Performance by split-brain humans on lateralized vigilance tasks.

Studies are described of the vigilance performance of the cerebral hemispheres of split-brain man. Investigations of visual, auditory and tactual performance are reported for six total commissurotomy patients. The vigilance tasks were lateralized to test the functions of the two hemispheres and make comparison between them. The right hemisphere emerges the superior in its vigilance performance in every case. The results support the association of sustained attention with the functions of the right hemisphere and they suggest that there are different mental mechanisms for selective and sustained attention.     

Release from central auditory competition in the split-brain patient

We used dichotic digits (DD), staggered spondaic words (SSW), and frequency patterns (PATT) to study central auditory function before and after two-stage callosotomy. Preoperatively, the patient demonstrated reduced scores bilaterally on all these tests, consistent with documented bilateral hemisphere lesions. After the first operation (sectioning the posterior half of the corpus callosum), the dichotic tests (DD and SSW) revealed the expected decrease in left-ear scores, but there was improvement on the right, perhaps because there was release from central auditory competition. Our findings also suggest that the "auditory" portion of the corpus callosum may be in the posterior half of this structure.     

Organization of auditory cortical areas in man.

Average responses to clicks were recorded from the exposed human cortex of 19 adult patients during operation for the treatment of intracranial diseases. Auditory evoked responses were obtained from two areas. Short latency potentials were recorded on the superior surface of the temporal lobe corresponding to the transverse temporal gyri. These responses consisted of two positive waves P1 and P2 separated by a negative wave N1. P1 had a mean latency of 14-7+/-1-5 ms, N1 a mean latency of 19-1+/-2-6 ms and P2 a mean latency of 32-2+/-4-1 ms. Responses to stimulation of the contralateral ear were of higher amplitude than responses to stimulation of the ipsilateral ear. Responses of smaller amplitude and longer latency were obtained from the superior temporal gyrus and the upper lip of the sylvian fissure. These responses had a mean peak latency for P1 of 40-2+/-2-6 ms, for N1 of 62-5+/-12-5 ms and for P2 of 97-7+/-17-2 ms. It is concluded that the cortical auditory region of man may be subdivided in two major areas: an area on the supratemporal plane representing the primary auditory area or A1 and a region surrounding A1 which perhaps comprises two areas, one on the superior temporal gyrus and one on the upper bank of the sylvian fissure including frontal and parietal operculi.     

Interhemispheric interaction in the split-brain.

An experiment is reported in which a split-brain patient (LB) was simultaneously presented with two words, one to the left and one to the right of fixation. He was instructed to categorize the right sided word (living vs non-living), and to ignore anything appearing to the left of fixation. LB's performance on this task closely resembled that of normal neurologically intact individuals. Manual response speed was slower when the unattended (left visual field) word belonged to the same category as the right visual field word. Implications of this finding for views of the split-brain syndrome are discussed.     

Arousal and auditory vigilance in individuals with aphasia during a linguistic and nonlinguistic task

Background: Attentional deficits have been observed in individuals with aphasia. Attention, specifically vigilance, is believed to be related to arousal. However, our understanding of arousal and its impact on vigilance performance in individuals with aphasia is very limited. Aims: The purpose of the current study was to investigate whether there is nonoptimal arousal in individuals with aphasia that would affect auditory processing of linguistic and nonlinguistic stimuli. Additionally, this study explored whether there was generalised, nonoptimal arousal and impaired vigilance or deficits specific to linguistic processing. Methods & Procedures: A total of 20 males (10 with left‐hemisphere stroke and aphasia and 10 nonbrain‐damaged controls) participated in this study. Physiologic arousal indexed by cardiovascular and neuroendocrine measures and vigilance performance was compared between the two groups during linguistic and nonlinguistic vigilance tasks. Outcomes & Results: Results indicated that arousal levels and vigilance performance differed between the two groups. However, within groups arousal and vigilance did not differ between the linguistic and nonlinguistic tasks. Conclusions: The present findings suggest that individuals with aphasia have decreased overall vigilance and nonoptimal arousal regardless of the linguistic or nonlinguistic nature of the stimuli.     

Rapid auditory evoked vascular response in man

Plethysmographic signals were recorded in four body regions (fingertip, forearm, cerebral cortex, and ophthalmic artery) of normal, healthy subjects during the presentation of brief auditory tones. Tones were presented during either systolic or diastolic phases of the cardiac cycle. Using data processing techniques similar to those employed in cortical evoked potential studies, averaged waveforms were derived which revealed the presence of a polyphasic volumetric response beginning as early as 150 ms (after onset of tones) in some subjects. Grand average waveforms suggested a similar morphology in all four body regions. The rapid onset argued for neural mediation of this response which may represent a sudden, transient flexure of vascular smooth muscle. The presence of this evoked vascular response in the brain suggests a previously unreported responsiveness of the cerebral vasculature in man to simple auditory stimulation.     

Effects of time and tasks upon auditory and somatosensory evoked potentials in man.

Auditory evoked potentials (AEPs) recorded from the vertex and somatosensory evoked potentials (SEPs) recorded from the somatosensory and motor areas and vertex were examined during visual and auditory discrimination tasks, with and without motor responses, and during motor tasks alone in normal human subjects. These procedures allowed the separation of sensory discrimination from motor activity effects on the evoked potentials. It was observed that EPs were modified systematically by task and by temporal variables, even though vigilance, as evaluated through EEG recording and performance level, was stable throughout the experimental recording session. 1. AEPs were minimally influenced by time, but very sensitive to task. Inversely, the SEP amplitude decreased considerably with time and less with task. There was evidence of time/task interaction. 2. The magnitude of SEP attenuation in time was reduced by intervening rest periods. 3. The sensory modality in which the discrimination task was performed did not influence the effect on EPs. A discrimination task involving a motor response reduced EPs more than a pure discrimination or a pure motor task. The task effect seems to involve a general, mechanism (load imposed upon the subject) not dependent on the particular sensory channel used to deliver task-relevant information.     

Tactual discrimination learning in monkeys

Results of studies of tactual discrimination learning in monkeys indicate that tactual learning is more difficult than visual for the naive monkey. The posterior parietal cortex, but not other cortical association areas, appears to be necessary for full utilization of tactual input, and lesions of this area produce deficits in discrimination learning which are analogous to tactual agnosia in man. A model is presented in terms of which discrimination learning, and deficits in discrimination learning, are described.     

Chronic effects of phenytoin on brain-stem auditory evoked potentials in man.

The effects of phenytoin (PHT) on brain-stem auditory evoked potentials (BAEPs) were studied in 65 epileptic patients who received long-term PHT monotherapy at therapeutic and supra-therapeutic levels with no clinical evidence of brain-stem toxicity. Abnormal BAEPs were found in 7.5% and 33.3% of patients with therapeutic and supra-therapeutic PHT levels respectively. Serum PHT levels had a trend towards a positive relationship with the I-V interpeak latency (IPL), and a significant negative relationship with the amplitudes of waves I and V. At supra-therapeutic levels, both I-V and I-III IPLs were significantly prolonged while at therapeutic levels only I-III IPLs were prolonged. The absolute latency of wave I was prolonged in both the therapeutic and the supra-therapeutic groups. These results suggest that PHT acts both peripherally on either the auditory nerve or the cochlea, and centrally on brain-stem conduction.     

Effect of voluntary self-paced movements upon auditory and somatosensory evoked potentials in man.

The effect of voluntary self-paced movements upon auditory (AEPs) and somatosensory (SEPs) evoked potentials has been investigated according to the temporal relationship between movement and delivery of test stimuli. EPs were recorded in 7 subjects and averaged in 10 successive epochs extending from 880 msec before to 2500 msec after movement. AEPs were attenuated in all epochs. The decrease was greatest in the 220 msec epoch just following movement and involved components N85 and P170. SEPs were attenuated similarly to AEPs when movements were performed by the hand contralateral to somatosensory stimulation. Of the 5 SEP components, only P40 failed to reflect the attenuation, while P95 showed the greatest amplitude decrease. When stimulation was ipsilateral, SEP amplitude was attenuated only when close to the movement. N65 and P95 decreased while N130 increased. In all subjects the results were consistent for treatments of AEP and SEP (with contralateral movements), whereas large inter-individual differences were observed for the SEP with ipsilateral movements.     

Glucocorticoid influences on the auditory brain-stem responses in man

The present study was designed to test whether human auditory brain-stem responses (ABRs) are sensitive to peripheral endocrine signals as reflected by changes in the plasma glucocorticoid concentration. ABRs were recorded following treatment with 50 mg hydrocortisone or placebo in 13 healthy male adults. Experiments were double-blind and designed according to a latin-square within-subject comparison. Treatment was applied 1 h prior to testing as an i.v. infusion over a period of 20 min. Subjects were tested in each session under 3 X 3 conditions, i.e., ABRs were obtained to series of clicks presented at 3 different intensities (80, 60, 40 dB HL) and with 3 different stimulus rates (39, 10, 5 clicks/sec). Additionally, oral temperature was monitored. Hydrocortisone significantly reduced latencies of waves III, V, and VI (and subsequent negative troughs) when low stimulus intensities were used. Furthermore, the glucocorticoid reduced latencies of waves Vn, VI and VIn when the clicks were presented at a slow stimulus rate. Further studies have to specify the underlying mechanisms of these changes.     

Localisation of auditory evoked potentials in man during awakeness and sleep

Auditory evoked potentials after click, speech and musical stimulation during sleep and awakeness were used in order to determine the existence of a topographical dominance of single cortical areas. The results showed a lateralization in awake subjects dependent on the semantic content of the stimulus. During deep sleep the lateralization of the potential was absent and the activity was extremely symmetrically concentrated around the vertex. The findings support the concept of cerebral dominances which has to be more than a pure metaphorical one.     

Unilateral pleasure-center stimulation in split-brain cats

Unilateral bipolar stimulation at seven positively reinforcing sites ranging from the ventral mesencephalic reticular formation to the preoptic region, in five split-brain cats, was shown to reinforce pattern discrimination learning in both hemispheres. The training method used permitted simultaneous presentation of discriminanda to both hemispheres of the split-brain animal. The separated hemispheres were subsequently tested individually for establishment of learning, and it was found that learning had occurred in both hemispheres, in most cases. Monocular training showed reinforcement of behavior can be produced in both hemispheres at every electrode site tested. It is concluded that unilateral pleasure-center stimulation may exert a bilateral central effect via direct and crossed projections from the brain stem.