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Poster B82, Tuesday, August 20, 2019, 3:15 – 5:00 pm, Restaurant Hall

Morphological decomposition in the ventral stream

Clare Lally1, Simon Fischer-Baum2, Tibor Auer1, Kathleen Rastle1;1Royal Holloway University of London, 2Rice University, Houston Texas

Morphology reflects one of the few systematic relationships between orthography and semantics, as letter combinations known as affixes are able to systematically modify the word meaning (e.g. un-: unlock, unable, unsure). Morphological information is accessed rapidly during skilled reading, and research has shown that morphological analysis arises at different levels, based on orthographic and semantic overlap (Gold & Rastle, 2007). Previous fMRI research has indicated that morphological analysis shows progressive abstraction in the ventral stream; an area extending from the visual cortex, through the left occipital temporal cortex to the superior temporal gyrus (Gold & Rastle, 2007). We used representational similarity analysis (RSA) to localise and characterise neural representations of morphologically complex words within the ventral stream. Participants silently read words which varied in orthographic, morphological and semantic overlap while we recorded neural responses using fMRI. We constructed a priori prediction matrices based on the morphological properties of words, which were expected to elicit different patterns of activation at different levels of the hierarchically organized pathway. The stimuli consisted of words which allowed between-stem (late, plate, relate, lately, lateness) and between-affix (regret, refuse, refund, redial) comparisons. Words with high orthographic overlap were expected to show similar patterns of activation in posterior regions, whereas words that were close in meaning were expected to show similar patterns of activation in anterior regions as representations became more abstract. Most importantly, we expected to observe an intervening shift in the representations of words based on their morphological properties; for example whether a word contained not only plausible stem but also a viable affix, and further, whether this affix provided a semantic connection to the stem. Our analyses included region of interest (ROI) analyses, using orthographic ROIs based on Vinckier et al (2007) and morphological/semantic ROIs based on Gold & Rastle (2007). We also conducted whole-brain searchlight analyses to explore morphological representations throughout the ventral stream. ROI analyses identified representations based on orthographic overlap in the most posterior ROI within the ventral occipital cortex (-18 -96 -10), whereas representations based on orthographic segmentation of morpheme units were found further anteriorly in the left ventral stream (-36 -80 -12). These results are consistent with findings from Vinckier et al. (2007), who found graded activation based on increasingly large orthographic units, such as frequent bigrams and quadigrams, and extend to indicate that this shallow orthographic processing also applies to morphemes, regardless of whether the unit appears in a morphologically viable word. Representations based on shared semantics were found in the middle temporal gyrus (-55 -42 -5), replicating the findings of Gold & Rastle (2007). Overall, RSA informs our understanding of the transformation of form to meaning in visual word recognition, and provides evidence of a graded hierarchy of abstraction within the ventral stream.

Themes: Reading, Morphology
Method: Functional Imaging

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