Split-brain Subjects Research Articles

Revisiting the attentional bias in the split brain

Previous research has revealed a strong right bias in allocation of attention in split brain subjects, suggesting that a pathological attention bias occurs not only after unilateral (usually right-hemispheric) damage but also after functional disconnection of intact right-hemispheric areas involved in allocation of attention from those in the left hemisphere. Here, we investigated the laterality bias in spatial attention, as measured with the greyscales task, in two split-brain subjects (D.D.C. and D.D.V.) who had undergone complete callosotomy. The greyscales task requires participants to judge the darker (or brighter) of two left-right mirror-reversed luminance gradients under conditions of free viewing, and offers an efficient means of quantifying pathological attentional biases in patients with unilateral lesions. As predicted, the results of the two split-brain subjects revealed a pathological rightward bias in allocation of attention, suggesting strong dependence on a single hemisphere (the left) in spatial attention, which is opposite to what one expects from people with intact commissures, and is remarkable in that it occurs in free viewing. In that sense both split-brain patients are behaving as though the brain is indeed split, especially in D.D.C. who had undergone partial resection of the anterior commissure in addition to complete callosotomy, whereas the anterior commissure is still intact in D.D.V. The findings support the view that the commissural pathways play a significant role in integration of attentional processes across cerebral hemispheres.

Emotion’s role in the unity of consciousness

ABSTRACT In subjects with the split-brain syndrome, some conscious states appear to be disunified (e.g., visual states), while others remain unified (e.g., affective states). While placing emphasis on the disunities, disunity accounts conclude that split-brain subjects have two subjective perspectives and not one. I argue that affective unity is more important than perceptual disunity in delineating our subjective perspective. Unlike enjoying an objective perspective, enjoying a subjective perspective entails experiencing aspects of your phenomenal field in terms of their overall relation to you. What it’s like to be you at any given time entails experiencing certain aspects of the phenomenal field as peripheral to others. Emotion creates and sustains a center/periphery structure in our phenomenal field by signaling orders of perceived importance as well as orders of perceived changeability (e.g., to someone feeling grief, the experience of getting a ticket is felt as peripheral/unimportant to the experience of grief; to someone feeling depressed, the experience of reaching a hilltop is felt as peripheral/unattainable to the experience of laying down to rest). Since emotion plays a greater role than perception in creating and sustaining this periphery/center structure, and split-brain patients remain affectively unified, split-brain patients retain a unified subjective perspective on the world.

Split-brain syndrome and extended perceptual consciousness

In this paper I argue that split-brain syndrome is best understood within an extended mind framework and, therefore, that its very existence provides support for an externalist account of conscious perception. I begin by outlining the experimental aberration model of split-brain syndrome and explain both: why this model provides the best account of split-brain syndrome; and, why it is commonly rejected. Then, I summarise Susan Hurley’s argument that split-brain subjects could unify their conscious perceptual field by using external factors to stand-in for the missing corpus callosum. I next provide an argument that split-brain subjects do unify their perceptual fields via external factors. Finally, I explain why my account provides one with an experimental aberration model which avoids the problems typically levelled at such views, and highlight some empirical predictions made by the account. The nature of split-brain syndrome has long been considered mysterious by proponents of internalist accounts of consciousness. However, in this paper I argue that externalist theories can provide a straightforward explanation of the condition. I therefore conclude that the ability of externalist accounts to explain split-brain syndrome gives us strong reason to prefer them over internalist rivals.

Intentions and Unified Agency: Insights from the Split‐brain Phenomenon

Under experimental conditions, behavior suggesting dual agency is easily elicited from split‐brain subjects who usually behave in an unremarkable fashion. This article presents a model of split‐brain agency that accounts for this apparent tension. Right and left hemisphere are associated with distinct agents, R and L, each of whose unity is grounded in the special inferential and experiential relations that its mental states bear to each other. These same relations do not hold interhemispherically; rather, unified behavior is largely the result of a split‐brain subject's having a single body, much of whose coordination in action is maintained by forces operating downstream of reasons and intentions and conscious experiences. The split‐brain subject as a whole is still a unified agent as well, however, in virtue of R's and L's intentions bearing, with respect to one and the same body, those special causal powers that an individual's intentions (and hers alone) bear to her body (and to her body alone).

The switch model of split-brain consciousness

The attempt to model the structure of consciousness in split-brain subjects is ongoing. This paper concerns the recently proposed “switch model” of split-brain consciousness, according to which a split-brain subject possesses only a single stream of consciousness, unified at and across time, that shifts from one hemisphere to the other from moment to moment. The paper argues that while the central explanatory element of the switch model may account for some aspects of split-brain consciousness, the best general picture of split-brain consciousness is still offered by some version of the “conscious duality model.”

An oculomotor and computational study of a patient with diagonistic dyspraxia

Diagonistic dyspraxia (DD) is a behavioural disorder encountered in split-brain subjects in which the left arm acts against the subject’s will, deliberately counteracting what the right arm does. We report here an oculomotor and computational study of a patient with a long lasting form of DD. A first series of oculomotor paradigms revealed marked and unprecedented saccade impairments. We used a computational model in order to provide information about the impaired decision-making process: the analysis of saccade latencies revealed that variations of decision times were explained by adjustments of response criterion. This result and paradoxical impairments observed in additional oculomotor paradigms allowed to propose that this adjustment of the criterion level resulted from the co-existence of counteracting oculomotor programs, consistent with the existence of antagonist programs in homotopic cortical areas. In the intact brain, trans-hemispheric inhibition would allow suppression of these counter programs. Depending on the topography of the disconnected areas, various motor and/or behavioural impairments would arise in split-brain subjects. In motor systems, such conflict would result in increased criteria for desired movement execution (oculomotor system) or in simultaneous execution of counteracting movements (skeletal motor system). At higher cognitive levels, it may result in conflict of intentions.

Individuating Mental Tokens: The Split-Brain Case

Some philosophers have argued that so long as two neural events, within a subject, are both of the same type and both carry the same content, then these events may jointly constitute a single mental token, regardless of the sort of causal relation to each other that they bear. These philosophers have used this claim—which I call the “singularity-through-redundancy” position—in order to argue that a split-brain subject normally has a single stream of consciousness, disjunctively realized across the two hemispheres. This paper argues, against this position, that the kind of causal relations multiple neural events bear to each other constrains the mental tokens with which functionalists who are realists can identify them.

Spatial attention and interhemispheric visuomotor integration in the absence of the corpus callosum

In the lateralized simple reaction time (SRT) task with unimanual responses (Poffenberger paradigm), reaction times (RTs) are faster with ipsilateral (uncrossed) than with contralateral (crossed) response hand–target hemifield combinations. The difference between crossed and uncrossed responses (CUD) has typically been interpreted to reflect callosal transfer time. Indeed, acallosal subjects and split-brain subjects have longer CUDs than control subjects. However, a few recent studies have demonstrated that, contrary to classical findings, the CUD is also affected by non-anatomical factors. Here we show that the CUD is also affected by non-anatomical factors in patients with agenesis of the corpus callosum and complete commissurotomy where interhemispheric transfer must be subcallosal. We tested acallosal subject M.M. and split brain patient A.A. on a lateralized SRT task with their arms alternately uncrossed (natural arms position) or crossed (unnatural arms position) across blocks of trials. The results revealed a significant effect of arms crossing on the size and direction of the CUD as previously found in normal subjects [Mooshagian, E., Iacoboni, M., & Zaidel, E. (2008). The role of task history in simple reaction time to lateralized light flashes. Neuropsychologia, 46(2), 659–664]. This suggests that non-anatomical factors that modulate interhemispheric visuomotor integration may occur in absence of the corpus callosum. Anterior commissure and interhemispheric cortico-subcortical pathways are likely implicated in these effects.

Consciousness and Persons: Unity and Identity, MICHAEL TYE. Cambridge, MA, and London, UK.

In Consciousness and Persons: Unity and Identity, Michael Tye takes on the thorny issue of the unity of consciousness and answers these important questions: What exactly is the unity of consciousness? Can a single person have a divided consciousness? What is a single person? Tye argues that unity is a fundamental part of human consciousness -- something so basic to everyday experience that it is easy to overlook. For example, when we hear the sound of waves crashing on a beach and at the same time see a red warning flag, there is an overall unity to our experience; the sound and the red shape are presented together in our consciousness. Similarly, when we undergo a succession of thoughts as we think something through, there is an experience of succession that unifies the thoughts into a conscious whole. But, Tye shows, consciousness is not always unified. Split-brain subjects, whose corpus callosum has been severed, are usually taken to have a divided or disunified consciousness. Their behavior in certain situations implies that they have lost the unity normal human subjects take for granted; it is sometimes even supposed that a split-brain subject is really two persons.Tye begins his account by proposing an account of the unity of experience at a single time; this account is extended over the succeeding chapters to cover bodily sensations at a single time and perceptual experience, bodily sensations, conscious thoughts, and felt moods at a single time. Tye follows these chapters with a discussion of the unity of experience through time. Turning to the split-brain phenomenon, he proposes an account of the mental life of split-brain subjects and argues that certain facts about these subjects offer support for his theory of unity. Finally, addressing the topic of the nature of persons and personal identity, Tye finds the two great historical accounts -- the ego theory and the bundle theory -- lacking and he makes an alternative proposal. He includes an appendix on the general representational approach to consciousness and its many varieties, because of the relevance of representationalism to the theory of unity being adanced.

Now you see it, now you don't: Variable hemineglect in a commissurotomized man

We describe the case of a callosotomized man, D.D.V., who shows unusual neglect of stimuli in the left visual field (LVF). This is manifest in simple reaction time (RT) to stimuli flashed in the LVF and in judging whether pairs of filled circles in the LVF are of the same or different color. It may reflect strong left-hemispheric control and consequent attention restricted to the right side of space. It is not evident in simple RT when there are continuous markers in the visual fields to indicate the locations of the stimuli. In this condition, his RTs are actually faster to LVF than to right visual field (RVF) stimuli, suggesting a switch to right-hemispheric control that eliminates the hemineglect. Neglect is also not evident when D.D.V. responds by pointing to or touching the locations of the stimuli, perhaps because these responses are controlled by the dorsal rather than the ventral visual system. Despite his atypical manifestations of hemineglect, D.D.V. showed evidence of functional disconnection typical of split-brained subjects, including prolonged crossed–uncrossed different in simple reaction time, inability to match colors between visual fields, and enhanced redundancy gain in simple RT to bilateral stimuli even when the stimulus in the LVF was neglected.

Redundancy gain in simple reaction time following partial and complete callosotomy

Four subjects with partial or complete section of the corpus callosum were tested on simple reaction time (RT) to visual stimuli presented either singly in one or other visual field, or simultaneously in both visual fields. The subject with posterior callosal section showed evidence of redundancy gain with bilateral stimuli beyond that attributable to probability summation (“enhanced” redundancy gain), and prolonged interhemispheric transfer. One of the two subjects with anterior section, like normals, showed little evidence of enhanced redundancy gain, and no evidence of prolonged interhemispheric transfer. The other did show some enhanced redundancy gain at the fast end of the RT distribution. These and other results suggest that the posterior corpus callosum provides the principal route or routes of interhemispheric transfer of the information required for simple visuomotor responses, and is also responsible for the much reduced redundancy gain in normal subjects relative to that in split-brained subjects. The subject with complete callosal section was unusual in that he responded only very rarely to stimuli in the left visual field (LVF), yet he showed markedly reduced RTs to bilateral relative to right visual field (RVF) stimuli.

Interhemispheric neural summation in the split brain: effects of stimulus colour and task

Four split-brained subjects, two subjects with agenesis of the corpus callosum, and 14 normal subjects performed two tasks requiring responses to red or green disks, briefly presented either singly in the left visual field, singly in the right visual field, or simultaneously in both visual fields. In Experiment 1, simple reaction times to these stimuli, regardless of colour, were recorded (the Go-Both Task), and found to be faster to bilateral-redundant stimulus pairs, than to single stimuli. This so-called “redundancy gain” was much larger for acallosal or split-brained subjects than for normal subjects and exceeded the predictions of a race model, implying neural summation. Experiment 2 used the same stimuli, but subjects were required to respond only to stimuli of a designated colour (the Go/No-Go Task). Redundant target stimuli produced neural summation, while stimuli pairs that included a non-target stimulus did not. These results suggest that neural summation in the acallosal or split brain involves the convergence of response-associated activation, and that redundant sensory processes are not sufficient.

Paradoxical interhemispheric summation in the split brain.

We measured simple reaction time (RT) to light flashes, presented either singly or in pairs, in two people who had undergone callosotomy, one person with agenesis of the corpus callosum, and 17 normal subjects. The three split-brained subjects' RTs were decreased to bilateral pairs beyond predictions based on a simple race between independent unilateral processes, while those of the normal subjects were actually longer than predicted by the race model. This effect was present whether the bilateral pairs were in mirror-image locations or not, but was not present when the pairs were presented unilaterally. Since summation does not depend on close spatial correspondence, and also occurs when inputs are staggered in time, we suggest that it is due to cortical projection to a subcortical arousal system, and is normally inhibited by the corpus callosum.

Effects of callosal lesions in a model of letter perception.

During cognitive tasks, the cerebral hemispheres cooperate, compete, and in general, interact via the corpus callosum. Although behavioral studies in normal and split-brain subjects have revealed a great deal about the transcallosal exchange of information, a fundamental question remains unanswered and controversial: Are transcallosal interhemispheric influences primarily excitatory or inhibitory? In this context, we examined the effects of simulating sectioning of the corpus callosum in a computational model of visual letter recognition. Differences were found, following simulated callosal sectioning, in the performance of each individual hemisphere, in the mean activation levels of hemispheres, and in the specific patterns of activity, depending on the nature of the callosal influences. Together with other recent computational modeling results, the findings are most consistent with the hypothesis that transcallosal influences are predominantly excitatory, and they suggest measures that could be examined in future experimental studies to help resolve this issue.

Interhemispheric visual matching in the split brain

Three split-brained subjects, one (N.G.) with full forebrain commissurotomy and two (V.P. and J.W.) with callosotomy, made same-different judgments about pairs of visual stimuli that were flashed either unilaterally or bilaterally. In separate blocks of trials, the stimuli could differ in luminance, size, or color. In the bilateral condition, only J.W. scored above chance, and only minimally, on the luminance and size tasks, and none of the subjects scored above chance on the color task. Accuracy was generally much higher, especially for V.P. and J.W., when the stimuli were unilateral. These results confirm that there is little or no interhemispheric transfer of the visual attributes of luminance, size or color in the split brain.

Effect of luminance on successiveness discrimination in the absence of the corpus callosum

Three split-brained subjects, one with full forebrain commissurotomy and two with callosotomy, were impaired at judging whether pairs of lights in opposite visual fields were successive or simultaneous. This impairment did not vary with luminance when the lights were grey against a dark background, but was more pronounced when the lights were equiluminant with a yellow background. All three subjects were also better able to discriminate succession from simultaneity when the lights were both in the left visual field than when they were both in the right. A fourth subject with callosal agenesis was only slightly impaired relative to normal subjects, who were virtually errorless.

Interhemispheric transfer of colour and shape information in the presence and absence of the corpus callosum

Two split-brained subjects, one (L.B.) with full forebrain commissurotomy and one (R.B.) with callosal agenesis, and a group of twenty neurologically intact subjects were tested in three discrimination tasks: a go–no go task, a two-choice task, and a three-choice task. The discriminations were based on colour in Experiment 1, and on shape in Experiment 2. The stimuli were presented in one or other visual field, and the subjects responded with the fingers of one or other hand, allowing the differences in reaction time between crossed and uncrossed responses (CUD) to be calculated. For the normal subjects the CUD tended to diminish with the complexity of the tasks, suggesting that both hemispheres were increasingly involved. Unlike R.B. and the normal controls, who made virtually no errors, L.B. had increasing difficulty as task complexity increased. He was better able to transfer information from the right to the left hemisphere than vice versa, but an analysis of his accuracy under the crossed conditions showed that the amount transferred was always well under one bit. This confirms previous evidence that L.B. has very limited subcortical transfer of either colour or shape.

Who tells one hand what the other is doing: the neurophysiology of bimanual movements.

Research into neural mechanisms of bimanual coordination sheds light on questions raised by psychophysics. While the results we review are merely suggestive, the overall structure of the neural mechanisms may be emerging slowly.Different mechanisms may serve the spatial and temporal coupling of bimanual movements. Temporal coupling depends on a lateralized pace-keeping mechanism possibly associated with spinal CPGs, is preferentially controlled by the dominant hemisphere, and does not depend directly on the corpus callosum. Spatial coupling, in contrast, arises through interhemispheric interaction. Individual neurons in each hemisphere reflect this interaction, as does interhemispheric synchronization. The interaction allows decoupling of the arms, as substantiated by the PV analysis. Furthermore, bimanual proximal arm movements and finger movements are controlled differently, as shown in differences in the patterns of neural activity in M1 and SMA.Taken as a whole, the research we review has not yet culminated in a conclusive model. However, by combining psychophysics and neurophysiology, we raise concrete, testable hypotheses that may help realize this goal.*To whom correspondence should be addressed (e-mail: eilon@hbf.huji.ac.il).

Same/different judgements about the direction and colour of apparent-motion stimuli after commissurotomy

One subject with complete forebrain commissurotomy (L.B.), another with posterior callosotomy (D.K.) and 6 normal controls were tested for their ability to make same/different judgements about the direction (Experiment 1) and colour (Experiment 2) of apparent-motion stimuli. In the first experiment, all of the subjects were able to judge whether the direction of apparent motion of two stimulus sequences was the same or different, regardless of whether the two sequences were presented unilaterally or bilaterally. In the second experiment, however, L.B. was unable to judge whether the colours of two lights comprising an apparent motion sequence were the same or different when the two lights were presented bilaterally although he was able to do so when both lights were in the same visual field. D.K. was able to make the discrimination on both bilateral and unilateral presentations but he performed more poorly in the bilateral condition than in either visual field. The control subjects performed consistently in all three locations. Given that same/different judgement tasks are a rigorous test of interhemispheric integration, the results indicate that the ability to discriminate apparent motion across the retinal midline is intact in split-brained subjects. Furthermore, the results support a dissociation in the interhemispheric integration of the direction and colour of apparent-motion stimuli after commissurotomy.

Unilateral and bilateral temperature comparisons in acallosal and split-brain subjects

Therapeutic section of the corpus callosum in adult epileptic patients typically results in their incapacity to carry out interhemispheric comparisons of lateralized information. The fact that acallosal and early split-brain subjects display few of these symptoms when tested in the tactile modality has led to the suggestion that these patients may use ipsilateral projections of the somatosensory system more effectively. Compensation, however, is limited by the fact that the lemniscal pathway is strongly lateralized, especially for the distal parts of the body, where few ipsilaterally projecting fibres have been demonstrated. The pathway carrying temperature information has a larger ipsilateral component. Bilateral comparisons within the same hemisphere in subjects who are lacking the corpus callosum should be more common and the development of compensatory mechanisms in early-sectioned or acallosal subjects should be more likely. The objective of the present experiment was to evaluate differential thresholds for thermal stimuli applied on a number of regions either on the same side or on corresponding sites on opposite sides of the body. One subject callosotomized as an adult and one split-brain subject who underwent callosotomy in childhood, as well as three acallosal subjects, were compared to IQ-matched and normal-IQ control subjects. The fingers, forearm and trunk were tested. The comparison temperature was 30 C and the other was varied in an ascending or descending fashion using a modified method of limits. Differential thresholds were similar for within- and between-side comparisons, and comparable to those of the IQ-matched subjects. The results indicate that comparisons involving temperature discrimination for stimuli applied to the two sides of the body do not require the integrity of the corpus callosum.