Speech, reading and language
Back to The Guide after a bit of time off, thanks for understanding. For a glimpse of what I've been up to, check out Sunday's blog and the new videos posted at TeddRoberts.com.
Following the discussions of visual and auditory systems, I promised a discussion of speech and language. In truth this should wait until *after* discussing the motor and sensory cortices, but seems more appropriate in this place. To fill in that extra information, refer to the figure at right which labels several key sensory and motor areas of the brain. At the back of the brain we have occipital lobe and exclusively the primary visual system. Forward (anterior) and to the top is parietal lobe which contains many of the association cortices that perform advanced sensory processing as well as combine and process sensory information from different areas. To the bottom is Temporal lobe, with the memory areas (inside) and auditory cortex. At the anterior edge of Parietal lobe is the somatosensory cortex - the sensory (touch and position) cortex for most of the body (soma). Forward of the somatosensory cortex is motor cortex in the Frontal Lobe.
Two primary areas have historically been associated with speech and language. Damage to Wernicke's Area would result is loss of language comprehension, while damage to Broca's Area resulted in loss of speech. Both areas are found in the *dominant* hemisphere - the left side (controlling the right side of the body) in 90% of humans. Interesting, damage in either area could result in loss of reading comprehension, but we will get to that later.
We now know that these regions are not exclusive. It is true that sudden injury frequently results in these losses, but slow-growing tumors and gradual degeneration can leave speech and language intact despite serious degradation due to the brain's own "plasticity" - the ability to shift functions to adjacent areas or even the opposite hemisphere. However, the location of these key areas is telling. Wernicke's area, the blue region in the figure at left, lies firmly in the Parietal Lobe association areas, near the auditory cortex. Broca's area (orange), lies in the motor association areas just anterior to the region that controls muscles in face and mouth.
While these areas were originally identified as a result of lesion, or damage to the respective brain area, the approximate areas have been confirmed by imaging that identifies active brain regions during listening, reading and speaking. With these imaging studies, though, came the realization that speech and language was not *limited* to these regions, and we find a lot of involvement of association cortices surrounding Broca's and Wernicke's areas. It is also not entirely true that these functions are strictly lateralized to one half of the brain. Imaging also shows *some* involvement of the corresponding areas in the nondominant hemisphere, especially if the person is not really paying attention to what they are hearing or hearing/speaking nonsense syllables.
*Reading* is an interesting specialization of lamnguage, and it would appear to make sense for it to be localized near the intersection of vision and hearing - but it is not restricted to those areas. Reading also seems to invoke Broca's Area and the eye muscle control areas in Frontal Lobe. Distribution between the two areas is also curious - with poor readers having more activity surrounding Broca's Area (and perhaps explaining people whose lips move during reading) while good readers have more activity surrounding Wernicke's area. Current explanations indicate that good readers "hear" words as they are read, while poor readers have to sound them out.
Thus language really does involve the areas of brain that process the mechanisms of language - hearing and speaking. Even the written word harkens back to oral tradition in the involvement of Broca's and Wernicke's Areas. Of course the ultimate brain activity is reading and reciting - as in poetry and author readings at a con (been there, done, that, no thanks). Such activity activates visual cortex and association areas for detection of words on the page (or screen), hearing, Wernicke's Area, the somatosensory and motor cortex areas for face, mouth, tongue, pharynx and larynx, Broca's area, Frontal Lobe eye fields for control of eye muscles to scan the eyes across the text, memory areas of the temporal lobe, and executive function areas of the prefrontal (most anterior) region of the Frontal lobe for decisions of what to read and when to speak.
In all, reading and reciting activates between 25% and 50% of the whole brain in one activity.
Take *that* ten-percenters!