Antje Kraft

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).

An initial transient-state and reliable measures of corticospinal excitability in TMS studies

OBJECTIVE The objective of this study was to determine if an initial transient state influences the acquisition of reliable estimates of corticospinal excitability in transcranial magnetic stimulation (TMS) studies. Whereas muscle evoked potential (MEP) amplitudes are an important index of cortical excitability, these are severely limited by sweep-to-sweep variability. Interesting in this context is the experimental observation that the first MEP amplitudes might be much larger than subsequent responses [Brasil-Neto JP, Cohen LG, Hallet M. Central fatigue as revealed by postexercise decrement of motor evoked potentials. Muscle Nerve 1994;17:713-9]. This led to the hypothesis that an initial transient-state of increased excitability affects MEP amplitude derived estimates of corticospinal excitability. METHODS To address this issue we acquired repeated measures of single pulse MEP amplitudes over the primary motor cortex with and without navigated brain stimulation (NBS) and with various TMS-coils. Importantly, NBS allows for the sweep-to-sweep differentiation of physical and physiological variability. RESULTS We found a significant decline in estimates of corticospinal excitability and a transition from log-Normal to Normal distributed state, after which reliable measures (British Standards Institute) could be acquired. CONCLUSIONS We argue that an initial transient state of physiological origin influences measures of corticospinal excitability. SIGNIFICANCE This has important implications for investigations of cortical excitability. For example, it could reduce variability over studies and within small group comparisons.

Dissociable neural effects of task order control and task set maintenance during dual-task processing

The functional relevance of the lateral prefrontal cortex (lPFC) for the ability to process two tasks simultaneously has been debated extensively in previous studies that employed functional magnetic resonance imaging (fMRI) to investigate the neural correlates of dual-task processing. In the present fMRI study, we shed new light on this debate by directly comparing the lPFC activity changes for two cognitive functions commonly associated with dual-task performance: task order control and task set maintenance. We manipulated both functions in a 2 2 integrated parametric design. The fMRI data revealed a functional-neuroanatomical dissociation for the lPFC. Regions surrounding the inferior frontal sulcus and the middle frontal gyrus were exclusively associated with task order control but not with increased demands on task set maintenance during dual-task processing. The only lPFC region associated with task set maintenance was located in the left anterior insula. Outside the lPFC, we found dissociable regions for task order control and task set maintenance bilaterally in the premotor cortices with more rostral premotor activity for task order control and more caudal premotor activity for task set maintenance. In addition, task order control activated the intraparietal sulci bilaterally. Our data clearly suggest that task order control is a separable cognitive mechanism in dual-task situations that is related to activity changes in the lPFC and that can be dissociated from task set maintenance.

A dysexecutive syndrome of the medial thalamus

Thalamic stroke is associated with neurological and cognitive sequelae. Resulting neuropsychological deficits vary with the vascular territory involved. Whereas sensory, motor and memory deficits following thalamic stroke are comparatively well characterized, the exact relationship between executive dysfunction and thalamic damage remains more ambiguous. To assess the pattern of executive-cognitive deficits following thalamic stroke and its possible association with distinct thalamic nuclei, 19 patients with focal thalamic lesions were examined with high-resolution structural imaging and neuropsychological testing. Twenty healthy individuals served as controls. Patient MRIs were co-registered to an atlas of the human thalamus. Lesion overlap and subtraction analyses were used for lesion-to-symptom mapping. In eight patients (42.1%), neuropsychological assessment demonstrated a disproportionate deficit in the Wisconsin Card Sorting Test (WCST), while other executive and memory functions were much less affected. Subtraction analysis revealed an area in the left medial thalamus, mainly consisting of the centromedian and parafascicular nuclei (CM-Pf complex) that was damaged in these patients and spared in patients with normal WCST performance. Thus, damage to the CM-Pf complex may yield a distinct dysexecutive syndrome in which deficient maintenance and shifting between cognitive sets predominates. We hypothesize that the CM-Pf complex may contribute to maintenance and shifting of cognitive sets by virtue of its dense connections with the striatum. The pattern of executive dysfunction following thalamic stroke may vary considerably with lesion location.

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.

Mechanisms and neuronal networks involved in reactive and proactive cognitive control of interference in working memory

Cognitive control can be reactive or proactive in nature. Reactive control mechanisms, which support the resolution of interference, start after its onset. Conversely, proactive control involves the anticipation and prevention of interference prior to its occurrence. The interrelation of both types of cognitive control is currently under debate: Are they mediated by different neuronal networks? Or are there neuronal structures that have the potential to act in a proactive as well as in a reactive manner? This review illustrates the way in which integrating knowledge gathered from behavioral studies, functional imaging, and human electroencephalography proves useful in answering these questions. We focus on studies that investigate interference resolution at the level of working memory representations. In summary, different mechanisms are instrumental in supporting reactive and proactive control. Distinct neuronal networks are involved, though some brain regions, especially pre-SMA, possess functions that are relevant to both control modes. Therefore, activation of these brain areas could be observed in reactive, as well as proactive control, but at different times during information processing.

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.

Electrophysiological evidence for cognitive control during conflict processing in visual spatial attention

Event-related potentials were measured to investigate the role of visual spatial attention mechanisms in conflict processing. We suggested that a more difficult target selection leads to stronger attentional top-down control, thereby reducing the effects of arising conflicts. This hypothesis was tested by varying the selection difficulty in a location negative priming (NP) paradigm. The difficult task resulted in prolonged responses as compared to the easy task. A behavioral NP effect was only evident in the easy task. Psychophysiologically the easy task was associated with reduced parietal N1, enhanced frontocentral N2 and N2pc components and a prolonged P3 latency for the conflict as compared to the control condition. The N2pc effect was also obvious in the difficult task. Additionally frontocentral N2 amplitudes increased and latencies of N2pc and P3 were delayed compared to the easy task. The differences at frontocentral and parietal electrodes are consistent with previous studies ascribing activity in the prefrontal and parietal cortex as the source of top-down attentional control. Thus, we propose that stronger cognitive control is involved in the difficult task, resulting in a reduced behavioral NP conflict.

Saccades to spatially extended targets: the role of eccentricity

Size and eccentricity of visual targets are known to modulate saccade parameters. Here we asked for a possible interaction between these target properties. We investigated latency and amplitude of saccades to targets of varying diameter presented at various eccentricities in the visual field. Effects of target size on saccadic eye movements highly depended on eccentricity of saccade targets. For large saccade targets, latencies increased and mean amplitudes decreased mainly at parafoveal eccentricities. By contrast, scatter of saccade amplitudes increased nearly linearly with target size and eccentricity. These effects are consistent with the known functional anatomy of the superior colliculus. Size- and eccentricity-related changes in saccade parameters may depend on distinct subpopulations of collicular neurons.

Compensatory eye and head movements of patients with homonymous hemianopia in the naturalistic setting of a driving simulation

Homonymous hemianopia (HH) is a frequent deficit resulting from lesions to post-chiasmal brain structures with a significant negative impact on activities of daily living. To address the question how patients with HH may compensate their visual field defect in a naturalistic environment, we performed a driving simulation experiment and quantitatively analyzed both eye and head movements using a head-mounted pupil camera. 14 patients with HH and 14 matched healthy control subjects participated in the study. Based on the detection performance of dynamically moving obstacles, which appeared unexpectedly along the sides of the road track, we divided the patient group into a high- and a low-performance group. Then, we compared parameters of eye and head movements between the two patient groups and the matched healthy control group to identify those which mediate successful detection of potentially hazardous objects. Differences in detection rates could not be explained by demographic variables or the extent of the visual field defect. Instead, high performance of patients with HH in the naturalistic setting of our driving simulation depended on an adapted visual exploratory behavior characterized by a relative increase in the amplitude and a corresponding increase in the peak velocity of saccades, widening horizontally the distribution of eye movements, and by a shift of the overall distribution of saccades into the blind hemifield. The result of the group comparison analyses was confirmed by a subsequent stepwise regression analysis which identified the horizontal spread of eye movements as single factor predicting the detection of hazardous objects.