Biomedical subjects

Chi-Hung Juan

Publications and source records attributed to Chi-Hung Juan.

6 recordsLinked to original sources

PubMed2005

Neural correlates of impulsive-violent behavior: an event-related potential study.

Impulsive-violent offenders are often less capable of controlling their behavior. A cued Go/No-go task was used to test the hypothesis that impulsive-violent behavior reflects a deficit in the ability to inhibit prepotent responses and could be reflected in the measurement of event-related potentials. Results showed that the amplitudes of the N2 component at Fz reflected different degrees of inhibition in impulsive-violent offenders compared with matched controls. The N2 amplitude (No-go minus Go) was significantly lower in the impulsive-violent offenders than in matched controls. The amplitude of N2 increased when effort was required to withhold the Go intention. A smaller N2 amplitude was seen in offenders, suggesting difficulties with inhibition of prepotent behavior.

Adult↗

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

Cortical interactions in vision and awareness: hierarchies in reverse.

The anatomical connections between visual areas can be organized in 'feedforward', 'feedback' or 'horizontal' laminar patterns. We report here four experiments that test the function of some of the feedback projections in visual cortex. Projections from V5 to V1 have been suggested to be important in visual awareness, and in the first experiment we show this to be the case in the blindsight patient GY. This demonstration is replicated, in principle, in the second experiment and we also show the timing of the V5-V1 interaction to correspond to findings from single unit physiology. In the third experiment we show that V1 is important for stimulus detection in visual search arrays and that the timing of V1 interference with TMS is late (up to 240 ms after the onset of the visual array). Finally we report an experiment showing that the parietal cortex is not involved in visual motion priming, whereas V5 is, suggesting that the parietal cortex does not modulate V5 in this task. We interpret the data in terms of Bullier's recent physiological recordings and Ahissar and Hochstein's reverse hierarchy theory of vision.

Adult↗

PubMed2003

Feedback to V1: a reverse hierarchy in vision.

We examined the involvement of visual area V1 in visual detection to assess the role of feedback connections to V1 as proposed in reverse hierarchy theory (Ahissar and Hochstein 2000). In Experiment 1, signal detection was decreased in feature and conjunction detection tasks by repetitive pulse TMS (rTMS) over V1 for 500 ms after stimulus onset. In Experiment 2, rTMS was delayed to allow uninterrupted signal processing for 100 ms after visual stimulus onset. TMS for the subsequent 500 ms now disrupted detection of conjunction but not feature targets. In Experiment 3 we applied double pulse TMS at varying intervals to assess the timing of V1 involvement in these tasks. Single feature detection involved V1 only at some point between 40 and 100 ms after visual array onset; detection of targets defined by conjunctions of features involved V1 throughout the first 100 ms and also between 200 and 240 ms after visual stimulus onset. We suggest that the early effects in the conjunction task are due to repeated sampling of the visual array to extract the signal from external noise. The later effects in conjunction search are attributed to the return projections from secondary visual areas back to V1, consistent with the reverse hierarchy theory. The effects in both tasks are consistent with early and repeated iterations of feed forward and feedback loops as hypothesised in recent neurophysiological experiments (see Foxe and Simpson 2002).

Adult↗

PubMed2003

Human frontal eye fields and visual search.

Recent physiological recording studies in monkeys have suggested that the frontal eye fields (FEFs) are involved in visual scene analysis even when eye movement commands are not required. We examined this proposed function of the human frontal eye fields during performance of visual search tasks in which difficulty was matched and eye movements were neither necessary nor required. Magnetic stimulation over FEF modulated performance on a conjunction search task and a simple feature search task in which the target was unpredictable from trial to trial, primarily by increasing false alarm responses. Simple feature search with a predictable target was not affected. The results establish that human FEFs are critical to visual selection, regardless of the need to generate a saccade command.

Adult↗