Biomedical subjects

Shihui Han

Publications and source records attributed to Shihui Han.

At least 19 recordsLinked to original sources

PubMed2006

Neural basis of cultural influence on self-representation.

Culture affects the psychological structure of self and results in two distinct types of self-representation (Western independent self and East Asian interdependent self). However, the neural basis of culture-self interaction remains unknown. We used fMRI to measured brain activity from Western and Chinese subjects who judged personal trait adjectives regarding self, mother or a public person. We found that the medial prefrontal cortex (MPFC) and anterior cingulate cortex (ACC) showed stronger activation in self- than other-judgment conditions for both Chinese and Western subjects. However, relative to other-judgments, mother-judgments activated MPFC in Chinese but not in Western subjects. Our findings suggest that Chinese individuals use MPFC to represent both the self and the mother whereas Westerners use MPFC to represent exclusively the self, providing neuroimaging evidence that culture shapes the functional anatomy of self-representation.

Adult↗

PubMed2006

Self-face recognition in attended and unattended conditions: an event-related brain potential study.

This study investigated whether neural mechanisms of self-face recognition are modulated by attention by recording event-related brain potentials associated with self-face recognition. Participants identified head orientations of self-faces and familiar or unfamiliar other faces presented briefly at the center of the visual field. Event-related brain potentials to self-faces and other faces were recorded when self-faces and familiar or unfamiliar other faces were either task relevant (attended) or irrelevant (unattended) in separate blocks of trials. We found that early face-specific event-related brain potential components such as the N170 and vertex positive potential did not differ between self-faces and other faces. Relative to familiar faces, however, self-faces induced an increased positivity over the frontocentral area at 220-700 ms. The increased positivity to self-faces relative to familiar faces between 500 and 700 ms was reduced in the attended relative to the unattended conditions, which arose from the fact that the amplitude to familiar faces during this time window was increased in the attended relative to the unattended conditions, whereas the event-related brain potential amplitude to self-faces was not influenced by attention. The event-related brain potential results suggest an automatic process of self-face recognition in human brains that occurs after face structure encoding and is independent of task relevance.

Adult↗

PubMed2006

Attention shift in human verbal working memory: priming contribution and dynamic brain activation.

When multiple items in working memory need to be accessed and manipulated, the internal attention should switch between them and, this switching process is time consuming. However, it is not clear how much of this switching cost is due to the existence or absence of the stimulus identification priming. With a figure identification and counting task, we demonstrate a small but significant priming contribution to this attention-switching cost. Furthermore, through 64-channel event-related potential (ERP) recordings, we found two ERP correlates (at 280 ms and 388 ms) of this internal attention-switching function. Source localization analysis shows dynamic brain activation starts from the temporal-occipital region and finishes in the left prefrontal cortex. The occipital-prefrontal and cingulate-prefrontal co-activations were orderly observed. We discuss the present ERP results along with our previous fMRI findings and suggest a dominant role of the left prefrontal cortex associated with attention shifts in verbal working memory.

Adult↗

PubMed2006

Neural correlates of within-level and across-level attention to multiple compound stimuli.

Event-related potentials (ERPs) were recorded to investigate the neural mechanisms of attention to the same or different levels of two compound letters presented concurrently in the left and right visual fields, respectively. Relative to the condition when attention was allocated to the global level of one compound stimulus and the local level of another one (across-level attention), attention to the same level of the two compound stimuli (within-level attention) increased an early positivity between 100 and 140 ms (P1) over the occipito-parietal cortex. A long-latency positivity between 320 and 560 ms (P3) over the central-parietal area was also increased in the within-level relative to across-level attention conditions. The ERP results suggest that, relative to across-level attention, within-level attention to multiple compound stimuli facilitates both early sensory-perceptual processing and late process of stimulus evaluation and identification in hierarchical analysis.

Attention↗

PubMed2006

Right hemisphere dominance in perceiving coherence of visual events.

The visual world consists of static pictures as well as of coherent visual events. The present study investigated neural substrates underlying the perception of coherence of visual events that evolves over time. We measured brain activity using functional magnetic resonance imaging (fMRI) while adults watched briefly presented static images that were extracted from movie clips depicting coherent visual events. The images were presented either in the coherent order as they were displayed in the movie clips or in a random order. Relative to the random order presentation condition, static images presented in the coherent order generated stronger activation in the right middle temporal cortex, the right posterior superior temporal cortex, and the right inferior postcentral gyrus. The results provide neuroimaging evidence for the dominance of the right hemisphere in perceiving coherent visual events.

Adult↗

PubMed2005

Neural mechanisms of attentional modulation of perceptual grouping by collinearity.

Psychophysical research showed that detection of an oriented visual target is facilitated when the target is grouped with collinear visual flankers. However, this collinear grouping effect is evident only when the flankers are attended. This study examined neural mechanisms underlying the interaction between attention and grouping by collinearity. Event-related potentials were recorded from study participants who judged whether oriented Gabor patches (i.e. visual elements consisting of a sinusoidal contrast modulation convolved with a Gaussian function) along the cued orientation were collinear or orthogonal. Event-related potentials showed an enhanced negativity over the posterior occipital cortex at 48-72 ms when collinear patches were congruent rather than incongruent with the cued orientation. A negative shift between 260 and 380 ms was observed over the occipital-parietal areas in the congruent rather than incongruent conditions. The long-latency effect, however, was evident only when the collinear patches were allocated along 45 degrees . The event-related potential results suggest that the interaction between attention and collinear grouping may take place as early as in the primary visual cortex and is independent of global orientations of perceptual groups.

Adolescent↗

PubMed2005

Neural mechanisms of global/local processing of bilateral visual inputs: an ERP study.

OBJECTIVE: Examine the neural mechanisms of global/local processing of multiple hierarchical stimuli. METHODS: Event-related brain potentials (ERPs) were recorded from adults who selectively attended to the global or local level of two compound letters that were simultaneously presented in the left and right visual fields, respectively. The compound stimuli were either broadband in spatial frequency (SF) spectrum or contrast balanced to remove low SFs. Subjects were asked to detect the presence of a global or local target that might appear in either the left or the right visual field in separate blocks of trials. RESULTS: Attention to the local level of broadband stimuli elicited a positivity over lateral occipital sites at 80-120 ms (P1) with larger amplitude than those in the global attention condition. However, global attention produced an enhanced positivity at 240-320 ms (P2) over lateral occipital sites relative to local attention. Both the P1 and P2 waves in the global condition were of larger amplitudes over the left than right hemispheres. Contrast balancing eliminated the P1 and P2 effects and modulated the hemispheric asymmetry of the long-latency occipital positivity. CONCLUSIONS: The results provide ERP evidence for modulations of neural activities in the visual cortex by global/local attention to concurrently presented multiple compound letters. Moreover, the modulation of brain activities by global/local attention depends upon the presence of low SFs in the compound stimuli. SIGNIFICANCE: The ERP results of this study contribute to the understanding of neural mechanisms of the processing of simultaneously-presented multiple compound stimuli.

Adult↗

PubMed2005

Attentional modulation of perceptual grouping in human visual cortex: functional MRI studies.

When presented with a complex visual scene, our visual system has to organize the discrete entities present into useful perceptual units. The current work investigated the neural substrates of perceptual grouping defined by Gestalt laws of proximity and similarity of shape, and whether the neural substrates underlying perceptual grouping are modulated by task relevance and spatial attention. In visual discrimination tasks, subjects identified the orientations of perceptual groups formed by proximity or similarity of local elements or alternatively identified colors of either dots around the grouped stimuli or the fixation cross. Using functional magnetic resonance imaging (fMRI), we identified that the calcarine cortex was involved in proximity grouping but not in the grouping process defined by similarity of shape. Moreover, we showed evidence that the neural correlates of proximity grouping in the calcarine cortex were weakened when the elements were of low task relevance and fell outside an attended area of field. The findings reveal the neural basis for basic grouping operations, as well as illustrating how attention and proximity grouping interact in human visual cortex.

Adult↗

PubMed2005

Attentional modulation of perceptual grouping in human visual cortex: ERP studies.

We sought to identify the neural substrates underlying perceptual grouping, and examined whether the grouping-related neural activities are modulated by task relevance and attention by recording high-density event-related potentials (ERPs). Participants were presented with stimulus arrays, in which local elements were either evenly distributed or grouped into rows or columns by proximity or similarity, and had to discriminate orientations of the perceptual groups, or to identify the colors of dots around stimulus displays or of the fixation cross. We found that proximity grouping was indexed by a positive activity over the medial occipital cortex with a peak latency of about 100 ms after stimulus onset (Pd100), whereas grouping by similarity of shape was reflected in a negative activity with longer latency over the occipito-temporal areas. Dipole modeling based on a realistic boundary element head model localized the Pd100 to the right calcarine cortex. Moreover, we showed that the grouping-related activities were weakened when stimulus arrays were of low task relevance and fell outside an attended area of field. The results suggest that human calcarine cortex is engaged in early grouping operations defined by proximity, reinforcing the previous fMRI findings. Moreover, our ERP results indicate that the neural bases underlying perceptual grouping in human visual cortex can be modulated by task relevance and attention as early as 100 ms after sensory stimulation. Hum Brain Mapp, 2005. (c) 2005 Wiley-Liss, Inc.

Adult↗

PubMed2005

Perceptual organization at attended and unattended locations.

This study examined the effects of attention on forming perceptual units by proximity grouping and by uniform connectedness (UC). In Experiment 1 a row of three global letters defined by either proximity or UC was presented at the center of the visual field. Participants were asked to identify the letter in the middle of stimulus arrays while ignoring the flankers. The stimulus onset asynchrony (SOA) between stimulus arrays and masks varied between 180 and 500 ms. We found that responses to targets defined by proximity grouping were slower than to those defined by UC at median SOAs but there were no differences at short or long SOAs. Incongruent flankers slowed responses to targets and this flanker compatibility effect was larger for UC than for proximity-defined flankers. Experiment 2 examined the effects of spatial precueing on discrimination responses to proximity- and UC-defined targets. The advantage for targets defined by UC over targets defined by proximity grouping was greater at uncued relative to cued locations. The results suggest that the advantage for UC over proximity grouping in forming perceptual units is contingent on the stimuli not being fully attended, and that paying attention to the stimuli differentially benefits proximity grouping.

Adolescent↗

PubMed2005

Shifts of spatial attention in perceived 3-D space.

Previous studies have shown that spatial attention can shift in three-dimensional (3-D) space determined by binocular disparity. Using Posner's precueing paradigm, the current work examined whether attentional selection occurs in perceived 3-D space defined by occlusion. Experiment 1 showed that shifts of spatial attention induced by central cues between two surfaces in the left and right visual fields did not differ between the conditions when the two surfaces were located at the same or different perceptual depth. In contrast, Experiment 2 found that peripheral cues generated a stronger cue validity effect when the two surfaces were perceived at a different rather than at the same perceptual depth. The results suggest that exogenous but not endogenous attention operates in perceived 3-D space.

Adolescent↗

PubMed2004

Spatial congruence in working memory: an ERP study.

The Simon effect refers to the finding that reaction times are faster when stimulus and response locations are congruent than when they are not, even stimulus locations are task irrelevant. Zhang and Johnson reported a Simon-like spatial congruence effect in working memory. This study examined the neural mechanisms of this memory-based spatial congruence effect by recording event-related potentials to probe stimuli. Behavioral results showed a clear congruence effect. The P300 amplitudes were larger in the congruent condition than in the incongruent condition. The results suggest that the memory-based congruence effect and the classical Simon effect were mediated by similar neural mechanisms and support theories assigning response selection an essential role in spatial congruence effects.

Adolescent↗

PubMed2004

Distinct neural substrates for the perception of real and virtual visual worlds.

Virtual environments have been frequently used for training and skill improvement. However, do real and virtual worlds engage the same brain states in human perceivers? We measured brain activity using functional magnetic resonance imaging (fMRI) while adults watched movie and cartoon clips, simulating real and virtual visual worlds, respectively. Relative to baselines using random static images, the medial prefrontal cortex (MPFC) and the cerebellum were activated only by movie clips of other humans. In contrast, cartoon clips of human and non-human agents activated the superior parietal lobes, while movie clips of animals also activated the superior parietal lobes. Our fMRI findings suggest that the perception of real-world humans is characterised by the involvement of MPFC and the cerebellum, most likely for on-line representation of the mental states of others, whereas the perception of virtual-world agents engages the parietal cortex in attention to actions.

Adult↗

PubMed2004

The parietal cortex and attentional modulations of activities of the visual cortex.

We recorded high density event-related brain potentials (ERPs) from a patient with focal left parietal damage in a covert visual orienting task requiring detection of targets in the attended or unattended hemifield. A positivity peaking at 120 ms (P1) to the left visual field stimuli was enlarged when attended than unattended and was localized to the right extrastirate cortex. However, spatial attention did not influence the ERPs to the right visual field stimuli. The leftward cue elicited an enlarged P1 relative to the rightward cue. The results suggest that human parietal cortex is critical for the attentional modulation of the neural activities in the extrastriate cortex associated with stimuli in the contralateral hemifield.

Adolescent↗

PubMed2004

Interactions between proximity and similarity grouping: an event-related brain potential study in humans.

The current work examined neural substrates underlying interactions between two types of Gestalt grouping by recording event-related brain potentials (ERPs). Subjects were presented with stimulus arrays in which local circles and squares were grouped into columns or rows. The cues dominating grouping (proximity and similarity) were either congruent or incongruent. Subjects responded faster to the identification of orientations of perceptual groups based on proximity than similarity cues. Responses were slowed by incongruent cues and this effect was larger in the similarity than proximity conditions. Proximity grouping generated enhanced positivity over the posterior occipital cortex between 100 and 140 ms relative to similarity grouping. Relative to congruent grouping cues, incongruent cues elicited enlarged positivity over the temporal-parietal areas between 180 and 220 ms only in the similarity condition, but generated smaller P3 amplitudes with longer latencies in both proximity and similarity conditions. The results provide ERP indices of the dominance of proximity over shape similarity in guiding perceptual grouping when two grouping principles are present in stimulus displays.

Adolescent↗

PubMed2004

Perceptual salience of global structures and the crowding effect in amblyopia.

BACKGROUND: The crowding effect refers to stronger deficits in linear acuity (e.g., letters in a line) than in single letter acuity in amblyopia. The current work investigated whether the salience of a global structure in which the target for identification is embedded influences the crowding effect in amblyopia. METHODS: Compound shapes were presented to the amblyopic and fellow eyes respectively of 12 anisometropic amblyopes. The compound stimuli were presented on either a blank or a cross background so that the salience of global structures were manipulated. Reaction times (RTs) and response error rates were recorded when subjects identified global or local shapes, respectively. RESULTS: RTs were shorter to global than local shapes for both the amblyopic and the fellow eyes. The global RT advantage was larger for the amblyopic than the fellow eye. Interestingly, when viewing the stimuli with the amblyopic eye, subjects made more errors to local targets when the compound stimuli were presented against the blank than the cross background. CONCLUSION: The results suggest that the salience of global structures of visual stimuli contributes to the crowding effect in amblyopia.

Adolescent↗

PubMed2004

The role of human parietal cortex in attention networks.

The parietal cortex has been proposed as part of the neural network for guiding spatial attention. However, it is unclear to what degree the parietal cortex contributes to the attentional modulations of activities of the visual cortex and the engagement of the frontal cortex in the attention network. We recorded behavioural performance and haemodynamic responses using functional MRI from a patient with focal left parietal damage in covert visual orienting tasks requiring detection of targets at the attended or unattended locations. While the patient's reaction times to left visual field stimuli were speeded by valid relative to invalid cues, attention to LVF stimuli was associated with enhanced activities in the right extrastriate cortex, right parietal and cingulate cortices, and bilateral frontal cortices. However, the patient's behavioural and neural responses to right visual field stimuli were not influenced by cue validity. The results are discussed in terms of the role of human parietal cortex in the neural network underlying voluntary attentional control.

Adolescent↗

PubMed2004

Neural substrates differentiating global/local processing of bilateral visual inputs.

We investigated neural substrates of global/local processing of bilateral hierarchical stimuli using functional magnetic resonance imaging (fMRI). Subjects were presented with two compound letters that were displayed simultaneously in the left and right visual fields, respectively. In a steady-state, block-design paradigm, hemodynamic responses were recorded while subjects detected infrequent global or local targets presented in one hemifield in separate epochs of trials. While behavioural responses were more accurate and faster to global than local targets, attention to the global level of bilateral visual inputs induced stronger activations in the left and right temporal cortex relative to attention to the local level. However, attention to the local level generated stronger activations in bilateral superior parietal cortex compared with attention to the global level. The results suggest that distinct neural substrates in the temporal and parietal cortices are preferentially engaged in the global and local processing of bilateral visual inputs, respectively.

Adult↗