verbal

VERBAL CENTER

"The question is," said Alice, "whether you can make words mean so many different things."
"The question is," said Humpty Dumpty, "which is to be the master--that's all." --Lewis Carroll (Through the Looking-Glass)

Neurologists have found a tiny area of tissue--about 1 centimetre square--near to Wernicke's area that lights up only when consonants are heard. --Rita Carter (1998:150)

Neuro term. A component of the brain, such as Broca's or Wernicke's area, which governs the use of manually articulated (i.e., signed) or vocally articulated (i.e., spoken) language. Also, an association (arcuate) fiber link, such as the arcuate fasciculus, connecting verbal components.

Usage: Verbal centers are used to control the production and/or comprehension of linguistic communication and words.

Hypothesis. Speech seems to have evolved its own specialized sensorimotor production-and-decoding system (see below, Embryology and Neuro-notes)--above and beyond that which is used for nonverbal expression (see below, Nonverbal speech areas). However, speech has not evolved its own semantic information content. The latter is housed in brain modules (e.g., of the parietal association areas and the frontal lobes) which are shared by verbal and nonverbal media alike. Thus, speech is special but not that special.

KNOWN VERBAL CENTERS

Angular gyrus. A visible bulge on the cerebral cortex marking regions of the occipital, parietal, and temporal lobes (behind Wernicke's area) which link visual word recognition with other linguistic abilities.

Arcuate fasciculus. A tract of association fibers connecting Broca's and Wernicke's areas. In a less robust form, the arcuate fasciculus may predate--and thus may be a preadaptation for--speech. Similar tracts of association fibers (the superior longitudinal fasciculus, inferior longitudinal fasciculus, and uncinate fasciculus) found in the right-brain hemisphere connect nonverbal centers of the cerebral cortex.

Basal ganglia. "It is likely that the enlargement of the prefrontal cortex reflects, in part, its role in speech production. The rewiring appears to involve the basal ganglia; data from recent comparative studies suggest that basal ganglia circuits may be the key to the unique brain bases of human speech and syntax" (Lieberman 1991:106-07).

Broca's area. A premotor module of the neocortex (in the lower lateral frontal lobe; specifically, Brodmann's areas 44 and 45) identified in 1861 by Paul Broca as essentially involved in the production and control of human speech. Damage to this area (called Broca's aphasia) produces problems in speaking (while comprehension of another's speech is left unimpaired). According to Philip Lieberman, Broca's area ". . . has no functional equivalent in nonhumans" (Lieberman 1991:24; but see below, Evolution I and II). Recently, a language module immediately anterior to Broca's area has been identified, which suggests that the Broca module may be involved in sequencing complex articulations which are not just limited to speech. Broca's area does not seem to control syntax (i.e., the combinatorial or grammatical arrangement of speech elements; see below, Neuro-notes II).

Insula. Some regard the insula as a verbal center (see, e.g., Ardila 1999). Damage to the left insula may result in language disturbances, including Broca's aphasia, conduction aphasia, speech apraxia, mutism, and the word-deafness of Wernicke's aphasia (Ardila 1999). ("Then on the other hand, recent studies of anatomical connections of the insula point to an important viscero-limbic role and it has been suggested that the insula may influence verbal motivation and verbal affect" [Ardila 1999].)

Planum temporale. "The planum temporale (PT) is a key site within Wernicke's posterior receptive language area in the left hemisphere of the human brain and is thought to be an epicenter within a dispersed mosaic of language-related regions in the cerebral cortex. The left hemisphere predominance of the PT is more pronounced than any other human brain asymmetry" (Gannon 1998:220). (See below, Neuro-notes.)

Wernicke's area. A supplementary-auditory module of the neocortex (in the left temporal lobe; specifically, Brodmann's areas 39, 40, posterior 21 and 22, and part of 37) identified as involved in the understanding of auditory words. Damage to this area (called Wernicke's aphasia) produces problems in deciphering the meanings of the speech sounds one hears (even of one's own speech sounds). According to a recent study, Wernicke's area is not unique to Homo (see below, Neuro-notes).

Apes. Magnetic resonance imaging (MRI) scans of chimpanzees, bonobos, and gorillas suggest that, like humans, these great apes also have an enlarged Brodmann's area 44 (part of Broca's area in the human brain). Writing in the journal Nature (2001), Claudio Cantalupo and William Hopkins (Emory University and Georgia State University) suggest the brain homologue may be due to a link between primate vocalization and gesture. Captive apes, the researchers note, usually gesture with the right hand as they vocalize.

Embryology. 1. "It is important to recognize that the speech areas of the human brain are already formed before birth . . ." (Eccles 1989:87). 2. The temporale plane is larger in the left fetal brain hemisphere than in the right (Stromswold 1995). 3. "Development of the cortical regions that subserve language in the left hemisphere consistently lags behind the development of the homologous regions in the right hemisphere [to await speech development]" (Stromswold 1995:860).

Evolution I. 1. "The evolutionary origin of human language may have been founded on this basal anatomic substrate, which was already lateralized to the left hemisphere in the common ancestor of chimpanzees and humans 8 million years ago" (Gannon 1998:220). 2. Regarding endocasts of Homo habilis skulls: "There was a further development of the inferior frontal lobule in the Broca area, but most remarkable was the rounded fullness of the inferior parietal lobule [corresponding to part of Wernicke's area]" (Eccles 1989:23).

Evolution II. In non-human primates, Broca's area controls muscles of the face and vocal tract. 1. "The homologue of Broca's area in nonhuman primates is the part of the lower precentral cortex that is the primary motor area for facial musculature" (Lieberman 1991:106). 2. In monkeys, the link between Broca-like and Wernicke-like areas is not as massively connected as it is in humans (Aboitiz and Garcia 1997).

Nonverbal speech areas. With regard to language, relationships between the right (nonverbal) and left (verbal) hemispheres are still poorly understood, with more deference being paid by researchers to the left-hand (i.e., dominant) side. 1. In the right cerebral hemisphere, modules control the production and interpretation of the nonverbal communication that accompanies words, e.g., facial expressions, voice tones, and gestures of the arms and hands. (Some of the latter, hand gestures are actually more verbal than nonverbal [see, e.g., MIME CUE].) 2. Prosody--the emotional content of speech--is right hemispheric in human beings with left-hemisphere verbal centers. 3. The right (or non-dominant) hemisphere is less involved in literal meanings of a speech element than it is with interpreting the figurative meanings conveyed by, e.g., hesitations, humor, metaphor, poetry, and voice tone. 4. Damage to the right parietal lobe's angular gyrus and supra-marginal gyrus results in a. problems using spatial concepts, b. difficulties dressing one's own body, c. feeling spatially disoriented, d. inability to draw simple 3D pictures, and e. neglect of left-handed body parts and objects to the left.

Stuttering. "But the stutterers were far less left-dominant; activation in their brains was shifted toward the right in both the motor and auditory language areas, revealing an inherent difference in the way the two groups [normal and stutterers] process language" (Barinaga 1995:1438).

E-Commentary: "I have two questions about the arcuate fasciculus, the fiber bundle from Wernicke's area to Broca's area. Can anyone help me? 1. Are there also fibers going in the opposite direction, from Broca's area to Wernicke's (we know that many cortico-cortical connections are bidirectional--what about this one?)? 2. How many fibers are we talking about? 3. A third question: What can anyone tell me about connections between Wernicke's area and the angular gyrus? (Bidirectional? How many fibers?) Thanx loads." --Syd Lamb, Linguistics and Cognitive Science, Rice University Houston TX 77251-1892 USA; smlamb@OWLNET.RICE.EDU (Sydney M Lamb) (Tue Jan 30 14:02:03 1996)

Neuro-notes I. In most humans, Wernicke's area is significantly larger in the left hemisphere than it is in the right. Its asymmetry dwarfs that of most other cerebral-cortex modules. And yet, though specialized for language, Wernicke's area is not unique to Homo. Recently, e.g., Patrick Gannon and his colleagues measured the corresponding area of chimpanzee brains. After spreading apart 15 chimp brains at the temporal lobe (i.e., at the sylvian fissure), they measured the planum temporale, and found it to be larger on the left than on the right in 14 cases (Gannon et al. 1998).

Neuro-notes II. "Lesions to Broca's area and its vicinity do not affect semantic abilities, nor do they disrupt basic syntactic abilities. Most notably, Broca's aphasics combine lexical meaning into propositions, create and analyze sentences of considerably complex structure, and are also able to synthesize and analyze words morphophonologically. It thus follows that most human linguistic abilities, including most syntax, are not localized in the anterior language areas--Broca's area and deeper white matter, operculum, and anterior insula" (Grodzinsky2000).

Neuro-notes III. 1. "We can assert unequivocally: no combinatorial language abilities reside in the non-dominant cerebral hemisphere" (Grodzinsky2000). 2. "Thus the evidence is that this side of the brain has an important an role in communication, but makes no syntactic contribution to language use" (Grodzinsky2000).

Neuro-notes IV. "However, it should be kept in mind that neither of the classical language areas, Broca's area and Wernicke's area, are cortical areas in the strict sense in which the term area is used by an [sic] neuroanatomist. For example, they are not defined according to the same strict and multiple criteria that are employed in defining primary visual cortex (area 17), and each includes more than one architectonically distinct area" (Killackey 1995:1248).