Split brain: divided perception but undivided consciousness.

In extensive studies with two split-brain patients we replicate the standard finding that stimuli cannot be compared across visual half-fields, indicating that each hemisphere processes information independently of the other. Yet, crucially, we show that the canonical textbook findings that a split-brain patient can only respond to stimuli in the left visual half-field with the left hand, and to stimuli in the right visual half-field with the right hand and verbally, are not universally true. Across a wide variety of tasks, split-brain patients with a complete and radiologically confirmed transection of the corpus callosum showed full awareness of presence, and well above chance-level recognition of location, orientation and identity of stimuli throughout the entire visual field, irrespective of response type (left hand, right hand, or verbally). Crucially, we used confidence ratings to assess conscious awareness. This revealed that also on high confidence trials, indicative of conscious perception, response type did not affect performance. These findings suggest that severing the cortical connections between hemispheres splits visual perception, but does not create two independent conscious perceivers within one brain.

The "consonance effect" and the hemispheres: a study on a split-brain patient.

The association between musical consonance and pleasantness, and between musical dissonance and unpleasantness ("consonance effect") is well established. Furthermore, a number of studies suggest the main involvement of the left hemisphere in the perception of dissonance and that of the right hemisphere in the perception of consonance. In the present study, the consonance effect was studied in a callosotomized patient, D. D. C. and in a control group. In binaural presentations, the patient did not attribute different pleasantness judgements to consonant and dissonant chords, differently from the control group who showed the consonance effect. However, in dichotic presentations (e.g. a chord in one ear and white noise in the other ear), a trend towards the consonance effect was found in D. D. C., but only when chords were presented in his right ear (left hemisphere), whereas the control group confirmed the known hemispheric asymmetry in labelling the pleasantness of consonant and dissonant chords. These results suggest that the right-hemispheric superiority in appreciating consonance might hide the inability of the right hemisphere to classify dissonant chords as unpleasant in the split-brain, whereas the left hemisphere seems capable to differently label the pleasantness of consonant and dissonant chords, even if it is more sensitive to dissonance.

Conscious and unconscious processing of facial expressions: evidence from two split-brain patients.

We investigated how the brain's hemispheres process explicit and implicit facial expressions in two 'split-brain' patients (one with a complete and one with a partial anterior resection). Photographs of faces expressing positive, negative or neutral emotions were shown either centrally or bilaterally. The task consisted in judging the friendliness of each person in the photographs. Half of the photograph stimuli were 'hybrid faces', that is an amalgamation of filtered images which contained emotional information only in the low range of spatial frequency, blended to a neutral expression of the same individual in the rest of the spatial frequencies. The other half of the images contained unfiltered faces. With the hybrid faces the patients and a matched control group were more influenced in their social judgements by the emotional expression of the face shown in the left visual field (LVF). When the expressions were shown explicitly, that is without filtering, the control group and the partially callosotomized patient based their judgement on the face shown in the LVF, whereas the complete split-brain patient based his ratings mainly on the face presented in the right visual field. We conclude that the processing of implicit emotions does not require the integrity of callosal fibres and can take place within subcortical routes lateralized in the right hemisphere.

Outcomes after anterior or complete corpus callosotomy in children.

BACKGROUND: Corpus callosotomy (CC) is a valuable palliative surgical option for children with medically refractory epilepsy due to generalized or multifocal cortical seizure onset. OBJECTIVE: To investigate the extent of CC resulting in optimal seizure control in a pediatric patient population and to evaluate the modification of seizure profile after various CC approaches. METHODS: The records of 58 children (3-22 years of age at the time of surgery) with medically refractory epilepsy who underwent CC between 1995 and 2011 were retrospectively reviewed. RESULTS: Anterior two thirds callosotomy resulted in resolution of absence (P = .03) and astatic (P = .03) seizures, whereas anterior two thirds callosotomy followed by second-stage completion resulted in resolution of generalized tonic-clonic (GTC) (P = .03), astatic (P = .005), and myoclonic (P = .03) seizures in addition to a trend toward resolution of absence seizures (P = .08). Single-stage upfront complete callosotomy resulted in resolution of absence (P = .002), astatic (P < .0001), myoclonic (P = .007), and complex partial (P = .008) seizures in addition to a trend toward resolution of GTC (P = .06). In comparing a composite of subjects who underwent anterior two thirds callosotomy alone or 2-stage complete callosotomy before the second stage to complete the callosotomy with subjects who underwent upfront complete CC, a more favorable outcome was found in those with the upfront complete CC (P = .02). CONCLUSION: Single-stage upfront complete callosotomy is effective in relieving a broader spectrum of seizure types than anterior two thirds callosotomy or 2-stage complete callosotomy in children. The advantages of single-stage complete callosotomy must be weighed against the potentially higher risk of neurological and operative complications.

Split-brain syndrome after hepatic transplantation: a tacrolimus-related vasculitis?

An unusual case of inter-haemispheric disconnection syndrome occurring in a patient who had undergone hepatic transplantation is presented. The underlying disorder, at first wrongly interpreted as encephalitis, was found to be severe, diffuse cerebral vasculitis. The hypothesis that treatment with tacrolimus might have caused, or at least favoured the vascular damage is discussed.

Abnormal moral reasoning in complete and partial callosotomy patients.

Recent neuroimaging studies suggest lateralized cerebral mechanisms in the right temporal parietal junction are involved in complex social and moral reasoning, such as ascribing beliefs to others. Based on this evidence, we tested 3 anterior-resected and 3 complete callosotomy patients along with 22 normal subjects on a reasoning task that required verbal moral judgments. All 6 patients based their judgments primarily on the outcome of the actions, disregarding the beliefs of the agents. The similarity in performance between complete and partial callosotomy patients suggests that normal judgments of morality require full interhemispheric integration of information critically supported by the right temporal parietal junction and right frontal processes.

Evaluation of efficacy and safety of anterior corpus callosotomy with keyhole in refractory seizures.

PURPOSE: This study was conducted to evaluate the efficacy and safety of anterior corpus callosotomy with a keyhole approach on refractory seizures and to evaluate this procedure for drop attacks (DAs) and generalized tonic-clonic seizures (GTCSs). METHODS: All refractory seizure patients underwent anterior corpus callosotomy (n=31) without other epilepsy surgery. Seizure response and procedure complications were evaluated. RESULTS: Seizure types included GTCS (n=26), and atonic or tonic seizures with DA (n=9). In GTCS patients, 84.6% had >or=50% decrease in seizure frequency, and 61.5% had >or=80% reduction. In DA patients, 77.8% had >or=50% decrease in seizure frequency, and 55.6% had >or=80% reduction. There were no statistically significant differences between the groups in final efficacy. Overall complication rate for corpus callosotomy was 12.9%, the permanent complication rate was 3.2%. CONCLUSIONS: Anterior corpus callosotomy with a keyhole approach produces highly favorable outcomes for both GTCS and atonic or tonic seizures with a low risk for complications.