Don't Assume that fMRI and MEG Will Give You Comparable Results

Accessibility: Intermediate/Advanced

There are three common methods of studying brain function in normal human populations: fMRI, MEG, an EEG. There is surprisingly little crosstalk between the techniques, mostly due to practical issues.For better or worse, labs tend to specialize in one technology.

It's often assumed that the relationship with techniques is straightforward, that it's simple to map results from one technique onto another. However, a recent study by Johanna Vartianen and colleagues suggests otherwise.



The group wanted to study reading using all three brain techniques. Participants performed the same experimental paradigm twice: once with simultaneous EEG and fMRI, and once with simultaneous EEG and MEG. Participants saw words, pseudowords, consonant strings, and symbol strings, and words embedded in noise. Their task was to detect immediate repetitions. The EEG results from the two sessions were comparable, so the researchers went on to compare the fMRI and MEG activation patterns for the experiment.

To summarize, activation patterns between MEG and fMRI did not show a straightforward relationship. In some regions, the two techniques showed the same pattern. For example, in the occipital lobe, both MEG and fMRI measures had more activation to noisy words than other types of stimuli.

If you look at the occipitaltemporal lobe however, the two techniques had opposite results. MEG showed more activation to real letters than symbols, while FMRI showed more activation to symbols then letters.

In the left frontal cortex the two regions had completely different patterns. FMRI activation was higher for words and pseudowords than symbols and noisy words. The MEG results showed no difference at all between stimulus types.

I guess this is one of these results that you don't see going in, but in hindsight make you hit yourself over the head. FMRI and MEG measure very different things, so it’s entirely possible that results would come out differently. FMRI measures cerebral blood flow on a timescale of several seconds, while MEG measures synchronous electrical activation with millisecond resolution. So ( as the authors suggest) non-synchronous activity may be lost in MEG. Meanwhile, fMRI picks up average activity over a longer time period and may miss short-term activity.

Interestingly, the authers mentioned that previous MEG results for the visual word form area were fairly robust to task differences, while fMRI results do seem to vary with task. Now I don't know the MEG literature well, but they're certainly right about the fMRI literature. In that case, I wonder what it is about the MEG that makes its results relatively task independent. Is it the better temporal resolution? Perhaps MEG analyses focus on early, bottom up processing, which may be relatively task independent?


Vartiainen J, Liljeström M, Koskinen M, Renvall H, & Salmelin R (2011). Functional magnetic resonance imaging blood oxygenation level-dependent signal and magnetoencephalography evoked responses yield different neural functionality in reading. The Journal of neuroscience : the official journal of the Society for Neuroscience, 31 (3), 1048-58 PMID: 21248130

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Multimodal Investigation of Reading in Children: More from Brem and Colleagues

Accessibility: Advanced

Last time we read an article from Brem and colleagues that compared word processing in adolescents (age 15-17) and adults (19-30). In follow-up paper from 2009, Brem expanded the report to include children (9-11).

If you didn’t read the last post, it’s probably a good idea to do that first. I won’t repeat any of the methodological details or background information here, just gonna make few quick notes on their results.



The 2006 paper found that adolescents had higher N1 amplitude than adults. Here, Brem reports that children have an even higher N1 amplitude with adolescents, thus suggesting a steady decrease in N1 amplitude from age 9 onwards.

For all groups, the N1 amplitude was higher for words than symbols. However, the difference between words and symbols declined with age. At first, I found this counterintuitive. I would have expected the opposite, with kids treating words and symbol similarly and the word/symbol difference getting larger as they matured and became better readers. The kids in this study, however, have already been reading for a few years. Perhaps they’re at the stage where they can process the words but are less efficient in doing so, thus resulting in a higher N1 amplitude for words than symbols.

On the fMRI front, Brem found the same  posterior to anterior gradient in the fusiform gyrus, with posterior regions being more responsive to symbols, and anterior regions being more responsive to words. There didn’t seem to be any difference between age groups there.

Brem also increases that a higher signal in anterior fusiform is correlated with slower reading.
(This is opposite of what was reported in other paper, perhaps I’m misreading the paper.)

There were some discrepancies between EEG and fMRI results. The N1 ERP component shows clear difference between words and symbols, but the fMRI analysis doesn’t show differences in the occipital temporal region, the calculated source of the N1. The could be due to temporal resolution. The N1 component only lasts about 100 ms.  EEG has good enough temporal resolution to pick up on the difference, but fMRI may not.


Brem S, Halder P, Bucher K, Summers P, Martin E, & Brandeis D (2009). Tuning of the visual word processing system: distinct developmental ERP and fMRI effects. Human brain mapping, 30 (6), 1833-44 PMID: 19288464

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Developmental Changes in Word Processing After Adolescence

When does brain development for reading stop? We often focus on school aged children, but what about the later teen years? To answer this question, Brem and colleagues tested adolescents (age 15-17) and adults (19-31) in a study using fMRI and EEG.



Participants were presented with words and symbols strings and asked to detect repeats. It’s an easy task, so it’s not surprising that the two groups had equal reading accuracy and speed. However, there were brain differences.

Brem focused on two early ERP components. The P1 component, a positive peak at 100 ms, is sensitive to low level stimulus characteristics like luminance and size. Brem found that this component had a higher amplitude for symbol strings and for words in both groups.

The N1 component occurs later (140-220ms) and is sensitive to higher level factors like stimulus category. Brem found that the later part of the N1 component was more pronounced to words than symbol strings. Source localization on the N1 component  found that the early part of the N1 localized to the temporal parietal occipital junction, while the late N1 localized to the left fusiform.

There were differences between the two groups. Adolescents had higher P1 and N1 amplitudes than adults. The N1 latency also became faster with age for words but not symbol strings.

Brem also used fMRI to look at the spatial organization of the fusiform gyrus*. Posterior fusiform regions responded more to symbol strings than words, while anterior regions responded more to words than symbol strings.

The left fusiform region seems to be related to reading skill. Bigger N1 amplitude was correlated with fewer mistakes in a reading test.  Higher fMRI signal in the anterior fusiform was correlated with faster reading.

It’s interesting that despite similar behavior between groups, brain measures still differ. I do wonder about differences within the adults as well. 19-31 is a pretty big range, so I'd like to see what happens after age 18.

*Using five regions of interest. 6 mm spheres based on Taleraich coordinates.


Brem S, Bucher K, Halder P, Summers P, Dietrich T, Martin E, & Brandeis D (2006). Evidence for developmental changes in the visual word processing network beyond adolescence. NeuroImage, 29 (3), 822-37 PMID: 16257546

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Development of Modality Tuning During Reading and Repetition

Accessibility Level:  Intermediate/Advanced

Today we’re again looking at the theme of increasing specialization in the brain over development. Rather than specialization in terms of spatial extent, as touched on in Brown 2004, Cerebral Cortex, this paper’s finding suggests specialization in processing of sensory modalities.



Church and colleagues tested children (age 7-10) and adults (18-35) in a word generation task. During the experiment they read words off a screen and repeated words presented aurally. Like the two papers previously discussed here by this group, the authors matched for behavior between children and adults.
They authors report several findings.

1. First, most brain regions did not change in activation over time. In well known language areas like the inferior frontal gyrus and superior temporal gyrus, the authors found no difference between children and adults. Also, they found no differences in lateralization (how much one side of the brain was favored over another) between children and adults.

2. The regions that differed between the two groups were mainly extrastriate visual regions, and all regions with differences had greater activation for children than adults. Unlike the Brown 2005 paper, where some frontal regions were found to have greater activation in adults, this paper found no such regions. This could be due to the different task (word generation vs. reading/repeating), or variation in the participant pools of the two the studies.

3. In several visual regions, including a cluster very close to the visual word form area, adults had more activation to the visual presentation than to auditory presentation, while children had similar activation to the two modalities. This suggests that these areas might be more specialized for the visual modality in adults. In other words, the region gets “tuned” to the visual modality as the children mature (However, the interaction between modality and age was not statistically significant).  The authors propose several possible mechanisms responsible for this modality tuning difference. Perhaps the kids are using a different strategy, visualizing more during the auditory task. Or perhaps their brains are just organized differently.

Together with the Brown 2004 paper, this paper presents an interesting story about increasing specialization and efficiency in the maturing brain, in which the immature brain starts out with relatively nonspecialized brain regions and recruits more brain regions to accomplish the tasks at hand. Then, maturation and expertise result in more specialization, finer tuning, and fewer recruited regions.



Church JA, Coalson RS, Lugar HM, Petersen SE, & Schlaggar BL (2008). A developmental fMRI study of reading and repetition reveals changes in phonological and visual mechanisms over age. Cerebral cortex (New York, N.Y. : 1991), 18 (9), 2054-65 PMID: 18245043

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