Dongchan Kim

Human brain response to visual fatigue caused by stereoscopic depth perception

In stereopsis, excessive screen disparity is known as one of the main causes of visual fatigue. Although the visual fatigue caused by stereoscopic depth perception has been reported in a large number of subjective assessments, there has been little effort to measure objective indicators for the visual fatigue. In this study, we investigated the relationship between the visual fatigue and the amount of binocular disparity by using functional magnetic resonance imaging (fMRI). As a result, we detected strong neuronal activities in frontal eye field (FEF) when the presented visual stimuli had disparities beyond the comfort zone. Since FEF is known to be involved in eye movement control, it also appears to have correlation with the visual fatigue caused by the stereoscopic depth perception.

fMRI analysis of excessive binocular disparity on the human brain

The aim of this study is to evaluate brain regions related with excessive binocular disparity that may be linked to stereoscopic visual fatigue. In stereoscopic displays, excessive binocular disparity may generate blurring or double vision in the stereovision and induce unnatural oscillations in accommodation and vergence. These phenomena may lead to visual fatigue and activation (or deactivation) of human brain related with sensory and eye movement functions. A functional magnetic resonance imaging (fMRI) method is used to investigate the effect of excessive binocular disparity on human brain. Subjective assessments of visual fatigue are also conducted with the same stimuli as the fMRI experiment. Based on the subjective assessment results, participants are classified into low‐ and high‐fatigue groups. From the fMRI experiments, the high‐fatigue group showed more activation at the intraparietal sulcus (IPS) than the low‐fatigue group, when viewing an excessive disparity stimulus. The results showed that the excessive binocular disparity stimulus may induce overload to the IPS region, which is related with stereo processing and saccadic eye movement. In addition, it could be possible to use fMRI as an objective measurement method for understanding the stereoscopic visual fatigue when stimuli with excessive binocular disparity are applied.

Subjective and objective measurements of visual fatigue induced by excessive disparities in stereoscopic images

As stereoscopic displays have spread, it is important to know what really causes the visual fatigue and discomfort and what happens in the visual system in the brain behind the retina while viewing stereoscopic 3D images on the displays. In this study, functional magnetic resonance imaging (fMRI) was used for the objective measurement to assess the human brain regions involved in the processing of the stereoscopic stimuli with excessive disparities. Based on the subjective measurement results, we selected two subsets of comfort videos and discomfort videos in our dataset. Then, a fMRI experiment was conducted with the subsets of comfort and discomfort videos in order to identify which brain regions activated while viewing the discomfort videos in a stereoscopic display. We found that, when viewing a stereoscopic display, the right middle frontal gyrus, the right inferior frontal gyrus, the right intraparietal lobule, the right middle temporal gyrus, and the bilateral cuneus were significantly activated during the processing of excessive disparities, compared to those of small disparities (< 1 degree).

Aberrant function of frontoamygdala circuits in adolescents with previous verbal abuse experiences.

Previous studies reported an association of depressive disorder and structural alteration of frontolimbic brain regions in subjects with emotional abuse experiences during childhood and adolescence. The results suggest that aberrant function of the frontolimbic circuit and its relation with psychiatric symptoms can be found in adolescents with preclinical status. We investigated functional changes of frontolimbic networks during implicit negative emotional face processing and their relationships with depressive symptoms in adolescents with previous verbal abuse experiences. We designed a gender discrimination task using emotional faces to induce an implicit level of emotional exposure, and was completed by 31 preclinical male adolescents during an fMRI scan. The right amygdala activity and its functional connectivity with the rostral anterior cingulate cortex (ACC) during implicit processing of negative emotional faces showed a significant relationship with previous verbal abuse experiences. The hierarchical regression analyses showed that their current depressive symptoms were associated with aberrant functional interaction between the right amygdala activity and right amygdala-rostral ACC connectivity. Our findings of verbal abuse-related functional changes in the right frontoamygdala circuit may be related to vulnerability to future mood disorder.

Non-classical response from quench-cooled solid helium confined in porous gold

The non-classical rotational inertia (NCRI) in solid helium was detected by a drop in the resonant period of a torsional oscillator. This non-classical response was interpreted as the first possible evidence of supersolidity. A number of subsequent experiments, however, reported unexpected phenomena within the supersolid context. Experimental and theoretical work have drawn attention to the role of disorder in solid helium to explain the inconsistency. We have investigated the non-classical response of solid 4He confined in porous gold set to torsional oscillation. When solid helium is grown rapidly, nearly 7% of the solid helium appears to be decoupled from the oscillation below about 200 mK. Dissipation appears at temperatures where the decoupling shows maximum variation. In contrast, the decoupling is substantially reduced in slowly grown solid helium. The dynamic response of solid helium was also studied by imposing a sudden increase in the amplitude of oscillation. Extended relaxation in the resonant period shift, suggesting the emergence of the pinning of low-energy excitations, was observed below the onset temperature of the non-classical response. The motion of a dislocation or a glassy solid is restricted in the entangled narrow pores and is not likely responsible for the period shift and long relaxation.

Multi-slice imAGe generation using intra-slice paraLLel imaging and Inter-slice shifting (MAGGULLI).

For acceleration of imaging time, multi-band imaging techniques (e.g. CAIPIRINHA) use the sensitivity differences of the multi-channel RF coils in the slice selection direction. To more effectively utilize the RF coil characteristics than the conventional multi-band imaging techniques, we propose a new imaging technique, called multi-slice image generation using intra-slice parallel imaging and inter-slice shifting (MAGGULLI). The proposed technique used an inter-slice shifting gradient in slice selection direction to make multi-slice images shift in the frequency encoding direction. Thus, aliasing caused by sub-sampling in the phase encoding direction is orthogonal to that by multi-band imaging with the inter-slice shifting, both of which are resolved by using the sensitivity information of the RF coil. Phantom and in vivo imaging experiments for the acceleration factors up to 10 demonstrate that the quality of the images reconstructed by MAGGULLI are better than that of CAIPIRINHA for high acceleration factors in the qualitative and quantitative analysis.

Multi-contrast MR image denoising for parallel imaging using multilayer perceptron

For clinical diagnosis in MRI, multiple examinations are commonly performed to acquire various contrast images. This article presents a learning‐based denoising method for parallel imaging to enhance the quality of multi‐contrast images so that the imaging time can be accelerated highly. Multi‐contrast images share structural information and coil geometry. The proposed method adopts the multilayer perceptron (MLP) model to save the sharable and redundant information among the multi‐contrast images. The images are divided into patches, which are used as the input and output of MLP. A geometry factor map is additionally used to provide noise amplification information of the accelerated MR images. Computer simulation demonstrates that the use of multi‐contrast images and geometry factor contributes to the quality of the reconstructed images. The proposed method reconstructs high‐quality images without impairing details from the subsampled intermediate images, and it shows better results than previous denoising methods.

An iterative reconstruction method of complex images using expectation maximization for radial parallel MRI

In MRI (magnetic resonance imaging), signal sampling along a radial k-space trajectory is preferred in certain applications due to its distinct advantages such as robustness to motion, and the radial sampling can be beneficial for reconstruction algorithms such as parallel MRI (pMRI) due to the incoherency. For radial MRI, the image is usually reconstructed from projection data using analytic methods such as filtered back-projection or Fourier reconstruction after gridding. However, the quality of the reconstructed image from these analytic methods can be degraded when the number of acquired projection views is insufficient. In this paper, we propose a novel reconstruction method based on the expectation maximization (EM) method, where the EM algorithm is remodeled for MRI so that complex images can be reconstructed. Then, to optimize the proposed method for radial pMRI, a reconstruction method that uses coil sensitivity information of multichannel RF coils is formulated. Experiment results from synthetic and in vivo data show that the proposed method introduces better reconstructed images than the analytic methods, even from highly subsampled data, and provides monotonic convergence properties compared to the conjugate gradient based reconstruction method.

Hippocampal Subfields Volume Reduction in High Schoolers with Previous Verbal Abuse Experiences

Objective Reduced hippocampal volume and alterations in white matter tracts have been frequently reported in adults having the history of emotional maltreatment. We investigated whether these structural change occur in adolescents with previous verbal abuse (VA) experiences. Methods Hippocampal subfield volume and white matter structural connectivity measures were assessed in 31 first year male high school students with various degrees of exposure to parental and peer VA. Results The high VA group showed significant volume reduction in the left cornu ammonis (CA) 1 and left subiculum compared to the low VA group (p<0.05). Volumes of left hippocampal subfields CA1 and subiculum were negatively correlated with previous VA experiences (p<0.05). Increased mean diffusivity (MD) of the splenium of the corpus callosum was related to high VA score across all subjects (p<0.05). There was an inverse relationship between volume of the CA1 and subiculum and MD of the splenium (p<0.05). Conclusion Exposure to parental and peer VA may affect development of the left hippocampal subfields and the splenium of corpus callosum. These structural alterations can be discernible during adolescence.

Self‐gated cardiac cine imaging using phase information

To obtain multiphase cardiac cine images with high resolution, a novel self‐gating method for both cardiac and respiratory motions is proposed.