Sir, In two recent articles, Pinto and colleagues (2017a, c) challenge the classic view that section of the forebrain commissures creates a division of consciousness, with each hemisphere operating as a distinct conscious entity. In an investigation focusing on visual perception in split-brained patients, they conclude that while perception is divided, consciousness is not. Volz and Gazzaniga (2017) reaffirm findings of divided consciousness in the split brain, and suggest a number of mechanisms whereby limited information might be transferred between hemispheres, noting evidence for attentional transfer (Holtzman et al., 1981) but emphasizing particularly the role of external cross-cueing. This is disputed in turn by Pinto et al. (2017b), but reiterated by Volz et al. (2018). Throughout this barrage there is only passing reference to the potential role of subcortical connections. Split-brained patients do not report any sense that the visual world is divided, and generally act with what Bogen (1993) described as ‘social ordinariness’ (p. 5). Where their visuospatial processing seems integrated, it may well be sustained by a subcortical system, sometimes called the ‘second visual system’ or the ‘ambient system’ (Trevarthen and Sperry, 1973), which courses through the superior colliculi, the pulvinar nuclei, and thence to the parietal lobes, bypassing the ‘focal’ geniculo-striate system, and is connected interhemispherically via the collicular commissure or other possible subcortical routes. The superior colliculi themselves play a role in implementing both saccadic and pursuit eye movements, and are also involved in the normal control of spatial attention during perceptual judgements (Krauzlis et al., 2013). In animals (including birds) with little or no cortical vision, vision is dependent on this system, and the corpus callosum itself is present only in placental mammals. Within the geniculo-striate system the two sides of space are integrated only at the cortical level, by the cerebral commissures. This system is specialized for perception of detail and the identification of objects, and is centred on foveal vision. Several lines of evidence suggest that the intact ambient system may be key to the sense of visuospatial unity in the split brain. For instance, Trevarthen and Sperry (1973) presented stimuli to split-brained patients in peripheral vision while they held fixation on a point. Two patients could easily tell whether two circles in opposite hemifields moved in the same direction (up or down) or in opposite directions. They could also describe the rhythm of movements, using terms such as ‘slow’, ‘fast’, ‘jerky’, and ‘bouncing about’ when the movements were in the left visual field, and therefore projected to the non-verbal right hemisphere. These results imply subcortical unification for motion perception. Interhemispheric integration is also necessary for the perception of apparent motion (‘phi motion’) across the vertical midline, which can be readily perceived by split-brained patients (Ramachandran et al., 1986). Gazzaniga (1987) suggested that the motion is inferred rather than perceived, but further study undermines this conclusion (Naikar and Corballis, 1996), although the perceived motion is less precise than in healthy controls (Forster et al., 2000). Split-brained patients can judge whether or not sloping lines in opposite hemifields are aligned (Sergent, 1987; Corballis and Trudel, 1993). Sergent (1987) also reported that a split-brained patient could readily judge whether or not an arrow in one hemifield pointed to a dot in the other, or whether the angle formed by lines in opposite hemifields is greater or less than 90°. Thus, line orientation and location are transferred subcortically, although again probably with less precision than in healthy controls. The patients can also respond to stimuli in either visual hemifield with the contralateral hand, implying interhemispheric transfer, although the estimated transfer time is longer than in controls (e.g. Berlucchi et al., 1995). It is delayed even further with short-wave visual stimuli (purple), undetectable by collicular neurons (Savazzi et al., 2007), implying a collicular role. Collicular input also contributes to ‘redundancy gain’—a lowering of reaction time when two stimuli are presented simultaneously in both hemifields rather than a single stimulus in just one. Redundancy gain is more pronounced in split-brained patients than in controls, but this added gain disappears when the stimuli are equiluminant with the background (Corballis, 1998) or shown in monochromatic purple (Savazzi and Marzi, 2004), again bypassing the collicular system. It also occurs when the cortical component is removed by hemi-spherectomy (Tomaiuolo et al., 1997), and in hemi-spherectomized patients it is enhanced when bilateral stimuli form a pattern, suggesting collicular sensitivity to gestalt-like properties (Georgy et al., 2016). These examples of interhemispheric integration, along with those reported by Pinto et al., are in stark contrast to the lack of ability of split-brained patients to judge whether pairs of letters, digits, colours, or faces presented in opposite hemifields are the same or different (Johnson, 1984; Corballis and Corballis, 2001). These judgements, though, require cortical analysis through the focal, geniculo-striate system. Although the collicular system appears to allow a unified sense of space and the perceptual integration of location, orientation, motion, and perhaps even pattern, it probably does so with less precision than in the focal system. Krauzlis et al. (2013) suggest that the expansion of the neocortex allowed for a proliferation of features for classifying objects and assigning meaning, but the ancient collicular system remains intact and functional for selecting the content of action or perception. The challenge by Pinto and colleagues reiterates the earlier one published in this journal by Sergent (1987), who drew the following conclusion from her data: ‘This subcortical coordination of hemisphere activity may thus underlie the behavioural integration displayed by commissurotomized patients in their daily activities, allowing them to relate different parts of the visual field and to maintain a unity of purpose in their action’ (p. 1389). 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Unifying control over the body: consciousness and cross-cueing in split-brain patients . Brain 2018 ; 141 : e15 . Google Scholar CrossRef Search ADS © The Author(s) (2018). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved. For permissions, please email: email@example.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)
Brain – Oxford University Press
Published: Apr 2, 2018
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