This commentary focuses on Findlay & Walker's model and more specifically, on its underestimation of the role of cognitive processes in eye movement control during complex activities such as text scanning. In particular, the issue of the complexity of the subject's task/behavior is discussed to stress the importance of the link between selection for perceptual processing on the one hand, and the selection of a target for a saccade, on the other. Future models will have to account for the fact that the goal of any saccade is to bring the eyes to a relevant object and that the selection of this saccade target is closely related to object recognition.

The Findlay & Walker target article emphasizes the role of the target-nonspecific “fixate” system while downplaying the role of the target-specific “move” system in determining saccade latency. We agree that disengagement of the fixate system is responsible for the target-nonspecific latency reduction associated with the gap effect. However, high target predictability and extensive training at a target location can also result in latency reductions, the culmination of this being express saccades. The target-specificity associated with the latter forms of latency reduction implicate a mechanism involving the move system. Recently discovered neurophysiological correlates underlying these behavioural phenomena reside in the superior colliculus.

The posterior parietal cortex and frontal eye field contain maps of visual salience on which the decision to choose a saccade may be based. However, an averaging express saccade is not represented by a victorious unimodal representation in the superior colliculus. Normalization as described by Findlay & Walker is not necessary for the generation of saccades.

We studied patients with left visual extinction following right hemisphere damage in a simple manual reaction time task using brief visual stimuli. With unilateral lateralized stimuli the patients showed a high proportion of unwanted, reflex-like saccades to either side of stimulation. In contrast, with bilateral stimuli there was an overall decrease in the proportion of unwanted saccades, and the vast majority of them were directed toward the ipsilesional side. The implications of these results for the Findlay & Walker model are discussed.

This commentary considers experimental material – some new, some from earlier studies – challenging the model presented by Findlay & Walker. It concentrates on the role of voluntary and involuntary visual attention versus fixation in saccade control and on the generation of antisaccades, reflexive prosaccades, and corrective saccades. The data of a large number of subjects are presented to show the systematic relationship between voluntary saccade generation, error production, and error correction in an antisaccade task.

The framework of Findlay & Walker's target article provides a first attempt to model the saccadic system at all levels. Their scheme is based on two main principles. These are “parallel processing of saccade timing and metrics” and “competitive inhibition through winner-take-all strategies.” In our opinion, however, both concepts are in their strictest sense at odds with the current knowledge of the saccadic system, and need to be refined to make the scheme more relevant.

The Findlay & Walker model of saccade generation does not appear to account fully for saccadic performance deficits observed in schizophrenia patients. It would be enhanced by inclusion of a frontally mediated, central executive function system. A review of schizophrenia patients' antisaccade task deficits provides an example of the role of higher cortical functioning in saccade generation.

In the model proposed by Findlay & Walker, the programming of saccadic eye movements is achieved by two parallel processes, one dedicated to the coding of saccade metrics (Where) and the other controlling saccade initiation (When). One outcome of the “winner-take-all” characteristics of the salience map, the main node of the model, is an independence between the metrics and the latency of saccades. We report on some observations, made in the head-unrestrained cat under pathological conditions, of a correlation between accuracy and latency of saccadic gaze shifts. To account for such a correlation, the link between metrics specification (Where) and saccade triggering (When) should be amended in the model.

Experiments using a modified gap paradigm, where regular trials are intermingled with catch trials (trials without saccade target), demonstrate that the relative frequency of express versus regular saccades distinctly depends on catch trial frequency. More specifically, it has been shown that the probability of an express saccade depends stochastically on the type of the preceding trial, that is, on the sequence of stimuli. We discuss whether such contextual effects can be accommodated within the framework of Findlay & Walker's model.

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.

In several recent experiments we have found that the eyes are often captured by the appearance of a sudden onset in a display, even though subjects intend to move their eyes elsewhere. Very brief fixations are made on the abrupt onset before the eyes complete their intended movement to the previously defined target. These results indicate concurrent programming of a voluntary saccade to the defined saccade target and an involuntary saccade to the sudden onset. This is inconsistent with the idea that a single salience map determines the location of a saccade in a winner-take-all fashion. Other results indicate that subjects attend to more than one location in a display during saccade preparation, contrary to the claim that covert attentional scanning plays no role in saccade generation.

Models of saccade generation and visual selective attention must explain how and why particular targets are selected. Findlay & Walker do an excellent job of explaining the how of visual selection, but not the why. For a salience map to be more than a description of the relative importance of potential targets, there must be some account of the learning and inheritance that fashion its peaks and troughs. Point of gaze is not necessarily region of attention, and it may be premature to downgrade the role of covert attention in models of visual selection.

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.

Winner-takes-all (WTA) typically describes a mechanism for selecting the highest peak of activity in a sensory map that encodes independent representations of potential targets. To Findlay & Walker, WTA is an inherent property of a motor map that is incapable of representing multiple targets independently. Although the output of a WTA system should be characteristic of only one target, actions can be influenced by multiple targets.

A model of when and where saccades are made necessarily incorporates a model of the “When” and “Where” of target selection. We suggest that the framework proposed by Findlay & Walker does not specify sufficiently how (and by what means) selection processes contribute to the spatial and temporal determinants of saccade generation. Examples from across-trial priming in visual search and from the inhibition of temporally segmented distractors show linkage between the processes involved in computing when and where selection operates, so that there is cooperation rather than competition between so-called Where and When pathways. Aspects of spatial selection may also determine the remote distractor effect on saccades. The detailed relations between the processes involved in selection and saccade generation may be best understood in relation to detailed computational accounts of the processes.

Recordings of neuronal activity in the monkey superior colliculus (SC) suggest that the two apparently independent effects of a visual distractor on both temporal (latency) and spatial (metrics) saccade parameters may be the result of lateral interactions between subpopulations of saccade-related neurons located at different sites on the motor map of the superior colliculus. One subpopulation is activated during the planing and initiation of a saccade; the other is activated by the appearance of a distractor. The inhibitory or facilitative nature of this interaction depends on the distance between the distractor and the target and is consistent with the complex pattern of intrinsic and commissural collicular connections.

We are largely in agreement with the Findlay & Walker model. However, they appear to dismiss the role of covert spatial attention in tasks in which people are free to move their eyes. We argue that an account of the facts about the perceptual span in reading requires a window of attention not centered around the fovea. Moreover, a computational model of reading that we (Reichle et al. 1998) developed gives a good account of eye movement control in reading and would be unable to do so without relying heavily on covert attention.

Deciding where to look is a very complicated process, but deciding when to look is probably much simpler. Once the brain is certain of where it wants to look, the saccade-generating pathway may simply start autonomously. Higher cognitive areas of the brain can only override this automatic start by preventing all saccades. Hence, the brain has no ability to say when to go; it can only say “go there” or “stay here.”

The postulate that events in peripheral vision enhance activity in the “fixate center” is called into question. An alternative explanation is used to account for the “remote distractor effect.” It is pointed out that a critical element of the model is missing.

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.