The adjustable pattern generator (APG) model addresses physiological detail in a manner that renders it eminently testable. However, the problem for which the APG was developed, namely, limb control, may be computationally too complex for this purpose. Instead, it is proposed that recent empirical and theoretical advances in understanding the role of the cerebellum in low-level saccadic control could be used to refine and extend the APG. [HOUK et al.].

E-Z Reader achieves an impressive fit of empirical eye movement data by simulating core processes of reading in a computational approach that includes serial word processing, shifts of attention, and temporal overlap in the programming of saccades. However, when common assumptions for the time requirements of these processes are taken into account, severe constraints on the time line within which these elements can be combined become obvious. We argue that it appears difficult to accommodate these processes within a largely (...) sequential modeling framework such as E-Z Reader. (shrink)

In their model, Findlay & Walker propose that where and when the eyes move is determined by two relatively independent processing streams. Whereas both saccade direction and amplitude result from a low-level visual analysis of the peripheral visual stimulation, saccade latency results mainly from higher-level processes related to processing of the central information. In the present commentary, reading eye movement data are put forward as evidence against a strict autonomy of “Where” and “When” processing streams. First, saccade direction and amplitude (...) might be modified by high-level processes related to word identification. Second, the direction of a saccade directly affects its latency. (shrink)

& The present study investigated saccadic inhibition in both voluntary and stimulus-elicited saccades. Two experiments examined saccadic inhibition caused by an irrelevant flash occurring subsequent to target onset. In each trial, participants were required to perform a single saccade following the presentation of a black target on a gray background, 48 to the left or to the right of screen center. In some trials (flash trials), after a variable delay, a 33-msec flash was displayed at the top and bottom (...) third of the monitor (these regions turned white). In all experimental conditions, histograms of flash-to-saccade latencies documented a decrease in saccadic frequency, forming a dip, time-locked to the flash and occurring as early as 60–70 msec following its onset. The fast latency of this effect strongly suggests a low-level, reflex-like, oculomotor effect, which was referred to as saccadic inhibition. A novel procedure was developed to allow comparisons of saccadic inhibition even across conditions, which in the absence of a flash (no-. (shrink)

Findlay & Walker's target article proposes a model of saccade generation related to the underlying neuroscience. A problem with such models is the number of brain areas showing oculomotor function. Traditionally, therefore, models have been partial, usually concentrating either on cortex (Liu et al. 1997; Pierrot Deseilligny et al. 1995) or on the superior colliculus and brainstem circuits (Moschovakis 1994; Van Gisbergen et al. 1993). Findlay & Walker's model attempts to integrate both levels within a functional framework. To some extent (...) it falls between two stools. For example, some functions that the authors ascribe to subcortical regions may actually occur at the cortical level. (shrink)

Several gaze contingent studies that used a fixed delay between physical eye movements and a display change documented a dip in the fixation duration distributions (e.g., Blanchard et al. 1984; McConkie et al. 1985; van Diepen et al. 1995). In a study by van Diepen et al. (1995), a moving mask paradigm was employed in which subjects searched line drawings of everyday scenes for non-objects. The appearance of the mask was delayed relative to the end of a saccade (beginning of (...) fixation) by 17, 46, 76 or 121 msec. All fixation duration distributions in the masking conditions exhibited a dip with longer masking delays resulting in the dip occurring at longer fixation durations. In contrast, a no-mask condition did not produce a dip. Similar effects in reading were reported by Blanchard et al. (1984), and McConkie et al. (1985). In both these studies the text was.. (shrink)

In 5 experiments, participants read text that was briefly replaced by a transient image for 33 ms at random intervals. A decrease in saccadic frequency, referred to as saccadic inhibition, occurred as early as 60 –70 ms following the onset of abrupt changes in visual input. It was demonstrated that the saccadic inhibition was influenced by the saliency of the visual event (Experiment 3) and was not produced in response to abrupt but irrelevant auditory stimuli (Experiment 1). Display changes restricted (...) to an area either inside or outside the perceptual span required for normal reading produced strong saccadic inhibition (Experiment 2). Finally, Experiments 4 and 5 demonstrated higher level cognitive or attentional modulation of the saccadic inhibition effect. (shrink)

During active vision, the eyes continually scan the visual environment using saccadic scanning movements. This target article presents an information processing model for the control of these movements, with some close parallels to established physiological processes in the oculomotor system. Two separate pathways are concerned with the spatial and the temporal programming of the movement. In the temporal pathway there is spatially distributed coding and the saccade target is selected from a Both pathways descend through a hierarchy of levels, the (...) lower ones operating automatically. Visual onsets have automatic access to the eye control system via the lower levels. Various centres in each pathway are interconnected via reciprocal inhibition. The model accounts for a number of well-established phenomena in target-elicited saccades: the gap effect, express saccades, the remote distractor effect, and the global effect. High-level control of the pathways in tasks such as visual search and reading is discussed; it operates through spatial selection and search selection, which generally combine in an automated way. The model is examined in relation to data from patients with unilateral neglect. (shrink)

To eliminate the leap of faith required to explain how visual consciousness arises from visual representation, O'Regan & Noë focus on the sensorimotor interaction with the outside world and ban internal representations from their account of vision. We argue that evidence for transsaccadic representations necessitates a central position for an internal, on-line stimulus rendition in any adequate theory of vision.

In this article we use proportional hazards models to examine how low-level processes affect the probability of making a saccade over time, through the period of fixation, during reading. We apply the Cox proportional hazards model to investigate how launch distance (relative to word beginning), fixation location (relative to word center), and word frequency affect the hazard of a saccadic response. This model requires that covariates have a constant impact on the hazard over time, the assumption of proportional hazards. We (...) show that this assumption is not supported. The impact of the covariates changes with the time passed since fixation onset. To account for the non-proportional hazards we fit step functions of time, resulting in a model with time-varying effects on the hazard. We evaluate the ability to predict the timing of saccades on held-out fixation data. The model with time-varying effects performs better in predicting the timing of saccades for fixations as short as 100 ms and as long as 500 ms, when compared both to a baseline model without covariates and a model which assumes constant covariate effects. This result suggests that the time-varying effects model better recovers the time course of low-level processes that influence the decision to move the eyes. (shrink)

We summarize several experiments indicating that the saccadic system is capable of simultaneously programming two movements toward different goals. This concurrent processing of saccades can lead to the execution of two saccades separated by an extremely short intersaccadic interval. This supports the idea of target competition proposed in Findlay & Walker's article, but suggests a greater degree of parallel processing. We provide evidence that concurrent processing of two saccades is not limited to higher-level planning subsystems; rather, it (...) also involves both regions close enough to the motor output that it can systematically affect saccade trajectory. (shrink)

Microsaccades, small saccadic eye movements occurring during fixation, have been suggested to be modulated by various sensory, cognitive, and affective processes relating to arousal. Although the modulation of fatigue-related arousal on microsaccade behavior has previously been characterized, the influence of other aspects of arousal, such as emotional arousal, is less understood. Moreover, microsaccades are modulated by cognitive processes that could also be linked to arousal. To investigate the influence of emotional arousal, saccade preparation, and global luminance levels on microsaccade behavior, (...) emotional auditory stimuli were presented prior to the onset of a fixation cue whose color indicated to look either at the peripheral stimulus or in the opposite direction of the stimulus. Microsaccade behavior was found to be significantly modulated by saccade preparation and global luminance level, but not emotional arousal. In the pro- and anti-saccade task, microsaccade rate was lower during anti-saccade preparation as compared to pro-saccade preparation, though microsaccade dynamics were comparable during both trial types. Our results reveal a differential role of arousal linked to emotion, fatigue, saccade preparation, and global luminance level on microsaccade behavior. (shrink)

The aim of the present study was to investigate how saccadic selection relates to people’s awareness of the saliency and identity of a saccade goal. Observers were instructed to make an eye movement to either the most salient line segment or the only right-tilted element in a visual search display. The display was masked contingent on the first eye movement and after each trial observers indicated whether or not they had correctly selected the target. Whereas people’s awareness concerning the saliency (...) of the saccade goal was generally low, their awareness concerning the identity was high. Observers’ awareness of the saccade goal was not related to saccadic performance. Whereas saccadic selection consistently varied as a function of saccade latency, people’s awareness concerning the saliency or identity of the saccade goal did not. The results suggest that saccadic selection is primarily driven by subconscious processes. (shrink)

In this commentary we describe findings in normal human subjects and in patients with visual hemineglect that support the importance of higher-level influences on saccade generation during visual exploration. As the duration of fixations increases with increases in the cognitive demand of the task, the timing of exploratory saccades is controlled more by centers of cognitive and perceptual processing at levels 4 and 5 than by reflex-like automatic processes at level 3. In line with this, unilateral frontal eye field (...) lesions impair systematic, intentional saccadic exploration of visual scenes, causing prolonged fixations and contralesional hemineglect, but leave visually triggered reflexive saccades largely intact. (shrink)

Vision substitution by electro-stimulation has been studied since the 60s beginning with P. Bach-y-Rita. Camera pictures or movies encoded in gray levels are displayed using an electro-stimulation display device on the surface of a body part, such as the skin or the tongue. Medical-technical devices have been developed on this principle to compensate for sensory-motor disabilities such as blindness or loss of balance, or to guide specific actions, such as surgery. However, the electrical signals of stationary or moving slowly moving (...) objects, displayed on a Tongue display unit , are quickly lost due to saturation of receptors undergoing electrostimulation. We propose to add random saccades or sparkle to the displayed visual scene to increase the quality of pattern recognition by the subjects. In the present experimental trimodal study , we show that the presence of a moderate sparkle level enhances the perception of the direction of lines drawn on a TDU and reduces the response time. (shrink)

The theoretical framework laid out by Findlay & Walker has direct implications for central topics in research on saccades in reading and other cognitive activities and these in turn may also have implications to be considered in the context of Findlay & Walker's model. The present commentary focuses on the problem of selecting a target for a saccade. It is argued that there are indirect and direct top-down influences on this process and that direct influences are not adequately represented (...) in Findlay & Walker's theory. (shrink)

Saccadic eye movements create (at least) two related but distinct problems for the visual system: they cause rapid image motion and a displacement of the retinal image. Although it is often assumed that the motion is too fast to be resolved, this is certainly not the case for low-spatial-frequency images. Recent experiments have suggested that the reason we are unaware of the motion during saccades is because motion channels are selectively suppressed, possibly by suppression of the magno-cellular (but not (...) the parvocellular) pathway. This suppression may explain why there is no sensation of motion during saccades, but it leaves open the problem of perceiving a stable world despite continual image displacements. (shrink)

Synchronized movements with external periodic rhythms, such as dancing to a beat, are commonly observed in daily life. Although it has been well established that some vocal learning species (including parrots and humans) spontaneously develop this ability, it has only recently been shown that monkeys are also capable of predictive and tempo-flexible synchronization to periodic stimuli. In our previous study, monkeys were trained to make predictive saccades for alternately presented visual stimuli at fixed stimulus onset asynchronies (SOAs) to obtain (...) a liquid reward. The monkeys generalized predictive synchronization to novel SOAs in the middle of trained range, suggesting a capacity for tempo-flexible synchronization. However, it is possible that when encountering a novel tempo, the monkeys might sample learned saccade sequences from those for the short and long SOAs so that the mean saccade interval matched the untrained SOA. To eliminate this possibility, in the current study we tested monkeys on novel SOAs outside the trained range. Animals were trained to generate synchronized eye movements for 600 and 900-ms SOAs for a few weeks, and then were tested for longer SOAs. The accuracy and precision of predictive saccades for one untrained SOA (1200 ms) were comparable to those for the trained conditions. On the other hand, the variance of predictive saccade latency and the proportion of reactive saccades increased significantly in the longer SOA conditions (1800 and 2400 ms), indicating that temporal prediction of periodic stimuli was difficult in this range, similar to previous results on synchronized tapping in humans. Our results suggest that monkeys might share similar synchronization mechanisms with humans, which can be subject to physiological examination in future studies. (shrink)

It is often assumed that the reaction time of a saccade toward visual and/or auditory stimuli reflects the sensitivities of our oculomotor-orienting system to stimulus saliency. Endogenous factors, as well as stimulus-related factors, would also affect the saccadic reaction time. However, it was not clear how these factors interact and to what extent visual and auditory-targeting saccades are accounted for by common mechanisms. The present study examined the effect of, and the interaction between, stimulus saliency and audiovisual spatial congruency (...) on the SRT for visual- and for auditory-target conditions. We also analyzed pre-target pupil size to examine the relationship between saccade preparation and pupil size. Pupil size is considered to reflect arousal states coupling with locus-coeruleus activity during a cognitive task. The main findings were that the pattern of the examined effects on the SRT varied between visual- and auditory-auditory target conditions, the effect of stimulus saliency was significant for the visual-target condition, but not significant for the auditory-target condition, Pupil velocity, not absolute pupil size, was sensitive to task set, and there was a significant correlation between the pre-saccade absolute pupil size and the SRTs for the visual-target condition but not for the auditory-target condition. The discrepancy between target modalities for the effect of pupil velocity and between the absolute pupil size and pupil velocity for the correlation with SRT may imply that the pupil effect for the visual-target condition was caused by a modality-specific link between pupil size modulation and the SC rather than by the LC-NE system. These results support the idea that different threshold mechanisms in the SC may be involved in the initiation of saccades toward visual and auditory targets. (shrink)

There are two accounts of how readers of unspaced writing systems know where to move their eyes: saccades are directed toward default targets ; or saccade lengths are adjusted dynamically, as a function of ongoing parafoveal processing. This article reports an eye-movement experiment supporting the latter hypothesis by demonstrating that the slope of the relationship between the saccade launch site on word N and the subsequent fixation landing site on word N + 1 is > 1, suggesting that (...) class='Hi'>saccades are lengthened from launch sites that afford more parafoveal processing. This conclusion is then evaluated and confirmed via simulations using implementations of both hypotheses, with a discussion of these results for our understanding of saccadic targeting during reading and existing models of eye-movement control. (shrink)

We identify two aspects of the problem of maintaining perceptual stability despite an observer's eye movements. The first, visual direction constancy, is the (egocentric) stability of apparent positions of objects in the visual world relative to the perceiver. The second, visual position constancy, is the (exocentric) stability of positions of objects relative to each other. We analyze the constancy of visual direction despite saccadic eye movements.Three information sources have been proposed to enable the visual system to achieve stability: the structure (...) of the visual field, proprioceptive inflow, and a copy of neural efference or outflow to the extraocular muscles. None of these sources by itself provides adequate information to achieve visual direction constancy; present evidence indicates that all three are used.Our final question concerns how information processing operations result in a stable world. The three traditionally suggested means have been elimination, translation, or evaluation. All are rejected. From a review of the physiological and psychological evidence we conclude that no subtraction, compensation, or evaluation need take place. The problem for which these solutions were developed turns out to be a false one. We propose a “calibration” solution: correct spatiotopic positions are calculated anew for each fixation. Inflow, outflow, and retinal sources are used in this calculation: saccadic suppression of displacement bridges the errors between these sources and the actual extent of movement. (shrink)