Despite its clinical relevance, direct electrical stimulation (DES) of the human brain is surprisingly poorly understood. Although we understand several aspects of electrical stimulation at the cellular level, surface DES evokes a complex summation effect in a large volume of brain tissue, and the effect is difficult to predict as it depends on many local and remote physiological and morphological factors. The complex stimulation effects are reflected in the heterogeneity of behavioural effects that are induced by DES, which range from evocation to inhibition of responses - sometimes even when DES is applied at the same cortical site. Thus, it is a misconception that DES - in contrast to other neuroscience techniques - allows us to draw unequivocal conclusions about the role of stimulated brain areas.

Both the corollary discharge of the oculomotor command and eye muscle proprioception provide eye position information to the brain. Two contradictory models have been suggested about how these two sources contribute to visual localization: (1) only the efference copy is used whereas proprioception is a slow recalibrator of the forward model, and (2) both signals are used together as a weighted average. We had the opportunity to test these hypotheses in a patient (R.W.) with a circumscribed lesion of the right postcentral gyrus that overlapped the human eye proprioceptive representation. R.W. was as accurate and precise as the control group (n ϭ 19) in locating a lit LED that she viewed through the eye contralateral to the lesion. However, when the task was preceded by a brief (Ͻ1 s), gentle push to the closed eye, which perturbed eye position and stimulated eye proprioceptors in the absence of a motor command, R.W.'s accuracy significantly decreased compared with both her own baseline and the healthy control group. The data suggest that in normal conditions, eye proprioception is not used for visual localization. Eye proprioception is, however, continuously monitored to be incorporated into the eye position estimate when a mismatch with the efference copy of the motor command is detected. Our result thus supports the first model and, furthermore, identifies the limits for its operation.

Since the first description of a systematic mis-reaching by Bálint in 1909, a reasonable number of patients showing a similar phenomenology, later termed optic ataxia (OA), has been described. However, there is surprising inconsistency regarding the behavioral measures that are used to detect OA in experimental and clinical reports, if the respective measures are reported at all. A typical screening method that was presumably used by most researchers and clinicians, reaching for a target object in the peripheral visual space, has never been evaluated. We developed a set of instructions and evaluation criteria for the scoring of a semi-standardized version of this reaching task. We tested 36 healthy participants, a group of 52 acute and chronic stroke patients, and 24 patients suffering from cerebellar ataxia. We found a high interrater reliability and a moderate test-retest reliability comparable to other clinical instruments in the stroke sample. The calculation of cut-off thresholds based on healthy control and cerebellar patient data showed an unexpected high number of false positives in these samples due to individual outliers that made a considerable number of errors in peripheral reaching. This study provides first empirical data from large control and patient groups for a screening procedure that seems to be widely used but rarely explicitly reported and prepares the grounds for its use as a standard tool for the description of patients who are included in single case or group studies addressing optic ataxia similar to the use of neglect, extinction, or apraxia screening tools.

During a grasping movement, the maximum grip aperture (MGA) is almost linearly scaled to the dimension of the target along which it is grasped. There is still a surprising uncertainty concerning the influence of the other target dimensions on the MGA. We asked healthy participants to grasp cuboids always along the object's width with their thumb and index finger. Independent from variations of object width, we systematically varied height and depth of these target objects. We found that taller objects were generally grasped with a larger MGA. At the same time, the slope of the regression of MGA on object width decreased with increasing target height. In contrast, we found no effect of varying target depth on the MGA. Simulating these movements with a grasping model in which the objective to avoid contact of the digits with the target object at positions other than the goal positions was implemented yielded larger effects of target height than of target depth on MGA. We concluded that MGA does not only depend on the dimension of the target object along which it is grasped. Furthermore, the effects of the other 2 dimensions are considerably different. This pattern of results can partially be explained by the aim to avoid contacting the target object at positions other than the goal positions.

No abstract