Pure Word Deafness is caused by bilateral damage to the posterior superior temporal lobe

Temporal lobe

The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....

s or disruption of connections between these areas. It exhibits itself as inability to comprehend the meaning of speech, but (in most cases) still being able to hear, speak, read, and write.

Presentation

The underlying problem seems to be that understanding speech requires the discrimination between specific sounds which are closely spaced in time. Note that this is not unique to speech; studies using non-speech sounds closely spaced in time (dog bark, phone ring, lightning, etc.) have shown that those with pure word deafness are unable to discriminate between those sounds in the majority of cases, though a few putative examples of speech-specific impairment have been documented in the literature. Additionally, those with pure word deafness are not deaf - they can (in the absence of other impairments) hear sounds, including speech (so it has nothing to do with words). As such, "pure word deafness" is something of a misnomer. "Pure word" is intended to mean that only words are affected (not accurate for many cases); "deafness" is intended to specify that these patients have an inability to hear (not accurate).

Causes

Pure word deafness is often associated with lesions to the left superior temporal lobe, but no such unilateral case has yet been documented without damage to the white matter tract connecting superior temporal lobes bilaterally or bilateral damage to the superior temporal lobe. These facts, in combination with the existence of cases of damage to these white matter tracts without detectable cortical damage, in combination with cases of pure word deafness resulting enlargement of the III ventricle alone suggest that the disorder results from damage to the left-right superior temporal circuit rather than the superior temporal area on one hemisphere or another.