Human Saccadic Eye Movements Research Articles

A non-iterative foveated single-pixel imaging using fast transformation algorithm

Inspired by the human eye retina, foveated single-pixel imaging employs an iterative algorithm, which, unfortunately, poses a challenge for real-time imaging due to its time-consuming nature. To address this issue, we present a non-iterative foveated single-pixel imaging method based on a computationally fast algorithm, reducing time consumption by five orders of magnitude. The fast algorithmic transform facilitates the acquisition of foveated reconstructions. In addition to mimicking human saccadic eye movements, the foveated strategy adapts to motion tracking through the foveal gaze control method. During high-resolution imaging in the fovea area, the maximum sensing area expands 3.14 times, similar to human vision. The proposed retina-like single-pixel imaging has great potential in future applications with the advantages of avoiding complex optics design, low time consumption, and wider vision field.

A Sensorimotor Numerosity System.

Incoming sensory input provides information for the planning and execution of actions, which yield motor outcomes that are themselves sensory inputs. One dimension where action and perception strongly interact is numerosity perception. Many non-human animals can estimate approximately the number of external elements as well as their own actions, and neurons have been identified that respond to both. Recent psychophysical adaptation studies on humans also provide evidence for neural mechanisms responding to both the number of externally generated events and self-produced actions. Here we advance the idea that these strong connections may arise from dedicated sensorimotor mechanisms in the brain, part of a more generalized system interfacing action with the processing of other quantitative magnitudes such as space and time.

Fast saccadic eye-movements in humans suggest that numerosity perception is automatic and direct.

Fast saccades are rapid automatic oculomotor responses to salient and ecologically important visual stimuli such as animals and faces. Discriminating the number of friends, foe, or prey may also have an evolutionary advantage. In this study, participants were asked to saccade rapidly towards the more numerous of two arrays. Participants could discriminate numerosities with high accuracy and great speed, as fast as 190 ms. Intermediate numerosities were more likely to elicit fast saccades than very low or very high numerosities. Reaction-times for vocal responses (collected in a separate experiment) were slower, did not depend on numerical range, and correlated only with the slow not the fast saccades, pointing to different systems. The short saccadic reaction-times we observe are surprising given that discrimination using numerosity estimation is thought to require a relatively complex neural circuit, with several relays of information through the parietal and prefrontal cortex. Our results suggest that fast numerosity-driven saccades may be generated on a single feed-forward pass of information recruiting a primitive system that cuts through the cortical hierarchy and rapidly transforms the numerosity information into a saccade command.

Two Distinct Types of Eye-Head Coupling in Freely Moving Mice

SummaryAnimals actively interact with their environment to gather sensory information. There is conflicting evidence about how mice use vision to sample their environment. During head restraint, mice make rapid eye movements coupled between the eyes, similar to conjugate saccadic eye movements in humans. However, when mice are free to move their heads, eye movements are more complex and often non-conjugate, with the eyes moving in opposite directions. We combined head and eye tracking in freely moving mice and found both observations are explained by two eye-head coupling types, associated with vestibular mechanisms. The first type comprised non-conjugate eye movements, which compensate for head tilt changes to maintain a similar visual field relative to the horizontal ground plane. The second type of eye movements was conjugate and coupled to head yaw rotation to produce a “saccade and fixate” gaze pattern. During head-initiated saccades, the eyes moved together in the head direction but during subsequent fixation moved in the opposite direction to the head to compensate for head rotation. This saccade and fixate pattern is similar to humans who use eye movements (with or without head movement) to rapidly shift gaze but in mice relies on combined head and eye movements. Both couplings were maintained during social interactions and visually guided object tracking. Even in head-restrained mice, eye movements were invariably associated with attempted head motion. Our results reveal that mice combine head and eye movements to sample their environment and highlight similarities and differences between eye movements in mice and humans.

Saccadic suppression measured by steady-state visual evoked potentials.

Visual sensitivity is severely impaired during the execution of saccadic eye movements. This phenomenon has been extensively characterized in human psychophysics and nonhuman primate single-neuron studies, but a physiological characterization in humans is less established. Here, we used a method based on steady-state visually evoked potential (SSVEP), an oscillatory brain response to periodic visual stimulation, to examine how saccades affect visual sensitivity. Observers made horizontal saccades back and forth, while horizontal black-and-white gratings flickered at 5-30 Hz in the background. We analyzed EEG epochs with a length of 0.3 s either centered at saccade onset (saccade epochs) or centered at fixations half a second before the saccade (fixation epochs). Compared with fixation epochs, saccade epochs showed a broadband power increase, which most likely resulted from saccade-related EEG activity. The execution of saccades, however, led to an average reduction of 57% in the SSVEP amplitude at the stimulation frequency. This result provides additional evidence for an active saccadic suppression in the early visual cortex in humans. Compared with previous functional MRI and EEG studies, an advantage of this approach lies in its capability to trace the temporal dynamics of neural activity throughout the time course of a saccade. In contrast to previous electrophysiological studies in nonhuman primates, we did not find any evidence for postsaccadic enhancement, even though simulation results show that our method would have been able to detect it. We conclude that SSVEP is a useful technique to investigate the neural correlates of visual perception during saccadic eye movements in humans. NEW & NOTEWORTHY We make fast ballistic saccadic eye movements a few times every second. At the time of saccades, visual sensitivity is severely impaired. The present study uses steady-state visually evoked potentials to reveal a neural correlate of the fine temporal dynamics of these modulations at the time of saccades in humans. We observed a strong reduction (57%) of visually driven neural activity associated with saccades but did not find any evidence for postsaccadic enhancement.

A Saccade Based Framework for Real-Time Motion Segmentation Using Event Based Vision Sensors.

Motion segmentation is a critical pre-processing step for autonomous robotic systems to facilitate tracking of moving objects in cluttered environments. Event based sensors are low power analog devices that represent a scene by means of asynchronous information updates of only the dynamic details at high temporal resolution and, hence, require significantly less calculations. However, motion segmentation using spatiotemporal data is a challenging task due to data asynchrony. Prior approaches for object tracking using neuromorphic sensors perform well while the sensor is static or a known model of the object to be followed is available. To address these limitations, in this paper we develop a technique for generalized motion segmentation based on spatial statistics across time frames. First, we create micromotion on the platform to facilitate the separation of static and dynamic elements of a scene, inspired by human saccadic eye movements. Second, we introduce the concept of spike-groups as a methodology to partition spatio-temporal event groups, which facilitates computation of scene statistics and characterize objects in it. Experimental results show that our algorithm is able to classify dynamic objects with a moving camera with maximum accuracy of 92%.

Bayesian Decision Theory Application for Double-step Saccades

Bayesian decision theory applied to the human saccadic eye movements elicited in double-step mode: primary and corrective saccade. Primary saccade performs large amplitude but not precise eye jump towards new target position and small amplitude corrective saccade brings line of sight precisely on the target. This behavior mach two-step Bayesian decision making process: first with large uncertainty and second, after getting additional information, – more precise. It explains how saccadic eye movements control system performs precise eye jump in the conditions of the noisy visual estimation of the new target position and eye jump errors made by eye globe muscles. Scatter of the landing places of the primary saccades was investigated theoretically using Bayesian statistics approach and experimentally. Ill. 5, bibl. 6 (in English; summaries in English, Russian and Lithuanian).

Human saccadic eye movements

Human saccadic eye movements

The odd human eye movements during oddity search are not suboptimal

Studies have shown that when humans search for a known target, eye movements are often guided towards target features (e.g., Findlay, 1997) with occasional fixational compensations for inhomogeneous processing across the visual field (Najemnik & Giesler, 2005). In a different type of search (oddity search), targets and distractors are not known beforehand and observers have to determine whether an element in the display differs from others (distractors) along some feature. Previous research has shown that oddity search has different behavioral properties from target known search, such as different effects of set-size (Bravo & Nakayama, 1992; Schoonveld et al., 2007). Here, we measure human saccadic eye movements during an oddity search task and develop an optimal foveated oddity searcher, which fixates locations that maximize search accuracy, to evaluate human eye movement strategies. Observers performed a yes/no search task in which they were to determine whether one of the five potential target locations contained a singleton target. Stimuli consisted of Gabor patches at three orientations (5° left, vertical, 5° right) embedded in white noise at five locations around an imaginary circle (6° radius). For target present trials, the target was randomly selected from the three possible elements, while one of the remaining elements occupied the other four locations (distractors). For target absent trials, all five locations contained the same randomly selected element. Participants viewed each stimulus for 700 ms (allowing approximately three eye movements) before making a decision. Unlike typical strategies for target known search, observers adopted an eye movement strategy for the oddity search in which they systematically fixated three locations regardless of the stimulus configuration. Such strategy shows little guidance towards the target location. We find that the optimal foveated oddity searcher, like humans, seeks to survey as many of the potential target locations as possible and shows little target guidance.

Three‐dimensional kinematics of saccadic eye movements in humans: is the “half‐angle rule” obeyed?

The "half-angle rule" must be satisfied for Listing's law to be obeyed during saccades: the eye velocity axis must tilt away from Listing's plane by half the angle of eye position eccentricity from primary position. We aimed to determine if this rule is satisfied during saccades. Horizontal and vertical saccades were recorded using the search coil technique, at different elevations and azimuths, in five normal humans. In all, eye positions were located on a twisted rather than planar surface. The mean tilt-angle coefficients (TACs; ratio of angle of eye velocity axis tilt to angle of eye position eccentricity) were 0.57 and 0.34 for horizontal and vertical saccades, respectively. TACs were significantly lower for vertical saccades. Thus, Listing's law is not perfectly obeyed during saccades. We suggest that the discrepancy in horizontal and vertical TACs causes eye positions to lie on a twisted rather than planar surface in three-dimensional space.

Effect of training to an area-cue on human saccadic eye movements

Introduction. Eye movement latency to targets can be shortened by advanced preparation of saccadic programs. Specifically, advanced saccade preparation can be enhanced through training to attend to a specific location in space (Paré & Munoz, 1996). However, people often know the area where a target will appear rather than its precise location. Here, we investigated how saccades were affected by training to attend to a circular area within which a target appeared at random locations. Additionally, we looked at how training to attend to an area of one size influenced saccades to targets presented in a larger circular area. Methods. Each trial began with a central fixation point, followed by simultaneously presenting a circular area-cue (6° or 10° diameter), for 400ms. These disappeared, and following a gap period (170ms or 220ms), the target was flashed for 68ms. Participants were required to quickly and accurately saccade to the target once it appeared. Saccade reaction time and position were recorded. To prevent anticipatory saccades, catch trials were included in pre- and post-training sessions where some targets were presented outside the area-cues. During the training sessions, the target was always presented within a 6° area at random locations. Results. Post-training goal-directed saccades were mostly anticipatory. Participants each developed a preferred region inside the trained area, where post-training anticipatory saccades were directed. This preferred region scaled with cued-area size, i.e. post-training distributions of saccade-end points greatly overlapped once normalized for cued-area position and size. From the preferred region subjects often generated visually driven corrective saccades to the target inside the area cue. Over all, there was no speed-accuracy trade off. Discussion. Present findings show that oculomotor preparations extend to areas, not just to a single target location. Compared to pre-training, the learned strategy assures that targets are acquired more quickly without loss of accuracy.

Stable individual differences across images in human saccadic eye movements.

Individual differences in eye movements during picture viewing were examined across image format, content, and foveal quality in 3 experiments. Experiment 1 demonstrated that an individual's fixation durations were strongly related across 3 types of scene formats and that saccade amplitudes followed the same pattern. In Experiment 2, a similar relationship was observed for fixation durations across faces and scenes, although the amplitude relationship did not hold as strongly. In Experiment 3, the duration and amplitude relationships were observed when foveal information was degraded and even removed. Eye movement characteristics differ across individuals, but there is a great deal of consistency within individuals when viewing different types of images.

Different modulation of medial superior temporal activity across saccades: a functional magnetic resonance imaging study

Studies on saccadic eye movements in humans and animals reported decreased cortical activation accompanying saccades in visual motion sensitive area MT+/V5, implying that the region is the neural basis of saccadic suppression. This, however, conflicts with findings that MT+/V5 is activated by saccades. As MT+/V5 can be subdivided into middle temporal (MT) and medial superior temporal (MST), these regions may have distinct functional roles that cause the discrepancy. To test this hypothesis, we compared the activation of MT with that of MST during exploratory saccades and visually guided saccades. MST was activated only during visually guided saccades, whereas MT was not activated by either. These findings support our hypothesis and suggest that the activity of these regions is differentially modulated depending on extraretinal information.

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.

Brain and Behavior: A Task-Dependent Eye Movement Study

Recent electrophysiological and behavioral studies have found similarities in the neurology of pursuit and saccadic eye movements. In a previous study on eye movements using closely matched paradigms for pursuit and saccades, we revealed that both exhibit bimodal distributions of latency to predictable (PRD) and randomized (RND) stimuli; however, the latency to each type of stimulus was different, and there was more segregation of latencies in saccades than pursuit (Burke MR, Barnes GR. 2006. Quantitative differences in smooth pursuit and saccadic eye movements in humans. Exp Brain Res. 175(4):596-608). To investigate the brain areas involved in these tasks, and to search for correlates of behavior, we used functional magnetic resonance imaging during equivalent PRD and RND target presentations. In the contrast pursuit > saccades, which reflects velocity-dependent versus position-dependent activities, respectively, we found higher activation in the dorsolateral prefrontal cortex (DLPFC) for pursuit and in the frontopolar region for saccades. In the contrast RND > PRD, which principally reflects activation related to visually driven versus memory-driven responses, respectively, we found a higher sustained level of activation in the frontal eye fields during visually guided eye movements. The reverse contrast revealed higher activity for the memory-guided responses in the supplementary eye fields and the superior parietal lobe. In addition, we found learning-related activation during the PRD condition in visual area V5, the DLPFC, and the cerebellum.

The use of working memory for task prediction: What benefits accrue from different types of foreknowledge?

The assumption that the deployment of executive processes invariably improves task performance is implicit to cognitive theory. In particular, working memory can be used to retain and update historical information about predictable trial sequences (foreknowledge) so that subjects can anticipate and prepare for the upcoming trial more effectively. We review the effects of different types of foreknowledge on response accuracy and latency, particularly in relation to experiments investigating saccadic eye movements in humans. While it is possible to make all aspects of an impending trial predictable, varying the predictability of different components of the trial independently can reveal which cognitive operations are potentially modifiable by foreknowledge. These operations include stimulus processing, retrieval of task-set rules, and response preparation, among others. The available data suggest that, while response preparation can be completed and the response even executed before the stimulus appears (i.e. anticipation) when the subject possesses complete task-foreknowledge (knowing both the stimulus to appear and the response required), foreknowledge of the task-set alone does not permit advance configuration of the task-set rules. A taxonomy for foreknowledge is proposed, including foreknowledge for timing, stimulus, set, response, and task. Work on differentiating these effects in neurophysiology, neuroimaging, and neuropsychology is still in the early stages.

Target size modulates saccadic eye movements in humans.

The authors investigated mechanisms involved in transformation of spatially extended targets into saccadic eye-movement vectors. Human subjects performed horizontal saccades to targets of varying diameter, which contained no conspicuous elements within the target shape. With increasing target size, express saccades and saccades with fast regular latencies decreased in frequency, whereas frequency of saccades with slow regular latencies increased. For all targets, saccade amplitude distributions showed a peak close to the geometric center of the targets. However, with large targets, increased scatter of saccade amplitudes and increased undershoot of the target center was observed. These effects may reflect distinct subprocesses involved in sensorimotor transformation to spatially extended targets, and may result from modulation of neuronal activity in the superior colliculus.

Canceling of Pursuit and Saccadic Eye Movements in Humans and Monkeys

The countermanding paradigm provides a useful tool for examining the mechanisms responsible for cancelling eye movements. The key feature of this paradigm is that, on a minority of trials, a stop signal is introduced some time after the appearance of the target, indicating that the subject should cancel the incipient eye movement. If the delay in giving the stop signal is too long, subjects fail to cancel the eye movement to the target stimulus. By modeling this performance as a race between a go process triggered by the appearance of the target and a stop process triggered by the appearance of the stop signal, it is possible to estimate the processing interval associated with cancelling the movement. We have now used this paradigm to analyze the cancelling of pursuit and saccades. For pursuit, we obtained consistent estimates of the stop process regardless of our technique or assumptions--it took 50-60 ms to cancel pursuit in both humans and monkeys. For saccades, we found different values depending on our assumptions. When we assumed that saccade preparation was under inhibitory control up until movement onset, we found that saccades took longer to cancel (humans: approximately 110, monkeys: approximately 80 ms) than pursuit. However, when we assumed that saccade preparation includes a final "ballistic" interval not under inhibitory control, we found that the same rapid stop process that accounted for our pursuit results could also account for the cancelling of saccades. We favor this second interpretation because cancelling pursuit or saccades amounts to maintaining a state of fixation, and it is more parsimonious to assume that this involves a single inhibitory process associated with the fixation system, rather than two separate inhibitory processes depending on which type of eye movement will not be made. From our behavioral data, we estimate that this ballistic interval has a duration of 9-25 ms in monkeys, consistent with the known physiology of the final motor pathways for saccades, although we obtained longer values in humans (28-60 ms). Finally, we examined the effect of trial sequence during the countermanding task and found that pursuit and saccade latencies tended to be longer if the previous trial contained a stop signal than if it did not; these increases occurred regardless of whether the preceding trial was associated with the same or different type of eye movement. Together, these results suggest that a common inhibitory mechanism regulates the initiation of pursuit and saccades.

Voluntary saccadic eye movements in humans studied with a double-cue paradigm

In the classic double-step paradigm, subjects are required to make a saccade to a visual target that is briefly presented at one location and then shifted to a new location before the subject has responded. The saccades in this situation are “reflexive” in that they are made in response to the appearance of the target itself. In the present experiments we adapted the double-step paradigm to study “voluntary” saccades. For this, several identical targets were always visible and subjects were given a cue to indicate that they should make a saccade to one of them. This cue was then changed to indicate another of the targets before the subject had responded: double-cue (DC) paradigm. The saccadic eye movements in our DC paradigm had many features in common with those in the double-step paradigm and we show that apparent differences can be attributed to the spatio-temporal arrangements of the cues/targets rather than to any intrinsic differences in the programming of these two kinds of eye movements. For example, a feature of our DC paradigm that is not seen in the usual double-step paradigm is that the second cue could cause transient delays of the initial saccade, and these delays still occurred when the second cue was reflexive––provided that it was at the fovea (as in our DC paradigm) and not in the periphery (as in the usual double-step paradigm). Thus, the critical factor for the delay was the retinal (foveal) location of the second cue/target––not whether it was cognitive or reflexive––and we argue that the second cue/target is here acting as a distractor. We conclude that the DC paradigm can be used to study the programming of voluntary saccades in the same way that the double-step paradigm can be used to study reflexive saccades.

Target selection for pursuit and saccadic eye movements in humans.

Eye movements were recorded from three subjects as they initiated tracking of a small circle ("target") moving leftward or rightward, above or below the horizontal meridian, either alone or in the presence of a small square ("distractor") moving leftward or rightward on the other side of the horizontal meridian. At the start of each trial, subjects were provided with either a "form" cue (always centrally positioned and having the circular shape and color of the upcoming moving target) or a "location" cue (a small white square positioned where the upcoming target would appear). The latency of pursuit increased in the presence of an oppositely moving distractor when subjects were provided the form cues but not when they were provided the location cues. The latency of saccades showed similar, but smaller, increases when subjects were given the form cues. On many trials with the form cues, pursuit started in the direction of the distractor and then reversed to follow the target. On these trials, the initial saccade often, but not always, also followed the distractor. These results indicate that the mechanisms of target selection for pursuit and saccades are tightly coordinated but not strictly yoked. The shared effects of the distractor on the latencies of pursuit and saccades probably reflect the common role of visual attention in filtering the inputs that guide these two types of eye movements. The differences in the details of the effects on pursuit and saccades suggest that the neural mechanisms that trigger these two movements can be independently regulated.