Error Pattern Effects

Feature Distance. 4.5 noted that 'feature* distances between target -> error tend to be small, though some studies showed a trend in PP towards three or four 'feature* gaps. Of course it must be remembered that 'features' are abstract, theory tied, descriptive entities, not explanatory terminology and it is far from clear what is meant in processing terms by *x features' away from target. This proximity is reconcilable with the proposal that the most closely competing candidates for a target are those sharing most likenesses (alveolar-alveolar; cluster-cluster; syllable onset with onset etc), and one expects these to be most susceptible to perceptual false evaluation. Greater 'feature* distances can 'stem* from displacement errors, out- of-plan derailments and lexical influence, (e.g. tiger/lion-> liger as above; /m <-> z/ in magazine -> zagamine). If this interpretation is correct the near versus distant dichotomy is seen to be an artefact of analysis.

Frequency Effect. 4.4 outlined arguments for derailments occurring on lower frequency targets and intruding errors representing higher frequency units (but see Stemberger 1991 for explanations of apparent antifrequency biases). This phenomenon is explained in interactive terms by the fact that more frequent targets have a higher resting level, thus reaching activation threshold more easily. Furthermore, they have a greater density of interconnections, giving them a heads tart in the 'rich get richer* race. It has long been noted that error direction tends to favour replacement by more familiar words and syllable patterns (whether in CV terms, or sonority gradients - Christman 1992). This is heard at its starkest in repetition of nonsense words where the urge to create real words is strong, and rarer syllable frames are 'simplified* to more familiar ones.

This operational feature also predicts that certain derailments are more likely than others. More complex syllable structure is not liable to replace less complex (but see Dressier et al 1990). Derailments that create non-words

are less likely than those causing alternative real words. For example, ^ in pretty girl would be more susceptible to perseveratory substitution than in nice girl, because bottom- up enrichment of superordinate nodes would happen in the case of pearl which would exist in the matrix, but not for *nirl.

The frequency effect is one source of normalisation of phonotactic transgressions. Out of language sequences would not receive further activation. The same can be observed in syntax where non-contextual word substitutions, additions etc and displacement errors are accommodated to the syntactic environment.

Another recorded phenomenon is worthy of mention here. In PP form words and numbers are negligibly affected compared with content words. Serial models posit extra levels and/or separate stores to explain this apparent dissociation. Within a connectionist model a more parsimonious solution is found by invoking the familiarity/frequency effect. This couples with the fact that form words constitute a closed class, so have inherently fewer competitors for their slots which are also strongly primed by syntactic-semantic

activation.

Length and Complexity Effects. Interactionist models account for this in two ways. The greater the look-ahead required in speech production, the wider the spread of activation, the more candidates competing for selection, the more scope there will be for derailment. Secondly, more complex syllables tend to occur more rarely, have fewer interconnections and hence are susceptible to the familiarity effect.

Rate Effects. Speculatively a lesioned system operates more slowly than an intact one. Spread and activation are slower; feedforward and back are less speedy, less clearcut. It takes longer to sort out the consequences of noise in the system. The result would be retarded output. Attempts to speed up would be thwarted simply because the system is incapable of operating more quickly. Acceleration would lead to increased errors. Slowing even further would not

necessarily give added accuracy in an inherently noisy, unstable system. At most it might alter the qualitative error picture to a predominance of errors arising at temporally late stages in sound genesis. No evidence exists for this supposition in disordered speech, but there are hints in normal induced slips that it is true (Dell 1988).

Automaticity-Volitional Effect. Similar to the situation in discrete point models, idioms, set phrases and overleamed material can be accessed and activated in toto by activation of only part of the whole partly because their 'prepackaged' nature requires less active processing and can be executed from 'downstream' sources of activation. Overleamed sequences are further favoured by the familiarity effect. Connectionist approaches assume that attempts to isolate subparts of an idiom or utter sequences in a non-familiar order will be more difficult, because sub-parts or revised order will have fewer interconnections to access or boost them.

Contextual facilitation, where a person is more accurate at a sound or word in context rather than in isolation (e.g. McCarthy, Warrington 1984), is also accounted for here. A facilitating phrase I want a cup of______ increases the feedforward and back required to raise tea to activation. One might expect differential effects according to whether the person has a concurrent dysphasia or not.

Another source of the automaticity effect in parallel distributed models, is that processing is not bound to proceed via a defective component. Access can proceed directly from one intact, or less impaired, level to another. Because such routes may require new or non-familiar interactions, conscious activation may not always/ever succeed, so non-conscious connections arise apparently out of the blue.

Variability. Apart from that conditioned by context, rate, affect etc, interactive systems provide further sources of variability. Activation does not spread through a passive or

donnant hierarchy. Resting and selection levels for nodes are not absolute. They fluctuate around a mean. Where spreading and accretion are normal and lead to clear winners, such noise is inconsequential. However, \diere activation and inhibition are impaired and competing networks equivocally differentiated, noise assumes selectional importance, and the instability leads to variability.

In an interactive framework context is to be understood as more than just immediate phonological environment. The state of arousal and vigilance of the organism as a whole enters consideration. Particular interconnections may receive added activation from distant (e.g. visual, auditory) sources, or distant lexical-semantic associations. Under normal circumstances these would be inhibited (poor get poorer principle) and not enter the selection pool, but their presence in pathology affects stability.

The same applies in repeated trials tasks (chapter 19; Appendix A3). Ordinarily repetition of a sound or word would strengthen its network (rich get richer). However, in an impaired system unclear activation, inefficient inhibition and noise fail to reject competing candidates. According to the type (site?) of breakdown in the system intrusions may be either or all of alternative whole words, syllable frames, movement trajectories, and so on.

7.4.2.4. Conclusion: Apart from interpretations of breakdowns alternative to hierarchical accounts, connectionist models appear to offer insight into derailments discrete point models do not cope with. What implications does an interactive description have for assessing pronunciation disorders?

Existing assessments (Chapter 8) seek all-or-nothing instances of neologisms, PP and AS. Errors that do not neatly fit supposed characteristics of these disorders are dismissed either as statistical noise, lesion size consequences, or listener artefacts, but not as challenges to the fundamental range of derailments and relationships

amongst disorders. Motivation for these claims derives from hierarchical models of language production and the units deemed to be processed at different levels. Mistakes arise from misapplication of linguistic rules and are constrained by the systematicity of language, (In fact it is not purely linguistic rules that are misapplied, but the effect on these of dysfunction of certain processing devices or translations - e.g. frames, buffers, scanners, mappers).

Reviews in Chapters 4, 5, showed that clinical reality did not correspond to theoretical optimism on syndrome assignment. Interactive models suggest why this search was destined to be fruitless. While errors may be attributed to misfunction of a given level/component, the output of that component is the product of input and interactions with multiple levels/components. Correspondingly, derailments may and do reflect this.

The implication is that assessment should shift from demarcation of discrete disorders to probabilistic descriptions of error trends. Which components tend to be in error; which assumed components appear to be misinfluencing one another to produce errors which seem to emanate from one 'level' but are products of interacting input from several; which of several 'levels' seem to be interacting to provoke multiple/complex error types.

In a distributed interactive system it is questionable where one 'level' ends and another starts. Similarly, in such a design it will be for empirical settlement whether PP and AS are delineable disorders or just convenient labels for patients where the cluster of error types leans towards a particular constellation. Far from being secondary artefacts, a mixed PP-AS could actually be a product of underlying (dis )organisation. As such diagnosis would not be on an all-or-nothing basis. There would be no unique PP, AS profile. Rather each patient would have an individual profile. Given sufficient numbers of standardisation subjects one could envisage that within the overall pool of possible errors particular error types would cluster closer

together - speculatively corresponding to the three main spheres of motor production intimated above (7.2).

Diagnosis then would assign a subject a probabilistic score relating their proximity to one of these empirically derived categories - categories which would, however, have a theoretical underpinning. For clinical and therapeutic purposes, however, it would still be the individual’s own profile that remained paramount.

This picture argues that the task of developing a standardised clinical assessment of pronunciation disorders needs to proceed by single case studies to a more global view of derailment trends. The growing data available would in turn feed back to single cases, enabling a better anchored quantification of their disability.

There remain, however, two big theoretical and practical issues to resolve: the mental-physical divide and units of control. These are addressed in the next sections. A third big issue, the relationship between the neural network analogy and central nervous system structures and operations proper is not tackled here.