Biomedical subjects

Jeffrey D Schall

Publications and source records attributed to Jeffrey D Schall.

At least 19 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

Executive control of countermanding saccades by the supplementary eye field.

The supplementary eye field registers the occurrence of conflict, errors and reward in macaque monkeys performing a saccade-countermanding task. Using intracortical microstimulation, we determined whether the supplementary eye field only monitors or can actually influence performance. Weak microstimulation of many sites in the supplementary eye field improved monkeys' performance on a 'stop signal' task by delaying saccade initiation. This effect depended on the context of the task because simple visually guided saccades were not delayed by the same stimulation. These results demonstrate that the supplementary eye field can exert contextual executive control over saccade generation.

Action Potentials↗

PubMed2006

Low-threshold Ca2+-associated bursts are rare events in the LGN of the awake behaving monkey.

It has been proposed that low-threshold Ca2+ (LT)-associated bursts in the lateral geniculate nucleus (LGN) of awake animals communicate significant or unexpected visual events to cortex. The present study investigated this hypothesis by examining the incidence of LT bursts in 146 cells recorded from the LGN of three macaque monkeys. Bursts were defined as clusters of two or more action potentials separated by not more than 4 ms and preceded by a > or = 100-ms quiescent interval. The incidence of bursts was examined in several intensive-training Go-NoGo and target selection tasks as well as in training-free tasks where natural scenes with both familiar and novel contents were shown. Our chief findings were as follows. 1) Bursts occur in the majority of cells under every condition tested, 2) burst incidence is very low (<1 burst every 10 s), 3) bursts occur in association with a receptive field stimulus on average only once every 23 times in 65% of cells tested, 4) cells responding with bursts to the stimulus also tended to exhibit higher levels of spontaneous bursting, 5) the presence of bursts did not depend on the novelty of the stimulus or its behavioral relevance. When the monkeys explored static natural scenes, 6) bursts were not correlated with short-term changes in the image sampled by the cell's receptive field during saccades. Burst incidence 7) did not increase when images were novel or when they evoked an emotional reaction, and 8) bursts did not decrease when images were familiar. 9) Bursts were not correlated with saccades in the dark, but 10) more spikes participated in bursts in the dark. Although these results confirm the occurrence of LT bursts in LGN cells of awake monkeys, they do not support the hypothesis that these bursts are a privileged means of transferring sensory information, that they signal unexpected or significant visual events, or that they are involved uniquely in the coding of natural scenes.

Action Potentials↗

PubMed2005

Chronometry of visual responses in frontal eye field, supplementary eye field, and anterior cingulate cortex.

The latency and variability of latency of single-unit responses to identical visual stimulation were measured in the frontal eye field (FEF), supplementary eye field (SEF), and anterior cingulate cortex (ACC) of macaque monkeys performing visually guided saccades. The mean visual response latency was significantly shorter in FEF (64 ms) than in SEF (81 ms) or ACC (100 ms), and latency values determined by four methods agreed. The latency variability of the visual response was respectively less in FEF (21 ms) than in SEF (37 ms) or ACC (41 ms). Latency, variability of latency, and magnitude of the visual responses were correlated within FEF and SEF but not ACC. These characteristics of the visual response are consistent with the degree of convergence of visual afferents to these areas and constrain hypotheses about visual processing in the frontal lobe.

Action Potentials↗

PubMed2005

Decision making.

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Choice Behavior↗

PubMed2004

Dissociation of spatial attention and saccade preparation.

The goal of this experiment was to determine whether the allocation of attention necessarily requires saccade preparation. To dissociate the focus of attention from the endpoint of a saccade, macaque monkeys were trained to perform visual search for a uniquely colored rectangle and shift gaze either toward or opposite this color singleton according to its orientation. A vertical singleton cued a prosaccade, a horizontal singleton, an antisaccade. Saccade preparation was probed by measuring the direction of saccades evoked by intracortical microstimulation of the frontal eye fields at variable times after presentation of the search array. Eye movements evoked on prosaccade trials deviated progressively toward the singleton that was also the endpoint of the correct eye movement. However, eye movements evoked on antisaccade trials never deviated toward the singleton but only progressively toward the location opposite the singleton. This occurred even though previous work showed that on antisaccade trials most neurons in frontal eye fields initially select the singleton while attention is allocated to distinguish its shape. Thus, sensorimotor structures can covertly orient attention without preparing a saccade.

Animals↗

PubMed2004

Effects of search efficiency on surround suppression during visual selection in frontal eye field.

Previous research has shown that visually responsive neurons in the frontal eye field of macaque monkeys select the target for a saccade during efficient, pop-out visual search through suppression of the representation of the nontarget distractors. For a fraction of these neurons, the magnitude of this distractor suppression varied with the proximity of the target to the receptive field, exhibiting more suppression of the distractor representation when the target was nearby than when the target was distant. The purpose of this study was to determine whether the variation of distractor suppression related to target proximity varied with target-distractor feature similarity. The effect of target proximity on distractor suppression did not vary with target-distractor similarity and therefore may be an endogenous property of the selection process.

Action Potentials↗

PubMed2004

On the role of frontal eye field in guiding attention and saccades.

The neural bases of shifting attention and directing gaze were investigated in macaque monkeys performing a singleton search that required a prosaccade, antisaccade, or no saccade cued by the shape of the singleton. In prosaccade trials, most neurons in frontal eye field selected the location of the singleton that was also the end point of the saccade. In antisaccade trials, most neurons selected the singleton followed by selection of the endpoint of the saccade. Other neurons selected only the endpoint of the saccade in antisaccade trials. When no saccade was produced, many of the first type of neuron still selected the singleton, and many but not all of both types of neurons later selected the stimulus opposite the singleton even though no saccade was produced. These patterns of activity are consistent with the hypotheses that covert shifts of attention can occur without saccade production and that FEF contributes to covert as well as overt orienting.

Animals↗

PubMed2004

Programming of double-step saccade sequences: modulation by cognitive control.

The capacity to detect and correct errors is thought to engage cognitive control. To probe the nature of such control in relation to eye movements, subjects performed a double-step task under different instructions: to FOLLOW the appearance of successive targets; or to cancel the initial saccade and REDIRECT gaze to the final target location. Saccade sequences occurred in the FOLLOW and REDIRECT conditions where they represented correct and corrective behaviour, respectively. We observed that corrective responses were faster than correct responses, and concurrent preparation of saccades was facilitated during error correction. These results are consistent with psychological theories that posit supervisory cognitive control over action during error correction.

Cognition↗

PubMed2004

On building a bridge between brain and behavior.

Cognitive neuroscience is motivated by the precept that a discoverable correspondence exists between mental states and brain states. This precept seems to be supported by remarkable observations and conclusions derived from event-related potentials and functional imaging with humans and neurophysiology with behaving monkeys. This review evaluates specific conceptual and technical limits of claims of correspondence between neural events, overt behavior, and hypothesized covert processes examined using data on the neural control of saccadic eye movements.

Brain↗

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

Effect of target-distractor similarity on FEF visual selection in the absence of the target.

We tested the hypothesis that frontal eye field (FEF) visual activity integrates visual information with a template of a target by examining whether a target that is not present in a search display influences the target selection in FEF. Neural activity was recorded in FEF of macaque monkeys performing visual search for a singleton target defined by color or direction of motion. The target remained constant throughout, but not across experimental sessions. Trials with distractors dissimilar to the target were interleaved with trials with distractors similar to the target. The hypothesis was tested by measuring the magnitude of activity in randomly interleaved trials with the target absent and only distractors in the display. We found that the response to the distractors was significantly greater when presented with displays consisting of distractors that resembled the absent target than when presented with displays consisting of distractors most different from the absent target. The influence of target-distractor similarity on FEF activity was also observed when the target was present, as reported previously. These data suggest that a template of the absent target can influence the selection process in FEF. This provides more direct evidence that FEF integrates visual information and knowledge of the target to determine the goal of a saccade.

Action Potentials↗

PubMed2003

Effects of stimulus-response compatibility on neural selection in frontal eye field.

We investigated the neural basis of visual and saccade selection in the frontal eye field of macaque monkeys using a singleton search task with prosaccade or antisaccade responses. Two types of neurons were distinguished. The first initially selected the singleton even in antisaccade trials, although most subsequently selected the endpoint of the saccade. The time the singleton was located was not affected by stimulus-response compatibility and did not vary with reaction time across trials. The second type of neuron selected only the endpoint of the saccade. The time of endpoint selection by these neurons accounted for most of the effect of stimulus-response compatibility on reaction time. These results indicate that visual selection and saccade selection are different processes.

Animals↗

PubMed2003

Neural correlates of decision processes: neural and mental chronometry.

Recent studies aim to explain the duration and variability of behavioral reaction time in terms of neural processes. The time taken to make choices is occupied by at least two processes. Neurons in sensorimotor structures accumulate evidence that leads to alternative categorizations, while other neurons within these structures prepare and initiate overt responses. These distinct stages of stimulus encoding and response preparation support variable but flexible behavior.

Animals↗

PubMed2002

Decision making: neural correlates of response time.

Recent studies have measured the time taken by neural processes to decide between alternative stimuli. Overt responses are produced only as soon as neural processes are completed. The brain throttles the mind.

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

The neural selection and control of saccades by the frontal eye field.

Recent research has provided new insights into the neural processes that select the target for and control the production of a shift of gaze. Being a key node in the network that subserves visual processing and saccade production, the frontal eye field (FEF) has been an effective area in which to monitor these processes. Certain neurons in the FEF signal the location of conspicuous or meaningful stimuli that may be the targets for saccades. Other neurons control whether and when the gaze shifts. The existence of distinct neural processes for visual selection and saccade production is necessary to explain the flexibility of visually guided behaviour.

Animals↗

PubMed2002

Priming in macaque frontal cortex during popout visual search: feature-based facilitation and location-based inhibition of return.

In popout search, humans and monkeys are affected by trial-to-trial changes in stimulus features and target location. The neuronal mechanisms underlying such sequential effects have not been examined. Single neurons were recorded in the frontal eye field (FEF) of monkeys performing a popout search during which stimulus features and target position changed unpredictably across trials. Like previous studies, repetition of stimulus features improved performance. This feature-based facilitation of return was manifested in the target discrimination process in FEF: neurons discriminated the target from distractors earlier and better with repetition of stimulus features, corresponding to improvements in saccade latency and accuracy, respectively. The neuronal target selection was mediated by both target enhancement and distractor suppression. In contrast to the repetition of features, repetition of target position increased saccade latency. This location-based inhibition of return was reflected in the neuronal discrimination process but not in the baseline activity in FEF. These results show adjustments of the target selection process in FEF corresponding to and therefore possibly contributing to changes in performance across trials caused by sequential regularities in display properties.

Action Potentials↗