Biomedical subjects

Richard C Shillcock

Publications and source records attributed to Richard C Shillcock.

8 recordsLinked to original sources

PubMed2005

Patients with hemianopic alexia adopt an inefficient eye movement strategy when reading text.

Patients with an acquired homonymous hemianopia often adapt over a period of a few months to compensate for some of the impairments caused by their visual field defect. Changes in their eye movement patterns have been demonstrated as performance on visual tasks improves with time; however, these patients often complain of persistent text reading problems. Using a video-based eye-movement tracking system, we investigated the text reading behaviour of patients with established hemianopic alexia (>6 months post deficit), a condition affecting left-to-right readers, with a homonymous field defect that encroaches into their right foveal/parafoveal visual field. Word-based analyses of text reading are standard in experiments involving normal readers, but this is the first time these methods have been extended to patients with hemianopic alexia. Using this method, we compared the patients' reading scanpaths to those generated by normal controls reading the same passages, and a random model generated by matching the patients' eye movement data to random permutations of the text they read. We demonstrate that patients adopt an inefficient reading strategy, fixating to the left of the preferred viewing location of words of four letters and longer. Fixating to the left of the normal preferred viewing location not only results in less of the fixated word being processed by the language system; ensuing fixations are also more likely to land within the same word (a refixation). It is this refixation rate that is the main factor in slowing reading times in these patients. Our data suggests that patients are able to extract some useful visual information from text to aid the planning of reading scanpaths as their behaviour differs critically from the random model. Potential reasons for this patient group failing to produce an effective reading strategy are discussed.

Adult↗

PubMed2005

The implications of foveal splitting for saccade planning in reading.

The human fovea and visual pathways are precisely split: information in one hemifield is initially projected to the contralateral visual cortex. This fundamental anatomical constraint on word recognition in reading has been largely ignored in eye movement research. We explore the consequences of this constraint through analyses of a large corpus of eye movement data, and demonstrate that aspects of saccade planning (target selection, initial landing position) are sensitive to both hemispheres, estimated uncertainty about the identity of the currently fixated word. We interpret these findings in terms of a hemispheric division of labour. We suggest that anatomical, visual and lexical factors all contribute to the decision of where to send the eyes next in reading.

Fixation, Ocular↗

PubMed2005

An anatomically constrained, stochastic model of eye movement control in reading.

This article presents SERIF, a new model of eye movement control in reading that integrates an established stochastic model of saccade latencies (LATER; R. H. S. Carpenter, 1981) with a fundamental anatomical constraint on reading: the vertically split fovea and the initial projection of information in either visual field to the contralateral hemisphere. The novel features of the model are its simulation of saccade latencies as a race between two stochastic rise-to-threshold LATER units and its probabilistic selection of the target for the next saccade. The model generates simulated eye movement behavior that exhibits important characteristics of actual eye movements made during reading; specifically, simulations produce realistic saccade target distributions and replicate a number of critical reading phenomena, including the effects of word frequency on fixation durations, the inverted optimal viewing position effect, the trade-off between first and second fixation durations of refixated words, and the dependence of parafoveal preview benefit on eccentricity.

Eye Movements↗

PubMed2005

Toward an appropriate baseline for measures of eye movement behavior during reading.

In empirical studies of human eye movement behavior during reading, it is common to compute various summary measures from the data, but these measures are typically not evaluated with respect to corresponding measures of baseline performance. The authors present a method for deriving an appropriate baseline by mapping the actual behavior to a random perturbation of the text being read, and they find surprising similarities between the baseline and the empirical data. The practical message from these findings is that the importance of a particular factor in explaining either empirical or simulated eye movement patterns should be evaluated using an appropriate baseline. In addition to this methodological point, the authors suggest that eye movement behavior in reading may be constrained by the properties of an eye-guidance system that has adapted to the coarse-grained statistical properties of written language.

Eye Movements↗

PubMed2004

Dyslexics' eye fixations may accommodate to hemispheric desynchronization.

We argue that the vertical division of the fovea and the hemispheric division of the brain condition reading, and that hemispheric desynchronization is a proximal cause of dyslexia. We predict that dyslexics' fixation behaviour in reading accommodates to problematic hemispheric transfer/coordination, with fixations projecting more letter-information directly to the left hemisphere to facilitate processing. We analysed eye movements of 24 dyslexics and 24 normal readers. Dyslexics fixated closer to word beginning than did normal readers, projecting more of the word directly to the left hemisphere. Both groups produced comparable fixation durations at the beginning of the word; further into the word the dyslexics produced longer fixation durations. The results support a model of dyslexia based on hemispheric desynchronization.

Adult↗

PubMed2004

The potential contribution of preplanned refixations to the preferred viewing location.

On the basis of recent evidence for the preplanning of refixation saccades in reading-like tasks (e.g., Beauvillain, Vergilino, & Dukic, 2000), we proposed that for a proportion of words refixated during natural reading, the refixation sequence is planned before the word is fixated, and that the saccade target for these cases is near the beginning of the word. The preferred viewing location (PVL) phenomenon can then be explained as a mixture of two populations of planned saccades: those that initiate a multiple-fixation sequence and single fixations. This account supplants saccadic undershooting as the primary determinant of the PVL. We used a corpus of eye movement data to show that the probability of refixation is strongly correlated with the estimated perceptibility of the target word from the current fixation location, and that the observed (re-)fixation behavior is justified when assessed in terms of a lexical uncertainty measure. Refixations may be preplanned in order to increase the chances of successful word identification.

Choice Behavior↗

PubMed2003

Low-level predictive inference in reading: the influence of transitional probabilities on eye movements.

We report the results of an investigation into the ability of transitional probability (word-to-word contingency statistics) to account for reading behaviour. Using a corpus of eye movements recorded during the reading of newspaper text, we demonstrate both the forward [P(n/n-1)] and backward [P(n/n+1)] transitional probability measures to be predictive of first fixation and gaze durations: the higher the transitional probability, the shorter the fixation time. Initial fixation position was also affected by the forward measure; we observed a small rightward shift for words that were highly predictable from the preceding word. Although transitional probability is sensitive to word class, with function words being generally more predictable from their context than content words, the measures accounted equally well for the data for both classes.

Eye Movements↗

PubMed2003

Eye movements reveal the on-line computation of lexical probabilities during reading.

Skilled readers are able to derive meaning from a stream of visual input with remarkable efficiency. In this article, we present the first evidence that statistical information latent in the linguistic environment can contribute to an account of reading behavior. In two eye-tracking studies, we demonstrate that the transitional probabilities between words have a measurable influence on fixation durations, and using a simple Bayesian statistical model, we show that lexical probabilities derived by combining transitional probability with the prior probability of a word's occurrence provide the most parsimonious account of the eye movement data. We suggest that the brain is able to draw upon statistical information in order to rapidly estimate the lexical probabilities of upcoming words: a computationally inexpensive mechanism that may underlie proficient reading.

Bayes Theorem↗