Science posts

See science posts on page 23 below.

    • 2007
    • Arias-Carrión O et al.
    • Dopamine, learning, and reward-seeking behavior.
    • Dopaminergic neurons of the midbrain are the main source of dopamine (DA) in the brain. DA has been shown to be involved in the control of movements, the signaling of error in prediction of reward, motivation, and cognition. Cerebral DA depletion is the hallmark of Parkinson's disease (PD). Other pathological states have also been associated with DA dysfunction, such as schizophrenia, autism, and attention deficit hyperactivity disorder in children, as well as drug abuse. DA is closely associated with reward-seeking behaviors, such as approach, consumption, and addiction. Recent researches suggest that the firing of DA neurons is a motivational substance as a consequence of reward-anticipation. This hypothesis is based on the evidence that, when a reward is greater than expected, the firing of certain DA neurons increases, which consequently increases desire or motivation towards the reward.
    • 1995
    • Heiner Deubel
    • Separate adaptive mechanisms for the control of reactive and volitional saccadic eye movements
    • Adaptive reduction of the gain of the saccadic system was induced by means of two basically different paradigms. In the first approach the subjects had to follow a step-wise moving target. During each follow-up saccade the target was systematically displaced by 25% of the initial step, into the opposite direction of the saccade. In the second approach the subjects scanned a display of six small items. During each scanning saccade the whole display was displaced by 25% into the opposite direction of the saccade. Both conditions lead to fast and consistent saccadic gain reductions. However, adaptation with the stepping target did not transfer to the saccades in the scanning situation, nor to delayed saccades in an overlap paradigm, nor to memory-guided saccades. Conversely, when saccades were adapted in the scanning situation, induced gain changes transferred to overlap and memory-guided saccades, but not to saccades following steps of a single target. The results suggest that two sepa..
    • 2003
    • Joshua W. Brown et al.
    • How laminar frontal cortex and basal ganglia circuits interact to control planned and reactive saccades
    • How does the brain learn to balance between reactive and planned behaviors? The basal ganglia (BG) and frontal cortex together allow animals to learn planned behaviors that acquire rewards when prepotent reactive behaviors are insufficient. This paper proposes a new model, called TELOS, to explain how laminar circuitry of the frontal cortex, exemplified by the frontal eye fields, interacts with the BG, thalamus, superior colliculus, and inferotemporal and parietal cortices to learn and perform reactive and planned eye movements. The model is formulated as fourteen computational hypotheses. These specify how strategy priming and action planning (in cortical layers III, Va and VI) are dissociated from movement execution (in layer Vb), how the BG help to choose among and gate competing plans, and how a visual stimulus may serve either as a movement target or as a discriminative cue to move elsewhere. The direct, indirect and hyperdirect pathways through the BG are shown to enable comple..
    • 2008
    • Magali Jaffard et al.
    • Proactive inhibitory control of movement assessed by event-related fMRI
    • Many neuronal processes play a role in the overall performance of inhibition tasks, often making it difficult to associate particular behavioral results to specific processes and structures. Indeed, in classical Go/NoGo, Stop or subliminal masked-prime tasks, inhibition is usually triggered at the same time as the sensorimotor processes involved in movement selection and conflict monitoring. To account for motor inhibition, many conflicting candidate structures, which depend on specific task requirements, have been proposed. In the present paper, first we used a simple reaction (RT) time task and, second, we took advantage of the fact that volitional inhibition is usually implemented before any stimulus occurs when subjects are aware that a warning signal will be presented before a target. This proactive inhibition would be intended to prevent anticipated responses and would be lifted as soon as the warning signal has been identified. In other words, we postulate that the same event ..
    • 2008
    • Todd S. Braver et al.
    • Flexible neural mechanisms of cognitive control within human prefrontal cortex
    • A major challenge in research on executive control is to reveal its functional decomposition into underlying neural mechanisms. A typical assumption is that this decomposition occurs solely through anatomically based dissociations. Here we tested an alternative hypothesis that different cognitive control processes may be implemented within the same brain regions, with fractionation and dissociation occurring on the basis of temporal dynamics. Regions within lateral prefrontal cortex (PFC) were examined that, in a prior study, exhibited contrasting temporal dynamics between older and younger adults during performance of the AX-CPT cognitive control task. The temporal dynamics in younger adults fit a proactive control pattern (primarily cue-based activation), whereas in older adults a reactive control pattern was found (primarily probe-based activation). In the current study, we found that following a period of task-strategy training, these older adults exhibited a proactive shift with..
    • 2012
    • Marion Criaud et al.
    • Proactive Inhibitory Control of Response as the Default State of Executive Control
    • Refraining from reacting does not only involve reactive inhibitory mechanisms. It was recently found that inhibitory control also relies strongly on proactive mechanisms. However, since most available studies have focused on reactive stopping, little is known about how proactive inhibition of response is implemented. Two behavioral experiments were conducted to identify the temporal dynamics of this executive function. They manipulated respectively the time during which inhibitory control must be sustained until a stimulus occurs, and the time limit allowed to set up inhibition before a stimulus occurs. The results show that inhibitory control is not set up after but before instruction, and is not transient and sporadic but sustained across time. Consistent with our previous neuroimaging findings, these results suggest that proactive inhibition of response is the default mode of executive control. This implies that top-down control of sensorimotor reactivity would consist of a tempor..
    • 2009
    • Bénédicte Ballanger
    • Top-Down Control of Saccades as Part of a Generalized Model of Proactive Inhibitory Control
    • Lo and colleagues have recently described a recurrent network model of inhibitory control of saccadic eye movements based on neurophysiological observations in the frontal eye field (FEF) and superior colliculus (SC) of rhesus monkeys. This model emphasizes the proactive, inhibition-based, tonic neuronal activity that prevents the eye from moving in a countermanding paradigm. In this review I discuss the model with respect to existing literature that the authors did not mention, suggesting that proactive inhibitory control extends far beyond saccadic control and provides an interesting framework to interpret several attentional and movement disorders in humans.
    • 2002
    • Andreas Meyer-Lindenberg et al.
    • Reduced prefrontal activity predicts exaggerated striatal dopaminergic function in schizophrenia
    • Both dopaminergic neurotransmission and prefrontal cortex (PFC) function are known to be abnormal in schizophrenia. To test the hypothesis that these phenomena are related, we measured presynaptic dopaminergic function simultaneously with regional cerebral blood flow during the Wisconsin Card Sorting Test (WCST) and a control task in unmedicated schizophrenic subjects and matched controls. We show that the dopaminergic uptake constant Ki in the striatum was significantly higher for patients than for controls. Patients had significantly less WCST-related activation in PFC. The two parameters were strongly linked in patients, but not controls. The tight within-patient coupling of these values, with decreased PFC activation predicting exaggerated striatal 6-fluorodopa uptake, supports the hypothesis that prefrontal cortex dysfunction may lead to dopaminergic transmission abnormalities.
    • 2007
    • Anthony A. Grace et al.
    • Regulation of firing of dopaminergic neurons and control of goal-directed behaviors
    • There are several brain regions that have been implicated in the control of motivated behavior and whose disruption leads to the pathophysiology observed in major psychiatric disorders. These systems include the ventral hippocampus, which is involved in context and focus on tasks, the amygdala, which mediates emotional behavior, and the prefrontal cortex, which modulates activity throughout the limbic system to enable behavioral flexibility. Each of these systems has overlapping projections to the nucleus accumbens, where these inputs are integrated under the modulatory influence of dopamine. Here, we provide a systems-oriented approach to interpreting the function of the dopamine system, its modulation of limbic–cortical interactions and how disruptions within this system might underlie the pathophysiology of schizophrenia and drug abuse.
    • 1995
    • Anthony A. Grace
    • The tonic/phasic model of dopamine system regulation: its relevance for understanding how stimulant abuse can alter basal ganglia function
    • The changes in dopamine system regulation occurring during stimulant administration are examined in relation to a new model of dopamine system function. This model is based on the presence of a tonic low level of extracellular dopamine that is released by the presynaptic action of corticostriatal afferents. In contrast, spike-dependent dopamine release results in a phasic, high concentration of dopamine in the synaptic cleft that is rapidly inactivated by reuptake. Tonic dopamine has the ability to down-modulate spike-dependent phasic dopamine release via stimulation of the very sensitive dopamine autoreceptors present on dopamine terminals. Stimulants are known to elicit locomotion and stimulate reward sites by releasing dopamine from terminals in the nucleus accumbens, which is followed by a rebound depression. It is proposed that the initial activating action of stimulants is caused by increasing the release of dopamine into the synaptic cleft to activate the phasic dopamine respo..

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