Sub Brocas Area

Language production tasks often activate the left anterior insula, or sub-Broca's area, located beneath BA 44/6. The anterior insula is most likely activated in phonological and articulatory planning of speech, as opposed to semantic or lexical processing, as a result of its proximity to Broca's area proper. Figure 7 shows a meta-analysis of functional imaging data for activation of anterior insulae, left and right, from paradigms involving language. The posterior portion of the insula

Figure 7 Activations in the anterior insula are shown in a meta-analysis of functional imaging data for left and right hemispheres from paradigms involving language. A set of mean coordinates was calculated from data reported from studies in Table II. The coordinates have been normalized and reported in standard three-dimensional (Talairach) space, plotted with the BrainMap database on an axial cross section at z=0.8 cm. Each relevant reference listed in Table II is represented here by a mean coordinate with the following symbols: ■, Becker (1994); □, Bookheimer (1995); •, Braun (1997); ▲, Fox (1999); ♦, Raichle (1994); O, Paulesu (1993); ▼, Fiez (1996); W, Price (1994); V, Rumsey (1997); }, Wise (1999). The distribution of activation points in both hemispheres shows that insular activation is not as strongly left lateralized as is activation of pre-Broca's area.

Figure 7 Activations in the anterior insula are shown in a meta-analysis of functional imaging data for left and right hemispheres from paradigms involving language. A set of mean coordinates was calculated from data reported from studies in Table II. The coordinates have been normalized and reported in standard three-dimensional (Talairach) space, plotted with the BrainMap database on an axial cross section at z=0.8 cm. Each relevant reference listed in Table II is represented here by a mean coordinate with the following symbols: ■, Becker (1994); □, Bookheimer (1995); •, Braun (1997); ▲, Fox (1999); ♦, Raichle (1994); O, Paulesu (1993); ▼, Fiez (1996); W, Price (1994); V, Rumsey (1997); }, Wise (1999). The distribution of activation points in both hemispheres shows that insular activation is not as strongly left lateralized as is activation of pre-Broca's area.

is associated with primary auditory and auditory association cortices and has been implicated in processing of auditory input. Braun related this function to a wide range of areas, noting that posterior insula serves as a parallel relay to prefrontal, motor, somatosen-sory, and cingulate regions of the brain. In addition to its role in speech, anterior insula has also been activated in overt and covert movement tasks. In general, functional imaging of the insulae is less abundant than that of BA 44/6 and 46/47. This trend may be due to the smaller size of the activated area combined with the less sensitive cameras and imaging equipment used in the past. In comparing language tasks that involve semantic, phonological, and articu-latory processing, evidence indicates that left anterior insula is responsible for phonological and articulatory planning, but the area's relationship with semantics and the details of its role in articulation are debated.

1. Speech Activation

Left anterior insula is activated, often along with BA 44/6, in language tasks involving picture naming, practiced verb generation, reading aloud, and word repetition. All these tasks connect the area with language production, but the exact relationship of the insular activation to the tasks follows two different trends of thought. Both agree that insula acts in articulatory planning, but they do not agree on its involvement in phonological and semantic analysis. The first set of opinions differentiated between automatic or rehearsed language tasks and novel tasks and proposed that insula is active in those tasks that are practiced. In a novel verb-generation task in a study by Raichle, the insula was inactivated relative to simple reading of nouns, and after the verb-generation task had been practiced the insula became active. In a silent counting task in a study by Fiez, insula was active, but when a verbal working memory component of retention of novel words was added to the task, insula became inactive. Fiez suggested that silent counting could be thought of as practiced speech. No distinction was made between phonological and articulatory functions in these studies; insular activation is attributed to the rehearsed nature of the speech, irrespective of the exact task being performed.

Another group of studies attempted to specify the purpose of insular activation based on the type of processing it performs or does not perform. Wise contrasted repetition of heard nouns with listening to nouns and anticipation of listening to nouns to differentiate between articulatory and phonological planning. Repetition is a task in which the phonetic plan (the selection and ordering of speech sounds and assignment of syllabic stress) is predetermined by input, and its execution is dependent only on the formulation of an articulatory plan. Left anterior insula was found to be active in both subtractions of listening and repeating nouns, which indicates its articulatory role. Rumsey showed that insula is inactive in orthographic decision making in a task in which subjects had to decide which of two homophones was a real word. She showed it is active in phonological processing in a task in which subjects had to decide which of two pseudowords would be the homophone of a real word. Price demonstrated that left anterior insula is active in a lexical decision-making task in which subjects had to identify real words among words with one incorrect letter substituted but not in a similar feature decision task in which subjects had to find false-font strings with ascending characters. Bookheimer argued that the insula is activated in articulatory coding. She differentiates between two types of motor speech pathways used in creating articulatory plans. Only one activates left anterior insula. She found activation in object naming, which she explained elicited a complete motor response: Subjects select a verbal label and the corresponding complete motor plan associated with that label. The alternate pathway that does not involve insula would be a sequential translation of orthographic units into sound patterns, as in word reading, with modification of the output based on auditory feedback. This differentiation of motor plan could explain Raichle's activation pattern in practiced word production if the practiced words are viewed and programmed as whole motor plans and not modified during production. Overall, activation in sub-Broca's area in speech production tasks indicates that the left anterior insula is active in articulatory planning and works closely with BA 44/6 in language tasks, but some details of the insula's role in language remain unclear.

2. Nonlanguage Activation

Nonlanguage tasks, such as overt and covert movement, activate the left anterior insula and demonstrate that the functions of the area extend to motor planning. Finger opposition and shoulder movement both activated insula in a PET study of movement by Colebatch, and imagined movement of a handheld joystick in a PET study by Stephan showed increased activation over a preparation to move baseline task. Right insula was activated by imagined movement of the hand in Parson's left/right hand decision study. He credited the insula with responding to somatosensory information from the body: The right insula activity observed is related to higher level somatic representation accompanying mentally simulated limb movement. Activation of left and right insulae in overt and covert movement shows that anterior insula is involved in complex nonspeech processes, and it demonstrates the need for more and varied functional imaging investigations of the relationship of language areas of the brain.

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