Secondary (micro-)saccades: The influence of primary saccade end point and target eccentricity on the process of postsaccadic fixation

More (corrective) consecutive saccades after a lesion to the posterior parietal cortex

To reach a target, primary saccades (S1s) are often followed by (corrective) consecutive saccades (S2, and potentially S3, S4, S5), which are based on retinal and extraretinal feedback. Processing these extraretinal signals was found to be significantly impaired by lesions to the posterior parietal cortex (PPC). Recent studies, however, added a more nuanced view to the role of the PPC, where patients with PPC lesions still used extraretinal signals for S2s and perceptual judgements (Fabius et al., 2020; Rath-Wilson & Guitton, 2015). Hence, it seems that a PPC lesion is not disrupting extraretinal processing per se. Yet, a lesion might still result in less reliable processing of extraretinal signals. Here, we investigated whether this lower reliability manifests as decreased or delayed S2 initiation. Patients with PPC lesions (n = 7) and controls (n = 26) performed a prosaccade task where the target either remained visible or was removed after S1 onset. When S1 is removed, accurate S2s (corrections of S1 error) rely solely on extraretinal signals. We analysed S2 quantity and timing using linear mixed-effects modelling and additive hazards analyses. Patients demonstrated slower S1 execution and lower S1 amplitudes than controls, but their S2s still compensated the S1 undershoot, also when they only relied on extraretinal information. Surprisingly, patients showed an increased amount of S2s. This deviation from control behaviour can be seen as suboptimal, but given the decreased accuracy of the primary saccade, it could be optimal for patients to employ more (corrective) consecutive saccades to overcome this inaccuracy.

Motor recalibration of visual and saccadic maps

How does the brain maintain an accurate visual representation of external space? Movement errors following saccade execution provide sufficient information to recalibrate motor and visual space. Here, we asked whether spatial information for vision and saccades is processed in shared or in separate resources. We used saccade adaptation to modify both, saccade amplitudes and visual mislocalization. After saccade adaptation was induced, we compared participants' saccadic and perceptual localization before and after we inserted 'no error' trials. In these trials, we clamped the post-saccadic error online to the predicted endpoints of saccades. In separate experiments, we either annulled the retinal or the prediction error. We also varied the number of 'no error' trials across conditions. In all conditions, we found that saccade adaptation remained undisturbed by the insertion of 'no error' trials. However, mislocalization decreased as a function of the number of trials in which zero retinal error was displayed. When the prediction error was clamped to zero, no mislocalization was observed at all. The results demonstrate the post-saccadic error is used separately to recalibrate visual and saccadic space.

Stronger saccadic suppression of displacement and blanking effect in children

Humans do not notice small displacements to objects that occur during saccades, termed saccadic suppression of displacement (SSD), and this effect is reduced when a blank is introduced between the pre- and postsaccadic stimulus (Bridgeman, Hendry, & Stark, 1975; Deubel, Schneider, & Bridgeman, 1996). While these effects have been studied extensively in adults, it is unclear how these phenomena are characterized in children. A potentially related mechanism, saccadic suppression of contrast sensitivity-a prerequisite to achieve a stable percept-is stronger for children (Bruno, Brambati, Perani, & Morrone, 2006). However, the evidence for how transsaccadic stimulus displacements may be suppressed or integrated is mixed. While they can integrate basic visual feature information from an early age, they cannot integrate multisensory information (Gori, Viva, Sandini, & Burr, 2008; Nardini, Jones, Bedford, & Braddick, 2008), suggesting a failure in the ability to integrate more complex sensory information. We tested children 7 to 12 years old and adults 19 to 23 years old on their ability to perceive intrasaccadic stimulus displacements, with and without a postsaccadic blank. Results showed that children had stronger SSD than adults and a larger blanking effect. Children also had larger undershoots and more variability in their initial saccade endpoints, indicating greater intrinsic uncertainty, and they were faster in executing corrective saccades to account for these errors. Together, these results suggest that children may have a greater internal expectation or prediction of saccade error than adults; thus, the stronger SSD in children may be due to higher intrinsic uncertainty in target localization or saccade execution.

Bold moves: Inevitable saccadic selection in visual short-term memory

Selection for visual short-term memory (vstm) provides a basis for many cognitive functions. Saccadic eye movements sway this selection in favor of stimuli previously seen at locations congruent with their target. In three experiments, we provide converging evidence that this saccadic selection is implemented as a fundamental, inevitable selection process, rather than a top-down strategy. In particular, benefits for congruent over incongruent items were largely constant across set sizes ranging from two to eight items (Experiment 1), showing that saccadic selection imposes priorities on vstm irrespective of memory load and is effective even when only few representations need to be maintained. Moreover, a decrement in performance for incongruent items occurred reliably, whether the congruent location contained a task-relevant item or an irrelevant noise patch (Experiment 2). Finally, saccadic selection was immune to a strong manipulation of the observer's attentional priorities (Experiment 3). Given the prevalence of saccades in natural vision, our results demonstrate a fundamental and ecologically relevant selection mechanism for vstm: Saccades systematically eliminate information seen at non-target locations, while information at the saccade target remains available to recall. This simple heuristic is effective in the absence of informative cues and may incapacitate voluntary selection mechanisms that are incongruent with ongoing movement plans.

Relationship of postsaccadic oscillation with the state of the pupil inside the iris and with cognitive processing

Recent studies using video-based eye tracking have presented accumulating evidence that postsaccadic oscillation defined in reference to the pupil center (PSOp) is larger than that to the iris center (PSOi). This indicates that the relative motion of the pupil reflects the viscoelasticity of the tissue of the iris. It is known that the pupil size controlled by the sphincter/dilator pupillae muscles reflects many aspects of cognition. A hypothesis derived from this fact is that cognitive tasks affect the properties of PSOp due to the change in the state of these muscles. To test this hypothesis, we conducted pro- and antisaccade tasks for human participants and adopted the recent physical model of PSO to evaluate the dynamic properties of PSOp/PSOi. The results showed the dependence of the elasticity coefficient of the PSOp on the antisaccade task, but this effect was not significant for the PSOi. This suggests that cognitive tasks such as antisaccade tasks affect elasticity of the muscle of the iris. We found that the trial-by-trial fluctuation in the presaccade absolute pupil size correlated with the elasticity coefficient of PSOp. We also found the task dependence of the viscosity coefficient and overshoot amount of PSOi, which probably reflects the dynamics of the entire eyeball movement. The difference in task dependence between PSOp and PSOi indicates that the separate measures of these two can be means to distinguish factors related to the oculomotor neural system from those related to the physiological states of the iris tissue.

NEW & NOTEWORTHY The state of the eyeball varies dynamically moment by moment depending on underlying neural/cognitive processing. Combining simultaneous measurements of pupil-centric and iris-centric movements and a recent physical model of postsaccadic oscillation (PSO), we show that the pupil-centric PSO is sensitive to the type of saccade task, suggesting that the physical state of the iris muscles reflects the underlying cognitive processes.

Gain control of saccadic eye movements is probabilistic

Saccades are rapid eye movements that orient the visual axis toward objects of interest to allow their processing by the central, high-acuity retina. Our ability to collect visual information efficiently relies on saccadic accuracy, which is limited by a combination of uncertainty in the location of the target and motor noise. It has been observed that saccades have a systematic tendency to fall short of their intended targets, and it has been suggested that this bias originates from a cost function that overly penalizes hypermetric errors. Here, we tested this hypothesis by systematically manipulating the positional uncertainty of saccadic targets. We found that increasing uncertainty produced not only a larger spread of the saccadic endpoints but also more hypometric errors and a systematic bias toward the average of target locations in a given block, revealing that prior knowledge was integrated into saccadic planning. Moreover, by examining how variability and bias covaried across conditions, we estimated the asymmetry of the cost function and found that it was related to individual differences in the additional time needed to program secondary saccades for correcting hypermetric errors, relative to hypometric ones. Taken together, these findings reveal that the saccadic system uses a probabilistic-Bayesian control strategy to compensate for uncertainty in a statistically principled way and to minimize the expected cost of saccadic errors.

Chances and challenges for an active visual search perspective

Using fixations as the fundamental unit of visual search is an appealing gear change in a paradigm that has long dominated attention research. To truly inform theories of search, however, additional challenges must be faced, including (1) an empirically motivated definition of fixation in the presence of fixational saccades and (2) the biases and limitations of transsaccadic perception and memory.

Decoding Target Distance and Saccade Amplitude from Population Activity in the Macaque Lateral Intraparietal Area (LIP)

Primates perform saccadic eye movements in order to bring the image of an interesting target onto the fovea. Compared to stationary targets, saccades toward moving targets are computationally more demanding since the oculomotor system must use speed and direction information about the target as well as knowledge about its own processing latency to program an adequate, predictive saccade vector. In monkeys, different brain regions have been implicated in the control of voluntary saccades, among them the lateral intraparietal area (LIP). Here we asked, if activity in area LIP reflects the distance between fovea and saccade target, or the amplitude of an upcoming saccade, or both. We recorded single unit activity in area LIP of two macaque monkeys. First, we determined for each neuron its preferred saccade direction. Then, monkeys performed visually guided saccades along the preferred direction toward either stationary or moving targets in pseudo-randomized order. LIP population activity allowed to decode both, the distance between fovea and saccade target as well as the size of an upcoming saccade. Previous work has shown comparable results for saccade direction (Graf and Andersen, 2014a,b). Hence, LIP population activity allows to predict any two-dimensional saccade vector. Functional equivalents of macaque area LIP have been identified in humans. Accordingly, our results provide further support for the concept of activity from area LIP as neural basis for the control of an oculomotor brain-machine interface.

Eye Gaze Behavior at Turn Transition: How Aphasic Patients Process Speakers' Turns during Video Observation

The human turn-taking system regulates the smooth and precise exchange of speaking turns during face-to-face interaction. Recent studies investigated the processing of ongoing turns during conversation by measuring the eye movements of noninvolved observers. The findings suggest that humans shift their gaze in anticipation to the next speaker before the start of the next turn. Moreover, there is evidence that the ability to timely detect turn transitions mainly relies on the lexico-syntactic content provided by the conversation. Consequently, patients with aphasia, who often experience deficits in both semantic and syntactic processing, might encounter difficulties to detect and timely shift their gaze at turn transitions. To test this assumption, we presented video vignettes of natural conversations to aphasic patients and healthy controls, while their eye movements were measured. The frequency and latency of event-related gaze shifts, with respect to the end of the current turn in the videos, were compared between the two groups. Our results suggest that, compared with healthy controls, aphasic patients have a reduced probability to shift their gaze at turn transitions but do not show significantly increased gaze shift latencies. In healthy controls, but not in aphasic patients, the probability to shift the gaze at turn transition was increased when the video content of the current turn had a higher lexico-syntactic complexity. Furthermore, the results from voxel-based lesion symptom mapping indicate that the association between lexico-syntactic complexity and gaze shift latency in aphasic patients is predicted by brain lesions located in the posterior branch of the left arcuate fasciculus. Higher lexico-syntactic processing demands seem to lead to a reduced gaze shift probability in aphasic patients. This finding may represent missed opportunities for patients to place their contributions during everyday conversation.

Revisiting corrective saccades: role of visual feedback

To clarify the role of visual feedback in the generation of corrective movements after inaccurate primary saccades, we used a visually-triggered saccade task in which we varied how long the target was visible. The target was on for only 100ms (OFF100ms), on until the start of the primary saccade (OFFonset) or on for 2s (ON). We found that the tolerance for the post-saccadic error was small (-2%) with a visual signal (ON) but greater (-6%) without visual feedback (OFF100ms). Saccades with an error of -10%, however, were likely to be followed by corrective saccades regardless of whether or not visual feedback was present. Corrective saccades were generally generated earlier when visual error information was available; their latency was related to the size of the error. The LATER (Linear Approach to Threshold with Ergodic Rate) model analysis also showed a comparable small population of short latency corrective saccades irrespective of the target visibility. Finally, we found, in the absence of visual feedback, the accuracy of corrective saccades across subjects was related to the latency of the primary saccade. Our findings provide new insights into the mechanisms underlying the programming of corrective saccades: (1) the preparation of corrective saccades begins along with the preparation of the primary saccades, (2) the accuracy of corrective saccades depends on the reaction time of the primary saccades and (3) if visual feedback is available after the initiation of the primary saccade, the prepared correction can be updated.

Age-related changes in visual exploratory behavior in a natural scene setting

Diverse cognitive functions decline with increasing age, including the ability to process central and peripheral visual information in a laboratory testing situation (useful visual field of view). To investigate whether and how this influences activities of daily life, we studied age-related changes in visual exploratory behavior in a natural scene setting: a driving simulator paradigm of variable complexity was tested in subjects of varying ages with simultaneous eye- and head-movement recordings via a head-mounted camera. Detection and reaction times were also measured by visual fixation and manual reaction. We considered video computer game experience as a possible influence on performance. Data of 73 participants of varying ages were analyzed, driving two different courses. We analyzed the influence of route difficulty level, age, and eccentricity of test stimuli on oculomotor and driving behavior parameters. No significant age effects were found regarding saccadic parameters. In the older subjects head-movements increasingly contributed to gaze amplitude. More demanding courses and more peripheral stimuli locations induced longer reaction times in all age groups. Deterioration of the functionally useful visual field of view with increasing age was not suggested in our study group. However, video game-experienced subjects revealed larger saccade amplitudes and a broader distribution of fixations on the screen. They reacted faster to peripheral objects suggesting the notion of a general detection task rather than perceiving driving as a central task. As the video game-experienced population consisted of younger subjects, our study indicates that effects due to video game experience can easily be misinterpreted as age effects if not accounted for. We therefore view it as essential to consider video game experience in all testing methods using virtual media.