An impairment in the recognition of stimuli in a particular sensory modality. True agnosias are associative defects, where the perceived stimulus fails to arouse a meaningful state. An unequivocal diagnosis of agnosia requires that the recognition failure not be due to sensory-perceptual deficits, to generalized intellectual impairment, or to impaired naming (as in aphasia). Because one or more of these conditions frequently occur with agnosia, some clinical scientists have questioned whether pure recognition disturbances genuinely exist; but careful investigation of appropriate cases has affirmed agnosia as an independent entity which may occur in the visual, auditory, or somesthetic modalities.