A systematic directional error in 2-D arm movements increases with increasing delay between visual target presentation and movement execution

Forty-seven normal subjects performed two-dimensional arm movements on a digitizer board using a mouse device. The movements were projected on a computer monitor. Subjects were instructed to move the mouse using the whole arm from a center position to a peripheral target so that the projected movement would pass over the target without stopping on the target. A large number of targets (360) were used to cover the entire directional continuum. The direction of the arm movement was the parameter of interest, which was measured at an initial position, at one third of the distance towards the target, and at the vicinity of the target. Four conditions of delay between target presentation and movement execution were used (0, 2, 4, 6 s). A systematic directional error was observed at the initial portion of the trajectory. This error resulted from a clustering of movement directions on an axis that was perpendicular to the axis of the resting forearm before movement onset. This pattern of errors can be explained by the initial inertial anisotropy of the arm. As the trajectory evolved, a different directional error emerged, resulting from a clustering of movement directions in two orthogonal axes. This pattern of directional error increased in amplitude as the delay increased, in contrast to the error at the initial portion of the trajectory which remained invariant with increasing delay. Finally, the information transmitted by the movement direction was shown to increase with the evolution of the trajectory. The increase in delay resulted in a decrease in directional-information transmission. It is proposed that the directional bias towards the end of the movement trajectory might reflect the action of 'movement primitives', that is patterns of muscle activation resulting from spinal interneuronal activation. It is further proposed that the directional bias observed at the vicinity of the target might reflect a loss of cortical directional information with increasing delay between target presentation and movement onset

Independent sources of anisotropy in visual orientation representation: a visual and a cognitive oblique effect

The representation of visual orientation is more accurate for cardinal orientations compared to oblique, and this anisotropy has been hypothesized to reflect a low-level visual process (visual, “class 1” oblique effect). The reproduction of directional and orientation information also leads to a mean error away from cardinal orientations or directions. This anisotropy has been hypothesized to reflect a high-level cognitive process of space categorization (cognitive, “class 2,” oblique effect). This space categorization process would be more prominent when the visual representation of orientation degrades such as in the case of working memory with increasing cognitive load, leading to increasing magnitude of the “class 2” oblique effect, while the “class 1” oblique effect would remain unchanged. Two experiments were performed in which an array of orientation stimuli (1–4 items) was presented and then subjects had to realign a probe stimulus within the previously presented array. In the first experiment, the delay between stimulus presentation and probe varied, while in the second experiment, the stimulus presentation time varied. The variable error was larger for oblique compared to cardinal orientations in both experiments reproducing the visual “class 1” oblique effect. The mean error also reproduced the tendency away from cardinal and toward the oblique orientations in both experiments (cognitive “class 2” oblique effect). The accuracy or the reproduced orientation degraded (increasing variable error) and the cognitive “class 2” oblique effect increased with increasing memory load (number of items) in both experiments and presentation time in the second experiment. In contrast, the visual “class 1” oblique effect was not significantly modulated by any one of these experimental factors. These results confirmed the theoretical predictions for the two anisotropies in visual orientation reproduction and provided support for models proposing the categorization of orientation in visual working memory. © 2015, Springer-Verlag Berlin Heidelberg

Frontal-parietal activation differences observed before the execution of remembered saccades: an event-related potentials study

Healthy subjects performed saccadic eye movements in one memory (MEM) and two (delay tasks delay, DEL and modified delay, M-DEL) while we recorded scalp event-related potentials (ERPs) from 25 electrode sites. In the MEM task the subjects were instructed to retain in memory the location of a visual target for a delay of 1-6 s and then perform a remembered saccade at the go signal. In the DEL task, the target remained on until movement completion and in the M-DEL task the target, that was visible during the delay period, disappeared synchronously with the go signal. A reduction in response latency and an increase in the percentage of dysmetric movements were observed fur the MEM task compared to the two delay tasks. An increased ERP activity at the central-frontal electrode sites compared to the parietal sites was significant only for the MEM task early on during the delay period (500-1000 ms). During the period preceding the onset of the saccade, a parietal increase of activity was observed for all tasks. Furthermore the activity was smaller for the frontal compared to the parietal areas only for the memory task thus indicating a near reversal of the previous pattern of activity observed during the early delay period. This specific activation pattern of frontal and parietal areas, observed for the MEM task only, requires further investigation focusing on the temporal pattern of activation of large brain areas involved in working memory processing. (C) 2001 Elsevier Science B.V. All rights reserved

Changes of presaccadic cortical activity when performing horizontal, visually guided saccades

When a visually guided saccade task is running, the presaccadic potential obtained in the initial period of the task differs from those obtained later, while the subject’s oculomotor performance remains unaffected. These time-related changes of cortical activity consist both of an overall decreasing electrical activity as well as a selective one over certain cortical areas. The generalised reduced activity already described in earlier studies is considered as an unspecified effect such as fatigue or decreased motivation. On the contrary, the pronounced selective changes of cortical activity obtained over cortical areas such as the centro-parietal and frontal cortices, should be related with more specific, that is, visuomotor function. We assume that at the beginning of the task the performance of the saccade needs the activation of several cortical areas but later on the same oculomotor plan runs sufficiently under subcortical control. (C) 1997 Elsevier Science Ireland Ltd

Mental maze solving: directional fMRI tuning and population coding in the superior parietal lobule.

The superior parietal lobule (SPL) of six human subjects was imaged at 4 T during mental traversing of a directed maze path. Here we demonstrate the orderly involvement of the SPL in this function, as follows. Forty-two percent of the voxels were tuned with respect to the direction of the maze path. This suggests a coherent tuning of local neuronal populations contributing to the change of the single-voxel BOLD signal. Preferred directions ranged throughout the directional continuum of 360 degrees. Voxels with similar preferred directions tended to cluster together: on average there were seven same-direction clusters per slice, with an average cluster membership of five voxels/cluster and an average nearest-neighbor same-direction intercluster distance of 13.1 mm. On the other hand, the average nearest-neighbor intercluster distance between a given direction and all other directions was 3.1 mm. This suggests a patchy arrangement such that patches of directionally tuned voxels, containing voxels with different preferred directions, alternate with patches of non-tuned voxels. Finally, the population vector predicted accurately the direction of the maze path (with an error of 12.7 degrees), and provided good estimates (with an error of 29 degrees) when calculated within parts of the SPL. Altogether, these findings document a new, orderly functional organization of the SPL with respect to mental tracing

Frontal lobe dysfunction in amyotrophic lateral sclerosis

The aim of the present study was to investigate the involvement of frontal lobe dysfunction in amyotrophic lateral sclerosis (ALS) using ocular motor paradigms and neuropsychological testing. Fifty-one patients with ALS participated in the following ocular motor tasks: (1) a three-choice task and (2) a remembered saccade task. The patients underwent a clinical and neuropsychological evaluation. One-third of ALS patients presented with signs of frontal dysfunction, as determined by their high distractibility factors (DF) in the three-choice task and their performances in both the Wisconsin and Stroop tests. ALS patients exhibited longer latencies to eye movement than controls in the performance of the remembered saccade task, specifically in performance of both remembered and delayed saccades, but saccade accuracy was not impaired. Finally, performance indices of the ocular motor tasks, in particular the DF, was correlated only with the degree of dysarthria. (C) 2002 Elsevier Science B.V. All rights reserved

Logarithmic transformation for high-field BOLD fMRI data.

Parametric statistical analyses of BOLD fMRI data often assume that the data are normally distributed, the variance is independent of the mean, and the effects are additive. We evaluated the fulfilment of these conditions on BOLD fMRI data acquired at 4 T from the whole brain while 15 subjects fixated a spot, looked at a geometrical shape, and copied it using a joystick. We performed a detailed analysis of the data to assess (a) their frequency distribution (i.e. how close it was to a normal distribution), (b) the dependence of the standard deviation (SD) on the mean, and (c) the dependence of the response on the preceding baseline. The data showed a strong departure from normality (being skewed to the right and hyperkurtotic), a strong linear dependence of the SD on the mean, and a proportional response over the baseline. These results suggest the need for a logarithmic transformation. Indeed, the log transformation reduced the skewness and kurtosis of the distribution, stabilized the variance, and made the effect additive, i.e. independent of the baseline. We conclude that high-field BOLD fMRI data need to be log-transformed before parametric statistical analyses are applied