Functions[edit source]
Language comprehension[edit source]
For a long time, it was assumed that the role of Broca's area was more devoted to language production than language comprehension. However, there is evidence to demonstrate that Broca's area also plays a significant role in language comprehension. Patients with lesions in Broca's area who exhibit agrammatical speech production also show inability to use syntactic information to determine the meaning of sentences.[8] Also, a number of neuroimaging studies have implicated an involvement of Broca's area, particularly of the pars opercularis of the left inferior frontal gyrus, during the processing of complex sentences.[9] Further, it has recently been found in functional magnetic resonance imaging (fMRI) experiments involving highly ambiguous sentences result in a more activated inferior frontal gyrus.[10] Therefore, the activity level in the inferior frontal gyrus and the level of lexical ambiguity are directly proportional to each other, because of the increased retrieval demands associated with highly ambiguous content.
There is also specialisation for particular aspects of comprehension within Broca's area. Work by Devlin et al. (2003)[11] showed in a repetitive transcranial magnetic stimulation (rTMS) study that there was an increase in reaction times when performing a semantic task under rTMS aimed at the pars triangularis (situated in the anterior part of Broca's area). The increase in reaction times is indicative that that particular area is responsible for processing that cognitive function. Disrupting these areas via TMS disrupts computations performed in the areas leading to an increase in time needed to perform the computations (reflected in reaction times). Later work by Nixon et al. (2004)[12] showed that when the pars opercularis (situated in the posterior part of Broca's area) was stimulated under rTMS there was an increase in reaction times in a phonological task. Gough et al. (2005)[13]performed an experiment combining elements of these previous works in which both phonological and semantic tasks were performed with rTMS stimulation directed at either the anterior or the posterior part of Broca's area. The results from this experiment conclusively distinguished anatomical specialisation within Broca's area for different components of language comprehension. Here the results showed that under rTMS stimulation:
- Semantic tasks only showed a decrease in reaction times when stimulation was aimed at the anterior part of Broca's area (where a decrease of 10% (50ms) was seen compared to a no-TMS control group)
- Phonological tasks showed a decrease in reaction times when stimulation was aimed at the posterior part of Broca's area (where a decrease of 6% (30ms) was seen compared to control)
To summarise, the work above shows anatomical specialisation in Broca's area for language comprehension, with the anterior part of Broca's area responsible for understanding the meaning of words (semantics) and the posterior part of Broca's area responsible for understanding how words sound (phonology).
Action recognition and production[edit source]
Recent experiments have indicated that Broca's area is involved in various cognitive and perceptual tasks. One important contribution of Brodmann's area 44 is also found in the motor-related processes. Observation of meaningful hand shadows resembling moving animals activates frontal language area, demonstrating that Broca's area indeed plays a role in interpreting action of others.[14] An activation of BA 44 was also reported during execution of grasping and manipulation.[15]
Speech-associated gestures[edit source]
It has been speculated that because speech-associated gestures could possibly reduce lexical or sentential ambiguity, comprehension should improve in the presence of speech-associated gestures. As a result of improved comprehension, the involvement of Broca's area should be reduced.[7]
Many neuroimaging studies have also shown activation of Broca's area when representing meaningful arm gestures. A recent study has shown evidence that word and gesture are related at the level of translation of particular gesture aspects such as its motor goal and intention.[16] This finding helps explain why, when this area is defective, those who use sign language also suffer from language deficits.[17] This finding that aspects of gestures are translated in words within Broca's area also explains language development in terms of evolution. Indeed, many authors have proposed that speech evolved from a primitive communication that arose from gestures.[14][18] (See below.)
Speaking without Broca's area[edit source]
Damage to Broca's area is commonly associated with telegraphic speech made up of content vocabulary. For example, a person with Broca's aphasia may say something like, "Drive, store. Mom." meaning to say, "My mom drove me to the store today." Therefore, the content of the information is correct, but the grammar and fluidity of the sentence is missing.[19]
The essential role of the Broca's area in speech production has been questioned since it can be destroyed while leaving language nearly intact. In one case of a computer engineer, a slow-growing glioma tumor was removed. The tumor and the surgery destroyed the left inferior and middle frontal gyrus, the head of the caudate nucleus, the anterior limb of the internal capsule, and the anterior insula. However, there were minimal language problems three months after removal and the individual returned to his professional work. These minor problems include the inability to create syntactically complex sentences including more than two subjects, multiple causal conjunctions, or reported speech. These were explained by researchers as due to working memory problems. They also attributed his lack of problems to extensive compensatory mechanisms enabled by neural plasticity in the nearby cerebral cortex and a shift of some functions to the homologous area in the right hemisphere.[5]
Mirror neurons[edit source]
Communication, both verbal and nonverbal, requires that the interacting individuals stay "tuned" to one another. Mirror neurons were discovered in the 1990s in frontal area F5 of the monkey cortex. These neurons are active during execution of object-related hand actions, but they are also active, importantly, when the monkey is just observing similar acts. For example, the mirror neurons are activated when the monkey takes a raisin from a tray and also when he views another monkey or the human experimenter doing the same. No information is yet available about possible hemispheric lateralization of the monkey mirror neurons.
Mirror neurons have visuomotor properties, being sensitive to goal-related motor acts, but they can also be activated by sounds that imply actions. Importantly, the mirror neurons do not only react to visual input and then project, via some transformational step, to motor-output-related neurons but are also part of a system that forms a neuronal representation of the observed motor acts. Similar to F5, the rostral part of the inferior parietal cortex contains neurons that are active during action observation and execution; this region receives input from the STS, which is known to contain neurons responding to biological motion.