Homotopic language reorganization in the right hemisphere after early left hemisphere injury

Speech production is inextricably linked to speech perception, yet they are usually investigated in isolation. In this study, we employed a verbal-repetition task to identify the neural substrates of speech processing with two ends active simultaneously using functional MRI. Subjects verbally repeated auditory stimuli containing an ambiguous vowel sound that could be perceived as either a word or a pseudoword depending on the interpretation of the vowel. We found verbal repetition commonly activated the audition-articulation interface bilaterally at Sylvian fissures and superior temporal sulci. Contrasting word-versus-pseudoword trials revealed neural activities unique to word repetition in the left posterior middle temporal areas and activities unique to pseudoword repetition in the left inferior frontal gyrus. These findings imply that the tasks are carried out using different speech codes: an articulation-based code of pseudowords and an acoustic-phonetic code of words. It also supports the dual-stream model and imitative learning of vocabulary.     

Rhyming and the right hemisphere

Subjects determined whether two successively presented and orthographically different words rhymed with each other. The first word was presented at fixation and the second was presented either to the left or to the right of fixation, either alone (unilateral presentation) or accompanied by a distractor word in the other visual hemifield (bilateral presentation). Subjects were more accurate when the words did not rhyme, when presentation was unilateral, and when the target was flashed to the right visual hemifield. It was predicted that bilateral presentation would produce interference when information from both visual fields was processed by one hemisphere (callosal relay), but not when each of the two hemispheres performed a task independently (direct access). That is, callosal relay tasks should show greater laterality effects with bilateral presentations, whereas direct access tasks should show similar laterality effects with both bilateral and unilateral presentations. Greater laterality effects were observed for bilaterally presented rhyming words, but nonrhyming words showed similar laterality effects for both bilateral and unilateral presentations. These results suggest that judgment of nonrhyming words can be performed by either hemisphere, but that judgment of rhyming words requires callosal relay to the left hemisphere. The absence of a visual field difference with nonrhyming word pairs suggests further that judgment of nonrhyming word pairs may be accomplished by the right hemisphere when presentation is to the left visual field.     

Hemihypokinesia after right hemisphere stroke

We tested the hypothesis that the right hemisphere is dominant for motor activation of "intention." If the right hemisphere is dominant for intention and if intention is a prerequisite for movement, motor impairment should be worse after an infarct in the right hemisphere than in the left. We asked nine subjects with right and nine with left hemisphere infarcts to elevate each shoulder independently. Subjects with right hemisphere lesions had more trouble moving the contralateral shoulder. The data are consistent with the hypothesis that the right hemisphere is dominant for motor activation or intention.     

Writing with the right hemisphere

We studied writing abilities in a strongly right-handed man following a massive stroke that resulted in virtually complete destruction of the language-dominant left hemisphere. Writing was characterized by sensitivity to lexical-semantic variables (i.e., word frequency, imageability, and part of speech), semantic errors in writing to dictation and written naming, total inability to use the nonlexical phonological spelling route, and agrammatism in spontaneous writing. The reliance on a lexical-semantic strategy in spelling, semantic errors, and impaired phonology and syntax were all highly consistent with the general characteristics of right hemisphere language, as revealed by studies of split-brain patients and adults with dominant hemispherectomy. In addition, this pattern of writing closely resembled the syndrome of deep agraphia. These observations provide strong support for the hypothesis that deep agraphia reflects right hemisphere writing.     

Right hemisphere participation in reading

This study examined whether the right hemisphere's contribution to lexical semantic processing is greatest when it is "disinhibited." Skilled reading may require the controlled modulation of interhemispheric interaction: the left hemisphere (or some other control mechanism) may regulate the subprocesses of reading by selectively "inhibiting" and "disinhibiting" right hemisphere function. Right-handed undergraduates concurrently performed two tasks: a lateralized semantic or rhyme task and a verbal memory task. It was hypothesized that right hemisphere reading processes would be disinhibited when the left hemisphere was "occupied" with the memory task. This hypothesis was supported for a subgroup of subjects who showed evidence of inhibition of right hemisphere function (i.e., left hemisphere dominance for lexical processing) when the lateralized semantic task was performed alone. Across subjects, there was a strong correlation between the degree of left hemisphere dominance in the single-task semantic conditions and the degree of disinhibition of right hemisphere function in the dual-task semantic condition.     

Left hemisphere sensitivity to consonant sounds not displayed by the right hemisphere: electrophysiological correlates

Auditory evoked responses (AER) to a series of consonant-vowel syllables were recorded from frontal, temporal, and parietal scalp locations from 14 right-handed college students. Averaged AERs were submitted to principal components analysis and analysis of variance. Six components of the group's AERs were found to reflect various aspects of the stimulus parameters. One component reflected changes over only the left hemisphere to different consonants. A second component changed systematically over both hemispheres but did not discriminate between all consonants.     

Right hemisphere specialization for color detection

Three experiments were carried out to investigate hemispheric asymmetry in color processing among normal participants. In Experiment 1, it was shown that the reaction times (RTs) of the dominant and non-dominant hands assessed using a visual target presented at the central visual field, were not significantly different. In Experiment 2, RTs of ipsilateral hands to lateralized chromatic stimuli revealed that the processing time was 17 ms shorter in the right hemisphere (RH) than that in the left hemisphere among the right-handed participants, whereas no significant difference was found among the left-handed participants. On the other hand, RTs to lateralized achromatic stimuli showed no such asymmetry among both the right- and left-handed participants (Experiment 3). These findings strongly suggest RH superiority for detection of color among right-handed individuals.     

Right hemisphere EEG sensitivity to speech

Recent speech perception work with normals and aphasics suggests that the right hemisphere may be more adept than the left at making the voicing discrimination, and the reverse for place of articulation. We examined this right hemisphere voicing effect with natural speech stimuli: stop consonants in pre-, mid-, and postvocalic contexts. Using a neuroelectric event-related potential paradigm, we found numerous effects indicating bilateral components reflecting the voicing and place contrast and unique right hemisphere discrimination of both voicing and place of articulation.     

A high-loaded hemisphere successfully ignores distractors

We used a response competition paradigm to investigate whether a distractor is effectively rejected under conditions where it is projected to a highly-loaded hemisphere. In two experiments we asked right-handed participants to identify a target among five task-relevant letters while they ignored a distractor. We manipulated both the distractor visual-field (low-load/high-load) and the compatibility of the target and the distractor. In the low-loaded visual-field, we presented a distractor with one task-relevant stimulus to one visual-field and the remaining task-relevant stimuli to the opposite visual-field. In the high-loaded visual-field, we presented a distractor and task-relevant stimuli in reverse. In Experiment 1 (left/right), we found a compatibility effect for the low-loaded, but not for the high-loaded visual-field. In Experiment 2, this modulation of the compatibility effect did not appear when the upper/lower visual-field was manipulated. These findings demonstrate that a distractor is successfully ignored when it is presented to a highly-loaded hemisphere.     

Probability matching in the right hemisphere

Previously it has been shown that the left hemisphere, but not the right, of split-brain patients tends to match the frequency of previous occurrences in probability-guessing paradigms (Wolford, Miller, & Gazzaniga, 2000). This phenomenon has been attributed to an "interpreter," a mechanism for making interpretations and forming hypotheses, thought to reside exclusively in the left hemisphere. In this study with a split-brain patient, we had him guess one of two types of faces, stimuli known to be preferentially processed in the right hemisphere of this patient. Unlike previous studies using other kinds of stimuli, the right hemisphere matched the frequency of the previous occurrences of a face-type, but the left hemisphere did not.     

Spontaneous humor among right hemisphere stroke survivors

We analyzed spontaneous conversational humor production and response among 11 right hemisphere-damaged (RHD) patients, 10 left hemisphere-damaged (LHD) patients, 7 normal controls (NC), and their spouses. RHD patients and their spouses reported a statistically significant decline in the patients' orientation to humor post-stroke. Also, we found a significant positive association between a RHD patient's ability to decode prosody and their self-reported orientation to humor post-stroke.     

Spatial probability cuing and right hemisphere damage

In this experiment we studied statistical learning, inter-trial priming, and visual attention. We assessed healthy controls and right brain damaged (RBD) patients with and without neglect, on a simple visual discrimination task designed to measure priming effects and probability learning. All participants showed a preserved priming effect for item color. Contrary to healthy controls and RBD participants without neglect, RBD participants with neglect did not show positional priming and both RBD groups learned the underlying spatial probability distribution of target locations to a lesser degree. To see if the latter deficiency could be improved, we tested a patient with long standing chronic spatial neglect on three separate days and observed improved identification times for left sided, high probability, targets. In summary, we found preservation of priming per se in people with spatial neglect. However, this was only clearly demonstrable for color priming and not for positional priming. Associated with this impairment was a difficulty in learning the overall statistical structure of target locations. In a patient with severe persistent neglect we were able to demonstrate that the deficit in statistical learning was not absolute, as this subject improved his identification times for targets appearing in high probability regions of the test display.