Contribution of default mode network to game and delayed-response task performance: Power and connectivity analyses of theta oscillation in the monkey

Neuroimaging studies have demonstrated the presence of a default mode network (DMN) which shows greater activity during rest, and an executive network (EN) which is activated during cognitive tasks. DMN and EN are thought to have competing functions. However, recent studies reported that the two networks show coactivation during some cognitive tasks. To clarify how DMN works and how DMN interacts with EN for cognitive control, we recorded EEG activities in the medial prefrontal (anterior DMN: aDMN), posterior cingulate/precuneus (posterior DMN: pDMN), and lateral prefrontal (EN) areas in the monkey. As cognitive tasks, we employed a monkey-monkey competitive video game (GAME) and a delayed-response (DR) task. We focused on theta oscillation because of its importance in cognitive control. We also examined theta band connectivity among the three network areas using the Granger causality analysis. DMN and EN were found to work cooperatively in both tasks. In all the three network areas, we found GAME-task-related, but no DR-task-related, increase in theta power from the resting level, maybe because of the higher cognitive demand associated with the GAME task performance. The information flow conveyed by the theta oscillation was directed more to aDMN than from aDMN for both tasks. The GAME-task-related increase in theta power in aDMN is supposed to be supported by more information flow conveyed by the theta oscillation from EN and pDMN.

Outcome Modulation Across Tasks in the Primate Dorsolateral Prefrontal Cortex

Animals need to learn and to adapt to new and changing environments so that appropriate actions that lead to desirable outcomes are acquired within each context. The prefrontal cortex (PF) is known to underlie such function that directly implies that the outcome of each response must be represented in the brain for behavioral policies update. However, whether such PF signal is context dependent or it is a general representation beyond the specificity of a context is still unclear. Here, we analyzed the activity of neurons in the dorsolateral PF (PFdl) recorded while two monkeys performed two perceptual magnitude discrimination tasks. Both tasks were well known by the monkeys and unexpected changes did not occur but the difficulty of the task varied from trial to trial and thus the monkeys made mistakes in a proportion of trials. We show a context-independent coding of the response outcome with neurons maintaining similar selectivity in both task contexts. Using a classification method of the neural activity, we also show that the trial outcome could be well predicted from the activity of the same neurons in the two contexts. Altogether, our results provide evidence of high degree of outcome generality in PFdl.

A loop-based neural architecture for structured behavior encoding and decoding

We present a new type of artificial neural network that generalizes on anatomical and dynamical aspects of the mammal brain. Its main novelty lies in its topological structure which is built as an array of interacting elementary motifs shaped like loops. These loops come in various types and can implement functions such as gating, inhibitory or executive control, or encoding of task elements to name a few. Each loop features two sets of neurons and a control region, linked together by non-recurrent projections. The two neural sets do the bulk of the loop's computations while the control unit specifies the timing and the conditions under which the computations implemented by the loop are to be performed. By functionally linking many such loops together, a neural network is obtained that may perform complex cognitive computations. To demonstrate the potential offered by such a system, we present two neural network simulations. The first illustrates the structure and dynamics of a single loop implementing a simple gating mechanism. The second simulation shows how connecting four loops in series can produce neural activity patterns that are sufficient to pass a simplified delayed-response task. We also show that this network reproduces electrophysiological measurements gathered in various regions of the brain of monkeys performing similar tasks. We also demonstrate connections between this type of neural network and recurrent or long short-term memory network models, and suggest ways to generalize them for future artificial intelligence research.

Saccade-related activity in the prefrontal cortex: its role in eye movement control and cognitive functions

Prefrontal neurons exhibit saccade-related activity and pre-saccadic memory-related activity often encodes the directions of forthcoming eye movements, in line with demonstrated prefrontal contribution to flexible control of voluntary eye movements. However, many prefrontal neurons exhibit post-saccadic activity that is initiated well after the initiation of eye movement. Although post-saccadic activity has been observed in the frontal eye field, this activity is thought to be a corollary discharge from oculomotor centers, because this activity shows no directional tuning and is observed whenever the monkeys perform eye movements regardless of goal-directed or not. However, prefrontal post-saccadic activities exhibit directional tunings similar as pre-saccadic activities and show context dependency, such that post-saccadic activity is observed only when monkeys perform goal-directed saccades. Context-dependency of prefrontal post-saccadic activity suggests that this activity is not a result of corollary signals from oculomotor centers, but contributes to other functions of the prefrontal cortex. One function might be the termination of memory-related activity after a behavioral response is done. This is supported by the observation that the termination of memory-related activity coincides with the initiation of post-saccadic activity in population analyses of prefrontal activities. The termination of memory-related activity at the end of the trial ensures that the subjects can prepare to receive new and updated information. Another function might be the monitoring of behavioral performance, since the termination of memory-related activity by post-saccadic activity could be associated with informing the correctness of the response and the termination of the trial. However, further studies are needed to examine the characteristics of saccade-related activities in the prefrontal cortex and their functions in eye movement control and a variety of cognitive functions.

Δ(9)Tetrahydrocannabinol impairs reversal learning but not extra-dimensional shifts in rhesus macaques

Expansion of medical marijuana use in the US and the recently successful decriminalization of recreational marijuana in two States elevates interest in the specific cognitive effects of Δ(9)tetrahydrocannabinol (Δ(9)THC), the major psychoactive constituent of marijuana. Controlled laboratory studies in nonhuman primates provide mixed evidence for specific effects of Δ(9)THC in learning and memory tasks, with a suggestion that frontal-mediated tasks may be the most sensitive. In this study, adult male rhesus monkeys were trained on tasks which assess reversal learning, extradimensional attentional shift learning and spatial delayed-response. Subjects were challenged with 0.1-0.5mg/kg Δ(9)THC, i.m., in randomized order and evaluated on the behavioral measures. Peak plasma levels of Δ(9)THC were observed 30min after 0.2mg/kg (69±29ng/ml) and 60min after 0.5mg/kg (121±23ng/ml) was administered and behavioral effects on a bimanual motor task persisted for up to 2h after injection. An increase in errors-to-criterion (ETC) associated with reversal learning was further increased by Δ(9)THC in a dose-dependent manner. The increase in ETC associated with extradimensional shifts was not affected by Δ(9)THC. Spatial delayed-response performance was impaired by Δ(9)THC in a retention-interval-dependent manner. Overall the pattern of results suggests a more profound effect of Δ(9)THC on tasks mediated by orbitofrontal (reversal learning) versus dorsolateral (extradimensional shifts) prefrontal mechanisms.

Clinical symptoms and alpha band resting-state functional connectivity imaging in patients with schizophrenia: implications for novel approaches to treatment

BACKGROUND

Schizophrenia (SZ) is associated with functional decoupling between cortical regions, but we do not know whether and where this occurs in low-frequency electromagnetic oscillations. The goal of this study was to use magnetoencephalography (MEG) to identify brain regions that exhibit abnormal resting-state connectivity in the alpha frequency range in patients with schizophrenia and investigate associations between functional connectivity and clinical symptoms in stable outpatient participants.

METHODS

Thirty patients with SZ and 15 healthy comparison participants were scanned in resting-state MEG (eyes closed). Functional connectivity MEG source data were reconstructed globally in the alpha range, quantified by the mean imaginary coherence between a voxel and the rest of the brain.

RESULTS

In patients, decreased connectivity was observed in left prefrontal cortex (PFC) and right superior temporal cortex, whereas increased connectivity was observed in left extrastriate cortex and the right inferior PFC. Functional connectivity of left inferior parietal cortex was negatively related to positive symptoms. Low left PFC connectivity was associated with negative symptoms. Functional connectivity of midline PFC was negatively correlated with depressed symptoms. Functional connectivity of right PFC was associated with other (cognitive) symptoms.

CONCLUSIONS

This study demonstrates direct functional disconnection in SZ between specific cortical fields within low-frequency resting-state oscillations. Impaired alpha coupling in frontal, parietal, and temporal regions is associated with clinical symptoms in these stable outpatients. Our findings indicate that this level of functional disconnection between cortical regions is an important treatment target in SZ.

Neural activity in frontal cortical cell layers: evidence for columnar sensorimotor processing

The mammalian frontal cortex (FCx) is at the top of the brain's sensorimotor hierarchy and includes cells in the supragranular Layer 2/3, which integrate convergent sensory information for transmission to infragranular Layer 5 cells to formulate motor system outputs that control behavioral responses. Functional interaction between these two layers of FCx was examined using custom-designed ceramic-based microelectrode arrays (MEAs) that allowed simultaneous recording of firing patterns of FCx neurons in Layer 2/3 and Layer 5 in nonhuman primates performing a simple go/no-go discrimination task. This unique recording arrangement showed differential encoding of task-related sensory events by cells in each layer with Layer 2/3 cells exhibiting larger firing peaks during presentation of go target and no-go target task images, whereas Layer 5 cells showed more activity during reward contingent motor responses in the task. Firing specificity to task-related events was further demonstrated by synchronized firing between pairs of cells in different layers that occupied the same vertically oriented "column" on the MEA. Pairs of cells in different layers recorded at adjacent "noncolumnar" orientations on the MEA did not show synchronized firing during the same task-related events. The results provide required evidence in support of previously suggested task-related sensorimotor processing in the FCx via functionally segregated minicolumns.

Reward acts as a signal to control delay-period activity in delayed-response tasks

Prefrontal delay-period activity represents a neural mechanism for the active maintenance of information and needs to be controlled by some signal to appropriately operate working memory. To examine whether reward-delivery acts as this signal, the effects of delay-period activity in response to unexpected reward-delivery were examined by analyzing single-neuron activity recorded in the primate dorsolateral prefrontal cortex. Among neurons that showed delay-period activity, 34% showed inhibition of this activity in response to unexpected reward-delivery. The delay-period activity of these neurons was affected by the expectation of reward-delivery. The strength of the reward signal in controlling the delay-period activity is related to the strength of the effect of reward information on the delay-period activity. These results indicate that reward-delivery acts as a signal to control delay-period activity.

Learning substrates in the primate prefrontal cortex and striatum: sustained activity related to successful actions

Learning from experience requires knowing whether a past action resulted in a desired outcome. The prefrontal cortex and basal ganglia are thought to play key roles in such learning of arbitrary stimulus-response associations. Previous studies have found neural activity in these areas, similar to dopaminergic neurons' signals, that transiently reflect whether a response is correct or incorrect. However, it is unclear how this transient activity, which fades in under a second, influences actions that occur much later. Here, we report that single neurons in both areas show sustained, persistent outcome-related responses. Moreover, single behavioral outcomes influence future neural activity and behavior: behavioral responses are more often correct and single neurons more accurately discriminate between the possible responses when the previous response was correct. These long-lasting signals about trial outcome provide a way to link one action to the next and may allow reward signals to be combined over time to implement successful learning.

The encoding of cocaine vs. natural rewards in the striatum of nonhuman primates: categories with different activations

The behavioral and motivational changes that result from use of abused substances depend upon activation of neuronal populations in the reward centers of the brain, located primarily in the corpus striatum in primates. To gain insight into the cellular mechanisms through which abused drugs reinforce behavior in the primate brain, changes in firing of neurons in the ventral (VStr, nucleus accumbens) and dorsal (DStr, caudate-putamen) striatum to "natural" (juice) vs. drug (i.v. cocaine) rewards were examined in four rhesus monkeys performing a visual Go-Nogo decision task. Task-related striatal neurons increased firing to one or more of the specific events that occurred within a trial represented by (1) Target stimuli (Go trials) or (2) Nogotarget stimuli (Nogo trials), and (3) Reward delivery for correct performance. These three cell populations were further subdivided into categories that reflected firing exclusively on one or the other type of signaled reward (juice or cocaine) trial (20%-30% of all cells), or, a second subpopulation that fired on both (cocaine and juice) types of rewarded trial (50%). Results show that neurons in the primate striatum encoded cocaine-rewarded trials similar to juice-rewarded trials, except for (1) increased firing on cocaine-rewarded trials, (2) prolonged activation during delivery of i.v. cocaine infusion, and (3) differential firing in ventral (VStr cells) vs. dorsal (DStr cells) striatum cocaine-rewarded trials. Reciprocal activations of antithetic subpopulations of cells during different temporal intervals within the same trial suggest a functional interaction between processes that encode drug and natural rewards in the primate brain.

Value-based modulations in human visual cortex

Economists and cognitive psychologists have long known that prior rewards bias decision making in favor of options with high expected value. Accordingly, value modulates the activity of sensorimotor neurons involved in initiating movements toward one of two competing decision alternatives. However, little is known about how value influences the acquisition and representation of incoming sensory information or about the neural mechanisms that track the relative value of each available stimulus to guide behavior. Here, fMRI revealed value-related modulations throughout spatially selective areas of the human visual system in the absence of overt saccadic responses (including in V1). These modulations were primarily associated with the reward history of each stimulus and not to self-reported estimates of stimulus value. Finally, subregions of frontal and parietal cortex represent the differential value of competing alternatives and may provide signals to bias spatially selective visual areas in favor of more valuable stimuli.

Dorsolateral prefrontal lesions do not impair tests of scene learning and decision-making that require frontal-temporal interaction

Theories of dorsolateral prefrontal cortex (DLPFC) involvement in cognitive function variously emphasize its involvement in rule implementation, cognitive control, or working and/or spatial memory. These theories predict broad effects of DLPFC lesions on tests of visual learning and memory. We evaluated the effects of DLPFC lesions (including both banks of the principal sulcus) in rhesus monkeys on tests of scene learning and strategy implementation that are severely impaired following crossed unilateral lesions of frontal cortex and inferotemporal cortex. Dorsolateral lesions had no effect on learning of new scene problems postoperatively, or on the implementation of preoperatively acquired strategies. They were also without effect on the ability to adjust choice behaviour in response to a change in reinforcer value, a capacity that requires interaction between the amygdala and frontal lobe. These intact abilities following DLPFC damage support specialization of function within the prefrontal cortex, and suggest that many aspects of memory and strategic and goal-directed behaviour can survive ablation of this structure.

Activity of primate orbitofrontal and dorsolateral prefrontal neurons: effect of reward schedule on task-related activity

Recent studies show that task-related activity in the dorsolateral prefrontal cortex (DLPFC) is modulated by the quality and quantity of the reward, suggesting that the subject's motivational state affects cognitive operations in the DLPFC. The orbito-frontal cortex (OFC) is a possible source of motivational inputs to the DLPFC. However, it is not well known whether these two areas exhibit similar motivational effects on task-related activity. We compared motivational effects on task-related activity in these areas while a monkey performed an oculomotor delayed-response (ODR) task under two reward schedules. In the ODR-1 schedule, reward was given only after the successful completion of four consecutive trials, whereas in the ODR-2 schedule, reward was given after every correct trial. Task-related activities in both areas showed spatial selectivity. The spatial characteristics of task-related activity remained constant in both schedules. Task-related activity in both areas, especially delay-period activity, was also affected by the reward schedule and the magnitude of the activity gradually increased depending on the proximity of the reward trial in the ODR-1 schedule. More task-related OFC activities were affected by reward schedules, whereas more task-related DLPFC activities were affected by spatial factors and reward schedules. These results indicate that the OFC plays a role in monitoring the proximity of the reward trial and detecting reward delivery, whereas the DLPFC plays a role in performing cognitive operations and integrating cognitive and motivational information. These results also indicate that spatial information and the animal's motivational state independently affect neuronal activity in both areas.

Neuronal firing rate, inter-neuron correlation and synchrony in area MT are correlated with directional choices during stimulus and reward expectation

Sensation, memories, and predictions contribute to choices in everyday life, and their relative impact should change with task constraints. To investigate how the impact from sensory cortex on decision making varies with task constraints we trained macaque monkeys in a direction discrimination task where they could maximize reward by waiting for sensory visual information early in a trial, while focusing on memory and reward prediction as a trial progressed. The task constraints caused animals to indicate decisions in complete absence of visual motion stimuli (stimulus independent decisions), as 25% of the trials were 'no stimulus' trials. On 'no stimulus' trials reward delivery depended on the current decision in relation to the decision history. Stimulus independent decisions occurred during an epoch when a stimulus could in principle have been presented, or afterwards when stimuli could not occur anymore. Stimulus independent decisions were significantly different during these two periods. Reward exploitation was more efficient late in the trial, but it was not associated with systematic activity changes in directionally selective neurons in area MT. Conversely, systematic changes of neuronal activity and firing rate correlation in directionally selective middle temporal area (MT) neurons were restricted to a short time period before early decisions. Changing task constraints in the course of a single trial thus determines how neurons in sensory areas contribute to decision making.

History- and current instruction-based coding of forthcoming behavioral outcomes in the striatum

Animals optimize behaviors by predicting future critical events based on histories of actions and their outcomes. When behavioral outcomes like reward and aversion are signaled by current external cues, actions are directed to acquire the reward and avoid the aversion. The basal ganglia are thought to be the brain locus for reward-based adaptive action planning and learning. To understand the role of striatum in coding outcomes of forthcoming behavioral responses, we addressed two specific questions. First, how are the histories of reward and aversion used for encoding forthcoming outcomes in the striatum during a series of instructed behavioral responses? Second, how are the behavioral responses and their instructed outcomes represented in the striatum? We recorded discharges of 163 presumed projection neurons in the striatum while monkeys performed a visually instructed lever-release task for reward, aversion, and sound outcomes, whose occurrences could be estimated by their histories. Before outcome instruction, discharge rates of a subset of neurons activated in this epoch showed positive or negative regression slopes with reward history (24/44), that is, to the number of trials since the last reward trial, which changed in parallel with reward probability of current trials. The history effect was also observed for the aversion outcome but in far fewer neurons (3/44). Once outcomes were instructed in the same task, neurons selectively encoded the outcomes before and after behavioral responses (reward, 46/70; aversion, 6/70; sound, 6/70). The history- and current instruction-based coding of forthcoming behavioral outcomes in the striatum might underlie outcome-oriented behavioral modulation.

Neural coding of reward magnitude in the orbitofrontal cortex of the rat during a five-odor olfactory discrimination task

The orbitofrontal cortex (OBFc) has been suggested to code the motivational value of environmental stimuli and to use this information for the flexible guidance of goal-directed behavior. To examine whether information regarding reward prediction is quantitatively represented in the rat OBFc, neural activity was recorded during an olfactory discrimination "go"/"no-go" task in which five different odor stimuli were predictive for various amounts of reward or an aversive reinforcer. Neural correlates related to both actual and expected reward magnitude were observed. Responses related to reward expectation occurred during the execution of the behavioral response toward the reward site and within a waiting period prior to reinforcement delivery. About one-half of these neurons demonstrated differential firing toward the different reward sizes. These data provide new and strong evidence that reward expectancy, regardless of reward magnitude, is coded by neurons of the rat OBFc, and are indicative for representation of quantitative information concerning expected reward. Moreover, neural correlates of reward expectancy appear to be distributed across both motor and nonmotor phases of the task.

Activity of primate orbitofrontal and dorsolateral prefrontal neurons: task-related activity during an oculomotor delayed-response task

The orbitofrontal cortex (OFC) has strong reciprocal connections to the dorsolateral prefrontal cortex (DLPFC), which is known to participate in spatial working memory processes. However, it is not known whether or not the OFC also participates in spatial working memory and whether the OFC and DLPFC contribute equally to this process. To address these issues, we collected single-neuron activity from both areas while a monkey performed an oculomotor delayed-response task, and compared the characteristics of task-related activities between the OFC and DLPFC. All of the task-related activities observed in the DLPFC were also observed in the OFC. However, the proportion and response characteristics of task-related activities were different between the two areas. While most delay-period activity observed in the DLPFC was directionally selective and showed tonic sustained activation, most delay-period activity observed in the OFC was omni-directional and showed gradually increasing activity. Reward-period activity was predominant among task-related activities in the OFC. The proportion of neurons showing reward-period activity was significantly higher in the OFC than in the DLPFC. These results suggest that, although both the OFC and DLPFC participate in spatial working memory processes, the OFC is related more to the expectation and the detection of reward delivery, while the DLPFC is related more to the temporary maintenance of spatial information and its processing.

Dynamic Changes in Representations of Preceding and Upcoming Reward in Monkey Orbitofrontal Cortex

We investigated how orbitofrontal cortex (OFC) contributes to adaptability in the face of changing reward contingencies by examining how reward representations in monkey orbitofrontal neurons change during a visually cued, multi-trial reward schedule task. A large proportion of orbitofrontal neurons were sensitive to events in this task (69/80 neurons in the valid and 48/58 neurons in the random cue context). Neuronal activity depended upon preceding reward, upcoming reward, reward delivery, and schedule state. Preceding reward-dependent activity occurred in both the valid and random cue contexts, whereas upcoming reward-dependent activity was observed only in the valid context. A greater proportion of neurons encoded preceding reward in the random than the valid cue context. The proportion of neurons with preceding reward-dependent activity declined as each trial progressed, whereas the proportion encoding upcoming reward increased. Reward information was represented by ensembles of neurons, the composition of which changed with task context and time. Overall, neuronal activity in OFC adapted to reflect the importance of different types of reward information in different contexts and time periods. This contextual and temporal adaptability is one hallmark of neurons participating in executive functions.

Neurons in the macaque orbitofrontal cortex code relative preference of both rewarding and aversive outcomes

Many studies have shown that the orbitofrontal cortex (OFC) is involved in the processing of emotional information. However, although some lines of study showed that the OFC is also involved in negative emotions, few electrophysiological studies have focused on the characteristics of OFC neuronal responses to aversive information at the individual neuron level. On the other hand, a previous study has shown that many OFC neurons code relative preference of available rewards. In this study, we aimed to elucidate how reward information and aversive information are coded in the OFC at the individual neuron level. To achieve this aim, we introduced the electrical stimulus (ES) as an aversive stimulus, and compared the neuronal responses to the ES-predicting stimulus with those to reward-predicting stimuli. We found that many OFC neurons showed responses to both the ES-predicting stimulus and the reward-predicting stimulus, and they code relative preference of not only the reward outcome but also the aversive outcome. This result suggests that the same group of OFC neurons code both reward and aversive information in the form of relative preference.