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For most people, the left half of the brain controls linguistic capabilities, but some people have their language centers in the right half. When a language center of the brain is damaged, for example by a stroke, linguistic capabilities are impaired in some way. Therefore, people who have suffered a serious stroke on the left side of the brain without suffering any such impairment must have their language centers in the right half.
Which of the following is an assumption on which the reasoning in the argument above depends?
| Text from Passage | Analysis |
| For most people, the left half of the brain controls linguistic capabilities, but some people have their language centers in the right half. |
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| When a language center of the brain is damaged, for example by a stroke, linguistic capabilities are impaired in some way. |
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| Therefore, people who have suffered a serious stroke on the left side of the brain without suffering any such impairment must have their language centers in the right half. |
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The argument starts with background info about where language centers are located, then explains what happens when they get damaged, and finally concludes what it means when someone doesn't have language problems despite left-brain damage.
People who have serious left-brain strokes without language problems must have their language centers in the right half of their brain.
The argument uses a process of elimination: if language centers are either left or right, and damage to language centers causes problems, then no problems after left-brain damage means the language center must be on the right.
Assumption - We need to find what the argument takes for granted but doesn't explicitly state. This unstated piece must be true for the conclusion to logically follow from the premises.
The argument makes specific claims about brain hemisphere locations (left vs right), stroke damage effects, and language impairment consequences. The conclusion is definitive - people with left brain strokes but no language problems 'must have' right-side language centers.
To find assumptions, we'll identify ways the conclusion could be false while keeping all the stated facts true. The argument jumps from 'left stroke + no language problems' to 'language center must be on right side.' We need to find what unstated conditions make this jump valid.
'No part of a person's brain that is damaged by a stroke ever recovers.' This deals with recovery after stroke damage, but the argument doesn't rely on this at all. The argument is about immediate damage and impairment, not about whether recovery is possible later. Someone could recover from language problems and the argument's logic would still hold - if they had problems initially after a left-brain stroke, that would still indicate their language center was on the left side. This isn't an assumption the argument needs.
'Impairment of linguistic capabilities does not occur in people who have not suffered any damage to any language center of the brain.' This says that language problems only happen when language centers are damaged. But the argument already states this premise explicitly when it says 'When a language center of the brain is damaged... linguistic capabilities are impaired.' Since this is already stated in the argument, it can't be an unstated assumption.
'Strokes tend to impair linguistic capabilities more severely than does any other cause of damage to language centers in the brain.' This compares strokes to other causes of brain damage, but the argument doesn't make any such comparison. The argument only deals with strokes and doesn't need to assume anything about how strokes compare to other types of brain damage. This is irrelevant to the logical flow.
'If there are language centers on the left side of the brain, any serious stroke affecting that side of the brain damages at least one of them.' This is exactly what the argument must assume! The argument concludes that people with serious left-brain strokes who have no language problems must have right-side language centers. But this only works if we assume that serious left-brain strokes would definitely damage any language centers that exist on the left side. Without this assumption, someone could have left-side language centers that simply weren't damaged by the stroke, making the conclusion invalid.
'It is impossible to determine which side of the brain contains a person's language centers if the person has not suffered damage to either side of the brain.' This is about determining language center location in healthy people, but the argument is specifically about people who have suffered strokes. The argument doesn't need to assume anything about how we determine language center locations in people without brain damage.