Investigating phonological processing by varying pronounceability: MEG Evidence Jeremy Bronheim, Martin Hackl, David Poeppel University of Maryland, CNL Lab, Departments of Biology and Linguistics, NACS Program Introduction Embick and colleagues (2001) performed an MEG study of visually presented words during a lexical decision task. They found that in the first 400 ms after stimulus onset, the MEG data showed 3 primary peaks. The first, at approximately 170ms, the M170, is thought to correspond to visual word form recognition. The next peak, the M250, occurs at approximately 250 ms, has not yet been found to correspond to a specific function. The M350 was found to have a latency that was inversely proportional to the frequency of the word stimulus. The M350 also shows priming effects (Pylkkänen et al. 2000) and other properties that suggest that it is an index of lexical access. In visual lexical decision, the orthography of a visually presented word must be converted into a phonological code before lexical search is performed. If the M170 reflects visual word form and the M350 lexical access, the M250 may reflect aspects of phonological processing, e.g. processes related to pronounceability. Hypothesis 1: The M250 peak is the activation associated with the process of converting the orthography into a phonological code, which is then used in the lexical decision task. Experiment 2: MEG Study Results (continued): Materials and Methods: All 3 peaks were found for all conditions. An example of a condition with all 3 peaks is shown in Figure 6. 11 right-handed subjects with 20:20 vision were run. 3 subjects’ data was discarded because of poor performance on the task or excessive noise in the MEG data. While in the MEG scanner, subjects performed a lexical decision task using the following items: •30 High Frequency (HF) words •30 Low Frequency (LF) words •30 items chosen from each category of Exp. 1 (PNW, Onset violation non-words, No vowel nonwords, and Consonant strings) Consonant Strings No vowel Non-words Onset violation Non-words Pronounceable Non-words Experiment 1: Stimulus norming Figure 6: Sample data showing M250 latency differences in two conditions, with all 3 peaks present. Conclusions: hard to pronounce Figure 2: The distribution of items chosen from Experiment 1 to be stimuli in Experiment 2. Materials and Methods: The 30 items from each category were controlled for letter length (8 4-letter, 8 5-letter, 8 6-letter and 6 8 letter items per group). • 4 categories of non-words: Each item was presented twice for a total of 480 trials. Non-Word Type Example A random ISI of between 1250 and 1750 ms in 50 ms intervals was used between items. Pronounceable Non-Word Onset Violation Non-Word No Vowels Non-Word Consonant String blicket Hypothesis: Peak Existence/Latency Expectations by Category M170 M250 M350 bficket M250 Latency Order (if possible peaks existent) Pronounceable NonWords ? High Frequency Words Onset Violation Nonwords ? Low Frequency Words Words HF & LF bfnckrt bvpxqtk Ratings •Scale of 1-7 Non-words w/o vowels, but with liquids and nasals Consonant Strings False Fonts e.g. Qellom • 23 Subjects rating 225 non-words each (approximately 56 non-words per category) ? ? Pronounceable Non-Words ? ? X 4 6 5 3 Onset Violation Non-Words X X Non-words w/o Vowels 2 Consonant Strings 1 Figure 3: a (left) shows expectation of the existence of each peak under each condition. some PNW may have pronounceabilities corresponding to HF words, and others corresponding to LF words. Results: Peak Latencies: Pronounceability Effect on M250 Latency 260 255 References: 255 250 No vowel Non-words Onset violation Non-words Latency (ms) 250 M250: 245 245 240 240 235 235 230 230 Embick, D., Hackl, M., Schaeffer, J., Kelepir, M. & Marantz, A. (2001). A magnetoencephalographic component whose latency reflects lexical frequency. Cognitive Brain Research, 10, 345-348. 225 225 Pronounceable Non-words 220 220 215 HF LF PNW OnVio NoVow Cons Pronounceable Items (HF, LF, PNW) FF Type Unpronounceable Items (Onset Violation, No Vowel, and Consonant Strings) False Fonts M350 Analysis by Type easy to pronounce The M250 latency data (Figure 4) only partially followed predicted trends. The words and pronounceable non-words follow the predicted trend (Figure 4a). Further, the fact that pronounceable items had longer latencies than unpronounceable items (Figure 4b) indicates that the orthography to phonology conversion hypothesis may, indeed, be correct. For the non-words, we hypothesized that greater violations would elicit earlier M250 latencies, since the conversion process would fail. We did not find this trend among the non-words (Figure 4a), although another possibly relevant trend was present. If we order the expected M250 latency for non-word categories based upon how early in the letter string the orthography or phonotactics of English are violated, we get the following order (from shortest to longest latency): False Fonts (non-English orthography), Onset violation non-words (2 letters), consonant strings (2-4 letters), then, finally no vowel non-words (3-4 letters). Despite not finding the M250 latency order that we expected, our hypotheses, that the M250 represents the orthography to phonology conversion process and thus, that its latency should depend on pronounceability, were tentatively confirmed. In order to test the validity of the interpretation leading to this confirmation, a follow-up study varying the point in a non-word at which it violates English phonotactics needs to be done. That it is effected by difficulty of pronounceability. PNW are above LF words because M250 Analysis by Type For words and pronounceable non-words, the M350 did appear to be inversely related to word frequency, as found in previous studies (Embick et. al., 2001). We did, however, find an M350 peak for the false fonts. This would not be expected if the M350 represents lexical access, because there should not be anything to search the lexicon with in the false font condition. Some other explanation for the M350 response to false fonts must be found. The one non-word type that does not follow this order are the no-vowel non-words. This can be explained by a confound we introduced into the stimuli – the no-vowel non-words also had onset violations. This feature of the no-vowel non-words makes sense if overall pronounceability of the non-words is what is being tested, but not if what needs to be varied is how early in the non-word the first violation occurs. Because they have onset violations, the no-vowel non-words are phonotactically bad just as early as the onset violation non-words, indicating that they should have the same latency, as they do. b (right) shows the expected latency of M250 peaks (if existent) based upon the hypothesis Consonant Strings M250 Contour Maps match across conditions Figure 2 Shows the distribution of the items chosen from Experiment 1. easy to pronounce 4 groups were rated as significantly different (p < 0.01). The separation is show in figure 1. M350 •30 Amharic Script items (e.g. Mover) used as False Fonts (FF) Hypothesis 2: M250 latency will be related to the pronounceability of the stimulus presented. Results: M250 •30 filler words to better balance word:non-word ratio In order to test this hypothesis, we need to show, therefore, that the time this task takes is affected by the difficulty of converting an item’s orthography into phonology. The difficulty of this conversion can also be though of as a measure the pronounceability of an item. Thus: Purpose: To find four, well separated groups of non-word stimuli (to be used in Exp. 2) based upon pronounceability. M170 Figure 4a: M250 Latency by Condition hard to pronounce Figure 4b: M250 Latency Phonotactic Legality 360 350 Latency (ms) Figure 1: Distribution of Results from Experiment 1 M350: Pylkkänen, L., Flagg, E., Stringfellow, A. & Marantz, A. (2000). A neural response sensitive to priming: An MEG study of lexical access. Poster Session presented at the annual meeting of Cognitive Neuroscience, San Francisco, CA. 340 330 Acknowledgments: 320 310 300 HF LF PNW OnVio Type NoVow Figure 5: M350 Latency by Condition Cons FF This work supported by NIH DC 05660 to DP.