Prefrontal cortical unit activity and delayed alternation performance in monkeys

Regional and cellular fractionation of working memory

This chapter recounts efforts to dissect the cellular and circuit basis of a memory system in the primate cortex with the goal of extending the insights gained from the study of normal brain organization in animal models to an understanding of human cognition and related memory disorders. Primates and humans have developed an extraordinary capacity to process information "on line," a capacity that is widely considered to underlay comprehension, thinking, and so-called executive functions. Understanding the interactions between the major cellular constituents of cortical circuits-pyramidal and nonpyramidal cells-is considered a necessary step in unraveling the cellular mechanisms subserving working memory mechanisms and, ultimately, cognitive processes. Evidence from a variety of sources is accumulating to indicate that dopamine has a major role in regulating the excitability of the cortical circuitry upon which the working memory function of prefrontal cortex depends. Here, I describe several direct and indirect intercellular mechanisms for modulating working memory function in prefrontal cortex based on the localization of dopamine receptors on the distal dendrites and spines of pyramidal cells and on interneurons in the prefrontal cortex. Interactions between monoamines and a compromised cortical circuitry may hold the key to understanding the variety of memory disorders associated with aging and disease.

Evidence for the importance of dopamine for prefrontal cortex functions early in life

There is considerable evidence that dorsolateral prefrontal cortex subserves critical cognitive abilities even during early infancy and that improvement in these abilities is evident over roughly the next 10 years. We also know that (a) in adult monkeys these cognitive abilities depend critically on the dopaminergic projection to prefrontal cortex and (b) the distribution of dopamine axons within dorsolateral prefrontal cortex changes, and the level of dopamine increases, during the period that infant monkeys are improving on tasks that require the cognitive abilities dependent on prefrontal cortex. To begin to look at whether these cognitive abilities depend critically on the prefrontal dopamine projection in humans even during infancy and early childhood we have been studying children who we hypothesized might have a selective reduction in the dopaminergic innervation of prefrontal cortex and a selective impairment in the cognitive functions subserved by dorsolateral prefrontal cortex. These are children treated early and continuously for the genetic disorder, phenylketonuria (PKU). In PKU the ability to convert the amino acid, phenylalanine (Phe), into another amino acid, tyrosine (Tyr), is impaired. This causes Phe to accumulate in the bloodstream to dangerously high levels and the plasma level of Tyr to fall. Widespread brain damage and severe mental retardation result. When PKU is moderately well controlled by a diet low in Phe (thus keeping the imbalance between Phe and Tyr in plasma within moderate limits) severe mental retardation is averted, but deficits remain in higher cognitive functions. In a four-year longitudinal study we have found these deficits to be in the working memory and inhibitory control functions dependent upon dorsolateral prefrontal cortex in PKU children with plasma Phe levels 3-5 times normal. The fact that even infants showed these impairments suggests that dopaminergic innervation to prefrontal cortex is critical for the proper expression of these abilities even during the first year of life. To test the hypothesis about the underlying biological mechanism we have created the first animal model of early and continuously treated PKU. As predicted, the experimental animals had reduced levels of dopamine and the dopamine metabolite, homovanillic acid (HVA), in prefrontal cortex and showed impaired performance on delayed alternation, a task dependent on prefrontal cortex function. Noradrenaline levels were unaffected; however some reduction in serotonin levels and in dopamine levels outside the prefrontal cortex was found. If prefrontal cortex functions are vulnerable in children with a moderate plasma Phe:Tyr imbalance because of the special properties of the dopamine neurons that project to prefrontal cortex, then other dopamine neurons that share those same properties should also be vulnerable in these children. The dopamine neurons in the retina share these properties (i.e. unusually high firing and dopamine turnover rates), and we have found that PKU children with plasma Phe levels 3-5 times normal are impaired in their contrast sensitivity, a behavioural measure sensitive to retinal dopamine levels.

Reward expectancy in primate prefrontal neurons

The prefrontal cortex is important in the organization of goal-directed behaviour. When animals are trained to work for a particular goal or reward, reward 'expectancy' is processed by prefrontal neurons. Recent studies of the prefrontal cortex have concentrated on the role of working memory in the control of behaviour. In spatial delayed-response tasks, neurons in the prefrontal cortex show activity changes during the delay period between presentation of the cue and the reward, with some of the neurons being spatially specific (that is, responses vary with the cue position). Here I report that the delay activity in prefrontal neurons is dependent also on the particular reward received for the behavioural response, and to the way the reward is given. It seems that the prefrontal cortex may monitor the outcome of goal-directed behaviour.

Synchronization and cooperative interaction in brain activity

A conception is advanced according to which synchronization and the cooperative interaction of plastic processes at the level of the individual cell and of cell units of varying degrees of complexity form a principle of cerebral integration. The triggering and unfolding of plastic reorganizations which take place with the participation of motivational-emotional structures are realized through the mechanism of alteration of cell excitability. These influences are widely distributed throughout the cerebral cortex, but are selective in relation to the current need of the organism.

Intrinsic circuit organization of the major layers and sublayers of the dorsolateral prefrontal cortex in the rhesus monkey

Intrinsic connections are likely to play important roles in cognitive information processing in the prefrontal association cortex. To gain insight into the organization of these circuits, intracortical connections of major laminar and sublaminar divisions were retrogradely labeled in Walker's area 9 and 46 in rhesus monkeys by using cholera toxin (B-subunit) conjugated to colloidal gold. Microinjections placed within particular cortical laminae produced unique patterns of retrograde labeling. Injections in layers II/III yielded labeling which was laterally widespread (2-7 mm) in supragranular layers, and more narrowly focused, i.e., conforming to a column, in layers IV-VI. In contrast, local circuits associated with layers IV and Vb displayed a regular, cylindrical organization, whereas intrinsic connections of layer Va were laterally extensive (3-5 mm) in layers III and Va. Finally, injections in layer VI gave rise to a narrow column of cell labeling traversing all layers, augmented by laterally extensive labeling (approximately 7 mm) in layer VI. The intrinsic connections of the prefrontal cortex were arrayed within mediolaterally elongated stripes which were often distributed asymmetrically in either the medial or lateral direction. In addition, labeled cells within these mediolaterally oriented fields were frequently grouped within discrete clusters or narrow bands. The intrinsic connections identified in this study differ from the local circuits of corresponding layers reported for primary visual cortex; the unique intrinsic wiring diagram of the prefrontal cortex may be related to its specialized cognitive and mnemonic functions.

A spatial oculomotor memory-task performance produces a task-related slow shift in human electroencephalography

Electroencephalographic (EEG) deflections in humans related to the performance of memory-guided saccades were studied in this work. The EEG deflections were recorded during 2 spatial oculomotor delayed response tasks in which the subject was instructed to make a saccade either to the right or to the left depending on the spatial location of the cue which had been shown in the beginning of the delay period. The EEG deflections were compared to those recorded during a control task in which the subject also made a saccade to the right or to the left after a delay but the requirement to keep spatial information actively in mind was minimized. A slow delay-related shift was recorded during all task conditions. The slow shift was positive in the most frontal and negative in the more posterior recording sites. The negative slow shift in the more posterior recording sites was larger in the memory tasks than in the control task. Since the memory and the control tasks differed mainly in their requirement to hold spatial information in mind it is suggested that the difference in the magnitude of slow shifts between the memory and the control tasks reflects neural activity related to spatial working memory. But although the oculomotor responses in all tasks were similar, the preparatory activities for the impending eye movements may not have been similar and in addition to working memory may have contributed to the observed differences in the slow shifts.

Topographically specific hippocampal projections target functionally distinct prefrontal areas in the rhesus monkey

The sources of ipsilateral projections from the hippocampal formation, the presubiculum, area 29a-c, and parasubiculum to medial, orbital, and lateral prefrontal cortices were studied with retrograde tracers in 27 rhesus monkeys. Labeled neurons within the hippocampal formation (CA1, CA1', prosubiculum, and subiculum) were found rostrally, although some were noted throughout the entire rostrocaudal extent of the hippocampal formation. Most labeled neurons in the hippocampal formation projected to medial prefrontal cortices, followed by orbital areas. In addition, there were differences in the topography of afferent neurons projecting to medial when compared with orbital cortices. Labeled neurons innervating medial cortices were found mainly in the CA1' and CA1 fields rostrally, but originated in the subicular fields caudally. In contrast, labeled neurons which innervated orbital cortices were considerably more focal, emanating from the same relative position within a field throughout the rostrocaudal extent of the hippocampal formation. In marked contrast to the pattern of projection to medial and orbital prefrontal cortices, lateral prefrontal areas received projections from only a few labeled neurons found mostly in the subicular fields. Lateral prefrontal cortices received the most robust projections from the presubiculum and the supracallosal area 29a-c. Orbital, and to a lesser extent medial, prefrontal areas received projections from a smaller but significant number of neurons from the presubiculum and area 29a-c. Only a few labeled neurons were found in the parasubiculum, and most projected to medial prefrontal areas. The results suggest that functionally distinct prefrontal cortices receive projections from different components of the hippocampal region. Medial and orbital prefrontal cortices may have a role in long-term mnemonic processes similar to those associated with the hippocampal formation with which they are linked. Moreover, the preponderance of projection neurons from the hippocampal formation innervating medial when compared with orbital prefrontal areas followed the opposite trend from what we had observed previously for the amygdala (Barbas and De Olmos [1990] (J Comp Neurol 301:1-23). Thus, the hippocampal formation, associated with mnemonic processes, targets predominantly medial prefrontal cortices, whereas the amygdala, associated with emotional aspects of memory, issues robust projections to orbital limbic cortices. Lateral prefrontal cortices receive robust projections from the presubiculum and area 29a-c and sparse projections from the hippocampal formation. These findings are consistent with the idea that the role of lateral prefrontal cortices in memory is distinct from that of either medial or orbital cortices. The results suggest that signals from functionally distinct limbic structures to some extent follow parallel pathways to functionally distinct prefrontal cortices.

Increased neuronal firing in the rat auditory cortex associated with preparatory set

Extracellular single neuronal firings were recorded in the auditory cortex of rats (n = 4) performing a visual reaction-time task with a warning tone (10 kHz, 10 ms duration), which preceded the imperative light stimulus by an interstimulus interval (ISI) of 1.4 s. Thirty-six neuronal firings were evoked by the warning tone, with the peak latency being between 15 and 55 ms. Among them, nine neurons (25%) showed an increased firing frequency following the evoked response during the ISI, which was, in average, 2.5 times as high as the firing frequency during the baseline period. When the tones were presented independent of the imperative stimulus, such sustained increase in neuronal firing was not observed. Activation of the sensory cortex during the ISI may constitute one of the neuronal modulations related to preparatory set.

Working memory and prefrontal cortex

Several different types of memory have recently been proposed, some of which are believed to operate within specific areas in the brain. In this article, we will discuss the relationship between the prefrontal cortex and working memory, which is a recently proposed type of short-term memory. The tight relationship between the prefrontal cortex and working memory has been supported by recent human and animal studies. This relationship provides good evidence that a particular type of memory is related to a particular brain structure, and can be used as an important model for understanding the neuronal mechanisms of memory. In this article, we will present a modular model based on recent neurophysiological results and discuss for spatial working memory processes in the prefrontal cortex.

Information processing for motor control in primate premotor cortex

Recent neurophysiological as well as neuropsychological studies provided evidences on how various informations are processed to generate motor programs in the central nervous system. In this article, functional specializations of two distinct cortical motor areas, the dorsal and ventral aspects of the premotor cortex (PMd and PMv, respectively) of macaque monkeys, are focused to review this issue. Three major conclusions emerged from recent neurophysiological studies. First, each of movement parameters such as amplitude and direction is distinctively programmed in PMd by serial integration, rather than by parallel distributed processing. Second, in performance of conditional motor behavior, conditionally presented sensory signals are processed for motor preparation and execution of an intended act in PMd, but not in PMv. Third, PMv may be specialized for motor execution under visual guidance.

Interconnections between the prefrontal cortex and the premotor areas in the frontal lobe

We examined interconnections between a portion of the prefrontal cortex and the premotor areas in the frontal lobe to provide insights into the routes by which the prefrontal cortex gains access to the primary motor cortex and the central control of movement. We placed multiple injections of one retrograde tracer in the arm area of the primary motor cortex to define the premotor areas in the frontal lobe. Then, in the same animal, we placed multiple injections of another retrograde tracer in and around the principal sulcus (Walker's area 46). This double labeling strategy enabled us to determine which premotor areas are interconnected with the prefrontal cortex. There are three major results of this study. First, we found that five of the six premotor areas in the frontal lobe are interconnected with the dorsolateral prefrontal cortex. Second, the major site for interactions between the prefrontal cortex and the premotor areas is the ventral premotor area. Third, the prefrontal cortex is interconnected with only a portion of the arm representation in three premotor areas (supplementary motor area, the caudal cingulate motor area on the ventral bank of the cingulate sulcus, and the dorsal premotor area), whereas it is interconnected with the entire arm representation in the ventral premotor area and the rostral cingulate motor area. These observations indicate that the output of the prefrontal cortex targets specific premotor areas and even subregions within individual premotor areas.

Prefrontal connections of medial motor areas in the rhesus monkey

Several areas on the medial surface of the frontal lobe in both monkeys and humans, including the supplementary motor area and specific areas within the ventral bank of the cingulate sulcus called the cingulate motor areas, have been implicated in the initiation and execution of skilled movements. These areas project directly to the motor cortex and spinal cord, and, on this basis alone, can be considered premotor areas. The present study investigated whether these premotor areas are specific targets of prefrontal cortical projections in the rhesus monkey and thereby provide links between this association cortex and motor effector pathways. Circumscribed injections of wheat germ agglutinin-conjugated horseradish peroxidase were placed into different cytoarchitectonic subdivisions of prefrontal cortex, and resultant retrograde and anterograde labeling examined with respect to designated premotor targets. Conversely, injections were also made in the supplementary and cingulate motor areas and labeled cells and terminals charted in the prefrontal cortex. A principal finding in this study is the identification of multiple prefrontal regions that project to the supplementary motor area, the cingulate motor areas, or both. Areas 46, 8a, 9, 11, and 12 are reciprocally connected with an area of the superior frontal gyrus in or near the supplementary motor area at its rostral margin. A smaller constellation of prefrontal areas, areas 46, 8a, and 11, is reciprocally connected with portions of cingulate cortex that have been classified as premotor arm and/or leg representations (Hutchins et al., Exp Brain Res 71:667-672, 1988). In accordance with numerous previous reports, prefrontal areas 46, 8a, 9, 10, 11, and 12 are reciprocally connected with "nonmotor" subdivisions of cingulate cortex. The results presented here specify the corticocortical connections by which prefrontal cortex may influence motor output.

The functional role of an increase in cell excitability and synaptic efficiency in the new cortex during learning

Data were obtained in experiments on nonimmobilized and nonanesthetized rabbits, during the development of an analog of a CR, with recording of the response of the pyramidal tract, suggesting temporal specificity in the manifestations of membrane and synaptic plasticity, the participation of these mechanisms in both representations of the combined stimuli, and primarily unidirectional changes in the degree of their participation in these points of the cortex. It is concluded that temporary membrane plasticity creates conditions through the mechanisms of synchronization and summation for the passage of excitation from the sensory link to the motor output of the new connection. The gradual reorganization of excitatory and inhibitory connections to the output elements of the conditioned reflex act, determined by the mechanisms of synaptic plasticity, determine and strengthen the specialized character of the developed reaction.

The dependance of neuronal reactions of the sensorimotor cortex to a simultaneous complex stimulus upon the level of differentiation of its components

The change in the neuronal activity of the sensorimotor area of the cerebral cortex of the cat was investigated in awake animals as a function of the level of differentiation of the components of a simultaneous heteromodal complex stimulus. Two groups of neurons in the sensorimotor cortex were distinguished on the basis of the character of this relationship and a number of other parameters. It was shown that the parameters of the reactions of all neurons recorded to the positive conditional stimulus following the consolidation of the conditioned motoric reaction are established first. Such parameters of the responses as degree of manifestation, intensity, duration, and the length of the latent period changed in the process of development. The reactions of neurons of both groups to inhibitory signals were stabilized only after the consolidation of the differentiation skill. In the process only the pattern of the discharge changed in the neurons of the first group, while in the neurons of the second group, the degree of manifestation of the response, its sign, duration, and length of the latent period could vary. Fluctuations in the level of differentiation following the development of the inhibitory conditioned reactions had an effect only on the responses of the neurons of the second group to the components of the complex.

Primate frontal cortex: neuronal activity following attentional versus intentional cues

We examined neuronal activity in three parts of the primate frontal cortex: the dorsal (PMd) and ventral (PMv) premotor cortex and a ventrolateral part of the dorsolateral prefrontal (PF) cortex. Two monkeys fixated a 0.2 degrees white square in the center of a video display while depressing a switch located between two touch pads. On each trial, a spatial-attentional/mnemonic (SAM) cue was presented first. The SAM cue consisted of one 2 degrees x 2 degrees square, usually red or green, and its location indicated where a conditional motor instruction would appear after a delay period. The stimulus event containing the motor instruction, termed the motor instructional/conditional (MIC) cue, could be of two general types. It might consist of a single 2 degrees x 2 degrees square stimulus identical to one of the SAM cues presented at the same location as the SAM cue on that trial. When the MIC cue was a single square, it instructed the monkey to move its forelimb to one of the two touch pads according to the following conditional rule: a green MIC cue meant that contact with the right touch pad would be rewarded on that trial and a red MIC cue instructed a movement to the left touch pad. Alternatively, the MIC cue might consist of two 2 degrees x 2 degrees squares, only one of which was at the SAM-cue location: in those cases, one square was red and the other was green. The colored square at the SAM cue location for that trial was the instructing stimulus, and the other part of the MIC cue was irrelevant. When, after a variable delay period, the MIC cue disappeared, the monkey had to touch the appropriate target within 1 s to receive a reward and could break visual fixation. The experimental design allowed comparison of frontal cortical activity when one stimulus, identical in retinocentric, craniocentric, and allocentric spatial location as well as all other stimulus parameters, had two different meanings for the animal's behavior. When a stimulus was the SAM cue, it led to either a reorientation of spatial attention to its location, or the storage of its location in spatial memory. By contrast, when it was the MIC cue, the same stimulus instructed a motor act to be executed after a delay period. For the majority of PMd neurons (55%), post-MIC cue activity exceeded post-SAM cue activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Dissociation of object and spatial processing domains in primate prefrontal cortex

Areas and pathways subserving object and spatial vision are segregated in the visual system. Experiments show that the primate prefrontal cortex is similarly segregated into object and spatial domains. Neurons that code information related to stimulus identity are dissociable, both by function and region, from those that code information related to stimulus location. These findings indicate that the prefrontal cortex contains separate processing mechanisms for remembering "what" and "where" an object is.

Topography of projections to the frontal lobe from the macaque frontal eye fields

Efferents from the frontal eye fields (FEF) to the ipsilateral frontal lobe were studied by autoradiography of tritiated tracers (leucine, proline, and fucose) in seven macaque monkeys that were used previously to describe subcortical connections. In four of the cases, tracer injection sites were confirmed by low thresholds for the electrical elicitation of saccadic eye movements. Cases were grouped as lFEF of sFEF cases according to large or small saccades that were characteristic of the injection site. Projections from the FEF terminated in five frontal regions: 1) area FD on the dorsomedial convexity; 2) area FC (containing SEF) medial to the upper limb of the arcuate sulcus; 3) areas FD and FD delta along the walls of the principal sulcus; 4) area FCBm on the deep, posterior wall of the arcuate sulcus inferior to the sulcal spur; and 5) the inferolateral cortex (area FDi) on the convexity and lateral two thirds of the anterior wall of the arcuate sulcus. Projections in sFEF cases tended to be confined to medial parts of dorsomedial FD and FC and the lateral wall of the principal sulcus and inferolateral convexity. Neither lFEF nor sFEF appeared to project to the SMA or pericingulate cortex. Label in these areas was found only in the cases in which tracer spread into non-FEF areas. FEF projections terminated in column-like patches of about 500-600 microns in diameter. Labeled axons and terminals were seen in all cortical layers regardless of location in the frontal lobe.

Effects of attention on visuomotor activity in the premotor and prefrontal cortex of a primate

We examined neuronal activity in the primate premotor (PM) and prefrontal (PF) areas during a demanding spatial matching task. On each behavioral trial, a rhesus monkey moved its forelimb when a visual stimulus, called the "prime stimulus," reappeared at a previously cued location. Because it triggered a movement, the part of space cued by the prime stimulus had to be either remembered or attended during the time between prime stimulus presentations. Between the first and second appearances of the prime stimulus, behaviorally irrelevant visual stimuli could appear at one or several locations other than that of the prime stimulus. We could thereby examine the activity that followed a stimulus when it was attended versus when it was irrelevant and presumably unattended. We found that visuospatial attention affected neuronal activity in both the motor and "nonmotor" parts of the frontal cortex. The magnitude of attention effects exceeded that previously reported--a finding that probably resulted from the intensive attentional demands of the present task.

Neuronal activity in the hippocampus during delayed non-match to sample performance in rats: evidence for hippocampal processing in recognition memory

Neuronal activity in the CA1 of rats was explored with regard to functional correlates of performance in an odor-guided continuous delayed non-match to sample task. Although different CA1 cells fired in association with each identifiable trial event, these analyses focused on cells that fired selectively during the period of odor cue sampling and response generation. The firing patterns of many of these cells reflected the match or non-match comparison between current and previous odor cues independent of the particular stimuli that composed those comparisons. Such cells were more prevalent in sessions when performance was highly accurate. Hippocampal cells did not demonstrate stimulus-evoked firing that persisted through the memory delay, nor did they fire differentially to session-novel vs. repeated odor presentations. These results suggest that the hippocampus contributes to recognition memory by processing comparisons between current information and representations of previous stimuli stored in parahippocampal and neocortical structures.

Functional properties of dorsolateral prefrontal cortical neurons in awake monkey

Electrophysiological single-cell responses were studied in 134 neurons in Walker's areas 46 and 9 of the prefrontal cortex of two stumptail macaques. The neurons were systematically tested for various visual, auditory and somatosensory stimuli. In addition, the rate of neuronal discharges were observed in relation to provoked or spontaneous eye or limb movements. More than half (52.2%) of the neurons responded to stimulation, and the majority of them gave visual responses. Eighty percent of the visual neurons responded to the presentation of various objects, the remaining being selective for meaningful objects or the appearance and movements of the experimenter. Auditory, somatosensory, somatomotor and oculomotor responses were also encountered; 9.0% of the recorded neurons were multimodal. Despite the large stimulus repertoire 47.8% of the neurons were found to be only spontaneously active.

Responses of monkey midbrain dopamine neurons during delayed alternation performance

Cognitive deficits are important components of the parkinsonian syndrome. In order to investigate the role of dopamine (DA) neurons in cognitive functions, we recorded the electrical activity of midbrain DA neurons in a monkey performing in a spatial delayed alternation task. Triggered by a light, the animal reached toward one of two levers to receive a drop of liquid reward. The lever associated with reward was alternated after each correct movement. Of 88 DA neurons, 65% and 52% showed phasic responses to the trigger light and reward, respectively. By contrast, sustained delay-related activity described for striatum and frontal cortex was not observed, suggesting that the activity of DA neurons does not reflect mnemonic or preparatory representational task components. Rather, DA neurons respond to the salient attentional and motivating stimuli guiding task performance.

Prefrontal projections to the mediodorsal nucleus of the thalamus in the rhesus monkey

The corticothalamic projections to the prefrontal cortex have been shown to be topographically organized. However, the underlying basis for this topography as it relates to the organization of the different architectonically defined areas of the prefrontal cortex has not been systematically studied. In the present investigation we have reassessed the thalamic projections from the different architectonic areas of the prefrontal cortex by using the technique of autoradiography in the rhesus monkey. The results show that the prefronto-mediodorsal projections are organized according to the architectonic differentiation of the prefrontal cortices. Thus architectonically less differentiated medial and orbital prefrontal regions project to the medial sector of the mediodorsal nucleus, the magnocellular subdivision. In contrast, highly differentiated prefrontal area 8 projects to the most lateral sector of the mediodorsal nucleus, the multiformis subdivision. Lateral prefrontal areas with intermediate architectonic features project to the central parvocellular sector of the mediodorsal nucleus. Additionally, these projections also reveal a dorsoventral topography. Thus areas in the medial and dorsolateral cortices project to the dorsal part of the mediodorsal nucleus. In contrast, areas in orbital and ventrolateral cortices project to the ventral part of the mediodorsal nucleus. The topographic organization of the corticothalamic connections described in this study corresponds to the progressive elaboration and differentiation of the architectonic features of the different prefrontal areas. This successive and dichotomous organization of prefrontothalamic connections may provide the basis for the observed differential functions of the prefrontal cortex and the mediodorsal nucleus.

Mastication and its control by the brain stem

This review describes the patterns of mandibular movements that make up the whole sequence from ingestion to swallowing food, including the basic types of cycles and their phases. The roles of epithelial, periodontal, articular, and muscular receptors in the control of the movements are discussed. This is followed by a summary of our knowledge of the brain stem neurons that generate the basic pattern of mastication. It is suggested that the production of the rhythm, and of the opener and closer motoneuron bursts, are independent processes that are carried out by different groups of cells. After commenting on the relevant properties of the trigeminal and hypoglossal motoneurons, and of internuerons on the cortico-bulbar and reflex pathways, the way in which the pattern generating neurons modify sensory feedback is discussed.

Vertical and horizontal coding of space in the monkey dorsolateral prefrontal cortex

Single cell activity in the dorsolateral prefrontal cortex was recorded in a monkey performing a delayed alternation (DA) task in 3 directions, to the left, to the right, and upwards. Among the 127 units studied in all three directions, 18 neurons were spatially selective in one direction (to the left, to the right or upwards), 37 neurons in two directions and 8 neurons in each 3 directions during the performance of the DA task. Of the 9 neurons that were spatially selective upwards, 8 had a specific pattern of activity during the delay period and one during the response period. When several spatial directions are studied in a DA task, as in this work, it becomes evident that the prefrontal cortex contains a large number of spatially selective neurons. The results of this study suggest that there is a spatial memory map in the prefrontal cortex which is needed not only when a DA task is performed to the left and to the right but also in the upward direction.

Prefrontal unit activity during associative learning in the monkey

Single unit activity was recorded from the dorsolateral prefrontal cortex of two monkeys which were trained on a stimulus-reward association task. The monkeys were trained on a reaction time task overlapped with a classical conditioning paradigm. The sequential events of the task were as follows: (1) lever pressing to start the trial; (2) presentation of a visual cue for 1 s; (3) delay period of 1 s; (4) imperative stimulus presentation; and (5) release of the lever by the animal. The visual cue signaled whether or not a drop of fruit juice would be given (its associative significance) for the animal's release response instead of signaling what response the animal should perform (its behavioral significance). In this task, the animal had to release the lever even on the trial where no juice was given in order to advance to the next trial. A total of 423 units showed activity changes in relation to one or more of the task events, such as the cue presentation, delay, release response and reward delivery. Among 313 units which showed cue-related activity changes, 179 units showed differential activity in relation to the different cues. A majority of them (Type M; n = 120) showed activity changes in relation to whether the cue indicated juice delivery or not, independent of its physical properties. The activity of 13 units (Type P) was related to the physical properties of the stimulus, and the activity of the remaining 46 units (Type MP) appeared to be related to both aspects of the stimulus. Sustained activity changes during the delay period were observed in 68 Type M, in 3 Type P and in 24 Type MP units. The results suggest that the prefrontal cortex plays important roles in the stimulus-reward association and that prefrontal units are involved in higher order information processing, extracting and retaining the "associative significance" of the stimulus independent of its physical properties.

Effects of cooling parietal cortex on prefrontal units in delay tasks

The effects of cooling posterior parietal cortex (areas 5 and 7) on behavior and on the activity of prefrontal neurons were assessed in monkeys performing two visual discrimination tasks with delayed choice. In both tasks, the visual cue for each trial was displayed for 0.5 s by rear projection through colored filters on a central 2.5-cm translucid button. After a variable delay, the choice stimuli were presented on two lower stimulus-response buttons; to obtain a reward, the animal had to press the correct button in accord with the cue. In one task, a red or a green cue called for the choice of that color when the two colors appeared after the delay; in the other task, a yellow or blue cue called for the choice of, respectively, the right or the left of the two white-illuminated choice buttons. Prefrontal single-unit activity (sulcus principalis area) and eye movements were recorded during task performance while parietal areas were at normal or subnormal (6-20 degrees C) temperature. Two-thirds of the units investigated showed significant spontaneous firing changes, most commonly a decrease, as a result of bilateral parietal cooling. A similar proportion of units showed cooling-related changes, excitatory or inhibitory, of their firing activity during the task; such firing changes could occur in any trial-epoch. Parietal cooling also induced misreaching, slow and inaccurate ocular movements, and longer choice reaction time, but did not alter performance in terms of correct responses. Our results suggest the involvement of posterior parietal cortex in spatial aspects of task performance (reaching speed and accuracy, eye movements, reaction time). They also suggest the existence of functional influences from parietal upon prefrontal cortex. Those influences, however, seem not essential for the basic role of the prefrontal cortex in the temporal integration of behavior.

Mode of [14C] 2-deoxy-D-glucose uptake into retrosplenial cortex and other memory-related structures of the monkey during a delayed response

Physiological studies on the monkey retrosplenial (RS) cortex have been few, and its functional role remains to be investigated. In the present study, activity of the RS cortex was investigated using radioactive 2-DG while the monkey was performing a visual tracking task with a delay (a delayed-response task) for 45 minutes. A remarkable increase in 2-DG uptake was observed equally in the left as well as in the right RS cortex. The anterior nucleus of the thalamus also showed increased 2-DG uptake. In addition, other memory-related structures (prefrontal cortex, dorsomedial nucleus of the thalamus, amygdala and hippocampus) showed a similar increase in 2-DG uptake compared to control monkeys, though their respective absolute values were different from one another. Since the RS cortex receives afferents from the anterior nucleus of the thalamus, which is one of the main nuclei of the Papez circuit, it is assumed that the RS cortex is important in memory function. Therefore, the remarkable increase in 2-DG uptake in the present study could reflect some aspects of memory or learning processes required to perform the delayed response.

Prefrontal representation of stimulus attributes during delay tasks. I. Unit activity in cross-temporal integration of sensory and sensory-motor information

The activity of 294 single units was recorded from the dorsolateral prefrontal cortex of monkeys performing two visual discrimination tasks with delayed response. One task, delayed matching-to-sample (DMS), required memory of a colored cue for later (18 s) matching and choice of color; the cue did not connote the location of the delayed response. The other task, delayed conditional position discrimination (DCPD), required memory of a colored cue for later (18 s) choice of spatial response; the cue did connote delayed-response location. All 4 cues (red and green in DMS, yellow and blue in DCPD) were isoluminous and appeared in identical location at trial start. Differential unit reactions to the two DCPD cues were more common than those to the two DMS cues (samples). During the delay period, 15% of all units showed, in one task or the other, differential discharge depending on the cue. In DCPD, a large proportion of the units showing direction-related activity at the time of motor response also reacted with a firing frequency change to one or both (spatially identical) trial-initiating cues. Some units showed coherence of cue-related and response-related changes in accord with the behavioral association between color and direction of response (i.e., yellow-right, blue-left). The reactivity of some units was correlated with the behavioral performance of the tasks in terms of correctness or incorrectness of response. The results indicate that, during visual delay tasks, neurons in the dorsolateral prefrontal cortex may process both spatial and non-spatial information. Because of their protracted differential discharge between cue and response (i.e., during the delay), some units seem involved in the transfer of sensory information across time. These findings suggest the role of prefrontal neurons in the representation of multiple attributes of sensory stimuli, including their associated motor connotations, and the overlap of the cortical representations of different attributes. They are also consistent with the role of the prefrontal cortex in the cross-temporal mediation of sensory-motor contingencies and, therefore, the temporal organization of behavior.

Prefrontal representation of stimulus attributes during delay tasks. II. The role of behavioral significance

Rhesus monkeys were trained to perform two visual discrimination tasks with delayed response. In both tasks, the response depended on the color of the cue, a lighted circle in the center of a panel. Red and green guided one task, yellow and blue the other. In the course of performance, a fifth color (violet), non-relevant and inconsequential, was presented at random in the same location as the cues. All 5 stimuli were of equal brightness. Many cells in the dorsolateral prefrontal cortex (sulcus principalis and superior convexity) treated the relevant cues differently than the irrelevant stimulus. In general, cellular reactions to that stimulus were of lesser magnitude than the reactions to the cues. Cell reaction differences as a function of stimulus significance outnumbered and overshadowed differences as a function of cue-color or any other task variable. The results indicate that, during visual delay tasks, units in the dorsolateral prefrontal cortex differentiate stimuli by their behavioral significance, as well as by other stimulus attributes, including color. Because the motivational evaluation of sensory stimuli is an integral part of the cognitive processes in delay tasks (together with short-term memory and motor set), these results support the notion that the prefrontal cortex integrates motivational inputs into the structure of behavioral action.

Neuronal activity preceding directional and nondirectional cues in the premotor cortex of rhesus monkeys

Pre-cue activity, the neuronal modulation that precedes a predictable stimulus, was studied in the premotor cortex of three rhesus monkeys. In one condition, a directional cue dictated the timing and target of a forelimb movement. In another condition, a nondirectional cue provided identical timing information but did not indicate the target. Of 501 task-related neurons recorded in premotor cortex, 168 showed pre-cue activity. The onset time of pre-cue activity varied markedly from trial to trial and cell to cell, ranging from trial initiation to 4.8 sec later. No pre-cue activity reflected the direction of limb movement; thus, the data argue against the hypothesis that pre-cue activity reflects preparation for specific limb movements. A small number of cells showed greater pre-cue activity before directional than before nondirectional cues, and this difference may reflect anticipation of the cue's directional information. However, the vast majority (84%) of neurons lacked such differences. We therefore hypothesize that most pre-cue activity reflects or contributes to a facet of behavior common to the two conditions: anticipation of the time and/or nature of events.

Single unit activity in cat prefrontal and parietal cortex during performance of a symmetrically reinforced go-no go task

Relationships between the performance in a symmetrically reinforced go-no go task and cellular firing patterns in prefrontal and parietal association areas of the neocortex were studied in six cats. During recordings, animals lay in a box, with their heads fixed to a stereotaxic frame, and performed an auditory go-no go task by pressing a retractable lever in front of them. Units obtained were classified into eight types according to the correlation of their activity changes with aspects of the task and/or with sensory stimuli. These types were (poly-) sensory, reward related, EMG-related, EOG-related, event-related, movement-initiating, expressing expectancy or novelty, and nonspecific or task-unrelated active units. Between the two recording areas a considerable degree of similarity was obtained in unit firing patterns. It was concluded that within the cerebral cortex, and especially within its association areas, a considerable functional overlap exists, that neurons may be involved in the processing of several and rather different phenomena, and that the processing of information at this level of the brain is generally done via widespread, interwoven neuronal nets so that only the average network activity, but not that of a particular, single neuron, represents a stimulus or an event.

Characteristics of neuronal activity in prefrontal cortex during performance of spatial delayed reactions in monkeys

Several types of neurons were differentiated on the basis of a study of neuronal activity in various parts of the cortex near the sulcus principalis during the execution of spatial delayed reactions by monkeys. It was established that the different types of neurons are represented in different numbers in different parts of the cortex near the sulcus principalis. The determination of several factors influencing the activity of these neurons and the comparison of data on their quantitative representation in the anterior, middle, and posterior parts of the cortex near the sulcus principalis with the existing behavioral data obtained after local ablations of identical regions of the brain made it possible to postulate that neurons belonging to the different types are involved in the analysis of different processes and represent different functional units.

Frontal eye field as defined by intracortical microstimulation in squirrel monkeys, owl monkeys, and macaque monkeys: I. Subcortical connections

Intracortical microstimulation was used to define the borders of the frontal eye fields in squirrel, owl, and macaque monkeys. The borders were marked with electrolytic lesions, and horseradish peroxidase conjugated to wheat germ agglutinin was injected within the field. Following tetramethyl benzidine histochemistry, afferent and efferent connections of the frontal eye field with subcortical structures were studied. Most connections were ipsilateral and were similar in all primates studied. These include reciprocal connections with the following nuclei: medial dorsal (lateral parts), ventral anterior (especially with pars magnocellularis), central lateral, paracentral, ventral lateral, parafascicular, medial pulvinar, limitans, and suprageniculate. The frontal eye field also projects to the ipsilateral pretectal nuclei, subthalamic nucleus, nucleus of the posterior commissure, superior colliculus (especially layer four), zona incerta, rostral interstitial nucleus of the medial longitudinal fasciculus, nucleus Darkschewitsch, dorsomedial parvocellular red nucleus, interstitial nucleus of Cajal, basilar pontine nuclei, and bilaterally to the paramedian pontine reticular formation and the nucleus reticularis tegmenti pontis. Many of these structures also receive input from deeper layers of the superior colliculus and are known to participate in visuomotor function. These results reveal connections that account for the parallel influence of the superior colliculus and the frontal eye field on visuomotor function; suggest that there has been little evolutionary change in subcortical connections, and therefore function, of the frontal eye fields since the time that these lines of primates diverged; and support the conclusion that the frontal eye fields are homologous in New and Old World monkeys.

The neurobiology of learning and memory

Study of the neurobiology of learning and memory is in a most exciting phase. Behavioral studies in animals are characterizing the categories and properties of learning and memory; essential memory trace circuits in the brain are being defined and localized in mammalian models; work on human memory and the brain is identifying neuronal systems involved in memory; the neuronal, neurochemical, molecular, and biophysical substrates of memory are beginning to be understood in both invertebrate and vertebrate systems; and theoretical and mathematical analysis of basic associative learning and of neuronal networks in proceeding apace. Likely applications of this new understanding of the neural bases of learning and memory range from education to the treatment of learning disabilities to the design of new artificial intelligence systems.

Localization in human thalamus of units triggered during 'verbal commands,' voluntary movements and tremor

Microelectrode recordings in the rostral (n. reticularis) and lateral (ventralis lateralis) human thalamus were carried out in locally anaesthetized diskinetic patients during stereotaxic operations. Their responses to voluntary motor tasks prompted by imperative verbal stimuli were tested. Spontaneous and evoked unit activities were studied using computer processing techniques. In the n. reticularis thalami and immediately adjacent thalamic zones, not only units reacting during the initiation of voluntary movements (100-200 msec before the movement), but also units responding to the verbal command itself ('triggered verbal command' units) were found. They proved to be concerned directly with the semantic meaning of the command. In the VL anterior area (Voa-Vop in German nomenclature) the majority of the units responded during the phases of initiation and/or realization of the voluntary motor act ('voluntary movement' units of Jasper and Bertrand 1966); when these units were not spontaneously rhythmic they were transiently transformed into rhythmic (5 +/- 1 Hz) ones. This transformation appeared during the preparation and realization of movement but also in some cells as a rebound phenomenon. In patients without tremor (akinetic and rigid forms of parkinsonism, torticollis), the transient rhythmogenic transformation was frequently provoked by the repetition of motor tasks. In the posterior part of VL (Vim), cells were driven by proprioceptive inflow coming from a specific peripheral region. They react also during the voluntary movement of the same region. The majority of these units were rhythmic at 5 +/- 1 Hz, and they presented a close correlation in phase and frequency with the tremor. The anatomical locations of the three main pools of neurons were determined. 'Triggered verbal command' units were placed more anteriorly and laterally. 'Voluntary movements' and 'rhythmic 5 +/- 1 Hz units' had identical spatial localizations. This fact supports the contributions of these two last types to the central mechanisms of both tremor and voluntary movement.

Role of bar press-related neurons in the dorsolateral prefrontal cortex during task performance by monkey

Extracellular single neuron activity of the dorsolateral prefrontal cortex (DL) was recorded in the monkey, during bar pressing for reward. The bar press-related neurons which exhibited excitation or inhibition during the bar press period were found to be scattered diffusely in the DL. Activity changes that arose during the bar press period also appeared when the experimenter pressed the bar for the monkey. When delivery of food was delayed for a random time after cue tone on, bar press responses were still confined to the bar press period and did not extend beyond the cue tone. These results, together with the lesion studies, suggest that bar press-related neurons are involved in the animal's concentration during the bar press period.

Corticocortical efferent systems in the monkey: a quantitative spatial analysis of the tangential distribution of cells of origin

The laminar and tangential distributions of association neurons projecting from areas 4 and 6 of the frontal lobe to area 5 of the superior parietal lobule were studied in macaque monkeys by using horseradish peroxidase histochemistry. In both areas 4 and 6 association neurons were medium-large pyramidal cells of layers II and III, and pyramidal and fusiform cells of layers V-VI. Tangentially, they were distributed unevenly over the cortical surface occupying only certain parts of areas 4 and 6, including the dorsomedial part of area 6, the proximal arm region of Woolsey's M1 map, parts of the postarcuate cortex, and the supplementary motor area. Within these projection zones, the number of projection cells waxed and waned in a periodic fashion. Quantitative methods, including spectral analysis techniques, were used to characterize precisely spatial periodicities along the rostrocaudal dimension. The same quantitative analyses were used to determine the nature of the tangential distribution of corticocallosal cells of area 5 projecting to contralateral area 5. Both association and callosal spectra contained a strong component in the range of low spatial frequencies, corresponding to periods greater than 2 mm. Moreover, a consistent peak was observed in both spectra at spatial frequencies corresponding to periods ranging from 0.85 to 1.28 mm. This peak is in accord with the hypothesis of a modular organization of the cells of origin of these projection systems.

Cortical afferent input to the principalis region of the rhesus monkey

The sources of ipsilateral cortical afferent projections to regions along both banks of the principalis sulcus in the prefrontal cortex were studied with horseradish peroxidase in macaque monkeys. The principalis cortex receives a substantial proportion of its projections from neighboring prefrontal regions. However, differences were noted in the distribution of labeled cells projecting to the various principalis regions. These differences were most marked with respect to the relative proportion of cells originating in visual, auditory, somatosensory, premotor and limbic cortical areas. The findings indicate that the caudal ventral principalis region receives projections from both visual and visuomotor regions, whereas the anterior tip of the principalis appears to be the major target of projections from auditory association regions. The ventral bank at the middle extent of the principalis was the only case with a significant proportion of labeled cells in somatosensory association and premotor regions. There was a consistent increase in the proportion of labeled cells in limbic cortical areas projecting to more rostral principalis sites, irrespective of whether the injection was placed in the dorsal or ventral bank. These findings suggest that the caudal principalis region has a visual-visuomotor and the rostral, an auditory-limbic bias with respect to the long projections they receive.

Comparative characteristics of unit activity in the prefrontal and parietal areas during delayed performance in monkeys

Neuronal mechanisms of prefrontal and parietal areas were compared in 3 monkeys during delayed performance. Spatioselective neurons were found in both areas in question. In the prefrontal cortex, they constitute 28% of all units sampled and in the parietal cortex they account for 21%. For the prefrontal area, spatial selectivity was particularly great during the delay (8%), and in the parietal area during the cue display (9%). During the delay, however, spatioselective parietal neurons accounted for 4% of all units sampled, i.e. their number was half that in the prefrontal area. The prefrontal cortex appears to play a major role in short-term memory proper, whereas the parietal area is more involved in assessing spatial relationships of emerging sensory stimuli. Spatioselective neurons of both areas were heterogeneous in their functions. Activity of some (11% in the prefrontal and 10% in the parietal area) was related only to the cue location. Activity of others (13% in the prefrontal cortex and 8% in the parietal cortex) was moreover coupled with the forthcoming movement. With lengthening of the delay, units related to the established temporal stereotype and some labile units which quickly rearranged to a new temporal task were recorded. Thus association area neurons reflect two concurrent processes linked with spatial and temporal memories. During cue displays, it is not only their spatial location that is described, but also a future motor act with its temporal and spatial properties programmed.