title: Imaging 'top-down' mobilization of visual information: a case study in a posterior split-brain patient.
abstract: During visual perception, automatic bottom-up and controlled top-down processes occur simultaneously and interact in a complex way, making them difficult to isolate and characterize. In rare neurological conditions, such a dissociation can be achieved more easily. In the present work, we studied a patient (AC) with a posterior lesion of the corpus callosum (CC), using a combination of behavioural, structural MRI and high-density scalp EEG measures. Given the complete disruption of the posterior half of the CC, we speculated that inter-hemispheric transfer of visual information was only possible through top-down mobilization across the preserved anterior segment of the CC. We designed a matching-to-sample visual task during which this patient was randomly presented with two successive numerical targets (T1 and T2) flashed with either a short or a long stimulus-onset asynchrony (SOA), each presented within one visual hemifield (HF). Intra-hemispheric processing of visual stimuli was essentially preserved. In sharp contrast, patient's performance was massively impaired during inter-HFs trials with a short-SOA, confirming the lack of fast inter-hemispheric transfer. Crucially, patient AC spontaneously improved his performance in inter-HFs trials with a long-SOA. This behavioral improvement was correlated with a mid-frontal ERP effect occurring during the T1-T2 interval, concomitant with an increase of functional connectivity of this region with distant areas including occipital regions. These results put to light a slow, non-automatic, and frontally mediated route of inter-hemispheric transfer dependent on top-down control.
authors.affiliation: INSERM, ICM Research Center, UMRS 975, Paris, France; Institut du Cerveau et de la Moëlle épinière, Paris, France; University Paris 6, Faculté de Médecine Pitié-Salpêtrière, Paris, France; AP-HP, Groupe hospitalier Pitié-Salpêtrière, Department of Neurology, Paris, France; AP-HP, Groupe hospitalier Pitié-Salpêtrière, Department of Neurophysiology, Paris, France. Electronic address: lionel.naccache@psl.aphp.fr.
authors.forename: Lionel
authors.initials: L
authors.lastname: Naccache
authors.affiliation: INSERM, ICM Research Center, UMRS 975, Paris, France; Institut du Cerveau et de la Moëlle épinière, Paris, France.
authors.forename: Sarah
authors.initials: S
authors.lastname: Sportiche
authors.affiliation: INSERM, ICM Research Center, UMRS 975, Paris, France; Institut du Cerveau et de la Moëlle épinière, Paris, France.
authors.forename: Mélanie
authors.initials: M
authors.lastname: Strauss
authors.affiliation: INSERM, ICM Research Center, UMRS 975, Paris, France; Institut du Cerveau et de la Moëlle épinière, Paris, France.
authors.forename: Imen
authors.initials: I
authors.lastname: El Karoui
authors.affiliation: INSERM, ICM Research Center, UMRS 975, Paris, France; Institut du Cerveau et de la Moëlle épinière, Paris, France; NeuroSpin Center, Institute of BioImaging Commissariat à l'Energie Atomique, F-91191 Gif/Yvette, France.
authors.forename: Jacobo
authors.initials: J
authors.lastname: Sitt
authors.affiliation: INSERM, ICM Research Center, UMRS 975, Paris, France; Institut du Cerveau et de la Moëlle épinière, Paris, France; University Paris 6, Faculté de Médecine Pitié-Salpêtrière, Paris, France; AP-HP, Groupe hospitalier Pitié-Salpêtrière, Department of Neurology, Paris, France.
authors.forename: Laurent
authors.initials: L
authors.lastname: Cohen
mesh_terms: D000328:Adult; D001921:Brain / Q000473:pathology / Q000503:physiopathology* / Q000601:surgery; D003337:Corpus Callosum / Q000473:pathology / Q000503:physiopathology*; D038524:Diffusion Magnetic Resonance Imaging; D004569:Electroencephalography; D005071:Evoked Potentials; D005260:Female; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008959:Models, Neurological; D009483:Neuropsychological Tests; D011930:Reaction Time; D050812:Split-Brain Procedure; D013647:Task Performance and Analysis; D013826:Theta Rhythm; D014795:Visual Pathways / Q000473:pathology / Q000503:physiopathology; D014796:Visual Perception / Q000502:physiology*
keywords: Anterior cingulate cortex; Behavior; EEG; Inter-hemispheric transfer; Split-brain; Top-down