The sides (hemispheres) of the brain are specialized. For most of us (even most left handed people) the left hemisphere is devoted to language related tasks whereas the right deals with special relationships, such as recognizing faces. Loss of language abilities is termed aphasia. The loss of ability to recognize objects is termed agnosia. One of the most common causes of either aphasia or agnosia is stroke.
The hemispheres are divided into four lobes, Frontal, Parietal, Occipital, Temporal, that have different functions. The frontal lobe is devoted to producing movement, the Parietal interprets sensation, the Occipital controls vision, and the Temporal serves hearing. Large strokes that damage the majority of the left hemisphere result in loss of both speech production and recognition and is termed global aphasia. However, strokes involving small branches of the arteries supplying the left hemisphere may cause very specific deficits in language. Primarily frontal lobe damage can result in the inability to speak while still being able to understand spoken and written words. Likewise, selective damage to only the Parietal lobe can affect our ability to monitor what we are saying. This results in a person who can form sounds but cannot monitor the words or content of what they say – so their sentences and words become gibberish or nonsense. This type of aphasia is called Fluent Aphasia.
In very rare instances when the area between the Parietal and Occipital lobes are damaged some very selective neurologic problems arise. For example, being able to speak and understand spoken words but not being able to read written words. Likewise, with involvement of the Temporal-Parietal region some rare forms of aphasia allow the person to read but not understand spoken words.
The above explanations are overly simplified, but give examples how listening to the ability of a patient processes language can help the neurologist understand what parts of the brain are not functioning properly.