Christian Scheiber

Two Cortical Systems for Reaching in Central and Peripheral Vision

Parietal lesions in humans can produce a specific disruption of visually guided hand movement, termed optic ataxia. The fact that the deficit mainly occurs in peripheral vision suggests that reaching in foveal and extrafoveal vision rely on two different neural substrates. In the present study, we have directly tested this hypothesis by event-related fMRI in healthy subjects. Brain activity was measured when participants reached toward central or peripheral visual targets. Our results confirm the existence of two systems, differently modulated by the two conditions. Reaching in central vision involved a restricted network including the medial intraparietal sulcus (mIPS) and the caudal part of the dorsal premotor cortex (PMd). Reaching in peripheral vision activated in addition the parieto-occipital junction (POJ) and a more rostral part of PMd. These results show that reaching to the peripheral visual field engages a more extensive cortical network than reaching to the central visual field.

Auditory motion perception activates visual motion areas in early blind subjects.

We have previously shown that some visual motion areas can be specifically recruited by auditory motion processing in blindfolded sighted subjects [Poirier, C., Collignon, O., De Volder, A.G., Renier, L., Vanlierde, A., Tranduy, D., Scheiber, C., 2005. Specific activation of V5 brain area by auditory motion processing: an fMRI study. Brain Res. Cogn. Brain Res. 25, 650-658]. The present fMRI study investigated whether auditory motion processing may recruit the same brain areas in early blind subjects. The task consisted of simultaneously determining both the nature of a sound stimulus (pure tone or complex sound) and the presence or absence of its movement. When a movement was present, blind subjects had to identify its direction. Auditory motion processing, as compared to static sound processing, activated the brain network of auditory and visual motion processing classically observed in sighted subjects. Accordingly, brain areas previously considered as specific to visual motion processing could be specifically recruited in blind people by motion stimuli presented through the auditory modality. This indicates that the occipital cortex of blind people could be organized in a modular way, as in sighted people. The similarity of these results with those we previously observed in sighted subjects suggests that occipital recruitment in blind people could be mediated by the same anatomical connections as in sighted subjects.

Pure retrograde amnesia following a mild head trauma: a neuropsychological and metabolic study.

After a minor closed head injury, a 33-year-old man acquired extensive retrograde amnesia (RA) covering the previous ten years and concerning autobiographical, semantic and procedural memories. The patients learning abilities remained excellent and he recovered considerable information from his wife, the media and personal documents. This relearned information did not, however, provide a sense of personal experience in the first weeks. CT and MRI failed to show brain damage, but EEG and SPECT examination showed a marked right temporal dysfunction. After three months the patient had almost completely recovered from RA. Interestingly, a parallel recovery was observed in the second SPECT obtained at this period. There was clearly a blockade of retrieval, while the stored engrams were probably intact. The mechanisms underlying such a functional amnesia are discussed in the light of previous reports of amnesia without brain lesions.

What neuroimaging tells us about sensory substitution.

A major question in the field of sensory substitution concerns the nature of the perception generated by sensory substitution prostheses. Is the perception determined by the nature of the substitutive modality or is it determined by the nature of the information transmitted by the device? Is it a totally new, amodal, perception? This paper reviews the recent neuroimaging studies which have investigated the neural bases of sensory substitution. The detailed analysis of available results led us to propose a general scheme of the neural mechanisms underlying sensory substitution. Two different main processes may be responsible for the visual area recruitment observed in the different studies: cross-modality and mental (visual) imagery. Based on our results analysis, we propose that cross-modality is the predominant process in early blind subjects whereas mental imagery is predominant in blindfolded sighted subjects. This model implies that, with training, sensory substitution mainly induces visual-like perception in sighted subjects and mainly auditory or tactile perception in blind subjects. This framework leads us to make some predictions that could easily be tested.

The neural bases of the constructive nature of autobiographical memories studied with a self-paced fMRI design.

In Conway and Pleydell-Pearces model (2000), autobiographical memories are viewed as transitory mental representations, more often generated in an effortful way. An important claim of the model concerns the dynamic process that evolves over time, from the left prefrontal areas to posterior regions, to retrieve specific memories. The present work aims at investigating, using fMRI, the temporal distribution of effortful autobiographical memory construction. In addition, a self-paced design was implemented to elucidate the question of the timing window required to evoke recollections. The results showed a large pattern of brain regions, which included the two major poles of activation predicted by Conway and Pleydell-Pearces model. Likewise, we were able to detect the earlier implication of the left dorso-lateral prefrontal cortex, by comparison with posterior structures, which seemed to confirm its involvement in the effortful retrieval process. Finally, the self-paced procedure allowed us to refine the timing window necessary to construct past events.

Pattern recognition using a device substituting audition for vision in blindfolded sighted subjects.

A major question in the field of sensory substitution concerns the nature of the perception generated by sensory substitution devices. In the present fMRI study, we investigated the neural substrates of pattern recognition through a device substituting audition for vision in blindfolded sighted subjects, before and after a short training period. Before training, pattern recognition recruited dorsal and ventral extra-striate areas. After training, the recruitment of these visual areas was found to have increased. These results suggest that visual imagery processes could be involved in pattern recognition and that perception through the substitution device could be visual-like.

Neural substrates of animal mental imagery: calcarine sulcus and dorsal pathway involvement: an fMRI study

Neural response was measured using fMRI in six healthy volunteers, performing a mental imagery task, using verbal cues exclusively. They listened to a list of animal names from which to generate a mental image, and listened passively to a list of abstract words. They were tested twice, using the same protocol. SPM99-processed results showed for both sessions activation in the calcarine sulcus and local activation foci, mainly in the occipito-parietal region. Other studies involving figurative mental imagery using verbal cues, have shown activation in the occipito-temporal area, but none in the calcarine sulcus or in the dorsal route. We account for the discrepancies relative to previous mental imagery studies using verbal cues, in terms of differences in the experimental conditions. In our opinion, restricting the stimuli to a single semantic category (animals) and increasing the time dedicated to the production of MI, may have enhanced the components of the pictures. This mental imagery generation protocol shows the importance of the design of experimental tasks on anatomo-functional responses.

Material-independent cerebral network of re-experiencing personal events: evidence from two parallel fMRI experiments.

Two parallel fMRI experiments were conducted with the aim to clarify the lateralisation issue of the cerebral network underlying autobiographical memory retrieval independently of the stimulus material and the refreshment of the memory trace. The verbal experiment required a pre-scanning interview, while the nonverbal version tested the subjects directly during the fMRI session. Both experiments were constructed using the same experimental design to eliminate methodological variables in order to render comparisons possible. We found a predominantly left-lateralised cerebral network independently of material and regardless of whether or not memory traces were reactivated prior to the scanning session. We discuss the results in the context of neuroimaging studies of autobiographical memory (AbM).

Auditory motion processing in early blind subjects

Although the neural substrates of visual motion processing have been extensively researched for several decades (for a review, see Culham et al. 2001), little is known about auditory motion processing. The few neuroimaging studies investigating auditory motion show the involvement of inferior and posterior parietal lobules, the dorsal and ventral pre-motor cortex (Bremmer et al. 2001; Griffiths et al. 1998, 2000; Lewis et al. 2000) and occasionally the additional involvement of the planum temporale (Baumgart et al. 1999; Bremmer et al. 2001; Lewis et al. 2000; Warren et al. 2002). These studies were conducted only on sighted subjects. In blind people, further functional neuroimaging has shown the involvement of striate and extra-striate cortices during Braille reading (Buchel 1998; Burton et al. 2002a; Sadato et al. 1996, 1998) and during language tasks (Burton et al. 2002b, 2003; Roder et al. 2002). However, fewer studies have investigated simpler auditory tasks (Alho et al. 1993; Kujala et al. 1995; Leclerc et al. 2000; Weeks et al. 2000). The present study investigated whether auditory motion perception could induce an occipital activation in early blind subjects (but not in sighted controls), indicating the developmental cross-modal reorganisation of their occipital cortex. Six early blind subjects and six blindfolded sighted controls matched in gender and age and who gave their informed consent participated in the study. The fMRI data were acquired by a 2 Tesla Brucker Imager. The experimental protocol was divided into 40 blocks each lasting 24 s, which were distributed over two sessions. The repetition time was 4.8 s, which corresponded to five scans per block. Two active conditions (one condition per block) were recorded with a rest period inbetween. The first condition was a fixed stimuli condition and the second one a motion stimuli condition. Auditory stimuli were composed of pure tones, or complex sounds, either fixed at various positions around the subject (fixed stimuli condition), or animated by a transverse movement in the horizontal plane (motion stimuli condition). In both conditions the auditory stimuli were composed of 10% pure tones (one sine wave) and 90% complex sounds (six sine waves). The presence of two different kinds of sound (pure tones and complex sounds) within each block allowed auditory recognition by the subjects both in the fixed stimuli and in the motion stimuli conditions. After having heard an individual stimulus, subjects were requested to determine the nature (i.e., ‘‘is it a pure tone or a complex sound ?‘’) and detect any movement of the sound. If movement was detected, the subjects were asked to determine the direction of the movement (i.e. from right to left ear or the reverse). Their answer was given by pressing buttons held in each hand. When the stimulus was identified as a fixed pure tone or as a pure tone moving towards the right, subjects had to press the button in their right hand. When the stimulus was identified as a fixed complex sound or as a complex sound moving towards the left, subjects had to press the button in their left hand. In all other cases (i.e. pure tone moving towards the left or complex sound moving towards the right), subjects had to press both buttons simultaneously. All subjects underwent a training period to learn this system before taking part in the study. Within a block, the same stimulus was repeated until subjects gave an answer (with a maximum of three repetitions); thereafter the next stimulus was given. Behavioural results showed no significant difference in the percentage of correct responses between early C. Poirier AE O. Collignon AE A. De Volder (&) Neural Rehabilitation Engineering Laboratory, Universite Catholique de Louvain, Ave. Hippocrate, 54, UCL-54.46, 1200 Brussels, Belgium E-mail: devolder@gren.ucl.ac.be Tel.: +32-2-7645482 Fax: +32-2-7649422

Perceptual encoding of fingerspelled and printed alphabet by deaf signers: An fMRI study

We measured brain activation during the perception of fingerspelled letters, printed letters, and abstract shapes (control condition) in six congenitally, profoundly deaf signers and six normal hearing subjects. Normal hearing subjects showed essentially extrastriate activation in the fingerspelled letters and printed letters conditions whereas deaf subjects showed activation of a broader network in printed letters and fingerspelled conditions, comprising supplementary frontal and posterior areas, and the supramarginal gyrus (Brodmann Area 6). These results suggest that, on one hand, different cerebral areas in deaf and hearing subjects mediate processing of printed letters and, on the other hand, common cerebral areas are activated in deaf signers when they are engaged in processing fingerspelled or printed letters.