Disruption of Columnar and Laminar Cognitive Processing in Primate Prefrontal Cortex Following Cocaine Exposure

Prefrontal cortical activity in primate brain plays a critical role in cognitive processes involving working memory and the executive control of behavior. Groups of prefrontal cortical neurons within specified cortical layers along cortical minicolumns differentially generate inter- and intra-laminar firing to process relevant information for goal oriented behavior. However, it is not yet understood how cocaine modulates such differential firing in prefrontal cortical layers. Rhesus macaque nonhuman primates (NHPs) were trained in a visual delayed match-to-sample (DMS) task while the activity of prefrontal cortical neurons (areas 46, 8 and 6) was recorded simultaneously with a custom multielectrode array in cell layers 2/3 and 5. Animals were reinforced with juice for correct responses. The first half of the recording session (control) was conducted following saline injection and in the second half of the same session cocaine was administered. Prefrontal neuron activity with respect to inter- and intra-laminar firing in layers 2/3 and 5 was assessed in the DMS task before and after the injection of cocaine. Results showed that firing rates of both pyramidal cells and interneurons increased on Match phase presentation and the Match Response (MR) in both control and cocaine halves of the session. Differential firing under cocaine vs. control in the Match phase was increased for interneurons but decreased for pyramidal cells. In addition, functional\u27 interactions between prefrontal pyramidal cells in layer 2/3 and 5 decreased while intra-laminar cross-correlations in both layers increased. These neural recordings demonstrate that prefrontal neurons differentially encode and process information within and between cortical cell layers via cortical columns which is disrupted in a differential manner by cocaine: administration

Muscarinic attenuation of mnemonic rule representation in macaque dorsolateral prefrontal cortex during a pro- and anti-saccade task

Maintenance of context is necessary for execution of appropriate responses to diverse environmental stimuli. The dorsolateral prefrontal cortex (DLPFC) plays a pivotal role in executive function, including working memory and representation of abstract rules, and is modulated by the ascending cholinergic system through nicotinic and muscarinic receptors. Muscarinic receptors’ effect on local primate DLPFC neural activity in vivo during cognitive tasks remains poorly understood. Here we examined the effects of muscarinic receptor blockade on rule-related activity in the macaque prefrontal cortex by combining iontophoretic application of the general muscarinic receptor antagonist scopolamine with single-unit recordings while monkeys performed a rule-guided saccade task. We found that scopolamine reduced overall neuronal firing rate and impaired rule discriminability of task-selective cells. Saccade and visual direction selectivity measures were also reduced by muscarinic antagonism. These results demonstrate that blockade of muscarinic receptors in dorsolateral prefrontal cortex creates deficits in working memory representation of rules in primates

Prefrontal cortical microcircuits bind perception to executive control

During the perception-to-action cycle, our cerebral cortex mediates the interactions between the environment and the perceptual-executive systems of the brain. At the top of the executive hierarchy, prefrontal cortical microcircuits are assumed to bind perceptual and executive control information to guide goal-driven behavior. Here, we tested this hypothesis by comparing simultaneously recorded neuron firing in prefrontal cortical layers and the caudate-putamen of rhesus monkeys, trained in a spatial-versus-object, rule-based match-to-sample task. We found that during the perception and executive selection phases, cell firing in the localized prefrontal layers and caudate-putamen region exhibited similar location preferences on spatial-trials, but less on object- trials. Then, we facilitated the perceptual-executive circuit by stimulating the prefrontal infra-granular-layers with patterns previously derived from supra-granular-layers, and produced stimulation-induced spatial preference in percent correct performance on spatial trials, similar to neural tuning. These results show that inter-laminar prefrontal microcircuits play causal roles to the perception-to-action cycle

Role of cholinergic receptors in prefrontal activity of nonhuman primates during an oculomotor rule-based working memory task

The ability to flexibly react to our dynamic environment is a cardinal component of cognition and our human identity. Millions across the globe are affected by disorders of cognition, affecting their ability to live independently. Prefrontal cortex is required for optimal cognitive functioning, but its circuitry is often disrupted in conditions of impaired cognition. In addition, the cholinergic system is vital to optimal executive function, but this is disrupted in a number of conditions, including Alzheimer’s disease and schizophrenia. The actions of cholinergic receptors were explored in this project with local application of cholinergic compounds onto prefrontal neurons as rhesus monkeys performed a rule-based saccadic task that requires working memory maintenance. The antisaccade task is a useful probe of prefrontal cortex function that elicits errors in neuropsychiatric conditions. Some prefrontal neurons respond to different task aspects of the antisaccade task, e.g., discharging preferentially for one task rule over the other (pro- or antisaccades), and are thought to be involved in the circuitry for correct behavioural responses. Chapter 2 explored the effect of general stimulation of cholinergic receptors on rhesus PFC neuronal activity during antisaccade performance. In Chapter 3, newly developed cholinergic receptor subtype-specific compounds were utilized to examine the actions of muscarinic M1 receptor stimulation on prefrontal activity. Cortical oscillations are emerging as an important aspect of cognitive circuitry, such as during working memory maintenance. Chapter 4 examined the influence of local cholinergic receptor stimulation and blockade on the power of local field potential in different frequency bands. This project characterized the role of cholinergic receptors in prefrontal cortical neurons that were actively involved in cognitive circuitry. This and future work on the cholinergic influence on prefrontal cortex will provide insights into the altered cognitive functioning in Alzheimer’s disease and schizophrenia, which are also affected by disrupted cholinergic systems

Human metabolic adaptations and prolonged expensive neurodevelopment: A review

1.	After weaning, human hunter-gatherer juveniles receive substantial (≈3.5-7 MJ day^-1^), extended (≈15 years) and reliable (kin and nonkin food pooling) energy provision.
2.	The childhood (pediatric) and the adult human brain takes a very high share of both basal metabolic rate (BMR) (child: 50-70%; adult: ≈20%) and total energy expenditure (TEE) (child: 30-50%; adult: ≈10%).
3.	The pediatric brain for an extended period (≈4-9 years-of-age) consumes roughly 50% more energy than the adult one, and after this, continues during adolescence, at a high but declining rate. Within the brain, childhood cerebral gray matter has an even higher 1.9 to 2.2-fold increased energy consumption. 
4.	This metabolic expensiveness is due to (i) the high cost of synapse activation (74% of brain energy expenditure in humans), combined with (ii), a prolonged period of exuberance in synapse numbers (up to double the number present in adults). Cognitive development during this period associates with volumetric changes in gray matter (expansion and contraction due to metabolic related size alterations in glial cells and capillary vascularization), and in white matter (expansion due to myelination). 
5.	Amongst mammals, anatomically modern humans show an unique pattern in which very slow musculoskeletal body growth is followed by a marked adolescent size/stature spurt. This pattern of growth contrasts with nonhuman primates that have a sustained fast juvenile growth with only a minor period of puberty acceleration. The existence of slow childhood growth in humans has been shown to date back to 160,000 BP. 
6.	Human children physiologically have a limited capacity to protect the brain from plasma glucose fluctuations and other metabolic disruptions. These can arise in adults, during prolonged strenuous exercise when skeletal muscle depletes plasma glucose, and produces other metabolic disruptions upon the brain (hypoxia, hyperthermia, dehydration and hyperammonemia). These are proportional to muscle mass.
7.	Children show specific adaptations to minimize such metabolic disturbances. (i) Due to slow body growth and resulting small body size, they have limited skeletal muscle mass. (ii) They show other adaptations such as an exercise specific preference for free fatty acid metabolism. (iii) While children are generally more active than adolescents and adults, they avoid physically prolonged intense exertion. 
8.	Childhood has a close relationship to high levels of energy provision and metabolic adaptations that support prolonged synaptic neurodevelopment. 
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Investigating the Primate Prefrontal Cortex Correlates of Cognitive Deficits In the Ketamine Model of Schizophrenia

The World Health Organization has classified schizophrenia as one of the five leading causes of disability worldwide. Afflicting almost 1% of the world’s population, the disease’s greatest impact stems from its reduction in patients’ cognitive faculties. In order to better study these impairments, a pharmacological model has been developed using the NMDA antagonist, ketamine. This disease model successfully recreates the cognitive dysfunction of schizophrenia, allowing researchers to search for associated electrophysiological changes. In this project I examined the behavioural and neurophysiological effects of ketamine on non-human primates performing the anti-saccade task. Success in this task requires a degree of cognitive control over behaviour and previous studies have described poor performance in both patients with schizophrenia and healthy controls administered ketamine. Our intracranial recordings are localized in the prefrontal cortex (PFC), a region associated with many of the cognitive functions impaired in schizophrenia. The first study shows that neurons in the PFC exhibit selectivity for the task rule. This rule selectivity is lost after ketamine administration due to an indiscriminate increase in the neuronal firing rate. These changes were also associated with an increased error rate and longer reaction times. The second study shows that neurons in the PFC are also sensitive to the outcome of the trial, firing more for either correct or erroneous responses. Once again, selectivity is lost following ketamine administration and the neurons show increased, nonspecific activity. Lastly, we recorded the local field potential of the PFC and found changes in the oscillatory patterns during the anti-saccade task. Prior to ketamine there was a significantly stronger beta-band activity after correct trials compared to error trials, but this selective activity was lost due to an overall decrease in the outcome selective oscillatory events. These findings show that ketamine’s effect on the PFC is one of selectivity reduction. Patients with schizophrenia have been shown to require increased PFC activity but only reach moderate performance levels in cognitive challenges. It is possible that their brains suffer the same changes highlighted in this research. Although the signals are still present in their PFC, they are being lost amongst the noise

Alterations of neural activity in the prefrontal cortex associated with deficits in working memory performance

Working memory (WM), a core cognitive function, enables the temporary holding and manipulation of information in mind to support ongoing behavior. Neurophysiological recordings conducted in nonhuman primates have revealed neural correlates of this process in a network of higher-order cortical regions, particularly the prefrontal cortex (PFC). Here, we review the circuit mechanisms and functional importance of WM-related activity in these areas. Recent neurophysiological data indicates that the absence of these neural correlates at different stages of WM is accompanied by distinct behavioral deficits, which are characteristic of various disease states/normal aging and which we review here. Finally, we discuss emerging evidence of electrical stimulation ameliorating these WM deficits in both humans and non-human primates. These results are important for a basic understanding of the neural mechanisms supporting WM, as well as for translational efforts to developing therapies capable of enhancing healthy WM ability or restoring WM from dysfunction

Human metabolic adaptations and prolonged expensive neurodevelopment: A review

1.	After weaning, human hunter-gatherer juveniles receive substantial (≈3.5-7 MJ day^-1^), extended (≈15 years) and reliable (kin and nonkin food pooling) energy provision.
2.	The childhood (pediatric) and the adult human brain takes a very high share of both basal metabolic rate (BMR) (child: 50-70%; adult: ≈20%) and total energy expenditure (TEE) (child: 30-50%; adult: ≈10%).
3.	The pediatric brain for an extended period (≈4-9 years-of-age) consumes roughly 50% more energy than the adult one, and after this, continues during adolescence, at a high but declining rate. Within the brain, childhood cerebral gray matter has an even higher 1.9 to 2.2-fold increased energy consumption. 
4.	This metabolic expensiveness is due to (i) the high cost of synapse activation (74% of brain energy expenditure in humans), combined with (ii), a prolonged period of exuberance in synapse numbers (up to double the number present in adults). Cognitive development during this period associates with volumetric changes in gray matter (expansion and contraction due to metabolic related size alterations in glial cells and capillary vascularization), and in white matter (expansion due to myelination). 
5.	Amongst mammals, anatomically modern humans show an unique pattern in which very slow musculoskeletal body growth is followed by a marked adolescent size/stature spurt. This pattern of growth contrasts with nonhuman primates that have a sustained fast juvenile growth with only a minor period of puberty acceleration. The existence of slow childhood growth in humans has been shown to date back to 160,000 BP. 
6.	Human children physiologically have a limited capacity to protect the brain from plasma glucose fluctuations and other metabolic disruptions. These can arise in adults, during prolonged strenuous exercise when skeletal muscle depletes plasma glucose, and produces other metabolic disruptions upon the brain (hypoxia, hyperthermia, dehydration and hyperammonemia). These are proportional to muscle mass.
7.	Children show specific adaptations to minimize such metabolic disturbances. (i) Due to slow body growth and resulting small body size, they have limited skeletal muscle mass. (ii) They show other adaptations such as an exercise specific preference for free fatty acid metabolism. (iii) While children are generally more active than adolescents and adults, they avoid physically prolonged intense exertion. 
8.	Childhood has a close relationship to high levels of energy provision and metabolic adaptations that support prolonged synaptic neurodevelopment. 
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Differential Effects of Macaque Dorsolateral Prefrontal Deactivations During Uncued and Cued Role Conditions

Cognitive control enables us to guide our behaviour in an appropriate, context-dependent manner. This behavioral flexibility is probed by task-switching paradigms, which require working memory to maintain relevant rules and flexibility to switch between rules. The dorsolateral prefrontal cortex (DLPFC) has been implicated in rule maintenance by neuroimaging and electrophysiological studies. While these studies have identified a correlation between DLPFC activity and rule maintenance, deactivation studies allow us to establish a causal relationship. Here we have examined the effect of bilateral deactivation of areas 46 and 9/46d on rule maintenance, while a monkey (Macacca mulatta) performed blocks of pro- and anti-saccades. Areas 46 and 9/46d were deactivated by pumping chilled methanol through bilaterally implanted cryoloops. Rule maintenance was tested while monkeys performed blocks of pro- and anti-saccades with and without instruction cues. Monkeys had to look toward the stimulus on pro-saccade trials and away from the stimulus to its mirror location on anti-saccade trials. After 15-25 correct responses, the task switched (e.g. from pro-saccades to anti-saccades) without any explicit signal to the monkey. Bilateral area 46 deactivation impaired performance throughout both blocks, while bilateral area 9/46d deactivation did not affect performance. Surprisingly, bilateral deactivation of both areas (46 and 9/46d) impaired performance on anti-saccade trials but recovered performance on pro-saccade trials. These results present a causal relationship between area 46 and rule maintenance and provide evidence for functional dissociation between subregions in the dorsolateral PFC for rule-guided behavior

Beta oscillations underlie top-down, feedback control while gamma oscillations reflect bottom-up, feedforward influences

Prefrontal cortex (PFC) is critical to behavioral flexibility and, hence, the top-down control over bottom-up sensory information. The mechanisms underlying this capacity have been hypothesized to involve the propagation of alpha/beta (8-30 Hz) oscillations via feedback connections to sensory regions. In contrast, gamma (30-160 Hz) oscillations are thought to arise as a function of bottom-up, feedforward stimulation. To test the hypothesis that such oscillatory phenomena embody such functional roles, we assessed the performance of nine monkeys on tasks of learning, categorization, and working memory concurrent with recording of local field potentials (LFPs) from PFC. The first set of tasks consisted of two classes of learning: one, explicit and, another, implicit. Explicit learning is a conscious process that demands top-down control, and in these tasks alpha/beta oscillations tracked learning. In contrast, implicit learning is an unconscious process that is automatic (i.e. bottom up), and in this task alpha/beta oscillations did not track learning. We next looked at dot-pattern categorization. In this task, category exemplars were generated by jittering the dot locations of a prototype. By chance, some of these exemplars were similar to the prototype (low distortion), and others were not (high distortion). Behaviorally, the monkeys performed well on both distortion levels. However, alpha/beta band oscillations carried more category information at high distortions, while gamma-band category information was greatest on low distortions. Overall, the greater the need for top-down control (i.e. high distortion), the greater the beta, and the lesser the need (i.e. low distortion), the greater the gamma. Finally, laminar electrodes were used to record from animals trained on working memory tasks. Each laminar probe was lowered so that its set of contacts sampled all cortical layers. During these tasks, gamma oscillations peaked in superficial layers, while alpha/beta peaked in deep layers. Moreover, these deep-layer alpha/beta oscillations entrained superficial alpha/beta, and modulated the amplitude of superficial-layer gamma oscillations. These laminar distinctions are consistent with anatomy: feedback neurons originate in deep layers and feedforward neurons in superficial layers. In summary, alpha/beta oscillations reflect top-down control and feedback connectivity, while gamma oscillations reflect bottom-up processes and feedforward connectivity