Strokes and Traumatic Brain Injury Can be Unpredictably Devastating Because They Disrupt Brain Networks

The effects of a stroke or traumatic brain injury can be unpredictable because damage doesn’t just disrupt an area of tissue, but also the networks it participates in.

When people learn to do a complex task, such as reading, they develop a network that focuses on doing that, and only that task. Individual neurons become more specialized: they read more and do other things less. At the level of the whole brain, activity becomes “focal”, with small groups of neurons instead of broad swaths of tissue participating in reading. These clusters of specialist neurons exist all over the brain. Thus, they frequently communicate with other members of the network over long distances, while communicating less with their neighbors.

What happens when one group of reading neurons is damaged or killed? Those neurons no longer send their usual signal to other members of the network, which disrupts their functioning. Severe enough damage can impair the whole network.

The human reading network is supported by a language network that covers all forms of language: speaking, listening, reading, writing, and for some people, signing. Disruptions to the language network cause a family of disabilities called aphasia. Depending on which regions of the language network are damaged, and how severely, people can develop different kinds of impairment.

In Broca’s Aphasia, damage to a part of the inferior frontal gyrus, called Broca’s Area, causes difficulty speaking without affecting understanding. That leads to long pauses while searching for words, lots of “um’s”, extremely simple language, and “telegraphic speech” (using a few key nouns and verbs instead of saying a whole sentence). To see how Broca’s Aphasia affects speech, watch the Tactus Therapy video below. It shows an interview with Mike Caputo, a stroke survivor who has Broca’s Aphasia.

By contrast, Wernicke’s Aphasia involves damage to a part of the superior temporal gyrus called Wernicke’s area. It causes difficulty understanding one’s own or others’ speech. People with Wernicke’s aphasia talk fluently, with typical grammar and expression, yet their speech doesn’t make sense. They seem unaware that they aren’t actually saying the words they mean. For an example, see this Tactus Therapy video below: an interview with Byron Peterson, a stroke survivor with Wernicke’s Aphasia.

Broca’s and Wernicke’s aphasia are examples of what brain researchers call a “double dissociation”: Broca’s aphasia involves difficulty speaking but not understanding, while Wernicke’s aphasia involves the opposite. A double dissociation between speaking and understanding language demonstrates that the brain treats them as separate functions. One task isn’t simply harder (thus more easily disrupted) than the other.

In Conduction Aphasia, neither speech nor comprehension regions have been significantly damaged. However, the two no longer communicate properly. As a result, people with conduction aphasia can speak fluently and understand others’ language. However, they have difficulty repeating words someone just said. Interestingly, they can hear when they repeat incorrectly, but that doesn’t help them correct the mistake.

Carl Wernicke, the first to describe and name conduction aphasia (1874), speculated that it came from a disconnection between the motor (Broca’s area) and sensory (Wernicke’s area) parts of the language network.

Thus, when asked to repeat a word, people with conduction aphasia hear the sound of the word, but cannot use it to guide and check what they say. They also know how to say the word, but not which word to say. Thus, they struggle to repeat what they hear.

People with conduction aphasia often have additional difficulty with other language skills that involve combining sensory imagery with motor output, such as writing.

Since Geschwind wrote in the 1960s, conduction aphasia has been considered an example of a “disconnection syndrome.” Geschwind believed that structural damage to the white matter connecting Broca’s and Wernicke’s areas causes functional disruption of communication between these regions.

The image above, made by me, shows the locations damaged in Broca’s aphasia in the frontal lobe (gray), Wernicke’s aphasia in the temporal lobe (blue), and conduction aphasia in the parietal lobe (red). The bidirectional arrow shows the information flow between Broca’s and Wernicke’s areas; the red area is an especially important part of this pathway.

Researchers have since challenged some of the anatomic details of Geschwind’s explanation. For example, different white matter tracts connect Broca’s and Wernicke’s areas than Geschwind believed. For our purposes, it doesn’t matter which white matter tracts connect these areas, only that some exist.

In practice, pure conduction aphasia rarely occurs, because of the messiness of brain damage and disease. People often have additional minor damage to Broca’s area that causes word finding difficulties (not as severe as in Broca’s aphasia, but much more frequent than the typical “tip of the tongue” feeling). Or, they may have damage to Wernicke’s area or executive function areas that leads them to, for example, carry out only the first step of multi-step instructions. However, people with conduction aphasia mainly, disproportionately struggle with tasks that require good communication between Broca’s and Wernicke’s area, like writing and spoken repetition.

In short, people can have difficulty doing things when parts of their brain network no longer communicate effectively, even if the regions themselves are intact.

In any brain network, some regions matter more than others. Imagine a human social group. Some people know everyone else in the network, and host group gatherings. Others may only know one or two people in the network. The well-connected people can be seen as “central” to the network. Similarly, some brain regions are network “hubs” that connect many regions, while others have very few direct connections.

Damage to a “hub” is especially damaging, because it can directly or indirectly affect most or all of the other regions in its networks.

So, when a person’s brain becomes damaged, it doesn’t just matter how much tissue is affected. It also matters which networks that tissue participated in and how interconnected it was with the rest of the network.

In short, the brain tends to form networks, and that principle has practical implications for how the brain falls apart and recovers.