Eye movements and attention: the role of pre-saccadic shifts of attention in perception, memory and the control of saccades.

Saccadic eye movements and perceptual attention work in a coordinated fashion to allow selection of the objects, features or regions with the greatest momentary need for limited visual processing resources. This study investigates perceptual characteristics of pre-saccadic shifts of attention during a sequence of saccades using the visual manipulations employed to study mechanisms of attention during maintained fixation. The first part of this paper reviews studies of the connections between saccades and attention, and their significance for both saccadic control and perception. The second part presents three experiments that examine the effects of pre-saccadic shifts of attention on vision during sequences of saccades. Perceptual enhancements at the saccadic goal location relative to non-goal locations were found across a range of stimulus contrasts, with either perceptual discrimination or detection tasks, with either single or multiple perceptual targets, and regardless of the presence of external noise. The results show that the preparation of saccades can evoke a variety of attentional effects, including attentionally-mediated changes in the strength of perceptual representations, selection of targets for encoding in visual memory, exclusion of external noise, or changes in the levels of internal visual noise. The visual changes evoked by saccadic planning make it possible for the visual system to effectively use saccadic eye movements to explore the visual environment.

Attention during sequences of saccades along marked and memorized paths.

Natural scenes are explored by combinations of saccadic eye movements and shifts of attention. The mechanisms that coordinate attention and saccades during ordinary viewing are not well understood because studies linking saccades and attention have focused mainly on single saccades made in isolation. This study used an orientation discrimination task to examine attention during sequences of saccades made through an array of targets and distractors. Perceptual measures showed that attention was distributed along saccadic paths when the paths were marked by color cues. When paths were followed from memory, attention rarely spread beyond the goal of the upcoming saccade. These different distributions of attention suggest the involvement of separate processes of attentional control during saccadic planning, one triggered by top-down selection of the saccadic target, and the other by activation linked to visual mechanisms not tied directly to saccadic planning. The concurrent activity of both processes extends the effective attentional field without compromising the accuracy, precision, or timing of saccades.

Attention during active visual tasks: counting, pointing, or simply looking.

Visual attention and saccades are typically studied in artificial situations, with stimuli presented to the steadily fixating eye, or saccades made along specified paths. By contrast, in real-world tasks saccadic patterns are constrained only by the demands of the motivating task. We studied attention during pauses between saccades made to perform three free-viewing tasks: counting dots, pointing to the same dots with a visible cursor, or simply looking at the dots using a freely-chosen path. Attention was assessed by the ability to identify the orientation of a briefly-presented Gabor probe. All primary tasks produced losses in identification performance, with counting producing the largest losses, followed by pointing and then looking-only. Looking-only resulted in a 37% increase in contrast thresholds in the orientation task. Counting produced more severe losses that were not overcome by increasing Gabor contrast. Detection or localization of the Gabor, unlike identification, were largely unaffected by any of the primary tasks. Taken together, these results show that attention is required to control saccades, even with freely-chosen paths, but the attentional demands of saccades are less than those attached to tasks such as counting, which have a significant cognitive load. Counting proved to be a highly demanding task that either exhausted momentary processing capacity (e.g., working memory or executive functions), or, alternatively, encouraged a strategy of filtering out all signals irrelevant to counting itself. The fact that the attentional demands of saccades (as well as those of detection/localization) are relatively modest makes it possible to continually adjust both the spatial and temporal pattern of saccades so as to re-allocate attentional resources as needed to handle the complex and multifaceted demands of real-world environments.

Visual memory during pauses between successive saccades.

Selective attention is closely linked to eye movements. Prior to a saccade, attention shifts to the saccadic goal at the expense of surrounding locations. Such a constricted attentional field, while useful to ensure accurate saccades, constrains the spatial range of high-quality perceptual analysis. The present study showed that attention could be allocated to locations other than the saccadic goal without disrupting the ongoing pattern of saccades. Saccades were made sequentially along a color-cued path. Attention was assessed by a visual memory task presented during a random pause between successive saccades. Saccadic planning had several effects on memory: (1) fewer letters were remembered during intersaccadic pauses than during maintained fixation; (2) letters appearing on the saccadic path, including locations previously examined, could be remembered; off-path performance was near chance; (3) memory was better at the saccadic target than at all other locations, including the currently fixated location. These results show that the distribution of attention during intersaccadic pauses results from a combination of top-down enhancement at the saccadic target coupled with a more automatic allocation of attention to selected display locations. This suggests that the visual system has mechanisms to control the distribution of attention without interfering with ongoing saccadic programming.

The relationship between spatial pooling and attention in saccadic and perceptual tasks.

Saccades aimed at spatially extended targets land reliably at central locations determined by pooling information across the target shape [Melcher, D., & Kowler, E. (1999). Shape, surfaces and saccades. Vision Research, 39, 2929-2946; Vishwanath, D., & Kowler, E. (2003). Localization of shapes: Eye movements and perception compared. Vision Research, 43, 1637-1653]. Previous findings of saccadic errors when attempting to look at a target in the midst of distractors encouraged suggestions that pooling occurs indiscriminately, with little or no influence of a selective filter to eliminate the influence of nearby distractors. To determine the effectiveness of filtering, saccadic localization was studied for saccades made to a set of target elements (discs) interleaved with an equivalent set of distractors of a different color. With such interleaved elements, selection and spatial pooling are constrained to occur over the same spatial region. The results showed that filtering was effective and saccadic landing position was determined mainly by the target elements. Concurrent perceptual judgments made about the same stimuli (estimating the mean size of either target or distractor discs) showed better performance for the target discs than distractors, confirming that perceptual attention was allocated to the set of target elements. These results: (1) support the role of attention in setting the input to the spatial pooling process that guides saccades to spatially extended targets, and (2) show that perceptual judgments of mean value, often thought to impose modest attentional demands, are not immune to the constraints of this pre-saccadic filter.

Eye movements during multiple readings of the same text.

People often read the same text more than once. Studying eye movements during multiple readings of the same texts provides a unique opportunity to observe the consistency of saccadic landing positions. Eye movements were recorded while 5 people read the same 4 texts more than 40 times, no more than 4 times/day, and never on consecutive trials. Other texts, read only once, were interspersed. Comprehension questions and a change-detection task helped maintain attention in the face of the repetition. There were two main findings: (1) repeated reading produced significant, but modest, changes in global saccadic patterns. The only change found in all readers was a reduction in the proportion of regressions. (2) Saccadic landing positions fell into clusters located at a variety of places with respect to word boundaries, and often across word boundaries. A mixed-strategy model of saccadic guidance (look to the center of words, while trying to maintain fairly uniform saccade lengths), could account for the overall strength of clustering, but not for the variability among cluster locations, suggesting that saccadic landing sites are selected in part on the basis of local text characteristics. The reliable clustering of saccadic landing positions found during multiple readings of the same text opens the way for cluster patterns to be used to study eye movement strategies during reading and overcome at least some of the variability associated with traditional global single-text measures.