1. Introduction
Extensive research using the gaze-contingent boundary paradigm [
1] has established the crucial role of parafoveal processing in normal reading. By manipulating the availability of valid parafoveal information, researchers have uncovered that fixation durations on critical words are shorter following valid parafoveal previews, compared with invalid preview conditions in which parafoveal information is masked. This effect, termed the parafoveal preview benefit [
2], demonstrates that information extracted parafoveally facilitates processing on the subsequent fixation [
3], and therefore aids efficient processing. Research has also aimed to understand the factors that influence the ability to effectively process parafoveal information. One hypothesis that the ease (or difficulty) with which the currently fixated word is processed may influence the processing of parafoveal word(s) known as the foveal load effect. This effect was initially demonstrated by Henderson et al. [
4], who found that when the fixated word is more difficult to process, such as when it is low frequency, fewer attentional resources are available to process upcoming words, therefore hindering parafoveal processing.
However, subsequent research has produced a mixed picture regarding the existence of a foveal load effect. A meta-analysis [
5] found only six out of sixteen published studies demonstrated foveal load effects. There are three important considerations regarding the interpretation of existing research, which need to be resolved in order to truly understand the nature of foveal load effects in reading. These are: (1) the role of writing script, (2) establishing appropriate neutral parafoveal mask, and (3) establishing how foveal load may influence parafoveal processing, in terms of both the amount and type of information extracted from the parafovea. It is these three issues that we aim to tackle in the current study.
First, considering the role of writing script, current knowledge of foveal load effects is based almost exclusively on alphabet languages. Indeed, all studies included in Veldre et al.’s [
5] meta-analysis explored only Latinate alphabetic reading (with the exception of one study that manipulated N + 1 load). However, there is good reason to speculate that script-specific processing demands may shape foveal load effects. Here, we focus on Chinese. Chinese is visually dense, with most words comprising one or two-characters [
6]. As a result, upcoming words fall closer to fixation [
7], allowing for more extensive extraction of parafoveal information [
8,
9]. Moreover, as written Chinese is unspaced, readers must segment words from the character’s string; therefore, they may prioritize the allocation of their attention to upcoming characters more than readers of alphabetic languages [
10]. Indeed, parafoveal preview appears to be more substantial, and processing of word N + 2 appears to be more common in Chinese, compared with alphabetic reading [
11]. These script-based differences may modulate the influence of foveal load, such that parafoveal processing in Chinese reading may be more robust and less susceptible to foveal load effects. Indeed, while there have been very few investigations of foveal load effects in Chinese reading, evidence to date suggests that foveal load effects may be absent. Zhang et al. [
12] examined effects of frequency-based foveal load on the processing of word N + 1. They found robust N + 1 processing in all conditions, and no evidence of a foveal load effect. As more efficient parafoveal processing during Chinese reading may allow the one-character N + 1 to be pre-processed relatively easily even when foveal load is high. Lv et al. [
10] investigated whether foveal load influenced the preview benefit of word N + 2. The results also revealed robust N + 2 processing in both conditions and no interactive effects. These findings provide evidence that foveal load does not affect Chinese reading, and upcoming words can be parafoveally processed effectively under both high and low foveal load.
To ensure that any differences observed across conditions are the result of differences in parafoveal processing, it is important that the parafoveal mask is neutral. In their meta-analysis, Veldre et al. [
5] hypothesized that the visually/orthographically unusual baseline preview conditions used in many studies may interfere with normal processing and contribute to mixed results found in alphabetic studies. In two experiments, they found a foveal load effect only when the preview consisted of a random consonant string, but found no interactions when using an alternating case preview or a nonword neighbor preview. They argued that interactive effects were driven by preview cost from the perceptually distinct illegal nonword preview. Further evidence that interactive effects reflect the presence of perceptually distinct parafoveal previews comes from Findelsberger et al. [
13], who found a foveal load effect with visually degraded previews, but not traditional letter previews. Visual degradation increased display change awareness and may capture the reader’s attention, leading to additional costs similar to illegal nonword previews [
14].
In the current study, we adopt unrelated pseudo-character previews as our baseline. Pseudo-characters are formed from components found in Chinese characters, but are not real characters. As such, they are designed to provide no meaningful information, while looking natural and not perceptually distinct from real characters. Studies using pseudo-character previews typically report low levels of display change awareness, suggesting that they do not capture attention in the same way as illegal letter strings or degraded previews [
10,
15]. Unrelated character previews that use real, legal characters may offer the opportunity for readers to activate and begin processing this irrelevant information and any processing of this unrelated character will then subsequently need to be corrected [
16]. We therefore consider the pseudo-character to be a good candidate for offering a truly neutral (not strongly associated with either disruption or facilitation) baseline. Pseudo-characters are also the mask type adopted by previous Chinese studies of foveal load, where the effect has been absent.
This adoption of an effective neutral baseline mask offers an important opportunity to further explore and consider the role of preview type in understanding foveal load effects.
Crucially, manipulations of foveal load have the potential to modulate parafoveal processing in two ways: by affecting the spatial extent of processing, or the linguistic depth of processing. Depth of processing can be considered a measure of the degree to which the parafoveal word has been efficiently processed. The majority of studies to date provide an estimate of processing depth by comparing the size of the preview benefit for an identical preview relative to a (typically orthographic) mask to infer the depth of processing, such that it is assumed that if the preview benefit across conditions is similar, a similar amount of processing must have been undertaken. One way of characterizing linguistic depth is in terms of the types of information that can be extracted parafoveally, that is, the degree to which a parafoveal word may be processed orthographically, phonologically, or semantically. Therefore, a similar preview benefit in these circumstances does not necessary mean that the processing undertaken across conditions is equivalent, as such an approach does not provide a measure of what type of information has been processed. Importantly, then, while pseudo-characters seem to offer an effective neutral baseline, because pseudo-characters do not contain meaningful information, studies that use only a pseudo-character condition cannot establish whether parafoveal processing is equivalent in terms of the type of information processed across foveal load conditions. Here, our aim is to explore more directly the types of linguistic information that can be parafoveally extracted under high and low foveal load conditions, as this will offer more nuanced understanding.
The “orthography-to-phonology-to-semantics” route in word recognition [
17] predicts that orthographic information is accessed earlier during the time course of processing than phonological information, which in turn is accessed prior to semantic information. Following these assumptions, evidence of the parafoveal preview benefits for phonology or semantics indicates greater depth of parafoveal processing, as more information has been extracted. In line with this, in English, there is robust evidence that orthographically related [
18,
19,
20,
21] and phonologically related [
22,
23] words in place of N + 1 yield strong preview benefits, indicating that readers extract orthographic and phonological information parafoveally. Parafoveal processing of high-level information such as semantics, on the other hand, appears to be uncommon in English [
3,
24,
25,
26], and where such effects are observed [
27,
28,
29], they may be determined by the plausibility of the preview given the sentence context, rather than the relationship between the preview and the target (i.e., plausibility preview effects [
30,
31,
32]). This suggests that English words are typically not pre-processed with sufficient depth to access semantic information, while words in German, a language with a shallow orthography, may allow for more frequent parafoveal semantic processing (Hohenstein et al. [
33], but see Rayner et al. [
34] for evidence that this may be driven by German noun capitalization).
In contrast to Latinate alphabetic languages, Chinese characters are mapped more closely to meaning than to phonology [
35]. As a result, skilled Chinese readers may have more direct access to semantics from orthography, and so the “orthography-to-phonology-to-semantics” route may be less dominant, and readers’ information processing priority may emphasize semantics over phonology [
36]. In support of this, robust parafoveal extraction of semantic information has been observed in Chinese, while phonological preview benefit appears to be a small and occurs at a relatively late stage of activation [
16,
23,
37]. The demands of the language may therefore shape the information processing priority, and so what constitutes efficient, or deep, parafoveal processing depends on the language being studied. Moreover, the visual characteristics of written Chinese, which may promote more extensive parafoveal processing [
11] make it the ideal language to observe subtle differences in processing type as a result of foveal load.
M. Yan et al. [
38] reported that in Chinese, semantic preview benefit is larger when pre-boundary fixations are shorter (perhaps indicating greater ease of processing), while phonological benefit is larger when pre-boundary fixations are longer (perhaps indicating greater processing difficulty). Developing readers show early activation of phonology, which diminishes with the development of reading skills [
39], while L2 learner’s parafoveal processing appears limited to low-level orthographic features [
40]. Taken together, these studies indicate that the types of information processed parafoveally can differ across conditions/readers, such that linguistic depth of processing may not be equivalent, even when substantial preview benefit is observed in both conditions. There is some evidence that the presence of a semantic preview benefit may be a marker of efficient parafoveal processing, and so exploring the type of information extracted from the parafovea under different foveal load conditions might be informative about the efficiency of processing. This study therefore provides the first comprehensive assessment of whether foveal load modulates preview benefit from orthographically, phonologically, and semantically related masks. We adopt an approach of comparing the preview benefit for related preview masks compared to the unrelated baseline. Similar approaches have been used in numerous studies to establish what types of information can be parafoveally extracted [
16,
36]. Moreover, parafoveal processing, as a fundamental process, may be more robust in Chinese reading, and it is reasonable to speculate that any influence of foveal load may be more likely to influence the type of information processed (rather than the presence of parafoveal processing).
This issue has important implications for models of eye movement control during Chinese reading. The Chinese Reading Model (CRM [
41]) predicts foveal load effects on parafoveal processing, but it only directly predicts them on the basis of evidence that saccades are longer when leaving high-frequency words than low-frequency words. Note, that several studies demonstrate that foveal load influences saccade targeting [
42], but this influence appears to occur independent of parafoveal preview [
12] and does not reflect reduced parafoveal processing. E-Z reader [
43] and SWIFT (saccade-generation with inhibition by foveal targets) [
44] also incorporate foveal load effects into their basic architecture, with the E-Z reader predicting effects on linguistic depth, while SWIFT predicts an effect on spatial extent, although neither make a specific prediction regarding foveal load in Chinese reading. Therefore, further research is needed to clarify the nature of foveal load effects and inform the future development of Chinese reading models. Understanding the influence of foveal load on parafoveal processing can optimize reading strategies and enhance reading comprehension and literacy instruction.
Above all, previous studies on the impact of foveal load on parafoveal processing depth have not considered the potential differences in the type of information processed under the conditions of high or low foveal load. Thus, the current study examines whether foveal load may modulate linguistic depth of parafoveal processing during Chinese reading, such that the types of parafoveal information extracted may differ as a function of foveal load. Experiment 1 examines the processing of word N + 1. Given the robust N + 2 preview benefits observed in Chinese reading, Experiment 2 additionally explores the processing of N + 2. Accordingly, sentences were created in which three consecutive words were manipulated, we refer to these as word N (the foveal word), word N + 1 (one-character word), and word N + 2 (one-character word). Word foveal load was manipulated using word frequency. The preview validity of word N + 1 (Experiment 1) or word N + 2 (Experiment 2) was manipulated with five preview types: identical, orthographically related word, phonologically related word, semantically related word, and unrelated pseudo-character.
Following previous research, we anticipated the main effects of foveal load on word N and effects of preview validity on word N + 1/N + 2. Given the substantial parafoveal processing observed in Chinese, we expect that readers will benefit from the availability of an orthographically, phonologically, or semantically related preview of N + 1, compared with the unrelated pseudo-character. As little research has investigated the depth of processing for N + 2, the predictions here are more tentative, but we anticipate robust N + 2 processing (at least in the low-load condition). Crucially, if there are interactive effects of foveal load and preview type, this would provide novel evidence that foveal load modulates parafoveal processing in Chinese reading. Given the findings of Zhang et al. [
12] and Lv et al. [
10], we predict additive effects of preview and frequency for comparisons of an identical preview with a pseudo-character preview. On this basis, we also anticipate an orthographic preview benefit in both conditions. If fewer attentional resources are allocated to the parafovea when foveal load is high, readers may be less likely to benefit from semantic information in this condition.
In order to investigate whether foveal processing load affected the depth of preview benefit for the subsequent word N + 1, we used a boundary paradigm and manipulated foveal load and different parafoveal preview types at the word N + 1 position. Experiment 1 adopted a 2 (foveal load: high-frequency, low-frequency) × 5 (preview type: identical, orthographically related, phonologically related, semantically related, and pseudo-character) within-subject design. Given that Chinese readers could access the information of parafoveal word N + 2 (when N + 1 is a single word), Experiment 2 further examines whether preview benefit for word N + 2 was reduced under conditions of increased foveal processing difficulty. Experiment 2 used the same experimental design as Experiment 1.
4. General Discussion
In the current study, we conducted two experiments examining the nature of foveal load effects in Chinese reading. To do this, we investigated preview effects under high and low foveal processing load (manipulated using word frequency) using a range of preview types. We assessed whether foveal load modulates the linguistic depth of parafoveal processing, measured as the types of parafoveal information obtained during parafoveal preview. Experiment 1 examined these issues for Word N + 1, and Experiment 2 examined these issues for Word N + 2.
Robust preview benefit effects were observed on both N + 1 and N + 2, contributing to a growing body of evidence indicating substantial parafoveal processing in Chinese reading [
11]. Moreover, Experiment 1 provided evidence to support the parafoveal extraction of orthographic, phonological, and semantic information from N + 1, in line with previous research [
16,
23,
37,
58]. The semantic preview benefit observed here was more limited than the semantic benefit reported by M. Yan et al. [
16]. However, Yan et al. used only visually and structurally simple pictographic and indicative characters as targets to maximize the chances of observing a semantic preview benefit, whereas many of the characters in the current study comprised two components/radicals [
59], and these more complex characters may be preprocessed parafoveally with different priority. It could be argued that because the pseudo-character does not contain any meaningful lexical content, that the difference between the unrelated and related conditions may reflect a general benefit relating to previewing a real character. However, we do not believe that this effect can fully account for the pattern of results observed, as if this were the case, we would anticipate a similar preview benefit for all related conditions relative to the unrelated conditions. This was not the case. Therefore, the differences among the related preview conditions in Experiment 1 strongly indicate that these specific linguistic characteristics were processed parafoveally. Nevertheless, there is considerable debate regarding the most suitable “neutral” baseline mask type [
11], and so the potential role of mask type should be further explored in future.
The pattern for Experiment 2 was slightly less clear. While the increase in fixation times for pseudo-character previews relative to identical previews provides strong evidence of N + 2 preview effects, readers did not benefit from the orthographic, phonological, or semantic relatedness of the masking character relative to the valid character. One possible explanation is that this preview benefit effect primarily reflects a preview cost associated with the unrelated pseudo-character mask. The large preview effects seen here in comparison with previous research [
10,
60,
61] may support such an interpretation. However, it should also be noted that reading times for all preview types (not only the unrelated pseudo-character preview) were substantially increased relative to the identical preview condition. Moreover, display change awareness was low, and although the pseudo-characters did not provide useful preview information, they were designed to be character-like and not perceptually distinct, although, as noted above, we acknowledge that the role of mask type warrants further consideration. Alternatively, it may also be the case that the benefit obtained from mask-target relatedness for word N + 2 is much more subtle and/or less consistent than for N + 1, and so was missed in the current study. Indeed, when considering the pattern of effects observed in Experiment 1, only orthographically related previews produced a robust benefit on early fixation measures, while the effects of phonologically related previews were restricted to total time and semantically related previews were restricted to word skipping. Word skipping for N + 2 may be a less sensitive measure of parafoveal extraction for two reasons. Firstly, following standard procedures, analyses of word N + 2 include both instances where N + 1 was fixated and instances where N + 1 was skipped [
60], and so skipping of N + 2 is likely to also be affected by processing that occurs at N + 1. Second, as N + 1 and N + 2 formed separate words, a skipping decision programmed on word N would involve skipping two consecutive words. Therefore, the opportunity to observe these preview benefits may have been limited. Further research is therefore needed to more fully characterize the nature of N + 2 parafoveal processing in Chinese reading. However, it remains clear that substantial N + 2 pre-processing is undertaken both when foveal load is low and when it is high.
Crucially, there was no evidence of a foveal load effect in either experiment. Taken together with the findings of Zhang et al. [
12] and Lv et al. [
10], it appears that foveal load does not modulate either the spatial extent or the linguistic depth of parafoveal processing during Chinese reading. Despite this, in line with previous research [
12,
42], foveal load did modulate N + 1 skipping rates. Foveal load also modulated first-pass fixation times on N + 1 (a spillover effect, see Marx et al. [
62]), adding further evidence that foveal load does not modulate the parafoveal preview, but does influence ongoing word processing and saccade targeting independently of the preview. The results of this study raise important theoretical considerations for models of eye movement control during reading [
41,
43,
44]. The CRM [
41] currently predicts foveal load effects during Chinese reading. In addition, in the EZ reader, spillover effects are hypothesized to arise from reduced parafoveal processing, an interpretation that is at odds with the current findings. In light of the growing evidence against such effects, this assumption may need to be revisited.
Additionally, comprehensive investigations of the spatial extent and linguistic depth of parafoveal processing remain absent in alphabetic languages, and so it will be important for future research to determine whether the lack of foveal load effects found in Chinese reflects a general lack of foveal load effects during reading or whether these findings are language specific. Lv et al. [
10] speculate that the need to segment unspaced text may lead Chinese readers to develop more efficient processing through necessity, and so readers of Chinese may prioritize this parafoveal processing more than readers of alphabetic languages, as the consequences of not conducting parafoveal processing may be greater (e.g., segmentation failure). This prioritization may minimize the effect of foveal load relative to spaced languages. This possibility requires further investigation. An additional consideration concerns the structure of the target stimuli used in the current experiments. Here, both N + 1 and N + 2 were single-character words. This decision was made in order to maximize processing of N + 2 [
15]. However, the majority of words in Chinese are composed of two characters (72%, Lexicon of common words in contemporary Chinese research team, 2008). Moreover, pre-processing of character N + 2 is more substantial when characters N + 1 and N + 2 form a single word (Xie et al., under review). Future research may seek to establish whether foveal load effects may be present for longer words and whether such effects depend on the lexical status of the characters, e.g., by comparing foveal load effects for character N + 3 when N + 3 is the final character of a three-character word and when character N + 3 is the first character of word N + 2 (following a two-character word N + 1). It is of course also important to acknowledge that despite conducting a well-powered study, our samples remain relatively small and homogeneous. We focused on skilled young adult readers, who may show particularly robust processing. To extrapolate to a more diverse range of readers, larger and more diverse sample sizes may be adopted in future studies to allow for a consideration of factors such as age and reading skill. Thus, how foveal load influences the type of information extracted during parafoveal processing remains a promising topic for future research.