BACKGROUND

1. The Effects of Phonological Complexity on Past Tense Processing: An FMRI StudyLisa L. Conant, Rutvik Desai, Jeffrey R. BinderDepartment of Neurology, Medical College of Wisconsin, Milwaukee, WI • BACKGROUND • Behavioral dissociations between regular and irregular past tense morphology have been frequently observed. Such dissociations have been taken as evidence of the existence of separate mechanisms underlying the processing of regular and irregular verbs. In such dual mechanism accounts, the transformation of a regular verb stem into its past tense form is thought to involve the application of a set of rules whereas irregular past tense forms, which have been learned by rote, are retrieved through a lexical-associative memory mechanism [1,2]. However, behavioral dissociations may also be explained by an alternative, single-mechanism account. In this view, all verbs are processed through a single integrated system, but component processes within the network such as phonology and semantics have distinct representations within the system and may differentially contribute to the processing of individual words depending on multiple features of those words [3]. One feature that tends to distinguish regular and irregular verb forms is phonological complexity (PC). Because the transformation from present to past tense in regular verbs requires the addition of phonemes while this is rarely true in irregular forms, the inflection of regular verbs involves greater phonological demands and may therefore be more vulnerable to disruption with phonological impairment [4]. This theory has received support from a connectionist simulation [3] as well as a behavioral study in which regularity effects in patients with nonfluent aphasia were no longer evident when regular and irregular past tense forms were matched on PC [4]. • In the current study, fMRI was used to examine activation patterns associated with regular and irregular past tense processing and to investigate the effect of PC on these patterns. We hypothesized that areas of greater activation for regular past tense processing would be minimized when regular and irregular past tense forms were matched on PC. Furthermore, we predicted that similar patterns of activation associated with PC would be seen when subjects were simply reading rather than generating past tense forms. • METHODS • Subjects • 20 healthy, right-handed, English speaking adults (15 women), mean age=27.5, age range=20-47. • Experimental Paradigm • Two tasks were used for the current analyses. A Read Stem Task was also completed but will not be discussed here. • Past Tense Generation Task • Regular and irregular verb stems were presented visually. • Subjects generated the past tense of each stem aloud. • Read Past Tense Task • Regular and irregular past tense forms were presented visually. • Subjects read aloud the past tense form. • Generate and read tasks were presented in alternating runs. Within each run, regular and irregular items, along with baseline fixation trials, were interspersed pseudorandomly. No more than three items of either past tense type were presented contiguously. • Stimuli and Subsets • For the Generate Irregular (GI)-Generate Regular (GR) contrast, the stimuli consisted of 50 irregular and 50 regular verbs matched on letter length of the stem, number of syllables in the stem and past tense forms, log frequency of the past tense form, and friend-enemy ratio. The past tense forms of GR items were more phonologically complex than those of the GI items as indicated by a higher number of phonemes in the GR past tense forms. • For the GI-Read Irregular Past (RIP) and GR-Read Regular Past (RRP) contrasts, each comparison consisted of 50 generate and 40 read items. The generate and read conditions for each past tense type were matched on number of syllables in the past tense form, log frequency of the past tense form, and friend-enemy ratio. • For the RIP-RRP contrast, the stimuli consisted of 40 regular and 40 irregular verbs matched on log frequency of the past tense form and friend-enemy ratio. They were not matched on PC. • In order to specifically examine the effects of PC in past tense generation, two subsets containing equal numbers of items and matched on log frequency were used. • A subset consisting of 40 irregular and 40 regular items matched not only on number of phonemes but also on the exact CV structure of the past tense form. • A subset consisting of 40 irregular and 40 regular items mismatched on number of phonemes. • In order to specifically examine the effects of PC in reading of past tense forms, two subsets containing equal numbers of items and matched on log frequency were used. • A subset consisting of 25 irregular and 25 regular past tense items matched on number of phonemes. • A subset of 25 irregular and 25 regular past tense items mismatched on number of phonemes. • Image Acquisition • Functional DataAnatomical Data • 1.5T GE Signa scanner, T2*-weighted GE-EPI 3D spoiled-gradient-echo-sequence • 21 contiguous sagittal slices 124 contiguous sagittal slices • TE=40ms, voxels=3.75 x 3.75 x 6.5mm T1-weighted images • Clustered Acquisition, TR=7s, Acquisition Time=2300ms • Image Analysis • AFNI software package • Within-subject multiple linear regression with reference functions for each condition • Incorrect trials removed from the analyses • Individual SPMs resampled in standard stereotaxic space and smoothed with a 5mm FWHM Gaussian kernel • Random effects analysis • Threshold at uncorrected p<0.001, corrected mapwise p<0.01 • RESULTS • Left Right Left Right • GI>RIP • Extensive activation of left IFG, SFG, precentral gyrus, medial SFG, SMA, cingulate cortex, angular gyrus, supramarginal gyrus, superior parietal lobule, precuneus, inferior temporal/fusiform gyrus, basal ganglia, and thalamus. • Right medial frontal, basal ganglia, and thalamic activation with lesser activation in right IFG, inferior parietal lobule, STS, and inferior temporal/fusiform. • RIP>GI • Small focus in right posterior MTG • Generate Irregular>Generate Regular • Bilateral inferior frontal gyrus (L>R) and left precentral gyrus • Bilateral posterior parietal (L>R) including the angular gyrus and intraparietal sulcus • Left ventral fusiform • Bilateral caudate and thalamus • Generate Regular>Generate Irregular • Left superior temporal gyrus (LSTG) • Right anterior cingulate • GR>RRP • Extensive activation of left IFG, SFG, precentral gyrus, medial SFG, SMA, cingulate cortex, and lentiform nucleus • Lesser activation in left inferior parietal lobule and inferior temporal/fusiform • Right medial frontal, caudate, and thalamic activation, and small foci in right IFG and precentral gyrus • RRP>GR • Small bilateral foci in posterior cingulate (L>R) and posterior MTG DISCUSSION Inthe current study, generation of irregular relative to regular past tense was associated with greater activation of a bilateral, distributed group of primarily frontal and parietal brain regions. In contrast, relative to irregular past tense processing, regular past tense processing was associated primarily with a single focus of greater activation in the LSTG, including the planum temporale and extending laterally and ventrally along the STG to the upper bank of the superior temporal sulcus. This region was highly related to PC and was not specific to morphological processing as indicated by its presence during both generation and reading of the regular past tense but only when PC was greater for regular than irregular verbs. The specific role this area may play in speech processing is not fully understood, but it contains regions potentially associated with the early stages of acoustic to phonetic recoding, possibly including the accessing of phonemic representations necessary for this process [5]. In the current study, greater activation for phonologically complex regulars was not seen in areas involved in later stages of phonological processing, such as the IFG. In fact, there was actually greater activation in the IFC associated with irregular past tense generation, which is seemingly inconsistent with the lesion data suggesting preferential disruption of regular past tense processing with damage to this area [2]. However, several factors are important to consider when interpreting this finding. First, these frontal regions are involved in multiple aspects of phonological and semantic processing, thereby complicating efforts to isolate IFG activation specifically associated with modulations in phonological complexity. In addition, preliminary analyses with the current data suggest that the preferential activation in both frontal and parietal areas associated with the irregulars may be significantly reduced when reaction time is considered, suggesting that the increased activation in these areas may be at least partially related to the greater difficulty associated with irregular past tense processing. In addition, when generation of each past tense type is contrasted with reading of that type, substantial overlap is seen in frontal regions, suggesting that these areas are important in the processing of both regular and irregular past tense. This overlap is consistent with the previous finding that, although lesions in this area may be associated with greater deficits in regular past tense morphology, irregular past tense processing is also significantly impaired in these patients [2]. Thus, these frontal regions may be important for the processing of both regular and irregular past tense but are potentially more critical for the former. In contrast, although there was some overlap in posterior temporoparietal regions as well, the activation in these regions was significantly greater in the contrast between generation and reading of irregular past tense forms. This finding may reflect the greater contribution of semantics for irregular past tense processing and is consistent with reports of more selective impairment of irregular past tense morphology with posterior lesions [2]. Overall, these results support a single-system account of past tense morphology in which all verbs are processed through a single integrated system, but component processes within the network such as phonology and semantics have distinct representations within the system and may differentially contribute to the processing of individual words depending on specific features of those words such as phonological complexity. • Phonologically Mismatched GI>GR • Similar to above • Phonologically Mismatched GR>GI • More extensive LSTG/STS (BA41, 42/22) • Small focus in left postcentral gyrus Flattened perisylvian patch showing the BA 41/42/22 activation STS STG • PC Matched GI>GR • Similar to above • PC Matched GR>GI • None STS STG In the contrasts between the Read Past conditions, there were no significant areas of activation in the right hemisphere, therefore only left hemisphere images are presented here. • RRP>RIP • LSTG • RIP>RRP • None • PC Mismatched RIP>RRP • None • PC Mismatched RRP>RIP • LSTG/STS (BA 41, 42/22) • Left postcentral gyrus REFERENCES 1. Pinker S. Rules of language. Science 1991;253:530-535. 2. Ullman MT, et al. A neural dissociation within language: evidence that the mental dictionary is part of a declarative memory, and that grammatical rules are processed by the procedural system. J Cog Neurosci 1997;9:266-276. 3. Joanisse MF, Seidenberg MS. Impairments in verb morphology after brain injury: a connectionist model. Proc Natl Acad Sci 1999;96: 7592-7597. 4. Bird H, Lambon Ralph MA, Seidenberg MS, McClelland JL, Patterson K. Deficits in phonology and past-tense morphology: what’s the connection? J Mem Lang 2003;48:502-526. 5. Binder J., Price C. Functional imaging of language processing. In: Cabeza R, Kingstone A, eds. Handbook on functional neuroimaging of cognition. Cambridge:MIT Press, 2001, p. 187-251. • PC Matched RIP>RRP • None • PC Matched RRP>RIP • None Acknowledgments: Supported by NINDS RO1 NS33576