Attentional sensitivity and asymmetries of vertical saccade generation in monkey

Peri-Saccadic Orientation Identification Performance and Visual Neural Sensitivity Are Higher in the Upper Visual Field

Visual neural processing is distributed among a multitude of sensory and sensory-motor brain areas exhibiting varying degrees of functional specializations and spatial representational anisotropies. Such diversity raises the question of how perceptual performance is determined, at any one moment in time, during natural active visual behavior. Here, exploiting a known dichotomy between the primary visual cortex (V1) and superior colliculus (SC) in representing either the upper or lower visual fields, we asked whether peri-saccadic orientation identification performance is dominated by one or the other spatial anisotropy. Humans (48 participants, 29 females) reported the orientation of peri-saccadic upper visual field stimuli significantly better than lower visual field stimuli, unlike their performance during steady-state gaze fixation, and contrary to expected perceptual superiority in the lower visual field in the absence of saccades. Consistent with this, peri-saccadic superior colliculus visual neural responses in two male rhesus macaque monkeys were also significantly stronger in the upper visual field than in the lower visual field. Thus, peri-saccadic orientation identification performance is more in line with oculomotor, rather than visual, map spatial anisotropies.SIGNIFICANCE STATEMENT Different brain areas respond to visual stimulation, but they differ in the degrees of functional specializations and spatial anisotropies that they exhibit. For example, the superior colliculus (SC) both responds to visual stimulation, like the primary visual cortex (V1), and controls oculomotor behavior. Compared with the primary visual cortex, the superior colliculus exhibits an opposite pattern of upper/lower visual field anisotropy, being more sensitive to the upper visual field. Here, we show that human peri-saccadic orientation identification performance is better in the upper compared with the lower visual field. Consistent with this, monkey superior colliculus visual neural responses to peri-saccadic stimuli follow a similar pattern. Our results indicate that peri-saccadic perceptual performance reflects oculomotor, rather than visual, map spatial anisotropies.

Regularities in vertical saccadic metrics: new insights, and future perspectives

Introduction

Asymmetries in processing by the healthy brain demonstrate regularities that facilitate the modeling of brain operations. The goal of the present study was to determine asymmetries in saccadic metrics during visual exploration, devoid of confounding clutter in the visual field.

Methods

Twenty healthy adults searched for a small, low-contrast gaze-contingent target on a blank computer screen. The target was visible, only if eye fixation was within a 5 deg. by 5 deg. area of the target's location.

Results

Replicating previously-reported asymmetries, repeated measures contrast analyses indicated that up-directed saccades were executed earlier, were smaller in amplitude, and had greater probability than down-directed saccades. Given that saccade velocities are confounded by saccade amplitudes, it was also useful to investigate saccade kinematics of visual exploration, as a function of vertical saccade direction. Saccade kinematics were modeled for each participant, as a square root relationship between average saccade velocity (i.e., average velocity between launching and landing of a saccade) and corresponding saccade amplitude (Velocity = S*[Saccade Amplitude]^0.5). A comparison of the vertical scaling parameter (S) for up- and down-directed saccades showed that up-directed saccades tended to be slower than down-directed ones.

Discussion

To motivate future research, an ecological theory of asymmetric pre-saccadic inhibition was presented to explain the collection of vertical saccadic regularities. For example, given that the theory proposes strong inhibition for the releasing of reflexive down-directed prosaccades (cued by an attracting peripheral target below eye fixation), and weak inhibition for the releasing of up-directed prosaccades (cued by an attracting peripheral target above eye fixation), a prediction for future studies is longer reaction times for vertical anti-saccade cues above eye fixation. Finally, the present study with healthy individuals demonstrates a rationale for further study of vertical saccades in psychiatric disorders, as bio-markers for brain pathology.

Superior colliculus saccade motor bursts do not dictate movement kinematics

The primate superior colliculus (SC) contains a topographic map of space, such that the anatomical location of active neurons defines a desired eye movement vector. Complementing such a spatial code, SC neurons also exhibit saccade-related bursts that are tightly synchronized with movement onset. Current models suggest that such bursts constitute a rate code dictating movement kinematics. Here, using two complementary approaches, we demonstrate a dissociation between the SC rate code and saccade kinematics. First, we show that SC burst strength systematically varies depending on whether saccades of the same amplitude are directed towards the upper or lower visual fields, but the movements themselves have similar kinematics. Second, we show that for the same saccade vector, when saccades are significantly slowed down by the absence of a visible saccade target, SC saccade-related burst strengths can be elevated rather than diminished. Thus, SC saccade-related motor bursts do not necessarily dictate movement kinematics.

Presaccadic attention enhances contrast sensitivity, but not at the upper vertical meridian

Visual performance has striking polar performance asymmetries: At a fixed eccentricity, it is better along the horizontal than vertical meridian and the lower than upper vertical meridian. These asymmetries are not alleviated by covert exogenous or endogenous attention, but have been studied exclusively during eye fixation. However, a major driver of everyday attentional orienting is saccade preparation, during which attention automatically shifts to the future eye fixation. This presaccadic attention shift is considered strong and compulsory, and relies on different neural computations and substrates than covert attention. Thus, we asked: Can presaccadic attention compensate for the ubiquitous performance asymmetries observed during eye fixation? Our data replicate polar performance asymmetries during fixation and document the same asymmetries during saccade preparation. Crucially, however, presaccadic attention enhanced contrast sensitivity at the horizontal and lower vertical meridian, but not at the upper vertical meridian. Thus, instead of attenuating performance asymmetries, presaccadic attention exacerbates them.

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Can presaccadic attention attenuate polar angle asymmetries in visual perception

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Presaccadic attention enhances sensitivity at horizontal and lower vertical meridians

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But presaccadic attention does not enhance sensitivity at the upper vertical meridian

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Thus, presaccadic attention even exacerbates polar angle asymmetries in perception

Active vision at the foveal scale in the primate superior colliculus

The primate superior colliculus (SC) has recently been shown to possess both a large foveal representation as well as a varied visual processing repertoire. This structure is also known to contribute to eye movement generation. Here, we describe our current understanding of how SC visual and movement-related signals interact within the realm of small eye movements associated with the foveal scale of visuomotor behavior. Within the SC's foveal representation, there is a full spectrum of visual, visual-motor, and motor-related discharge for fixational eye movements. Moreover, a substantial number of neurons only emit movement-related discharge when microsaccades are visually guided, but not when similar movements are generated toward a blank. This represents a particularly striking example of integrating vision and action at the foveal scale. Beyond that, SC visual responses themselves are strongly modulated, and in multiple ways, by the occurrence of small eye movements. Intriguingly, this impact can extend to eccentricities well beyond the fovea, causing both sensitivity enhancement and suppression in the periphery. Because of large foveal magnification of neural tissue, such long-range eccentricity effects are neurally warped into smaller differences in anatomical space, providing a structural means for linking peripheral and foveal visual modulations around fixational eye movements. Finally, even the retinal-image visual flows associated with tiny fixational eye movements are signaled fairly faithfully by peripheral SC neurons with relatively large receptive fields. These results demonstrate how studying active vision at the foveal scale represents an opportunity for understanding primate vision during natural behaviors involving ever-present foveating eye movements.NEW & NOTEWORTHY The primate superior colliculus (SC) is ideally suited for active vision at the foveal scale: it enables detailed foveal visual analysis by accurately driving small eye movements, and it also possesses a visual processing machinery that is sensitive to active eye movement behavior. Studying active vision at the foveal scale in the primate SC is informative for broader aspects of active perception, including the overt and covert processing of peripheral extra-foveal visual scene locations.

Properties of visually guided saccadic behavior and bottom-up attention in marmoset, macaque, and human

Saccades are stereotypic behaviors whose investigation improves our understanding of how primate brains implement precise motor control. Furthermore, saccades offer an important window into the cognitive and attentional state of the brain. Historically, saccade studies have largely relied on macaques. However, the cortical network giving rise to the saccadic command is difficult to study in macaques because relevant cortical areas lie in deep sulci and are difficult to access. Recently, a New World monkey. the marmoset, has garnered attention as an alternative to macaques because of advantages including its smooth cortical surface. However, adoption of the marmoset for oculomotor research has been limited due to a lack of in-depth descriptions of marmoset saccade kinematics and their ability to perform psychophysical tasks. Here, we directly compare free-viewing and visually guided behavior of marmoset, macaque, and human engaged in identical tasks under similar conditions. In the video free-viewing task, all species exhibited qualitatively similar saccade kinematics up to 25° in amplitude although with different parameters. Furthermore, the conventional bottom-up saliency model predicted gaze targets at similar rates for all species. We further verified their visually guided behavior by training them with step and gap saccade tasks. In the step paradigm, marmosets did not show shorter saccade reaction time for upward saccades whereas macaques and humans did. In the gap paradigm, all species showed similar gap effect and express saccades. Our results suggest that the marmoset can serve as a model for oculomotor, attentional, and cognitive research while we need to be aware of their difference from macaque or human.NEW & NOTEWORTHY We directly compared the results of a video free-viewing task and visually guided saccade tasks (step and gap) among three different species: marmoset, macaque, and human. We found that all species exhibit qualitatively similar saccadic kinematics and saliency-driven saccadic behavior albeit with different parameters. Our results suggest that the marmoset possesses similar neural mechanisms to macaque and human for saccadic control, and it is an appropriate model to study neural mechanisms for active vision and attention.

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.

Gaze direction as equilibrium: more evidence from spatial and temporal aspects of small-saccade triggering in the rhesus macaque monkey

Rigorous behavioral studies made in human subjects have shown that small-eccentricity target displacements are associated with increased saccadic reaction times, but the reasons for this remain unclear. Before characterizing the neurophysiological foundations underlying this relationship between the spatial and temporal aspects of saccades, we tested the triggering of small saccades in the male rhesus macaque monkey. We also compared our results to those obtained in human subjects, both from the existing literature and through our own additional measurements. Using a variety of behavioral tasks exercising visual and nonvisual guidance of small saccades, we found that small saccades consistently require more time than larger saccades to be triggered in the nonhuman primate, even in the absence of any visual guidance and when valid advance information about the saccade landing position is available. We also found a strong asymmetry in the reaction times of small upper versus lower visual field visually guided saccades, a phenomenon that has not been described before for small saccades, even in humans. Following the suggestion that an eye movement is not initiated as long as the visuo-oculomotor system is within a state of balance, in which opposing commands counterbalance each other, we propose that the longer reaction times are a signature of enhanced times needed to create the symmetry-breaking condition that puts downstream premotor neurons into a push-pull regime necessary for rotating the eyeballs. Our results provide an important catalog of nonhuman primate oculomotor capabilities on the miniature scale, allowing concrete predictions on underlying neurophysiological mechanisms.NEW & NOTEWORTHY Leveraging a multitude of neurophysiological investigations in the rhesus macaque monkey, we generated and tested hypotheses about small-saccade latencies in this animal model. We found that small saccades always take longer, on average, than larger saccades to trigger, regardless of visual and cognitive context. Moreover, small downward saccades have the longest latencies overall. Our results provide an important documentation of oculomotor capabilities of an indispensable animal model for neuroscientific research in vision, cognition, and action.

Asymmetries of the visual system and their influence on visual performance and oculomotor dynamics

Our representation of the visual field is not homogenous. There are differences in resolution not only between the fovea and regions eccentric to it, but also between the nasal and temporal hemiretinae, that can be traced to asymmetric distributions of photoreceptors and ganglion cells. We review evidence for differences in visual and attentional processing and oculomotor behaviour that can be traced to asymmetries of the visual system, mainly emphasising nasal-temporal asymmetries. Asymmetries in the visual system manifest in various measures, in basic psychophysical tests of visual performance, attentional processing, choice behaviour, saccadic peak velocity, and latencies. Nasal-temporal asymmetries on saccadic latency seem primarily to occur for express saccades. Neural asymmetries between the upper and lower hemifields are strong and cause corresponding differences in performance between the hemifields. There are interesting individual differences in asymmetric processing which seem to be related to the strength of eye dominance. These neurophysiological asymmetries and the corresponding asymmetries in visual performance and oculomotor behaviour can strongly influence experimental results in vision and must be considered during experimental design and the interpretation of results.

Perisaccadic perceptual mislocalization is different for upward saccades

Saccadic eye movements, which dramatically alter retinal images, are associated with robust perimovement perceptual alterations. Such alterations, thought to reflect brain mechanisms for maintaining perceptual stability in the face of saccade-induced retinal image disruptions, are often studied by asking subjects to localize brief stimuli presented around the time of horizontal saccades. However, other saccade directions are not usually explored. Motivated by recently discovered asymmetries in upper and lower visual field representations in the superior colliculus, a structure important for both saccade generation and visual analysis, we observed significant differences in perisaccadic perceptual alterations for upward saccades relative to other saccade directions. We also found that, even for purely horizontal saccades, perceptual alterations differ for upper vs. lower retinotopic stimulus locations. Our results, coupled with conceptual modeling, suggest that perisaccadic perceptual alterations might critically depend on neural circuits, such as superior colliculus, that asymmetrically represent the upper and lower visual fields. NEW & NOTEWORTHY Brief visual stimuli are robustly mislocalized around the time of saccades. Such mislocalization is thought to arise because oculomotor and visual neural maps distort space through foveal magnification. However, other neural asymmetries, such as upper visual field magnification in the superior colliculus, may also exist, raising the possibility that interactions between saccades and visual stimuli would depend on saccade direction. We confirmed this behaviorally by exploring and characterizing perisaccadic perception for upward saccades.

Dynamics of fixational eye position and microsaccades during spatial cueing: the case of express microsaccades

Microsaccades are systematically modulated by peripheral spatial cues, and these eye movements have been implicated in perceptual and motor performance changes in cueing tasks. However, an additional oculomotor factor that may also influence performance in these tasks, fixational eye position itself, has been largely neglected so far. Using precise eye tracking and real-time retinal-image stabilization, we carefully analyzed fixational eye position dynamics and related them to microsaccade generation during spatial cueing. As expected, during baseline fixation, microsaccades corrected for a foveal motor error away from the preferred retinal locus of fixation (the so-called ocular position "set point" of the oculomotor system). However, we found that this relationship was violated during a short period immediately after cue onset; a subset of cue-directed "express microsaccades" that were highly precise in time and direction, and that were larger than regular microsaccades, occurred. These movements, having <100-ms latencies from cue onset, were triggered when fixational eye position was already at the oculomotor set point when the cue appeared; they were thus error-increasing rather than error-decreasing. Critically, even when no microsaccades occurred, fixational eye position itself was systematically deviated toward the cue, again with ~100-ms latency, suggesting that the oculomotor system establishes a new set point at different postcue times. This new set point later switched to being away from the cue after ~200-300 ms. Because eye position alters the location of retinal images, our results suggest that both eye position and microsaccades can be associated with performance changes in spatial cueing tasks. NEW & NOTEWORTHY Covert spatial cueing tasks are a workhorse for studying cognitive processing in humans and monkeys, but gaze is not perfectly stable during these tasks. We found that minute fixational eye position changes, independent of the more studied microsaccades, are not random in cueing tasks and are thus not "averaged out" in analyses. These changes can additionally dictate microsaccade times. Thus, in addition to microsaccadic influences, retinal image changes associated with fixational eye position are relevant for performance in cueing tasks.

Rapid Processing of a Global Feature in the ON Visual Pathways of Behaving Monkeys

Visual objects are recognized by their features. Whereas, some features are based on simple components (i.e., local features, such as orientation of line segments), some features are based on the whole object (i.e., global features, such as an object having a hole in it). Over the past five decades, behavioral, physiological, anatomical, and computational studies have established a general model of vision, which starts from extracting local features in the lower visual pathways followed by a feature integration process that extracts global features in the higher visual pathways. This local-to-global model is successful in providing a unified account for a vast sets of perception experiments, but it fails to account for a set of experiments showing human visual systems' superior sensitivity to global features. Understanding the neural mechanisms underlying the "global-first" process will offer critical insights into new models of vision. The goal of the present study was to establish a non-human primate model of rapid processing of global features for elucidating the neural mechanisms underlying differential processing of global and local features. Monkeys were trained to make a saccade to a target in the black background, which was different from the distractors (white circle) in color (e.g., red circle target), local features (e.g., white square target), a global feature (e.g., white ring with a hole target) or their combinations (e.g., red square target). Contrary to the predictions of the prevailing local-to-global model, we found that (1) detecting a distinction or a change in the global feature was faster than detecting a distinction or a change in color or local features; (2) detecting a distinction in color was facilitated by a distinction in the global feature, but not in the local features; and (3) detecting the hole was interfered by the local features of the hole (e.g., white ring with a squared hole). These results suggest that monkey ON visual systems have a subsystem that is more sensitive to distinctions in the global feature than local features. They also provide the behavioral constraints for identifying the underlying neural substrates.

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.

Horizontal and vertical eye movement metrics: what is important?

OBJECTIVE

To assist other eye movement investigators in the design and analysis of their studies.

METHODS

We examined basic saccadic eye movements and smooth pursuit in the horizontal and vertical directions with video-oculography in a group of 145 healthy subjects between 19 and 82 years of age.

RESULTS

Gender and education level did not influence eye movement metrics. With age, the latency of leftward and vertical pro- and antisaccades increased (p<0.001), velocity of upward prosaccades decreased (p<0.001), gain of rightward and upward prosaccades diminished (p<0.001), and the error rate of antisaccades increased (p<0.001). Prosaccades and antisaccades were influenced by the direction of the target, resulting in a right/left and up/down asymmetry. The skewness of the saccade velocity profile was stable throughout the lifespan, and within the range of saccades analyzed in the present study, correlated with amplitude and duration only for antisaccades (p<0.001).

CONCLUSIONS

Some eye movement metrics must be separated by the direction of movement, others according to subject age, while others may be pooled.

SIGNIFICANCE

This study provides important information for new oculomotor laboratories concerning the constitution of subject groups and the analysis of eye movement metrics.

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.

Spatiotemporal profiles of visual processing with and without primary visual cortex

The spatiotemporal profiles of visual processing are normally distributed in two temporal phases, each lasting about 100 ms. Within each phase, cortical processing begins in V1 and traverses the visual cortical hierarchy. However, the causal role of V1 in starting each of these two phases is unknown. Here we used magnetoencephalography to study the spatiotemporal profiles of visual processing and the causal contribution of V1 in three neurologically intact participants and in a rare patient (GY) with unilateral destruction of V1, in whom residual visual functions mediated by the extra-geniculostriate pathways have been reported. In healthy subjects, visual processing in the first 200 ms post-stimulus onset proceeded in the two usual phases. Normally perceived stimuli in the left hemifield of GY elicited a spatiotemporal profile in the intact right hemisphere that closely matched that of healthy subjects. However, stimuli presented in the cortically blind hemifield produced no detectable response during the first phase of processing, indicating that the responses in extrastriate visual areas during this phase are determined by the feedforward progression of activity initiated in V1. The first responses occurred during the second processing phase, in the ipsilesional high-level visual areas. The activity then spread forward toward higher-level areas and backward toward lower-level areas. However, in contrast to responses in the intact hemisphere, the back-propagated activity in the early visual cortex did not exhibit the classic retinotopic organization and did not have well-defined response peaks.

Saccadic foveation of a moving visual target in the rhesus monkey

When generating a saccade toward a moving target, the target displacement that occurs during the period spanning from its detection to the saccade end must be taken into account to accurately foveate the target and to initiate its pursuit. Previous studies have shown that these saccades are characterized by a lower peak velocity and a prolonged deceleration phase. In some cases, a second peak eye velocity appears during the deceleration phase, presumably reflecting the late influence of a mechanism that compensates for the target displacement occurring before saccade end. The goal of this work was to further determine in the head restrained monkey the dynamics of this putative compensatory mechanism. A step-ramp paradigm, where the target motion was orthogonal to a target step occurring along the primary axes, was used to estimate from the generated saccades: a component induced by the target step and another one induced by the target motion. Resulting oblique saccades were compared with saccades to a static target with matched horizontal and vertical amplitudes. This study permitted to estimate the time taken for visual motion-related signals to update the programming and execution of saccades. The amplitude of the motion-related component was slightly hypometric with an undershoot that increased with target speed. Moreover, it matched with the eccentricity that the target had 40-60 ms before saccade end. The lack of significant difference in the delay between the onsets of the horizontal and vertical components between saccades directed toward a static target and those aimed at a moving target questions the late influence of the compensatory mechanism. The results are discussed within the framework of the "dual drive" and "remapping" hypotheses.

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.

Effect of vergence on human ocular following response (OFR)

The human ocular following response (OFR) is a preattentive, short-latency visual-field-holding mechanism, which is enhanced if the moving stimulus is applied in the wake of a saccade. Since most natural gaze shifts incorporate both saccadic and vergence components, we asked whether the OFR was also enhanced during vergence. Ten subjects viewed vertically moving sine-wave gratings on a video monitor at 45 cm that had a temporal frequency of 16.7 Hz, contrast of 32%, and spatial frequency of 0.17, 0.27, or 0.44 cycle/deg. In Fixation/OFR experiments, subjects fixed on a white central dot on the video monitor, which disappeared at the beginning of each trial, just as the sinusoidal grating started moving up or down. We measured the change in eye position in the 70- to 150-ms open-loop interval following stimulus onset. Group mean downward responses were larger (0.14 degrees) and made at shorter latency (85 ms) than upward responses (0.10 degrees and 96 ms). The direction of eye drifts during control trials, when gratings remained stationary, was unrelated to the prior response. During vergence/OFR experiments, subjects switched their fixation point between the white dot at 45 cm and a red spot at 15 cm, cued by the disappearance of one target and appearance of the other. When horizontal vergence velocity exceeded 15 degrees/s, motion of sinusoidal gratings commenced and elicited the vertical OFR. Subjects showed significantly (P<0.001) larger OFR when the moving stimulus was presented during convergence (group mean increase of 46%) or divergence (group mean increase of 36%) compared with following fixation. Since gaze shifts between near and far are common during natural activities, we postulate that the increase of OFR during vergence movements reflects enhancement of early cortical motion processing, which serves to stabilize the visual field as the eyes approach their new fixation point.

The control of vertical saccades in aged subjects

In real life we produce vertical saccades at different distances and eccentricities, and while our fixation is more or less actively engaged. The goal of this study is to examine vertical saccades in aged and young subjects, taking into consideration all these parameters. Eleven adults (20-28 years) and 11 aged subjects (63-83 years) were recruited. We used LED targets at 7.5 degrees or 15 degrees, up or down in four conditions: gap and overlap tasks, each done at two distances-at near (40 cm) and at far (150 cm). In the gap task fixation target extinguishes prior to target onset, while in the overlap condition it stays on after target onset; consequently, visual attention and fixation are employed differently in the two tasks. Eye movements were recorded with the Chronos video eye tracker. Results showed that vertical saccades were longer for aged subjects than for young adults under almost all conditions. For both aged and young subjects, latencies were shorter under the gap than under the overlap task. Latencies for eccentric targets at 15 degrees were significantly longer than those at 7.5 degrees but for aged subjects only; this effect was more pronounced for upward saccades under the overlap condition. Express type of latencies (80-120 ms) occurred frequently in the gap task and at similar rates for young adults (16%) and aged subjects (12%); in the overlap task express latencies were scarce in young adults (0.4%) and aged subjects (1.8%). Age deteriorates the ability to trigger regular volitional saccades but not the ability to produce express type of saccades. Latency increase with aging is attributed to the degeneration of central areas, e.g. oculomotor cortical areas involved in the initiation of vertical saccades.

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.

Why your "head is in the clouds" during thinking: the relationship between cognition and upper space

Higher-order cognition in humans has not generally been viewed as closely entwined with the brain mechanisms mediating more basic perceptual-motor interactions in 3-D space. However, recent findings suggest that perceptual and oculomotor mechanisms that are biased toward the upper field (which disproportionately represents radially distant space) are activated during complex mental operations, ranging from semantic processing to mental arithmetic and memory search. The particularly close affinity with upward conjugate eye deviations--further confirmed in a study of 24 schoolchildren who responded to various mental questions and demands--suggests that active, abstract thinking in humans may have expropriated the focal-extrapersonal brain systems involved in saccadic exploration of the distant environment in other primates.

Saccade dysmetria in Williams–Beuren syndrome

Numerous studies have described the poor visuo-spatial processing capacities of subjects with Williams-Beuren syndrome (WBS), a genetically based developmental disorder. Since visual perception and eye movements are closely related we hypothesized that the poor visuo-spatial processing capacities of subjects with WBS might be related to a poor saccadic control. Thereto, we recorded horizontal and vertical saccadic eye movements to targets using infrared video-oculography in 27 subjects with WBS and eight healthy controls. In the WBS group saccadic gains were highly variable, both between and within individual subjects, and they often needed more than one correction saccade to reach the target. Ten (out of a subgroup of 22) WBS subjects showed a large number of hypometric and/or hypermetric saccades, and, also a left-right asymmetry in saccadic gains was observed in WBS. We conclude that the observed impairments in saccadic control are likely to affect the proper processing of visuo-spatial information.