise firing-rate correlations within a population of simultaneously recorded neurons. The effects of behavioral state [quiet wakefulness, SWS, and rapid eye movement (REM)], interactions between two successive spatial experiences, and global modulation during 200 Hz electroencephalographic (EEG) "ripples" on pattern reinstatement were studied in CA1 pyramidal cell population recordings. Pairwise firing-rate correlations during often repeated experiences accounted for a significant proportion of the variance in these interactions in subsequent SWS or quiet wakefulness and, to a lesser degree, during SWS before the experience on a given day. The latter effect was absent for novel experiences, suggesting that a persistent memory trace develops with experience. Pattern reinstatement was strongest during sharp wave-ripple oscillations, suggesting that these events may reflect system convergence onto attractor states corresponding to previous experiences. When two different experiences occurred in succession, the statistically independent effects of both were evident in subsequent SWS. Thus, the patterns of neural activity reemerge spontaneously, and in an interleaved manner, and do not necessarily reflect persistence of an active memory (i.e., reverberation). Firing-rate correlations during REM sleep were not related to the preceding familiar experience, possibly as a consequence of trace decay during the intervening SWS. REM episodes also did not detectably influence the correlation structure in subsequent SWS, suggesting a lack of strengthening of memory traces during REM sleep, at least in the case of familiar experiences.

The "psychic" neuron of the cerebral cortex

Remarkable advances in the identification, cloning, and localization of ion channels and receptors in the central nervous system have opened up unprecedented possibilities for relating structure to physiological function at the subcellular level of analysis. A singularly advanced property of select central nervous system neurons is their ability to exhibit increases in firing rate in relation to the mnemonic trace of a preceding event, a property that has been referred to as "working memory." Single-cell recordings from the prefrontal cortex of nonhuman primates have revealed neurons in the prefrontal cortex that possess "memory fields" analogous to the receptive field properties of sensory neurons. The integrity of these neurons has been shown to be essential for accurate performance in memory tasks performed by trained monkeys (and humans). We can now show that the excitability and/or tuning of these prefrontal neurons are subject to modulatory influences by dopamine, serotonin, GABA, and glutamate among other peptides and conventional neurotransmitters. I will describe the dopaminergic, serotonergic, and GABAergic innervation of pyramidal neurons engaged in working memory and the localization of neurotransmitter receptors through which they exert their actions. The findings reveal a remarkable degree of diversity in the subcellular localization and functionality of the five cloned dopamine receptors (D1, D2, D3, D4, and D5) and two serotonin (5HT2A and 5HT3) receptors that have been examined to date. The potential now exists for linking systems neurobiology with molecular biophysics to comprehend the highest functions of information processing that distinguish our species.

Relative reward preference in primate orbitofrontal cortex

The orbital part of prefrontal cortex appears to be crucially involved in the motivational control of goal-directed behaviour. Patients with lesions of orbitofrontal cortex show impairments in making decisions about the expected outcome of actions. Monkeys with orbitofrontal lesions respond abnormally to changes in reward expectations and show altered reward preferences. As rewards constitute basic goals of behaviour, we investigated here how neurons in the orbitofrontal cortex of monkeys process information about liquid and food rewards in a typical frontal task, spatial delayed responding. The activity of orbitofrontal neurons increases in response to reward-predicting signals, during the expectation of rewards, and after the receipt of rewards. Neurons discriminate between different rewards, mainly irrespective of the spatial and visual features of reward-predicting stimuli and behavioural reactions. Most reward discriminations reflect the animals' relative preference among the available rewards, as expressed by their choice behaviour, rather than physical reward properties. Thus, neurons in the orbitofrontal cortex appear to process the motivational value of rewarding outcomes of voluntary action.

Control of resolution and perception in working memory

Mechanisms underlying and controlling resolution and perception in working memory are studied by means of a pulse-coupled network model. It is shown that the adaptivity, i.e. the degree to which previous activity affects the ability to fire, of the excitatory units can control several aspects of the network dynamics in a coordinated way to enable multiple items to be resolved and perceived in working memory. One basic aspect is the complexity of the dynamics that regulates the temporal resolution of several items. The slow NMDA-receptor-mediated component of synaptic couplings to excitatory units facilitates successive activations of a given item. The dimension of the activated subspace of the complete available neural representation space is gradually decreased as adaptivity is reduced. It is also shown that the formation of perception by sufficiently intense and coherent activation of different features of an object can be controlled concurrently with resolution by the adaptivity. The mechanisms derived can account for the observed capacity of working memory with respect to number of items consciously resolved and also for the observed temporal separation of different items. Numerous observations link neuromodulators to cognitive functions and to various brain disorders involving working memory. Based on the influence of various neuromodulators on neuronal adaptivity, the model can also account for neuromodulatory regulation of working memory functions.

Load-dependent roles of frontal brain regions in the maintenance of working memory

Brain imaging studies have suggested a critical role for prefrontal cortex in working memory (WM) tasks that require both maintainenance and manipulation of information over time in delayed-response WM tasks. In the present study, functional magnetic resonance imaging (fMRI) was used to examine whether prefrontal areas are activated when only maintenance is required in a delayed-response WM task, without the overt requirement to manipulate the stored information. In two scans, six subjects performed WM tasks in which, on each trial, they (1) encoded 1, 3, or 6 to-be-remembered letters, (2) maintained these letters across a 5-second unfilled delay, and (3) determined whether a single probe letter was or was not part of the memory set. Activation of left caudal inferior frontal gyrus was observed, relative to the 1-letter task, when subjects were required to maintain 3 letters in WM. When subjects were required to maintain 6 letters in WM, additional prefrontal areas, most notably middle and superior frontal gyri, were activated bilaterally. Thus, increasing the amount of to-be-maintained information, without any overt manipulation requirement, resulted in the recruitment of wide-spread frontal-lobe regions. Inferior frontal gyrus activation was left-hemisphere dominant in both the 3- and 6-letter conditions, suggesting that such activation reflected material-specific verbal processes. Activation in middle and superior frontal gyri appeared only in the 6-letter condition and was right-hemisphere dominant, suggesting that such activation reflected material-independent executive processes.

Cognitive and motivational operations in primate prefrontal neurons

To investigate neuronal mechanisms of higher cognitive activity, single neuronal activity has been recorded from the prefrontal cortex of monkeys trained in cognitive tasks. It has been well documented that there are prefrontal neurons which are involved in cognitive operations such as coding the meaning of a stimulus or retaining information in working memory. On the other hand, there are also prefrontal neurons which show reward-related activity changes. Two kinds of reward-related activities are found in the primate prefrontal cortex; one kind is concerned with processing post-trial events such as coding the reinforcement and/or error, and the other is concerned with the expectancy of the specific reward. These reward-related activities, which appear to be involved in motivational operations, are related also to cognitive operations such as coding the context in which the reward is given or omitted, or monitoring the context concerning the task situation with which the animal is faced. It seems that the prefrontal cortex is involved in the integration of cognitive and motivational information for goal-directed behavior. While the function of the prefrontal cortex is often associated with working memory, I introduce the idea that the prefrontal cortex plays important roles for coding, monitoring and providing the context for goal-directed behavior.

Memory fields of neurons in the primate prefrontal cortex

Many prefrontal (PF) neurons convey information about both an object's identity (what) and its location (where). To explore how they represent conjunctions of what and where, we explored the receptive fields of their mnemonic activity (i.e., their "memory fields") by requiring monkeys to remember both an object and its location at many positions throughout a wide portion of central vision. Many PF neurons conveyed object information and had highly localized memory fields that emphasized the contralateral, but not necessarily foveal, visual field. These results indicate that PF neurons can simultaneously convey precise location and object information and thus may play a role in constructing a unified representation of a visual scene.

A model of visuospatial working memory in prefrontal cortex: recurrent network and cellular bistability

We report a computer simulation of the visuospatial delayed-response experiments of Funahashi et al. (1989), using a firing-rate model that combines intrinsic cellular bistability with the recurrent local network architecture of the neocortex. In our model, the visuospatial working memory is stored in the form of a continuum of network activity profiles that coexist with a spontaneous activity state. These neuronal firing patterns provide a population code for the cue position in a graded manner. We show that neuronal persistent activity and tuning curves of delay-period activity (memory fields) can be generated by an excitatory feedback circuit and recurrent synaptic inhibition. However, if the memory fields are constructed solely by network mechanisms, noise may induce a random drift over time in the encoded cue position, so that the working memory storage becomes unreliable. Furthermore, a "distraction" stimulus presented during the delay period produces a systematic shift in the encoded cue position. We found that the working memory performance can be rendered robust against noise and distraction stimuli if single neurons are endowed with cellular bistability (presumably due to intrinsic ion channel mechanisms) that is conditional and realized only with sustained synaptic inputs from the recurrent network. We discuss how cellular bistability at the single cell level may be detected by analysis of spike trains recorded during delay-period activity and how local modulation of intrinsic cell properties and/or synaptic transmission can alter the memory fields of individual neurons in the prefrontal cortex.

Opioid effects on activation of neurons in the medial prefrontal cortex

1. The effects of opioids have been characterized in portions of the neural circuitry proposed to underly the development and maintenance of addiction. One possible mechanism is modulation of function of endogenous transmitters. 2. Cells in the prefrontal cortex, a brain area involved in cognitive function and processes relevant to addiction, are described that exhibit morphine-associated attenuation of activation response to glutamate but not acetylcholine. 3. The predominantly excitatory response of prefrontal cortical cells to local application of glutamate and acetylcholine were differentially modified by systemic and local application of opioids. 4. Local mu opioid effects mimic those of systemic morphine to a more limited degree. 5. Morphine attenuates the response of prefrontal cortical cells to activation of excitatory afferents from the mediodorsal thalamus, and to a lesser degree, from the basolateral amygdala and the hippocampus. 6. Morphine modulation of prefrontal excitatory activation is naloxone-reversible.

Lessons from the Human Brain: Neuronal Activity Related to Cognition

Several special clinical settings provide opportunities for extracellular recording of neuronal activity in the human brain during measures of cognition. The limited experience with recordings obtained from human temporal and frontal cortex, medial temporal lobe, and subcortical structures in association with language, visuospatial processes, memory, learning, and music is reviewed here. The frequency of activity in a high proportion of neurons changes with a specific behavior. These neurons are widely distributed in both hemispheres. Relative inhibition of activity is prominent in cortical recordings made during language measures, particularly in the dominant hemisphere. Widespread excitation is prominent in recordings made during measures of recent explicit memory and learning. However, any individual neuron often has a narrow behavioral repertory, with activation to only one specific behavior in a range of behaviors. Some of the behaviors associated with consistent changes are not intuitively obvious. Nearby neurons often have different behavioral repertories. A few patterns of activity that may represent specific coding for a behavior, in some cases even sparse coding, have been identified. Neuronal recording in humans during cognitive measures provides an additional perspective on the neurobiological substrate of cognition that complements findings from other techniques.

Functional MRI studies of spatial and nonspatial working memory

Single-unit recordings in monkeys have revealed neurons in the lateral prefrontal cortex that increase their firing during a delay between the presentation of information and its later use in behavior. Based on monkey lesion and neurophysiology studies, it has been proposed that a dorsal region of lateral prefrontal cortex is necessary for temporary storage of spatial information whereas a more ventral region is necessary for the maintenance of nonspatial information. Functional neuroimaging studies, however, have not clearly demonstrated such a division in humans. We present here an analysis of all reported human functional neuroimaging studies plotted onto a standardized brain. This analysis did not find evidence for a dorsal/ventral subdivision of prefrontal cortex depending on the type of material held in working memory, but a hemispheric organization was suggested (i.e., left-nonspatial; right-spatial). We also performed functional MRI studies in 16 normal subjects during two tasks designed to probe either nonspatial or spatial working memory, respectively. A group and subgroup analysis revealed similarly located activation in right middle frontal gyrus (Brodmann's area 46) in both spatial and nonspatial [working memory-control] subtractions. Based on another model of prefrontal organization [M. Petrides, Frontal lobes and behavior, Cur. Opin. Neurobiol., 4 (1994) 207-211], a reconsideration of the previous imaging literature data suggested that a dorsal/ventral subdivision of prefrontal cortex may depend upon the type of processing performed upon the information held in working memory.

Selective representation of relevant information by neurons in the primate prefrontal cortex

The severe limitation of the capacity of working memory, the ability to store temporarily and manipulate information, necessitates mechanisms that restrict access to it. Here we report tests to discover whether the activity of neurons in the prefrontal (PF) cortex, the putative neural correlate of working memory, might reflect these mechanisms and preferentially represent behaviourally relevant information. Monkeys performed a 'delayed-matching-to-sample' task with an array of three objects. Only one of the objects in the array was relevant for task performance and the monkeys needed to find that object (the target) and remember its location. For many PF neurons, activity to physically identical arrays varied with the target location; the location of the non-target objects had little or no influence on activity. Information about the target location was present in activity as early as 140ms after array onset. Also, information about which object was the target was reflected in the sustained activity of many PF neurons. These results suggest that the prefrontal cortex is involved in selecting and maintaining behaviourally relevant information.

Areal segregation of face-processing neurons in prefrontal cortex

A central issue in cognitive neuroscience concerns the functional architecture of the prefrontal cortex and the degree to which it is organized by sensory domain. To examine this issue, multiple areas of the macaque monkey prefrontal cortex were mapped for selective responses to visual stimuli that are prototypical of the brain's object vision pathway-pictorial representations of faces. Prefrontal neurons not only selectively process information related to the identity of faces but, importantly, such neurons are localized to a remarkably restricted area. These findings suggest that the prefrontal cortex is functionally compartmentalized with respect to the nature of its inputs.

Increase of extracellular dopamine in primate prefrontal cortex during a working memory task

Increase of extracellular dopamine in primate prefrontal cortex during a working memory task. J. Neurophysiol. 78: 2795-2798, 1997. The dopamine innervation of the prefrontal cortex is involved importantly in cognitive processes, such as tested in working memory tasks. However, there have been no studies directly investigating prefrontal dopamine levels in relation to cognitive processes. We measured frontal extracellular dopamine concentration using in vivo microdialysis in monkeys performing in a delayed alternation task as a typical working memory paradigm and in a sensory-guided control task. We observed a significant increase in dopamine level in the delayed alternation task as compared both with the sensory-guided control task and the basal resting level. The increase was seen in the dorsolateral prefrontal but not in the arcuate or orbitofrontal areas. The increase appeared to reflect the working memory component of the task and was observed mainly in the lip areas of principal sulcus. Although there was no significant difference in dopamine level between delayed alternation and sensory-guided control tasks in the premotor area, significant increases in dopamine concentration were observed during both tasks as compared with the basal resting level, indicating the importance of premotor dopamine for the motor response itself.

Integration of what and where in the primate prefrontal cortex

The visual system separates processing of an object's form and color ("what") from its spatial location ("where"). In order to direct action to objects, the identity and location of those objects must somehow be integrated. To examine whether this process occurs within the prefrontal (PF) cortex, the activity of 195 PF neurons was recorded during a task that engaged both what and where working memory. Some neurons showed either object-tuned (what) or location-tuned (where) delay activity. However, over half (52 percent, or 64/123) of the PF neurons with delay activity showed both what and where tuning. These neurons may contribute to the linking of object information with the spatial information needed to guide behavior.

Single neuron activity in human hippocampus and amygdala during recognition of faces and objects

The hippocampus and its associated structures play a key role in human memory, yet the underlying neuronal mechanisms remain unknown. Here, we report that during encoding and recognition, single neurons in the medial temporal lobe discriminated faces from inanimate objects. Some units responded selectively to specific emotional expressions or to conjunctions of facial expression and gender. Such units were especially prevalent during recognition, and the responses depended on stimulus novelty or familiarity. Traces of exposure to faces or objects were found a few seconds after stimulus removal as well as 10 hr later. Some neurons maintained a record of previous stimulus presentation that was more accurate than the person's conscious recollection. We propose that the human medial temporal lobe constructs a "cognitive map" of stimulus attributes comparable to the map of the spatial environment described in the rodent hippocampus.

Delay-period activity in the primate prefrontal cortex encoding multiple spatial positions and their order of presentation

To investigate whether prefrontal neurons temporarily retain information regarding multiple spatial positions, single-neuron activity was recorded while monkeys performed a delayed sequential reaching task, in which they needed to remember two cue positions out of three and their temporal order of presentation. Most neurons were also tested on a conventional delayed reaching task, in which they needed to remember one cue position during the delay. Among 72 neurons that exhibited significant delay-period activity, one group of neurons (n = 19) exhibited delay-period activity only when a visual cue was presented at one of the three positions (position-dependent). Of these, 6 neurons exhibited this activity when a cue was presented at that position independent of the temporal order, whereas 13 neurons exhibited this activity only when a cue was presented at that position in a particular temporal order (e.g., as the first cue or the second cue). Another group of neurons (n = 39) exhibited delay-period activity only when visual cues were presented at two positions out of three (pair-dependent). Of these, 7 neurons exhibited pair-dependent activity independent of the temporal order of cue presentation. However, 32 neurons exhibited this activity only when two cues were presented in a particular temporal order. The remaining 11 neurons exhibited non-differential activity during the delay period and 3 neurons exhibited miscellaneous activity. These results show that a single prefrontal neuron can retain information regarding two spatial positions, and that, to retain two spatial positions and the temporal order of cue presentation, new types of delay-period activity emerged; i.e., pair-dependent activity and temporal order-dependent activity. Both types of activity could be a mechanism for simultaneously retaining two items of spatial information and for effectively combining multiple spatial information by a single neuron. In addition, the presence of delay-period activity with position-dependency, pair-dependency and temporal order-dependency suggests that the dorsolateral prefrontal cortex plays an important role in planning sequential behaviors.

Dissociation of mnemonic coding and other functional neuronal processing in the monkey prefrontal cortex

Single-neuron activity was recorded in the prefrontal cortex of three monkeys during the performance of a spatial delayed alternation (DA) task and during the presentation of a variety of visual, auditory, and somatosensory stimuli. The aim was to study the relationship between mnemonic neuronal processing and other functional neuronal responsiveness at the single-neuron level in the prefrontal cortex. Recordings were performed in both experimental situations from 152 neurons. The majority of the neurons (92%) was recorded in the prefrontal cortex. Nine of the neurons were recorded in the dorsal bank of the anterior cingulate sulcus and two in the premotor cortex. Of the total number of neurons recorded in the prefrontal area, 32% fired in relation to the DA task performance and 39% were responsive to sensory stimulation or to the movements of the monkey outside of the memory task context. Altogether 42% of the recorded neurons were neither activated by the various stimuli nor by the DA task performance. Three types of task-related neuronal activity were recorded: delay related, delay and movement related, and movement related. The majority of the task-related neurons (n = 33, 73%) fired in relation to the delay period. Of the delay-related neurons, 26 (79%) were spatially selective. The number of spatially selective delay-related neurons of the whole population of recorded neurons was 18%. Twelve task-related neurons (27%) fired in relation to the response period of the DA task. Five of these neurons changed their firing rate during the delay period and were classified as delay/movement-related neurons. Contrary to the delay-related neurons, less than half (42%) of the response-related neurons were spatially selective. The majority (70%) of the delay-related neurons could not be activated by any of the sensory stimuli used and did not fire in relation to the movements of the monkey. The remaining portion of the delay-related neurons was activated by stationary and moving visual stimuli or by visual fixation of an object. In contrast to the delay-related neurons, the majority (66%) of the task-related neurons firing in relation to the movement period were also responsive to sensory stimulation outside of the task context. The majority of these neurons responded to visual stimulation, visual fixation of an object, or tracking eye movements. One neuron gave a somatomotor and another a polysensory response. The majority (n = 37, 67%) of all neurons responding to stimulation outside of the task context did not fire in relation to the DA task performance. The majority of their responses was elicited by visual stimuli or was related to visual fixation of an object or to eye movements. Only six neurons fired in relation to auditory, somatosensory, or somatomotor stimulation. This study provides further evidence about the significance of the dorsolateral prefrontal cortex in spatial working memory processing. Although a considerable number of all DA task-related neurons responded to visual, somatosensory, and auditory stimulation or to the movements of the monkey, most delay-related neurons engaged in the spatial DA task did not respond to extrinsic sensory stimulation. These results indicate that most prefrontal neurons firing selectively during the delay phase of the DA task are highly specialized and process only task-related information.

Patterns of intrinsic and associational circuitry in monkey prefrontal cortex

Both local and long-range connections are critical mediators of information processing in the cerebral cortex, but little is known about the relationships among these types of connections, especially in higher-order cortical regions. We used quantitative reconstructions of the label arising from discrete (approximately 350 microns diameter) injections of biotinylated dextran amine and cholera toxin B to determine the spatial organization of the axon collaterals and principal axon projections furnished by pyramidal neurons in the supragranular layers of monkey prefrontal cortex (areas 9 and 46). Both terminals and cell bodies labeled by transport along axon collaterals in the gray matter formed intrinsic clusters which were arrayed as a series of discontinuous stripes of similar size and shape. The co-registration of anterograde and retrograde transport confirmed that these convergent and divergent intrinsic connections also were reciprocal. Transport from the same injection sites along principal axons through the white matter formed associational clusters which were also arrayed as a series of discontinuous stripes. The dimensions of the anterogradely- and retrogradely-labeled associational stripes were very similar to each other and to the intrinsic stripes. These findings demonstrate that divergence, convergence, and reciprocity characterize both the intrinsic and associational excitatory connections in the prefrontal cortex. These patterns of connections provide an anatomical substrate by which activation of a discrete group of neurons would lead to the recruitment of a specific neuronal network comprised of both local and distant groups of cells. Furthermore, the consistent size of the intrinsic and associational stripes (approximately 275 by 1,800 microns) suggests that they may represent basic functional units in the primate prefrontal cortex.

Neural mechanisms of visual working memory in prefrontal cortex of the macaque

Prefrontal (PF) cells were studied in monkeys performing a delayed matching to sample task, which requires working memory. The stimuli were complex visual patterns and to solve the task, the monkeys had to discriminate among the stimuli, maintain a memory of the sample stimulus during the delay periods, and evaluate whether a test stimulus matched the sample presented earlier in the trial. PF cells have properties consistent with a role in all three of these operations. Approximately 25% of the cells responded selectively to different visual stimuli. Half of the cells showed heightened activity during the delay after the sample and, for many of these cells, the magnitude of delay activity was selective for different samples. Finally, more than half of the cells responded differently to the test stimuli depending on whether they matched the sample. Because inferior temporal (IT) cortex also is important for working memory, we compared PF cells with IT cells studied in the same task. Compared with IT cortex, PF responses were less often stimulus-selective but conveyed more information about whether a given test stimulus was a match to the sample. Furthermore, sample-selective delay activity in PF cortex was maintained throughout the trial even when other test stimuli intervened during the delay, whereas delay activity in IT cortex was disrupted by intervening stimuli. The results suggest that PF cortex plays a primary role in working memory tasks and may be a source of feedback inputs to IT cortex, biasing activity in favor of behaviorally relevant stimuli.