This chapter adds to the body of work on spillover frequency eects in reading by showing that they persist in the LLDT in spite of its wide word spacing and even when parafoveal preview is masked. As such, it provides evidence for spillover as a cognitive rather than perceptual phenomenon and puts it in the explanatory domain of models such as the one advanced in the thesis. In addition, this chapter embarks on a modeling exercise in E-Z Reader, the model of eye movement control on which the architecture of the thesis model is based. It shows that there are in fact two dierent mechanisms for spillover eects possible under the E-Z Reader assumptions: one relying on parafoveal preview and another using a post-perceptual delay (though only some reported parameter ts use both). This suggests a key revision to the model that guides remainder of modeling work in the dissertation.
5.1.1 Spillover frequency eects
As discussed in Chapter 2, spillover frequency eects are de ned as the speedup of the reading time of a word as a function of the frequency of the preceding word. One explanation for these eects is that they are a result of parafoveal preview in a resource-constrained model. Such a model must make the tradeo between foveal and parafoveal processing. Easier foveal processing (e.g. on higher frequency words) makes more resources available for parafoveal processing, speeding up the eventual recognition of the next word.
This explanation is supported by evidence that spillover eects disappear when parafoveal preview is incorrect (Inho & Rayner,1986; Henderson & Ferreira, 1990).
This evidence comes from experiments using gaze-contingent boundary paradigms (McConkie & Rayner, 1975): a target word is replaced with a non-useful preview (typically a set of consonants), and then changed to the correct preview when the eye crosses an invisible boundary placed before the target word. The measurement is of the xation time on the target word as a function of the frequency of the pre-target word.
However, the disappearance of spillover eects with incorrect preview is not always attested. In particular, Kennison & Clifton (1995) show only a reduction but not a disappearance of the eect, andSchroyens et al.(1999) show little eect of masking on spillover. Most interestingly, White et al. (2005) show no disappearance of spillover eect at all in participants who were aware that something unusual was going on in how the words were displayed, but an eect about twice the size as the ones shown in the other paper in unaware participants.
One explanation for this contrast, in light of the ndings by White et al., is that participants who are not aware that the display change occasionally provides inaccurate preview attempt to take advantage of preview, whereas participants aware of the preview's occasional uselessness adapt to this constraint and nd another way to extend processing one word into the next. This explanation also provides a new way of making sense of a nding byMorrison & Rayner (1981), who showed that saccade distances depend on characters skipped independent of the visual angle traversed. If participants target saccades the same character distance forward regardless of where in their acuity function the target lies, they might be able to vary the amount of processing they apportion to dierent parts of their perceptual span and therefore choose to use or ignore parafoveal information.
5.1.2 Spillover in the unmasked LLDT
The LLDT was explicitly designed to minimize parafoveal preview and other word- to-word dependencies. Under the explanation for preview provided above, one might therefore expect not to see spillover eects in the LLDT. However, this is not the case: gure 5.1 shows single xation durations by condition, split into bins based on frequency of the current and previous word, with both foveal and spillover fre- quency eects readily apparent. Statistical analysis (using the methods described in section 4.2.3) bears this out (foveal β = 4.6633, χ2 = 219.85, p < 0.0001, spillover
acc bal spd ● ● ● ● ● ● ● ● ● ● ● ● 230 250 270 290
high low high low high low
Previous word frequency (median split)
Single Fixation Dur
ation (ms)
Current word Frequency (median split) ● high ● low
Single Fixation Duration by Frequency
Figure 5.1: Plot shows single xation durations by condition in the unmasked LLDT, by foveal and spillover (previous) frequency bin. Both foveal frequency (indicated by separation between blue and red lines) and parafoveal frequency (indicated by line slopes) have a signi cant eect on single xation durations.
This evidence is consistent with one of two possibilities: the rst is that this is a parafoveal preview eect and that participants are able to preview more distant words than expected. In the LLDT experiment in the previous chapter, participants' heads were not xed, so they could have been sitting farther from the screen than 25 inches and therefore tting adjacent words into their reading span in spite of the wide spacing. Even if their screen distance was as intended, perhaps the reading span estimates in the literature are variable or incorrect, again allowing for unintended preview.
To mitigate against both of those possibilities, another LLDT experiment was run, this time in a gaze-contingent moving window paradigm. Two dierent imple- mentations of the gaze-contingent paradigm were implemented, one a novel predictive unmasking design. The experiment is described in the next section, with both a com- parison of the two masking variants and an investigation of spillover.