Gap duration and location of attention focus modulate the occurrence of left/right asymmetries in the saccadic reaction times of human subjects

The adaptive global effect: Luminance contrast modulates the global effect zone

When two peripheral objects are presented in close proximity, saccades towards one of these objects land at a weighted average location between the two objects. This phenomenon, known as the 'global effect' or 'saccade averaging', disappears when the distance between the objects increases. When objects are further apart, outside the averaging zone, saccades land on one of the objects with little or no saccade averaging. Although it is known that the strength of the global effect is dependent on the specific features of the two objects, it is unclear if the size of the zone in which averaging can occur (i.e., the averaging zone) is adaptive. The aim of the current study was to investigate whether the size of the averaging zone adapts to variations in object luminance contrast of the objects. In order to systematically assess changes in the averaging zone, in two experiments, observers made saccadic eye movements while the luminance of the target and the distractor varied. We report three major findings: 1) When a distractor was more luminant relative to the target, the averaging zone increased (Exp. 1). Notably, saccade averaging never entirely ceased to exist, even for remote distractors. 2) When target and distractor were equiluminant, the averaging zone did not change with absolute luminance (Exp. 2). 3) Higher (relative and absolute) luminance increased the averaging zone especially for shorter saccadic response times (SRT). We conclude that the averaging zone is adaptive and becomes larger with increasing relative luminance and especially when SRTs are short.

Eye movement patterns in complex tasks: Characteristics of ambient and focal processing

Analyzing the time course of eye movements during scene viewing often indicates that people progress through two distinct modes of visual processing: an ambient mode, which is associated with overall spatial orientation in a scene, followed by a focal mode, which requires central vision of an object. However, the shifts between ambient and focal processing modes have mainly been identified relative to changes in the environment, such as relative to the onset of various visual stimuli but also following scene cuts or subjective event boundaries in dynamic stimuli. The results so far do not allow conclusions about the nature of the two processing mechanisms beyond the influence of externally triggered events. It remains unclear whether people shift back and forth from ambient to focal processing also based on internal triggers, such as switching between different tasks while no external event is given. The present study therefore investigated ambient to focal processing shifts in an active task solving paradigm. The Rubik’s Cube task introduced here is a multi-step task, which can be broken down into smaller sub-tasks that are performed serially. The time course of eye movements was analyzed at multiple levels of this Rubik’s Cube task, including when there were no external changes to the stimuli but when internal representations of the task were hypothesized to change (i.e., switching between different sub-tasks). Results suggest that initial ambient exploration is followed by a switch to more focal viewing across various levels of task processing with and without external changes to the stimuli. More importantly, the present findings suggest that ambient and focal eye movement characteristics might serve as a probe for the attentional state in task processing, which does not seem to be influenced by changes in task performance.

Multimodal Neurocognitive Screening of Military Personnel With a History of Mild Traumatic Brain Injury Using the Bethesda Eye & Attention Measure

OBJECTIVE

To evaluate a neurocognitive eye tracking task, the Bethesda Eye & Attention Measure (BEAM), for use in cognitive screening of patients with a history of mild traumatic brain injury (TBI).

SETTING

US military TBI rehabilitation clinic.

DESIGN/PARTICIPANTS

Cross-sectional study of 191 military personnel receiving outpatient services related to history of mild TBI. Main measures: BEAM; neuropsychological screening measures of attention, processing speed, executive function, and memory.

RESULTS

Medium effect sizes were found for partial correlations (controlling for age) between key BEAM metrics and neuropsychological screening tests. Linear regression analyses demonstrated that BEAM saccadic eye movements and manual (button press) metrics each provided complementary value in measurement of cognitive performance, above and beyond effects of demographic factors and clinical characteristics.

CONCLUSION

This study provides initial support for the use of BEAM neurocognitive eye tracking in cognitive screening of adults with a history of mild TBI. BEAM saccadic metrics appear to be particularly well-suited for the assessment of visual attention. Study findings also highlight opportunities for greater cognitive sensitivity or testing efficiency that may be missed by tests measuring only one response modality at a time.

Differential diagnosis of vergence and saccade disorders in dyslexia

Previous studies suggest vergence and saccade abnormalities in dyslexic adolescents. However, these studies are mainly clinically based and do not provide objective measurements of eye movements, but rather subjectively evaluate vergence using haplosopic conditions in which the two eyes are dissociated (via polarizers, prisms, or intermittent spectacles). Other studies have identified deficits with binocular coordination during reading in dyslexics. Yet, there are few studies that provide objective measurements of eye movements in the dyslexic population to help provide more information regarding if these deficits could be due to an intrinsic motor problem or if they are the consequence of poor reading. 47 dyslexic adolescents (18 female, 29 male; mean age 15.5) and 44 non-dyslexic adolescents (22 female, 22 male; mean age 14.8) wore a head-based eye tracker (PupilCore, Pupil Labs, Berlin) which recorded wide angle saccade and vergence eye movements at 200 Hz. Tests were run using the REMOBI device, which produced a saccade or vergence audiovisual target. Analysis of eye movements was performed with lab-developed software, AIDEAL. The results showed statistically significant abnormalities in vergence and saccades. In vergence, dyslexics displayed a reduced amplitude of the visually driven portion of convergence and a longer duration in the initial phase of divergence. In saccades, dyslexic adolescents demonstrated slower saccades in both directions. They also had an increased disconjugate drift in the first 80 or 160 ms following saccades to the right, suggesting poor binocular coordination. For both vergence and saccades, the peak velocity and time to peak velocity was higher and earlier, respectively, in non-dyslexics compared to dyslexics; yet the average velocity of both movements was lower in dyslexics. Thus, these results indicate peculiar velocity profiles in dyslexics, particularly a slow deceleration phase in both vergence and saccades. The study provides an objective method to diagnose vergence and saccade abnormalities while viewing targets in the real three-dimensional space in a dyslexic population. Vergence abnormalities are demonstrated to be a problem in dyslexics, occurring independently from reading. We hypothesize these disconjugate drifts following saccades are the result of slow vergence capacity. Rehabilitation programs, such as those using REMOBI, should aim to target these deficits in vergence velocity, as this has been shown to improve binocular control.

Eye tracking to explore attendance in health-state descriptions

INTRODUCTION

A crucial assumption in health valuation methods is that respondents pay equal attention to all information components presented in the response task. So far, there is no solid evidence that respondents are fulfilling this condition. The aim of our study is to explore the attendance to various information cues presented in the discrete choice (DC) response tasks.

METHODS

Eye tracking was used to study the eye movements and fixations on specific information areas. This was done for seven DC response tasks comprising health-state descriptions. A sample of 10 respondents participated in the study. Videos of their eye movements were recorded and are presented graphically. Frequencies were computed for length of fixation and number of fixations, so differences in attendance were demonstrated for particular attributes in the tasks.

RESULTS

All respondents completed the survey. Respondents were fixating on the left-sided health-state descriptions slightly longer than on the right-sided. Fatigue was not observed, as the time spent did not decrease in the final response tasks. The time spent on the tasks depended on the difficulty of the task and the amount of information presented.

DISCUSSION AND CONCLUSION

Eye tracking proved to be a feasible method to study the process of paying attention and fixating on health-state descriptions in the DC response tasks. Eye tracking facilitates the investigation of whether respondents fully read the information in health descriptions or whether they ignore particular elements.

Multimodal assessment of visual attention using the Bethesda Eye & Attention Measure (BEAM)

INTRODUCTION

Computerized cognitive tests measuring manual response time (RT) and errors are often used in the assessment of visual attention. Evidence suggests that saccadic RT and errors may also provide valuable information about attention. This study was conducted to examine a novel approach to multimodal assessment of visual attention incorporating concurrent measurements of saccadic eye movements and manual responses.

METHOD

A computerized cognitive task, the Bethesda Eye & Attention Measure (BEAM) v.34, was designed to evaluate key attention networks through concurrent measurement of saccadic and manual RT and inhibition errors. Results from a community sample of n = 54 adults were analyzed to examine effects of BEAM attention cues on manual and saccadic RT and inhibition errors, internal reliability of BEAM metrics, relationships between parallel saccadic and manual metrics, and relationships of BEAM metrics to demographic characteristics.

RESULTS

Effects of BEAM attention cues (alerting, orienting, interference, gap, and no-go signals) were consistent with previous literature examining key attention processes. However, corresponding saccadic and manual measurements were weakly related to each other, and only manual measurements were related to estimated verbal intelligence or years of education.

CONCLUSIONS

This study provides preliminary support for the feasibility of multimodal assessment of visual attention using the BEAM. Results suggest that BEAM saccadic and manual metrics provide divergent measurements. Additional research will be needed to obtain comprehensive normative data, to cross-validate BEAM measurements with other indicators of neural and cognitive function, and to evaluate the utility of these metrics within clinical populations of interest.

Temporal and Spatial Characteristics of Attention to Facilitate Manual and Eye-Movement Responses

Previous investigation found that the speed of saccadic eye movements is enhanced when a temporal interval (gap) is introduced between the disappearance of a foveal fixation mark and the appearance of a peripheral target (the gap paradigm). Attention was shown to be involved in the gap paradigm. Here, we investigated relevant temporal and spatial characteristics of attention, manipulating central fixation marks and peripheral targets. Results from three experiments indicate that (i) the speed of manual and eye-movement detection is accelerated when a fixation mark changes abruptly (in less than 100 ms) before its termination in the gap paradigm; (ii) the speed is further accelerated when a peripheral target location is pre-cued; (iii) sufficient time for fixation (1000 ms) is necessary for the facilitation. These results suggest that fast and transient attention at the fixation spot facilitates attentional disengagement process that urges a spatial-orienting mechanism. Sustained attention is required in the engagement process during the fixation.

Perimetric evaluation of saccadic latency, saccadic accuracy, and visual threshold for peripheral visual stimuli in young compared with older adults

PURPOSE

Using a novel automated perimetry technique, we tested the hypothesis that older adults will have increased latency and decreased accuracy of saccades, as well as higher visual thresholds, to peripheral visual stimuli when compared with younger adults.

METHODS

We tested 20 healthy subjects aged 18 to 30 years ("young") and 21 healthy subjects at least 60 years old ("older") for detection of briefly flashed peripheral stimuli of differing sizes in eight locations along the horizontal meridian (±4°, ±12°, ±20°, and ±28°). With the left eye occluded, subjects were instructed to look quickly toward any seen stimuli. Right eye movements were recorded with an EyeLink 1000 infrared camera system. Limiting our analysis to the four stimulus positions in the nasal hemifield (-4°, -12°, -20°, and -28°), we evaluated for group-level differences in saccadic latency, accuracy, and visual threshold at each stimulus location.

RESULTS

Saccadic latency increased as stimulus size decreased in both groups. Older subjects had significantly increased saccadic latencies (at all locations; P < 0.05), decreased accuracies (at all locations; P < 0.05), and higher visual thresholds (at the -12°, -20°, and -28° locations; P < 0.05). Additionally, there were significant relationships between visual threshold and latency, visual threshold and accuracy, and latency and accuracy (P < 0.0001).

CONCLUSIONS

Older adults have increased latency and decreased accuracy of saccades, as well as higher visual thresholds, to peripheral visual stimuli when compared with younger adults. Saccadic latency and accuracy are related to visual threshold, suggesting that saccadic latency and accuracy could be useful as perimetric outcome measures.

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.

Fall prevention modulates decisional saccadic behavior in aging

As society ages and frequency of falls increases in older adults, counteracting motor decline is a challenging issue for developed countries. Physical activity based on aerobic and strength training as well as motor activity based on skill learning both help benefit balance and reduce the risk of falls, as assessed by clinical or laboratory measures. However, how such programs influence motor control is a neglected issue. This study examined the effects of fall prevention (FP) training on saccadic control in older adults. Saccades were recorded in 12 participants aged 64–91 years before and after 2.5 months training in FP. Traditional analysis of saccade timing and dynamics was performed together with a quantitative analysis using the LATER model, enabling us to examine the underlying motor control processes. Results indicated that FP reduced the rate of anticipatory and express saccades in inappropriate directions and enhanced that of express saccades in the appropriate direction, resulting in decreased latency and higher left-right symmetry of motor responses. FP reduced within-participant variability of saccade duration, amplitude, and peak velocity. LATER analysis suggested that FP modulates decisional thresholds, extending our knowledge of motor training influence on central motor control. We introduce the Threshold Interval Modulation with Early Release-Rate of rIse Deviation with Early Release (TIMER-RIDER) model to account for the results.

Saccades to a remembered location elicit spatially specific activation in human retinotopic visual cortex

The possible impact upon human visual cortex from saccades to remembered target locations was investigated using functional magnetic resonance imaging (fMRI). A specific location in the upper-right or upper-left visual quadrant served as the saccadic target. After a delay of 2,400 msec, an auditory signal indicated whether to execute a saccade to that location (go trial) or to cancel the saccade and remain centrally fixated (no-go). Group fMRI analysis revealed activation specific to the remembered target location for executed saccades, in the contralateral lingual gyrus. No-go trials produced similar, albeit significantly reduced, effects. Individual retinotopic mapping confirmed that on go trials, quadrant-specific activations arose in those parts of ventral V1, V2, and V3 that coded the target location for the saccade, whereas on no-go trials, only the corresponding parts of V2 and V3 were significantly activated. These results indicate that a spatial-motor saccadic task (i.e., making an eye movement to a remembered location) is sufficient to activate retinotopic visual cortex spatially corresponding to the target location, and that this activation is also present (though reduced) when no saccade is executed. We discuss the implications of finding that saccades to remembered locations can affect early visual cortex, not just those structures conventionally associated with eye movements, in relation to recent ideas about attention, spatial working memory, and the notion that recently activated representations can be "refreshed" when needed.

Neurophysiology and neuroanatomy of reflexive and volitional saccades: evidence from studies of humans

This review provides a summary of the contributions made by human functional neuroimaging studies to the understanding of neural correlates of saccadic control. The generation of simple visually guided saccades (redirections of gaze to a visual stimulus or pro-saccades) and more complex volitional saccades require similar basic neural circuitry with additional neural regions supporting requisite higher level processes. The saccadic system has been studied extensively in non-human (e.g., single-unit recordings) and human (e.g., lesions and neuroimaging) primates. Considerable knowledge of this system's functional neuroanatomy makes it useful for investigating models of cognitive control. The network involved in pro-saccade generation (by definition largely exogenously-driven) includes subcortical (striatum, thalamus, superior colliculus, and cerebellar vermis) and cortical (primary visual, extrastriate, and parietal cortices, and frontal and supplementary eye fields) structures. Activation in these regions is also observed during endogenously-driven voluntary saccades (e.g., anti-saccades, ocular motor delayed response or memory saccades, predictive tracking tasks and anticipatory saccades, and saccade sequencing), all of which require complex cognitive processes like inhibition and working memory. These additional requirements are supported by changes in neural activity in basic saccade circuitry and by recruitment of additional neural regions (such as prefrontal and anterior cingulate cortices). Activity in visual cortex is modulated as a function of task demands and may predict the type of saccade to be generated, perhaps via top-down control mechanisms. Neuroimaging studies suggest two foci of activation within FEF - medial and lateral - which may correspond to volitional and reflexive demands, respectively. Future research on saccade control could usefully (i) delineate important anatomical subdivisions that underlie functional differences, (ii) evaluate functional connectivity of anatomical regions supporting saccade generation using methods such as ICA and structural equation modeling, (iii) investigate how context affects behavior and brain activity, and (iv) use multi-modal neuroimaging to maximize spatial and temporal resolution.

The presence of visual gap affects the duration of stopping process

A milestone on which relies the voluntary control of behavior is the ability to shape our motor output to meet the needs of the context which we are continuously facing. Even though it is solidly established that contextual information influence movement generation few studies have so far explored their effects on inhibitory processes. We compared the inhibitory control of arm movements of ten healthy right-handed volunteers in a countermanding reaching paradigm with and without the presence of a temporal gap between the offset of the central target and the peripheral target appearance. We found that this perceptual gap reduces the reaction times of hand movements and, at the same time, increases the duration of the stop process, the stop signal reaction time. The two effects are not correlated implying that inhibition and execution of reaching movement are two independent processes influenced by a common factor: the disengagement of selective attention from the central target. Therefore our results support the idea of the existence of a link between spatial selective attention and inhibitory processes.

The spatial scale of attention strongly modulates saccade latencies

We have previously shown that when a stimulus consisting of two concentric rings moves, saccade latencies are much longer (by 150 ms) when attention is directed to the larger ring than to the smaller ring. Here, we investigated whether this effect can be explained by a deferral of the "cost" of making a saccade while the target remains inside the attentional field, or by purely visual factors (eccentricity or contrast). We found 1) latencies were shorter when attention was directed to small features irrespective of retinal eccentricity; 2) saccade latency distributions were systematically determined by the ratio between the amplitude of the stimulus step and the diameter of the attended ring: stimulus steps that were larger than the attended ring resulted in short latencies, whereas steps smaller than the attended ring resulted in proportionally longer and more variable latencies; 3) this effect was not seen in manual reaction times to the same target movement; and 4) suprathreshold changes in the contrast of targets, mimicking possible attentional effects on perceived contrast and saliency, had little effect on latency. We argue that the spatial scale of attention determines the urgency of saccade motor preparation processes by changing the rate and rate variability of the underlying decision signal, to defer the cost of saccades that result in little visual benefit.

The influence of stimulus direction and eccentricity on pro- and anti-saccades in humans

We examined the sensory and motor influences of stimulus eccentricity and direction on saccadic reaction times (SRTs), direction-of-movement errors, and saccade amplitude for stimulus-driven (prosaccade) and volitional (antisaccade) oculomotor responses in humans. Stimuli were presented at five eccentricities, ranging from 0.5 degrees to 8 degrees , and in eight radial directions around a central fixation point. At 0.5 degrees eccentricity, participants showed delayed SRT and increased direction-of-movement errors consistent with misidentification of the target and fixation points. For the remaining eccentricities, horizontal saccades had shorter mean SRT than vertical saccades. Stimuli in the upper visual field trigger overt shifts in gaze more easily and faster than in the lower visual field: prosaccades to the upper hemifield had shorter SRT than to the lower hemifield, and more anti-saccade direction-of-movement errors were made into the upper hemifield. With the exception of the 0.5 degrees stimuli, SRT was independent of eccentricity. Saccade amplitude was dependent on target eccentricity for prosaccades, but not for antisaccades within the range we tested. Performance matched behavioral measures described previously for monkeys performing the same tasks, confirming that the monkey is a good model for the human oculomotor function. We conclude that an upper hemifield bias lead to a decrease in SRT and an increase in direction errors.

Fixation offset facilitates saccades and manual reaching for single but not multiple target displays

Turning off a fixation point, typically for 200 ms, before the onset of a peripheral target substantially reduces saccadic reaction times. This facilitatory effect generated by an inserted temporal gap between fixation offset and the target appearance is called the "gap" effect [J Opt Soc Am 57:1030-1033, 1967]. We show that the gap reduces the initial latency of both saccades and manual pointing in single and multiple target displays. Yet, in multiple target displays, the gap increased the movement duration because eye or hand movements were frequently misdirected toward distractors so that the trajectory had to be corrected. Thus, in spite of the shortened latency, the total time for trial completion was not shortened in multiple target displays, whereas it was reduced in single target displays. This selective gap effect for a single target was not restricted to goal-directed motor tasks because perceptual discrimination tasks, where no motor response is required, also demonstrated the gap effect only for single target displays. Our results suggest that the gap may facilitate attentional disengagement, but it does not help target selection in motor and perceptual discrimination tasks, where the allocation of attention to the target is required.

Effects of uncertainty and target displacement on the latency of express saccades in man

Saccadic eye movements generated in response to a gap paradigm in which the fixation light spot was extinguished 200 ms prior to presentation of the target light spot showed appreciably shorter latencies than for the overlap paradigm in which the target light spot was presented 200 ms prior to extinction of the fixation light spot. When there was unpredictability in the direction of target presentation, i.e., to the left or right of the fixation light spot, the gap paradigm evoked mainly fast regular saccades of peak latency of 155 ms with relatively few express saccades which were defined as having latencies of less than 120 ms. By contrast, when the target always appeared to the right, a substantial population of express saccades with peak latency 95 ms was now generated. There was also a change in the relationship between saccadic latency and target angular displacement which covered the range 5-35 degrees . With the overlap paradigm and unpredictability of target direction, the latencies of the slow regular saccades increased markedly with target angular displacement. This was not the case with the same target direction when the latency of slow regular, fast regular, and express saccades remained constant with increasing target angular displacement. This indicates for targets appearing in the same hemifield that the ocular motor system operates with shortest latency irrespective of target angular displacement.

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.

Binocular coordination of saccades in 7 years old children in single word reading and target fixation

The purpose of the present study was to examine in 7 years old normal children who just learnt to read, saccade and fixation characteristics during reading single words. Eight children were studied and their results were compared to those of eight normal adults doing the same task. For each group word reading data were also compared with data in a task requiring saccades and fixations to target-LEDs. Horizontal saccades from both eyes were recorded with a photoelectric device (Oculometer, Bouis). Latencies of saccades both to words and to LEDs presented at predictable location were similar, and they were also similar between children and adults. In contrast, disconjugacy of saccades was significantly increased for children and similar in the two tasks (LEDs or words). Disconjugate post-saccadic drift and its velocity were also significantly higher in children and similar in the two tasks. Substantial conjugate leftward drift was present for children only, and for the word task only. Finally, fixation duration on words was significantly longer in children than in adults. We conclude that binocular coordination and fixation stability is poor in children and that it could be partially responsible for the long fixation duration. Binocular coordination does not depend on the task (LEDs or words) neither for adults, nor for children; this contrasts prior reports.

An invariant for timing of saccades during visual search

The variable latency of a saccade to the onset of a single target reveals our brain's hypothesis testing about the target's presence. Search in complex scenes involves multiple objects that compete to become fixated. The initiation of a saccade in this case involves two hypotheses: (1) a potential target is present outside the fovea and (2) the currently fixated object is not the target. Previous models suggest that these hypotheses are evaluated independently, each involving a decision signal that races towards threshold. We show here that the skewed latency distributions during search comply with strong competition between these decision signals rather than independence. Moreover, the thresholds for the two competing processes are not independent either but conform to an invariant that suggests that saccades in complex scenes are made when the odds for the target's presence outside the fovea versus within the fovea are about four.

Differential effects of target probability on saccade latencies in gap and warning tasks

Saccade latencies are significantly reduced by extinguishing a foveal fixation stimulus before the appearance of a saccade target. It has been shown recently that this "fixation offset effect" (FOE) can be modulated by varying target probability. Cortico-collicular top-down effects have been assumed to mediate this strategic FOE modulation. Here, we have investigated strategic FOE modulation in 14 healthy human subjects performing gap and warning tasks. In the former task, the central fixation point was extinguished 200 ms before target onset. In the latter task, the central fixation point changed its colour 200 ms before target onset, but remained illuminated until the target appeared. Target probability was varied block-wise between 25 and 75%. In both tasks, mean latencies decreased with increasing target probability. However, in contrast with what can be expected from preceding studies, we found no differential modulation of mean latencies by target probability between tasks. Instead, we observed differential probability-dependent changes in latency distributions. In the gap task, discrete changes of saccade latencies were found, with a probability-dependent change in frequency of express and regular latencies. By contrast, in the warning task a shift of the entire latency distribution towards longer latencies with low target probability was found. We conclude that strategic modulation of saccade latencies by target probability may be mediated by two distinct neural mechanisms. Selection of either mechanism seems to depend critically on activation of the fixation system.

Saccadic instabilities and voluntary saccadic behaviour

Primary gaze fixation is never perfectly stable but can be interrupted by involuntary, conjugate saccadic intrusions (SI). SI have a high prevalence in the normal population and are characterised by a horizontal fast eye movement away from the desired eye position, followed, after a variable duration, by a return saccade or drift. Amplitudes are usually below 1 degrees and they often exhibit a directional bias. The aim of the present study was to investigate the aetiology of SI in relation to saccadic behaviour. It was hypothesised that if SI resulted from deficits in the saccadic system (i.e. reduced inhibitory mechanisms), changes in voluntary saccade behaviour may be apparent and related to SI frequency. To examine this, synchrony (no gap), gap, overlap and antisaccade tasks were conducted on ten normal subjects. No significant correlations were found between SI frequency and voluntary saccade latencies, the percentage of express saccades, or the percentage of antisaccade errors. In addition, no significant correlations were found between SI directional biases and saccade latency directional biases, express saccade biases or antisaccade error biases. These results suggest that an underlying alteration to saccadic behaviour is unlikely to be involved in SI production, and that the SI command signal may arise from the influence of attention on an intact saccadic system. Specifically, descending corticofugal signals relating to attention level and orientation may alter the balance between fixation and saccade generation, so determining SI characteristics.

An examination of the variables that affect express saccade generation

The frequency with which express saccades are generated under a variety of conditions in rhesus monkeys was examined. Increasing the gap time between fixation spot termination and target onset increased express saccade frequency but was progressively less effective in doing so as the number of target positions in the sample was increased. Express saccades were rarely produced when two targets were presented simultaneously and the choice of either of which was rewarded; a temporal asynchrony of only 17 ms between the targets reinstated express saccade generation. Express saccades continued to be generated when the vergence or pursuit systems was coactivated with the saccadic system.

Influences of motor instructions on the reaction times of saccadic eye movements

When a gap period is inserted between the fixation point extinction and the target presentation, the distribution of saccadic reaction times has two distinct peaks: one at 150-250 ms (ordinary saccades) and another at approximately 100 ms (express saccades). The distribution of saccadic reaction times can be explained by the linear approach to threshold with ergodic rate (LATER) model, in which the value of a decision signal increases linearly from a start level to initiate a saccade when the signal value reaches a threshold. We hypothesized that a gap period and/or an instruction signal can modulate the parameters of the model to determine when a saccade is initiated. Two reciprobit plots of reaction times, one for ordinary and the other for express saccades, for a task with both a gap period and visuospatial instruction, were constrained by a common infinite-time intercept, although no such constraint was observed during task performance without a visuospatial instruction. We interpreted the results that either the threshold, the start level, or the rate of increase of the decision signal of the model was switched in a bistable manner by both the visuospatial instruction and a gap period, but not by the gap period alone.

Fixation stability and saccadic latency in élite shooters

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

Effects of direction on saccadic performance in relation to lateral preferences

A sample of 676 healthy young males performed visually guided saccades and antisaccades and completed the Porac-Coren questionnaire measuring lateral preferences. There was no difference in mean latency between rightward versus leftward saccades or for saccades executed in the left versus right hemispace. There was also no right/left asymmetry for individuals with left or right dominance as assessed by the lateral preferences questionnaire. The same results were observed for the latency of antisaccades and for the error rate in the antisaccade task. Finally, we did not confirm any substantial subpopulation of individuals with idiosyncratic left/right latency asymmetries that persisted both in the saccade and antisaccade task. These results suggest that neither latency nor antisaccade error rate are good indicators of lateral preferences in these tasks. Other oculomotor tasks might be more sensitive to hemifield differences, or cerebral hemispheric asymmetry is not present at the level of cortical organization of saccades and antisaccades.

Idiosyncratic left-right asymmetries of saccadic latencies: examination in a gap paradigm

Subjects were given reflexive- and voluntary-saccade tasks using five different gap intervals (0-500 ms) between the fixation point offset and the target onset and an overlap paradigm (i.e., the fixation point remained on during the target presentation). In the first experiment, targets were monocularly presented, and the latencies of reflexive saccades to a peripheral target were compared between the left and right visual fields in which targets were presented. The data averaged over subjects did not show a significant difference of saccade latencies between the two visual fields. However, individual subjects showed strong left/right asymmetries of saccade latencies: six out of the 12 subjects that participated made a saccade more rapidly to the right than to the left, and two other subjects showed the reverse result. In these cases, the left/right asymmetry was observed on both gap and overlap trials. The saccade latencies were not affected by the hemiretina to which a target was projected. The second experiment was conducted to identify conditions under which the left/right asymmetry can be reproduced. For this purpose, five subjects were given both reflexive-saccade and voluntary-saccade tasks. In the latter task, a cue stimulus for generating saccades was given at the central fixation point. Regardless of whether saccades were made reflexively or voluntarily, and whether the targets were viewed monocularly or binocularly, each subject showed the same pattern of left/right asymmetries of saccade latencies. The results were interpreted as showing that a visuo-spatial attentional bias specific to individual subjects is involved in generating idiosyncratic left/right asymmetries of saccade latencies.

On the production and correction of involuntary prosaccades in a gap antisaccade task

In an antisaccade task, where saccades in the direction opposite of a suddenly presented stimulus are required, certain numbers of prosaccades can occur. The hypothesis is put forward that poor fixation and poor voluntary saccade control constitute two independent sources for the errors. This possibility is investigated by including the corrections of the errors in the analysis. First, the eye movements of 346 normal subjects (group N) performing a gap antisaccade and an overlap prosaccade task were measured. For each subject the proportion of express saccades in the overlap prosaccade task and the proportion of prosaccades in the gap antisaccade task were determined. The data of 150 subjects with more than 20% proerrors were divided into two groups: group A with relatively many, group B with relatively few express saccades in the overlap prosaccade task. Group A subjects produced their errors after significantly shorter reaction times and they corrected their errors significantly faster and more often than group B subjects. Second, we analysed the data of three groups of subjects: the complete normal group N, a group D of dyslexic subjects (n=343), and a group T containing all subjects irrespective of their cognitive achievements (n=780). A highly significant negative correlation exists between the correction rates and the error rates. A factor analysis of the variables performed for each group separately results in only two factors, one describing prosaccade the other antisaccade control. Only the error rate contributes significantly to both factors indicating that high errors may have two independent reasons.

Poor saccadic control correlates with dyslexia

A large group of subjects, either average readers or reading/spelling disabled subjects (n = 185; age between 8-25 years; M = 13 +/- 4 years), were tested in various standardized cognitive tasks including reading/spelling assessment and in non-cognitive saccadic eye movement tasks. Dyslexics were separated into a subgroup (D1) with deficits in the serial auditory short-term memory and a subgroup (D2) with an isolated low achievement in reading/writing. Control subjects had no relevant cognitive deficit of any type. Saccadic eye movements were measured in a single target and in a sequential-target task. A significant correlation was found between abnormal saccadic control and reading disability. The two dyslexic groups showed only slight differences. As compared to the control group, the mean values of the standard deviations of the saccadic reaction times (SRT) and the amount of late saccades (SRT > 700) were significantly increased in both dyslexic groups and especially in group D1 who also showed an increased amount of anticipatory saccades. The number of express saccades (SRT = 80-134 ms) was increased, but not significantly, in D2 dyslexics. Both dyslexic groups produced significantly more regressive saccades in the sequential-target task. The correlation between saccadic variables and "reading factor" was 0.4. Significant deviations from normal performance of the saccadic variables were found in an estimated 50% of the dyslexics as compared to 20% of the control subjects. In spite of their worse level in saccadic control, dyslexics also developed with age in the eye movement performance as the control subjects did. Yet, the development was slower in group D1. It is suggested that reading process and saccade system are both controlled by visuo spatial attention and fixation systems that maybe impaired or develop slowly in many dyslexic subjects.

The gap effect during visual and auditory stimulation using manual responses

Six experiments were conducted in naive human participants to examine any facilitation produced in manual reaction time (RT) by the interposition of a temporal gap between a warning signal and an imperative signal. Peripheral visual stimuli and monoaural auditory stimuli were used as targets. Participants showed a facilitation of RTs to the targets for both auditory and visual stimuli in the five experiments in which RTs were the dependent variable. In addition, the gap effect increased over successive blocks of trials, suggesting learning. RTs were facilitated only when the gap had predictive value and was salient. Using a variable temporal gap or visual warning stimulus did not change the facilitation in RTs. A further experiment demonstrated that the gap can be perceived by the participants. The dissociation between a learned and a non-learned component in the gap effect suggests that the temporal gap induces two independent processes: warning and disengagement of attention.

The spatial relationship between scanning saccades and express saccades

When monkeys interrupt their saccadic scanning of a visual scene to look at a suddenly appearing target, saccades to the target are made after an "express" latency or after a longer "regular" latency. The purpose of this study was to analyze the spatial patterns of scanning, express, and regular saccades. Scanning patterns were spatially biased. Express saccade patterns were biased, too, and were directly correlated with scanning patterns. Regular saccade patterns were more uniform and were not directly correlated with scanning patterns. Express saccades, but not regular saccades, seemed to be facilitated by preparation to scan. This study contributes to a general understanding of how monkeys examine scenes containing both unchanging and suddenly appearing stimuli.

Saccadic reaction times: a statistical analysis of multimodal distributions

The distributions of saccadic reaction times (SRT) often deviate from unimodal normal distributions. An excess-mass procedure was used to detect peaks in 963 data sets containing 90,927 reaction times from 170 subjects. About 55% showed one, 30% two, 12% three and 3% four peaks. According to their clustering along the reaction time scale the modes could be classified into express (90-120 msec), fast regular (135-170 msec) and slow regular (200-220 msec) modes. Among the unimodal distributions 29% had peaks in the range of the express mode and 46% had peaks in the range of the fast regular mode. Therefore, 87% of the data sets support the notion of saccadic reaction time distributions being the superposition of three modes. All experimental distributions were fitted by as many gamma distributions as determined by the excess-mass test. The significance of the multimodality for saccade generation processes is discussed.

On the development of voluntary and reflexive components in human saccade generation

The saccadic performance of a large number (n = 281) of subjects of different ages (8-70 years) was studied applying two saccade tasks: the prosaccade overlap (PO) task and the antisaccade gap (AG) task. From the PO task, the mean reaction times and the percentage of express saccades were determined for each subject. From the AG task, the mean reaction time of the correct antisaccades and of the erratic prosaccades were measured. In addition, we determined the error rate and the mean correction time, i.e. the time between the end of the first erratic prosaccade and the following corrective antisaccade. These variables were measured separately for stimuli presented (in random order) at the right or left side. While strong correlations were seen between variables for the right and left sides, considerable side asymmetries were obtained from many subjects. A factor analysis revealed that the seven variables (six eye movement variables plus age) were mainly determined by only two factors, V and F. The V factor was dominated by the variables from the AG task (reaction time, correction time, error rate) the F factor by variables from the PO task (reaction time, percentage express saccades) and the reaction time of the errors (prosaccades!) from the AG task. The relationship between the percentage number of express saccades and the percentage number of errors was completely asymmetric: high numbers of express saccades were accompanied by high numbers of errors but not vice versa. Only the variables in the V factor covaried with age. A fast decrease of the antisaccade reaction time (by 50 ms), of the correction times (by 70 ms) and of the error rate (from 60 to 22%) was observed between age 9 and 15 years, followed by a further period of slower decrease until age 25 years. The mean time a subject needed to reach the side opposite to the stimulus as required by the antisaccade task decreased from approximately 350 to 250 ms until age 15 years and decreased further by 20 ms before it increased again to approximately 280 ms. At higher ages, there was a slight indication for a return development. Subjects with high error rates had long antisaccade latencies and needed a long time to reach the opposite side on error trials. The variables obtained from the PO task varied also significantly with age but by smaller amounts. The results are discussed in relation to the subsystems controlling saccade generation: a voluntary and a reflex component the latter being suppressed by active fixation. Both systems seem to develop differentially. The data offer a detailed baseline for clinical studies using the pro- and antisaccade tasks as an indication of functional impairments, circumscribed brain lesions, neurological and psychiatric diseases and cognitive deficits.

Reaction times of vertical prosaccades and antisaccades in gap and overlap tasks

Horizontal saccadic reaction times (SRTs) have been extensively studied over the past 3 decades, concentrating on such topics as the gap effect, express saccades, training effects, and the role of fixation and attention. This study investigates some of these topics with regard to vertical saccades. The reaction times of vertical saccades of 13 subjects were measured using the gap and the overlap paradigms in the prosaccade task (saccade to the stimulus) and the antisaccade task (saccade in the direction opposite to the stimulus). In the gap paradigm, the initial fixation point (FP) was extinguished 200 ms before stimulus onset, while, in the overlap paradigm, the FP remained on during stimulus presentation. With the prosaccade overlap task, it was found that most subjects (10/13)-whether they were previously trained making horizontal saccades or naive-had significantly faster upward saccades compared with their downward saccades. One subject was faster in the downward direction and two were symmetrical. The introduction of the gap reduced the reaction times of the prosaccades, and express saccades were obtained in some naive and most trained subjects. This gap effect was larger for saccades made to the downward target. The strength of the updown asymmetry was more pronounced in the overlap as compared to the gap paradigm. With the antisaccade task, up-down asymmetries were much reduced. Express antisaccades were absent even with the gap paradigm, but reaction times were reduced as compared to the antisaccade overlap paradigm. There was a slight tendency for a larger gap effect of downward saccades. All subjects produced a certain number of erratic prosaccades in the antitasks, more with the gap than with the overlap paradigm. There was a significantly larger gap effect for the erratic prosaccades made to the downward, as compared to the upward, target, due to increased downward SRTs in the overlap paradigm. Three subjects trained in both the horizontal and the vertical direction showed faster SRTs and more express saccades in the horizontal directions as compared to the vertical. It is concluded that different parts of the visual field are differently organized with both directional and nondirectional components in saccade preparation.

Presaccadic attention allocation and express saccades

Express saccades are visually-guided saccades that are characterized by an extremely short latency of about 100 ms. The present experiments tested the hypothesis that a disengagement of visual attention is necessary for the generation of express saccades. All subjects produced large numbers of express saccades in the gap paradigm, in which the fixation stimulus is removed 200 ms before target onset (Exp. 1), but not in the overlap paradigm, in which the fixation stimulus remained on during the entire trial (Exp. 2). By means of peripheral cues (Exps. 3-5) and central cues (Exps. 6-7), visual attention was directed at the target location for the saccade before the actual appearance of the saccade target. In all experiments, the location cues facilitated rather than abolished express saccades. The generation of express saccades was facilitated even when the currently fixated visual stimulus was not removed before target onset (fixation-overlap; Exps. 5-7). The results are explained by the hypothesis that a disengagement of a separate fixation system is necessary for the generation of express saccades, a hypothesis that is in line with current neurobiological findings.

Response latencies and event-related potentials during the gap paradigm using saccadic responses in human subjects

An experiment was conducted in naive human subjects to measure the time benefits of the latencies of saccadic eye movements to peripheral targets when the offset of a central fixation point precedes the switching on of the peripheral target by 200 ms. Naive subjects produced a shift advancement of the eye movement latencies to the targets with respect to when there is no such temporal gap. Simultaneously, the event-related potentials produced by visual stimuli and saccadic eye movements were recorded. The switching on of the central fixation point induced a negative component that could be considered a contingent negative variation. Subsequently, in the control non-gap condition visual evoked potentials and P300 appear. The temporal gap paradigm induced offset visual-evoked potentials and a frontal negativity; it also induced a higher P300 than the non-gap condition. The saccadic ERPs also showed a frontal negativity preceding the saccade during the gap condition. The results suggest that fast regular saccades during the gap paradigm occur by a priming of premotor and motor frontal circuits indexed by the recorded negativity during the gap paradigm.

Intertrial effects of randomization on saccadic reaction times in human observers

We investigated the effect of randomizing different spatial and temporal parameters on saccadic reaction times (SRTs) in five human subjects, to explore the relative occurrence of express and regular saccades. Parameters randomized in various test sessions were: target direction (right/left), intertrial interval, fixation foreperiod and gap duration (two or three 400 msec) in gap and overlap trials. For the sake of comparison the parameters under consideration were kept constant in non-random control sessions. We found that express saccades were still present in the random test sessions but their relative frequency (and the number of regular saccades) obtained in a given test session depends on the type of randomized parameters: randomizing the intertrial interval or the fixation foreperiod in the gap task yielded modest but significant changes in the SRT distributions, express and fast regular saccades being present in both the control and the random conditions. Randomization of the fixation foreperiod in the overlap task, on the other hand, caused a quite drastic increase in the SRTs. Randomization of gap and overlap trials did not cause considerable effects on express and fast regular saccades in the gap trials, and the SRTs in the overlap trials were significantly increased only in two subjects. When two or three gap durations were randomly interleaved, we found effects that ranged from "negligible" (usually for the longest gap in a given test session) to highly significant differences as compared with the corresponding control condition. The results suggest that express saccades--as fast regular saccades--are visually guided saccades which occur when a certain state of saccade preparation has been reached before target onset. This state depends on the amount of activation in the brain structures involved in the control of attention and fixation, and the decision processes involved in saccade preparation.