Positron emission tomography (PET) was used to investigate the functional anatomy of the foveal fixation system in 10 subjects scanned under three different conditions: at rest (REST), during the fixation of a central point (FIX), and while fixating the same foveal target during the presentation of peripheral visual distractors (DIS). Compared with the REST condition, both FIX and DIS tasks activated a common set of cortical areas. First, in addition to the involvement of the occipital visual cortex, both the frontal eye field (FEF) and the intraparietal sulcus (IPS) were bilaterally activated. Right frontal activation was also found in the dorsolateral prefrontal cortex, the inferior part of the precentral gyrus, and the inferior frontal gyrus. These results suggest that both FEF and IPS may constitute the main cortical regions subserving bilaterally the foveal fixation system in humans. The remaining right frontal activations may be considered as part of the anterior attentional network, supporting a role for the right frontal lobe in the allocation of the attentional mechanisms. Compared with the FIX condition, the DIS task also revealed the perceptual and cognitive processes related to the presence of peripheral visual distractors during foveal fixation. In addition to a bilateral activation of the V5/MT motion-sensitive area, a right FEF-IPS network was activated which may correspond to the engagement of the visuospatial attention. Finally, normalized regional cerebral blood flow (NrCBF) decreases were also observed during both DIS and FIX condition performance. Such NrCBF decreases were centered in the superior and middle temporal gyri, the prefrontal cortex, and the precuneus and the posterior retrosplenial part of the cingulate gyrus.

Case reportThe patient was a white male aged 10 years at the time of the first metabolic study. Early psychomotor development was uneventful, with an apparently normal acquisition of language. A progressive decline of verbal fluency was first noted at age five, and complete mutism became established in a few months. This severe aphasia was associated with verbal auditory agnosia, aggressive behaviour, hyperkinesis, and clumsiness. During this period at least one atypical absence with palpebral myoclonic jerks was seen. The child was considered to be psychotic, however, and remained unmedicated. A CT scan, performed at age seven, revealed the presence of an extraparenchymal cystic mass containing fluid in the left middle cranial fossa, extending over the cerebral convexity to the frontoparietal region. Although the portion of the skull covering the cyst was bulging and thinned, there was no distortion of the midline, nor apparent displacement of adjacent brain structures and the collection was considered to be a benign arachnoid cyst. Studies by EEG disclosed bilateral multifocal spike and spike-wave complexes, with a slight predominance over the left temporal region.

No abstract

Visuomotor tasks elicit neuronal activity in primate motor areas at relatively short latencies. Although this early activity embodies features of visual responses (short latency, stimulusdependency), its sensory nature has been questioned. We investigated neural correlates of visuomotor performance in human motor areas using scalp and intracranial event-related potential measures. A simple visuomanual reaction-time task evoked early potentials at 133±145 ms post-stimulus which occurred much earlier than the motor potentials of the same region. The amplitude of the early potentials covaried with stimulus location and was independent of parameters of the motor response. Because of their timing, stimulus-dependency and characteristics of our behavioral task, the early potentials are suggested to re¯ect neuronal responses of sensory nature rather than processing related to pure motor aspects of the task.

No abstract