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Single-unit activity was recorded from the hand areas of the somatosensory cortex of monkeys trained to perform a haptic delayed matching to sample task with objects of identical dimensions but different surface features. During the memory retention period of the task (delay), many units showed sustained firing frequency change, either excitation or inhibition. In some cases, firing during that period was significantly higher after one sample object than after another. These observations indicate the participation of somatosensory neurons not only in the perception but in the short-term memory of tactile stimuli. Neurons most directly implicated in tactile memory are (i) those with object-selective delay activity, (ii) those with nondifferential delay activity but without activity related to preparation for movement, and (iii) those with delay activity in the haptic-haptic delayed matching task but no such activity in a control visuo-haptic delayed matching task. The results indicate that cells in early stages of cortical somatosensory processing participate in haptic short-term memory.

A delayed match-to-sample (DMS) task of abstract, visual memory was performed during the uptake period of 18F-fluorodeoxyglucose. The increase in glucose uptake of cortical and subcortical regions ("activation") during the DMS task was compared with that during a control, immediate match-to-sample task using positron emission tomography. Both discriminant analysis and paired t tests supported the observation that the dorsolateral prefrontal area underwent the greatest activation, while a factor analysis revealed the functional correlation matrices of the tasks. Activations in the ventral premotor cortex and supramarginal and angular gyri were highly correlated with the change in the dorsolateral prefrontal cortex. The basal forebrain/ventral pole region showed a smaller but independently significant change. The findings support the role of the dorsal prefrontal region in the nonspatial working memory of humans.

Purpose – To show that Hayek's prescient concepts on the cerebral cortex have received substantial support from modern neuroscience. Methodology – Update the terminology of The Sensory Order to adjust it to prevalent concepts of cognitive network, plasticity, association, connectivity, and cortical dynamics. Extend his concepts of perception to other cognitive functions, notably memory. Reveal significance of modern methods to study the formation and organization of cognitive cortical networks (cognits), applying the same basic methodologies that he applied to perception. He also applied those methodologies to knowledge transactions in economics and the social order. Findings – As Hayek proposed or assumed in his theoretical monograph:•Cognitive networks are spontaneously formed by associations (connections) between neuronal assemblies representing simultaneous elementary sensations.•Perceiving is classifying the world into categories of objects defined by those associations, in accord with a relational code.•Networks are hierarchically organized, with smaller networks constituting, and nested within, larger ones.•After formed and organized, a network becomes memory, which will make and shape future perception.•The interactions between the organism and its environment are governed by the perception/action (PA) cycle, a concept intuited by Hayek. This is the cybernetic interplay between the mammalian organism and its environment that courses through perceptual and executive networks of the cortex.•The dialog with an interlocutor epitomizes the PA cycle of language, unique to the human. Social Implications – The brain embodies structure and dynamics similar to those relating the individual to society. They include a complex adaptive system, the cerebral cortex, which engages the brains of others through the PA cycle. Language is the highest operation of that cycle at interpersonal level. Transactions of knowledge within the cortex are similar to those of the market place, with their attributes of spontaneity, self-organization, and incompleteness. Originality/Value of paper – This paper is unusual in that it highlights: (a) the insight of Hayek in cognitive neuroscience, anticipating by several decades the verification of his thinking on the role of the cerebral cortex in knowledge utilization and storage; and (b) the value for brain science of the principles of organization of knowledge that Hayek successfully applied to social sciences.

Working memory is critical to the integration of information across time in goal-directed behavior, reasoning and language, yet its neural substrate is unknown. Based on recent research, we propose a mechanism by which the brain can retain working memory for prospective use, thereby bridging time in the perception/action cycle. The essence of the mechanism is the activation of 'cognits', which consist of distributed, overlapping and interactive cortical networks that in the aggregate encode the long-term memory of the subject. Working memory depends on the excitatory reentry between perceptual and executive cognits of posterior and frontal cortices, respectively. Given the pervasive role of working memory in the structuring of purposeful cognitive sequences, its mechanism looms essential to the foundation of behavior, reasoning and language.

The effects of reversible lesion--by cooling--of dorsolateral prefrontal and posterior parietal cortex were studied in rhesus monkeys performing a cognitive visuomotor integration task. Correct performance required the use of a learned set of cue-response contingencies, some spatial and some nonspatial; in some cases, the task required the short-term retention, through a delay, of the color of the cue or its implicit response direction. Prefrontal cooling impaired performance of the task regardless of its spatial demands, an effect that increased with delay. Parietal cooling, on the other hand, only impaired performance if the task demanded the processing and retention of spatial information (i.e., if spatial active memory was required). Parietal effects were not related to delay. Both prefrontal and, even more, parietal cooling increased response time in all task contingencies. Thus, the results dissociate the respective contributions of the prefrontal and the posterior parietal cortex to the temporal and spatial aspects of information processing in visuomotor performance. They indicate that posterior parietal areas participate in spatial processing and in active memory of spatial information, whereas prefrontal areas subserve a broader role of visuomotor processing and cross-temporal integration of both spatial and nonspatial information. Language: en

Neurons in the cortex exhibit a number of patterns that correlate with working memory. Specifically, averaged across trials of working memory tasks, neurons exhibit different firing rate patterns during the delay of those tasks. These patterns include: 1) persistent fixed-frequency elevated rates above baseline, 2) elevated rates that decay throughout the tasks memory period, 3) rates that accelerate throughout the delay, and 4) patterns of inhibited firing (below baseline) analogous to each of the preceding excitatory patterns. Persistent elevated rate patterns are believed to be the neural correlate of working memory retention and preparation for execution of behavioral/motor responses as required in working memory tasks. Models have proposed that such activity corresponds to stable attractors in cortical neural networks with fixed synaptic weights. However, the variability in patterned behavior and the firing statistics of real neurons across the entire range of those behaviors across and within trials of working memory tasks are typical not reproduced. Here we examine the effect of dynamic synapses and network architectures with multiple cortical areas on the states and dynamics of working memory networks. The analysis indicates that the multiple pattern types exhibited by cells in working memory networks are inherent in networks with dynamic synapses, and that the variability and firing statistics in such networks with distributed architectures agree with that observed in the cortex.

An expanding body of literature describes the existence of concerted brain activations in the absence of any external stimuli. Resting-state networks have been identified and demonstrated to be modulated during the performance of specific cognitive operations. However, despite mounting evidence the possibility still remains that those correlated signal fluctuations reflect non-neural phenomena. In order to isolate functionally relevant spontaneous coactivations, we utilized a multi-level sampling approach to obtain co-registered brain signals across a range of sampling resolution and sensitivity. Surface and local field potentials, hemodynamic signals (near-infrared spectroscopy, NIRS), and cell spiking were recorded from dorsolateral prefrontal and posterior parietal cortices in four monkeys trained to remain motionless in a primate chair. The use of an optical recording technique (NIRS) allows measurement of a signal that is physiologically equivalent to that obtained using BOLD fMRI, though with millisecond temporal resolution and minimal technical or environmental constraints. The different signal types exhibited correlations between the two regions of interest in both the frequency and time domains. This evidence suggests that the resting-state network activations detected by fMRI do in fact reflect functional coactivations of areas across multiple levels of network communication.

The role of prefrontal cortex in working memory (WM) is well established. However, questions remain regarding the topography and “domain-specific differentiation” of different types of information processing in the cortex. While it has been theorized that dorsolateral (DPFC) and ventrolateral (VPFC) prefrontal cortex preferentially process spatial and object WM, respectively, both electrophysiological evidence in the monkey and neuroimaging in the human have largely failed to demonstrate such regional differentiation. In this study we use near-infrared spectroscopy (NIRS) to detect functional changes, across relatively large cortical cell populations, simultaneously from prefrontal and posterior parietal cortices. Imaging data were recorded from a Rhesus macaque performing two types of WM tasks: a spatial task in which the animal had to retain the spatial position of a visual stimulus, and a non-spatial task where he had to retain its color (red or green) during a 20s delay. During performance of the spatial WM task, cerebral activation trends were found in which DPFC exhibited stronger activation than did the VPFC, and posterior parietal cortex maintained higher delay activation than did frontal regions. These differences were less pronounced during performance of the non-spatial task. Additionally, incorrect trials generally elicited lower activations during the delay period than did trials ending with a correct response. Furthermore, NIRS data collected during the performance of a haptic WM task also appear to exhibit inter-regional differences in delay activation. The data thus suggest the presence of preferential cognitive processing between and within posterior and frontal cortical regions.

Publisher Summary This chapter focuses on use of neuroimaging to study parts of brains, including prefrontal cortex. By use of noninvasive methods, it is now possible to assess regional cerebral blood flow (rCBF) or metabolism, and in this manner, indirectly, the levels of neuronal activity in various parts of the brain. Functional neuroimaging provides indirect records of activity simultaneously in various regions of the brain—in other words, functional maps of the brain. All neuroimaging is still subject to unresolved methodological problems that basically stem from the uncertain relationships between neuronal firing and the imaged variables, especially blood flow (neurovascular coupling). Despite these problems, remarkable progress has been made toward unraveling, by essentially noninvasive methods, the functions and dysfunctions of the human prefrontal cortex. Using positron emission tomography (PET), changes and differences in the concentration of radioactive tracers, and thus in neural activity, can be discerned in the millimeter range. For structural imaging of brain, the definition obtained by magnetic resonance imaging (MRI) is unsurpassed. In recent years, functional MRI has been developed and applied to the analysis of temporal changes in oxygenation or blood flow. Functional MRI (fMRI), by the blood oxygen level-dependent (BOLD) method, has thus become the preferred method for imaging the course of oxygenation, and by inference neuronal activity, in nervous tissue. Neuroimaging during sensory stimulation of various modalities substantiates the convergence of cortical sensory pathways upon prefrontal cortex that anatomical and physiological studies indicate.

Publisher Summary This chapter focuses on studies of the functions of the prefrontal cortex in animals. An experimental lesion in the prefrontal cortex remains a prime tool of neuropsychology. Lesions of the prefrontal cortex elicit characteristic behavioral abnormalities. These fall into three major categories: disorders of motility, disorders of emotion and social behavior, and deficits in performance of cognitive tasks, notably delay tasks. Some of the abnormalities are closely interrelated and denote the alteration of common functions, such as attention. Large ablations of the prefrontal cortex generally result in the impoverishment of emotional life and social isolation of the animal. Certain prefrontal lesions involving medial or basal prefrontal cortex have been seen to induce behavioral changes suggesting disinhibition of aggression and hunger drives, especially in carnivores. Hyperactivity effect of ablation is the most consistently observed in the macaque with an orbital lesion. Dorsolateral lesions may lead to increased aggressiveness, often accompanied by blunted emotional expression and communication. This deficit may be based largely on a cognitive impairment. Among the most consistent cognitive impairments of the animal with a prefrontal lesion are those of attention. The aging of the prefrontal cortex is accompanied by diminishing capacity to learn and perform cognitive tasks that depend on the functional integrity of this cortex, especially working short-term memory tasks.

Publisher Summary This chapter deals with neuropsychological effects of prefrontal damage in humans. The main sources of empirical data on the effects of prefrontal damage in the human are diseases and traumatic lesions of the frontal lobe, and cases of frontal psychosurgery. The neuropsychological effects of prefrontal damage vary greatly depending on the location and the extent of that damage. Apathy and general disinterest are common results of a large prefrontal lesion, especial if it involves lateral or medial cortex. Depression is the most common affective disorder from more circumscribed prefrontal lesions, especially if they involve the left lateral and polar cortex, although some affective disorders can result from orbital damage as well. Orbital lesions are accompanied by hyperreactivity to extraneous or irrelevant stimuli, yet low reactivity to emotional stimuli. They also lead to the weakening of autonomic or visceral signals that are concomitant to normal emotion. All prefrontal lesions, by reason of the emotional and cognitive changes they produce, tend to affect adversely the social life of the patient, usually constricting it. Orbital lesions, however, generally induce the most dramatic changes in social behavior, and these changes are usually opposite to social restraint. In the cognitive sphere, disorders of attention are the most common disorders of executive function caused by prefrontal damage. Those abnormalities of the control of attention may take several forms: loss of general alertness, sensory neglect, excessive distractibility, set-shifting disorder, disorder of ocular control, difficulty in sustaining attention, internal interference, and faulty executive set (executive attention).

Publisher Summary This chapter presents a discussion on chemical neurotransmission and role of chemical neurotransmitters in the prefrontal cortex. It begins with an introduction to neurotransmitters and neurotransmitter receptors. Cells produce certain chemical substances called neurotransmitters and neuromodulators that, through specific receptors embedded in pre- and postsynaptic membranes, modify the electrical activity of other nerve cells. The most important classes of neurotransmitters thus far identified in the cerebral cortex include: amino acids (i.e. glutamate (Glu), aspartate, γ -aminobutyric acid [GABA]); the monoamines, including two catecholamines, dopamine (DA) and norepinephrine (NE), and an indoleamine, 5-hydroxytriptamine (5-HT) or serotonin; Acetylcholine (ACh); and neuropeptides (e.g., enkephalins, substance P, somatostatin, neurotensin). Among the six best-known transmitters in prefrontal transmitters include Glu, GABA, NE, DA, 5-HT, and Ach. GABA is the prime inhibitory neurotransmitter in the central nervous system. Glutamate is the prime excitatory neurotransmitter in the cortex. It serves not only the local prefrontal circuitry but also the excitatory connectivity of the prefrontal cortex with striatal, thalamic, and limbic structures. The three monoaminergic systems, with their cells of origin in the brainstem, innervate the prefrontal cortex by way of ascending fiber paths that bypass the thalamus: the noradrenergic system (its transmitter NE) from the nucleus coeruleus, the dopaminergic system (its transmitter DA) from the ventral tegmentum, and the serotonergic system (its transmitter, serotonin, 5-HT) from the nuclei of the raphe. In addition to the monoamines, the prefrontal cortex, like the rest of the neocortex, receives profuse afferents from the subcortical components of the cholinergic system. Also active in the prefrontal cortex are a number of neuropeptides (somatostatin, substance P, CCK, angiotensin, neurotensin, and others) that act there as neurotransmitters or neuromodulators.

Rhesus monkeys were trained on a within-subjects design to assess whether they could perform concurrently visual-to-haptic (V-H) and haptic-to-visual (H-V) crossmodal delayed matching-to-sample (DMS). A parametric analysis was conducted of the effect of delay between presentation and re-presentation of the test discriminanda (three-dimensional geometric objects). The results indicate that (a) monkeys are capable of concurrent V-H and H-V crossmodal matching of objects by shape, size, and texture; (b) monkeys acquire faster and perform better crossmodal matching in the V-H direction than in the H-V direction; (c) as they learn to perform DMS with successive object pairs, monkeys transfer some--procedural--knowledge from the use of one pair to the use of the next; and (d) in the monkey, crossmodal short-term memory, as measured by DMS performance, has a temporal decline.

Executive actions are represented and hierarchically organized in the cortex of the frontal lobe. The representation and coordination of an action or series of actions have the same anatomical substrate: an executive neuronal network (cognit) in forntal cortex. That network interacts structurally and dynamically with perceptual networks of posterior cortex at the highest levels of the perception–action cycle.

Cells in the somatosensory cortex of the monkey are known to exhibit sustained elevations of firing frequency during the short-term mnemonic retention of tactile information in a haptic delay task. In this study, we examine the possibility that those firing elevations are accompanied by changes in firing pattern. Patterns are identified by the application of a pattern-searching algorithm to the interspike intervals of spike trains. By sequential use of sets of pattern templates with a range of temporal resolutions, we find patterned activity in the majority of the cells investigated. In general, the degree of patterning significantly increases during active memory. Surrogate analysis suggests that the observed patterns may not be simple linear stochastic functions of instantaneous or average firing frequency. Therefore, during the active retention of a memorandum, the activity of a 'memory cell' may be characterized not only by changes in frequency but also by changes in pattern.

The neural substrate for behavioral, cognitive and linguistic actions is hierarchically organized in the cortex of the frontal lobe. In their methodologically impeccable study, Koechlin et al. reveal the neural dynamics of the frontal hierarchy in behavioral action. Progressively higher areas control the performance of actions requiring the integration of progressively more complex and temporally dispersed information. The study substantiates the crucial role of the prefrontal cortex in the temporal organization of behavior.

Neurons in the monkey’s anterior parietal cortex (Brodmann’s areas 3a, 3b, 1, and 2) have been reported to retain information from a visual cue that has been associated with a tactile stimulus in a haptic memory task. This cross-modal transfer indicates that neurons in somatosensory cortex can respond to non-tactile stimuli if they are associated with tactile information needed for performance of the task. We hypothesized that neurons in somatosensory cortex would be activated by other non-tactile stimuli signaling the haptic movements—of arm and hand—that the task required. We found such cells in anterior parietal areas. They reacted with short-latency activity changes to an auditory signal (a click) that prompted those movements. Further, some of those cells changed their discharge in temporal correlation with the movements themselves, with the touch of the test objects, and with the short-term memory of those objects for subsequent tactile discrimination. These findings suggest that cells in the somatosensory cortex participate in the behavioral integration of auditory stimuli with other sensory stimuli and with motor acts that are associated with those stimuli.

Recent studies show that cells in the somatosensory cortex are involved in the short-term retention of tactile information. In addition, some somatosensory cells appear to retain visual information that has been associated with the touch of an object. The presence of such cells suggests that nontactile stimuli associated with touch have access to cortical neuron networks engaged in the haptic sense. Thus, we inferred that somatosensory cells would respond to behaviorally associated visual and tactile stimuli. To test this assumption, single units were recorded from the anterior parietal cortex (Brodmann's areas 3a, 3b, 1, and 2) of monkeys performing a visuo-haptic delay task, which required the memorization of a visual cue for a tactile choice. Most cells responding to that cue responded also to the corresponding object presented for tactile choice. Significant correlations were observed in some cells between their differential reactions to tactile objects and their differential reactions to the associated visual cues. Some cells were recorded in both the cross-modal task and a haptic unimodal task, where the animal had to retain a tactile cue for a tactile choice. In most of these cells, correlations were observed between stimulus-related firing in corresponding cue periods of the two tasks. These findings suggest that cells in somatosensory cortex are the components of neuronal networks representing tactile information. Associated visual stimuli may activate such networks through visuo-haptic associations established by behavioral training.

The neuronal networks of the frontal lobe that represent motor or executive memories are probably the same networks that cooperate with other cerebral structures in the temporal organization of behavior. The prefrontal cortex, at the top of the perception-action cycle, plays a critical role in the mediation of contingencies of action across time, an essential aspect of temporal organization. That role of cross-temporal mediation is based on the interplay of two short-term cognitive functions: one retrospective, of short-term active perceptual memory, and the other prospective, of attentive set (or active motor memory). Both appear represented in the neuronal populations of dorsolateral prefrontal cortex. At least one of the mechanisms for the retention of active memory of either kind seems to be the reentry of excitability through recurrent cortical circuits. With those two complementary and temporally symmetrical cognitive functions of active memory for the sensory past and for the motor future, the prefrontal cortex seems to secure the temporal closure at the top of the perception-action cycle.

The prefrontal cortex is essential for the temporal integration of sensory information in behavioural and linguistic sequences. Such information is commonly encoded in more than one sense modality, notably sight and sound. Connections from sensory cortices to the prefrontal cortex support its integrative function. Here we present the first evidence that prefrontal cortex cells associate visual and auditory stimuli across time. We gave monkeys the task of remembering a tone of a certain pitch for 10 s and then choosing the colour associated with it. In this task, prefrontal cortex cells responded selectively to tones, and most of them also responded to colours according to the task rule. Thus, their reaction to a tone was correlated with their subsequent reaction to the associated colour. This correlation faltered in trials ending in behavioural error. We conclude that prefrontal cortex neurons are part of integrative networks that represent behaviourally meaningful cross-modal associations. The orderly and timely activation of neurons in such networks is crucial for the temporal transfer of information in the structuring of behaviour, reasoning and language.

Memory networks are formed in the cerebral cortex by associative processes, following Hebbian principles of synaptic modulation. Sensory and motor memory networks are made of elementary representations in cell assemblies of primary sensory and motor cortex (phyletic memory). Higher-order individual memories, e.g. episodic, semantic, conceptual — are represented in hierarchically organized neuronal networks of the cortex of association. Perceptual memories are organized in posterior (post-rolandic) cortex, motor (executive) memories in cortex of the frontal lobe. Memory networks overlap and interact profusely with one another, such that a cellular assembly can be part of many memories or networks. Working memory essentially consists in the temporary activation of a memory network, as needed for the execution of successive acts in a temporal structure of behavior. That activation of the network is maintained by recurrent excitation through reentrant circuits. The recurrent reentry may occur within local circuits as well as between separate cortical areas. In either case. recurrence binds together the associated components of the network and thus of the memory it represents.

In the primate, the cortex of the frontal lobe appears devoted in its entirety to the representation and execution of actions. The frontal cortex as a whole can therefore be considered “motor cortex” in the broadest sense of the word. It coordinates actions in practically all the domains of adaptation of the organism to its environment; skeletal and ocular motility, logical reasoning, communication and the spoken language. Even visceral actions and emotional behavior are regulated by certain orbital and medial areas of the frontal cortex. This chapter outlines the rationale for the role of the dorsolateral prefrontal cortex in the temporal organization of action, as well as some of the mechanisms that support it. It begins with certain basic assumptions about the cortex in general and the frontal cortex in particular. The cognitive functions of the cortex of the frontal lobe, as those of any other part of the neocortex, consist in the activation and processing within and between networks of representation, or memory networks. Those networks are widely distributed and highly specific, defined by their synaptic structure and connectivity. Thus, the memory code is a relational code, and all memory is associative. The cortical networks of memory extend across modules and areas by any anatomical definition. Memory networks overlap and are profusely interconnected with one another. Thus, one neuron or group of neurons anywhere in the cortex can be a part of many networks and thus many memories. This is why it is virtually impossible, by any method, to localize a memory.

The dorsolateral prefrontal cortex (DPFC) and the posterior parietal cortex (PPC) are anatomically and functionally interconnected, and have been implicated in working memory and the preparation for behavioral action. To substantiate those functions at the neuronal level, we designed a visuomotor task that dissociated the perceptual and executive aspects of the perception-action cycle in both space and time. In that task, the trial-initiating cue (a color) indicated with different degrees of certainty the direction of the correct manual response 12 s later. We recorded extracellular activity from 258 prefrontal and 223 parietal units in two monkeys performing the task. In the DPFC, some units (memory cells) were attuned to the color of the cue, independent of the response-direction it connoted. Their discharge tended to diminish in the course of the delay between cue and response. In contrast, few color-related units were found in PPC, and these did not show decreasing patterns of delay activity. Other units in both cortices (set cells) were attuned to response-direction and tended to accelerate their firing in anticipation of the response and in proportion to the predictability of its direction. A third group of units was related to the determinacy of the act; their firing was attuned to the certainty with which the animal could predict the correct response, whatever its direction. Cells of the three types were found closely intermingled histologically. These findings further support and define the role of DPFC in executive functions and in the temporal closure of the perception-action cycle. The findings also agree with the involvement of PPC in spatial aspects of visuomotor behavior, and add a temporal integrative dimension to that involvement. Together, the results provide physiological evidence for the role of a prefrontal-parietal network in the integration of perception with action across time. Language: en

One example of “emergence” is the development, as a result of neural ontogeny and living experience, of cortical networks capable of representing and retaining cognitive information. A large body of evidence from neuropsychology, electrophysiology and neuroimaging indi­cates that so-called working memory and long-term memory share the same neural substrate in the cerebral cortex. That substrate consists in a system of widespread, overlapping and hierarchically organized networks of cortical neurons. In this system, any neuron or group of neurons can be part of many networks, and thus many memories. Working memory is the temporary activation of one such network of long-term memory for the purpose of executing an action in the near future. The activation of the network may be brought about by stimuli that by virtue of prior experience are in some manner associated with the cognitive content of the network, including the response of the organism to those stimuli. The mechanisms by which the network stays activated are presumed to include the recurrent re-entry of impulses through associated neuronal assemblies of the network. Consistent with this notion is the following evidence: (1) working memory depends on the functional integrity of cortico-corti-cal connective loops; and (2) during working memory, remarkable similarities -including “attractor behavior” -have been observed between firing patterns in real cortex and in an artificial recurrent network.

Neuropsychology points to the wide distribution of cortical memory networks. Electrophysiology and neuroimaging indicate that working memory, like long-term memory, is a widely distributed function, largely neocortical. Most of the evidence available from those three methodologies suggests that both working memory and long-term memory share the same substrate: a system of broad, partly overlapping and interconnected neocortical networks. Working memory appears mostly, if not completely, characterized by the sustained activation of one widely distributed network of long-term memory. That activation is at least in part sustained by reentrant excitatory loops through the different neuronal assemblies that constitute the network and that represent the associated features of the memorandum.

Single-unit spike trains recorded from parietal cortex of monkeys performing a tactile short-term memory task show characteristic fluctuations (transitions) in their firing frequency that are related to memory. Spike trains recorded during the memory period, when the animal must retain information for the short term, show a higher rate of such transitions than spike trains recorded during intertrial baseline periods. In the present study, an analysis of multiple temporal resolutions over which these transitions are observed reveals that the memory-related transitions occur most prominently in the 25–50 Hz range. The results of this study suggest that, in the monkey, high frequency fluctuations of neuronal discharge in the parietal cortex are correlated with haptic short-term memory. The presence of such fluctuations are also consistent with theoretical models of short-term memory.

Studies have shown that in the monkey′s associative cerebral cortex, cells undergo sustained activation of discharge while the animal retains information for a subsequent action. Recent work has revealed the presence of such ″memory cells″ in the anterior parietal cortex (Brodmann′s areas 3a, 3b, 1, and 2) – the early stage of the cortical somatosensory system. Here we inferred that, in a cross-modal visuo-haptic short-term memory task, somatosensory cells would react to visual stimuli associated with tactile features. Single-unit discharge was recorded from the anterior parietal cortex – including areas of hand representation – of monkeys performing a visuo-haptic delayed matching-to-sample task. Units changed firing frequency during the presentation of a visual cue that the animal had to remember for making a correct tactile choice between two objects at the end of a delay (retention period). Some units showed sustained activation during the delay. In some of them that activation differed depending on the cue. These findings suggest that units in somatosensory cortex react to visual stimuli behaviorally associated with tactile information. Further, the results suggest that some of these neurons are involved in short-term active memory and may, therefore, be part of cross-modal memory networks.

The trion model is a highly structured representation of cortical organization, which predicts families of symmetric spatial-temporal firing patterns inherent in cortical activity. The symmetries of these inherent firing patterns are used by the brain in short-term memory to perform higher level computations. In the present study, symmetric temporal patterns were searched for in spike trains recorded from cells in parietal cortex of a monkey performing a short-term memory task. A new method of analysis was used to map neuronal firing into sequences of integers representing relative levels of firing rate about the mean (i.e. -1, 0 and 1). The results of this analysis show families of patterns related by symmetry operations. These operations are: i. the interchanging of all the +1's and -1's in a given pattern sequence (C symmetry), ii. the inverting of the temporal sequence of the mapping (T symmetry), and iii. the combination of the two previous operations (CT symmetry). Patterns of a given family are found across cells, especially in the memory periods of the task; in most cases they reoccur within a given spike train. The pattern families predicted by the model and reported here should be further investigated in multiple microelectrode and EEG recordings.

Bodner, Mark, Yong-Di Zhou, and Joaquı́n M. Fuster. Binary mapping of cortical spike trains in short-term memory. J. Neurophysiol. 77: 2219–2222, 1997. Microelectrode studies in monkeys performing short-term memory tasks show the sustained elevated discharge of cortical neurons during the retention of recalled sensory information. Cortical cells that are part of memory networks are assumed to receive numerous inputs of excitatory as well as inhibitory nature and local as well as remote. Thus it is reasonable to postulate that the temporal and spatial summation of diverse inputs on any cell in an activated network will result in temporally discrete groups of spikes in its firing. The activation of a network in active memory supposedly increases the magnitude and diversity of those inputs and thus increases the discontinuities and frequency fluctuations in the firing of cells in the network. In this study we use a new method of analysis that allows the quantification of firing discontinuities in a spike train. We apply it to parietal cells recorded from monkeys during the performance of a tactile short-term memory task. In our method, time is divided into bins of equal duration and the measure of discontinuities is the total count of the number of transitions between consecutive time bins with and without spikes. The results of the analysis show that in many of the cells studied, discontinuities (transitions between spiking and nonspiking) reflect memory-related activity obscured in the measures of raw spike frequency over a wide range of frequencies. These cells show more firing transitions in active short-term memory than in baseline (intertrial) conditions.

The activity of single neurons was recorded extracellularly from dorsolateral prefrontal cortex (DPC) of monkeys during the performance of a cross-modal audio-visual short-term memory task. Cells in DPC show sustained elevated firing levels (higher than spontaneous discharge) during the retention of the auditory stimulus. In some cells this elevated firing was significantly different depending on the particular auditory memorandum of each trial. These results support the notion that DPC participates in auditory short-term memory and the integration of auditory and visual information for prospective action.

Motor representations are hierarchically organized in dorsolateral frontal cortex. The highest, most global plans and schemes of action appear to be represented in prefrontal cortex, intermediate ones in premotor cortex, and the most elementary motor acts in primary motor cortex. The confluence of external and internal inputs on frontal cortex leads to the activation of frontal neuron networks representing different categories of action. The activation of these networks is the physiological substrate for the initiation and execution of behavioral action.

The main purpose of this study was to explore the role of dorsolateral prefrontal cortex in skilled and sequential haptic performance. Monkeys were trained to perform a delayed matching-to-sample task that required the memorization of three-dimensional objects perceived either by palpation (haptically) or by sight. At the start of a trial the animal was allowed to touch or view an object, the sample; after a period of delay, during which the object remained out of touch and out of sight, the animal was presented with two side-by-side objects--one of them the sample--for either tactile or visual recognition, and the choice of the sample (correct match) was rewarded. Three variants of the task were used: (1) visual sample, haptic match; (2) haptic sample, visual match; and (3) haptic sample, haptic match. The temporary bilateral cooling of dorsolateral prefrontal cortex to 15 degrees C induced a reversible deficit in performance of all three tasks. Cooling to the same degree a portion of posterior parietal cortex of equivalent size did not significantly alter either performance or reaction time. These findings indicate that the functional integrity of the dorsolateral prefrontal cortex is important for performance of sequential behavior dependent on haptic skill. Further, the results suggest that the role of this cortex in active memory, already well documented for spatially and nonspatially defined visual information, extends also to tactile information and associated motor acts.

Studies of subhuman primates and man have shown that the prefrontal cortex is important for spatial working memory. We have used 18fluorodeoxyglucose positron emission tomography (18FDG-PET) to study a non-spatial, abstract visual memory task of in man. Using a regions-of-interest approach with discriminant analysis of the relative regional cerebral metabolic rate of glucose consumption (rCMRGlc), we found that changes in dorsal prefrontal, premotor/motor frontal and posterior cingulate areas differentiated the primary memory task from the control task. Less robust increases in glucose uptake were observed in lateral parietal cortex, while some subcortical and limbic regions showed decreases. This is the first activation study with a non-spatial, visual task. These results complement previous studies in that they substantiate the role of the prefrontal cortex in the mediation of cross-temporal contingencies of behavior, and point to a role of the premotor region in this mediation as well.

Introduccion. Los avances de la neurociencia cognitiva en los ultimos anos nos obligan a cambiar radicalmente el modelo tradicional de representacion de memoria en la corteza cerebral. El viejo modelo (modular) postulaba un area distinta para cada forma de representacion cognitiva (memoria operante, visual, auditiva, tactil, fisonomica, semantica, etc.). En el nuevo paradigma, las memorias y objetos mentales de conocimiento estan constituidos por amplias redes de neuronas corticales ligadas sinapticamente por la experiencia. Desarrollo. Se presentan los principios fundamentales de este paradigma, con enfasis en sus aspectos estructurales, clinicos y de desarrollo. A partir del nacimiento, y con cada nueva experiencia, estas redes o cognitos se van formando o reformando por medio de procesos asociativos sinapticos que siguen gradientes filogeneticos, ontogeneticos y conectivos, desde las areas sensoriales y motoras hacia las cortezas asociativas. Los cognitos nuevos se van autoorganizando en dos jerarquias de redes, con base sensorial y motora. La jerarquia perceptual, en la corteza posterior, representa cognitos definidos por parametros sensoriales en areas sensoriales primarias, y los cognitos perceptivos individuales (por ejemplo, memoria autobiografica y episodica, conocimiento semantico), en areas asociativas posteriores. La jerarquia ejecutiva, por otra parte, representa movimientos concretos en las areas motoras frontales, y acciones mas complejas (p. ej., planes de conducta) en la corteza prefrontal. Conclusiones. La investigacion reciente nos obliga a abandonar los modelos tradicionales, 'modulares' o 'geograficos', de la memoria cortical. En su lugar, se impone con creciente vigor su paradigma reticular, el cual tiene importantes implicaciones con respecto al desarrollo cognitivo del individuo, la clinica de las lesiones corticales y la rehabilitacion del enfermo con tales lesiones.

Single-neuron discharge was recorded from prefrontal and posterior parietal cortex in monkeys performing a visuo-motor memory task with temporal and spatial separation between cue (color) and directional manual response. During the delay interval between cue and response, neurons in both cortices engaged in two concurrent and reciprocal trends of discharge: (a) sensory-coupled, decelerating firing apparently related to color retention, or (b) motor-coupled, accelerating firing apparently related to the anticipated response direction. In both cortices, the acceleration of the direction-anticipating activity was related to the probability with which the animal could predict, and prepare for, the correct response site. Our findings suggest that neurons from prefrontal and parietal cortex are part of distributed networks, with representational and operational properties, for visuomotor cognitive processing.

The question is raised whether the sequence of spikes of a cortical neuron, i.e. its spike train, is related to cognitive functions. Neuronal patterns of firing in the inferotemporal cortex of monkeys performing visual delayed-matching tasks showed that short-term memory was accompanied by decreased bursting and changes in the incidence of recurrent spike-interval patterns. The temporal structure of the spike train suggests an inverse relationship between the incidence of repeated patterns and the degree of selectivity of sustained firing frequency elicited by the memorandum (sample stimulus).

The prefrontal cortex is critical for temporal organization of behavior. It mediates cross-temporal sensory-motor contingencies, integrating motor action (including speech) with recent sensory information. It performs this role through cooperation of two cognitive functions represented in its dorsolateral areas: short-term memory (STM) and preparatory set. Supporting data have been obtained from monkeys performing delay tasks, which epitomize the principle of cross-temporal contingency. In a given trial, the animal performs an act contingent on a sensory cue given a few seconds or minutes earlier. During the delay between cue and response, cells in dorsolateral prefrontal cortex show sustained activation. Two cell categories can be identified in tasks in which cue and response are spatially separate. Cells of the first participate in STM: Their activation tends to diminish as the delay progresses; in some, the activation level depends on the particular cue received. Similar cells are found elsewhere in cortex. Cells of the second category seem to take part in preparation of motor response: Their activation tends to increase in anticipation of it and may be attuned to the particular movement the cue calls for. This cell type is rare outside of frontal cortex. The temporally integrative function of the prefrontal cortex is probably based on local interactions between "memory" and "motor-set" cells, as well as on neural associations between prefrontal cortex and posterior cortical areas.

The prefrontal cortex (PFC) is critical for temporal organization of behavior. It mediates cross-temporal sensorimotor contingencies, integrating motor action (including speech) with recent sensory information. It performs this role through cooperation of 2 cognitive functions represented in its dorsolateral areas: short-term memory (STM) and preparatory set. Supporting data have been obtained from monkeys performing delay tasks, which epitomize the principle of cross-temporal contingency. In a given trial, the animal performs an act contingent on a sensory cue given a few seconds or minutes earlier. During the delay between cue and response, cells in dorsolateral PFC show sustained activation. Two cell categories can be identified in tasks in which cue and response are spatially separate. Cells of the first participate in STM: Their activation tends to diminish as the delay progresses; in some, the activation level depends on the particular cue received. Similar cells are found elsewhere in the cortex. Cells of the second category seem to take part in preparation of motor response: Their activation tends to increase in anticipation of it and may be attuned to the particular movement the cue calls for. This cell type is rare outside of the frontal cortex. The temporally integrative function of the PFC is probably based on local interactions between "memory" and "motor-set" cells, as well as on neural associations between PFC and posterior cortical areas.

The correlative coactivation of sensory inputs, Hebb's “second rule,” probably plays a critical role in the formation of word representations in the neocortex. It is essential to the acquisition of word meaning. The acquisition of semantic memory is inseparable from that of individual memory, and therefore the two probably share the same neural connective substrate. Thus, “content” words are represented mainly in postrolandic cortex, where individual perceptual memories are also represented, whereas “action” words are represented in frontal cortex, with executive memories. The activation of a memory network may not necessarily entail the high-frequency oscillatory firing of its cells, though reverberation remains a plausible mechanism of short-term memory.

1. This research was designed to further clarify how, in the primate, the neurons of the inferotemporal (IT) cortex support the cognitive functions of visually guided behavior. Specifically, the aim was to determine the role of those neurons in 1) selective attention to behaviorally relevant features of the visual environment and 2) retention of those features in temporary memory. Monkeys were trained in a memory task in which they had to discriminate and retain individual features of compound stimuli, each stimulus consisting of a colored disk with a gray symbol in the middle. A trial began with brief presentation of one such stimulus, the sample for the trial. Depending on the symbol in it, the monkey had to memorize the symbol itself or the background color; after 10-20 s of delay (retention period), two compound stimuli appeared, and the animal had to choose the one with the symbol or with the color of the sample. Thus the test required attention to the symbol, in some trials also to the color, and short-term retention of the distinctive feature for each trial, either a symbol or a color. Single-unit activity was recorded from cortex of the IT convexity, lower and upper banks of the superior temporal sulcus (STS), and from striate cortex (V1). Firing frequency was analyzed during intertrial periods and during the entirety of every trial, except for the (match) choice period. 2. In IT cortex, as in V1, many units responded to the sample stimulus. Some responded indiscriminately to all samples, whereas others responded selectively to one of their features, i.e., to one symbol or to one color. Fifteen percent of the IT units were symbol selective and 21% color selective. These neurons appeared capable of extracting individual features from complex stimuli. Some color cells (color-attentive units) responded significantly more to their preferred color when it was relevant (i.e., had to be retained) than when it was not. 3. The latency of IT-unit response to the sample stimulus was, on the average, relatively short in unselective units (mean 159 ms), longer in symbol units (mean 203 ms), and longest in color-attentive units (mean 270 ms). This order of latencies corresponds to the presumed order of participation of those three types of units in the selective attention to the component features of the sample as required by the task. It suggests intervening steps of serial processing before color information reached color-attentive cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Normal behavior is characterized by a constant circular flow of influences from sensory receptors to motor effectors, to the physical environment, back to sensory receptors, and so on. This cybernetic cycle of influences (the perception-action cycle) governs all sequences of behavior to make them adaptive and goal directed. In the primate (including the human primate), considerable evidence indicates that dorsolateral prefrontal cortex is essential for the bridging of temporal gaps in the perception-action cycle, in other words, for mediating cross-temporal contingencies of behavior. This chapter summarizes some neuropsychological and neurophysiological evidence in support of this conclusion. The evidence has been obtained from monkeys performing delay tasks, which epitomize the principle of cross-temporal contingency.