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ABSTRACT: We present an MEG study of homonym recognition in reading, identifying effects of a semantic measure of homonym ambiguity. This measure sheds light on two competing theories of lexical access: the “early access” theory, which entails that lexical access occurs at early (pre 200 ms) stages of processing; and the “late access” theory, which interprets this early activity as orthographic wordform identifcation rather than genuine lexical access. A correlational analysis method was employed to examine effects of the lexical frequencies of distinct words that share the same orthography (homographs) on brain activity. We find that lexical frequency did not affect processing until after 300 ms, while earlier activation was primarily modulated by orthographic form frequency.
Language and Cognitive Processes 02/2012; 27(2):275-287. · 1.54 Impact Factor
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ABSTRACT: To explore whether individuals with aphasia exhibit differences in the M350, an electrophysiological marker of lexical activation, compared with healthy controls.
Seven people with aphasia, 9 age-matched controls, and 10 younger controls completed an auditory lexical decision task while cortical activity was recorded with magnetoencephalography. There were 2 stimulus conditions of interest: identity primed (i.e., a word preceded by itself) and semantic primed (i.e., a word preceded by a semantically related word). Latency and amplitude of the M350 response as well as reaction time were measured.
Consistent with the age-matched control group, the group with aphasia showed both identity and semantic priming behaviorally. In contrast to the control groups, the group with aphasia did not show either semantic or identity priming of the M350 response. This group also demonstrated longer M350 latencies than either control group. Furthermore, within this group, M350 latency was positively correlated with a measure of semantic impairment.
These findings highlight the usefulness of temporally sensitive measures when studying aphasia and demonstrate that the latency of electrophysiological markers is of interest in this population. In particular, increased M350 latency appears to be indicative of a semantic processing impairment.
Journal of Speech Language and Hearing Research 08/2011; 54(6):1577-96. · 1.88 Impact Factor
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ABSTRACT: Recent neurolinguistic studies present somewhat conflicting evidence concerning the role of the inferior temporal cortex (IT) in visual word recognition within the first 200 ms after presentation. On the one hand, fMRI studies of the Visual Word Form Area (VWFA) suggest that the IT might recover representations of the orthographic form of words. On the other hand, influential MEG studies of responses from the occipito-temporal regions around 150 ms post-stimulus onset indicate recognition of letters as opposed to symbols but not a sensitivity to statistical properties of letter strings associated with word form representations. Recent MEG experiments support the position that the IT does represent the visual word forms of morphemes and performs morphological decomposition modulated by the statistical relations between morphemes by 170 ms post presentation (at the M170 response). Responses to heteronyms show that the M170 does not make contact with the mental lexicon where word forms are connected to meanings. We report here an MEG study of pseudo-affixed words like brother, which masked priming studies have shown are decomposed in recognition. If the M170 response from IT does index obligatory morphological decomposition based on visual word forms but not lexical entries, we should find that the statistical relation between pseudo-stem and pseudo-suffix modulates the M170 for pseudo-affixed words, as for truly affixed words. The results of this experiment confirm this prediction. In addition, surface form frequency for these words also modulates the M170, providing some support for dual route recognition for words for which decomposition is a garden path.
Brain and Language 05/2011; 118(3):118-27. · 3.12 Impact Factor
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ABSTRACT: We employ a linear mixed-effects model to estimate the effects of visual form and the linguistic properties of Chinese characters on M100 and M170 MEG responses from single-trial data of Chinese and English speakers in a Chinese lexical decision task. Cortically constrained minimum-norm estimation is used to compute the activation of M100 and M170 responses in functionally defined regions of interest. Both Chinese and English participants' M100 responses tend to increase in response to characters with a high numbers of strokes. English participants' M170 responses show a posterior distribution and only reflect the effect of the visual complexity of characters. On the other hand, the Chinese participants' left hemisphere M170 is increased when reading characters with high number of strokes, and their right hemisphere M170 is increased when reading characters with small combinability of semantic radicals. Our results suggest that expertise with words and the decomposition of word forms underlies processing in the left and right occipitotemporal regions in the reading of Chinese characters by Chinese speakers.
Brain and Language 11/2010; 117(1):1-11. · 3.12 Impact Factor
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ABSTRACT: Semantic priming has long been used to investigate how concepts and ideas are related at the level of language, and has become a convenient tool for assessing conceptual and semantic dysfunction in cognitive disorders, including schizophrenia. The study of semantic priming in schizophrenia has led to diverse results: enhanced priming, reduced priming, and priming equivalent to that found in nonpsychiatric comparison groups. A number of hypotheses have been proposed to explain some of the observed deficits in schizophrenia patients. For example, difficulties in word recognition may be due to hyperactivation of too many lexical representations or to a failure to inhibit lexical competitors. One way to distinguish between these possible explanations is to move beyond reliance on behavior alone and to examine the neural processes involved in lexical recognition. Here we present a magnetoencephalographic study of semantic priming in schizophrenia. Importantly, schizophrenia patients and healthy controls did not differ in performance on a priming task. We show that normal behavioral performance can occur in a context of aberrant neural responses. These findings suggest that normal behavioral responses in schizophrenia can be achieved through neural mechanisms that differ from those seen in the psychiatrically well brain.
Journal of Neurolinguistics 05/2010; 23(3):223-239. · 1.97 Impact Factor
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ABSTRACT: We employ a single-trial correlational MEG analysis technique to investigate early processing in the visual recognition of morphologically complex words. Three classes of affixed words were presented in a lexical decision task: free stems (e.g., taxable), bound roots (e.g., tolerable), and unique root words (e.g., vulnerable, the root of which does not appear elsewhere). Analysis was focused on brain responses within 100-200 msec poststimulus onset in the previously identified letter string and visual word-form areas. MEG data were analyzed using cortically constrained minimum-norm estimation. Correlations were computed between activity at functionally defined ROIs and continuous measures of the words' morphological properties. ROIs were identified across subjects on a reference brain and then morphed back onto each individual subject's brain (n = 9). We find evidence of decomposition for both free stems and bound roots at the M170 stage in processing. The M170 response is shown to be sensitive to morphological properties such as affix frequency and the conditional probability of encountering each word given its stem. These morphological properties are contrasted with orthographic form features (letter string frequency, transition probability from one string to the next), which exert effects on earlier stages in processing ( approximately 130 msec). We find that effects of decomposition at the M170 can, in fact, be attributed to morphological properties of complex words, rather than to purely orthographic and form-related properties. Our data support a model of word recognition in which decomposition is attempted, and possibly utilized, for complex words containing bound roots as well as free word-stems.
Journal of Cognitive Neuroscience 08/2009; 22(9):2042-57. · 5.18 Impact Factor
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ABSTRACT: We present an MEG study of heteronym recognition, aiming to distinguish between two theories of lexical access: the 'early access' theory, which entails that lexical access occurs at early (pre 200 ms) stages of processing, and the 'late access' theory, which interprets this early activity as orthographic word-form identification rather than genuine lexical access. A correlational analysis method was employed to examine effects of the heteronyms' form and lexical properties on brain activity. We find support for the 'late access' view, in that lexical properties did not affect processing until after 300 ms, while earlier activation was primarily modulated by orthographic form.
Brain and Language 12/2008; 108(3):191-6. · 3.12 Impact Factor
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ABSTRACT: Production studies have shown that speakers of languages with larger phoneme inventories expand their acoustic space relative to languages with smaller inventories [Bradlow, A. (1995). A comparative acoustic study of English and Spanish vowels. Journal of the Acoustical Society of America, 97(3), 1916-1924; Jongman, A., Fourakis, M., & Sereno, J. (1989). The acoustic vowel space of Modern Greek and German. Language Speech, 32, 221-248]. In this study, we investigated whether this acoustic expansion in production has a perceptual correlate, that is, whether the perceived distance between pairs of sounds separated by equal acoustic distances varies as a function of inventory size or organization. We used magnetoencephalography, specifically the mismatch field response (MMF), and compared two language groups, French and Spanish, whose vowel inventories differ in size and organization. Our results show that the MMF is sensitive to inventory size but not organization, suggesting that speakers of languages with larger inventories perceive the same sounds as less similar than speakers with smaller inventories.
Brain and Language 04/2007; 100(3):295-300. · 3.12 Impact Factor
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ABSTRACT: Against longstanding assumptions in the psycholinguistics literature, we ar-gue for a model of morphological complexity that has all complex words as-sembled by the grammar from lexical roots and functional morphemes. This assembly occurs even for irregular forms like gave. Morphological related-ness is argued to be an identity relation between repetitions of a single root, distinguishable from semantic and phonological relatedness. Evidence for the model is provided in two MEG priming experiments that measure root activation prior to lexical decision. Both regular and irregular allomorphs of a root are shown to prime the root equally. These results are incompat-ible both with connectionist models that treat all morphological relatedness as similarity and with dual mechanism models in which only regular forms involve composition.
The Mental Lexicon 01/2006; 1.
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Brain and Language 06/2005; 93(2):243-7; discussion 248-52. · 3.12 Impact Factor
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ABSTRACT: To reconstruct neuromagnetic sources, the minimum-variance beamformer has been extended to incorporate the three-dimensional vector nature of the sources, and two types of extensions-the scalar- and vector-type extensions-have been proposed. This paper discusses the asymptotic signal-to-noise ratio (SNR) of the outputs of these two types of beamformers. We first show that these two types of beamformers give exactly the same output power and output SNR if the beamformer pointing direction is optimized. We then compare the output SNR of the beamformer with optimum direction to that of the conventional vector beamformer formulation where the beamformer pointing direction is not optimized. The comparison shows that the beamformer with optimum direction gives an output SNR superior to that of the conventional vector beamformer. Numerical examples validating the results of the analysis are presented.
IEEE Transactions on Biomedical Engineering 11/2004; 51(10):1726-34. · 2.28 Impact Factor
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ABSTRACT: Schreuder and Baayen (Schreuder. R., & Baayen, R. H. (1997). How complex simplex words can be. Journal of Memory and Language 37, 118-139) report that lexical decision times to nouns are not sensitive to the cumulative frequency of the noun's morphological derivatives in its "morphological family", even though such a cumulative frequency effect is obtained in the domain of inflection. Under a decomposition view of derivational morphology, this constitutes a puzzling exception to the robust finding that lexical frequency is one of the major determinants of behavioral response latencies. If morphologically complex words are decomposed, each occurrence of a member of a noun's morphological family should add to its root-frequency. We investigated the effects of morphological family frequency on the magnetoencephalographic response component M350, which shows sensitivity to factors affecting early stages of lexical processing, including lexical frequency. We hypothesized that high morphological family frequency should have a facilitory effect on the M350, even though no such effect can be seen in response time, presumably due to competition among possible root-affix combinations. Contrary to this hypothesis, we found that high family frequency elicits an M350 inhibition, suggesting that competition among morphological family members occurs at the M350. The result is significant, since there is evidence that competition among phonologically similar words occurs after, not at, the M350. Thus, our results suggest that competition within a morphological family precedes competition within a phonological similarity neighborhood.
Cognition 04/2004; 91(3):B35-45. · 3.16 Impact Factor
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ABSTRACT: The influence of external interference on neuromagnetic source reconstruction by adaptive beamformer techniques was investigated. In our analysis, we assume that the interference has the following two properties: First, it is additive and uncorrelated with brain activity. Second, its temporal behavior can be characterized by a few distinct activities, and as a result, the spatio-temporal matrix of the interference has a few distinctly large singular values. Namely, the interference can be modeled as a low-rank signal. Under these assumptions, our analysis shows that the adaptive beamformer techniques are insensitive to interference when its spatial singular vectors are so different from a lead field vector of a brain source that the generalized cosine between these two vectors is much smaller than unity. Four types of numerical examples verifying this conclusion are presented.
IEEE Transactions on Biomedical Engineering 02/2004; 51(1):90-9. · 2.28 Impact Factor
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ABSTRACT: Twenty years ago it was discovered that recognition of semantically unexpected words is associated with a special ERP signature - the N400. Pinpointing the precise functional significance of the N400 has, however, been difficult. Recent MEG studies of word processing reveal that, in fact, the N400 decomposes into several functionally distinct subcomponents, allowing for more fine-grained investigation of its significance.
Trends in Cognitive Sciences 06/2003; 7(5):187-189. · 12.59 Impact Factor
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ABSTRACT: The influence of temporarily correlated source activities on neuromagnetic reconstruction by adaptive beamformer techniques was investigated. It is known that the spatial filter weight of an adaptive beamformer cannot perfectly block correlated signals. This causes two major influences on the reconstruction results: time course distortions and reductions in reconstructed signal intensities. Our theoretical analysis and numerical experiments both showed that the reduction in signal intensity for sources with a medium degree of correlation is small. The time-course distortion for such sources, however, may be discernible. Our analysis also showed that the magnitude correlation coefficient between two correlated sources can be accurately estimated by using the beamformer outputs. A method of retrieving the original time courses using estimated correlation coefficients was developed. Our numerical experiments demonstrated that reasonably accurate time courses can be retrieved from considerably distorted time courses even when the signal-to-noise ratio is low.
IEEE Transactions on Biomedical Engineering 01/2003; 49(12 Pt 2):1534-46. · 2.28 Impact Factor
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ABSTRACT: We have applied the eigenspace-based beamformer to reconstruct spatio-temporal activities of neural sources from MEG data. The weight vector of the eigenspace-based beamformer is obtained by projecting the weight vector of the minimum-variance beamformer onto the signal subspace of a measurement covariance matrix. This projection removes the residual noise-subspace component that considerably degrades the signal-to-noise ratio (SNR) of the beamformer output when errors in estimating the sensor lead field exist. Therefore, the eigenspace-based beamformer produces a SNR considerably higher than that of the minimum-variance beamformer in practical situations. The effectiveness of the eigenspace-based beamformer was validated in our numerical experiments and experiments using auditory responses. We further extended the eigenspace-based beamformer so that it incorporates the information regarding the noise covariance matrix. Such a prewhitened eigenspace beamformer was experimentally demonstrated to be useful when large background activity exists.
Human Brain Mapping 05/2002; 15(4):199-215. · 5.88 Impact Factor
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Proceedings of the 2002 IEEE International Symposium on Biomedical Imaging, Ritz-Carlton Hotel, Washington, DC, USA, 7-10 June 2002; 01/2002
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ABSTRACT: A covariance matrix-based subtraction method has recently been proposed to remove interference using two MEG measurements: The first has both target and interfering activities and the second only has the interference. This paper compared covariance matrix-based subtraction with conventional waveform-based subtraction, which requires that the waveforms of interference be equal at every time point between the two measurements. Our analysis showed that covariance-subtraction only requires that the time-average of the squared intensity of interference be equal between the two measurements. As a result, the method is still effective when the onset of interference differs or even their measured waveforms differ between the two measurements. The covariance- and waveform-subtraction methods were both applied to remove the interference caused by response-button-pressing finger movements in auditory-evoked MEG measurements. The results of this application demonstrated the superiority of the covariance-subtraction method over the conventional waveform-subtraction method.
Brain Topography 11/1998; 11(2):95-102. · 3.45 Impact Factor
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ABSTRACT: The auditory evoked neuromagnetic fields elicited by synthesized vowels of two different fundamental frequencies F0 were recorded in six subjects over the left and right temporal cortices using a 37-channel biomagnetometer. Single equivalent current dipole modeling of the fields elicited by all vowel types localized activity to a well-circumscribed area in supratemporal auditory cortex in both hemispheres. There were hemisphere asymmetries in the amplitude and latency of the M100 response. We also observed changes in M100 latency related to vowel type, but not to F0. There was no clear effect of vowel type or F0 on dipole localization for the M100, but a possible vowel type by latency interaction. These M100 data provide further evidence that vowels are processed independently of their pitch.
Neuroscience Letters 02/1997; · 2.11 Impact Factor
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ABSTRACT: Visually presented letter strings consistently yield three MEG response components: the M170, associated with letter-string processing (Tarkiainen, Helenius, Hansen, Cornelissen, & Salmelin, 1999); the M250, affected by phonotactic probability, (Pylkkänen, Stringfellow, & Marantz, 2002); and the M350, responsive to lexical frequency (Embick, Hackl, Schaeffer, Kelepir, & Marantz, 2001). Pylkkänen et al. found evidence that the M350 reflects lexical activation prior to competition among phonologically similar words. We investigate the effects of lexical and sublexical frequency and neighborhood density on the M250 and M350 through orthogonal manipulation of phonotactic probability, density, and frequency. The results confirm that probability but not density affects the latency of the M250 and M350; however, an interaction between probability and density on M350 latencies suggests an earlier influence of neighborhoods than previously reported.
Brain and Language 90(1-3):88-94. · 3.12 Impact Factor
