Ongoing Cognitive Processing Influences Precise Eye-Movement Targets in Reading

1,2 Klinton Bicknell, Roger Levy3, and Keith Rayner4 1AI Research Group, Duolingo, Pittsburgh, Pennsylvania; 2Department of Linguistics, Northwestern University; 3Department of Brain & Cognitive Sciences, Massachusetts Institute of Technology; and 4Department of Psychology, University of California, San Diego

Abstract

Reading is a highly complex learned skill in which humans move their eyes three to four times every second in response to visual and cognitive processing. The consensus view is that the details of these rapid eye-movement decisions—which part of a word to target with a saccade—are determined solely by low-level oculomotor heuristics. But maximally efficient saccade targeting would be sensitive to ongoing word identification, sending the eyes farther into a word the farther its identification has already progressed. Here, using a covert text-shifting paradigm, we showed just such a statistical relationship between saccade targeting in reading and trial-to-trial variability in cognitive processing. This result suggests that, rather than relying purely on heuristics, the human brain has learned to optimize eye movements in reading even at the fine-grained level of character-position targeting, reflecting efficiency-based sensitivity to ongoing cognitive processing.

Keywords

reading, eye movements, psycholinguistics, motor control, open data, open materials

Introduction

Reading is a complex learned skill that is near-ubiquitous in industrialized societies, requiring tight coordination among vision, language processing, and motor control. It is highly practiced, and although it emerged only 4,000 years ago—too recently and (until the past few centuries) in too small a fraction of the population of any society to have exerted selective pressure on brain evolution—skilled readers have a specialized brain area for it (Dehaene & Cohen, 2011). Given recent research suggesting near-optimality in other complex tasks requiring a tight link between information processing and motor control (Körding & Wolpert, 2004; Najemnik & Geisler, 2005; Todorov & Jordan, 2002), it is of considerable interest to what extent motor control in reading is optimized for ongoing processing.

Saccade Targeting

The basic facts about where saccades land on words are straightforward. Given a particular launch site—the origin of a saccade into a word—the distribution of landing sites on the word is unimodal, relatively broadly distributed across the word. The mode of this distribution depends on the launch site: the nearer to the word, the farther forward the mode of the distribution. Researchers agree that this distribution is the consequence of (possibly implicit) selection of a target position within the word, overlaid with roughly normally distributed random error from the motor system. The question taken up in this research was what factors influence a saccade’s precise target position.

Saccade target position (henceforth, target) is commonly thought to be determined by a fast heuristic composed of two components (McConkie et al., 1988). The functional target of a saccade is always the center of the word, which is presumed to be a reasonable position from which to process the word on average on the basis of single-word-recognition experiments (cf. O’Regan et al., 1984). Then, systematic error biases the saccade length toward a preferred saccade size (e.g., seven characters), yielding an actual target.

Cognitive-Processing Account

An alternative to this fast-heuristic account is one in which readers target saccades to the most useful part of an upcoming word, given their current state of cognitive processing (Bicknell & Levy, 2010, 2012; Legge et al., 1997; Rayner et al., 1980). We refer to this as the cognitive-processing account of saccade targeting. To understand how this account works, note first that readers often start identifying an upcoming word while still fixating on a previous word.

Method

In our experiments, we tracked readers’ eyes while they read individual sentences in fixed-width font. Each sentence contained a seven-letter target word. When the readers’ eyes crossed an invisible boundary immediately prior to the space preceding the target word (Rayner, 1975), the display was updated with one of three possible outcomes: no shift, in which no change occurred; text-right shift, in which the text was shifted three characters to the right, so that the eyes landed farther back in the text than they normally would have; or text-left shift, in which the text was shifted three characters to the left, so that the eyes landed farther forward in the text than they normally would have.

Results

Overall analysis Figure 2 shows gaze durations and refixation probabilities broken down according to the 4 × 2 × 2 factorial analysis described above. For every actual-landing-site-controlled comparison, the original destination that was farther behind showed longer gaze durations and higher refixation probabilities on the target word, as predicted by the cognitive-processing account.

Discussion

These data provide strong evidence against the fast-heuristic account, in which all saccades to a word from a particular launch site are aimed at the same position. Our key result—that saccades directed farther forward in the target word were associated with less subsequent target-word processing on landing—is highly consistent with the predictions of the cognitive-processing account of saccade targeting.

Author Contributions

K. Bicknell, R. Levy, and K. Rayner designed and performed the research and analyzed the data. K. Bicknell and R. Levy wrote the manuscript. All the authors approved the final manuscript for submission.