In this study, patients with CFL had to read single-line French sentences. As assessed with microperimetry, all patients had a dense macular scotoma covering the fovea so that eccentric viewing was necessarily involved in the reading process. We first showed that number of fixations and average fixation duration both have significant effects on reading speed as assessed with zero-order regressions. The effect of fixation duration is consistent with evidence in normally sighted observers that slower readers have longer fixation durations than faster readers.
36,37 It is also consistent with a previous gaze-contingent scotoma study showing that fixation duration and reading speed are negatively correlated.
38 However, when compared with studies of CFL patients, evidence for a correlation between average fixation duration and reading speed is a new result that was never observed in previous studies.
4–8 The reasons of these discrepancies are currently unclear, especially because the effect of fixation duration reported here has a large amplitude and is still present when partialling out the effect of the total number of fixations. One possible reason, as is often the case in clinical investigations, is that experimental paradigms are very different across studies. For instance, in some previous studies, oculo-motor parameters and reading speed were recorded in different experimental sessions and with different texts.
Interestingly, our finding that reading speed is negatively correlated with fixation duration is compatible with a recent study.
38 These authors show that reading speed (measured with a Rapid Serial Visual Presentation paradigm [RSVP]) is negatively correlated with temporal threshold for letter recognition as estimated in another experiment. Therefore, their results combined with ours suggest that average fixation duration during reading with eye movements is proportional to the temporal threshold estimated in the Cheong et al.
38 study.
Our results confirm the relationship between L/FS (number of letters per forward saccade) and reading speed reported previously with CFL patients.
4–8 This confirmation was needed because the measure used to assess L/FS in these studies actually did not directly measure L/FS. Instead, L/FS was assessed as the ratio between the number of letters for a given sentence and the number of forward saccades. We previously discussed why using this measure, which covaries with the number of fixations, might reflect effects due to the number of fixations rather than to L/FS per se.
37
The authors of these eye-movement studies have interpreted the correlation between L/FS and reading speed as evidence that the perceptual span is a major determinant of reading speed (assuming that L/FS reflects the average perceptual span).
4–8 In parallel with these studies (where reading required eye movements), studies of eccentric reading with the RSVP paradigm (i.e., when eye movements are not required) have accumulated evidence concerning the predominant influence of the visual span on reading speed.
3,22,39–42 There have been many discussions and controversies about the correct characterizations of different types of span in the eye-movement literature, especially in relation to reading.
9,12,14 Although the terminology is sometimes confusing, it is acknowledged that two different spans must be distinguished: the visual span and the perceptual span (sometimes called the span of effective vision). For instance, O'Regan
10 calls visual span “what can be seen without making use of lexical knowledge and contextual constraints,” and perceptual span “what can be perceived by additionally making use of them.” Many studies have shown that the perceptual span differs in several ways from the visual span; for instance, it is dynamic (i.e., it can change from fixation to fixation), of wider extent, and asymmetric (larger to the right
43 ). Another important difference is that the perceptual span is attentionally constrained.
44–48 This is suggested for instance by the reduction of the perceptual span when foveal processing difficulty increases (thus increasing foveal attentional engagement).
The shrinking perceptual span hypothesis assumes that L/FS reflects the average perceptual span. The underlying logic is that the size of each forward saccade is calculated by the oculo-motor system so that the next fixation's span is adjacent to the current fixation's span. In this view, the whole chain of causal links is thus the following: a reduced span induces smaller forward saccades, hence an increase in the total number of fixations (a step that is overlooked in the literature), and eventually a slower reading rate. Ignoring the “number of fixations” step leads one to think that a positive correlation between L/FS and reading speed is sufficient to support the perceptual span hypothesis.
4–8 However, as made explicit by our mediation model of reading speed, this effect of L/FS on reading speed might also be due to an indirect effect of L/FS through fixation duration, which would not be consistent with the shrinking perceptual span hypothesis. Such a mediation through fixation duration could be related to some evidence in the literature on free viewing of visual scenes that there are two interspersed modes of viewing: global and local scanning. Global scanning periods are associated with large-amplitude saccades and short-duration fixations, whereas local scanning periods are associated with smaller-amplitude saccades and longer fixations.
49 One could argue that this mixture of scanning strategies also occurs in reading, with different proportions of the two different modes leading to different reading speeds. Thus, low reading speeds would correspond to a predominance of the local scanning strategy, whereas higher reading speeds would be induced by a greater predominance of the global scanning strategy.
Our results clearly distinguish between the two predictions presented above (mediation through number of fixations or through fixation duration) and are consistent with the shrinking perceptual span hypothesis: there is a large indirect effect of L/FS on reading speed through the total number of fixations, and this effect is close to the total effect of L/FS on reading speed. In contrast, there is no indirect effect of L/FS on reading speed through fixation duration (although the effect of fixation duration on reading speed is large). In sum, the effect of L/FS on reading speed is fully mediated by the total number of fixations, as predicted by a mediation formalization of the shrinking perceptual span hypothesis.
Our study and several previous eye-movement investigations of this hypothesis present some limitations. It is known that people who are forced to read magnified text do so by moving the text being read and/or their head.
50 This introduces a pattern of slow eye movements interleaved with saccades, which bears some resemblance to the reflexive optokinetic nystagmus. In our study, this pattern was not observed mainly because the text was fixed on the monitor. It is thus possible that the characteristics of saccades might be different in our study compared with situations where text and head are free to move, an issue that should be investigated in future research. Another limitation might be due to the short sentences that were used in the present work. It is possible that long sentences or texts would increase the number and size of both backward and forward saccades.
51 For instance, a long-range backward saccade might have a confirmatory purpose, such as checking information present several words behind, which in turn would induce a subsequent large forward saccade, skipping over already identified words.
Future research also should extend our work by investigating new hypotheses on the possible causal determinants of the factors included in our model. For instance, additional experiments could be performed with different levels of luminance contrast that are known to alter reading speed. Then, the contrast factor could be included in a mediation model to test whether its effect on reading speed is mediated by the number of fixations or by average fixation duration. Our model also could be extended to look for the factors that are causal determinants of L/FS. For instance, variations in the L/FS factor could be directly affected by factors known to influence the visual span, and this could be tested by a measure of each patient's visual span with the RSVP trigram method.
22 On the other hand, some authors argue that it is the speed of comprehension that affects eye movements note vice-versa.
20 It seems therefore possible that this cognitive factor might be the cause of variations in the L/FS factor. This hypothesis could be investigated by measuring an index of speed of comprehension for each patient, and by including this factor in our model as a potential cause of variations in the L/FS factor. In summary, future research should investigate determinants of reading speed by integrating within a mediation model the complex causal relationships linking visual, oculo-motor, and cognitive parameters with each other.
Finally, we advocate the use of mediation analysis to help develop programs of visual rehabilitation or training for low-vision patients,
7,52–58 along lines similar to the strategies used in health behavior intervention research programs.
16,59 Mediation analysis should allow researchers to target the mechanisms by which different interventions or training programs improve the dependent variable of interest, here reading speed (when eye movements are required).
Supported by a French Ministry of Research and Technology grant (AC) and a Bourse de Doctorat pour Ingénieur (BDI) grant from the Centre National de la Recherche Scientifique (CNRS) (J-BB).
Disclosure: A. Calabrèse, None; J.-B. Bernard, None; G. Faure, None; L. Hoffart, None; E. Castet, None