Yei-Yu Yeh

Cortical control of gait in healthy humans: an fMRI study

This study examined the cortical control of gait in healthy humans using functional magnetic resonance imaging (fMRI). Two block-designed fMRI sessions were conducted during motor imagery of a locomotor-related task. Subjects watched a video clip that showed an actor standing and walking in an egocentric perspective. In a control session, additional fMRI images were collected when participants observed a video clip of the clutch movement of a right hand. In keeping with previous studies using SPECT and NIRS, we detected activation in many motor-related areas including supplementary motor area, bilateral precentral gyrus, left dorsal premotor cortex, and cingulate motor area. Smaller additional activations were observed in the bilateral precuneus, left thalamus, and part of right putamen. Based on these findings, we propose a novel paradigm to study the cortical control of gait in healthy humans using fMRI. Specifically, the task used in this study—involving both mirror neurons and mental imagery—provides a new feasible model to be used in functional neuroimaging studies in this area of research.

Functional Connectivity during Top-Down Modulation of Visual Short-Term Memory Representations

Recent evidence has revealed that short-lived internal representations held in visual short-term memory (VSTM) can be modulated by top-down control via retrospective attention which impacts subsequent behavioral performance. However, the functional inter-regional interactions underlying these top-down modulatory effects are not fully characterized. Here we used event-related functional magnetic imaging to investigate whether the strength of functional connectivity between the frontal cortex and posterior visual areas varies with the efficacy of top-down modulation of memory traces. Top-down modulation was manipulated by the timing of retro-cuing (early or late) in a VSTM task. Univariate analyses revealed that more effective top-down modulation (early cueing vs. late cueing) increased activity in early visual areas. Importantly, coherency analyses revealed that top-down modulation produced stronger functional connectivity between frontal and posterior occipital regions. Also, participants with stronger functional connectivity exhibit better memory performance. These results suggest that augmented functional connectivity between frontal and posterior visual areas strengthens the VSTM representations of importance to behavioral goals.

Binding or prioritization: The role of selective attention in visual short-term memory

Whether selective attention binds features in visual short-term memory or prioritizes selection for memory consolidation and decision was investigated with a change detection paradigm. Two types of change were manipulated: Feature or conjunctions of features. Previous work suggests that the allocation of attentional resources affects binding; hence attentional shifts during retention should affect the detection of conjunction changes more than feature changes. The results of Experiments 1 and 2 showed that attention shifts had a similar impact on detecting feature and conjunction changes. Experiment 3 showed a performance benefit with a post-cue occurring 200 or 550 ms after stimulus offset, but no improvement was found when prioritization occurred with a delay of 800 ms. The results of Experiment 4 suggested that signals from both feature changes and conjunction changes contribute to detection. The theoretical implications are discussed.

The neural correlates of attention orienting in visuospatial working memory for detecting feature and conjunction changes

The neural mechanisms of attentional orienting in visuospatial working memory for change detection were investigated. A spatial cue was provided with the onset time manipulated to allow more effective top-down control with an early cue than with a late cue. The change type was also manipulated so that accurate detection depended on color or the binding of color and location. The results showed that both the frontal and parietal regions subserved the change detection task without cueing. When data were collapsed over the two change types, early cueing increased activation in the right inferior frontal gyrus (IFG) and middle frontal gyrus (MFG) while late cueing increased activation in the right inferior parietal lobule (IPL) and temporoparietal junction (TPJ) as compared with the no-cue condition. The cue onset time led to different levels of enhancement in the frontal and posterior cortices related to top-down control and stimulus-driven orienting. For feature detection, early cueing increased activation in the right MFG and late cueing increased activation in the bilateral precuneus (PCu), right TPJ, and right cuneus. The neural correlates of conjunction detection involved the right PCu and cerebellum without cueing, were associated with the anterior MFG, left IFG, and the left STG with early cueing, and involved the right MFG, left IFG, and right IPL with late cueing. The left IFG was correlated with memory retrieval of the cued representation for conjunction detection, and the right posterior PCu was associated with maintenance and memory retrieval among competing stimuli.

The set switching function of nonclinical dissociators under negative emotion.

Rapid switching may underlie the disruption of some integrated thought processes that characterize dissociation in both nonclinical and clinical populations. We investigated the set switching function under negative emotion with three groups of nonclinical participants that had different degrees of dissociation proneness. In the experiment, participants judged whether the digit in a predefined target color was odd or even on the preswitch trials. In a perseverance condition, participants were required to switch to a new target color while the previous target color became the distractor color. In a learned irrelevance condition, the previously ignored color became the new target color. The results showed that the three groups did not differ in focusing attention in the preswitch trials, for set switching in the baseline condition (in which emotion was not engaged), or for switching in the learned irrelevance condition under negative emotion. However, high dissociators under negative emotion showed faster switching in the perseverance condition. This enhanced ability to divert attention to a new mental set under negative emotion may be a coping strategy related to cognitive symptoms in dissociative disorders.

Unintentional memory inhibition is weakened in non-clinical dissociators.

Although participants with dissociation proneness showed inefficient cognitive inhibition, whether and under what stimulus-task contexts dissociators show inefficient memory inhibition remains inconclusive. This study investigated the relationship between trait dissociation and basic operation of memory control using a non-clinical sample. To reduce the involvement of strategic control and the influence of emotionality, the retrieval-practice paradigm was adopted to examine unintentional memory inhibition of neutral materials. Both the low- and middle-dissociation groups showed the forgetting effect, resulting from suppressing competing memories while retrieving a target. In contrast, the high-dissociation group did not show the forgetting effect although their performance in the baseline condition and in recalling practiced items was comparable to the other two groups. High dissociation proneness is linked with weakened memory inhibition that may cause diverse memory problems in clinical patients.

SEGREGATION BY COLOR AND STEREOSCOPIC DEPTH IN THREE-DIMENSIONAL VISUAL SPACE

Two experiments were conducted to investigate how color and stereoscopic depth information are used to segregate objects for visual search in three-dimensional (3-D) visual space. Eight observers were asked to indicate the alphanumeric category (letter or digit) of the target which had its unique color and unique depth plane. In Experiment 1, distractors sharing a common depth plane or a common color appeared in spatial contiguity in thexy plane. The results suggest that visual search for the target involves examination of kernels formed by homogeneous items sharing the same color and depth. In Experiment 2, thexy contiguity of distractors sharing a common color or a common depth plane was varied. The results showed that when target-distractor distinction becomes more difficult on one dimension, the other dimension becomes more important in performing visual search, as indicated by a larger effect on search time. This suggests that observers can make optimal use of the information available. Finally, color had a larger effect on search time than did stereoscopic depth. Overall, the results support models of visual processing which maintain that perceptual segregation and selective attention are determined by similarity among objects in 3-D visual space on both spatial and nonspatial stimulus dimensions.

Location negative priming in identity discrimination relies on location repetition

Negative priming manifests when a previously ignored stimulus becomes a target. The contingency of identity negative priming on repeated stimuli has been demonstrated, implying a crucial role for distractor competition. In this study, a naming task was used to examine whether location negative priming also relies on the repetition of locations. In Experiment 1, location negative priming was observed only when a small set of repeated locations was used. Positive priming was found instead when a large set of nonrepeated locations was used. Experiment 2 demonstrated that target-to-distractor distance modulated location priming effects, with priming effects observed only for a far distance. Experiments 3 and 4 demonstrated that the effect of location negative priming increased as locations repeated. Like identity negative priming, location negative priming depends on location repetition.

Forgetting the unforgotten affective autobiographical memories in nonclinical dissociators.

Inefficient memory inhibition has been observed in nonclinical and clinical dissociators. Paradoxically, dissociators also report unusual forgetfulness. Investigating how forgetting emerges in dissociators may uncover the antecedents for their self-report memory problems. We postulated that set switch can link inefficient memory inhibition to forgetting. Recollection detour, which involves an affect switch, may elicit forgetting of previously uninhibited memories in nonclinical dissociators. This hypothesis was verified in participants with high- and low-dissociation proneness via a retrieval practice paradigm using positive and negative autobiographical memories. After the study and retrieval-practice phases, memories of the practiced affect category were tested without and with intervening recall of the unpracticed affect category in the control and detour condition, respectively. Nondissociators showed reduced recall in the control condition, replicating the retrieval-induced forgetting (RIF) effect and recollection detour did not alter the RIF effect. By contrast, nonclinical dissociators showed the RIF effect in the detour condition but not in the control condition. Detour to recollecting memories of another affect category rendered an aftereffect of forgetting of previously uninhibited memories in nonclinical dissociators.

Relative salience affects the process of detecting changes in orientation and luminance.

This study tests the effect of relative saliency on perceptual comparison and decision processes in the context of change detection in which distinct visual mechanisms process two features (e.g., luminance and orientation). Townsend and Nozawas (1995) systems factorial technology was used to investigate the process architecture and stopping rule when deciding whether luminance or orientation of a Gabor patch had changed. Experiment 1 found individual differences in decision strategies when we did not control relative saliency. One group of participants adopted co-active processing, and the other group adopted serial self-terminating processing to detect the change signals. When Experiment 2 eliminated the relative saliency, all but one observer adopted parallel processing and followed a self-terminating rule. These results support the relative saliency hypothesis and highlight the fact that observers adopt different change-detection strategies for two features, especially when relative saliency exists between the two feature dimensions.