Attention modulates neuronal correlates of interhemispheric integration and global motion perception

Abstract

In early retinotopic areas of the human visual system, information from the left and right visual hemifields (VHFs) is processed contralaterally in two hemispheres. Despite this segregation, we have the perceptual experience of a unified, coherent, and uninterrupted single visual field. How exactly the visual system integrates information from the two VHFs and achieves this perceptual experience still remains largely unknown. In this study using fMRI, we explored candidate areas that are involved in interhemispheric integration and the perceptual experience of a unified, global motion across VHFs. Stimuli were two-dimensional, computer-generated objects with parts in both VHFs. The retinal image in the left VHF always remained stationary, but in the experimental condition, it appeared to have local motion because of the perceived global motion of the object. This perceptual effect could be weakened by directing the attention away from the global motion through a demanding fixation task. Results show that lateral occipital areas, including the medial temporal complex, play an important role in the process of perceptual experience of a unified global motion across VHFs. In early areas, including the lateral geniculate nucleus and V1, we observed correlates of this perceptual experience only when attention is not directed away from the object. These findings reveal effects of attention on interhemispheric integration in motion perception and imply that both the bilateral activity of higher-tier visual areas and feedback mechanisms leading to bilateral activity of early areas play roles in the perceptual experience of a unified visual field.

Investigations on bottom-up and top-down processing in early visual cortex with high-resolution fMRI

“…the rotating 'mouth' of the Pac-Man, and the rotating wedge of the dynamic control stimulus) caused a positive signal change in their cortical representations in V1, V2, and V3 ( Figure 3.6 A, C, D, F, G, I). More surprisingly, but consistent with a previous report using the same stimuli (Akin et al, 2014), the surface region of all three stimuli in the left visual hemifield caused a negative signal change in the right hemisphere in all regions of interest ( Figure 3.6 A-I). The band of positive signal change seen on the inflated brain (Figure 3.3 C) is also apparent in the visual field projections (particularly in Figure 3.6 D, E, F).…”

“…Thus, we conducted a control experiment, in which we discovered that the negative surface response only occurs when a uniform surface stimulus is presented on a texture pattern background, irrespective of the shape of the stimulus. The decision to present the stimuli on a texture pattern had originally been motivated by an earlier study (Akin et al, 2014), which used the texture background as a means to improve perceptual figure-ground segregation.…”

“…All stimuli were presented on a textured random noise background in order to enhance figure-ground segregation. The stimuli, including the texture background, were adapted from Akin et al (2014). The texture pattern was created by randomly drawing pixel intensity values from a Gaussian distribution, and filtering the resulting image with a uniform kernel (kernel size 6 x 6 pixel).…”

“…All stimuli were presented on a static, textured random noise background to enhance figure-ground segmentation. The stimuli, including the texture background, were adapted from Akin et al (2014). Videos of the stimuli are available online (https://doi.org/10.5281/zenodo.2583017).…”

“…In the current study, instead of investigating a form of interpolation in an illusory display, we used ultra-high field 7T fMRI to determine whether the perceived motion of a luminance-defined surface was associated with enhanced activity in early visual cortex. The stimulus design (see Akin et al, 2014) and analysis methods were optimised to separate signal due to the figure surface from signal due to local discontinuities or boundaries. The stimulus consisted of a luminance-defined disk that was centrally fixated (Figure 3.1).…”