Word and letter reading and the mechanism of the third alexia.

Four patients, two with global aphasia and two with Broca's aphasia, demonstrated the seemingly paradoxical ability to read words but not their component letters. Picture naming was only moderately impaired, and repetition of word and letter names was intact, excluding both a generalized dysnomia and an articulatory disturbance as the cause of the literal alexia. Matching tests revealed processing deficits in three of the patients, more severe for letters than for words. Oral reading of word lists showed that short, high-frequency, and picturable words were read best, whereas nonsense trigrams, which require phonetic processing, were the most difficult. The residual reading of patients with severe Broca's or global aphasia and the "third alexia" appears to involve purely visual, nonphonetic mechanisms for word recognition, using posterior left hemisphere or even minor hemisphere centers. Letter reading, by contrast, along with phonetic reading of syllables, appears to be a more specialized, anterior left hemisphere process.