Jean François Démonet

Alzheimer's patients engage an alternative network during a memory task

We conducted an event‐related functional magnetic resonance imaging experiment to better understand the potentially compensatory alternative brain networks activated by a clinically relevant face‐name association task in Alzheimers disease (AD) patients and matched control subjects. We recruited 17 healthy subjects and 12 AD patients at an early stage of the disease. They underwent functional magnetic resonance imaging scanning in four sessions. Each of the sessions combined a “study” phase and a “test” phase. Face/name pairs were presented in each study phase, and subjects were asked to associate faces with names. In the test phase, a recognition task, faces seen in the study phase were presented each with four different names. The task required selection of appropriate previously associated names from the study phase. Responses were recorded for post hoc classification into those successfully or unsuccessfully encoded. There were significant differences between the groups in accuracy and reaction time. Comparison of correctly versus incorrectly encoded and recognized pairs in the two groups indicated bilateral hippocampal hypoactivation both when encoding and recognizing in the AD group. Moreover, patients showed bilateral hyperactivation of parts of the parietal and frontal lobes. We discuss whether hyperactivation of a frontoparietal network reflects compensatory strategies for failing associative memory in AD patients. Ann Neurol 2005;58:870–879

Volume and iron content in basal ganglia and thalamus

Magnetic resonance imaging (MRI) studies have highlighted the possibility to investigate brain iron content in vivo. In this study, we combined T2* relaxometry and automatic segmentation of basal ganglia based on T1‐weighted images in healthy subjects, with the aim of characterizing age related changes in volume and iron‐related relaxivity values (R2*) of these structures. Thirty healthy subjects underwent MR imaging at 3 Tesla. Mean R2* values and volumes were calculated for the selected subcortical structures (pallidum, putamen, thalamus and caudate nucleus). Our results showed a correlation between R2* values and iron concentration as calculated from published post‐mortem data. Furthermore, we observed a shrinkage/iron increase with a different pattern in the anatomical regions selected in this work, suggesting that the age‐related changes on these MR parameters are specific to the subcortical structure considered. In particular, the putamen demonstrated a decrease of volume and an increase of iron level, with the posterior region of this structure appearing more disposed to iron deposition. Our work suggests that combining volumetry and iron estimation in MRI permits to investigate in vivo neurophysiological and neuropathological changes of basal ganglia. Hum Brain Mapp 2009.

Enhanced response of the left frontal cortex to slowed down speech in dyslexia: an fMRI study

Language difficulties of dyslexic subjects may result partly from a basic deficit in processing rapidly changing sensory inputs. In this fMRI study, we compared brain activities in adult dyslexics and controls during implicit categorical perception of phonemes with normal and slowed down stimuli. Perception of phonemic contrasts activated a frontal parietal network (Brocas area and the left supramarginal gyrus) in which the frontal component was down-regulated by slowed speech in controls and enhanced in dyslexic subjects. No modulation by speech rate was observed in the left supra-marginal gyrus. Enhancement of activity in Brocas area for slowed speech in dyslexic subjects might represent a neural basis of the improvement of performance that has been observed after remediation using this type of stimuli.

Monitoring cognitive and emotional processes through pupil and cardiac response during dynamic versus logical task.

The paper deals with the links between physiological measurements and cognitive and emotional functioning. As long as the operator is a key agent in charge of complex systems, the definition of metrics able to predict his performance is a great challenge. The measurement of the physiological state is a very promising way but a very acute comprehension is required; in particular few studies compare autonomous nervous system reactivity according to specific cognitive processes during task performance and task related psychological stress is often ignored. We compared physiological parameters recorded on 24 healthy subjects facing two neuropsychological tasks: a dynamic task that require problem solving in a world that continually evolves over time and a logical task representative of cognitive processes performed by operators facing everyday problem solving. Results showed that the mean pupil diameter change was higher during the dynamic task; conversely, the heart rate was more elevated during the logical task. Finally, the systolic blood pressure seemed to be strongly sensitive to psychological stress. A better taking into account of the precise influence of a given cognitive activity and both workload and related task-induced psychological stress during task performance is a promising way to better monitor operators in complex working situations to detect mental overload or pejorative stress factor of error.

Mental representations of action: The neural correlates of the verbal and motor components

Recent theories have hypothesized that semantic representations of action verbs and mental representations of action may be supported by partially overlapping, distributed brain networks. An fMRI experiment in healthy participants was designed to identify the common and specific regions in three different tasks from a common set of object drawings (manipulable man-made objects (MMO) and biological objects (MBO)): the generation of action words (GenA), the mental simulation of action (MSoA) and the mime of an action with the right hand (MimA). A fourth task, object naming (ON), was used as control for input/output effects. A null conjunction identified a common neural network consisting of nine regions distributed over premotor, parietal and occipital cortices. Within this common network, GenA elicited significantly more activation than either ON or MSoA in the left inferior frontal region, while MSoA elicited significantly more activation than either ON or GenA in the left superior parietal lobule. Both MSoA and GenA activated the left inferior parietal lobule more than ON. Furthermore, the left superior parietal cortex was activated to a greater extent by MMO than by MBO regardless of the tasks. These results suggest that action-denoting verbs and motor representations of the same actions activate a common frontal-parietal network. The left inferior parietal cortex and the left superior parietal cortex are likely to be involved in the retrieval of spatial-temporal features of object manipulation; the former might relate to the grasping and manipulation of any object while the latter might be linked to specific object-related gestures.

Cerebral Blood Flow, Cerebral Blood Flow Reactivity to Acetazolamide, and Cerebral Blood Volume in Patients with Leukoaraiosis

We have used single photon emission computed tomography to study cerebral blood flow (CBF), CBF reactivity to acetazolamide, and cerebral blood volume (CBV) in 15 subjects presenting widespread leukoaraiosis, with the aim of answering the question if chronic ischaemia or hypo metabolism is associated with leukoaraiosis. We compared these subjects to 9 controls presenting no leukoaraiosis but with similarly distributed factors for leukoaraiosis. The subjects with leukoaraiosis showed a low regional (r)CBF in their white matter, while CBV and CBF reactivity to acetazolamide were not significantly affected. rCBF, rCBV and rCBF reactivity to acetazolamide did not differ between the cortex of patients and controls. Our results suggest that the low white matter CBF of the patients with leukoaraiosis was related mainly to hypometabolism and not to oligaemia. The capacity of the cerebral vessels to vasodilate does not appear to be affected by leukoaraiosis.

Abnormal activation in the visual cortex after corneal refractive surgery for myopia: demonstration by functional magnetic resonance imaging.

OBJECTIVE To try to correlate subjective photophobic symptoms with visual pathway modifications (from the retinal image to the visual cortex) after refractive surgery by exploring brain activation on photic stimulation. DESIGN Noncomparative case series. PARTICIPANTS Four subjects reporting discomfort produced by luminance (glare, halos, starbursts, or a combination thereof) in one eye after laser in situ keratomileusis (LASIK) were enrolled. The contralateral myopic eye (control) had no visual impairment and had undergone LASIK without complications or had not had previous surgery. METHODS Functional magnetic resonance imaging was performed during photic stimulation, delivered by an optical fiber, of the affected and unaffected eyes. RESULTS Functional magnetic resonance imaging provided evidence that most subjective visual symptoms correlated with anatomic flap abnormalities are associated with a higher signal increase in the visual association cortices compared with a nonsymptomatic eye. CONCLUSIONS Functional magnetic resonance imaging of the visual cortex may help in exploring the mechanisms involved in glare effects after refractive surgery.

Preserved Striate Cortex is Not Sufficient to Support the McCollough Effect: Evidence from two Patients with Cerebral Achromatopsia

The McCollough effect (ME) is a colour aftereffect contingent on pattern orientation. This effect is generally thought to be mediated by primary visual cortex (V1) although this has remained the subject of some debate. To determine whether V1 is in fact sufficient to subserve the ME, we compared McCollough adaptation in controls to adaptation in two patients with damage to ventrotemporal cortex, resulting in achromatopsia, but who have spared V1. Each of these patients has some residual colour abilities of which he is unaware. Participants performed a 2AFC orientation-discrimination task for pairs of oblique and vertical/horizontal gratings both before and after adaptation to red/green oblique induction gratings. Successful ME induction would manifest itself as an improvement in oblique-orientation discrimination owing to the additional colour cue after adaptation. Indeed, in controls oblique grating discrimination improved post-adaptation. Further, a subdivision of our control group demonstrated successful ME induction despite a lack of conscious awareness of the added colour cue, indicating that conscious colour awareness is not required for ME induction. The patients, however, did not show improvement in oblique-orientation discrimination, indicating a lack of ME induction. This suggests that V1 must be connected to higher cortical colour areas to drive ME induction.