Biomedical subjects

Oliver Zafiris

Publications and source records attributed to Oliver Zafiris.

3 recordsLinked to original sources

PubMed2004

Neural mechanism underlying impaired visual judgement in the dysmetabolic brain: an fMRI study.

An altered brain metabolism in the parietal and prefrontal regions of the cerebral cortex as well as cognitive alterations have been found in patients suffering from hepatic encephalopathy. The neural mechanisms underlying these metabolically induced cognitive alterations, however, are not known. Since patients with liver cirrhosis without clinically overt encephalopathy already show an impaired performance in a flicker light test, the aim of this study was to analyze the normal and pathologically impaired neural mechanisms of these patients using functional magnetic resonance imaging (fMRI). Nine subjects at the early stage of encephalopathy [nonmanifest hepatic encephalopathy (nmHE)] and ten controls underwent scanning, while they indicated the apparent transition from a steady light to the onset of a flicker light, that is, the critical flicker frequency (CFF). Judgement-related blood oxygenation level-dependent (BOLD) activation was decreased in nmHE compared to controls in the right inferior parietal cortex (IPL). Furthermore, the analysis of psychophysiologic interaction suggests impaired neural interaction in patients with nmHE, especially between the IPL and the parietooccipital cortex (Poc), the intraparietal sulcus, the anterior cingulate cortex (ACC), the right prefrontal cortex (PFC), the medial temporal lobe, and the extrastriate cortex V5. In contrast, nonmanifest patients revealed an enhanced coupling between IPL and the postcentral cortex. Our findings provide evidence of an early-impaired and compensatory neural mechanism during visual judgement already in the earliest stages of hepatic encephalopathy and suggest an aberrant coupling between cerebral regions in the dysmetabolic brain.

Brain↗

PubMed2003

Functional anatomy and differential time courses of neural processing for explicit, inferred, and illusory contours. An event-related fMRI study.

The perception of shape does not necessarily require viewing an explicit outline figure. Using event-related functional magnetic resonance imaging we examined the time courses of neural activations provoked by shapes defined by (1) lines, (2) illusory contour inducers, and (3) reversed inducers. SPM99 was used to analyze the common and differential neural responses associated with the stimuli and their temporal derivatives. Illusory figures versus reversed inducers activated extrastriate cortex. Reversed inducers versus illusory figures activated the right parietal cortex. For both illusory and line contours versus reversed inducers, analysis of the temporal derivatives showed earlier activations in extrastriate and left parietal cortex and for line contours also in the extrastriate cortex bilaterally and in the right parietal cortex; these earlier activations were mirrored by differences in reaction times with subjects responding more slowly to shapes defined by reversed inducers. The results show substantial bottom-up effects (in occipital cortex) in the recognition of illusory and explicit shapes. By contrast, in stimuli where the shape must be inferred, there is greater reliance on right parietal cortex, consistent with increased attentional demands and top-down processing. The temporal derivatives provide useful information on the differential timing of the associated hemodynamic responses in occipital, parietal, and motor cortex.

Adult↗

PubMed2002

Posterior parietal cortex is implicated in continuous switching between verbal fluency tasks: an fMRI study with clinical implications.

We investigated whether posterior parietal cortex controls attentional switching when the tasks involve neither spatial nor visual cognition. Normal volunteers were scanned using functional MRI (fMRI). In all conditions, subjects were required to covertly produce words in verbal fluency tasks. They did so at a rate of one every 2 s (with eyes closed) in response to an auditory beep. In the non-switching (NS) trials, subjects responded with a series of items from a prespecified semantic category (SC) (e.g. fruits or cars) and from overlearned sequences (OSs) (days of the week, months of the year or letters of the alphabet). Instructions as to which category items should be drawn from on a given run of trials were presented over fMRI-compatible earphones prior to each run. In the switching (S) trials, subjects produced a series of word triads from three SCs: for example, fruits, cars and furniture (e.g. pear, Mercedes, table.); and from three OSs: days of the week, months of the year and letters of the alphabet (e.g. Monday, January, A.). This design is factorial, with the factors verbal class (SC or OSs) and switching conditions (S or NS). Increases in neural activity (P < 0.05, corrected for multiple comparisons) were observed only in superior posterior parietal cortex bilaterally as a main effect of the S conditions compared with the NS conditions. When SC fluency was compared with OS fluency, significant activations were found in anterior cingulate cortex bilaterally, the left inferior frontal gyrus, the middle frontal gyrus bilaterally, frontal operculum bilaterally and in the cerebellar vermis. These results support the hypothesis that superior posterior parietal cortex is a supramodal area implicated in task switching, even when no visual or spatial component is implicated in the tasks. Task switching, frequently used to examine 'frontal' executive functions, may also be clinically relevant to the assessment of patients with superior posterior parietal lesions.

Adult↗