Models based upon abstract ahistoric representations predict that there should be no differences in the magnitude of repetition effects with changes in format. This prediction contrasts with that made by processing accounts which suggest that the magnitude of the repetition effects observed will decrease as changes in format across presentations, and hence differences in processing, become more pronounced. Jacoby and Dallas (1981) argued that an attenuation of a repetition effect as a result of changing the modality of stimulus presentation from visual to auditory, or vice versa, would be particularly problematic for abtractionist models. I therefore begin by describing the experiments which have investigated the effects of such modality changes.
1.3.1 Repetition across visual and auditory modalities
In the original formulation of the logogen model of word recognition a single set of logogens could be activated by input from either the visual or auditory modality. If repetition priming effects are entirely a product of changes in the activation of logogens, then the repetition effects should be as great when a word is repeated in the same modality as when a word is repeated across modality. The results of a number of studies using a variety of tasks suggest that this is not the case.
Winnick and Daniel (1970) showed that the repetition effects on a perceptual identification task were sensitive to the modality of presentation. While repetition effects in the identification task were obtained when a word was presented at study in its printed form, there was no increase in the probability of identifying a repeated word when it had earlier been generated by subjects in response to a printed definition. This study however confounds the modality of the encounter with whether the word was being comprehended or produced.
A large number of studies have examined the sensitivity of the perceptual identification task to changes in modality between study and test presentations of a word. (e.g. Clarke & Morton,
1983; Hashtroudi, et al., 1988; Jacoby & Dallas, 1981, experiment 6; Light, et al., 1992; Rajaram, 1993; Weldon, 1991). Although these studies have generally failed to find reliable across-modal priming, some studies have demonstrated reliable effects (e.g. Clarke & Morton,
1983), whilst in others the across-modal effect was up to 40% of the magnitude of the within- modality effect (e.g. Rajaram, 1993).
The absence of reliable across-modality effects in perceptual identification suggest that this task is mainly sensitive to overlap of visual form. The occasionally reliable across-modal effect may indicate that there is a modality independent component, but Jacoby and Dallas (1981) argue that in one study which demonstrated across-modality effects (Lee, Tzeng, Garro, & Hung, 1978), these effects may have arisen because an extended study period allowed conceptual processing of the words.
Across-modality priming has also been investigated in the word stem completion task (e.g. Basilli, Smith, & MacLeod, 1989; Craik, Moscovitch, & McDowd, 1994; Graf, et al., 1985; Jacoby, et al., 1993; Rajaram, 1993). In each of these studies, there were reliable effects of across- and within-modality repetition but with the within-modality effects being reliably greater than the across-modality effects. A similar pattern of effects was reported for the word fragment completion task (Craik, et al., 1994; Rajaram, 1993; Roediger & Blaxton, 1987; Weldon, 1991).
Studies by Kirsner and Smith (1974) and by Monsell (1985) have investigated within- and across- modality repetition effects in the LDT. Kirsner and Smith (1974) found reliable effects of both within- and across-modality repetition for words but the within-modality effects were greater than the across modality effects. For non-words there were reliable effects of within- modality repetition but not of across modality repetition. Monsell (1985) found that when words were presented visually in a LDT there were reliable repetition priming effects only for words which had originally been seen. In contrast when the LDT was conducted in the auditory modality there were reliable priming effects irrespective of the modality of the first encounter. The effects in the within modality conditions were reliably greater than the priming effects in the across modality condition.
Roediger and Blaxton (1987) reviewed the effects of manipulations of surface similarity on the magnitude of priming effects. They noted the reduction in the priming effect caused by changes of surface form and argued that this reduction indicated the importance of surface features in eliciting repetition effects. However for changes of auditory and visual modality, the magnitude of the across-modal effects were some 60% of the magnitude of the within-modality effects (see also Brown, Neblett, Jones, & Mitchell, 1991 for other estimates of the relative magnitude of across-modal effects). It might be equally appropriate therefore to consider reasons why the across-modal effect remained at a relatively high level.
Morton (1979; Morton & Patterson, 1980) amended the logogen model to account for the lack of equivalence of within and across modal effects. This revised model incorporated separate logogens for words encountered in visual and auditory modalities. Under this amended logogen model it is the modality specific component that reflects the operation of a lexicon of modality specific word forms. Although not designed to account for modality non-specific effects, such effects could be mediated through links between each logogen system and the cognitive system.
These modality non-specific processes may include processing which allows for the conscious recollection of repeating stimuli (Jacoby, et al., 1993). Such a 'conceptual' account of modality independent effects contrast with the accounts provided by Monsell (1985) and Kirsner et al (1989), neither of which invoke conceptual processing to account for modality independent repetition effects.
Kirsner (Kirsner, et al., 1989; Kirsner & Smith., 1974) has argued that the repetition effect has two components. A modality specific component which reflected perceptual processing prior to a 'lexical' level of representation, whilst a modality non-specific component reflected changes to an amodal lexical representation. Whilst both components contribute to the within-modality effect, only the modality non-specific component contributes to the across-modality effect. The within-modality effect results from changes to event-specific perceptual processing, whilst across-modality effects arise from event specific mappings between processes involved with the perception and production of words.
Monsell (1985) proposed separate logogen systems for visual and auditory modalities, as well as for perception and production. Across-modality effects occur because of links between logogen systems. Although such indirect activation passing across these links facilitates subsequent word recognition, it does so to a lesser degree than does direct activation of a logogen. The links between visual and auditory input lexica are asymmetric. Whilst activity in the visual input logogen system results in significant activity in the auditory input logogen system, activity in the auditory input lexicon produces little activity in the visual input lexicon. This asymmetry accounts for why across-modal priming effects are greater when the first presentation is visual and the second presentation is auditory than vice versa.
Models based on abstract representations can thus account for the modality-dependence of repetition effects by making the 'unit of analysis' more specific, but these modified models are no longer based upon truly abstract representations. Activity within a system of auditory input logogens, for example, provides information concerning the likely modality of input.
Nonetheless the division of lexical representations by modality can be defended as the end result of the different sensory mechanisms and neural pathways through which spoken and written language is apprehended, and as a reflection of relatively recent development of language in a written form compared to the considerably more ancient spoken form. Thus there seems to be good reason for visual and spoken language to have a different representational basis (but see Bradley & Forster, 1987). If so, then the claim that the sensitivity of repetition effects to modality counts against an abstractionist view of lexical representation is weakened. Greater difficulty might be caused for the abtractionist position if a diminution of the repetition effect were brought about by changes in the format of stimulus presentation which could not be parsimoniously accounted for by a further sub-division of a system of abstract representations. A number of studies have attempted to demonstrate such effects.
1.3.2 Manipulations of visual format
A number of studies have varied the physical appearance of visually presented words between a first and a repeated presentation. Thus Scarborough et al (1977) found equivalent facilitation in the LDT when the letter case of the first and second presentations was in a different letter case as when they were in the same case. Clarke and Morton (1983) investigated the effects of format change in a perceptual identification task. Since this task has been thought to be
particularly sensitive to perceptual processing, it may have proved to be sensitive to changes of visual form. Clarke and Morton (1983) found an equivalent enhancement of performance in a perceptual identification task when the first presentation of a word was in a cursive script and the second was printed text than they did when both presentations were in printed text.
A number of other studies investigating the sensitivity of repetition effects to changes in surface form have used the perceptual identification task, or variants of it (Feustel, Shiffrin, & Salasoo, 1983; Graf & Ryan, 1990; Jacoby & Hayman, 1987; Jacoby & Witherspoon, 1982; Standen, Kirsner & Dunn, cited in Kirsner, et al., 1983). As with auditory to visual across- modal effects, these studies have generally found that a change in format of presentation reduces the size of the repetition effect. Nonetheless the magnitude of the across-format effects may be some 70% (Jacoby & Hayman, 1987; Jacoby & Witherspoon, 1982) or 50% (Graf & Ryan, 1990) of the magnitude of the same-format effect.
Levy and Kirsner (1989) and Carr, Brown and Charalambous (1989) investigated the effects of changing the format of presentation on the time required to read a prose passage. Both studies suggested that this measure was insensitive to changes in presentation format, a result that is consistent with an abstractionist position. However Carr et al (1989, experiment 3) also found a similar effect with texts made up largely of nonsense words. Such a result is potentially
problematic for an abstractionist view such as the logogen model since non-words have no pre existing representations (see Section 1.6).
Masson (1986, experiment 2) required subjects to read word triplets, the constituent letters of which were mirror reversed and presented in either upper or lower case depending upon the identity of the letter. Subsequently subjects read words which were either repeated from the training phase or were words new to the experiment. Old words were either presented in a visually identical format to that in which they had been presented previously, or in a format in which letters that had been presented in lower case in the training phase were now presented in upper case and
vice versa.
Repeated words in the same format and different format conditions each showed reliable priming effects, but the magnitude of the effect in the case of the visually identical condition was greater than that in the visually non-identical condition. Masson (1986) argued that there were form-specific and form non-specific components to the repetition effect with these components acting independently of one another. Woltz (1990) studied the effects of changes in surface form on repetition effects in a task in which subjects judged whether or notpairs of simultaneously presented words were synonyms. He found that whilst changes in letter-case from study to test did result in a reduction in repetition effects, this reduction was only some 15% of the magnitude of the repetition effect.
In summary, changes in the visual form of items across encounters have a similar effect as does a change in the modality of presentation. When the format changes across encounters, the magnitude of the effects is reduced but the repetition effect is still relatively large and usually statistically reliable. This is the case even in the perceptual identification task; a task which is thought to be particularly sensitive to changes in perceptual processing. Furthermore in some tasks, e.g. the LDT, there is little evidence for any reduction in the magnitude of the effect as a result of changes in visual format. Word repetition effects therefore seem to reflect both format specific and format non-specific levels of processing. The conclusion that there are multiple components to repetition effects is consistent with the results of studies which have
investigated the effects of the repetition of perceptually degraded stimuli to which I turn next.