In the first part of this study we have compared the frequencies of disturbances of language, gestures, and perception, according to the hemisphere involved, in series of left- and right-handed subjects with verified unilateral lesions (left-handers: 73 cases, 26 with right-sided lesions and 47 with left-sided lesions; right-handers: 487 cases, 194 with right-sided lesions and 293 with left-sided lesions).
The left hemisphere syndrome in left-handed subjects consists of aphasic disturbances similar to those found in right-handers with the exception that there is lower frequency of defects of verbal comprehension and writing and a higher frequency of disturbances of reading. The pattern of disorganization of calculation, praxis and visual gnosis is also similar to that seen with lesions of the left hemisphere in right-handers. However, some of the symptoms that are generally seen with right-sided lesions in right-handers are observed here (hemiasomatognosia, unilateral spatial agnosia).
The right hemisphere syndrome in left-handed subjects is characterized by the comparatively high frequency of disturbances of oral and written language, while disturbances of calculation, praxis and visual gnosis are similar to those found in right handers with right hemisphere lesions.
The comparison between the right and left hemisphere syndromes in left-handers shows much less difference between the frequencies of symptoms than the same comparison in right-handers. These results indicate that there is a certain cerebral ambilaterality in the left-handed subjects. However, the left-handers do not seem to represent a single group.
In the second part of the study we have evaluated the language disturbances in a series of 49 left-handed subjects with verified unilateral cerebral lesions, dividing this group according to the side of the lesion (17 right; 32 left) and to the presence or absence of a family history of left-handedness. In the “familial” type of left-handers, disturbances of oral language and of reading occurred with similar frequency following either right or left-sided lesions. However, in the “non-familial” type of left-handers these disturbances were almost absent with right-sided lesions.
On the basis of these results, which are consistent with the findings of the dichotic test of Broadbent in normal left-handers, we suggest that cerebral ambilaterality is not a characteristic of all left-handers, but only of those who belong to the so-called “familial” type.