28-3 What brain areas are involved in language processing and speech?

We think of speaking and reading, or writing and reading, or singing and speaking as merely different examples of the same general ability—language. But consider this curious finding: Aphasia, an impairment of language, can result from damage to any of several cortical areas. Even more curious, some people with aphasia can speak fluently but cannot read (despite good vision), while others can comprehend what they read but cannot speak. Still others can write but not read, read but not write, read numbers but not letters, or sing but not speak. These cases suggest that language is complex, and that different brain areas must serve different language functions.

Indeed, in 1865, French physician Paul Broca reported that after damage to an area of the left frontal lobe (later called Broca’s area) a person would struggle to speak words while still being able to sing familiar songs and comprehend speech.

In 1874, German investigator Carl Wernicke discovered that after damage to an area of the left temporal lobe (Wernicke’s area) people could speak only meaningless words. Asked to describe a picture that showed two boys stealing cookies behind a woman’s back, one patient responded: “Mother is away her working her work to get her better, but when she’s looking the two boys looking the other part. She’s working another time” (Geschwind, 1979). Damage to Wernicke’s area also disrupts understanding.

Today’s neuroscience has confirmed brain activity in Broca’s and Wernicke’s areas during language processing (FIGURE 28.2). But language functions are distributed across other brain areas as well. Functional MRI scans show that different neural networks are activated by nouns and verbs (or objects and actions); by different vowels; and by reading stories of visual versus motor experiences (Shapiro et al., 2006; Speer et al., 2009). Different neural networks also enable one’s native language and a second language (Perani & Abutalebi, 2005).

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The big point to remember: In processing language, as in other forms of information processing, the brain operates by dividing its mental functions—speaking, perceiving, thinking, remembering—into subfunctions. Your conscious experience of reading this page seems indivisible, but you are engaging many different neural networks in your brain to compute each word’s form, sound, and meaning (Posner & Carr, 1992). Different brain areas also process information about who spoke and what was said (Perrachione et al., 2011). We can also see this distributed processing in our visual system, for which the brain engages specialized subtasks, such as discerning color, depth, movement, and form. And in vision as in language, a localized trauma that destroys one of these neural work teams may cause people to lose just one aspect of processing. In visual processing, a stroke may destroy the ability to perceive movement but not color. In language processing, a stroke may impair the ability to speak distinctly without harming the ability to read. What we experience as a continuous, unified stream of experience is actually but the visible tip of a subdivided information-processing iceberg. E pluribus unum: Out of many, one.