Plasticity in Speech Organization Following Commissurotomy

Abstract

For three-and-a-half years we have been studying the cognitive and conscious mechanisms in a remarkable 18-year-old man: Case P.S. This unique individual had his corpus callosum divided in order to control intractable epilepsy. Although for some time after the operation he appeared like other split-brain patients, unable to describe verbally stimuli directed to his mute right hemisphere, he behaved as if he was capable of comprehending a wide range of language-related stimuli directed to that hemisphere. Spelling by choosing the appropriate letters with his left hand, he could process nouns, verbs, rhymes, antonyms, and superordinate concepts. When asked about tachistoscopic presentations delivered to his left visual field, he either said he had seen nothing, or only a flash of light. He was also unable to identify verbally tactile 'sterognostic' inputs to his left hand. In the last year P.S. has begun to speak about stimuli directed to his right hemisphere. This series of experiments suggests that this speech is not interhemispheric transfer within the visual modality. Further, plotting the relative increased proficiency of verbal description of inputs directed to the right hemisphere, this speech system seems to be in a process of continuing development.

Preserved speech abilities and compensation following prefrontal damage.

“…We hypothesized that LF1 might have been using brain areas in the right hemisphere to compensate. Past research, using both lesion-behavior analysis and imaging techniques, has suggested that the right hemisphere may perform atypical language function following stroke or when disconnected from the left hemisphere by resection of the corpus callosum (22)(23)(24)(25)(26)(27). Alternatively, LF1 could have been using an insular cortex pathway to compensate.…”

Language and Reading in the Right Hemisphere: Highways or Byways of the Brain?

“…It recognized antonyms and associations and was able to rhyme. By one and a half years after surgery, P. S. was able to initiate speech from his right hemisphere (Gazzaniga et al 1979). He was not only able to read left visual field stimuli, but P. S. could read aloud homophones of real words (core) presented to his right hemisphere, demonstrating grapheme-phoneme correspondence rules.…”

“…Hence these investigators concluded that the right hemisphere was able to express itself linguistically through the left hand, but not vocally. P. S. was the first split brain subject to be able to name lateralized stimuli vocally (Gazzaniga et al 1979). P. S. was able to demonstrate good right hemisphere ability in all linguistic tasks that he was tested on soon after his surgery, but dfd not demonstrate an ability to initiate speech until a year and a half after surgery.…”

“…Gazzaniga (1983b) has emphasized the variability of language skills represented in the right hemisphere of callosotomy patients. Although no language can be demonstrated in most of these patients, in a select subgroup, right hemisphere language skills that include the ability to make a variety of semantic and syntactic judgments, generate (at least) single word utterances, and make rhyme judgments has been demonstrated (Gazzaniga et al 1977(Gazzaniga et al , 1979(Gazzaniga et al , 1984Sidtis et al 1981). Patterson and Besner (1984) have argued strongly against the role of the right hemisphere in the reading of deep dyslexics, although recent evidence has caused Patterson to question that position (Patterson et al 1989).…”

Orthographic and Phonological Facilitation from Unattended Words: Evidence for Bilateral Processing

“…These patients are unable to m atch a printed word in the LVF to another rhym ing word or picture (Baynes et al, 1995 ;Sidtis et al, 1981;Zaidel & Peters, 1981). Interestingly the two atypical com m issurotom y patients who have shown som e ability to match visual rhym es in the RH have also been able to generate speech in response to LVF/RH input (Gazzaniga et al, 1979). This would appear to suggest that phonol ogical coding of print is at least partially dependent on speech mechanism s, and this is consistent with the idea of a unilateral LH locus.…”