Causal evidence for the role of the sensory visual cortex in visual short-term memory maintenance

The role of the sensory visual cortex during visual short-term memory (VSTM) remains controversial. This controversy is possibly due to methodological issues in previous attempts to investigate the effects of transcranial magnetic stimulation (TMS) on VSTM. This study aims to use TMS, while covering previous methodological deficits. Young adults will be recruited to participate in two experiments using a VSTM orientation change-detection under TMS. Monocular vision will be ensured using red-blue goggles combined with red-blue stimuli. Double-pulse TMS will be delivered at different times (Experiment 1: 0ms, 200ms, or 1000ms; Experiment 2: 200ms, 1000ms) during a 2 s retention phase, on one side of the occipital hemisphere (right hemisphere for 50% of the participants). In experiment 2, a sham-TMS condition will be introduced. Behavioural effects in the ipsilateral occipital hemisphere to visual hemifield will indicate a causal involvement of the sensory visual cortex during a specific temporal point in VSTM.

Parietal alpha tACS shows inconsistent effects on visuospatial attention

Transcranial alternating current stimulation (tACS) is a popular technique that has been used for manipulating brain oscillations and inferring causality regarding the brain-behaviour relationship. Although it is a promising tool, the variability of tACS results has raised questions regarding the robustness and reproducibility of its effects. Building on recent research using tACS to modulate visuospatial attention, we here attempted to replicate findings of lateralized parietal tACS at alpha frequency to induce a change in attention bias away from the contra- towards the ipsilateral visual hemifield. 40 healthy participants underwent tACS in two separate sessions where either 10 Hz tACS or sham was applied via a high-density montage over the left parietal cortex at 1.5 mA for 20 min, while performance was assessed in an endogenous attention task. Task and tACS parameters were chosen to match those of previous studies reporting positive effects. Unlike these studies, we did not observe lateralized parietal alpha tACS to affect attention deployment or visual processing across the hemifields as compared to sham. Likewise, additional resting electroencephalography immediately offline to tACS did not reveal any notable effects on individual alpha power or frequency. Our study emphasizes the need for more replication studies and systematic investigations of the factors that drive tACS effects.

Optic Tract Shrinkage Limits Visual Restoration After Occipital Stroke

Background and Purpose: Damage to the adult primary visual cortex (V1) causes vision loss in the contralateral visual hemifield, initiating a process of trans-synaptic retrograde degeneration. The present study examined functional implications of this process, asking if degeneration impacted the amount of visual recovery attainable from visual restoration training in chronic patients, and if restoration training impacted optic tract (OT) shrinkage. Methods: Magnetic resonance imaging was used to measure OT volumes bilaterally in 36 patients with unilateral occipital stroke. From OT volumes, we computed laterality indices (LI), estimating the stroke-induced OT shrinkage in each case. A subset of these chronic patients (n=14, 13±6 months poststroke) underwent an average of nearly 1 year of daily visual restoration training, which repeatedly stimulated vision in their blind field. The amount of visual field recovery was quantified using Humphrey perimetry, and post training magnetic resonance imaging was used to assess the impact of training on OT shrinkage. Results: OT LI was correlated with time since stroke: it was close to 0 (no measurable OT shrinkage) in subacute participants (<6 months poststroke) while chronic participants (>6 months poststroke) exhibited LI >0, but with significant variability. Visual training did not systematically alter LI, but chronic patients with baseline LI≈0 (no OT shrinkage) exhibited greater visual field recovery than those with LI>0. Conclusions: Unilateral OT shrinkage becomes detectable with magnetic resonance imaging by ≈7 months poststroke, albeit with significant interindividual variability. Although visual restoration training did not alter the amount of degeneration already sustained, OT shrinkage appeared to serve as a biomarker of the potential for training-induced visual recovery in chronic cortically blind patients.

Lateralization of lexical processing in monolinguals and bilinguals

Aims and objectives: This study aims to investigate how lexical processing (LP) is organized in early Turkish–English bilinguals and Turkish monolinguals. Methodology: We used a visual hemifield paradigm where bilingual ( n = 48) and monolingual ( n = 53) participants performed a lexical decision task. Bilingual participants performed the task in both their languages. Data and analysis: We recorded response times (RTs) and the accuracy rates (ARs) of the participants. An analysis of variance and t-test were run to analyze the bilingual and monolingual data, respectively. Findings: The results obtained from the analysis of the RTs and ARs for the Turkish and English words showed a balanced hemispheric organization in LP in bilingual speakers. The RTs for Turkish words in the monolingual group provided supportive evidence for the predominant role of the left hemisphere in LP. However, no significant difference was found in the accuracy of their answers, suggesting that the monolingual participants’ performance was not influenced by visual field of presentation of the words. Finally, the comparison of the two groups revealed that bilingual participants’ performance was inferior to monolinguals’ in speed and accuracy of processing of words presented in both visual fields. This result gives further support for the differential representation of LP in monolinguals and bilinguals. Originality: The psycholinguistic literature abounds with studies of LP in bilinguals and monolinguals from a variety of language backgrounds; however, there is much less data regarding the brain correlates of LP in Turkish–English bilinguals and Turkish monolinguals. Implications: Since Turkish–English bilinguals and Turkish monolinguals are underrepresented in the literature as compared to the population who speak other languages with alphabetic writing, this study provides preliminary data for future studies. Limitations: We did not control for gender or lexical factors such as orthographic neighbors when designing the word sets used as stimuli.

Inducing lateralized phosphenes over the occipital lobe using transcranial magnetic stimulation to navigate a virtual environment

Electrical stimulation involving visual areas of the brain produces artificial light percepts called phosphenes. These visual percepts have been extensively investigated in previous studies involving intracortical microsimulation (ICMS) and serve as the basis for developing a visual prosthesis for the blind. Although advances have been achieved, many challenges still remain with implementing a functional ICMS for visual rehabilitation purposes. Transcranial magnetic stimulation (TMS) over the primary occipital lobe offers an alternative method to produce phosphenes non-invasively. A main challenge facing blind individuals involves navigation. Within the scientific community, methods to evaluate the ability of a visual prosthesis to facilitate in navigation has been neglected. In this study, we investigate the effectiveness of evoking lateralized phosphenes to navigate a computer simulated virtual environment. More importantly, we demonstrate how virtual environments along with the development of a visual prosthesis share a mutual relationship benefiting both patients and researchers. Using two TMS devices, a pair of 40mm figure-of-eight coils were placed over each occipital hemisphere resulting in lateralized phosphene perception. Participants were tasked with making a series of left and right turns using peripheral devices depending on the visual hemifield in which a phosphene is present. If a participant was able to accurately perceive all ten phosphenes, the simulated target is able to advance and fully exit the virtual environment. Our findings demonstrate that participants can interpret lateralized phosphenes while highlighting the integration of computer based virtual environments to evaluate the capability of a visual prosthesis during navigation.

α-Band activity tracks a two-dimensional spotlight of attention during spatial working memory maintenance

A substantial body of work has shown that patterns of EEG α-band activity track the angular coordinates of attended and remembered stimuli around fixation, but whether these patterns track the two-dimensional coordinates of stimuli presented within a visual hemifield remains an open question. Here, we demonstrate that α-band activity tracks the two-dimensional coordinates of remembered stimuli within a hemifield, showing that α-band activity reflects a spotlight of attention focused on locations maintained in working memory.

Improved correspondence of fMRI visual field localizer data after macroanatomical alignment

The study of the visual system and its role for human cognition in health and disease with fMRI often requires the use of localizer paradigms to define anatomical regions of interest (ROIs). However, the considerable degree of interindividual variability of the cerebral cortex represents an important confound, especially when analyzing visual localizer data on the group level. Cortex-based alignment (CBA) techniques lead to a reliable reduction of interindividual anatomical variability. Yet, the potential benefits of CBA has not been investigated for visual field localizer paradigms used to map specific parts of the visual field within retinotopically organized early visual areas. We evaluated CBA for an attention-enhanced visual field localizer mapping a homologous part of each visual quadrant in a cohort of 50 participants. After CBA, group ROIs showed markedly increased spatial consistency. CBA also led to an increase in the probability of activation overlap of up to forty percent. Furthermore, the size of group ROIs for the lower visual hemifield was larger than for the upper visual hemifield after CBA. This asymmetry, which mirrors previous findings from electrophysiological and fMRI studies, was not detectable before CBA. Our results confirm and extend the utility of CBA for the study of the visual system particularly in the context of group analyses. This method should be particularly important for the study of neuropsychiatric disorders with abnormally increased interindividual anatomical variability.

Conflict processing networks: A directional analysis of stimulus-response compatibilities using MEG

The suppression of distracting information in order to focus on an actual cognitive goal is a key feature of executive functions. The use of brain imaging methods to investigate the underlying neurobiological brain activations that occur during conflict processing have demonstrated a strong involvement of the fronto-parietal attention network (FPAN). Surprisingly, the directional interconnections, their time courses and activations at different frequency bands remain to be elucidated, and thus, this constitutes the focus of this study. The shared information flow between brain areas of the FPAN is provided for frequency bands ranging from the theta to the lower gamma band (4–40 Hz). We employed an adaptation of the Simon task utilizing Magnetoencephalography (MEG). Granger causality was applied to investigate interconnections between the active brain regions, as well as their directionality. Following stimulus onset, the middle frontal precentral cortex and superior parietal cortex were significantly activated during conflict processing in a time window of between 300 to 600ms. Important differences in causality were found across frequency bands between processing of conflicting stimuli in the left as compared to the right visual hemifield. The exchange of information from and to the FPAN was most prominent in the beta band. Moreover, the anterior cingulate cortex and the anterior insula represented key areas for conflict monitoring, either by receiving input from other areas of the FPAN or by generating output themselves. This indicates that the salience network is at least partly involved in processing conflict information. The present study provides detailed insights into the underlying neural mechanisms of the FPAN, especially regarding its temporal characteristics and directional interconnections.