Pars triangularis asymmetry and language dominance.

Abstract

The pars triangularis is a portion of Broca's area. The convolutions that form the inferior and caudal extent of the pars triangularis include the anterior horizontal and anterior ascending rami of the sylvian fissure, respectively. To learn if there are anatomic asymmetries of the pars triangularis, these convolutions were measured on volumetric magnetic resonance imaging scans of 11 patients who had undergone selective hemispheric anesthesia (Wada testing) to determine hemispheric speech and language lateralization. Of the 10 patients with language lateralized to the left hemisphere, 9 had a leftward asymmetry of the pars triangularis. The 1 patient with language lateralized to the right hemisphere had a significant rightward asymmetry of the pars triangularis. Our data suggest that asymmetries of the pars triangularis may be related to speech-language lateralization.Paul Broca (1) described eight right-handed patients who lost the facility of speech and were found to have left hemispheric lesions. Broca (1) thought that the left third frontal convolution, including pars triangularis, was critical for speech. Although subsequent studies have demonstrated that left-sided lesions restricted to the third frontal convolution are probably associated with only transient loss of speech (2, 3) lesions of the right third frontal convolution rarely produce any speech deficits. These observations suggest that although the left inferior frontal lobe appears to be important in speech production, other undamaged areas can either compensate or substitute for this area. Both cortical stimulation (4, 5) and functional imaging studies (6-9) have provided support for the role of the left inferior frontal lobe in speech production. Wernicke (10) demonstrated that a lesion in the posterior portion of the left superior temporal lobe produced a syndrome different from that described by Broca (1). Whereas Broca's patients lost the capacity to speak fluently but retained comprehension, Wernicke's patients retained the ability to speak fluently but were impaired at comprehending speech. Subsequently, morphological cerebral asymmetries in the posterior superior temporal speech region (planum temporale) were demonstrated on postmortem brains (11-15), with a predominant leftward asymmetry of the planum temporale. However, consistent asymmetries of the inferior frontal region have been more difficult to demonstrate on postmortem studies (13,(16)(17)(18). Therefore, the distinctive morphological asymmetries of the planum temporale have been thought to be the neuroanatomical substrate for speech and language. Furthermore, the planum temporale constitutes part of cytoarchitectonic areas TA and TB, which are auditory association cortices important in higher order processing of auditory language input. However, direct evidence supporting this structurefunction relationship has been lacking.With the advent of three-dimensional volumetric magnetic resonance imaging (MRI), in vivo quantitative analysis of the The publication costs of this ...

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