Yul-Wan Sung

Progression of neuronal processing of visual objects

We studied, with functional MRI, how neuronal processing for visual objects progresses in the human brain by measuring suppressive responses to stimulus pairs. Two concentric rectangular frames of slightly different sizes were presented such that the smaller frame was shown first, followed by the larger dominant frame, with interstimulus intervals of 0, 50 and 240-ms. The functional MRI response at 50-ms decreased along the ventral visual pathway. The response at 240-ms interstimulus interval was suppressed after a fusiform area at which the response at 50-ms interstimulus interval was highly suppressed already and the responses at 0, 50 and 240-ms interstimulus intervals showed a V-shaped dependence on those interstimulus intervals. These observations show a way of progression of the information processing along visual object recognition pathway.

A dynamic system model-based technique for functional MRI data analysis

Signals in functional magnetic resonance imaging (fMRI) are influenced by physiological fluctuations in addition to local brain activity. We have proposed a dynamic system model-based technique for separation of signal changes related to brain activation inputs from those related to physiological fluctuations. We applied this technique to a visual fMRI experiment to determine the validity and feasibility of this technique for fMRI data analyses. Gradient-echo echo planar images were obtained from 12 healthy volunteers with a Siemens ALLEGRA operating at 3 T, with a repetition time of 500 ms, echo time of 20 ms, field of view of 200-210 mm, matrix size of 64 x 64, and slice thickness of 5 mm. Twelve runs with two stimulation periods of varied duration (2-8 s) with 8-Hz flickering illumination were obtained for each subject. Local signal changes were modeled by an autoregressive model with two exogenous inputs, a visual stimulation input and a global reference signal. Local signal changes were appropriately predicted not only for stimulation periods but also resting periods. A significant linear relationship was found between model static gain based on the dynamic system modeling and beta coefficient based on a general linear model (GLM) analysis for active voxels in the primary visual cortex (analysis of covariance [ANCOVA], P < 0.001; estimated parameter, 0.967; 95% confidence interval, 0.734-1.201). This dynamic system model-based technique is sufficiently accurate and feasible for use in extracting signal changes related to brain activation inputs from measured signals with physiological fluctuations.

Involvement of low-level visual areas in hemispheric superiority for face processing.

Previous studies on laterality in face processing have indicated superiority of the right hemisphere in discriminating and recognizing faces; however, the reasons for this feature are poorly understood. We employed functional MRI (fMRI) to elucidate the origin of this feature and used a paired-stimulus paradigm in which face pairs were presented unilaterally at the left or right visual hemifield of the participants. Each face in a pair was at a different position in the visual field. Refractory suppression in the fMRI response was observed bilaterally at the fusiform face area (FFA) for the same face pairs when pictures were presented in the left visual hemifield. In contrast, suppression was observed bilaterally at the FFA for the different as well as for the same face pairs when pictures were presented in the right visual hemifield. This pattern indicated inferior discrimination ability for paired stimuli presented to the right visual hemifield. These observations, along with changes in bilaterally interlocked responses at the FFA, suggest that low-level visual areas, and not high-level face areas, are strongly associated with the superiority of the right hemisphere in face processing.

Alteration of blood oxygenation level-dependent signaling by local circulatory condition

To determine regional differences in the relationship between neuronal activation and blood oxygenation level‐dependent (BOLD) signal changes.

An fMRI study of the functional distinction of neuronal circuits at the sites on ventral visual stream co-activated by visual stimuli of different objects

In functional studies of the human brain using functional magnetic resonance imaging (fMRI) we often observe some functional areas that are commonly activated by different stimulus inputs even when the inputs are of different categories. It is difficult to distinguish by fMRI whether the neuronal circuits activated for processing these inputs are separate and independent ones within the functional area or whether they are mutually interactive and possibly sharing a part of the circuits for processing some common features of the input information. In order to elucidate such property of the neuronal circuits, we used a novel paradigm in which paired input stimuli with varied inter stimulus interval (ISI) were presented during a stimulation period in fMRI experiments. The refractory suppression induced by an input pair depends on ISI as well as the differing characteristics of the input pair. The extent of suppression, an indicator for the interaction between the processing activities of the input pair, can be evaluated by the shape of the time-course of fMRI responses. We examined the functional characteristics of the neuronal circuits in areas that were activated by different inputs of inter-category types, namely face stimuli and building stimuli and also by intra-category input pairs such as different faces. In the ventral visual occipitotemporal region, we found functional areas where neuronal circuits were interacting to process these inputs rather than functioning as separate independent circuits. With this approach, one can probe functional system activity at the neuronal circuit level to learn the characteristics that determine the functional roles of certain brain areas.

A property of face representation at the categorical level.

Face processing is unique to human beings and is likely to have special neuronal properties. We studied one of these properties, categorical representation of the face in perception processing (perception) and in memory recall processing (imagery), using the face and building stimuli. The same building-selective areas showed categorical selectivity in imagery as well as in perception, but face-selective areas did not. The selective areas to face stimuli in perception were completely different from those in imagery. This indicates that conceptual category selectivity in top-down processing is based on schemes different from that in bottom-up processing in the case of face processing. This may explain an aspect of specialized processing mechanisms of the face.

Structural and functional plasticity specific to musical training with wind instruments

Numerous neuroimaging studies have shown structural and functional changes resulting from musical training. Among these studies, changes in primary sensory areas are mostly related to motor functions. In this study, we looked for some similar functional and structural changes in other functional modalities, such as somatosensory function, by examining the effects of musical training with wind instruments. We found significant changes in two aspects of neuroplasticity, cortical thickness, and resting-state neuronal networks. A group of subjects with several years of continuous musical training and who are currently playing in university wind ensembles showed differences in cortical thickness in lip- and tongue-related brain areas vs. non-music playing subjects. Cortical thickness in lip-related brain areas was significantly thicker and that in tongue-related areas was significantly thinner in the music playing group compared with that in the non-music playing group. Association analysis of lip-related areas in the music playing group showed that the increase in cortical thickness was caused by musical training. In addition, seed-based correlation analysis showed differential activation in the precentral gyrus and supplementary motor areas (SMA) between the music and non-music playing groups. These results suggest that high-intensity training with specific musical instruments could induce structural changes in related anatomical areas and could also generate a new functional neuronal network in the brain.

Intermixed structure of voxels with different hemispheric characteristics in the fusiform face area.

The fusiform face area, a high-level visual area, is pivotal in processing facial information. This area receives inputs from the left and right visual fields unlike the primary visual area, which only receives inputs from its contralateral visual field. Response of the fusiform face area to ipsilateral stimulation depends on the signals crossing over at the corpus callosum. We investigated the distribution of voxel-wise activation to determine whether ipsilateral-dominant voxels exist in the fusiform face area using high spatial resolution functional MRI at 7 T. We further examined the possible functional differences between ipsilateral-dominant and contralateral-dominant voxels. By unilateral visual field stimulation, we detected ipsilateral-dominant voxels in the right fusiform face area. Their distribution was spatially heterogeneous. We tested upright and inverted facial stimulation confined to unilateral visual fields and found that these ipsilateral-dominant voxels had a different functional nature from contralateral-dominant voxels.

Building-specific categorical processing in the retrosplenial cortex

This study investigates by functional MRI (fMRI) the characteristics of processing activities at the retrosplenial cortex (RSC) and compares them to activities at the parahippocampal place area (PPA). A special categorical selectivity of the RSC in the recognition of buildings was elucidated. RSC activation-which is related to the processing of information vis-à-vis buildings, as well as the perception and recollection thereof-deals with buildings per se, or the conceptual aspects of buildings; it is not involved in distinguishing information regarding individual buildings, as is the case with PPA.

Steady-state and dynamic network modes for perceptual expectation

Perceptual expectation can attenuate repetition suppression, the stimulus-induced neuronal response generated by repeated stimulation, suggesting that repetition suppression is a top-down modulatory phenomenon. However, it is still unclear which high-level brain areas are involved and how they interact with low-level brain areas. Further, the temporal range over which perceptual expectation can effectively attenuate repetition suppression effects remains unclear. To elucidate the details of this top-down modulatory process, we used two short and long inter-stimulus intervals for a perceptual expectation paradigm of paired stimulation. We found that top-down modulation enhanced the response to the unexpected stimulus when repetition suppression was weak and that the effect disappeared at 1,000 ms prior to stimulus exposure. The high-level areas involved in this process included the left inferior frontal gyrus (IFG_L) and left parietal lobule (IPL_L). We also found two systems providing modulatory input to the right fusiform face area (FFA_R): one from IFG_L and the other from IPL_L. Most importantly, we identified two states of networks through which perceptual expectation modulates sensory responses: one is a dynamic state and the other is a steady state. Our results provide the first functional magnetic resonance imaging (fMRI) evidence of temporally nested networks in brain processing.