Woochul Choi

Role of inserting layer controlling wavelength in InGaAs quantum dots

Emission wavelength from the self-assembled In(Ga)As QDs on GaAs is typically around 1.0 /spl mu/m. In order to be applied to optical fiber communication, the extension of its optical emission wavelength to 1.3 /spl mu/m and further is necessary. Several groups have demonstrated GaAs-based InGaAs QDs with 1.3 /spl mu/m photoluminescence (PL). During the formation of such ternary dots, the variation of composition and dot size make it difficult to reproducibly achieve long wavelength emission. Long emission wavelength up to 1.3 /spl mu/m at room temperature cannot be realized until the In(Ga)As dots are placed in or below and InGaAs matrix. Among the proposed origins of achieving long wavelength emission from InAs quantum dots, we believe that the residual strain in quantum dots plays a key role. In this study, we have investigated the role of inserting layers tuning emission wavelength in InGaAs quantum dots.

Symmetry of learning rate in synaptic plasticity modulates formation of flexible and stable memories

Spike-timing-dependent plasticity (STDP) is considered critical to learning and memory functions in the human brain. Across various types of synapse, STDP is observed as different profiles of Hebbian and anti-Hebbian learning rules. However, the specific roles of diverse STDP profiles in memory formation still remain elusive. Here, we show that the symmetry of the learning rate profile in STDP is crucial to determining the character of stored memory. Using computer simulations, we found that an asymmetric learning rate generates flexible memory that is volatile and easily overwritten by newly appended information. Moreover, a symmetric learning rate generates stable memory that can coexist with newly appended information. In addition, by combining these two conditions, we could realize a hybrid memory type that operates in a way intermediate between stable and flexible memory. Our results demonstrate that various attributes of memory functions may originate from differences in the synaptic stability.

Sexually dimorphic behavior, neuronal activity, and gene expression in Chd8-mutant mice

Autism spectrum disorders (ASDs) are four times more common in males than in females, but the underlying mechanisms are poorly understood. We characterized sexually dimorphic changes in mice carrying a heterozygous mutation in Chd8 (Chd8+/N2373K) that was first identified in human CHD8 (Asn2373LysfsX2), a strong ASD-risk gene that encodes a chromatin remodeler. Notably, although male mutant mice displayed a range of abnormal behaviors during pup, juvenile, and adult stages, including enhanced mother-seeking ultrasonic vocalization, enhanced attachment to reunited mothers, and isolation-induced self-grooming, their female counterparts do not. This behavioral divergence was associated with sexually dimorphic changes in neuronal activity, synaptic transmission, and transcriptomic profiles. Specifically, female mice displayed suppressed baseline neuronal excitation, enhanced inhibitory synaptic transmission and neuronal firing, and increased expression of genes associated with extracellular vesicles and the extracellular matrix. Our results suggest that a human CHD8 mutation leads to sexually dimorphic changes ranging from transcription to behavior in mice.Autism is ~4 times more common in males. Jung et al. reveal male-preponderant abnormal behaviors in mice lacking CHD8, a chromatin remodeler, accompanying sexually dimorphic changes in neuronal firing, synaptic transmission, and gene expression.

Automated 3-D mapping of single neurons in the standard brain atlas using single brain slices

Recent breakthroughs in neuroanatomical tracing methods have helped unravel complicated neural connectivity in whole brain tissue at a single cellular resolution. However, analysis of brain images remains dependent on highly subjective manual processing. In the present study, we introduce AMaSiNe, a novel software for automated mapping of single neurons in the standard mouse brain atlas. The AMaSiNe automatically calibrates alignment angles of each brain slice to match the Allen Reference Atlas (ARA), locates labeled neurons from multiple brain samples in a common brain space, and achieves a standardized 3D-rendered brain. Due to the high fidelity and reliability of AMaSiNe, the retinotopic structures of neural projections to the primary visual cortex (VISp) were determined from single and dual injections of the rabies virus onto different visual areas. Our results demonstrate that distinct retinotopic organization of bottom-up and top-down projections could be precisely mapped using AMaSiNe.

Intrinsic timescales of sensory integration for motion perception

A subject-specific process of accumulation of information may be responsible for variations in decision time following visual perceptions in humans. A detailed profile of this perceptual decision making, however, has not yet been verified. Using a coherence-varying motion discrimination task, we precisely measured the perceptual decision kernel of subjects. We observed that the kernel size (decision time) is consistent within subjects, independent of stimulus dynamics, and the observed kernel could accurately predict each subject’s performance. Interestingly, the performance of most subjects was optimized when stimulus duration was matched to their kernel size. We also found that the observed kernel size was strongly correlated with the perceptual alternation in bistable conditions. Our result suggests that the observed decision kernel reveals a subject-specific feature of sensory integration.

Auditory noise influences human visual perception of ambiguous information: multi-modal integration during bistable perception

When the sensory system receives an ambiguous signal, human perception often switches spontaneously between two different interpretations. This phenomenon is called bistable perception, and has been considered important to understanding sensory system. In this study, we investigated the intervals of spontaneous switching, defined as reversal time τ, to examine the temporal dynamics of bistable perception. We also studied the multi-modal feature of bistable switching by applying auditory noise with the visual stimuli. Our hypothesis is that auditory noise would significantly alter the reversal time of bistable visual perception. By building a computational model, we could explain the influence of auditory noise on the reversal time. In the human psychophysical experiments, we first measured the reversal time with visual stimulus only, using two types of bistable visual movies: the racetrack [1] and the rotating 3D cylinder (Figure ​(Figure1A).1A). We observed that the reversal times are widely varied across the subjects but fairly consistent within the subject in both cases. Interestingly, we also found that the reversal time for the racetrack and the rotating cylinder were highly correlated (N = 9, R2 = 0.84, Figure ​Figure1C).1C). Next, we performed the experiment with auditory noise and visual stimuli together, and found that the reversal times are significantly altered. Importantly, when auditory noise was given, we found a systematic change such that a fast switching subjects (short τ) tend to slow down the switching while slow switching subjects (long τ) tend to speed up, so that the difference of τ between the two groups become insignificant (Figure ​(Figure1D).1D). Lastly, we designed a double-well energy model with destabilization/restabilization processes [2], and the model could well explain the observed phenomena. Figure 1 Effect of auditory noise in visual bistable perception. A. Two bistable visual movies. B. Example response and τ distribution. C. Peak of τ distribution in no noise condition. D. Peak of τ distribution with auditory noise condition. ...