E. Lobel

A fronto-parietal system for computing the egocentric spatial frame of reference in humans

Abstract Spatial orientation is based on coordinates referring to the subject’s body. A fundamental principle is the mid-sagittal plane, which divides the body and space into the left and right sides. Its neural bases were investigated by functional magnetic resonance imaging (fMRI). Seven normal subjects pressed a button when a vertical bar, moving horizontally, crossed the subjective mid-sagittal plane. In the control condition, the subjects’ task was to press a button when the direction of the bar movement changed, at the end of each leftward or rightward movement. The task involving the computation of the mid-sagittal plane yielded increased signal in posterior parietal and lateral frontal premotor regions, with a more extensive activation in the right cerebral hemisphere. This direct evidence in normal human subjects that a bilateral, mainly right hemisphere-based, cortical network is active during the computation of the egocentric reference is consistent with neuropsychological studies in patients with unilateral cerebral lesions. Damage to the right hemisphere, more frequently to the posterior-inferior parietal region, may bring about a neglect syndrome of the contralesional, left side of space, including a major rightward displacement of the subjective mid-sagittal plane. The existence of a posterior parietal-lateral premotor frontal network concerned with egocentric spatial reference frames is also in line with neurophysiological studies in the monkey.

Human taste cortical areas studied with functional magnetic resonance imaging: evidence of functional lateralization related to handedness.

Whole-brain functional magnetic resonance imaging was used to detect local hemodynamic changes reflecting cortical activation in five left handed and five right handed human subjects during bilateral stimulation of the tongue with various tastes. Activation was found bilaterally in the insula and the perisylvian region. These regions correspond to the primary taste cortical areas identified with electrophysiological recordings in monkeys and suggested from former clinical observations in human subjects. Moreover, a unilateral projection was described for the first time in the inferior part of the insula of the dominant hemisphere, according to the subjects handedness.

fMRI study of voluntary saccadic eye movements in humans

fMRI Study of Voluntary Saccadic Eye Movements in Humans E. Lobel(1,2), A. Berthoz(1), A. Leroy-Will ig(2) , D. Le Bihan(2) 1. LPPA, Collbge de France / CNRS, Paris, France 2. SHFJ, Department of Medical Research, CEA, Orsay, France Introduction Previous studies in animals, in brain-damaged patients, and in healthy humans (1-4), have shown that several cortical areas are involved in the production and control of voluntary saccades. Our aim in this study is to use functional MRI to compare the individual loci of activation during a self-paced self-triggered saccade experiment in healthy humans. Methods Four right-handed healthy volunteers, aged 20 to 29, were studied. 45 minutes before the experiment, subjects were trained to execute horizontal saccades to visual targets at + 20 ~ All experiments were performed on a 3T whole-body system (Bruker, Germany) equipped with a head-gradient coil insert designed for echoplanar imaging (EPI). Sets of high resolution images were acquired for 3D reconstruction and anatomical identification of activated areas, as well as sets of images highlighting blood vessels. Functional images were obtained with a T2*-weighted EPI sequence. Twelve slices (voxel: 5x3x3 mm 3) covering the frontal and parietal lobes were acquired every 3.6 seconds. During the experiment, 5 periods of rest alternated with 4 periods of saccades executed in total darkness with eyes open (25 seconds per period). Performance was followed online using filtered EOG. Functional maps were obtained by pixel correlation and cluster activation analysis, using a dedicated software running under IDL (Research System, Inc.). Results and Discussion Saccades were performed with frequency 1.42 + 0.15 Hz, and with mean amplitude about 50 ~ The main areas of cortical activation are summarized in table 1. left hemisphere right hemisphere anterior precentral gyms 4 2 posterior precentral gyms 3 2 medial superior frontal gyms 4 4 inferior parietal lobule 4 3 insula 3 1 median cingulate gyms 2 0 table 1: number of volunteers showing activation in cortical regions of interest during saccadic eye movements (SEM) In one subject, we also found activation of putamen and caudate nucleus. Compared to previous PET results (2,4) which showed involvment of the left and right precentral gyms, left and right medial superior frontal gyms, and left median cingulate gyms, three main findings arise from this study : 1) There is a strong bilateral involvement of the medial superior frontal gyms in all 4 volunteers : this area always located under the paracentral lobule could be the human equivalent of the monkeys SEF. 2) Two distinct areas are activated in the precentral gyms. A posterior area, located on the bank of the central sulcus, next to the hand region, could correspond to a purely motor component of SEM. The anterior area, located at the intersection of the precentral sulcus and of the superior frontal sulcus could correspond to a premotor component of SEM. 3) Activations were found in the inferior parietal lobule, but the loci are very variable among volunteers. These activations could partly result from the visual recall of the training targets (1). They could also correspond to reafference activation from the oculomotor command updating the visuo-spatial integration realized in this region. New experiments are in progress to further investigate these preliminary results. References 1. Berthoz et al., Soc Neurosci Abstr, 18: 214, 1992 2. Petit et al., J Neurophysiol, 69: 1009-1017, 1993 3.Anderson et al., Brain, 117: 1073-1084, 1994 4. Lang et al., NeuroReport, 5: 921-924, 1994