Biomedical subjects

Joshua W Brown

Publications and source records attributed to Joshua W Brown.

8 recordsLinked to original sources

PubMed2006

Influence of history on saccade countermanding performance in humans and macaque monkeys.

The stop-signal or countermanding task probes the ability to control action by requiring subjects to withhold a planned movement in response to an infrequent stop signal which they do with variable success depending on the delay of the stop signal. We investigated whether performance of humans and macaque monkeys in a saccade countermanding task was influenced by stimulus and performance history. In spite of idiosyncrasies across subjects several trends were evident in both humans and monkeys. Response time decreased after successive trials with no stop signal. Response time increased after successive trials with a stop signal. However, post-error slowing was not observed. Increased response time was observed mainly or only after cancelled (signal inhibit) trials and not after noncancelled (signal respond) trials. These global trends were based on rapid adjustments of response time in response to momentary fluctuations in the fraction of stop signal trials. The effects of trial sequence on the probability of responding were weaker and more idiosyncratic across subjects when stop signal fraction was fixed. However, both response time and probability of responding were influenced strongly by variations in the fraction of stop signal trials. These results indicate that the race model of countermanding performance requires extension to account for these sequential dependencies and provide a basis for physiological studies of executive control of countermanding saccade performance.

Animals↗

PubMed2006

A computational model of fractionated conflict-control mechanisms in task-switching.

A feature of human cognition is the ability to monitor and adjust one's own behavior under changing circumstances. A dynamic balance between controlled and rapid responding is needed to adapt to a fluctuating environment. We suggest that cognitive control may include, among other things, two distinct processes. Incongruent stimuli may drive top-down facilitation of task-relevant responses to bias performance toward exploitation vs. exploration. Task or response switches may generally slow responses to bias toward accuracy vs. speed and exploration vs. exploitation. Behavioral results from a task switching study demonstrate these two distinct processes as revealed by higher-order sequential effects. A computational model implements the two conflict-control mechanisms, which allow it to capture many complex and novel sequential effects. Lesion studies with the model demonstrate that the model is unable to capture these effects without the conflict-control loops and show how each monitoring component modulates cognitive control. The results suggest numerous testable predictions regarding the neural substrates of cognitive control.

Adult↗

PubMed2005

Learned predictions of error likelihood in the anterior cingulate cortex.

The anterior cingulate cortex (ACC) and the related medial wall play a critical role in recruiting cognitive control. Although ACC exhibits selective error and conflict responses, it has been unclear how these develop and become context-specific. With use of a modified stop-signal task, we show from integrated computational neural modeling and neuroimaging studies that ACC learns to predict error likelihood in a given context, even for trials in which there is no error or response conflict. These results support a more general error-likelihood theory of ACC function based on reinforcement learning, of which conflict and error detection are special cases.

Brain Mapping↗

PubMed2004

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 complex gating functions, including deferred execution of selected plans, and switching among alternative sensory-motor mappings. Notably, the model can learn and gate the use of a What-to-Where transformation that enables spatially invariant object representations to selectively excite spatially coded movement plans. Model simulations show how dopaminergic reward and non-reward signals guide monkeys to learn and perform saccadic eye movements in the fixation, single saccade, overlap, gap, and delay (memory-guided) saccade tasks. Model cell activation dynamics quantitatively simulate seventeen established types of dynamics exhibited by corresponding real cells during performance of these tasks.

Animals↗

PubMed2003

Performance monitoring by the anterior cingulate cortex during saccade countermanding.

Consensus is emerging that the medial frontal lobe of the brain is involved in monitoring performance, but precisely what is monitored remains unclear. A saccade-countermanding task affords an experimental dissociation of neural signals of error, reinforcement, and conflict. Single-unit activity was monitored in the anterior cingulate cortex of monkeys performing this task. Neurons that signaled errors were found, half of which responded to the omission of earned reinforcement. A further diversity of neurons signaled earned or unexpected reinforcement. No neurons signaled the form of conflict engendered by interruption of saccade preparation produced in this task. These results are consistent with the hypothesis that the anterior cingulate cortex monitors the consequences of actions.

Animals↗

PubMed2003

Principles of pleasure prediction: specifying the neural dynamics of human reward learning.

Accumulating evidence from nonhuman primates suggests that midbrain dopamine cells code reward prediction errors and that this signal subserves reward learning in dopamine-receiving brain structures. In this issue of Neuron, McClure et al. and O'Doherty et al. use event-related fMRI to provide some of the strongest evidence to date that the reward prediction error model of dopamine system activity applies equally well to human reward learning.

Animals↗

PubMed2002

Monitoring and control of action by the frontal lobes.

Success requires deciding among alternatives, controlling the initiation of movements, and judging the consequences of actions. When alternatives are difficult to distinguish, habitual responses must be overcome, or consequences are uncertain, deliberation is necessary and a supervisory system exerts control over the processes that produce sensory-guided movements. We have investigated these processes by recording neural activity in the frontal lobe of macaque monkeys performing a countermanding task. Distinct neurons in the frontal eye field respond to visual stimuli or control the production of the movements. In the supplementary eye field and anterior cingulate cortex, neurons appear not to control directly movement initiation but instead signal the production of errors, the anticipation and delivery of reinforcement, and the presence of processing conflict. These signals form the core of current models of supervisory control of sensorimotor processes.

Animals↗

PubMed2002

Computational perspectives on dopamine function in prefrontal cortex.

Dopamine and the prefrontal cortex are critical for thought and behaviour. Recently, computational models have tried to elucidate the specific and intricate roles of dopamine in the prefrontal cortex, at the neurophysiological, system and behavioral levels, with varying degrees of success.

Animals↗