We summarise the main results from a series of Finnish studies dealing with single-word experiments with aphasics as well as lexical decision and eyemovement registration tests performed on normals. On the basis of our experimental results, we propose a processing model of Finnish nouns. For the input and central lexicons, this Stem Allomorph/Inflectional Decomposition (SAID) model assumes morphological decomposition of inflected (with the exception of the most frequently encountered inflected noun forms) but not derived noun forms. For the output lexicon, it predicts that both inflected and productive derived forms have decomposed representations. In the case of marked stem variation (resulting from stem formation and/or morphophonological alternation), the model assumes that the stems are represented by their allomorphs, and not by a single morph. In this respect, our model postulates more suppletion in the input/output lexicons than would be predicted on the basis of formal morphological analyses. However, among the allomorphs, the nominative singular of nouns appears to have a special status.

The effect of word frequency on the processing of monomorphemic vs. inflected words was investigated in a morphologically relatively limited language, Swedish, with two participant groups: early Finnish-Swedish bilinguals and Swedish monolinguals. The visual lexical decision results of the monolinguals suggest morphological decomposition with low-frequency inflected nouns, while with medium- and high-frequency inflections, full-form processing was apparently employed. The bilinguals demonstrated a similar pattern. The results suggest that morpheme-based recognition is employed even in a morphologically limited language when the inflectional forms occur rarely. With more frequent inflectional forms, full-form representations have developed for both mono- and bilingual speakers. In a comparable study employing a morphologically rich language, Finnish, Lehtonen and Laine (2003, Bilingualism: Language and Cognition, 6, 213-225) observed full-form access only at the high-frequency range and only for monolinguals. These differences suggest that besides word frequency and language background, the morphological richness of a language affects the processing mode employed with polymorphemic words.

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This study explores the relevance of suffix allomorphy for processing complex words. The question is whether structural invariance of the morphological category (i.e., lack of allomorphy) would affect the processing of Finnish derived words. A series of four visual lexical decision experiments in which alternatively surface and base frequency was manipulated showed that the two invariant suffixes, namely denominal -stO and deadjectival -hkO, showed reliable effects of base frequency, whereas for the two categories with suffix allomorphy, deverbal -Us and deadjectival -(U)Us, only surface frequency played a role. A further experiment showed that even with the most frequent variant of -(U)Us, namely -Ude-, response latencies were a function of surface frequency only. It is shown that neither the results from the experiments here nor previous findings from processing Finnish words can be accounted for by suffix frequency, the frequency ratio between the derived word and its base, or morphological productivity in any straightforward manner. We conclude that the lack of allomorphy, that is, structural invariance, significantly adds to affixal salience and therefore enhances morphological decomposition. The implications of this finding for models of lexical processing are discussed.

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