Biomedical subjects

Shihui Han

Publications and source records attributed to Shihui Han.

24 records · Page 2Linked to original sources

PubMed2004

Engagement of the prefrontal cortex in representational momentum: an fMRI study.

Behavioral studies have identified a robust phenomenon that an observer's memory of the final position of a moving target is shifted a little further in its motion direction, which is usually called representational momentum (RM). However, the neural substrates underlying RM are poorly understood. The current study measured hemodynamic responses in association with RM using functional magnetic resonance imaging (fMRI). Two experiments using block and event-related designs, respectively, were conducted in which subjects compared the orientation of a probe rectangle with the remembered orientation of the final inducing figures in a set of rotating rectangles. Both experiments showed that, relative to the control task in which behavioral data did not show RM effects, RM task induced stronger activation in the prefrontal cortex. However, no activation was found in MT/MST complex in association with RM. The fMRI results suggest that RM may not simply reflect implicit motion perception and high level cognitive mechanisms underpinned by the prefrontal cortex may be involved in the RM effect.

Adult↗

PubMed2003

Modulation of neural activities by enhanced local selection in the processing of compound stimuli.

The global precedence effect refers to the findings that responses are faster to a global structure than to its local parts and local responses are slowed by incongruent global information. We recorded high-density event-related potentials (ERPs) to study the role of enhanced local selection in the global precedence effect. Hierarchical stimuli were compound letters in which the local letters were either identical (homogeneous stimuli) or the central local letter was brighter than (bright stimuli) or different in color from the others (red stimuli). Subjects were asked to attend to the pop-out local letter of the red and bright stimuli during the local task whereas there was no such instruction for the homogeneous stimuli. Top-down attention to the pop-out local item weakened the global reaction time advantage and the interference effect. The enhanced local selection decreased the amplitude of an occipito-temporal negativity between 240-360 msec but increased the amplitude of a frontal-central negativity between 260-320 msec related to local processing. The incongruency between global and local letters enlarged the posterior N2 in the local condition and this effect was eliminated by enhanced local selection. These effects were evident regardless of whether the pop-out local letter was defined by color or luminance difference. The results support the proposal that distinct neural mechanisms over the posterior and anterior areas are engaged in the selection process that contributes to local processing of compound stimuli.

Adult↗

PubMed2003

An ERP study of the global precedence effect: the role of spatial frequency.

OBJECTIVE: This study investigated the neural mechanisms underlying the effects of removal of low spatial frequency (SF) contents from stimulus displays on the processing of global and local properties of compound stimuli. METHODS: Event-related brain potentials (ERPs) were recorded from 16 subjects who selectively attended to the global or local features of compound letters, which were either white on a gray background containing broadband SFs or were contrast-balanced (CB) to eliminate low SFs, and were randomly presented in the left or right visual fields. ERPs were analyzed to examine how global/local attention modulations of neural substrates were influenced by SF manipulations. RESULTS: We found that an early process of global recognition was indexed by a negativity peaking at 190 ms over contralateral occipito-temporal cortex and was eliminated by contrast balancing. The late stage of global recognition was reflected in a late negativity peaking at 300 ms and was only retarded by contrast balancing. Global-to-local interference was characterized by enhanced occipito-temporal negativities and was evident for both broadband and CB stimuli. CONCLUSIONS: The results clarify distinct cognitive and neural mechanisms underlying the global precedence and interference effects, which were different in terms of the independence of low SFs in compound stimuli.

Adult↗

PubMed2002

Neural mechanisms of perceptual grouping in humans as revealed by high density event related potentials.

Behavioral studies suggest that perceptual grouping by proximity occurs earlier than grouping by similarity. This notion is supported by recent electrophysiological evidence that proximity grouping generates earlier occipital activation relative to grouping by shape similarity. The current work further investigated neural substrates that differentiate grouping by proximity and grouping by colour similarity by recording high density event related potentials. Subjects discriminated perceptual groups defined by either proximity or colour similarity. Proximity grouping resulted in short-latency modulations of medial occipital activity followed by longer latency modulations in the occipito-parietal cortex. Grouping by colour similarity, however, produced only long-latency occipito-temporal modulations. The results support the proposal that grouping by proximity and grouping by similarity have neural substrates over distinct time courses and cortical areas.

Adult↗

PubMed2002

Hemispheric asymmetry in global/local processing: effects of stimulus position and spatial frequency.

We examined the neural mechanisms of functional asymmetry between hemispheres in the processing of global and local information of hierarchical stimuli by measuring hemodynamic responses with functional magnetic resonance imaging (fMRI). In a selective attention task, subjects responded to targets at the global or local level of compound letters that were (1) broadband in spatial-frequency spectrum and presented at fixation; (2) broadband and presented randomly to the left or the right of fixation; or (3) contrast balanced (CB) to remove low spatial frequencies (SFs) and presented at fixation. Central broadband stimuli induced stronger activation in the right middle occipital cortex under global relative to local attention conditions but in the left inferior occipital cortex, stronger activation was induced under local relative to global attention conditions. The asymmetry over the occipital cortex was weakened by unilateral presentation and by contrast balancing. The results indicate that the lateralization of global and local processing is modulated by the position and SF spectrum of the compound stimuli. The global attention also produced stronger activation over the medial occipital cortex relative to the local attention under all the stimulus conditions. The nature of these effects is discussed.

Adult↗

PubMed2002

Segmentation and selection contribute to local processing in hierarchical analysis.

The present study examined the role of segmentation and selection processes when we respond to local elements in hierarchical stimuli. The ease of segmentation and selection of an individual local element from hierarchical patterns was manipulated by making one local element substantially distinct from the others in colour. Experiment 1 showed that, when attention was spread across the global and local levels in a divided attention task, the introduction of the local red element speeded responses to local targets but slowed responses when targets appeared at the global level. Experiment 2 used a selective attention task in which subjects responded only to the local or the global shapes across a block of trials. Under these circumstances, the local red element reduced global-to-local interference in addition to speeding local responses. The results suggest that the efficiency with which local elements are segmented and selected affects responses to local aspects of hierarchical patterns; furthermore, the effect of local pop-out on global processing is contingent on top-down attentional control settings.

Adolescent↗