We report the performance of a patient who, as a consequence of left f
rontal and temporoparietal strokes, makes far more errors on nouns tha
n on verbs in spoken output tasks, but makes far more errors on verbs
than on nouns in written input tasks. This double dissociation within
a single patient with respect to grammatical category provides evidenc
e for the hypothesis that phonological and orthographic representation
s of nouns and verbs are processed by independent neural mechanisms. F
urthermore, the opposite dissociation in the verbal output modality, a
n advantage for nouns over verbs in spoken tasks, by a different patie
nt using the same stimuli has also been reported (Caramazza & Hillis,
1991). This double dissociation across patients on the same task indic
ates that results cannot be ascribed to ''greater difficulty'' with on
e type of stimulus, and provides further evidence for the view that gr
ammatical category information is an important organizational principl
e of lexical knowledge in the brain.