Fujita, I. | Search bioRxiv

bioRxiv2018

Microstructural properties of the vertical occipital fasciculus explain the variability in human stereoacuity

Stereopsis is a fundamental visual function that has been studied extensively. However, it is not clear why depth discrimination (stereoacuity) varies more significantly among people than other modalities. Previous studies reported the involvement of both dorsal and ventral visual areas in stereopsis, implying that not only neural computations in cortical areas but also the anatomical properties of white matter tracts connecting those areas can impact stereopsis and stereoacuity. Here, we studied how human stereoacuity relates to white matter properties by combining psychophysics, diffusion MRI (dMRI), and quantitative MRI (qMRI). We performed a psychophysical experiment to measure stereoacuity, and in the same participants we analyzed the microstructural properties of visual white matter tracts based on two independent measurements, dMRI (fractional anisotropy, FA) and qMRI (macromolecular tissue volume; MTV). Microstructural properties along the right vertical occipital fasciculus (VOF), a major tract connecting dorsal and ventral visual areas, were highly correlated with measures of stereoacuity. This result was consistent for both FA and MTV, suggesting that the structural-behavioral relationship reflects differences in neural tissue density, rather than differences in the morphological configuration of fibers. fMRI confirmed that binocular disparity stimuli activated the dorsal and ventral visual regions near VOF endpoints. No other occipital tracts explained the variance in stereoacuity. In addition, the VOF properties were not associated with differences in performance on a different psychophysical task (contrast detection). These series of experiments suggest that stereoscopic depth discrimination performance is, at least in part, constrained by dorso-ventral communication through the VOF.

neuroscience↗

bioRxiv2017

Rapid responses of amygdala neurons discriminate facial expressions

The amygdala plays a critical role in detecting potential danger through sensory input [1, 2]. In the primate visual system, a subcortical pathway through the superior colliculus and the pulvinar is thought to provide the amygdala with rapid and coarse visual information about facial emotions [3-6]. A recent electrophysiological study in human patients supported this hypothesis by showing that intracranial event-related potentials discriminated fearful faces from other faces very quickly (within [~]74 ms) [7]. However, several aspects of the hypothesis remain debatable [8]. Critically, evidence for short-latency, emotion-selective responses from individual amygdala neurons is lacking [9-12], and even if this type of response existed, how it might contribute to stimulus detection is unclear. Here, we addressed these issues in the monkey amygdala and found that ensemble responses of single neurons carry robust information about emotional faces-- especially threatening ones--within [~]50 ms after stimulus onset. Similar rapid response was not found in the temporal cortex from which the amygdala receives cortical inputs [13], suggesting a subcortical origin. Additionally, we found that the rapid amygdala response contained excitatory and suppressive components. The early excitatory component might be useful for quickly sending signals to downstream areas. In contrast, the rapid suppressive component sharpened the rising phase of later, sustained excitatory input (presumably from the temporal cortex) and might therefore improve processing of emotional faces over time. We thus propose that these two amygdala responses that originate from the subcortical pathway play dual roles in threat detection.

neuroscience↗