Manuel C. Olma

Transcranial direct current stimulation affects visual perception measured by threshold perimetry.

In this study, we aimed to characterize the effect of anodal and cathodal direct current stimulation (tDCS) on contrast sensitivity inside the central 10 degrees of the visual field in healthy subjects. Distinct eccentricities were investigated separately, since at the cortical level, more central regions of the visual field are represented closer to the occipital pole, i.e. closer to the polarizing electrodes, than are the more peripheral regions. Using a double-blind and sham-controlled within-subject design, we measured the effect of stimulation and potential learning effect separately across testing days. Anodal stimulation of the visual cortex compared to sham stimulation yielded a significant increase in contrast sensitivity within 8° of the visual field. A significant increase in contrast sensitivity between the conditions “pre” and “post” anodal stimulation was only obtained for the central positions at eccentricities smaller than 2°. Cathodal stimulation of the visual cortex did not affect contrast sensitivity at either eccentricity. Perceptual learning across testing days was only observed for threshold perimetry before stimulation. Measuring contrast sensitivity changes after tDCS with a standard clinical tool such as threshold perimetry may provide an interesting perspective in assessing therapeutic effects of tDCS in ophthalmological or neurological defects (e.g. with foveal sparing vs. foveal splitting).

Inhibitory non-invasive brain stimulation to homologous language regions as an adjunct to speech and language therapy in post-stroke aphasia: a meta-analysis

Chronic communication impairment is common after stroke, and conventional speech and language therapy (SLT) strategies have limited effectiveness in post-stroke aphasia. Neurorehabilitation with non-invasive brain stimulation techniques (NIBS)—particularly repetitive transcranial magnetic stimulation (rTMS) or transcranial direct current stimulation (tDCS)—may enhance the effects of SLT in selected patients. Applying inhibitory NIBS to specific homologous language regions may induce neural reorganization and reduce interhemispheric competition. This mini review highlights randomized controlled trials (RCTs) and randomized cross-over trials using low-frequency rTMS or cathodal tDCS over the non-lesioned non-language dominant hemisphere and performs an exploratory meta-analysis of those trials considered combinable. Using a random-effects model, a meta-analysis of nine eligible trials involving 215 participants showed a significant mean effect size of 0.51 (95% CI = 0.24–0.79) for the main outcome “accuracy of naming” in language assessment. No heterogeneity was observed (I2 = 0%). More multicenter RCTs with larger populations and homogenous intervention protocols are required to confirm these and the longer-term effects.

Excitability changes in the visual cortex quantified with signal detection analysis

PURPOSE TDCS can increase excitability in the visual cortex. It is a matter of current debate if tDCS can improve visual performance. Promising parameters to measure detection sensitivity may be those of the signal detection theory ( = SDT), as it allows differentiating between response bias and detection sensitivity changes. The measure of detection sensitivity can be used to predict actual performance under a wide variety of different response criteria. METHODS Here we test if the SDT can quantify tDCS-induced effects in a visual contrast discrimination task in healthy subjects. RESULTS Anodal stimulation of the visual cortex improved performance, as calculated by detection sensitivity for stimuli presented in the center of the visual field. More peripheral locations in the visual field were unaffected by anodal stimulation. Cathodal stimulation and sham stimulation of the visual cortex had no consistent effect on detection sensitivity. The response bias was not affected by any type of stimulation. CONCLUSIONS Neuroplastic changes in the visual cortex induced by anodal tDCS can be measured by SDT, suggesting SDT could prospectively be a useful approach for monitoring restorative tDCS-effects on visual function in patients with central visual deficits.

Long-term effects of serial anodal tDCS on motion perception in subjects with occipital stroke measured in the unaffected visual hemifield

Transcranial direct current stimulation (tDCS) is a novel neuromodulatory tool that has seen early transition to clinical trials, although the high variability of these findings necessitates further studies in clinically relevant populations. The majority of evidence into effects of repeated tDCS is based on research in the human motor system, but it is unclear whether the long-term effects of serial tDCS are motor-specific or transferable to other brain areas. This study aimed to examine whether serial anodal tDCS over the visual cortex can exogenously induce long-term neuroplastic changes in the visual cortex. However, when the visual cortex is affected by a cortical lesion, up-regulated endogenous neuroplastic adaptation processes may alter the susceptibility to tDCS. To this end, motion perception was investigated in the unaffected hemifield of subjects with unilateral visual cortex lesions. Twelve subjects with occipital ischemic lesions participated in a within-subject, sham-controlled, double-blind study. MRI-registered sham or anodal tDCS (1.5 mA, 20 min) was applied on five consecutive days over the visual cortex. Motion perception was tested before and after stimulation sessions and at 14- and 28-day follow-up. After a 16-day interval an identical study block with the other stimulation condition (anodal or sham tDCS) followed. Serial anodal tDCS over the visual cortex resulted in an improvement in motion perception, a function attributed to MT/V5. This effect was still measurable at 14- and 28-day follow-up measurements. Thus, this may represent evidence for long-term tDCS-induced plasticity and has implications for the design of studies examining the time course of tDCS effects in both the visual and motor systems.

Dynamic spatial coding within the dorsal frontoparietal network during a visual search task.

To what extent are the left and right visual hemifields spatially coded in the dorsal frontoparietal attention network? In many experiments with neglect patients, the left hemisphere shows a contralateral hemifield preference, whereas the right hemisphere represents both hemifields. This pattern of spatial coding is often used to explain the right-hemispheric dominance of lesions causing hemispatial neglect. However, pathophysiological mechanisms of hemispatial neglect are controversial because recent experiments on healthy subjects produced conflicting results regarding the spatial coding of visual hemifields. We used an fMRI paradigm that allowed us to distinguish two attentional subprocesses during a visual search task. Either within the left or right hemifield subjects first attended to stationary locations (spatial orienting) and then shifted their attentional focus to search for a target line. Dynamic changes in spatial coding of the left and right hemifields were observed within subregions of the dorsal front-parietal network: During stationary spatial orienting, we found the well-known spatial pattern described above, with a bilateral hemifield representation in the right hemisphere and a contralateral preference in the left hemisphere. However, during search, the right hemisphere had a contralateral preference and the left hemisphere equally represented both hemifields. This finding leads to novel perspectives regarding models of visuospatial attention and hemispatial neglect.

Long-Lasting Enhancement of Visual Perception with Repetitive Noninvasive Transcranial Direct Current Stimulation

Understanding processes performed by an intact visual cortex as the basis for developing methods that enhance or restore visual perception is of great interest to both researchers and medical practitioners. Here, we explore whether contrast sensitivity, a main function of the primary visual cortex (V1), can be improved in healthy subjects by repetitive, noninvasive anodal transcranial direct current stimulation (tDCS). Contrast perception was measured via threshold perimetry directly before and after intervention (tDCS or sham stimulation) on each day over 5 consecutive days (24 subjects, double-blind study). tDCS improved contrast sensitivity from the second day onwards, with significant effects lasting 24 h. After the last stimulation on day 5, the anodal group showed a significantly greater improvement in contrast perception than the sham group (23 vs. 5%). We found significant long-term effects in only the central 2–4° of the visual field 4 weeks after the last stimulation. We suspect a combination of two factors contributes to these lasting effects. First, the V1 area that represents the central retina was located closer to the polarization electrode, resulting in higher current density. Second, the central visual field is represented by a larger cortical area relative to the peripheral visual field (cortical magnification). This is the first study showing that tDCS over V1 enhances contrast perception in healthy subjects for several weeks. This study contributes to the investigation of the causal relationship between the external modulation of neuronal membrane potential and behavior (in our case, visual perception). Because the vast majority of human studies only show temporary effects after single tDCS sessions targeting the visual system, our study underpins the potential for lasting effects of repetitive tDCS-induced modulation of neuronal excitability.

P 155. Serial anodal tDCS over V1 induces long-term effects on colour discrimination in V4 measured in the unimpaired hemifield of patients with occipital stroke

Question Little is known about the impact of tDCS on higher level visual functions such as colour perception. Previous studies have primarily investigated the effects of anodal and cathodal direct current stimulation over the occipital cortex on basic visual functions (Antal and Paulus, 2008). In a recent single-session tDCS study in healthy young subjects, anodal stimulation was ineffective in transiently improving red/green discrimination (Costa, 2012). One possible explanation for this may be a ceiling effect of red/green discrimination in the healthy visual system. Given the high degree of plasticity in the sensory cortices of adults (Fahle and Poggie, 2002) we examined whether serial anodal tDCS can induce long-lasting improvements in colour discrimination in the unimpaired visual hemifield of patients with occipital stroke. Materials and methods Twelve chronic stroke patients with unilateral visual cortex lesions (mean age 54.0years, 5 male) participated in a within-subject, sham-controlled, double-blind study. MRI-registered anodal(1.5mA, 20min.)or sham tDCS was applied on 5 consecutive days above the ipsilesional calcarine sulcus. Campimetric testing of age matched colour discrimination was performed in the unaffected hemifield before and after each stimulation session and at 14-and 28-day follow-up. After a 14-day interval, an identical stimulation block with the other stimulation parameter was performed. The visual task was a forced choice task involving binocular detection of red circular stimuli, presented for 200ms in one visual field quadrant at 5° offset from fixation against a green background. Results The age matched overall deviation of colour discrimination was entered in a 2-way ANOVA. In order to control for assumable learning effects, a covariate was added to the ANOVA model. The repeated measures ANOVA attained significance for the factor stimulation ( p =0.004) (stimulation∗learning: p =0.006). The factor time reached significance by trend ( p =0.065) (time∗learning: p =0.181).No significant interaction was revealed for the factor stimulation∗time (stimulation∗time∗learning).Due to a positive learning effect post hoc tests were performed within the first block. No significant difference was seen between the baselines of anodal and sham conditions ( p =0.21).In the anodal condition colour discrimination improved compared to sham on day 5 ( p =0.045) and at two ( p =0.066) and four week ( p =0.041) follow-up. Conclusion Long-term modulation in colour perception following serial anodal tDCS may represent evidence of inducible long-term plasticity in distantly connected components of the visual system (V1 and V4) in patients with occipital stroke measured in the unimpaired hemifield. This builds on studies describing the immediate effects of tDCS in the visual system in healthy volunteers. The temporal dynamics of serial anodal tDCS seem to interact with learning processes, and may yield potential support for neuroplastic adaptation processes following cerebral lesions. Future studies should examine the long-term outcomes and dynamics of tDCS-induced neuromodulation.

101. Visual selection during covert visual search in the human cortex

2 h with considerably reduced discomfort. Ophthalmic examinations and saccade recordings were used for comparison with the conventional SSC recording technique. In three experiments, subjects were examined using SSCs with a commercially available cornea bandage lens on the top of the search coil for up to 120 min of recording time. Ophthalmic testing revealed no apparent harmful effects on eyes or on lid surface. Saccade parameters (main sequence) remained unchanged when compared to SSC and CEPD recordings. Subjects rated less discomfort by using the CEPD. For the first time we show that SSC recordings can be extended over about 120 min without hazard to the eye, when using an eyelid protection lens. This advanced method allows new applications such as eye movement recordings during sleep (rapid eye movements) or perceptional or motor learning tasks, e.g. saccade adaptation paradigms.