Delay-related activity of prefrontal neurons in rhesus monkeys performing delayed response

Medial prefrontal cortex lesions abolish contextual control of competing responses

There is much debate as to the extent and nature of functional specialization within the different subregions of the prefrontal cortex. The current study was undertaken to investigate the effect of damage to medial prefrontal cortex subregions in the rat. Rats were trained on two biconditional discrimination tasks, one auditory and one visual, in two different contexts. At test, they received presentations of audiovisual compounds of these training stimuli in extinction. These compounds had dictated either the same (congruent trials) or different (incongruent trials) responses during training. In sham-operated controls, contextual cues came to control responding to conflicting information provided by incongruent stimulus compounds. Experiment 1 demonstrated that this contextual control of responding was not evident in individual rats with large amounts of damage that included the prelimbic and cingulate subregions of the prefrontal cortex. Experiment 2 further dissociated the result of Experiment 1, demonstrating that lesions specific to the anterior cingulate cortex were sufficient to produce a deficit early on during presentation of an incongruent stimulus compound but that performance was unimpaired as presentation progressed. This early deficit suggests a role for the anterior cingulate cortex in the detection of response conflict, and for the medial prefrontal cortex in the contextual control of competing responses, providing evidence for functional specialization within the rat prefrontal cortex.

Behavioral inhibition and prefrontal cortex in decision-making

Every day we make innumerable decisions; some require no effort at all whereas others require considerable deliberation and weighing of various options. Despite the importance of decision-making in our lives and increased research interest, the specific neural mechanisms underlying decision-making remain unclear. We propose that the brain has at least two cortical pathways that independently generate a decision about appropriate behavior in given circumstances. These two pathways are extensions of the dorsal and ventral streams of the visual processing pathways. The parieto-premotor (extended dorsal) pathway makes decisions about motor actions in a largely autonomous and automatic fashion whereas the temporo-ventrolateral prefrontal (extended ventral) pathway is involved primarily in deliberate decisions and inhibitory control over behavior through the inhibitory function of the ventrolateral prefrontal cortex.

Delay period activity of monkey prefrontal neurones during duration-discrimination task

Evidence from brain imaging studies has indicated involvement of the prefrontal cortex (PFC) in time perception; however, the role of this area remains unclear. To address this issue, we recorded single neuronal activity from the PFC of two monkeys while they performed a duration-discrimination task. In the task, two visual cues (a blue or red square) were presented consecutively followed by delay periods and subjects then chose the cue presented for the longer duration. Durations of both cues, order of cue duration [long-short (LS) or short-long (SL)] and order of cue colour (blue-red or red-blue) were randomized on a trial-by-trial basis. We found that subjects responded differently between LS and SL trials and that most prefrontal neurones showed significantly different activity during either the first or the second delay period when comparing activity in LS and SL trials. The present result offers new insights into neural mechanisms of time perception. It appears that, during the delay periods, the PFC contributes to implement a strategic process in temporal processing associated with a trial type (LS or SL) such as representation of the trial type, retention of cue information and anticipation of the forthcoming cue.

Interactions among the medial prefrontal cortex, hippocampus and midline thalamus in emotional and cognitive processing in the rat

The medial prefrontal cortex (mPFC) participates in several higher order functions including selective attention, visceromotor control, decision making and goal-directed behaviors. We discuss the role of the infralimbic cortex (IL) in visceromotor control and the prelimbic cortex (PL) in cognition and their interactions in goal-directed behaviors in the rat. The PL strongly interconnects with a relatively small group of structures that, like PL, subserve cognition, and together have been designated the 'PL circuit.' These structures primarily include the hippocampus, insular cortex, nucleus accumbens, basolateral nucleus of the amygdala, the mediodorsal and reuniens nuclei of the thalamus and the ventral tegmental area of the midbrain. Lesions of each of these structures, like those of PL, produce deficits in delayed response tasks and memory. The PL (and ventral anterior cingulate cortex) (AC) of rats is ideally positioned to integrate current and past information, including its affective qualities, and act on it through its projections to the ventral striatum/ventral pallidum. We further discuss the role of nucleus reuniens of thalamus as a major interface between the mPFC and the hippocampus, and as a prominent source of afferent limbic information to the mPFC and hippocampus. We suggest that the IL of rats is functionally homologous to the orbitomedial cortex of primates and the prelimbic (and ventral AC) cortex to the lateral/dorsolateral cortex of primates, and that the IL/PL complex of rats exerts significant control over emotional and cognitive aspects of goal-directed behavior.

Representation of future and previous spatial goals by separate neural populations in prefrontal cortex

The primate prefrontal cortex plays a central role in choosing goals, along with a wide variety of additional functions, including short-term memory. In the present study, we examined neuronal activity in the prefrontal cortex as monkeys used abstract response strategies to select one of three spatial goals, a selection that depended on their memory of the most recent previous goal. During each trial, the monkeys selected a future goal on the basis of events from the previous trial, including both the symbolic visual cue that had appeared on that trial and the previous goal that the monkeys had selected. When a symbolic visual cue repeated from the previous trial, the monkeys stayed with their previous goal as the next (future) goal; when the cue changed, the monkeys shifted from their previous goal to one of the two remaining locations as their future goal. We found that prefrontal neurons had activity that reflected either previous goals or future goals, but only rarely did individual cells reflect both. This finding suggests that essentially separate neural networks encode these two aspects of spatial information processing. A failure to distinguish previous and future goals could lead to two kinds of maladaptive behavior. First, wrongly representing an accomplished goal as still pending could cause perseveration or compulsive checking, two disorders commonly attributed to dysfunction of the prefrontal cortex. Second, mistaking a pending goal as already accomplished could cause the failures of omission that occur commonly in dementia.

Verbal and spatial working memory in autism

Verbal and spatial working memory were examined in high-functioning children, adolescents, and adults with autism compared to age and cognitive-matched controls. No deficit was found in verbal working memory in the individuals with autism using an N-back letter task and standardized measures. The distinction between the N-back task and others used previously to infer a working memory deficit in autism is that this task does not involve a complex cognitive demand. Deficits were found in spatial working memory. Understanding the basis for the dissociation between intact verbal working memory and impaired spatial working memory and the breakdown that occurs in verbal working memory as information processing demands are increased will likely provide valuable insights into the neural basis of autism.

Prefrontal and parietal contributions to spatial working memory

Functional neuroimaging studies consistently implicate a widespread network of human cortical brain areas that together support spatial working memory. This review summarizes our recent functional magnetic resonance imaging studies of humans performing delayed-saccades. These studies have isolated persistent activity in dorsal prefrontal regions, like the frontal eye fields, and the posterior parietal cortex during the maintenance of positional information. We aim to gain insight into the type of information coded by this activity. By manipulating the sensory and motor demands of the working memory task, we have been able to modulate the frontal eye fields and posterior parietal cortex delay-period activity. These findings are discussed in the context of other neurophysiological and lesion-based data and some hypotheses regarding the differential contributions of frontal and parietal areas to spatial working memory are offered. Namely, retrospective sensory coding of space may be more prominent in the posterior parietal cortex, while prospective motor coding of space may be more prominent in the frontal eye fields.

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.

Functional specialization within the dorsolateral prefrontal cortex: A review of anatomical and physiological studies of non-human primates

The dorsolateral prefrontal cortex (DLPFC) possesses cortico-cortical connections with the parietal and premotor cortices that are involved in visuomotor control of actions. Studies have shown that the DLPFC, especially the caudal part, has a crucial role in cognitive control of motor behavior, and that it uses spatial information in conjunction with information such as object identity, behavioral rules, and rewards. Current anatomical and physiological studies indicate that the DLPFC may not be a single entity. Anatomical studies show that preferential anatomical connections exist between subregions of the DLPFC and the parietal/premotor cortices. Physiological studies based on data obtained from monkeys performing a variety of cognitive tasks report region-specific neuronal activity within the DLPFC. In this article, I review evidence for functional segregation within the DLPFC and postulate that at least two distinct subregions, i.e., the dorsal and ventral parts, can be identified.

A neural network model for trace conditioning

We studied the dynamics of a neural network that has both recurrent excitatory and random inhibitory connections. Neurons started to become active when a relatively weak transient excitatory signal was presented and the activity was sustained due to the recurrent excitatory connections. The sustained activity stopped when a strong transient signal was presented or when neurons were disinhibited. The random inhibitory connections modulated the activity patterns of neurons so that the patterns evolved without recurrence with time. Hence, a time passage between the onsets of the two transient signals was represented by the sequence of activity patterns. We then applied this model to represent the trace eye blink conditioning, which is mediated by the hippocampus. We assumed this model as CA3 of the hippocampus and considered an output neuron corresponding to a neuron in CA1. The activity pattern of the output neuron was similar to that of CA1 neurons during trace eye blink conditioning, which was experimentally observed.

Repetitive transcranial magnetic stimulation of dorsolateral prefrontal cortex increases tolerance to human experimental pain

Dorsolateral prefrontal cortex (DLPFCx) has been implicated in pain perception and in a pain modulation pathway. However, the precise participation of this region is not completely understood. The aim of this study was to evaluate whether 1 Hz rTMS of DLPFCx modifies threshold and tolerance in experimental pain. The effect of 1 Hz rTMS during 15 min at 100% motor threshold was tested in one hundred and eighty right-handed healthy volunteers, using a parallel-group stimulation design. The stimulation sites were right or left DLPFCx, right or left motor cortex, vertex or sham. rTMS was applied in two experimental contexts: (1) To evaluate its transitory effect (interference or facilitation) during cold pressor threshold (CPTh) and tolerance (CPTt) and (2) to evaluate its long-term effect by stimulating before CPTh, CPTt, pain heat thermal threshold, pain pressure threshold and tolerance. During rTMS of right DLPFCx, an increase in left hand CPTt (mean +/- SD; 17.63 s +/- 5.58 to 30.94 s +/- 14.84, P < 0.001) and in right hand CPTt (18.65 s +/- 6.47 to 26.74 s +/- 11.85, P < 0.001) were shown. No other stimulation site modified any of the pain measures during or after rTMS. These results show that 1 Hz rTMS of right DLPFCx has a selective effect by increasing pain tolerance and also sustains a right hemisphere preference in pain processing.

A role for prefrontal calcium-sensitive protein phosphatase and kinase activities in working memory

The prefrontal cortex is involved in the integration and interpretation of information for directing thoughts and planning action. Working memory is defined as the active maintenance of information in mind and is thought to lie at the core of many prefrontal functions. Although dopamine and other neurotransmitters have been implicated, the intracellular events activated by their receptors that influence working memory are poorly understood. We demonstrate that working memory involves transient changes in prefrontal G(q/11)-signaling and in calcium-dependent intracellular protein phosphatase and kinase activity. Interestingly, inhibition of the calcium activated phosphatase calcineurin impaired, while calcium/calmodulin dependent kinase II (CaMKII) and calcium-dependent protein kinase C (PKC) enhanced, working memory. Our findings suggest that the active maintenance of information required for working memory involves transient changes in the balance of these enzymes' activities.

Control networks and hemispheric asymmetries in parietal cortex during attentional orienting in different spatial reference frames

Neuropsychological research has consistently demonstrated that spatial attention can be anchored in one of several coordinate systems, including those defined with respect to an observer (viewer-centered), to the gravitational vector (environment-centered), or to individual objects (object-centered). In the present study, we used hemodynamic correlates of brain function to investigate the neural systems that mediate attentional control in two competing reference frames. Healthy volunteers were cued to locations defined in either viewer-centered or object-centered space to discriminate the shape of visual targets subsequently presented at the cued locations. Brain responses to attention-directing cues were quantified using event-related functional magnetic resonance imaging. A fronto-parietal control network was activated by attention-directing cues in both reference frames. Voluntary shifts of attention produced increased neural activity bilaterally in several cortical regions including the intraparietal sulcus, anterior cingulate cortex, and the frontal eye fields. Of special interest was the observation of hemispheric asymmetries in parietal cortex; there was significantly greater activity in left parietal cortex than in the right, but this asymmetry was more pronounced for object-centered shifts of attention, relative to viewer-centered shifts of attention. Measures of behavioral performance did not differ significantly between the two reference frames. We conclude that a largely overlapping, bilateral, cortical network mediates our ability to orient spatial attention in multiple coordinate systems, and that the left intraparietal sulcus plays an additional role for orienting in object-centered space. These results provide neuroimaging support for related claims based on findings of deficits in object-based orienting in patients with left parietal lesions.

Distinct prefrontal molecular mechanisms for information storage lasting seconds versus minutes

The prefrontal cortex (PFC) is known to actively hold information "online" for a period of seconds in working memory for guiding goal-directed behavior. It has been proposed that relevant information is stored in other brain regions, which is retrieved and held in working memory for subsequent assimilation by the PFC in order to guide behavior. It is uncertain whether PFC stores information outside the temporal limits of working memory. Here, we demonstrate that although enhanced cAMP-dependent protein kinase A (PKA) activity in the PFC is detrimental to working memory, it is required for performance in tasks involving conflicting representations when memory storage is needed for minutes. This study indicates that distinct molecular mechanisms within the PFC underlie information storage for seconds (working memory) and for minutes (short-term memory). In addition, our results demonstrate that short-term memory storage within the prefrontal cortex is required for guiding behavior in tasks with conflicts and provides a plausible mechanism by which the prefrontal cortex executes cognitive control.

Functional significance of delay-period activity of primate prefrontal neurons in relation to spatial working memory and reward/omission-of-reward expectancy

The lateral prefrontal cortex (LPFC) is important in cognitive control. During the delay period of a working memory (WM) task, primate LPFC neurons show sustained activity that is related to retaining task-relevant cognitive information in WM. However, it has not yet been determined whether LPFC delay neurons are concerned exclusively with the cognitive control of WM task performance. Recent studies have indicated that LPFC neurons also show reward and/or omission-of-reward expectancy-related delay activity, while the functional relationship between WM-related and reward/omission-of-reward expectancy-related delay activity remains unclear. To clarify the functional significance of LPFC delay-period activity for WM task performance, and particularly the functional relationship between these two types of activity, we examined individual delay neurons in the primate LPFC during spatial WM (delayed response) and non-WM (reward-no-reward delayed reaction) tasks. We found significant interactions between these two types of delay activity. The majority of the reward expectancy-related neurons and the minority of the omission-of-reward expectancy-related neurons were involved in spatial WM processes. Spatial WM-related neurons were more likely to be involved in reward expectancy than in omission-of-reward expectancy. In addition, LPFC delay neurons observed during the delayed response task were not concerned exclusively with the cognitive control of task performance; some were related to reward/omission-of-reward expectancy but not to WM, and many showed more memory-related activity for preferred rewards than for less-desirable rewards. Since employing a more preferred reward induced better task performance in the monkeys, as well as enhanced WM-related neuronal activity in the LPFC, the principal function of the LPFC appears to be the integration of cognitive and motivational operations in guiding the organism to obtain a reward more effectively.

Neural correlates of executive control in the avian brain

Executive control, the ability to plan one's behaviour to achieve a goal, is a hallmark of frontal lobe function in humans and other primates. In the current study we report neural correlates of executive control in the avian nidopallium caudolaterale, a region analogous to the mammalian prefrontal cortex. Homing pigeons (Columba livia) performed a working memory task in which cues instructed them whether stimuli should be remembered or forgotten. When instructed to remember, many neurons showed sustained activation throughout the memory period. When instructed to forget, the sustained activation was abolished. Consistent with the neural data, the behavioural data showed that memory performance was high after instructions to remember, and dropped to chance after instructions to forget. Our findings indicate that neurons in the avian nidopallium caudolaterale participate in one of the core forms of executive control, the control of what should be remembered and what should be forgotten. This form of executive control is fundamental not only to working memory, but also to all cognition.

The effect of memory load on cortical activity in the spatial working memory circuit

Accumulating evidence from electrophysiology and neuroimaging studies suggests that spatial working memory is subserved by a network of frontal and parietal regions. In the present study, we parametrically varied the memory set size (one to four spatial locations) of a delayed-response task and applied time-resolved fMRI to study the influence of memory load upon the spatial working memory circuit. Our behavioral results showed that performance deteriorates (lower accuracy and longer reaction time) as memory load increases. Memory load influenced cortical activity during the cue, delay, and response phases of the delayed-response task. Although delay-related activity in many regions increased with increasing memory load, it also was significantly reduced in the middle frontal gyrus and frontal eye fields and leveled off in the parietal areas when memory load increased further. Delay-related activity in the left posterior parietal cortex was also lower during the error trials, in comparison with the correct trials. Our findings indicate that the delay period activity in the spatial working memory circuit is load sensitive and that the attenuation of this signal is the neural manifestation of performance limitation in the face of excessive memory load.

Cortico-striatal circuits and interval timing: coincidence detection of oscillatory processes

Humans and other animals demonstrate the ability to perceive and respond to temporally relevant information with characteristic behavioral properties. For example, the response time distributions in peak-interval timing tasks are well described by Gaussian functions, and superimpose when scaled by the criterion duration. This superimposition has been referred to as the scalar property and results from the fact that the standard deviation of a temporal estimate is proportional to the duration being timed. Various psychological models have been proposed to account for such responding. These models vary in their success in predicting the temporal control of behavior as well as in the neurobiological feasibility of the mechanisms they postulate. A review of the major interval timing models reveals that no current model is successful on both counts. The neurobiological properties of the basal ganglia, an area known to be necessary for interval timing and motor control, suggests that this set of structures act as a coincidence detector of cortical and thalamic input. The hypothesized functioning of the basal ganglia is similar to the mechanisms proposed in the beat frequency timing model [R.C. Miall, Neural Computation 1 (1989) 359-371], leading to a reevaluation of its capabilities in terms of behavioral prediction. By implementing a probabilistic firing rule, a dynamic response threshold, and adding variance to a number of its components, simulations of the striatal beat frequency model were able to produce output that is functionally equivalent to the expected behavioral response form of peak-interval timing procedures.

Dopamine D4 receptors: beyond schizophrenia

Dopamine D4 receptors mediate a wide range of neuronal signal transduction cascades. Malfunctions of these mechanisms may contribute to the pathophysiology of neuropsychiatric disorders, and their modification underlies the actions of many psychotropic drugs. Postmortem neuropathological and genetic studies provide inconclusive associations between D4 receptors and schizophrenia. Clinical trials of partially selective lead D4 antagonists have proved them to be ineffective against psychotic symptoms in patients diagnosed with schizophrenia. However, associations are emerging between D4 receptors and other neuropsychiatric disorders, including attention-deficit hyperactivity disorder as well as specific personality traits such as novelty seeking. Preclinical studies indicate that D4 receptors play a pivotal role in the cellular mechanisms of hyperactivity, impulsivity, and working memory. Accordingly, D4 receptors have broader implications for human illnesses than has been suggested by early focus on psychotic illness as a clinical target, and selective D4 agents may yield clinically useful drugs for several neuropsychiatric disorders that require improved treatments.

Representation of Immediate and Final Behavioral Goals in the Monkey Prefrontal Cortex during an Instructed Delay Period

We examined neuronal activity in the lateral prefrontal cortex of monkeys performing a path-planning task in a maze that required the planning of actions in multiple steps. The animals received an instruction that prompted them to prepare to move a cursor in the maze stepwise from a starting position to a goal position by operating manipulanda with either arm. During a delay period in which the animal prepared to start the first of three cursor movements to approach the pre-instructed goal, we identified two types of neuronal activity: the first type reflected the position within the maze to which the animal intended to move the cursor as an initial step (an immediate goal) and the second type reflected the position within the maze that was to be captured as a final goal. Neither type reflected motor responses. We propose that these two types of neuronal activity are neuronal correlates that represent immediate and ultimate behavioral goals. This finding implicates the prefrontal cortex in governing goal-oriented sequential behavior rather than sensorimotor transformation.

Afferent projections to nucleus reuniens of the thalamus

The nucleus reuniens (RE) is the largest of the midline nuclei of the thalamus and the major source of thalamic afferents to the hippocampus and parahippocampal structures. Nucleus reuniens has recently been shown to exert powerful excitatory actions on CA1 of the hippocampus. Few reports on any species have examined afferent projections to nucleus reuniens. By using the retrograde anatomical tracer Fluorogold, we examined patterns of afferent projections to RE in the rat. We showed that RE receives a diverse and widely distributed set of afferents projections. The main sources of input to nucleus reuniens were from the orbitomedial, insular, ectorhinal, perirhinal, and retrosplenial cortices; CA1/subiculum of hippocampus; claustrum, tania tecta, lateral septum, substantia innominata, and medial and lateral preoptic nuclei of the basal forebrain; medial nucleus of amygdala; paraventricular and lateral geniculate nuclei of the thalamus; zona incerta; anterior, ventromedial, lateral, posterior, supramammillary, and dorsal premammillary nuclei of the hypothalamus; and ventral tegmental area, periaqueductal gray, medial and posterior pretectal nuclei, superior colliculus, precommissural/commissural nuclei, nucleus of the posterior commissure, parabrachial nucleus, laterodorsal and pedunculopontine tegmental nuclei, nucleus incertus, and dorsal and median raphe nuclei of the brainstem. The present findings of widespread projections to RE, mainly from limbic/limbic-associated structures, suggest that nucleus reuniens represents a critical relay in the transfer of limbic information (emotional/cognitive) from RE to its major targets, namely, to the hippocampus and orbitomedial prefrontal cortex. RE appears to be a major link in the two-way exchange of information between the hippocampus and the medial prefrontal cortex.

Prefrontal cortex function, quasi-physiological stimuli, and synaptic plasticity

The prelimbic area of rat medial frontal cortex may be functionally analogous to human/primate dorsolateral prefrontal cortex. This area may be involved in selective attention to the external stimuli and the coupling of the attention to a repertory of actions. It was suggested that this function may rely on a form of long-term memory [Biol. Rev. 77 (2002) 563]. Indeed, during learning of this type of behavior, a portion of prelimbic neurons persistently change their firing characteristics [Prog. Brain Res. 126 (2000) 287]. It is therefore important to study long-term potentiation (LTP) and depression (LTD) in rat prelimbic neurons. In this article, the author first briefly reviews recent findings on the prefrontal cortex function and discusses that the prefrontal cortex may be involved in long-term memory. Second, the author will show some new results which indicate that quasi-physiological patterns of stimuli mimicking prelimbic neuronal activity during behavior can induce LTP in prelimbic pyramidal neuron synapses. These results suggest that prelimbic neuronal activity during behavior may lastingly modify prelimbic synaptic efficacy.

Climbing neuronal activity as an event-based cortical representation of time

The brain has the ability to represent the passage of time between two behaviorally relevant events. Recordings from different areas in the cortex of monkeys suggest the existence of neurons representing time by increasing (climbing) activity, which is triggered by a first event and peaks at the expected time of a second event, e.g., a visual stimulus or a reward. When the typical interval between the two events is changed, the slope of the climbing activity adapts to the new timing. We present a model in which the climbing activity results from slow firing rate adaptation in inhibitory neurons. Hebbian synaptic modifications allow for learning the new time interval by changing the degree of firing rate adaptation. This event-based representation of time is consistent with Weber's law in interval timing, according to which the error in estimating a time interval is proportional to the interval length.

Neural representation of interval time

Animals can predict the time of occurrence of a forthcoming event relative to a preceding stimulus, i.e. the interval time between those two, given previous learning experience with the temporal contingency between them. Accumulating evidence suggests that a particular pattern of neural activity observed during tasks involving fixed temporal intervals might carry interval time information: the activity of some cortical and subcortical neurons ramps up slowly and linearly during the interval, like a temporal integrator, and peaks around the time at which the event is due to occur. The slope of this climbing activity, and hence the peak time, adjusts to the length of a temporal interval during repetitive experience with it. Various neural mechanisms for producing climbing activity with variable slopes, representing the length of learned intervals, are discussed.

Learning-related changes in response patterns of prefrontal neurons during instrumental conditioning

A crucial aspect of organizing goal-directed behavior is the ability to form neural representations of relationships between environmental stimuli, actions and reinforcement. Very little is known yet about the neural encoding of response-reward relationships, a process which is deemed essential for purposeful behavior. To investigate this, tetrode recordings were made in the medial prefrontal cortex (PFC) of rats performing a Go-NoGo task. After task acquisition, a subset of neurons showed a sustained change in firing during the rewarded action sequence that was triggered by a specific visual cue. When these changes were monitored in the course of learning, they were seen to develop in parallel with the behavioral learning curve and were highly sensitive to a switch in reward contingencies. These sustained changes correlated with the reward-associated action sequence, not with sensory or reward-predicting properties of the cue or individual motor acts per se. This novel type of neural plasticity may contribute to the formation of response-reinforcer associations and of behavioral strategies for guiding goal-directed action.

Prefrontal cortical up states are synchronized with ventral tegmental area activity

The innervation of the prefrontal cortex (PFC) by the ventral tegmental area (VTA) has an important role in incentive-motivation and cognitive functions. Although this projection has been extensively studied, the precise actions of its transmitters, dopamine (DA) and GABA, on PFC pyramidal neurons remain to be determined. We have recently shown that VTA stimulation elicits a sustained depolarization in PFC pyramidal neurons resembling the periodic depolarizations (up states) these neurons exhibit. This response was shortened by a D1 antagonist, suggesting that DA may sustain depolarized up states in PFC neurons. Here, we tested whether spontaneous PFC up states in vivo require spontaneous VTA activity. Intracellular recordings from PFC neurons conducted simultaneously with VTA local field potentials (LFPs) revealed PFC membrane potential fluctuations occurring synchronously with VTA field potential transitions. Extracellular PFC recordings performed simultaneously with VTA LFPs also indicated a high coherence between these two regions, with VTA oscillations trailing PFC oscillations by a few milliseconds. Furthermore, blockade of VTA activity with lidocaine transiently eliminated PFC LFPs, but not PFC cell up states; instead, up states became irregular during intra-VTA lidocaine administration. These results suggest that baseline levels of VTA activity are necessary for synchronizing PFC pyramidal neurons in the up-down oscillations observed in the anesthetized preparation, allowing the emergence of slow EEG components.

Interval timing and the encoding of signal duration by ensembles of cortical and striatal neurons

This study investigated the firing patterns of striatal and cortical neurons in rats in a temporal generalization task. Striatal and cortical ensembles were recorded in rats trained to lever press at 2 possible criterion durations (10 s or 40 s from tone onset). Twenty-two percent of striatal and 15% of cortical cells had temporally specific modulations in their firing rate, firing at a significantly different rate around 10 s compared with 40 s. On 80% of trials, a post hoc analysis of the trial-by-trial consistency of the firing rates of an ensemble of neurons predicted whether a spike train came from a time window around 10 s versus around 40 s. Results suggest that striatal and cortical neurons encode specific durations in their firing rate and thereby serve as components of a neural circuit used to represent duration.

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

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

Information processes in the primate prefrontal cortex in relation to working memory processes

Working memory is a mechanism for short-term active storage of information as well as for processing stored information. Although evidence for neuronal mechanisms of temporary storage of information has accumulated for the prefrontal cortex, little is known about neuronal mechanisms for processing information. To understand how information is processed by prefrontal neurons, we first need to know what information is represented by single-neuron activity, and then examine how information represented by single-neuron activity or a population of activities changes along the temporal sequence of the trial. By examining task-related single-neuron activities while monkeys performed various working memory tasks, delay-period activity observed in the prefrontal cortex is considered to be a neuronal correlate of the mechanism for temporary active storage of information. Delay-period activity represents a variety of information including the spatial position and the physical feature of the stimulus, the forthcoming behavioral response, the quality of reward that the subject would receive, the difference of the task, or the rule of the task. Although delay-period activity could represent this variety of information, the information represented by delay-period activity is only the information relevant for task performance. In addition, using complex conditional tasks, delay-period activity has been shown to represent several kinds of information simultaneously. Based on these results, we examined how information represented by a population of prefrontal activities changes along the temporal sequence of the trial. Using two kinds of oculomotor delayed-response tasks, we first identified what information each task-related activity represents. Then, using population vector analysis, we could not only visualize information represented by a population of prefrontal activities, but also demonstrate the temporal change of information represented by a population of prefrontal activities. These attempts are important to understand information processes for working memory.

Flutter discrimination: neural codes, perception, memory and decision making

Recent studies combining psychophysical and neurophysiological experiments in behaving monkeys have provided new insights into how several cortical areas integrate efforts to solve a vibrotactile discrimination task. In particular, these studies have addressed how neural codes are related to perception, working memory and decision making in this model. The primary somatosensory cortex drives higher cortical areas where past and current sensory information are combined, such that a comparison of the two evolves into a behavioural decision. These and other observations in visual tasks indicate that decisions emerge from highly-distributed processes in which the details of a scheduled motor plan are gradually specified by sensory information.

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.

Memory trace in prefrontal cortex: theory for the cognitive switch

The dorsolateral prefrontal cortex in human and non-human primates functions as the highest-order executor for the perception-action cycle. According to this view, when perceptual stimuli from the environment are novel or complex, the dorsolateral prefrontal cortex serves to set consciously a goal-directed scheme which broadly determines an action repertory to meet the particular demand from the environment. In this respect, the dorsolateral prefrontal cortex is a short-term activation device with the properties of a cognitive switch', because it couples a particular set of perceptual stimuli to a particular set of actions. Here, I suggest that, in order for the organism to react systematically to the environment, neural traces for the switch function must be stored in the brain. Thus, the highest-order, perception-action interface function of the dorsolateral prefrontal cortex per se depends on permanently stored neural traces in the dorsolateral prefrontal cortex and related structures. Such a memory system may be located functionally between two of the well-documented memory systems in the brain: the declarative memory system and the procedural memory system. Finally, based on available neurophysiological data, the possible mechanisms underlying the formation of cognitive switch traces are proposed.

Active maintenance in prefrontal area 46 creates distractor-resistant memory

How does the brain maintain information in working memory while challenged by incoming distractions? Using functional magnetic resonance imaging (fMRI), we measured human brain activity during the memory delay of a spatial working memory task with distraction. We found that, in the prefrontal cortex (PFC), the magnitude of activity sustained throughout the memory delay was significantly higher on correct trials than it was on error trials. By contrast, the magnitude of sustained activity in posterior areas did not differ between correct and error trials. The correlation of activity between posterior areas was, however, associated with correct memory performance after distraction. On the basis of these findings, we propose that memory representations gain resistance against distraction during a period of active maintenance within working memory. This may be mediated by interactions between prefrontal and posterior areas.

Dissociating top-down attentional control from selective perception and action

Research into the neural mechanisms of attention has revealed a complex network of brain regions that are involved in the execution of attention-demanding tasks. Recent advances in human neuroimaging now permit investigation of the elementary processes of attention being subserved by specific components of the brain's attention system. Here we describe recent studies of spatial selective attention that made use of positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and event-related brain potentials (ERPs) to investigate the spatio-temporal dynamics of the attention-related neural activity. We first review the results from an event-related fMRI study that examined the neural mechanisms underlying top-down attentional control versus selective sensory perception. These results defined a fronto-temporal-parietal network involved in the control of spatial attention. Activity in these areas biased the neural activity in sensory brain structures coding the spatial locations of upcoming target stimuli, preceding a modulation of subsequent target processing in visual cortex. We then present preliminary evidence from a fast-rate event-related fMRI study of spatial attention that demonstrates how to disentangle the potentially overlapping hemodynamic responses elicited by temporally adjacent stimuli in studies of attentional control. Finally, we present new analyses from combined neuroimaging (PET) and event-related brain potential (ERP) studies that together reveal the timecourse of activation of brain regions implicated in attentional control and selective perception.

Analysis of projections from the medial prefrontal cortex to the thalamus in the rat, with emphasis on nucleus reuniens

The medial prefrontal cortex (mPFC) is involved in high-order cognitive processes, including, but not limited to, decision making, goal directed behavior, and working memory. Although previous reports have included descriptions of mPFC projections to the thalamus in overall examinations of mPFC projections throughout the brain, no previous study has comprehensively examined mPFC projections to the thalamus. The present report compares and contrasts projections from the four divisions of the mPFC, i.e., the infralimbic, prelimbic, anterior cingulate and medial agranular cortices, to the thalamus in the rat by using the anterograde anatomic tracer Phaseolus vulgaris-leucoagglutinin. We showed that (1) the infralimbic, prelimbic, anterior cingulate cortices distribute heavily and selectively to midline/medial structures of the thalamus, including the paratenial, paraventricular, interanteromedial, anteromedial, intermediodorsal, mediodorsal, reuniens, and the central medial nuclei; (2) the medial agranular cortex distributes strongly to the rostral intralaminar nuclei (central lateral, paracentral, central medial nuclei) as well as to the ventromedial and ventrolateral nuclei of thalamus; and (3) all four divisions of the mPFC project densely to the nucleus reuniens (RE) of the thalamus. The nucleus reuniens is the major source of thalamic afferents to the hippocampal formation. There are essentially no direct projections from the mPFC to the hippocampus. The present demonstration of pronounced mPFC projections to RE suggests that the nucleus reuniens is a critical relay in the transfer of information from the medial prefrontal cortex to the hippocampus. Our further demonstration of strong mPFC projections to several additional thalamic nuclei, particularly to the mediodorsal nucleus, suggests that these thalamic nuclei, like RE, represent important output stations (or gateways) for the actions of mPFC on diverse subcortical and cortical structures of the brain.

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.

Touch and go: decision-making mechanisms in somatosensation

A complex sequence of neural events unfolds between sensory receptor activation and motor activity. To understand the underlying decision-making mechanisms linking somatic sensation and action, we ask what components of the neural activity evoked by a stimulus are directly related to psychophysical performance, and how are they related. We find that single-neuron responses in primary and secondary somatosensory cortices account for the observed performance of monkeys in vibrotactile discrimination tasks, and that neuronal and behavioral responses covary in single trials. This sensory activity, which provides input to memory and decision-making mechanisms, is modulated by attention and behavioral context, and microstimulation experiments indicate that it may trigger normal perceptual experiences. Responses recorded in motor areas seem to reflect the output of decision-making operations, which suggests that the ability to make decisions occurs at the sensory-motor interface.

Single-cell recording from the brain of freely moving monkeys

Single-cell recording from the brain of non-human primates has traditionally been performed in monkeys seated in a primate chair. However, this arrangement makes long-term recordings difficult, causes stress that may confound the data, and prevents the manifestation of natural behaviors. Extending our previous neurophysiological studies in non-human primates (Ludvig et al. Brain Res. Protocols 2000;5:75-85), we have developed a method for recording the electrical activity of single hippocampal neurons in freely moving squirrel monkeys (Saimiri sciureus). The recording sessions lasted for up to 6 h, during which the monkeys moved freely around on the walls and the floor of a large test chamber and collected food pellets. Stable action potential waveforms were readily kept throughout the sessions. The following factors proved to be critical in this study: (a) selecting squirrel monkeys for the experiments, (b) using a driveable bundle of microwires for the recordings, (c) using a special recording cable, (d) implanting the microwires into the brain without causing neurological deficits, and (e) running the recording sessions in a special test chamber. The described method allows long-term extracellular recordings from the brain of non-human primates, without the stress of chairing, during a wide range of natural behaviors. Using this model, new insights can be obtained into the unique firing repertoire of the neurons of the primate brain.

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.

Visual, saccade-related, and cognitive activation of single neurons in monkey extrastriate area V3A

Area V3A is an extrastriate visual area that provides a major input to parietal cortex. To identify the sensory, saccade-related, and cognitive signals carried by V3A neurons, we recorded from single units in alert monkeys during performance of fixation and memory guided saccade tasks. We found that visual responses to stationary stimuli in area V3A were affected by the behavioral relevance of the stimulus. The amplitude of the visual response differed between the memory-guided saccade task, in which the monkey had to use the information provided by the stimulus to guide its behavior, and the fixation task. For 18% (29/163) of V3A neurons, the response was significantly enhanced in the memory-guided saccade task as compared with that in the fixation task. For 8% (13/163) of V3A neurons, the amplitude of response in the memory-guided saccade task was significantly suppressed. We also observed task-related modulation of activity prior to stimulus onset. Among the V3A neurons (37/163) that showed significant differences between tasks in prestimulus activity, the majority (89%; 33/37) showed greater prestimulus activity in the memory-guided saccade task. Task-related increases in prestimulus activity in the memory-guided saccade task were not always matched by increases in the sensory response, indicating that visual responses and prestimulus activity can be modulated independently. Activity in the memory period was suppressed compared with prestimulus activity for 83% (49/59) of the V3A neurons that showed a significant difference in activity (59/197) between these two epochs. For some neurons, memory-period activity dropped even below the baseline level in the fixation task, indicating that there may be an active suppression mechanism. Many V3A neurons (75%, 148/197) also had activity in the saccade epoch. This activity was most prominent immediately after the saccade. Postsaccadic activity was observed even when testing was carried out in total darkness, indicating that this activity reflects, at least in part, extraretinal signals and is not simply a response to visual reafference. These results indicate that several kinds of signals are carried by single neurons in extrastriate area V3A. Moreover, activity in V3A is subject to modulation by extraretinal factors, including attention, anticipation, memory, and saccadic eye movements.

Executive function oculomotor tasks in girls with ADHD

OBJECTIVE

To assess executive function in girls with attention-deficit/hyperactivity disorder (ADHD) using oculomotor tasks as possible trait markers for neurobiological studies.

METHOD

Thirty-two girls aged 6 to 13 years with DSM-IV ADHD and 20 age-matched, normal control girls were tested on a variety of oculomotor tasks requiring attention, working memory, and response inhibition, which included smooth pursuit, delayed response, and go-no go tasks.

RESULTS

Girls with ADHD performed the delayed response task correctly on 32% of trials as measured by number of memory-guided saccades, in contrast to 62% of trials for control subjects (p = .0009). Patients made twice as many commission errors to no go stimuli (p = .0001) and 3 times as many intrusion errors (saccades in the absence of go or no go stimuli; p = .004) during the go-no go task compared with controls. Smooth pursuit performance was statistically equivalent across subject groups. Repeated testing in a subgroup of 15 patients revealed substantial practice effects on go-no go performance.

CONCLUSIONS

The data confirm that girls with ADHD exhibit impairments in executive function, as has been reported in boys, implying a similar pathophysiology of ADHD in both sexes. However, practice effects may limit the utility of the oculomotor go-no go task for some neurobiological studies.

The neural mechanisms of top-down attentional control

Selective visual attention involves dynamic interplay between attentional control systems and sensory brain structures. We used event-related functional magnetic resonance imaging (fMRI) during a cued spatial-attention task to dissociate brain activity related to attentional control from that related to selective processing of target stimuli. Distinct networks were engaged by attention-directing cues versus subsequent targets. Superior frontal, inferior parietal and superior temporal cortex were selectively activated by cues, indicating that these structures are part of a network for voluntary attentional control. This control biased activity in multiple visual cortical areas, resulting in selective sensory processing of relevant visual targets.

Properties of delay-period neuronal activity in the monkey dorsolateral prefrontal cortex during a spatial delayed matching-to-sample task

The dorsolateral prefrontal cortex (PFC) has been implicated in visuospatial memory, and its cellular basis has been extensively studied with the delayed-response paradigm in monkeys. However, using this paradigm, it is difficult to dissociate neuronal activities related to visuospatial memory from those related to motor preparation, and few studies have provided evidence for the involvement of PFC neurons in visuospatial memory of a sensory cue, rather than in motor preparation. To extend this finding, we examined neuronal activities in the dorsolateral PFC while a rhesus monkey performed a spatial delayed matching-to-sample (SDMTS) task, which allows us to adequately access visuospatial memory independent of any sensorimotor components. The SDMTS task required the subject to make a lever-holding NOGO response or a lever-releasing GO response when a visuospatial matching cue (white spot, one of four peripheral locations, 15 degrees in eccentricity) matched or did not match a sample cue (physically the same as the matching cue) that had been presented prior to a delay period (3 s). Thus, the SDMTS task requires the subject to remember visuospatial information regarding the sample cue location during the delay period and is suitable for accessing visuospatial memory independent of any sensorimotor components, such as motor preparation, for directed movements. Of a total of 385 task-related neurons, 184 showed a sustained increase in activity during the delay period ("delay-period activity"). Most of these neurons (n = 165/184, 90%) showed positional delay-period activity, i.e., delay-period activity where the magnitude differed significantly with the position of the sample cue. This activity appears to be involved in visuospatial memory and to form a "memory field." To quantitatively examine the properties of positional delay-period activity, we introduced a tuning index (TI) and a discriminative index (DI), which represent the sharpness of tuning and the discriminative ability, respectively, of positional delay-period activity. Both TI and DI varied among neurons with positional delay-period activity and were closely related to the time from the onset of the sample cue to the onset of positional delay-period activity; positional delay-period activity with sharper tuning and a greater discriminative ability had a slower onset. Furthermore, at the population level, both TI and DI were increased during the delay period in the neuronal population with a high DI value. These results extend previous findings to suggest that integrative, convergent processes of neuronal activities for increasing the accuracy of visuospatial memory may occur in the dorsolateral PFC. Thus, a critical role of the dorsolateral PFC in visuospatial memory may be to sharpen it to guide behaviors/decisions requiring accurate visuospatial memory.

Prefrontal cortical involvement in visual working memory

Studies of human amnesia provide evidence for a short-term memory store with information transfer to long term memory occurring within 60 s of sensory encoding. Human and nonhuman primate research has shown that maintenance of this short-term or working memory store is dependent upon frontal cortical activation, although the critical temporal parameters of frontal involvement throughout this 60-s window are undetermined. We examined prefrontal contributions to rapid (under 2 s) and sustained (over 4 s) visual working memory by recording behavioral performance and event-related potentials (ERPs) in patients with lesions in dorsolateral frontal cortex and age-matched control subjects. Prefrontal lesioned patients generated a reduced sustained frontal positivity at all delays. At short delays, patients generated reduced performance to stimuli presented in the contralesional field. Patients generated a negative potential (N400), greatest to contralesionally presented stimuli, that was observed in the control subjects only at long delays. The results indicate that prefrontal lesions impair the frontal component of an anterior-posterior working memory network activated during rapid and sustained visual memory processing. Frontal patients may require activation of limbic cortex, indexed by N400, for maintenance of both rapid and sustained working memory.

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.

A versatile software system for controlling various within-subject tasks in behaving monkeys and humans: application to an attention task

A multi-purpose software system is described for controlling a variety of tasks used in experiments on behaving monkeys and humans. It involved two programs, a real-time unit and a task editor. The former used editable pixel maps flashed on a computer screen as visual stimuli and operated according to a control list of trial parameters specifying the various components of each trial in the behavioural session. It involved an interrupt-driven driver interface for clock control and response analysis, and a main code managing the session process, disk access and stimulus display. The task editor was used to generate the list of trial parameters and edit the pixel-map of signals. It generated a task file used by the real-time unit. Experimental manipulations could be implemented through the organisation of the behavioural session by trial types corresponding to the different behavioural conditions. The system's capabilities are illustrated by behavioural results from an attention task in which monkeys increased their attention as a function of task difficulty.

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.

Neuronal correlates of parametric working memory in the prefrontal cortex

Humans and monkeys have similar abilities to discriminate the difference in frequency between two mechanical vibrations applied sequentially to the fingertips. A key component of this sensory task is that the second stimulus is compared with the trace left by the first (base) stimulus, which must involve working memory. Where and how is this trace held in the brain? This question was investigated by recording from single neurons in the prefrontal cortex of monkeys while they performed the somatosensory discrimination task. Here we describe neurons in the inferior convexity of the prefrontal cortex whose discharge rates varied, during the delay period between the two stimuli, as a monotonic function of the base stimulus frequency. We describe this as 'monotonic stimulus encoding', and we suggest that the result may generalize: monotonic stimulus encoding may be the basic representation of one-dimensional sensory stimulus quantities in working memory. Thus we predict that other behavioural tasks that require ordinal comparisons between scalar analogue stimuli would give rise to monotonic responses similar to those reported here.

Neural activity in the primate prefrontal cortex during associative learning

The prefrontal (PF) cortex has been implicated in the remarkable ability of primates to form and rearrange arbitrary associations rapidly. This ability was studied in two monkeys, using a task that required them to learn to make specific saccades in response to particular cues and then repeatedly reverse these responses. We found that the activity of individual PF neurons represented both the cues and the associated responses, perhaps providing a neural substrate for their association. Furthermore, during learning, neural activity conveyed the direction of the animals' impending responses progressively earlier within each successive trial. The final level of activity just before the response, however, was unaffected by learning. These results suggest a role for the PF cortex in learning arbitrary cue-response associations, an ability critical for complex behavior.