Visual Word Form Recognition: An MEG study using Masked Priming

1. Sensors of Interest Sensors of Interest M100 window: 80-150ms M170 window: 155-225ms Prime (-34ms) Target (0ms) Visual Word Form Recognition: An MEG study using Masked Priming Heejeong Ko1, Michael Wagner1, Linnaea Stockall1, Sid Kouider2, Alec Marantz1 1Department of Linguistics and Philosophy, MIT & KIT-MIT MEG Laboratory, 2Ehess/CNRS, Paris 1 4 6 Testing Identity Priming with MEG Testing Morphological Priming with MEG • Goals of the Study • To establish the neuropsychological correlates of identity priming in visual word processing in the masked priming paradigm without conscious perception, using Magnetoencephalography (MEG). • To identify the MEG components that dissociate the priming effects of morphological and orthographical relatedness in the masked priming paradigm. • Stimuli. (240 Items: 120 word targets, 120 nonword targets): mostly taken from Rastle et al. (2000, 2003). (1) 40 tokens of control and identity condition (abyss - BOWEL (control); bowel – BOWEL (identity)). 40 tokens of semantically related fillers (gut - BOWEL). (2) 40 tokens of nonword target triplets (hora - ABOL, abol - ABOL, tomato - ABOL). • Hypothesis. If morphological decomposition applies blindly to all potentially complex words in early visual processing (e.g broth-er), we expect to find identity priming effects when two words are potentially morphologically related in early visual processing (brother-BROTH). • Procedure. DMDX presentation with masked priming (resolution 1024*768 , 60Hz). Prime duration = 34 ms. ISI=500ms. Items were divided in three blocks and counterbalanced in each block. Items were scrambled randomly by DMDX. Lexical Decision Task. • Stimuli. (240 items:120 word targets, 120 nonword targets): stimuli are mostly taken from Rastle et al. (2000, 2003). (1) 40 tokens of control, morphology, and orthography condition (trifle-BROTH (control), brother-BROTH (morphology), brothel-BROTH (orthography)). (2) 40 tokens of nonword target triplets (supply-ANG, anger-ANG, angle-ANG). 2 GIFT Target + Fixation 500ms gift Prime 34ms ###### Forward Mask 500 ms Early Visual Word Responses in the Brain • Procedure, Subject, and MEG data analysis are the same as in our experiment 1 (see # 4 in the poster). M100: Early Visual Response M170: Visual Word Activation • Subjects.14 right-handed native English speakers with normal vision gave informed consent to participate in the experiment. MEG data from 11 subjects. • M170 is sensitive to visual form processing in the brain (e.g. letter, symbol, face) (Tarkiainen et al. 1999). In our laboratory, M170 has not been sensitive to lexicality or frequency of words. • M100 is the first large MEG time component evoked by visual stimuli. Generated by primary visual cortex bi-laterally. More activity for noisier/longer letter and symbol strings (Tarkiainen et al. 1999). 7 Results: Morphological Priming in the Brain • MEG Data collection. Neuromagnetic fields were recorded using a 93-channel axial gradiometer whole-head system for 6 subjects and a 157-channel system for 5 subjects (KIT, Kanazawa, Japan). Data were acquired in a band between DC and 200Hz, at a 1000Hz sampling frequency. • Behavioral Results.N=11. RT in the morphology condition is significantly faster than RT in the control condition (p=0.019). No significant difference between the orthography and control condition in RT (p=.310). The RT difference between the morphology and orthography condition is approaching significance (p= 0.065), replicating Rastle et al. (2003). p=0.019* • MEG Data analysis. 25 sensors from 157 MEG channels, and 13 sensors from 93 MEG channels in the left hemisphere were selected as the Sensors of Interest (SOI). SOI had the negative field for M100 and the positive field for M170 (see # 2 in the poster).15 data points from the time window for M100 (80ms-150ms) and 15 data points from the time window for M170 (155ms-225 ms) were included from each SOI for statistical analysis. Outlier data that do not match the typical polarity of the relevant components were filtered out. One-way ANOVA (DV= amplitude at each time point, IV = subject, time, condition) and planned comparisons for time*condition interactions were conducted through SPSS. p<.0001*** 5 p=.031* Results: Identity Priming in the Brain • Behavioral Results. N=11. • Planned comparisons show that RT in the identity condition is significantly faster than RT in the control condition (p=0.009). p=0.009*** p<.0001*** p<.0001*** Discussion • MEG Results. N=11.The M100 shows significantly more activation for the morphology condition than the control (p <. 0001) and the orthography conditions (p <. 0001). The M170 shows significantly less activation for the morphology condition than the control (p=.031). The amplitude in the orthography condition is significantly lower thanthecontrol condition in M100 (p<.0001). No significant difference between orthography and control in M170 (p=.073) or between morphology and orthography in M170 (p=.737). 3 Masked Priming Paradigm • The Masked Priming Paradigm provides a method to examine priming effects without conscious perception in early visual word form processing (Forster & Davis 1984, Rastle et al. 2000, 2003, among others). • In masked priming, a prime is sandwiched between a forward mask (#####) and a target. With short prime duration, the prime cannot be consciously perceived by the subject. • Identity and morphological priming effects in the Masked Priming Paradigm have been observed in the absence of semantic and orthographic priming effects (cf. Rastle et al. 2000, 2003 for behavioral studies; Dehaene et al. 2001 for a fMRI and EEG study). • In the current study, we investigate the effects of marked priming without conscious perception in the brain by examining the early MEG time components (M100 and M170). In particular, the effects of identity priming (experiment 1) and morphological priming (experiment 2) are of special interest. 8 • Our MEG data show that the M100 and M170 components are sensitive to identity and morphological priming effects in the masked priming paradigm. • The identity and morphological priming conditions show the same pattern in the M100 and M170 components. This supports our hypothesis that morphological decomposition occurs prior to lexical access. • Unlike RT results, orthographical and morphological priming show a significantly different pattern in the brain (M100). This suggests that priming due to morphological decomposition significantly differs from priming due to pure graphemic overlap in visual word processing in the brain. • Localization and source activation analyses are required before we can definitely assign effects from masked priming to either the M100 (visual cortex) or M170 (letter string/visual word form area). p=.029* p<.0001*** • MEG results. N=11.The amplitude in theidentity priming condition is significantly lower than the amplitude in thecontrol condition (p<.0001) at the M100 (negative polarity, more activation). The amplitude in the identity condition is also significantly lower than the control condition (p=.029) at the M170 (positive polarity, less activation). 9 • Dehaene, S., L. Naccache, L. Cohen, D. L.Bihan, J-F. Mangin, J-B. Poline, and D. Rivière (2001). Cerebral mechanisms of word masking and unconscious repetition priming. nature. neuroscience 4(7): 752-758. • Feldman, L. B. (2000). Are morphological effects distinguishable from the effects of shared meaning and shared form? Journal of Experimental Psychology: Learning, Memory, and Cognition 26. • Forster, K.I. and C. Davis (1984). Repetition priming and frequency attenuation in lexical access. Journal of Experimental Psychology: Learning, Memory, and Cognition, 10, 680-698. • Rastle, K., M.H. Davis, W.D Marslen-Wilson and L.K. Tyler (2000). Morphological and semantic effects in visual word recognition: A time-course study. Language and Cognitive Processes, 15 (4/5), 507-537. • Rastle, K. and M.H. Davis (2003). Reading morphologically-complex words: Some thoughts from masked priming. In Kinoshita, S. & Lupker, S.J. (eds.) Masked priming: State of the art. Psychology Press. • Tarkiainen, A, P. Helenius, P.C. Hansen, P.L. Cornelissen, R. Salmelin (1999). Dynamics of letter string perception in the human occipitotemporal cortex. Brain 122: 2119-2131.