Of Minds and Language (34 page)

Fig. 13.1. Cytoarchitectonically segregated brain areas in the frontal cortex (indicated by numbers). Gray-shaded are those areas that make up the language-related Broca's area in the human brain and their homologue areas in the macaque brain.

Source
: adapted from Petrides and Pandya1994

What does granularity mean in this context? The cortex consists of six layers. Layer IV of the cortex is characterized by the presence of particular cells. These cells are very sparse or not present in BA 6, but they become more and more numerous as one moves further anterior in the brain. The dark-colored
areas are the dysgranular part (BA 44) and the granular part (BA 45), and as you may have recognized already, this is what makes up Broca's area in humans. With respect to the evolution of these parts, the neuroanatomist Sanides (1962) has proposed a “principle of graduation,” claiming that brain evolution proceeded from agranular to dysgranular and then to completely granular cortex. That is, the agranular cortex (BA 6) is not a well-developed cortex with respect to layer IV, whereas the dysgranular area (BA 44) and granular area (BA 45) are.

Fig. 13.2. Structure of the two grammar types. General structure and examples of stimuli in the FSG (Grammar I) and PSG (Grammar II). Members of the two categories (A and B) were coded phonologically with category “A” syllables containing the vowels “i” or “e” with category “B” syllables containing the vowels “o” or “u”. The same syllables were used for both types of grammar.

Source
: adapted from Friederici et al. 2006a

What could that mean with respect to the questions we are considering here? Could it be that the underlying structures of these two types of brains have something to do with the capacity to process either a simple probabilistic grammar or a hierarchical grammar? Let us assume that an FSG may be processed by a brain area that is phylogenetically older than the area necessary to process a PSG. In order to test this hypothesis, we (Friederici et al. 2006a) conducted an fMRI experiment using two types of grammars quite similar to those used by Fitch and Hauser in their experiment (see
Fig. 13.2
). We made the grammars a bit more complicated, but not too much. Note that we have two conditions, namely short and long sequences. This should allow us to see whether the difficulty or length of these particular sequences could be an explanation for a possible difference between the two grammar types. In our study, unlike the study with the cotton-top tamarins, we decided to use a visual presentation mode. Disregarding further details,
¹
what might be of interest is that we had correct and incorrect sequences in each of the grammar types, and
we had two subject groups. One subject group learned the FSG and the other learned the PSG. The subjects learned these grammars two days before entering the scanner, where they were given correct and incorrect sequences. We then compared the brain activation of the two groups.

Fig. 13.3. Brain activation pattern for the two grammar types. Statistical parametric maps of the group-averaged activation during processing of violations of two different grammar types (P<0.001, corrected at cluster level) are displayed for the frontal operculum (FOP) and Broca's area. (
Left
) The contrast of incorrect vs. correct sequences in the FSG (Grammar I) is shown. (
Right
) The same contrast in the PSG (Grammar II) is shown. (
Bottom
) Time courses (% signal change) in corresponding voxels of maximal activation in FOP and Broca's area are displayed.

Source
: adapted from Friederici et al. 2006a

For the group that learned the FSG, we found activation in the frontal operculum, an area that is phylogenetically older than Broca's area, for the comparison between grammatically correct and incorrect sequences (
Fig. 13.3
, left). However, Broca's area is not active. Interestingly enough, difficulty cannot be an explanation here because behaviorally, no difference was found between the short sequences of the FSG and the long sequences. In the imaging data a difference was observed in the delay of the activation peak with an early peak for the short, and a later peak for the long FSG sequences. But what do we find for the PSG learning group? Here again, not surprisingly, the frontal operculum is active, but now additionally Broca's area comes into play (
Fig. 13.3
, right). And again, when we compare the short sequences and the long sequences, difficulty does not matter. For this first study, we concluded that the processing of FSG, or more precisely what one should call it the processing of local dependencies, only recruits the frontal operculum (a phylogenetically older
cortex), whereas for the processing of minimal hierarchies as used in the present PSG, the phylogenetically younger cortex (Broca's area) comes into play.

Fig. 13.4. Structure of the two grammar types. General structure and examples of stimuli of FSG (Grammar I) and hierarchical PSG (Grammar III) are displayed. Grammar III implies a rule that characterizes the dependency between related A and B elements by the phonetic feature voiced/unvoiced.

Source
: adapted from Friederici et al. 2006a

However, there is more than one caveat to this conclusion. One argument could be the following: subjects did not really process the hierarchies, as the present PSG could be processed by a counting mechanism “plus something.” I remember that Noam said this once,
²
and furthermore that this “plus something” could be memory. So, if you have a good memory, you can work with this sort of mechanism and be successful in processing such a grammar.

In order to see whether we could find a similar brain activation pattern when forcing subjects to really process the hierarchies, we conducted a second fMRI study including a more complex hierarchical grammar (Grammar III,
Fig. 13.4
).
³
In this study again we used two grammar types: a probabilistic and a hierarchical grammar. But the hierarchical grammar was realized such that there was a defined relation between the members of categories A and B in the sequence. In the syllables used, the consonants were either voiced or unvoiced and the fixed relation was defined over this phonological feature. This forced the subjects to establish the relation between A1 and B1, and A2 and B2. In order to learn this grammar, it took the subjects quite a bit longer (actually a couple of hours longer), but nonetheless they managed quite well after about five hours of learning. Again, learning took place two days before subjects went into the scanner, where they were given a quick refresher lesson immediately
before the scanning session. The task was once again to judge whether the sequence they were viewing was grammatical according to the rule they had learned. Moreover, and this is a second caveat you might want to raise with respect to the first experiment, we tested two different subject groups. Therefore, in the second study our subjects had to learn both grammar types in the time window of two weeks. This allowed us to do a within-subject comparison. So any difference we see now cannot be attributed to group differences. Thus, in this second fMRI study, we were able to compare directly the brain activation for the FSG and the PSG, in a within-subject design. When comparing the two grammars directly, by subtracting the activation for one grammar from the other, one should not see the frontal operculum active, because that should be active for both of the grammars. Instead, what one should see is activation in the Broca's area only.

What we found is shown in
Fig. 13.5
. From these functional neuroanatomical data, we concluded that two different areas (i.e., the frontal operculum and Broca's area) are supporting different aspects of sequence and grammatical processing. The frontal operculum is able to process local dependencies, whereas whenever hierarchical dependencies have to be processed, Broca's area (BA
₄₄
and BA
₄₅
) comes into play.

Fig. 13.5. Brain activation pattern for Hierarchical PSG (Grammar III) minus FSG (Grammar I). Statistical parametric map of group-averaged activation is shown.

Source
: Bahlmann et al., in press.

However, as these two areas are located pretty close neuroanatomically in the prefrontal cortex, we thought it would be good to have additional evidence for a differentiation between these two areas in the prefrontal cortex. As one possibility, we considered structural neuroanatomy, in particular information about the structural connectivity between different brain areas. I'll explain what
that means. With the advent of the diffusion tensor imaging technique, we are able to image the brain fibers connecting two or more areas. Using this technique we looked at the connectivity of the two areas of interest, namely the frontal operculum and Broca's area, in order to see whether they differed with respect to their connectivity pattern (Friederici et al. 2006a).
Fig. 13.6
displays the connectivity patterns for four subjects.