Prefrontal cortical unit activity and delayed alternation performance in monkeys

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

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

Single-neuron theory of consciousness

By most accounts, the mind arises from the integrated activity of large populations of neurons distributed across multiple brain regions. A contrasting model is presented in the present paper that places the mind/brain interface not at the whole brain level but at the level of single neurons. Specifically, it is proposed that each neuron in the nervous system is independently conscious, with conscious content corresponding to the spatial pattern of a portion of that neuron's dendritic electrical activity. For most neurons, such as those in the hypothalamus or posterior sensory cortices, the conscious activity would be assumed to be simple and unable to directly affect the organism's macroscopic conscious behavior. For a subpopulation of layer 5 pyramidal neurons in the lateral prefrontal cortices, however, an arrangement is proposed to be present such that, at any given moment: (i) the spatial pattern of electrical activity in a portion of the dendritic tree of each neuron in the subpopulation individually manifests a complexity and diversity sufficient to account for the complexity and diversity of conscious experience; (ii) the dendritic trees of the neurons in the subpopulation all contain similar spatial electrical patterns; (iii) the spatial electrical pattern in the dendritic tree of each neuron interacts non-linearly with the remaining ambient dendritic electrical activity to determine the neuron's overall axonal response; (iv) the dendritic spatial pattern is reexpressed at the population level by the spatial pattern exhibited by a synchronously firing subgroup of the conscious neurons, thereby providing a mechanism by which conscious activity at the neuronal level can influence overall behavior. The resulting scheme is one in which conscious behavior appears to be the product of a single macroscopic mind, but is actually the integrated output of a chorus of minds, each associated with a different neuron.

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

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

Synaptic mechanisms of persistent reverberatory activity in neuronal networks

For brain functions such as working memory and motor planning, neuronal circuits are able to sustain persistent activity after transient inputs. Theoretical studies have suggested that persistent activity can exist in recurrently connected networks as active reverberation. However, the actual cellular processes underlying such reverberation are not well understood. In this study, we investigated the basic synaptic mechanisms responsible for reverberatory activity in small networks of rat hippocampal neurons in vitro. We found that brief stimulation of one neuron in a network could evoke, in an all-or-none fashion, reverberatory activity lasting for seconds. The reverberation was likely to arise from recurrent excitation because it was eliminated by partial inhibition of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors (but not by blockade of NMDA receptors). In contrast, blocking inhibitory transmission with bicuculline enhanced the reverberation. Furthermore, paired-pulse stimuli with interpulse intervals of 200-400 ms were more effective than single pulses in triggering reverberation, apparently by eliciting higher levels of asynchronous transmitter release. Suppressing asynchronous release by EGTA-AM abolished reverberation, whereas elevating asynchronous release by strontium substantially enhanced reverberation. Finally, manipulating calcium uptake into or release from intracellular stores also modulated the level of reverberation. Thus, the oft-overlooked asynchronous phase of synaptic transmission plays a central role in the emergent phenomenon of network reverberation.

Discovering spatial working memory fields in prefrontal cortex

This essay looks at the historical significance of one APS classic paper that is freely available online:

Funahashi S, Bruce CJ, and Goldman-Rakic PS. Mnemonic coding of visual space in the monkey's dorsolateral prefrontal cortex. J Neurophysiol 61: 331–349, 1989 ( http://jn.physiology.org/cgi/reprint/61/2/331 ).

Directing the mind's eye: prefrontal, inferior and medial temporal mechanisms for visual working memory

Human and nonhuman primates have a remarkable ability to recall, maintain and manipulate visual images in the absence of external sensory stimulation. Evidence from lesion, single-unit neurophysiological and neuroimaging studies shows that these visual working memory processes are consistently associated with sustained activity in object-selective inferior temporal neurons. Furthermore, results from these studies suggest that mnemonic activity in the inferior temporal cortex is, in turn, supported by top-down inputs from multimodal regions in prefrontal and medial temporal cortex, and under some circumstances, from the hippocampus.

Neural correlates of executive control in the avian brain

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

Functional connectivity during working memory maintenance

Neurophysiological experiments with monkeys have demonstrated that working memory (WM) is associated with persistent neural activity in multiple brain regions, such as the prefrontal cortex (PFC), the parietal cortex, and posterior unimodal association areas. WM maintenance is believed to require the coordination of these brain regions, which do not function in isolation but, rather, interact to maintain visual percepts that are no longer present in the environment. However, single-unit physiology studies and traditional univariate analyses of functional brain imaging data cannot evaluate interactions between distant brain regions, and so evidence of regional integration during WM maintenance is largely indirect. In this study, we utilized a recently developed multivariate analysis method that allows us to explore functional connectivity between brain regions during the distinct stages of a delayed face recognition task. To characterize the neural network mediating the on-line maintenance of faces, the fusiform face area (FFA) was defined as a seed and was then used to generate whole-brain correlation maps. A random effects analysis of the correlation data revealed a network of brain regions exhibiting significant correlations with the FFA seed during the WM delay period. This maintenance network included the dorsolateral and ventrolateral PFC, the premotor cortex, the intraparietal sulcus, the caudate nucleus, the thalamus, the hippocampus, and occipitotemporal regions. These findings support the notion that the coordinated functional interaction between nodes of a widely distributed network underlies the active maintenance of a perceptual representation.

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

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

Coherence between fMRI time-series distinguishes two spatial working memory networks

Widespread and distributed brain regions are thought to form networks that together support working memory. We recently demonstrated that different cortical areas maintain relatively different codes across a memory delay (Curtis et. al., J Neurosci, 2004; 24:3944-3952). The frontal eye fields (FEF), for example, were more active during the delay when the direction of the memory-guided saccade was known compared to when it was not known throughout the delay. Other areas showed the opposite pattern. Despite these task-dependent differences in regional activity, we could only assume but not address the functional interactions between the identified nodes of the putative network. Here, we use a bivariate technique, coherence, to formally characterize functional interactions between a seed region and other brain areas. We find that the type of representational codes that are being maintained in working memory biases frontal-parietal interactions. For example, coherence between FEF and other oculomotor areas was greater when a motor representation was an efficient strategy to bridge the delay period. However, coherence between the FEF and higher-order heteromodal areas, e.g., dorsolateral prefrontal cortex, was greater when a sensory representation must be maintained in working memory.

A neural circuit basis for spatial working memory

The maintenance of a mental image in memory over a time scale of seconds is mediated by the persistent discharges of neurons in a distributed brain network. The representation of the spatial location of a remembered visual stimulus has been studied most extensively and provides the best-understood model of how mnemonic information is encoded in the brain. Neural correlates of spatial working memory are manifested in multiple brain areas, including the prefrontal and parietal association cortices. Spatial working memory ability is severely compromised in schizophrenia, a condition that has been linked to prefrontal cortical malfunction. Recent computational modeling work, in interplay with physiological studies of behaving monkeys, has begun to identify microcircuit properties and neural dynamics that are sufficient to generate memory-related persistent activity in a recurrent network of excitatory and inhibitory neurons during spatial working memory. This review summarizes recent results and discusses issues of current debate. It is argued that understanding collective neural dynamics in a recurrent microcircuit provides a key step in bridging the gap between network memory function and its underlying cellular mechanisms. Progress in this direction will shed fundamental insights into the neural basis of spatial working memory impairment associated with mental disorders.

Phase locking of single neuron activity to theta oscillations during working memory in monkey extrastriate visual cortex

Working memory has been linked to elevated single neuron discharge in monkeys and to oscillatory changes in the human EEG, but the relation between these effects has remained largely unexplored. We addressed this question by measuring local field potentials and single unit activity simultaneously from multiple electrodes placed in extrastriate visual cortex while monkeys were performing a working memory task. We describe a significant enhancement in theta band energy during the delay period. Theta oscillations had a systematic effect on single neuron activity, with neurons emitting more action potentials near their preferred angle of each theta cycle. Sample-selective delay activity was enhanced if only action potentials emitted near the preferred theta angle were considered. Our results suggest that extrastriate visual cortex is involved in short-term maintenance of information and that theta oscillations provide a mechanism for structuring the recurrent interaction between neurons in different brain regions that underlie working memory.

The Role of the Prefrontal Cortex in the Maintenance of Verbal Working Memory: An Event-Related fMRI Analysis

Neuroimaging studies have been inconclusive in characterizing the role of the prefrontal cortex (PFC) for maintaining increasingly larger amounts of information in working memory (WM). To address this question, the authors collected event-related functional MRI data while participants performed an item-recognition task in which the number of to-be-remembered letters was parametrically modulated. During maintenance of information in WM, the dorsolateral and the ventrolateral PFC exhibited linearly increasing activation in response to increasing WM load. Prefrontal regions could not be distinguished from one another on the basis of load sensitivity, but the dorsolateral PFC had stronger functional connectivity with the parietal and motor cortex than the ventrolateral PFC. These results suggest an increasingly important role for the PFC in actively maintaining information as the amount of that information increases.

Modular biased-competition and cooperation: a candidate mechanism for selective working memory

Prefrontal cortex (PFC) has been suggested to play an important role in executive cognitive functions, participating in planning and controlling behaviour. The results of several recent electrophysiological studies indicate that PFC might be involved not only in the active maintenance of information but in doing so in a context- or task-dependent manner. In a delayed-match-to-sample paradigm, recordings from neurons in the PFC showed their ability to selectively represent information, which is needed for task completion, suggesting that task-irrelevant information does not access working memory. We present a neurodynamical computational model of a part of the PFC to account for the selective representation of information in working memory. We show that a network of biological realistic integrate-and-fire excitatory and inhibitory neurons, implementing the mechanisms of local or modular biased-competition, which is transmitted through cooperation to different subsets of neuronal pools, can explain the formation of selective context-dependent working memory. The modes of operation of the network are characterized and the corresponding parameter settings revealed. Modular competition and cooperation might constitute general mechanisms for implementing context-dependent formation of working memory.

Representation of attended versus remembered locations in prefrontal cortex

A great deal of research on the prefrontal cortex (PF), especially in nonhuman primates, has focused on the theory that it functions predominantly in the maintenance of short-term memories, and neurophysiologists have often interpreted PF's delay-period activity in the context of this theory. Neuroimaging results, however, suggest that PF's function extends beyond the maintenance of memories to include aspects of attention, such as the monitoring and selection of information. To explore alternative interpretations of PF's delay-period activity, we investigated the discharge rates of single PF neurons as monkeys attended to a stimulus marking one location while remembering a different, unmarked location. Both locations served as potential targets of a saccadic eye movement. Although the task made intensive demands on short-term memory, the largest proportion of PF neurons represented attended locations, not remembered ones. The present findings show that short-term memory functions cannot account for all, or even most, delay-period activity in the part of PF explored. Instead, PF's delay-period activity probably contributes more to the process of attentional selection.

Persistent activity of neurons in an area of the frontal lobe - the prefrontal cortex - is often proposed to underlie short- term memory. Mikhail Lebedev and colleagues provide an alternative explanation

Jogging improved performance of a behavioral branching task: implications for prefrontal activation

We studied the effect of habitual jogging on the performance of a frontal lobe functioning test. Fourteen subjects were divided into a jogging trained group (TG) or a jogging untrained group (NG). The TG jogged for 12 weeks, for 30 min, 2.6 times per week, while the NG did not. We administered a prefrontal branching task (BR) combining a Spatial Delayed-Response Test (DR) and a Go/No-Go Test (GNG). Each test alone and a Simple Reaction Time Test (SR) were given as controls. All tests were given three times at 6 week intervals over 12 weeks in both groups. In the TG, the tests were given two times after termination of the jogging. The maximal oxygen uptake (VO2max) was measured in the TG during the 12 weeks. After 12 weeks, the correct performance rates in the BR task were more improved in the TG than in the NG. The control and reaction time tests were unchanged in both groups. The improved performance in the BR task in the TG decreased after stopping the jogging. The VO2max increased significantly during the 12 weeks of jogging in the TG. Thus, the habitual jogging improved performance in a prefrontal BR.

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

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

Maintenance of spatial and motor codes during oculomotor delayed response tasks

The most compelling neural evidence for working memory is persistent neuronal activity bridging past sensory cues and their contingent future motor acts. This observation, however, does not answer what is actually being remembered or coded for by this activity. To address this fundamental issue, we imaged the human brain during maintenance of spatial locations and varied whether the memory-guided saccade was selected before or after the delay. An oculomotor delayed matching-to-sample task (match) was used to measure maintained motor intention because the direction of the forthcoming saccade was known throughout the delay. We used a nonmatching-to-sample task (nonmatch) in which the saccade was unpredictable to measure maintained spatial attention. Oculomotor areas were more active during match delays, and posterior parietal cortex and inferior frontal cortex were more active during nonmatch delays. Additionally, the fidelity of the memory was predicted by the delay-period activity of the frontal eye fields; the magnitude of delay-period activity correlated with the accuracy of the memory-guided saccade. Experimentally controlling response selection allowed us to functionally separate nodes of a network of frontal and parietal areas that usually coactivate in studies of working memory. We propose that different nodes in this network maintain different representational codes, motor and spatial. Which code is being represented by sustained neural activity is biased by when in the transformation from perception to action the response can be selected.

Stimulus duration in working memory is represented by neuronal activity in the monkey prefrontal cortex

Humans are capable of memorizing several attributes of a presented stimulus as well as its duration of presentation. However, the neuronal representation of stimulus duration in memory remains unknown. This study investigated activities of single neurons in the prefrontal cortex of monkeys while they were performing a behavioral task in which working memory for stimulus duration was needed. Here we describe specific neurons whose discharge rates reflect encoding or retention of the duration of the presentation of stimuli to be remembered. We also describe other specific neurons whose activities reflect encoding or retention of fixed duration, similar but unrelated to the stimulus duration presented in each trial. Some of these specific neurons showed the same duration-related discharges even while the monkeys were performing a different task, in which working memory for stimulus duration was no longer needed. From these results, we suggest that neurons in the prefrontal cortex play roles in encoding and retention of temporal information in working memory and that some of those neurons are dedicated to representation of temporal information attributed to stimuli even when the temporal information is unnecessary for correct performance.

Working Memory: Imaging the Magic Number Four

Many brain regions have been implicated in memory performance, but the relationship between memory capacity and neural activity has not been clear. Recent studies show that activity in the posterior parietal cortex increases with working memory load, implicating this region in the storage of representations in visual memory.

Neurons in monkey prefrontal cortex whose activity tracks the progress of a three-step self-ordered task

The self-ordered task is a powerful tool for the analysis of dorsal prefrontal deficits. Each trial consists of a number of steps, and subjects must remember their choices in previous steps. The task becomes more difficult as the number of objects to be remembered increases. We recorded the activity of 156 neurons in the mid-dorsal prefrontal cortex of two rhesus monkeys performing an oculomotor version of the task. Although the task requires working memory, there was no convincing evidence for activity selective for the working memory of the objects that the monkey had to remember. Instead, nearly one-half of neurons (47%, 74/156) showed activity that was modulated according to the step of the task in any one or more task periods. Although the monkey's reward also increased with step, the neurons exhibited little or no step modulation in a reward control task in which reward increased without a concurrent increase in task difficulty. The activity of some neurons was also selective for the location of saccade target that the monkey voluntarily chose. Neurons showed less step modulation in error trials, and there was no increase between the second and third step responses on trials in which the error was on the third step. These results suggest that the mid-dorsal prefrontal cortex contributes to the self-ordered task, not by providing an object working memory signal, but by regulating some general aspect of the performance in the difficult task.

Prefrontal Neuronal Activity Encodes Spatial Target Representations Sequentially Updated After Nonspatial Target-Shift Cues

We examined prefrontal neuronal activity while monkeys performed a sequential target-shift task, in which, after a positional cue indicated the initial saccade target among 8 peripheral positions, the monkeys were required to internally shift the target by one position on every flash of a target-shift cue. The target-shift cue appeared in the center 0 to 3 times within a single trial and was always the same in shape, size, and color. We found selective neuronal activity related to the target position: when the target-shift cue implied the target shift to particular peripheral positions, neurons exhibited early-dominant and late-dominant activity during the following delay period. The early-dominant target-selective activity emerged early in the delay just after the presentation of the target-shift cue, whereas the late-dominant activity gradually built up toward the end of the delay. Because the target-shift cue was not related to any specific target location, the early-dominant target-selective activity could not be a mere visual response to the target-shift cue. We suggest that the early-dominant activity reflects the transitory representation for the saccade target that was triggered by the nonspatial target-shift cue, whereas the late-dominant activity reflects the target representation in the spatial working memory or the preparatory set for the possible impending saccade, being repeatedly updated during sequential target shifts.

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

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

Impaired Decision Making Related to Working Memory Deficits in Individuals With Substance Addictions

This study examined whether individuals with substance dependence (ISDs) show impairments in working memory and whether there is a relationship between their impairments in decision making as measured by the gambling task (GT) paradigm and working memory as measured by a delayed nonmatching to sample (DNMS) task. Using the GT, 11% of healthy control participants and 61% of ISDs opted for choices with high immediate gains in spite of higher future losses. For the ISDs and controls with equal GT impairments, the ISDs performed significantly lower than controls on the DNMS task. The nonimpaired ISDs on the GT also performed significantly worse than matched controls on the DNMS task. The DNMS task deficit in ISDs was across all delay times, suggesting the deficit may lie in the "executive" process of working memory, which supports earlier findings (E. M. Martin et al., 2003). The authors suggest that the prefrontal cortex hosts multiple distinct mechanisms of decision making and inhibitory control and that ISDs may be affected in any one or combination of them.

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

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

Spatially directed movement and neuronal activity in freely moving monkey

The abilities to plan a series of movements and to navigate within the environment require the functions of the frontal and ventromedial temporal lobes, respectively. Neuropsychological studies posit the existence of egocentric (prefrontal) and allocentric (ventromedial temporal) spatial frames of reference that mediate these functions. To examine neural mechanisms underlying egocentric and allocentric guidance of movement, we have developed behavioral and neurophysiological techniques for freely moving monkey. In this chapter, we provide evidence that the dorsolateral prefrontal cortex is important for egocentric spatial tasks in both the visual and tactile modalities, but it does not contribute to performance of an allocentric spatial task. Moreover, neurophysiological recordings indicate that prefrontal neurons are involved in monitoring the spatial nature of behavioral sequences in an egocentric memory task. In contrast, hippocampal neurons are active during spatially directed locomotion, apparently reflecting the monkey's location in a testing room. This discharge is independent of the task's contingencies.

Maturation of layer V pyramidal neurons in the rat prefrontal cortex: intrinsic properties and synaptic function

Layer V pyramidal neurons in the rat medial prefrontal cortex (PFC) were examined with whole cell patch-clamp recording in acute slices from postnatal day 1 (P1) to P36. In the first few days after birth, layer V pyramidal neurons had low resting potentials, high-input resistance, and long membrane time constant. During the next 2 wk, the resting potential shifted by -14 mV, while the input resistance and time constant decreased by 15- and 4-fold, respectively. Between P3 and P21, the surface area of the cell body doubled, while the total lengths of apical and basal dendrites increased by 5- and 13-fold, respectively. Action potentials (APs) were observed at all aged tested. The peak amplitude of APs increased by 30 mV during the first 3 wk, while AP rise time and half-maximum duration shortened significantly. Compared with neurons at P21 or older, neurons in the first week required much smaller currents to reach their maximum firing frequencies, but the maximum frequencies were lower than those at older ages. Stimulation of layer II/III induced monosynaptic responses in neurons older than P5. Paired-pulse responses showed a short-term depression at P7, which shifted progressive to facilitation at older ages. These results demonstrate that, similar to other neurons in the brain, layer V pyramidal neurons in the PFC undergo a period of rapid development during the first 3 wk after birth. These findings suggest that the intrinsic properties of neurons and the properties of synaptic inputs develop concomitantly during early life.

Temporally irregular mnemonic persistent activity in prefrontal neurons of monkeys during a delayed response task

An important question in neuroscience is whether and how temporal patterns and fluctuations in neuronal spike trains contribute to information processing in the cortex. We have addressed this issue in the memory-related circuits of the prefrontal cortex by analyzing spike trains from a database of 229 neurons recorded in the dorsolateral prefrontal cortex of 4 macaque monkeys during the performance of an oculomotor delayed-response task. For each task epoch, we have estimated their power spectrum together with interspike interval histograms and autocorrelograms. We find that 1). the properties of most (about 60%) neurons approximated the characteristics of a Poisson process. For about 25% of cells, with characteristics typical of interneurons, the power spectrum showed a trough at low frequencies (<20 Hz) and the autocorrelogram a dip near zero time lag. About 15% of neurons had a peak at <20 Hz in the power spectrum, associated with the burstiness of the spike train; 2). a small but significant task dependency of spike-train temporal structure: delay responses to preferred locations were characterized not only by elevated firing, but also by suppressed power at low (<20 Hz) frequencies; and 3). the variability of interspike intervals is typically higher during the mnemonic delay period than during the fixation period, regardless of the remembered cue. The high irregularity of neural persistent activity during the delay period is likely to be a characteristic signature of recurrent prefrontal network dynamics underlying working memory.

Dynamics of population code for working memory in the prefrontal cortex

Some neurons (delay cells) in the prefrontal cortex elevate their activities throughout the time period during which the animal is required to remember past events and prepare future behavior, suggesting that working memory is mediated by continuous neural activity. It is unknown, however, how working memory is represented within a population of prefrontal cortical neurons. We recorded from neuronal ensembles in the prefrontal cortex as rats learned a new delayed alternation task. Ensemble activities changed in parallel with behavioral learning so that they increasingly allowed correct decoding of previous and future goal choices. In well-trained rats, considerable decoding was possible based on only a few neurons and after removing continuously active delay cells. These results show that neural activity in the prefrontal cortex changes dynamically during new task learning so that working memory is robustly represented and that working memory can be mediated by sequential activation of different neural populations.

Attention and working memory: a dynamical model of neuronal activity in the prefrontal cortex

Cognitive behaviour requires complex context-dependent mapping between sensory stimuli and actions. The same stimulus can lead to different behaviours depending on the situation, or the same behaviour may be elicited by different cueing stimuli. Neurons in the primate prefrontal cortex show task-specific firing activity during working memory delay periods. These neurons provide a neural substrate for mapping stimulus and response in a flexible, context- or rule-dependent, fashion. We describe here an integrate-and-fire network model to explain and investigate the different types of working-memory-related neuronal activity observed. The model contains different populations (or pools) of neurons (as found neurophysiologically) in attractor networks which respond in the delay period to the stimulus object, the stimulus position ('sensory pools'), to combinations of the stimulus sensory properties (e.g. the object identity or object location) and the response ('intermediate pools'), and to the response required (left or right) ('premotor pools'). The pools are arranged hierarchically, are linked by associative synaptic connections, and have global inhibition through inhibitory interneurons to implement competition. It is shown that a biasing attentional input to define the current rule applied to the intermediate pools enables the system to select the correct response in what is a biased competition model of attention. The integrate-and-fire model not only produces realistic spiking dynamicals very similar to the neuronal data but also shows how dopamine could weaken and shorten the persistent neuronal activity in the delay period; and allows us to predict more response errors when dopamine is elevated because there is less different activity in the different pools of competing neurons, resulting in more conflict.

Dissociation of egocentric and allocentric spatial processing in prefrontal cortex

Monkeys with lesions of areas 9 and 46 performed three variants of the spatial delayed response (SDR) task. There were no impairments in allocentric spatial memory in which geometrical relationships between environmental cues were used to identify spatial location; thus, memory of a 3D environmental map is intact. In contrast, there were severe impairments in egocentric spatial memory guided by visual or tactile cues that monkeys can relate to their viewing perspective during testing. These results strongly suggest that dorsolateral prefrontal cortex selectively mediates spatial memory tasks that are solved by referencing the location of targets to the body's orientation.

Persistent activity in the prefrontal cortex during working memory

The dorsolateral prefrontal cortex (DLPFC) plays a crucial role in working memory. Notably, persistent activity in the DLPFC is often observed during the retention interval of delayed response tasks. The code carried by the persistent activity remains unclear, however. We critically evaluate how well recent findings from functional magnetic resonance imaging studies are compatible with current models of the role of the DLFPC in working memory. These new findings suggest that the DLPFC aids in the maintenance of information by directing attention to internal representations of sensory stimuli and motor plans that are stored in more posterior regions.

Basic mechanisms for graded persistent activity: discrete attractors, continuous attractors, and dynamic representations

Persistent neural activity is observed in many systems, and is thought to be a neural substrate for holding memories over time delays of a few seconds. Recent work has addressed two issues. First, how can networks of neurons robustly hold such an active memory? Computer systems obtain significant robustness to noise by approximating analogue quantities with discrete digital representations. In a similar manner, theoretical models of persistent activity in spiking neurons have shown that the most robust and stable way to store the short-term memory of a continuous parameter is to approximate it with a discrete representation. This general idea applies very broadly to mechanisms that range from biochemical networks to single cells and to large circuits of neurons. Second, why is it commonly observed that persistent activity in the cortex can be strongly time-varying? This observation is almost ubiquitous, and therefore must be taken into account in our models and our understanding of how short-term memories are held in the cortex.

Functional topography of a distributed neural system for spatial and nonspatial information maintenance in working memory

We investigated the degree to which the distributed and overlapping patterns of activity for working memory (WM) maintenance of objects and spatial locations are functionally dissociable. Previous studies of the neural system responsible for maintenance of different types of information in WM have reported seemingly contradictory results concerning the degree to which spatial and nonspatial information maintenance leads to distinct patterns of activation in prefrontal cortex. These inconsistent results may be partly attributable to the fact that different types of objects were used for the "object WM task" across studies. In the current study, we directly compared the patterns of response during WM tasks for face identity, house identity, and spatial location using functional magnetic resonance imaging (fMRI). Furthermore, independence of the neural resources available for spatial and object WM was tested behaviorally using a dual-task paradigm. Together, these results suggest that the mechanisms for the maintenance of house identity information are distributed and overlapping with those that maintain spatial location information, while the mechanisms for maintenance of face identity information are relatively more independent. There is, however, a consistent functional topography that results in superior prefrontal cortex producing the greatest response during spatial WM tasks, and middle and inferior prefrontal cortices producing their greatest responses during object WM tasks, independent of the object type. These results argue for a dorsal-ventral functional organization for spatial and nonspatial information. However, objects may contain both spatial and nonspatial information and, thus, have a distributed but not equipotent representation across both dorsal and ventral prefrontal cortex.

The pyramidal neuron in occipital, temporal and prefrontal cortex of the owl monkey (Aotus trivirgatus): regional specialization in cell structure

Recent studies have revealed marked regional variation in pyramidal cell morphology in primate cortex. In particular, pyramidal cells in human and macaque prefrontal cortex (PFC) are considerably more spinous than those in other cortical regions. PFC pyramidal cells in the New World marmoset monkey, however, are less spinous than those in man and macaques. Taken together, these data suggest that the pyramidal cell has become more branched and more spinous during the evolution of PFC in only some primate lineages. This specialization may be of fundamental importance in determining the cognitive styles of the different species. However, these data are preliminary, with only one New World and two Old World species having been studied. Moreover, the marmoset data were obtained from different cases. In the present study we investigated PFC pyramidal cells in another New World monkey, the owl monkey, to extend the basis for comparison. As in the New World marmoset monkey, prefrontal pyramidal cells in owl monkeys have relatively few spines. These species differences appear to reflect variation in the extent to which PFC circuitry has become specialized during evolution. Highly complex pyramidal cells in PFC appear not to have been a feature of a common prosimian ancestor, but have evolved with the dramatic expansion of PFC in some anthropoid lineages.

Dissociation of spatial-, object-, and sound-coding neurons in the mediodorsal nucleus of the primate thalamus

The mediodorsal nucleus (MD) is the thalamic gateway to the prefrontal cortex, an area of the brain associated with spatial and object working memory functions. We have recorded single-neuron activities from the MD nucleus in monkeys trained to perform spatial tasks with peripheral visual stimuli and a nonspatial task with foveally presented pictures of objects and faces-tasks identical to those we have previously used to map regional specializations in the dorso- and ventro-lateral prefrontal cortex, respectively. We found that MD neurons exhibited categorical specificity-either responding selectively to locations in the spatial tasks or preferentially to specific representations of faces and objects in the nonspatial task. Spatially tuned neurons were located in parts of the MD connected with the dorsolateral prefrontal cortex while neurons responding to the identity of stimuli mainly occupied more ventral positions in the nucleus that has its connections with the inferior prefrontal convexity. Neuronal responses to auditory stimuli were also examined, and vocalization sensitive neurons were found in more posterior portions of the MD. We conclude that MD neurons are dissociable by their spatial and nonspatial coding properties in line with their cortical connections and that the principle of information segregation in cortico-cortical pathways extends to the "association" nuclei of the thalamus.

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

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

Memory trace in prefrontal cortex: theory for the cognitive switch

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

The "psychic cell" of Ramón y Cajal

Santiago Ramón y Cajal might have envisioned, but likely could not have anticipated, the scientific advances that have allowed the functional validation of the existence of a "psychic cell" in the prefrontal cortex and its extension to human cognition at the end of the 20th century. This achievement rests not only on the shoulders of giants but on many small steps in the development of primate cognition, single and multiple unit recording in behaving monkeys, light and electron microscopic analysis of cortical circuitry no less than on the evolution of concepts about memory systems and parallel processing networks, among other advances. We can only wonder what the next generation of neuroscientists will bring to our understanding of brain-behavior relationships and human information capacity.

Neural correlates of visual working memory: fMRI amplitude predicts task performance

We used fMRI to investigate how moment-to-moment neural activity contributes to success or failure on individual trials of a visual working memory (WM) task. We found that different nodes of a distributed cortical network were activated to a greater extent for correct compared to incorrect trials during stimulus encoding, memory maintenance during delays, and at test. A logistic regression analysis revealed that the fMRI signal amplitude during the delay interval in a network of frontoparietal regions predicted successful performance on a trial-by-trial basis. Differential delay activity occurred even for only those trials in which BOLD activity during encoding was strong, demonstrating that it was not a simple consequence of effective versus ineffective encoding. Our results indicate that accurate memory depends on strong sustained signals that span the delay interval of WM tasks.

The computational role of dopamine D1 receptors in working memory

The prefrontal cortex (PFC) is essential for working memory, which is the ability to transiently hold and manipulate information necessary for generating forthcoming action. PFC neurons actively encode working memory information via sustained firing patterns. Dopamine via D1 receptors potently modulates sustained activity of PFC neurons and performance in working memory tasks. In vitro patch-clamp data have revealed many different cellular actions of dopamine on PFC neurons and synapses. These effects were simulated using realistic networks of recurrently connected assemblies of PFC neurons. Simulated D1-mediated modulation led to a deepening and widening of the basins of attraction of high (working memory) activity states of the network, while at the same time background activity was depressed. As a result, self-sustained activity was more robust to distracting stimuli and noise. In this manner, D1 receptor stimulation might regulate the extent to which PFC network activity is focused on a particular goal state versus being open to new goals or information unrelated to the current goal.

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

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

Timecourse of object-related neural activity in the primate prefrontal cortex during a short-term memory task

We studied the timecourse of neural activity in the primate (Macacca mulatta) prefrontal (PF) cortex during an object delayed-matching-to-sample (DMS) task. To assess the effects of experience on this timecourse, we conducted the task using both novel and highly familiar objects. In addition, noise patterns containing no task-relevant information were used as samples on some trials. Comparison of average PF ensemble activity relative to baseline activity generated by objects and noise patterns revealed three distinct activity periods. (i) Sample onset elicited a transient sensory visual response. In this sensory period, novel objects elicited stronger average ensemble activity than both familiar objects and noise patterns. (ii) An intermediate period of elevated activity followed, which began before sample offset, and continued well into the delay period. In the intermediate period, activity was elevated for noise patterns and novel objects, but near baseline for familiar objects. (iii) Finally, after average ensemble activity reached baseline activity at the end of the intermediate period, a reactivation period occurred late in the delay. Experience had little effect during reactivation, where activity was elevated for both novel and familiar objects compared to noise patterns. We show that the ensemble average resembles the activity timecourse of many single prefrontal neurons. These results suggest that PF delay activity does not merely maintain recent sensory input, but is subject to more complex experience-dependent dynamics. This has implications for how delay activity is generated and maintained.

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

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

An integrative theory of prefrontal cortex function

The prefrontal cortex has long been suspected to play an important role in cognitive control, in the ability to orchestrate thought and action in accordance with internal goals. Its neural basis, however, has remained a mystery. Here, we propose that cognitive control stems from the active maintenance of patterns of activity in the prefrontal cortex that represent goals and the means to achieve them. They provide bias signals to other brain structures whose net effect is to guide the flow of activity along neural pathways that establish the proper mappings between inputs, internal states, and outputs needed to perform a given task. We review neurophysiological, neurobiological, neuroimaging, and computational studies that support this theory and discuss its implications as well as further issues to be addressed

Age differences in prefrontal cortical activity in working memory

Working memory (WM) declines with advancing age. Brain imaging studies indicate that ventral prefrontal cortex (PFC) is active when information is retained in WM and that dorsal PFC is further activated for retention of large amounts of information. The authors examined the effect of aging on activation in specific PFC regions during WM performance. Six younger and 6 older adults performed a task in which, on each trial, they (a) encoded a 1- or 6-letter memory set, (b) maintained these letters over 5-s. and (c) determined whether or not a probe letter was part of the memory set. Comparisons of activation between the 1- and 6-letter conditions indicated age-equivalent ventral PFC activation. Younger adults showed greater dorsal PFC activation than older adults. Older adults showed greater rostral PFC activation than younger adults. Aging may affect dorsal PFC brain regions that are important for WM executive components.

Multiple effects of dopamine on layer V pyramidal cell excitability in rat prefrontal cortex

The mechanisms underlying the inhibitory effects of dopamine (DA) on layer V pyramidal neuron excitability in the prelimbic region of the rat medial prefrontal cortex were investigated. Under control conditions, DA depressed both action potential generation (driven by somatic current injection) and input resistance (R(N)). The presence of GABA(A) receptor antagonists blocked DA-induced depression of action potential generation and revealed a delayed increase in excitability that persisted for the duration of experimental recording, up to 20 min following the washout of DA. In contrast to spike generation, disinhibition did not affect the transient depression of R(N) produced by DA, suggesting independent actions of DA on spike generation and R(N). Consistent with the hypothesis that DA acts to decrease pyramidal cell output via a GABAergic mechanism, DA increased the frequency of spontaneous inhibitory postsynaptic currents in both the absence and presence of TTX. Furthermore focal application of GABA to a perisomatic region mimicked the inhibitory effect of DA on spike production without affecting R(N). Focal application of bicuculline to the same location reversed the inhibitory effect of bath-applied DA on spike generation, while again having no effect on R(N). The depression of R(N) by DA was both occluded and mimicked by the Na(+) channel blocker TTX, suggesting the involvement of a Na(+) conductance in reducing pyramidal cell R(N) during the acute presence of DA. Together these data demonstrate that the acute presence of DA decreases pyramidal neuron excitability by two independent mechanisms. At the same time DA triggers a delayed and longer-lasting increase in excitability that is partially masked by synaptic inhibition.