Dominique Hasboun

Direct Intracranial, fMRI, and Lesion Evidence for the Causal Role of Left Inferotemporal Cortex in Reading

Models of the visual word form system postulate that a left occipitotemporal region implements the automatic visual word recognition required for efficient reading. This theory was assessed in a patient in whom reading was explored with behavioral measures, fMRI, and intracranial local field potentials. Prior to surgery, when reading was normal, fMRI revealed a normal mosaic of ventral visual selectivity for words, faces, houses, and tools. Intracranial recordings demonstrated that the left occipitotemporal cortex responded with a short latency to conscious but also to subliminal words. Surgery removed a small portion of word-responsive occipitotemporal cortex overlapping with the word-specific fMRI activation. The patient developed a marked reading deficit, while recognition of other visual categories remained intact. Furthermore, in the post-surgery fMRI map of visual cortex, only word-specific activations disappeared. Altogether, these results provide direct evidence for the causal role of the left occipitotemporal cortex in the recognition of visual words.

Hemispheric asymmetry and corpus callosum morphometry: a magnetic resonance imaging study.

Previous post-mortem studies (Aboitiz, F., Scheibel, A.B., Fisher, R.S., Zaidel, E., 1992. Brain Res. 598, 154-161 and Aboitiz, F., Scheibel, A.B., Zaidel, E., 1992. Brain 115, 1521-1541) have shown an inverse association between asymmetry in perisylvian areas and the size of a specific segment, the isthmus, of the corpus callosum (CC) in males. The purpose of this work was to study in vivo the association between hemispheric asymmetry and the total size of the CC in 35 right-handed subjects (16 males, 19 females; mean age 24.9 +/- 3.9). An MRI scan was performed for each subject. The area of the right (RH) and left (LH) hemispheres were measured from images in the sagittal plane and the area of the CC from images in the mid-sagittal plane. The index of hemispheric asymmetry was absolute value((LH - RH)/[(LH + RH)/2]). There was a negative correlation between the absolute value of hemispheric asymmetry and the size of the CC in males (r = -0.55, P = 0.03) but not in females (r = -0.20, P = 0.42). These findings, like those of Aboitiz et al. (Aboitiz, F., Scheibel, A.B., Zaidel, E., 1992. Brain 115, 1521-1541), suggest a sex-dependent decrease in interhemispheric connectivity with increasing hemispheric asymmetry.

Relationship between attentional performance and corpus callosum morphometry in patients with Alzheimer’s disease

There has been considerable interest in cognitive deficits associated with Alzheimers disease (AD) and relationships between these impairments and specific cortical atrophies. Two previous studies [Neuropsychologia 28 (1990) 1197; Dementia 3 (1992) 350] have found that AD patients exhibit significant impairments in the attentional ID/ED set-shifting tasks of the CANTAB battery which involved attentional shifting abilities. But, at present, no study has examined the neural bases of these abilities in AD patients. In the present study, the relationship between performances on this attentional test and morphometry of the anterior and posterior corpus callosum is examined in AD patients in the mild to moderate stages of the disease (n=30, mean age=74.1+/-4.9 years, mean MMSE score=23.9+/-2.6). A control group is constituted (n=20, mean age=73.15+/-5.5 years) for comparison of cerebral measurements. The stepwise multiple regression analysis indicates that the relative contribution for the total callosal and the anterior CC areas of the simple discrimination subtest is significantly positive whereas for the posterior callosal areas the relative contribution of the more complex subtest is significantly positive. AD patients from the subgroup low, who failed to do the nine subtests of the attentional set-shifting tasks, exhibit smaller callosal areas than control subjects. There is no significant difference for all callosal measurements between AD patients from the subgroup high, who completely succeeded the test, and control subjects. Our findings suggest that the anterior corpus callosum would be related to attentional shifting abilities in AD patients. Moreover, these patients with probable AD seem heterogeneous for performances in the attentional test of the CANTAB and for callosal atrophies.

Event-Related Potential, Time-frequency, and Functional Connectivity Facets of Local and Global Auditory Novelty Processing: An Intracranial Study in Humans

Auditory novelty detection has been associated with different cognitive processes. Bekinschtein et al. (2009) developed an experimental paradigm to dissociate these processes, using local and global novelty, which were associated, respectively, with automatic versus strategic perceptual processing. They have mostly been studied using event-related potentials (ERPs), but local spiking activity as indexed by gamma (60-120 Hz) power and interactions between brain regions as indexed by modulations in beta-band (13-25 Hz) power and functional connectivity have not been explored. We thus recorded 9 epileptic patients with intracranial electrodes to compare the precise dynamics of the responses to local and global novelty. Local novelty triggered an early response observed as an intracranial mismatch negativity (MMN) contemporary with a strong power increase in the gamma band and an increase in connectivity in the beta band. Importantly, all these responses were strictly confined to the temporal auditory cortex. In contrast, global novelty gave rise to a late ERP response distributed across brain areas, contemporary with a sustained power decrease in the beta band (13-25 Hz) and an increase in connectivity in the alpha band (8-13 Hz) within the frontal lobe. We discuss these multi-facet signatures in terms of conscious access to perceptual information.

Single-unit activities during the transition to seizures in deep mesial structures: Seizures and Single-Unit Activities

Focal seizures are assumed to arise from a hypersynchronous activity affecting a circumscribed brain region. Using microelectrodes in seizure‐generating deep mesial regions of 9 patients, we investigated the firing of hundreds of single neurons before, during, and after ictal electroencephalogram (EEG) discharges. Neuronal spiking activity at seizure initiation was highly heterogeneous and not hypersynchronous. Furthermore, groups of neurons showed significant changes in activity minutes before the seizure with no concomitant changes in the corresponding macroscopic EEG recordings. Altogether, our findings suggest that only limited subsets of neurons in epileptic depth regions initiate the seizure‐onset and that ictogenic mechanisms operate in submillimeter‐scale microdomains. Ann Neurol 2017 Ann Neurol 2017;82:1022–1028