Nature of variability in saccades

Occipital gamma-oscillations modulated during eye movement tasks: simultaneous eye tracking and electrocorticography recording in epileptic patients

We determined the spatio-temporal dynamics of cortical gamma-oscillations modulated during eye movement tasks, using simultaneous eye tracking and intracranial electrocorticography (ECoG) recording. Patients with focal epilepsy were instructed to follow a target moving intermittently and unpredictably from one place to another either in an instantaneous or smooth fashion during extraoperative ECoG recording. Target motion elicited augmentation of gamma-oscillations in the lateral, inferior and polar occipital regions in addition to portions of parietal and frontal regions; subsequent voluntary eye movements elicited gamma-augmentation in the medial occipital region. Such occipital gamma-augmentations could not be explained by contaminations of ocular or myogenic artifacts. The degree of gamma-augmentation was generally larger during saccade compared to pursuit trials, while a portion of the polar occipital region showed pursuit-preferential gamma-augmentations. In addition to the aforementioned eye movement task, patients were asked to read a single word popping up on the screen. Gamma-augmentation was elicited in widespread occipital regions following word presentation, while gamma-augmentation in the anterior portion of the medial occipital region was elicited by an involuntary saccade following word presentation rather than word presentation itself. Gamma-augmentation in the lateral, inferior and polar occipital regions can be explained by increased attention to a moving target, whereas gamma-augmentation in the anterior-medial occipital region may be elicited by images in the peripheral field realigned following saccades. In functional studies comparing brain activation between two tasks, eye movement patterns during tasks may need to be considered as confounding factors.

An adaptive algorithm for fixation, saccade, and glissade detection in eyetracking data

Event detection is used to classify recorded gaze points into periods of fixation, saccade, smooth pursuit, blink, and noise. Although there is an overall consensus that current algorithms for event detection have serious flaws and that a de facto standard for event detection does not exist, surprisingly little work has been done to remedy this problem. We suggest a new velocity-based algorithm that takes several of the previously known limitations into account. Most important, the new algorithm identifies so-called glissades, a wobbling movement at the end of many saccades, as a separate class of eye movements. Part of the solution involves designing an adaptive velocity threshold that makes the event detection less sensitive to variations in noise level and the algorithm settings-free for the user. We demonstrate the performance of the new algorithm on eye movements recorded during reading and scene perception and compare it with two of the most commonly used algorithms today. Results show that, unlike the currently used algorithms, fixations, saccades, and glissades are robustly identified by the new algorithm. Using this algorithm, we found that glissades occur in about half of the saccades, during both reading and scene perception, and that they have an average duration close to 24 msec. Due to the high prevalence and long durations of glissades, we argue that researchers must actively choose whether to assign the glissades to saccades or fixations; the choice affects dependent variables such as fixation and saccade duration significantly. Current algorithms do not offer this choice, and their assignments of each glissade are largely arbitrary.

Right visual field advantage in parafoveal processing: evidence from eye-fixation-related potentials

Readers acquire information outside the current eye fixation. Previous research indicates that having only the fixated word available slows reading, but when the next word is visible, reading is almost as fast as when the whole line is seen. Parafoveal-on-foveal effects are interpreted to reflect that the characteristics of a parafoveal word can influence fixation on a current word. Prior studies also show that words presented to the right visual field (RVF) are processed faster and more accurately than words in the left visual field (LVF). This asymmetry results either from an attentional bias, reading direction, or the cerebral asymmetry of language processing. We used eye-fixation-related potentials (EFRP), a technique that combines eye-tracking and electroencephalography, to investigate visual field differences in parafoveal-on-foveal effects. After a central fixation, a prime word appeared in the middle of the screen together with a parafoveal target that was presented either to the LVF or to the RVF. Both hemifield presentations included three semantic conditions: the words were either semantically associated, non-associated, or the target was a non-word. The participants began reading from the prime and then made a saccade towards the target, subsequently they judged the semantic association. Between 200 and 280ms from the fixation onset, an occipital P2 EFRP-component differentiated between parafoveal word and non-word stimuli when the parafoveal word appeared in the RVF. The results suggest that the extraction of parafoveal information is affected by attention, which is oriented as a function of reading direction.

Eye movements of laterally eyed birds are not independent

Most birds have laterally placed eyes with two largely separated visual fields. According to studies in pigeons laterally eyed birds move their eyes independently in most situations, eye coordination just occurred during converging saccades towards frontal stimuli. Here we demonstrate for the first time that laterally eyed zebra finches show coordinated eye movements, regarding direction and amplitude. Spontaneous and visually elicited movements of the two eyes were recorded simultaneously, using a newly developed eye tracking system. We found that, if one eye moves in a certain direction, the other eye simultaneously performs a counter-movement in the opposite direction. Based on these data we developed a hypothesis of how laterally eyed birds cope with the situation in which the left and right eye simultaneously obtain images with different content. We suggest that the counter-movements maintain the spatial relationship of the two visual fields. ;Oculospatial constancy', as we call it, facilitates the combination of the left and right visual percept on the level of peripheral or unattended viewing, and the localization of appearing stimuli within the whole visual field. As soon as two visual stimuli simultaneously appear in the left and right visual field, the birds decide on one stimulus and direct the fovea of the appropriate eye towards it for high resolution analysis, the other eye simultaneously performing a counter-saccade. This leads to the assumption that, in contrast to simultaneous peripheral perception with two eyes, the processing of foveal information is possible only for one eye at one time.

The significance of microsaccades for vision and oculomotor control

Over the past decade several research groups have taken a renewed interest in the special role of a type of small eye movement, called 'microsaccades', in various visual processes, such as the activation of neurons in the central nervous system, or the prevention of image fading. As the study of microsaccades and their relation to visual processes goes back at least half a century, it seems appropriate to review the more recent reports in light of the history of research on maintained oculomotor fixation, in general, and on microsaccades in particular. Our review shows that there is no compelling evidence to support the view that microsaccades (or, fixation saccades more generally) serve a necessary role in improving oculomotor control or in keeping the visual world visible. The role of the retinal transients produced by small saccades during fixation needs to be evaluated in the context of both the brisk image motions present during active visual tasks performed by freely moving people, as well as the role of selective attention in modulating the strength of signals throughout the visual field.

The sources of variability in saccadic eye movements

Our movements are variable, but the origin of this variability is poorly understood. We examined the sources of variability in human saccadic eye movements. In two experiments, we measured the spatiotemporal variability in saccade trajectories as a function of movement direction and amplitude. One of our new observations is that the variability in movement direction is smaller for purely horizontal and vertical saccades than for saccades in oblique directions. We also found that saccade amplitude, duration, and peak velocity are all correlated with one another. To determine the origin of the observed variability, we estimated the noise in motor commands from the observed spatiotemporal variability, while taking into account the variability resulting from uncertainty in localization of the target. This analysis revealed that uncertainty in target localization is the major source of variability in saccade endpoints, whereas noise in the magnitude of the motor commands explains a slightly smaller fraction. In addition, there is temporal variability such that saccades with a longer than average duration have a smaller than average peak velocity. This noise model has a large generality because it correctly predicts the variability in other data sets, which contain saccades starting from very different initial locations. Because the temporal noise most likely originates in movement planning, and the motor command noise in movement execution, we conclude that uncertainty in sensory signals and noise in movement planning and execution all contribute to the variability in saccade trajectories. These results are important for understanding how the brain controls movement.

Using double-magnetic induction to measure head-unrestrained gaze shifts. I. Theory and validation

So far, the double-magnetic induction (DMI) method has been successfully applied to record eye movements from head-restrained humans, monkeys and cats. An advantage of the DMI method, compared to the more widely used scleral search coil technique, is the absence of vulnerable lead wires on the eye. A disadvantage, however, is that the relationship between the eye-in-head orientation and the secondary induction signal is highly non-linear and non-monotonic. This limits the effective measuring range to maximum eye orientations of about +/-30 degrees . Here, we analyze and test two extensions required to record the full eye-head orienting range, well exceeding 90 degrees from straight-ahead in all directions. (1) The use of mutually perpendicular magnetic fields allows for the disambiguation of the non-monotonic signal from the ring. (2) The application of an artificial neural network for offline calibration of the signals. The theoretical predictions are tested for horizontal rotations with a gimbal system. Our results show that the method is a promising alternative to the search coil technique.

Intra-individual variability of saccadic velocity measured with the infrared reflection and magnetic scleral search coil methods

BACKGROUND

The infrared (IR) and the magnetic scleral search coil (MSC) systems for eye tracking were studied with regard to the intra-individual variability in saccadic eye movement recordings.

METHOD

Three healthy subjects performed similar saccadic eye movement tasks at five different occasions with both the IR (Orbit XY-1000) and the MSC (Skalar Medical) techniques. The maximum velocity (VMAX) and slope constant (C) of the main sequence plots were analyzed with regard to the coefficient of variation (CV) and the intraclass correlation coefficient (Ricc). In addition, the possible reasons for variability in the IR recordings, especially different causes for noise, were analyzed and discussed.

RESULTS

The main sequence data showed intra-individual variation with both recording systems, but the coefficient of variation was higher for VMAX with the IR compared to the MSC method. Ricc analysis showed that 36% of the variance of VMAX and 49% of the variance of C resulted from intra-individual variability in recordings of the IR system. The corresponding results for the MSC recordings regarding VMAX and C were 48% and 88%.

CONCLUSIONS

Saccadic eye movement recordings yielded a larger intra-individual variability with the IR system than with the MSC system. The effect that the MSC annulus may have on the ocular motor command signal and the possible low pass filter caused by the coil slipping on the surface of the eye may partly explain the relatively lower velocity in the MSC recordings. Also, noise in the IR recordings induces peaks of eye velocity, which can be reduced considerably by filtering. However, the variability in the recordings, which was larger in the IR than in the MSC recordings, did not seem to be decreased by filtering. The basic level of noise in the recordings was not clearly associated with the amount of reduction of VMAX when the IR recordings were filtered. We suggest that artefacts of the saccadic signal, which can be related to changes in the reflecting surface of the eyes and eyelids, are important factors for explaining the variability and high-velocity peaks in the IR recordings. Lighting conditions was confirmed as a cause for noise, but temperature and air humidity changes in the goggles were not suspected to influence data in the normal experimental setting. Although noise, shortcomings of the recording technique and procedure may offer explanations for the intra-individual variability, the calibration procedure and changes in attention and fatigue of the subject should also be considered.

The temporal and spatial limits of compensation for fixational eye movements

High-fidelity eye tracking is combined with a perceptual grouping task to provide insight into the likely mechanisms underlying the compensation of retinal image motion caused by movement of the eyes. The experiments describe the covert detection of minute temporal and spatial offsets incorporated into a test stimulus. Analysis of eye motion on individual trials indicates that the temporal offset sensitivity is actually due to motion of the eye inducing artificial spatial offsets in the briefly presented stimuli. The results have strong implications for two popular models of compensation for fixational eye movements, namely efference copy and image-based models. If an efference copy model is assumed, the results place constraints on the spatial accuracy and source of compensation. If an image-based model is assumed then limitations are placed on the integration time window over which motion estimates are calculated.

Sensory input from the moving hand influences saccadic eye movements during reading

Reading involves saccadic eye movements. Measured reading time and the number of mistakes made while reading reflect the speed and accuracy of the saccades in target localization, if all other factors influencing these parameters are kept constant. The observed phenomenon that reading a book is easier when it is held in an individual's hand than when it is not, especially when movement of the reading text occurs while travelling in a vehicle, raises the question of the role of sensory input from the moving arms in guiding saccades in the direction in which the text is moved. To address this question, 12 healthy subjects (6 males and 6 females), aged 19-21 years took part in this study where reading time and the number of mistakes made while reading a non-moving standardized printed text was noted. Similar printed texts were read by the subjects while mechanically moving them at different fixed speeds in the horizontal plane, with and without the subject's arms moving with the text. At each speed, the reading time recorded when the subject's arms moved with the text was significantly lesser than when they did not (P<0.05). The number of mistakes made were significantly more when not moving the arms than when moving them with the text, at higher speed of text movement (P<0.05). The results indicate that sensory input from the passively moving arms guided saccades in the direction of movement of the text during reading.

How important is lateral masking in visual search?

Five experiments are presented, providing empirical support of the hypothesis that the sensory phenomenon of lateral masking may explain many well-known visual search phenomena that are commonly assumed to be governed by cognitive attentional mechanisms. Experiment I showed that when the same visual arrays are used in visual search and in lateral masking experiments, the factors (1) number of distractors, (2) distractor density, and (3) search type (conjunction vs disjunction) have the same effect on search times as they have on lateral masking scores. Experiment II showed that when the number of distractors and eccentricity is kept constant in a search task, the effect of reducing density (which reduces the lateral masking potential of distractors on the target) is to strongly reduce the disjunction-conjunction difference. In experiment III, the lateral masking potential of distractors on a target was measured with arrays that typically yield asymmetric search times in visual search studies (a Q among Os vs. an O among Qs). The lateral masking scores showed the same asymmetry. Experiment IV was a visual search study with such asymmetric search arrays in which the number of distractors and eccentricity was kept constant, while manipulating density. Reducing density (i.e., reducing lateral masking) produced a strong reduction of the asymmetry effect. Finally in experiment V, we showed that the data from experiment IV cannot be explained due to a difference between a fine and a coarse grain attentional mechanism. Taken together with eye movement data and error scores from experiment II and with similar findings from the literature, these results suggest that the sensory mechanism of lateral masking could well be a very important (if not the main) factor causing many of the well-known effects that are traditionally attributed to higher level cognitive or attentional mechanisms in visual search.

A comparison between the magnetic scleral search coil and infrared reflection methods for saccadic eye movement analysis

BACKGROUND

A non-invasive eye tracking system, based on pulsed infrared light (IR), was compared with the magnetic scleral search coil method (MSC) for saccadic eye movement recordings.

METHODS

Ten normal subjects performed horizontal and vertical saccades recorded with both methods. Eight recordings were complete and analysed for maximum peak velocity (V(MAX)) and constant (C) of the main sequence curve.

RESULTS

The IR recordings showed significantly higher peak velocity values than the MSC system and generally more inter-individual variability. No significant difference regarding peak velocity was detected between abducting and adducting saccades or between upward and downward saccades with either of the systems. Horizontal saccades had higher peak velocities with both techniques.

CONCLUSIONS

Comparison of the main sequence plots of the IR and MSC eye tracking techniques reveals that the IR system yields higher values of maximum peak velocity and the constant, the differences being similar for eye movements in different directions. There are various possible explanations for the lower maximum velocity of the MSC recordings, e.g. slipping of the coil annulus on the surface of the eye and a change of the oculomotor command signal induced by wearing the coil. Also, artefacts associated with the IR recording system may cause over-estimations of the saccadic velocity and, furthermore, contribute to the higher variability of the IR recordings.

Predictive strategies in interception tasks: differences between eye and hand movements

To investigate how the sensorimotor systems of eye and hand use position, velocity, and timing information of moving targets, we conducted a series of three experiments. Subjects performed combined eye-hand catch-up movements toward visual targets that moved with step-ramp-like velocity profiles. Visual feedback of the hand was prevented by blanking the target at the onset of the hand movement. A multiple regression was used to determine the effects of position, velocity, and timing accessed before each movement on the movement amplitudes of eye and hand. The following results were obtained: 1. The predictive strategy of eye movements could be modeled by a linear regression on the basis of the position error and the target velocity. This was not the case for hand movements, for which there was a significant partial correlation between the movement amplitude and the product of target velocity and movement duration. This correlation was not observed for eye movements suggesting that the predictive strategy of hand movements takes movement duration into account, in contrast to the strategy used in eye movements. 2. To determine whether the movement amplitudes of eye and hand depend on a categorical classification between a discrete number of movement types, we compared an experiment in which target position and velocity were distributed continuously with an experiment using only four different combinations of target position and velocity. No systematic differences between these experiments were observed. This shows that the system output is a function of continuous, interval-scaled variables rather than a function of discrete categorical variables. 3. We also analyzed the component of the movement amplitudes not explained by the regression, i.e., the residual error. The residual errors between subsequent trials were correlated more strongly for eye than for hand movements, suggesting that short-term temporal fluctuations of the predictive strategy were stronger for the eye than for the hand.