Neuroscience When different brains speak

No brain is like the other. In some people it is slightly larger or slightly heavier than in others; some have more grey, others have more white matter. In academic studies, brain researchers typically lump all brains together after all data had been collected and calculate a mean value. Not so Patrick Friedrich. The PhD student at the biopsychology work unit is interested in the differences between brains of different individuals – and not just on the anatomic level. Using functional magnetic resonance imaging (functional MRI), he also records differences in brain activity.

Friedrich focuses on Broca’s area. It is responsible for the production of speech and supplies input to the brain motoric regions that control the speech apparatus with the vocal cords. Broca’s area is involved even if people only think of words without pronouncing them – albeit to a different extent in different individuals. In some people, Broca activation spreads across a larger surface than in others. Patrick Friedrich set out to find out the reason for this phenomenon.

Thick nerve fibre bundles

To this end, the biopsychologist from the team headed by Prof Dr Dr h. c. Onur Güntürkün examined not only Broca’s area, a region located in the frontal lobe. He also analysed areas further back in the brain that are associated with speech: the primary auditory cortex, i.e. the entry point for acoustic signals in the cerebral cortex, and two regions that together form Wernicke’s area, which, in turn, is involved in the comprehension of speech.

Thick nerve fibre bundles lead into Broca’s area in the frontal region from the primary auditory cortex and from Wernicke’s area. Can the mass of these fibre bundles shed light on the differences in Broca activation? It is conceivable that individuals with a highly developed link between the frontal lobe and the temporal lobe exhibit increased activation in Broca’s area.

In addition to the structural link, another parameter must be considered: the functional link. “We can examine in what way Broca activity correlates with the activity of another area,” explains Friedrich. Thus, he is able to analyse whether Broca’s area is active at precisely the same time as the primary auditory cortex.

In order to collect the structural and functional data, Patrick Friedrich performed MRI scans of 105 participants. The participants’ task was to think of as many words beginning with a specific letter as possible. In the control condition, the biopsychologist recorded the brain activity while the respective person looked at a cross without being given a specific task. “We asked them not to think of any words during that time,” he reports. “Naturally, we cannot verify whether they succeeded or not.” But considering that the analysis yielded a significant difference between the control condition and the word task, it is very likely that the participants followed the instruction.

Analysis with six influencing factors

For the analysis, Patrick Friedrich focused on the activity in Broca’s area, in the primary auditory cortex as well as in both Wernicke’s subparts. He identified to what extent the activity in Broca’s area was determined by the three areas located further back. Moreover, he recorded structural images of the brain and calculated the volume of the fibre bundles between the analysed regions.

Thus, six influencing factors came together that might shed light on the extent of the activity in Broca’s area: the thickness of the fibre bundles from Broca’s area to the primary auditory cortex and to both regions of Wernicke’s fractions, and the intensity of the functional link between Broca’s area and the three speech areas further back.

Unexpected results

“We had assumed that a certain section of Wernicke’s area in particular could be used to explain the extent of activity in Broca’s area. This is because that region is connected with Broca’s area via a main fibre tract,” explains Patrick Friedrich.

The results to date show: among six potential influencing factors, there was one that stood out, and it was a functional rather than a structural link. The activity in the primary auditory cortex affected the extent of Broca activity the strongest. The stronger the communication between Broca’s area and the primary auditory cortex, the greater the active section of Broca’s area during the production of speech.

But why is the primary auditory cortex relevant at all if the participants didn’t speak aloud? They only thought of the words without pronouncing them. A speech model by Frank Guenther might provide an explanation. According to that model, Broca’s area is involved in two functional process loops. The first loop creates a link between Broca’s area and the motoric cortex that initiates the articulation of language via the vocal cords. The second one is a feedback loop. The brain uses it to verify whether the words it hears are identical with those, which were supposed to be pronounced. If that is not the case, the primary auditory cortex sends out an error signal.

As the participants only thought of the words, there was nothing for the auditory cortex to hear. Consequently, an error signal had to be generated. The team at the biopsychology work unit assumes that the correlation between Broca activity and the auditory cortex is caused by the latter’s function as an error detector.

“At present, we cannot yet say whether this interpretation is correct,” admits Patrick Friedrich. “Still, it does make sense. Language is a dynamic process, after all. When we speak, we wait for feedback, we listen to our own word flow or wait for the reaction of the person we talk to.” The researcher sees this dynamics reflected in the observed activity patterns. And it finds a slightly different expression in each individual.