Initial Saccade Research Articles

Eye movements when reading spaced and unspaced Thai and English: A comparison of Thai–English bilinguals and English monolinguals

The study investigated the eye movements of Thai–English bilinguals when reading both Thai and English with and without interword spaces, in comparison with English monolinguals. Thai is an alphabetic orthography without interword spaces. Participants read sentences with high and low frequency target words embedded in same sentence frames with and without interword spaces. Interword spaces had a selective effect on reading in Thai, as they facilitated word recognition, but did not affect eye guidance and lexical segmentation. Initial saccade landing positions were similar in spaced and unspaced text. As expected, removal of spaces severely disrupted reading in English, as reflected by the eye movement measures, in both bilinguals and monolinguals. Here, initial landing positions were significantly nearer the beginning of the target words when reading unspaced rather than spaced text. Effects were more accentuated in the bilinguals. In sum, results from reading in Thai give qualified support for a facilitatory function of interword spaces.

Firing Patterns in Superior Colliculus of Head-Unrestrained Monkey during Normal and Perturbed Gaze Saccades Reveal Short-Latency Feedback and a Sluggish Rostral Shift in Activity

The superior colliculus (SC) encodes a saccade via the spatial position of an ensemble of active neurons on its motor map. Downstream circuits control muscles with the temporal code of firing frequency and duration. The moving hill hypothesis resolves the SC-to-brainstem spatiotemporal transformation (STT) enigma by proposing feedback to the SC which "pushes" a hill of activity (height = frequency) caudorostrally such that its instantaneous position encodes the angular error [gaze-position error (GPE)] between gaze and target. This mechanism, proposed for cat but controversial in primate, has not been tested in the head-unrestrained monkey. We do this here. During large approximately 60 degrees control gaze shifts in the dark, a hill of activity began in the caudal SC and moved rostrally, but too sluggishly to encode veridical GPE. At gaze end the peak had not reached the rostral pole and only arrived there approximately 80 ms later. No moving hill accompanied approximately 20 degrees gaze shifts, in agreement with previous studies of head-fixed monkeys. To investigate feedback to the SC, we perturbed large gaze shifts producing initial and corrective gaze saccades separated by 50-800 ms of gaze immobility, the gaze plateau. Map activity was dramatically remodeled: the sluggishly moving hill stopped during the plateau, at the site encoding the corrective gaze saccade, thereby providing a tonic stationary spatially encoded memory signal of plateau GPE. A burst occurred before the corrective saccade. Feedback to map moves activity which encodes a low-pass filtered GPE signal, a process too slow to implement the STT but adequate for corrective gaze saccades.

Short article: Does conspicuity enhance distraction? Saliency and eye landing position when searching for objects

While visual saliency may sometimes capture attention, the guidance of eye movements in search is often dominated by knowledge of the target. How is the search for an object influenced by the saliency of an adjacent distractor? Participants searched for a target amongst an array of objects, with distractor saliency having an effect on response time and on the speed at which targets were found. Saliency did not predict the order in which objects in target-absent trials were fixated. The within-target landing position was distributed around a modal position close to the centre of the object. Saliency did not affect this position, the latency of the initial saccade, or the likelihood of the distractor being fixated, suggesting that saliency affects the allocation of covert attention and not just eye movements.

Correction of Saccade-Induced Midline Errors in Responses to Pure Disparity Vergence Stimuli.

Purely symmetrical vergence stimuli aligned along the midline (cyclopean axis) require only a pure vergence response. Yet, in most responses saccades are observed and these saccades must either produce an error in the desired midline response or correct an error produced by asymmetry in the vergence response. A previous study (Semmlow, et al. 2008) has shown that the first saccade to appear in a response to a pure vergence stimulus usually increased the deviation from the midline, although all subjects (N = 12) had some responses where the initial saccade corrected a vergence induced midline error. This study focuses on those responses where the initial saccade produces an increased midline deviation and the resultant compensation that ultimately brings the eyes to the correct binocular position. This correction is accomplished by a higher level compensatory mechanism that uses offsetting asymmetrical vergence and/or corrective saccades. While responses consist of a mixture of the two compensatory mechanisms, the dominant mechanism is subject-dependent. Since fixation errors are quite small (minutes of arc), some feedback controlled physiological process involving smooth eye movements, and possibly saccades, must move the eyes to reduce binocular error to fixation disparity levels.

Viewpoint and center of gravity affect eye movements to human faces

In everyday life, human faces are encountered in many different views. Despite this fact, most psychological research has focused on the perception of frontal faces. To address this shortcoming, the current study investigated how different face views are processed, by measuring eye movements to frontal, mid-profile and profile faces during a gender categorization (Experiment 1) and a free-viewing task (Experiment 2). In both experiments observers initially fixated the geometric center of a face, independent of face view. This center-of-gravity effect induced a qualitative shift in the features that were sampled across different face views in the time period immediately after stimulus onset. Subsequent eye fixations focused increasingly on specific facial features. At this stage, the eye regions were targeted predominantly in all face views, and to a lesser extent also the nose and the mouth. These findings show that initial saccades to faces are driven by general stimulus properties, before eye movements are redirected to the specific facial features in which observers take an interest. These findings are illustrated in detail by plotting the distribution of fixations, first fixations, and percentage fixations across time.

Suboptimal selection of initial saccade in a visual search task

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Suboptimal selection of initial saccade in a visual search task

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Development of Working Memory Maintenance

The neural circuitry supporting mature visual spatial working memory (VSWM) has been well delineated in nonhuman primates and in human adults. However, we still have limited understanding about developmental change through adolescence in this network. We present results from a fast event-related functional MRI (fMRI) study aimed at characterizing developmental changes in brain mechanisms supporting VSWM across different delay periods. Forty-three healthy subjects (17 adults, 18-30 yr; 13 adolescents, 13-17 yr; 13 children, 8-12 yr) were scanned as they performed an oculomotor delayed response (ODR) task with short (2.5 s) and long (10 s) delay period trials. Results showed that all age groups recruited a common network of regions to support both delay trials, including frontal, parietal, and temporal regions, indicative of a core circuitry needed to perform the task. Several age-related differences were found in the recruitment of regions, supporting short delay trials, including fronto-caudal areas, which could contribute to known differences in initial memory-guided saccade precision. To support extended delay trials, adults primarily recruited additional posterior parietal cortex (PPC), whereas children and adolescents recruited a considerably more extensive distributed circuitry. Our findings indicate that brain processes supporting basic aspects of working memory across cortex are established by childhood. We also find evidence for continued immaturities in systems supporting working memory precision, reflected by differences in the circuitry recruited by children and by continued refinement of fronto-insular-temporal regions recruited by adolescents. Taken together, these results suggest distinct developmental changes in the circuitry supporting visual spatial working memory.

The planning of a sequence of saccades in pro- and antisaccade tasks: Influence of visual integration time and concurrent motor processing

Previous studies have shown that a saccade is coded in a specific reference frame according to its goal: to aim for a new object or to explore an object which has already been fixated. In a two saccade sequence, the second saccade aiming for a new object is programmed in a retinocentric reference frame in which the spatial location of the second object is stored in spatial memory before the first saccade and updated after its execution. The second saccade exploring the same object is coded in an oculocentric reference frame in which object size is directly transformed into a fixed motor vector, encoded in motor memory before the first saccade and simply applied after its execution. The integration of parafoveal visual information appears to be crucial in the selection of the appropriate reference frame. The two experiments presented here investigate how and when the saccadic system integrates visual information to plan a sequence of saccades. In separate blocks, subjects were asked to execute a sequence of prosaccades directed toward a single object or two short objects, or to execute a sequence of antisaccades in the opposite direction of the stimuli. The latency of the initial saccade was modulated by using the Gap-200, Gap-0 and Overlap-600 ms paradigms. The results show that the time available for segmenting the visual stimulation into discrete objects and application of a specific reference frame according to this segmentation is critical for saccadic planning.

Dynamics of attention during the initiation of smooth pursuit eye movements

Many studies indicate that saccades are necessarily preceded by a shift of attention to the target location. There is no direct evidence for the same coupling during smooth pursuit. If smooth pursuit and attention were coupled, pursuit onset should be delayed whenever attention is focused on a stationary, non-target location. To test this hypothesis, observers were instructed to shift their attention to a peripheral location according to a location cue (Experiments 1 and 2) or a symbolic cue (Experiment 3) around the time of smooth pursuit initiation. Attending to static targets had only negligible effects on smooth pursuit latencies and the early open-loop response but lowered pursuit velocity substantially about the onset of closed-loop pursuit. Around this time, eye velocity reflected the competition between the to-be-tracked and to-be-attended object motion, entailing a reduction of eye velocity by 50% compared to the single task condition. The precise time course of attentional modulation of smooth pursuit initiation was at odds with the idea that an attention shift must precede any voluntary eye movement. Finally, the initial catch-up saccades were strongly delayed with attention diverted from the pursuit target. Implications for models of target selection for pursuit and saccades are discussed.

Control of Predictive Error Correction During a Saccadic Double-Step Task

We explored the nature of control during error correction using a modified saccadic double-step task in which subjects cancelled the initial saccade to the first target and redirected gaze to a second target. Failure to inhibit was associated with a quick corrective saccade, suggesting that errors and corrections may be planned concurrently. However, because saccade programming constitutes a visual and a motor stage of preparation, the extent to which parallel processing occurs in anticipation of the error is not known. To estimate the time course of error correction, a triple-step condition was introduced that displaced the second target during the error. In these trials, corrective saccades directed at the location of the target prior to the third step suggest motor preparation of the corrective saccade in parallel with the error. To estimate the time course of motor preparation of the corrective saccade, further, we used an accumulator model (LATER) to fit the reaction times to the triple-step stimuli; the best-fit data revealed that the onset of correction could occur even before the start of the error. The estimated start of motor correction was also observed to be delayed as target step delay decreased, suggesting a form of interference between concurrent motor programs. Taken together we interpret these results to indicate that predictive error correction may occur concurrently while the oculomotor system is trying to inhibit an unwanted movement and suggest how inhibitory control and error correction may interact to enable goal-directed behaviors.

Eye-catching: Right hemisphere attentional bias for emotional pictures

To examine whether healthy individuals spontaneously direct attention to emotional pictures, eye movements were monitored continuously during the presentation of pairs of emotional and neutral pictures. A right hemisphere advantage was expected for initial orienting to emotional pictures. When an emotionally arousing picture was presented in the left visual hemifield, i.e., to the right hemisphere, the initial saccade was more often issued towards it, irrespective of valence. Negative pictures did not lead to avoidance throughout the 8-second trials; rather, both positive and negative valence held attention. This study adds to the evidence that overt attention to visual stimuli is boosted by emotion and that there is a right hemisphere advantage for orienting to emotional cues.

Head impulse test in unilateral vestibular loss

Quantitative head impulse test (HIT) measures the gain of the angular vestibulo-ocular reflex (VOR) during head rotation as the ratio of eye to head acceleration. Bedside HIT identifies subsequent catch-up saccades after the head rotation as indirect signs of VOR deficit. To determine the VOR deficit and catch-up saccade characteristics in unilateral vestibular disease in response to HIT of varying accelerations. Eye and head rotations were measured with search coils during manually applied horizontal HITs of varying accelerations in patients after vestibular neuritis (VN, n = 13) and unilateral vestibular deafferentation (UVD, n = 15) compared to normal subjects (n = 12). Normal VOR gain was close to unity and symmetric over the entire head-acceleration range. Patients with VN and UVD showed VOR gain asymmetry, with larger ipsilesional than contralesional deficits. As accelerations increased from 750 to 6,000 degrees /sec(2), ipsilesional gains decreased from 0.59 to 0.29 in VN and from 0.47 to 0.13 in UVD producing increasing asymmetry. Initial catch-up saccades can occur during or after head rotation. Covert saccades during head rotation are most likely imperceptible, while overt saccades after head rotation are detectable by clinicians. With increasing acceleration, the amplitude of overt saccades in patients became larger; however, initial covert saccades also became increasingly common, occurring in up to about 70% of trials. Head impulse test (HIT) with high acceleration reveals vestibulo-ocular reflex deficits better and elicits larger overt catch-up saccades in unilateral vestibular patients. Covert saccades during head rotation, however, occur more frequently with higher acceleration and may be missed by clinicians. To avoid false-negative results, bedside HIT should be repeated to improve chances of detection.

Directing visual attention with spatially informative and spatially noninformative tactile cues

We investigated the tactile cuing of visual spatial attention using spatially-informative (75% valid) and spatially-noninformative (25% valid) tactile cues. The participants performed a visual change detection task following the presentation of a tactile spatial cue on their back whose location corresponded to one of the four visual quadrants on a computer monitor. The participants were explicitly instructed to use the spatially-informative tactile cues but to ignore the spatially-noninformative cues. In addition to reaction time data, participants' eye-gaze was monitored as a measure of overt visual attention. The results showed that the spatially-informative tactile cues resulted in initial saccades toward the cued visual quadrants, and significantly reduced the visual change detection latencies. When spatially-noninformative tactile cues were used, the participants were largely successful at ignoring them as indicated by a saccade distribution that was independent of the quadrant that was cued, as well as the lack of a significant change in search time as compared to the baseline measure of no tactile cuing. The eye-gaze data revealed that the participants could not always completely ignore the spatially-noninformative tactile cues. Our results suggest that the tactile cuing of visual attention is natural but not automatic when the tactile cue and visual target are not collocated spatially, and that it takes effort to ignore the cues even when they are known to provide no useful information. In addition, our results confirm previous findings that spatially-informative tactile cues are especially effective at directing overt visual attention to locations that are not typically monitored visually, such as the bottom of a computer screen or the rearview mirror in an automobile.

Square Wave Jerks in Children and Adolescents

Square wave jerks are involuntary, horizontal, saccadic intrusions that interrupt fixation. Each square wave jerk consists of an initial saccade that moves the fovea away from the intended position of fixation, followed by a second saccade in the opposite direction, which refoveates the fixation position. Square wave jerks reportedly occur in 24-60% of healthy adults. No previous study of square wave jerks in children and adolescents is available. We recorded eye movements using an infrared eye tracker in 38 participants aged 8-19 years while they fixated on a visual target for 1 minute. The frequency of square wave jerks, and the durations, amplitudes, and peak velocities of their saccades, were calculated and correlated with age. Ninety percent of participants had square wave jerks. Their median frequency was 3 per minute (range, 1-18), median duration was 249 milliseconds, the median amplitude of their saccades was 0.81 degrees, and the median peak velocity was 60 degrees/second. No parameter of square wave jerks correlated with age. The prevalence of square wave jerks is high in children and adolescents. This finding may be a feature of the less mature brain, and may reflect an inability to suppress unwanted supranuclear triggers for saccades.

Double saccadic pulses and macrosaccadic oscillations from a focal brainstem lesion

Double saccadic pulses (DSP) are saccadic intrusions that consist of an initial saccade away from a fixation followed immediately by a return saccade back to the fixation. DSP have been reported in patients with presumed multiple sclerosis and metabolic encephalopathy. However, DSP have not been described in a circumscribed brain lesion. We report a man who developed almost continuous DSP with intervening macrosaccadic oscillations from a circumscribed lesion in the dorsal pontine tegmentum which extended up to the midbrain level and spared the nucleus raphe interpositus where the pause cells reside. Damage to the projections from the superior colliculus to omnipause neurons and resultant dysfunction of omnipause neuron may be a mechanism of saccadic intrusions and oscillations observed in our patient with a circumscribed brainstem lesion.

Two Distinct Visual Motion Mechanisms for Smooth Pursuit: Evidence from Individual Differences

Smooth-pursuit eye velocity to a moving target is more accurate after an initial catch-up saccade than before, an enhancement that is poorly understood. We present an individual-differences-based method for identifying mechanisms underlying a physiological response and use it to test whether visual motion signals driving pursuit differ pre- and postsaccade. Correlating moment-to-moment measurements of pursuit over time with two psychophysical measures of speed estimation during fixation, we find two independent associations across individuals. Presaccadic pursuit acceleration is predicted by the precision of low-level (motion-energy-based) speed estimation, and postsaccadic pursuit precision is predicted by the precision of high-level (position-tracking) speed estimation. These results provide evidence that a low-level motion signal influences presaccadic acceleration and an independent high-level motion signal influences postsaccadic precision, thus presenting a plausible mechanism for postsaccadic enhancement of pursuit.

Eye movements during rapid pointing under risk

We recorded saccadic eye movements during visually-guided rapid pointing movements under risk. We intended to determine whether saccadic end points are necessarily tied to the goals of rapid pointing movements or whether, when the visual features of a display and the goals of a pointing movement are different, saccades are driven by low-level features of the visual stimulus. Subjects pointed at a stimulus configuration consisting of a target region and a penalty region. Each target hit yielded a gain of points; each penalty hit incurred a loss of points. Late responses were penalized. The luminance of either target or penalty region was indicated by a disk which differed significantly from the background in luminance, while the other region was indicated by a thin circle. In subsequent experiments, we varied the visual salience of the stimulus configuration and found that manual responses followed near-optimal strategies maximizing expected gain, independent of the salience of the target region. We suggest that the final eye position is partially pre-programmed prior to hand movement initiation. While we found that manipulations of the visual salience of the display determined the end point of the initial saccade we also found that subsequent saccades are driven by the goal of the hand movement.

Saccade trajectory under simultaneous inhibition for two locations

A saccade trajectory often curves away from the location of a non-target stimulus that appears before saccade execution. Spatial inhibition may prevent the saccade from moving toward the non-target stimulus. However, little is known about how simultaneous inhibition for multiple locations affects saccade trajectories. In this study, we examined the effects from two inhibited locations on saccade trajectories. The results show that the saccade trajectories depend on the inhibited locations, and the effect of inhibiting two locations on the trajectory was a summation of the effect of inhibiting each location. A simulation study using the initial interference model also suggests that the effect of each inhibition was summed up to modulate the initial saccade direction.

Being “slow to see” is a dynamic visual function consequence of infantile nystagmus syndrome: Model predictions and patient data identify stimulus timing as its cause

The objective of this study was to investigate the dynamic properties of infantile nystagmus syndrome (INS) that affect visual function; i.e., which factors influence latency of the initial reflexive saccade (Ls) and latency to target acquisition (Lt). We used our behavioral ocular motor system (OMS) model to simulate saccadic responses (in the presence of INS) to target jumps at different times within a single INS cycle and at random times during multiple cycles. We then studied the responses of 4 INS subjects with different waveforms to test the model’s predictions. Infrared reflection was used for 1 INS subject, high-speed digital video for 3. We recorded and analyzed human responses to large and small target-step stimuli. We evaluated the following factors: stimulus time within the cycle (Tc), normalized Tc (Tc%), initial orbital position (Po), saccade amplitude, initial retinal error ( e i), and final retinal error ( e f). The ocular motor simulations were performed in MATLAB Simulink environment and the analysis was performed in MATLAB environment using OMLAB software. Both the OMS model and OMtools software are available from http://http:www.omlab.org. Our data analysis showed that for each subject, Ls was a fixed value that is typically higher than the normal saccadic latency. Although saccadic latency appears somewhat lengthened in INS, the amount is insufficient to cause the “slow-to-see” impression. For Lt, Tc% was the most influential factor for each waveform type. The main refixation strategies employed by INS subjects made use of slow and fast phases and catch-up saccades, or combinations of them. These strategies helped the subjects to foveate effectively after target movement, sometimes at the cost of increased target acquisition time. Foveating or braking saccades intrinsic to the nystagmus waveforms seemed to disrupt the OMS’ ability to accurately calculate reflexive saccades’ amplitude and refoveate. Our OMS model simulations demonstrated this emergent behavior and predicted the lengthy target acquisition times found in the patient data.