The influence of eye dominance on saccade characteristics and slow presaccadic potentials

Characteristics of saccades and presaccadic slow potentials were studied in 36 right-handed men with right (the RE group) and left (the LE group) eye dominance. Three light-emitting diodes located in the center of the visual field (the central fixation stimulus, CFS) and 10 deg to the left and to the right of the center (peripheral stimuli, PSs) were used for stimulation. The subjects performed a task with simple saccades to a PS and a task with antisaccades to the horizontal mirror position of the PS. Monopolar EEGs at 19 derivations and electrooculograms (EOGs) were recorded. Back averaging of the EEG time-locked to the PS onset or the saccade onset was used to obtain slow presaccadic potentials. The saccade characteristics in the RE and LE groups were similar. Differences between them were found only in the antisaccade task. The amplitude of negative presaccadic potentials (NPPs) time-locked to the PS in the frontal cortex was lower in the LE group compared to the RE group. Analysis of potentials time-locked to the saccade onset showed that changes in the slow potentials during the last 50 s before the saccade depended on the saccade direction and reflected the activation of the hemisphere opposite to the saccade direction. The activation of the right hemisphere before left-side saccades was higher in the LE than the RE group. In addition, the amplitude of NPPs was decreased in the frontal area and increased in the left posterior temporal area in the LE group compared to the RE group. The obtained results indicate that the involvement of the frontal cortex in cognitive and motor processes is decreased in subjects with the left eye dominance.     

The effects of simulated microgravity on characteristics of slow presaccadic potentials

The parameters of saccades and presaccadic slow potentials were studied in seven right-handed male volunteers with a dominant right eye before and after exposure to 6-day dry immersion. Visual stimuli were presented using three light diodes, which were located in the center of the visual field (the central fixation stimulus) and 10° to the right and left of it (peripheral stimuli (PSs)). The subjects performed a test with simple saccades to a PS and a test with antisaccades to the point located symmetrically in the opposite visual field. The EEG (19 monopolar leads) and electrooculogram were recorded. To isolate slow potentials, backward EEG averaging was performed, with the moment of switching on the PS serving as a trigger for the averaging. It was found that the characteristics of saccadic eye movements did not substantially change after exposure to immersion. However, both tests revealed a change in topography and a decrease in the amplitude of presaccadic slow negative potentials (PSNPs) during immersion. Characteristically, the focus of presaccadic negativity shifted to the right hemisphere so that the PSNP amplitude sharply decreased in the left and increased in the right hemisphere. A significant decrease in the PSNP amplitude on day 6 of immersion was found in the midline and left-hemispheric frontal and parietal leads. It may be suggested that, because of support unloading and a decrease in proprioceptive input, exposure to microgravity causes a decrease in the activity of the left hemisphere and prefrontal and parietal cortices, initially involved in preparation and realization of motor responses. The activation of the right hemisphere could be of compensatory character.     

The influence of support withdrawal on presaccadic EEG potentials in subjects with different asymmetry profiles

Characteristics of saccades and parameters of slow presaccadic potentials were studied in 12 volunteers, including seven subjects with a leading right eye (the RE group) and five subjects with a leading left eye (the LE group) before and on the sixth day of dry immersion. For visual stimulation, three light-emitting diodes were used; one of them was located in the center of the visual field, and two other, 10° away from the horizontal axis to the right and left of the first one. The subjects performed an anti-saccade test, which included making saccades at the spot that was symmetrically located in the visual field opposite to the stimulus. The EEG (19 standard unipolar derivations) and electrooculogram were recorded. To obtain slow presaccadic potentials (PSPs), backward averaging triggered by switching on a peripheral stimulus was performed. Before the immersion, there were no significant differences in the characteristics of saccades in both groups of subjects. At the same time, the amplitude of presaccadic negativity (PSN) in the LE group was decreased, especially in the frontal region, and had considerable asymmetry during the analysis. During the immersion, the latent periods of the saccades and the percentage of incorrect reactions did not change in RE subjects and were increased in LE subjects. Both groups demonstrated a decrease in the PSN amplitude and its shift to the right hemisphere; intergroup differences decreased in immersion conditions. The characteristic feature of the RE group was a significant decrease in PSN in frontal leas at immersion, apparently caused by sensory disintegration and a decrease in the tonic afferent input. In the LE group, the maximal amplitude of PSN was observed in the central region.     

Cortical positive potentials in the period of eye fixation prior to saccades and antisaccades

The EEG of 10 right-handed healthy subjects preceding saccade and antisaccade with mean values of latency in the eye fixations period were selected and averaged. The positive potential P2 appearing on the fixation stimuly switching on and slow positive wave following after it were more prominent before antisaccades than normal saccades. Space-temporal analyses of presaccadic potentials showed that right frontal cortex was activated more before antisaccades. These findings suggest that right cortical hemisphere dominate in spatial attention and inhibition of automatic saccades to visual stimuli in the period of antisaccades preparing. During the period of central fixations "intermediate" positivity potentials, developing in 600-400 ms prior to saccade or antisaccade onset, were find out. These potentials were predominantly recorded in the left frontal and frontosagittal cortical areas. The obtained evidence suggest that "intermediate" positive potentials a period related to the process of motor attention, anticipation and decision making in the period of eyes fixation.     

Potential of the human cerebral cortex prior to antisaccades

Latencies and other parameters of presaccadic potentials preceding antisaccades and normal saccades to visual stimuli were studied in 10 right-handed healthy subjects. The EEG was recorded in F3, F4, Fz, C3, C4, Cz, P3, P4, O1 and O2 derivation. EEG records preceding saccades and antisaccades with mean latencies were selected and averaged. The latencies of the leftward antisaccades were shorter than of the rightward antisaccades. The slow presaccadic negativity (in the period of central eye fixations) and fast N -2 and P -1 potentials within the latent period were more prominent before antisaccades than normal saccades. Spatiotemporal analyses of presaccadic potentials showed that the right frontal cortex was activated to a greater extent before antisaccades than before saccades. These findings suggest that right-hemispheric dominance in the spatial attention and inhibition of automatic saccades to visual stimuli in the period of antisaccades preparation.     

Latent period of antisaccades in various conditions of visual stimulation in man

The latent periods (LP) of normal saccades and antisaccades were studied in 10 right-handed healthy subjects in two series of experiments. Peripheral visual stimuli were located at an angle of 10 degrees with respect to the central fixation stimulus in the left and right visual semifields. Two standard schemes of visual stimulation: 1) SS (single step), i.e., switching the peripheral stimulus on immediately after switching the central stimulus of; 2) GAP, i.e., the same with the interstimulus interval in 200 ms. It was shown that in the GAP stimulation condition, the LP of both saccades and antisaccades was 30-50 shorter than in the SS condition. The LP of antisaccades was longer than that of saccades by 145-300 ms. The LP of the leftward antisaccades was by 10-100 ms shorter than that of the rightward ones. Probably, this phenomenon reflects the dominance of the right hemisphere in spatial attention.     

The effect of the visual stimulation of the leading and nonleading eye on the value of the saccadic latency period and on the peak latency of rapid presaccadic potentials]

Visual targets were presented monocularly to the leading and nonleading eyes. The complex of rapid positive and negative potentials was studied using the reverse summation from the onset of saccades. The latencies of saccades and peak latencies of the averaged presaccadic potentials were measured. The dependence of the saccade latencies and peak latencies of the complex of potentials on stimulation of the leading or nonleading eye and saccade direction was not simple and was largely determined by the individual profile of asymmetry. It is suggested that during stimulation of the leading eye the processes of attention fixation and switching as well as of the space visual processing are faster than during stimulation of the nonleading eye. Thus, the leading role of the right eye is reflected not only in fixation processes but also in movement anticipation.     

Positive potentials of the human brain at different stages of preparation of a visually triggered saccade

The EEG of 10 right-handed subjects preceding saccades with mean values of latent periods were selected and averaged. Two standard paradigms of presentation of visual stimuli (central fixation stimulus-peripheral target succession): with a 200-ms inerstimulus interval (GAP) and successive single step (SS). During the period of central fixation, two kinds of positive potentials were observed: fast potentials of "inermediate" positivity (IP) developing 600-400 ms prior to saccade onset and fast potentials of "leading" positivity (LP), which immediately preceded the offset of the central fixation stimulus. Peak latency of the LP potentials was 300 ms prior to saccade onset in the SS paradigm and 400 ms in the GAP paradigm. These potentials were predominantly recorded in the frontal and frontosagittal cortical areas. Decrease in the latency by 30-50 ms in the GAP paradigm was associated with more pronounced positive potentials during the fixation period and absence of the initiation potential P-1' (or decrease in its amplitude). The obtained evidence suggest that the fast positive presaccadic potentials are of a complex nature related to attention, anticipation, motor preparation, decision making, saccadic initiation, and backward afferentation.     

Negative Potentials of the Human Brain Elicited in Saccadic Latency during Stimulation of the Leading and Nonleading Eyes with an Overlap

Fast presaccadic EEG potentials in saccadic latency were studied with the use of inverse averaging during monocular stimulation of the leading or nonleading eye. Two paradigms were followed, with presentation of visual stimuli consecutively or with a 200-ms overlap. Irrespective of the paradigm and the stimulated eye, the negative N ^–1 potential in the interval of 50–20 ms preceding the beginning of the saccade predominated in the hemisphere contralateral to the saccade direction, reflecting the command processes of saccadic initiation. The N ^–2 potential was more pronounced in the case of direct averaging, starting from the stimulus. Its amplitude increased with increasing concentration of attention on the fixation stimulus under the overlap conditions, and its foci predominated in the left hemisphere, in the frontal, central, and parietosagittal regions. Hence, the N ^–2 potential was assumed to reflect spatial perception and attention as initial stages of saccadic programming. The findings testify to the priority of the leading eye both in fixation and in spatial attention.     

Directed attention influence on the human brain potentials under conditions of probability visual stimulation

Saccadic latency and averaged EEG-potentials connected with switching on of the set and cue visual stimuli were examined in 12 right-handed healthy subjects in M. Posner's "cost-benefit" experimental paradigm. It was shown that attention was reflected in parameters of positive potential P100 evoked by switching on of set and cue stimuli and P300 and slow positive wave PMP1 evoked by switching on of the set stimulus in the relevant conditions. The spatiotemporal pattern of P100 probably reflects the involvement of the frontoparietal network of spacial attention in the perception of a relevant stimulus. Prevalence of the P300 and PMP1 potentials in the right parietal cortex suggests that these potentials reflect processes of space attention and visual fixation. Late positive potentials in a 600-900-ms interval after switching on of the set stimulus were found. Their amplitude was higher in backward averaging and they were predominantly localized in the left frontal cortex. These findings suggest that the late potentials reflect the anticipation and motor attention processes.     

Cortical slow negative potentials during eye fixation and preparing of visually triggered saccades in a man

In 10 right-handed healthy subjects EEGs preceding saccades with mean latent periods were selectively averaged. Two standard schemes of visual stimulation were used: with immediate presentation of a peripheral target stimuli after the central fixation stimulus (a single step paradigm) and with the interval between the stimuli in 200 ms (GAP paradigm). Two waves of slow premotor negativity (early PMN1 and late PMN2) that appeared 930 +/- 79 and 609 +/- 82 ms, respectively, prior to a saccade onset were observed. The PMN2 was followed by the negative potentials N-3, N-2, and N-1 (saccadic initiation potential). It was found that in GAP stimulation condition the PMN1 was less pronounces and N-1 was increased as compared to the single step. These findings suggest that disengage of attention from the central point during the GAP period clears the saccadic system for decision making and initiation of a saccade. Under such conditions, the expectation of a visual target does not require a high level of nonspecific activation and motor attention.     

Human fast negative EEG potentials before express saccades

Fast negative EEG potentials preceding fast regular saccades and express saccades were studied by the method of backward averaging under conditions of monocular stimulation of the right and left eye. "Step" and "gap" experimental paradigms were used for visual stimulation. Analysis of parameters of potentials and their spatiotemporal dynamics suggests that, under conditions of the increased attention and optimal readiness of the neural structures, express saccades appear when the previously chosen program of the future eye movement coincides with the actual target coordinates. We assumed that the saccade latency decreases at the expense of the involvement of the main oculomotor areas of motor and saccadic planning in its initiation; an express saccade can be initiated also by means of direct transmission of the signal from the cortex to the brainstem saccadic generator passing by the superior colliculus. Moreover, anticipating release from the central fixation and attention distraction are necessary for the successful initiation of an express saccade.     

Positive potentials ot the human brain in the latent presaccadic period under conditions of stimulation of the leading and nonleading eye

We used backward averaging method to study fast positive presaccadic EEG-potentials under conditions of the monocular stimulation of the leading and nonleading eye. Two schemes of the visual stimulus presentation ("no gap" and "overlap") were used. In the "no gap" condition, potential P1 dominated in the hemispere ipsilateral to a saccade direction. In the "overlap" condition, when the gaze was fixed at the central point, foci of this potential were localized in the sagittal derivations or in the same sites as in the "no gap" conditions. Irrespective on the stimulation scheme, the P2 foci were localized in the hemisphere contralateral to a saccade direction. We assume that the fast positive potentials involve both activation and inhibition processes in visuomotor structures and can be also associated with cognitive presaccadic processes (such as fixation disengage, attention lateralization and a preliminary extraction of motor programs from memory).     

Concurrent TMS-fMRI and psychophysics reveal frontal influences on human retinotopic visual cortex

BACKGROUND: Regions in human frontal cortex may have modulatory top-down influences on retinotopic visual cortex, but to date neuroimaging methods have only been able to provide indirect evidence for such functional interactions between remote but interconnected brain regions. Here we combined transcranial magnetic stimulation (TMS) with concurrent functional magnetic resonance imaging (fMRI), plus psychophysics, to show that stimulation of the right human frontal eye-field (FEF) produced a characteristic topographic pattern of activity changes in retinotopic visual areas V1-V4, with functional consequences for visual perception. RESULTS: FEF TMS led to activity increases for retinotopic representations of the peripheral visual field, but to activity decreases for the central field, in areas V1-V4. These frontal influences on visual cortex occurred in a top-down manner, independently of visual input. TMS of a control site (vertex) did not elicit such visual modulations, and saccades, blinks, or pupil dilation could not account for our results. Finally, the effects of FEF TMS on activity in retinotopic visual cortex led to a behavioral prediction that we confirmed psychophysically by showing that TMS of the frontal site (again compared with vertex) enhanced perceived contrast for peripheral relative to central visual stimuli. CONCLUSIONS: Our results provide causal evidence that circuits originating in the human FEF can modulate activity in retinotopic visual cortex, in a manner that differentiates the central and peripheral visual field, with functional consequences for perception. More generally, our study illustrates how the new approach of concurrent TMS-fMRI can now reveal causal interactions between remote but interconnected areas of the human brain.     

Causal Inference for Spatial Constancy across Saccades

Our ability to interact with the environment hinges on creating a stable visual world despite the continuous changes in retinal input. To achieve visual stability, the brain must distinguish the retinal image shifts caused by eye movements and shifts due to movements of the visual scene. This process appears not to be flawless: during saccades, we often fail to detect whether visual objects remain stable or move, which is called saccadic suppression of displacement (SSD). How does the brain evaluate the memorized information of the presaccadic scene and the actual visual feedback of the postsaccadic visual scene in the computations for visual stability? Using a SSD task, we test how participants localize the presaccadic position of the fixation target, the saccade target or a peripheral non-foveated target that was displaced parallel or orthogonal during a horizontal saccade, and subsequently viewed for three different durations. Results showed different localization errors of the three targets, depending on the viewing time of the postsaccadic stimulus and its spatial separation from the presaccadic location. We modeled the data through a Bayesian causal inference mechanism, in which at the trial level an optimal mixing of two possible strategies, integration vs. separation of the presaccadic memory and the postsaccadic sensory signals, is applied. Fits of this model generally outperformed other plausible decision strategies for producing SSD. Our findings suggest that humans exploit a Bayesian inference process with two causal structures to mediate visual stability.     

Latency of visually evoked saccadic eye movements

The paper deals with the initiation of visually guided saccades, in order to break down the saccadic reaction time into functionally different periods of time. It takes into account that spatial processing of information is so basic that modelling of saccadic control properties should include spatio-temporal arrangements. The output signal of the saccadic system was measured in response to visual stimuli in which the time between the appearance of a visual stimulus in the peripheral field and the disappearance of the central fixation point was varied. The variation of the mean saccadic latency time, measured with respect to the onset of the peripheral stimulus, as a function of stimulus asynchrony was highly significant. This variation may be represented by a so-called gap-overlap curve, which is characterized here by means of five parameters. A facilitation model is introduced to fit the results of the gap-overlap experiments. The facilitation model for the initiation of visually evoked saccades incorporates a mechanism which governs the efficiency of processing of signals that arise from a stimulus presented at a particular position in space. It explains how visual information may be affected by other sensory information before it is used to command further saccades. It allows determination of saccadic system parameters, such as the peripheral and the foveal afferent processing time, the central processing time for a saccade and the degree of facilitation. These quantities were found to be characteristic for the given test subjects, and where these data could be compared with neurophysiological data, the agreement was within the experimental error.     

Asymmetry of the latent periods of saccades in human depending on visual space complicity

Saccadic latencies of visually-guided saccades of 10 right-handed subjects with right-leading eyes were studied. Stimulation paradigm was spatially bidimentional, and stimuli were shown along horizontal and vertical meridians. Three traditional single step GAP - NO DELAY - OVERLAP temporal paradigms were used. In the first experiment, each paradigm was applied separately (simple visual space). In the second experiment, all the three paradigms were varied pseudo-random order and equiprobably, which complicated the time parameters of visual stimulation (complicated visual space). Asymmetry of visually-guided saccades along the vertical and horizontal meridians was revealed. The character of this asymmetry varied between subjects. MANOVA showed that the factor of visual space complicity (simple or complicated visual space) affected the latent period of saccades to a greater extent than the factor of stimulus lateralization (stimulus presentation in the left/right or upper/lower visual hemifields).     

The influence of recognition of a visual stimulus on evoked potentials of the projection and non-projection areas of the cerebral cortex]

Evoked potentials (EP) were recorded in the projection and non-projection areas of the cerebral cortex in juveniles in response to exposures of structured visual stimuli with subthreshold and supraliminal durations. The data obtained have shown that recognition of the presented stimulus is attended with intensification of the EP late complex. This effect is most pronouned in the central and frontal parts of the cortex. The Nv component with a 240 to 300 msec latency has a more regular connection with recognition as compared with other components.     

Hemispheric asymmetry of visual evoked potentials during human recognition of facial emotional expressions

Visual evoked potentials (VEP) in standard 16 EEG derivations were recorded in 26 young men and 20 women during recognition of facial emotional expressions and geometric figures. The stimuli were presented on a computer screen in the center of the visual field or randomly in the right or left vision hemifields. Peak VEP latency and mean amplitude in 50-ms epochs were measured; spatiotemporal VEP dynamics was analyzed in a series of topographic maps. The right hemisphere was shown to be more important in processing emotional faces. The character of the asymmetry was dynamic: at earlier stages of emotion processing the electrical activity was higher in the right inferior temporal region compared to the left symmetrical site. Later on the activity was higher in the right frontal and central areas. The dynamic mapping of "face-selective" component N180 of VEPs revealed the onset of activation over the right frontal areas that was followed by the fast activation of symmetrical left zones. Notably, this dynamics didn't correlate with the hemifield of stimuli exposition. The degree of asymmetry was lower during presentation of figures, especially in the inferior temporal and frontal regions. The prominent asymmetry of information processes in the inferior temporal and frontal areas was suggested to be specific for recognition of facial expression.     

Clinical evaluation of VEPs to interleaved checkerboard reversal stimulation of central,hemi- and peripheral fields

The VEPs of 195 patients referred for supportive evidence of multiple sclerosis or optic neuritis were studied by a new method of interleaved checkerboard reversal stimulation of different areas of the visual field. In the first group of 95 patients checks of 40′ subtense reversed in the whole field (28° × 20°), alternately in the left and right hemifields and alternately in the central (5° radius) and peripheral fields. In the second group of 100 patients checks reversed in the whole field and in interleaved mode in 3 visual field areas, comprising the central (4° radius) and left and right hemisurround fields.In the first group abnormal responses were recorded from 52 eyes and there was partial disagreement among the stimulus conditions in 10 of the 52. Abnormalities were seen uniquely to central field stimulation in 3 eyes but never to whole field stimulation alone. In the second group abnormal responses were recorded in 58 eyes, again never uniquely to whole field stimulation, while abnormalities confined to one or two areas of the visual field were seen in 24, providing evidence of peripheral field involvement alone in 8 eyes.In the first group, waveforms created from the sum of the left and right hemifield and central and peripheral field responses showed quite close conformity to the whole field VEP, although amplitudes were significantly lower and latencies significantly shorter. In 7 eyes responses would have been differently classified (normal or abnormal) using the sum as compared with the whole fields. The sum of the 3 interleaved stimuli was less reliable, its morphology often not closely approximating whole field responses.It is suggested that interleaved stimulation of two or more areas of the visual field is a sensitive and reliable method which reduces the time necessary to perform the test and helps control the patients' concentration, fixation and alertness. Whole field stimulation is probably necessary only in patients with severely degraded responses.