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An optimized-sequence event-related morphological paradigm reveals Bayesian evidence of brain activation during Spanish verb-tense operations Jorge Gutiérrez, Roger Carrillo, Francisco Robles
Antoinette Hawayek
Research Division Instituto Nacional de Neurología y Neurocirugía (INNN) Mexico City, D.F. Mexico
[email protected]
Linguistics Department Universidad Autónoma Metropolitana (UAM) / INNN Mexico City, D.F. Mexico
[email protected]
Abstract—A Bayesian-inference group analysis,
On the other hand, syntax is not the only component of grammar production that has combinatorial properties [2]: as syntax combines words to construct phrases or phrases to build sentences, morphology combines minimal lexical entities to build words. These minimal blocks (words or affixes) are stored in long term memory. Morphology allows us then to observe the combinatoriality as an online process of assembling linguistic elements to create a more complex linguistic structure [2] [3].
done on functional magnetic resonance images corresponding to healthy Mexican Spanishspeaking subjects to whom an optimizedsequence rapid-presentation event-related morphological paradigm was applied, revealed statistically the activation of key brain regions (e.g. the inferior frontal gyrus including areas such as Brodmann 44 and 45, left hemisphere, for verbal inflections, the anterior region of the opercular and right superior temporal gyrus for infinitives, etc.). The sequence of stimuli that allowed us to statistically reveal loci of Spanish verbal computational operations such us rapid verbal inflections was optimized by using a statistical efficiency metrics based on the covariance of the set of regressor coefficients with respect to the matrix of such regressors (basis functions in correspondence to the occurrence of stimuli). This way, by launching a Gaussianbased random search we ended up with one of the best combinatory sequences for presentation of the stimuli.
Morphological processing has the advantage of being executed in word context and, in many cases, in a syllable context. Therefore, memory working demands are minimal avoiding compound processing of language and memory (a major problem in language production research). Morphology comprises derivative and inflectional operations. This study explores exclusively inflectional morphology in verbs and nouns. Thus in order to reveal brain regions implicated in such morphological inflections within a Bayesian framework [4], a rapid-presentation version of an event-related functional paradigm [5] has been designed and optimized in its sequence of stimuli presentation [6]. And to illustrate such a paradigm, the corresponding verb-trial component is reported in detail.
Keywords: verbal inflections; event-related paradigm; stimuli-sequence optimization; fMRI; Bayesian inference I.
INTRODUCTION
Thus, in section 2 we explain the design of the complete morphological paradigm focused on verb and noun inflections. In section 3, we describe the fMRI data acquisition procedure. Results regarding activated brain regions revealed within a Bayesian framework are covered in section 4. Conclusions and future work are treated in section 5.
Traditionally, syntax has been the center of grammar studies due to its recursion property achieved through combination of elements. But it has been impossible to separate syntax from other core processes in language production research as semantic selection, meaning relation and working memory [1].
Research done under the 26/09/2006 INNN – UAM agreement. March 16 - 20, 2009, Mexico City, Mexico
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ISBN 978 - 1 - 4244 - 3669 - 9 IEEE Catalog Number CFP0918G Library of Congress 2008911781
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II.
THE MORPHOLOGICAL PARADIGM MORPHOLOGICAL TRIALS SESSION TRIAL NUMBER NULLS TOTAL LENGTH DURATION OF TRIALS Verb 3500 ms 80 80 7m 00s Noun 3500 ms 60 60 5m 15s Reading 1750 ms 140 70 6m 7.5s Table 1. The morphological paradigm is composed of verb, noun and reading trials to be applied in three sessions
The stimuli within the morphological frame were designed to activate primitive grammatical operations. Thus we carefully chose a set of pertinent stimuli, i.e. 40 verbs and 40 nouns, which would be inflected within the context of referential phrases: infinitive, present tense, past tense, verb reading, diminutive, singular, plural and noun reading (Fig. 1).
In order to apply such a verb paradigm, a computational design was done under the rapid-presentation version of event-related functional paradigms [5] (RPER for short): 1 trial equals two 1750-ms segments (one segment for the stimulus and the other one for the contextual referential phrase). 1750-ms null segments were interleaved in a random way (Fig. 2). Six Mexican individuals composed the sample of in-average 30-year-old right-handed native Spanish-speaking healthy participant subjects.
Figure 1. The morphological testing set comprises 16 conditions under a 2×4×2 design (class, task and regularity respectively). In RPER paradigms, events are presented in a random way to avoid subject habituation and are so temporally close to each other that their corresponding hemodynamic responses are overlapped. Besides, RPER can be considered as a stochastic technique since there is a large number of combinatory presentation sequences: 16 factorial plus 30% of base-line nulls in the present case. This leads us to search for one of the best sequences of event and null onsets following a certain optimization technique. For practical reasons, the whole paradigm was intended to be applied to subjects in three sessions (Table 1).
Figure 2. 1750-second null periods occurred randomly in the sequence of 3500-second verb trials.
(1)
denote under a linear hypothesis the formula for the sampled fMRI signal at each voxel where X is the matrix of regressors (basis functions) corresponding to the occurrence of simuli, that is to say X defines the order in which trials take place for each condition, B is the set of regressor coefficients, and r the unexplained noise.
In the specific case of the verb session, functional evaluation was carried out with a 6-condition 2-factor morphological paradigm focused on verbal morphological inflections only: present tense, past tense and infinitives for regular and irregular verbs. Such a paradigm consists of instructing the subjects to do verb operations silently according to the exposed stimuli (regular or irregular verbs) and within the context of tense-referential phrases.
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For the optimization of the sequence of stimuli, let
Y = XB + r
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(1)
ISBN 978 - 1 - 4244 - 3669 - 9 IEEE Catalog Number CFP0918G Library of Congress 2008911781
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The optimization technique would consist of using the statistical efficiency metrics [6], Eh, which can be based on the covariance of B with respect to X while considering as weight matrix the contrast C regarding the centered additive noise r:
T
Eh = 1/ [ trace(C (X X) – 1) ]
corrected, structurally coregistered, segmented, normalized and statistically processed to model subjects’ brain performance and visualize most-likely statistical activated regions within a Bayesian framework [4], i.e. using a spatial prior where the spatial regularity in the signal was estimated from the existing data.
(2)
IV. RESULTS
Thus, launching a Gaussian-based random search
guided by (2) we ended up with one of the best combinatory sequences (Table 2) for presentation of the stimuli: the orthogonal design matrix shows an acceptable diagonal. The found sequence was implemented computationally to be presented to the subjects during the fMRI-scanner session. Stimuli exposition duration was defined by means of testing the paradigm on a set of 10 subjects prior to the fMRI sessions.
The Bayesian-inference [4] group analysis done on functional magnetic resonance images corresponding to healthy Mexican Spanish-speaking subjects to whom an optimized-sequence rapid-presentation event-related morphological paradigm was applied, revealed statistically the activation of the inferior frontal gyrus including areas such as Brodmann 44 and 45 (Fig. 3), left hemisphere, for verbal inflections. Infinitive activates the anterior region of the opercular and right superior temporal gyrus (Fig. 4). As such Bayesian studies are non existent for the Spanish language, search of evidence clues of brain statistical activation was focused on rather-expected areas according to recent advances for statistical parametric maps of the Spanish [7] and English [1] languages.
Figure 3. Activation of Brodmann 44 & 45 areas. Table 2. Sample of three sequences of stimuli where the timing corresponds to sequence 1. III. FMRI DATA ACQUISITION Imaging data was acquired on a 3.0-Tesla General electric Signa scanner with an EPI sequence (64x64 matrix; flip angle=90; TR=1.75 s; TE=40 ms; FOV=24 cm; 30 continuous axial slices for each of the 240 volumes) for the 7-minute fMRI verb session. Functional-scan images were motion and slice-time March 16 - 20, 2009, Mexico City, Mexico
Figure 4. Activation of opercular and right superior temporal gyrus. 13
ISBN 978 - 1 - 4244 - 3669 - 9 IEEE Catalog Number CFP0918G Library of Congress 2008911781
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V.
CONCLUSIONS AND FUTURE WORK
This study leads us to consider Bayesian-inference optimized-sequence RPER functional paradigms as a strong complement to traditional-functionality epoch-related functional paradigms since the former allows us to statistically reveal loci of Spanish verbal computational operations such us rapid verbal inflections. The importance of identifying such loci resides in providing neurosurgeons with information not only on patients’ known language areas (traditional language functional paradigms’ arena) but also on those regions where verbal inflections take place: key brain information to reduce surgical risks on patients with epilepsy or tumors. As a future work, we look forward to do a random-effect group analysis in robust samples of both healthy subjects and neurological patients, with the extended version of our paradigm which includes the class factor, e.g. 2 conditions: for noun and verbal inflections (Fig. 1), so that generalizations to the Mexican Spanish-speaking population could be done.
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ISBN 978 - 1 - 4244 - 3669 - 9 IEEE Catalog Number CFP0918G Library of Congress 2008911781
