Spatial anisotropy of saccadic latency in normal subjects and brain-damaged patients

Shorter fixation durations for up-directed saccades during saccadic exploration: A meta-analysis

Utilizing 23 datasets, we report a meta-analysis of an asymmetry in presaccadic fixation durations for saccades directed above and below eye fixation during saccadic exploration. For inclusion in the meta-analysis, saccadic exploration of complex visual displays had to have been made without gaze-contingent manipulations. Effect sizes for the asymmetry were quantified as Hedge's g. Pooled effect sizes indicated significant asymmetries such that during saccadic exploration in a variety of tasks, presaccadic fixation durations for saccades directed into the upper visual field were reliably shorter than presaccadic fixation durations for saccades into the lower visual field. It is contended that the asymmetry is robust and important for efforts aimed at modelling when a saccade is initiated as a function of ensuing saccade direction.

Eye-Movement Training Results in Changes in qEEG and NIH Stroke Scale in Subjects Suffering from Acute Middle Cerebral Artery Ischemic Stroke: A Randomized Control Trial

Context

Eye-movement training (EMT) can induce altered brain activation and change the functionality of saccades with changes of the brain in general.

Objective

To determine if EMT would result in changes in quantitative electroencephalogram (qEEG) and NIH Stroke Scale (NIHSS) in patients suffering from acute middle cerebral artery (MCA) infarction. Our hypothesis is that there would be positive changes in qEEG and NIHSS after EMT in patients suffering from acute MCA ischemic stroke.

Design

Double-blind randomized controlled trial.

Setting and participants

Thirty-four subjects with acute MCA ischemic stroke treated at university affiliated hospital intensive care unit.

Interventions

Subjects were randomized into a “control” group treated only with aspirin (125 mg/day) and a “treatment” group treated with aspirin (125 mg/day) and a subject-specific EMT.

Main outcome measures

Delta–alpha ratio, power ratio index, and the brain symmetry index calculated by qEEG and NIHSS.

Results

There was strong statistical and substantive significant improvement in all outcome measures for the group of stroke patients undergoing EMT. Such improvement was not observed for the “control” group, and there were no adverse effects.

Conclusion

The addition of EMT to a MCA ischemic stroke treatment paradigm has demonstrated statistically significant changes in outcome measures and is a low cost, safe, and effective complement to standard treatment.

When do you look where you look? A visual field asymmetry

Pre-saccadic fixation durations associated with saccades directed in different directions were compared in three endogenous-attention oriented saccadic scanning tasks (i.e. visual search and scene viewing). Pre-saccadic fixation durations were consistently briefer before the execution of upward saccades, than downward saccades. Saccades also had a higher probability of being directed upwards than downwards. Pre-saccadic fixation durations were symmetric and longer for horizontally-directed saccades. The vertical visual field asymmetry in pre-saccadic fixation durations reflects an influence of factors not directly related to currently fixated elements. The ability to predict pre-saccadic fixation durations is important for computational modelling of real-time saccadic scanning, and the findings make a case for including directional constraints in computational modelling of when the eyes move.

Differential auditory-oculomotor interactions in patients with right vs. left sided subjective tinnitus: a saccade study

Subjective tinnitus (ST) is a frequent but poorly understood medical condition. Recent studies demonstrated abnormalities in several types of eye movements (smooth pursuit, optokinetic nystagmus, fixation, and vergence) in ST patients. The present study investigates horizontal and vertical saccades in patients with tinnitus lateralized predominantly to the left or to the right side. Compared to left sided ST, tinnitus perceived on the right side impaired almost all the parameters of saccades (latency, amplitude, velocity, etc.) and noticeably the upward saccades. Relative to controls, saccades from both groups were more dysmetric and were characterized by increased saccade disconjugacy (i.e., poor binocular coordination). Although the precise mechanisms linking ST and saccadic control remain unexplained, these data suggest that ST can lead to detrimental auditory, visuomotor, and perhaps vestibular interactions.

Cortical activity preceding vertical saccades: a MEG study

Previous studies have shown that upward saccade latencies are faster than downward saccade latencies in certain tasks. This asymmetry does not appear to represent a general main effect of the visual, or the vertical oculomotor system. In this study we examined the cortical activity underlying this latency asymmetry. We used MEG to assess cortical activity related to horizontal and vertical saccade preparation, and eye movement recordings to assess saccade latencies in a modified delay task. The reconstructed cortical activity was examined with respect to the onset of the target stimulus and the onset of the saccade. Upward saccades were faster than downward saccades, in agreement with previous studies. Although to a large extent, horizontal and vertical targets activated similar areas, there were also some differences. The earlier difference was found 100-150 ms after target onset over the right supramarginal gyrus when subjects attended to location-cues. Down cues activated this area faster than up cues. Moreover, cue-related activity was stronger over the left frontal cortex for up than down cues. In contrast, saccade-related activity over the same area was stronger when preceding downward than upward saccades. The results suggest that stimuli in the upper and lower visual field may have different impacts on accessing networks related to visual attention and motor preparation resulting in different behavioral asymmetries.

The effect of transcranial magnetic stimulation on the latencies of vertical saccades

In this study, we investigated the effect of transcranial magnetic stimulation (TMS) over the right posterior parietal cortex (PPC) on the latency of two different types of visually-guided vertical saccades: reflexive saccades triggered by the sudden onset of a target, and saccades towards target locations known in advance. For this reason, we used two oculomotor tasks: a gap and a delay task, respectively. Nine normal subjects performed vertical saccades at +/-7.5 and +/-15 degrees . TMS was applied at 80 and 100 ms after target onset in the gap task, and after fixation offset in the delay task. Without TMS, we confirmed a latency asymmetry in the gap task favouring upward saccades at the lower eccentricity (7.5 degrees ), and a latency symmetry in the delay task. TMS increased the latencies of all saccades in the delay task, when delivered at 100 ms. This effect was mostly pronounced for downward saccades at 7.5 degrees . As a result, saccade latencies showed an asymmetry in this condition, similar to the one observed in the gap task without TMS. The gap task with TMS resulted in a variable latency distribution and no significant overall effect on saccade latency. Our results indicate that the right PPC is involved in the initiation of vertical saccades in the delay task, and that this involvement appears to be enhanced for downward saccades. A conclusion for the involvement of this area in the gap task could not be drawn from this study.

Loss of visual information in neglect: the effect of chromatic- versus luminance-contrast stimuli in a "what" task

One to four vertical bars were tachistoscopically presented at various eccentricities along the horizontal meridian to patients with right brain damage and neglect (n=7) and to a control group of right brain damaged patients without neglect (n=4). Luminance contrast stimuli and isoluminant chromatic contrast stimuli were used. The patients' task was to report the number of bars. With luminance stimuli the neglect patients' discrimination in the left visual field was reduced, particularly at the most leftward position tested (ca 20 degrees ) where about 20% of the bars were omitted. The loss of information was limited to luminance contrast stimuli. When chromatic contrast stimuli were used, accuracy was comparable in the two hemifields and between groups of patients. The differential pattern of results for chromatic and luminance contrast stimuli is consistent with the hypothesis of a selective deficit of the magno-cellular pathway in neglect.

Underestimation of contralateral space in neglect: a deficit in the “where” task

Space perception was investigated in right brain damaged patients with ( N=13) and without neglect ( N=5; control group). Patients were requested to localise a target tachistoscopically flashed at various eccentricities along the horizontal meridian. All patients had an intact visual field and spared ability to manually point to a target. To segregate magno- and parvo-pathway activity, stimuli modulated in either luminance or chromatic contrast were used. Patients were required to verbally report the stimulus position (verbal task) or to manually point to the stimulus (pointing task). Neglect patients reported the stimuli in the left visual field closer to the centre than they actually were. In the verbal test, underestimation was about 7 deg at the most eccentric position tested (20 deg), and decreased linearly for smaller eccentricities. The effect was similar but less marked in the pointing task. No difference was found for stimuli with luminance or chromatic contrast. Space underestimation was confined to the contralesional space; no evidence of misperception was detected in the ipsilesional hemifield. The present findings are consistent with the view that contralesional space representation is compressed in neglect patients.

Fixation stability and saccadic latency in élite shooters

This study tested the hypothesis that elementary visuo-motor functions involved in visual scanning, as measured by fixation and saccadic tasks, are better in a group of high-level clay target shooters (N=7) than in a control group (N=8). In the fixation task, subject were told to keep fixation as still as possible on a target for 1 min, both in the presence and absence of distracters. For shooters, time did not have an effect on fixation stability, and they had more stable fixation than controls in the distracters condition. Results indicate a difference between groups on both the temporal span of attention and selective attention. In the saccadic task, subjects were asked to saccade, as fast as possible, towards a peripherally displayed target. Two conditions were used: simple reaction to target onset and discrimination between targets and distracters. Shooters had faster saccadic latency to targets than controls in both conditions. Finally, to evaluate the effect of exercise on saccadic latency, we trained one control subject to saccade to a target displayed at a constant spatial position. At the end of the training, saccadic latency reached a value comparable to that recorded in shooters. Learning was largely retinotopic, not showing transfer to untrained spatial positions.

Neuropsychological perspectives on eye-hand coordination in visually-guided reaching

Substantial progress has been made in understanding the neural control of movement in the past 30 years. Lower cost technology for tracking movements of the eyes and the hands has increased our understanding of these two systems and their interactions in both neurologically intact individuals and non-human primates. Nevertheless the neuropsychology of eye-hand coordination during visually-guided tasks such as reaching and grasping remains relatively understudied. This chapter reviews some of the relevant neurophysiology and neuropsychology of eye-hand coordination during visually-guided reaching. Current models emphasising coordinate transformations are discussed in light of new patient data showing a particular type of failure of eye-hand coordination during reaching.