Biomedical subjects

L Muckli

Publications and source records attributed to L Muckli.

3 recordsLinked to original sources

PubMed2006

Retinotopic effects during spatial audio-visual integration.

The successful integration of visual and auditory stimuli requires information about whether visual and auditory signals originate from corresponding places in the external world. Here we report crossmodal effects of spatially congruent and incongruent audio-visual (AV) stimulation. Visual and auditory stimuli were presented from one of four horizontal locations in external space. Seven healthy human subjects had to assess the spatial fit of a visual stimulus (i.e. a gray-scaled picture of a cartoon dog) and a simultaneously presented auditory stimulus (i.e. a barking sound). Functional magnetic resonance imaging (fMRI) revealed two distinct networks of cortical regions that processed preferentially either spatially congruent or spatially incongruent AV stimuli. Whereas earlier visual areas responded preferentially to incongruent AV stimulation, higher visual areas of the temporal and parietal cortex (left inferior temporal gyrus [ITG], right posterior superior temporal gyrus/sulcus [pSTG/STS], left intra-parietal sulcus [IPS]) and frontal regions (left pre-central gyrus [PreCG], left dorsolateral pre-frontal cortex [DLPFC]) responded preferentially to congruent AV stimulation. A position-resolved analysis revealed three robust cortical representations for each of the four visual stimulus locations in retinotopic visual regions corresponding to the representation of the horizontal meridian in area V1 and at the dorsal and ventral borders between areas V2 and V3. While these regions of interest (ROIs) did not show any significant effect of spatial congruency, we found subregions within ROIs in the right hemisphere that showed an incongruency effect (i.e. an increased fMRI signal during spatially incongruent compared to congruent AV stimulation). We interpret this finding as a correlate of spatially distributed recurrent feedback during mismatch processing: whenever a spatial mismatch is detected in multisensory regions (such as the IPS), processing resources are re-directed to low-level visual areas.

Acoustic Stimulation↗

PubMed2001

Sustained extrastriate cortical activation without visual awareness revealed by fMRI studies of hemianopic patients.

Patients with lesions in the primary visual cortex (V1) may show processing of visual stimuli presented in their field of cortical blindness even when they report being unaware of the stimuli. To elucidate the neuroanatomical basis of their residual visual functions, we used functional magnetic resonance imaging in two hemianopic patients, FS and GY. In the first experiment, a rotating spiral stimulus was used to assess the responsiveness of dorsal stream areas. Although no response was detectable within denervated or destroyed early visual cortex, motion-sensitive areas (hMT+/V5) ipsilateral to the lesion showed a strong sustained hemodynamic response. In GY, this activation was at least as strong as that of his contralesional hMT+/V5 to the stimulus in the normal hemifield. In the second experiment, coloured images of natural objects were used to assess the responsiveness of ventral stream areas. Again, no activity was detectable in ipsilesional early visual areas, but extrastriate areas in the lateral occipital cortex (hMT+/V5 and LO) and within the posterior fusiform gyrus (V4/V8) showed a robust sustained hemodynamic response. In both experiments, we observed that ipsilesional areas responded to stimuli presented in either hemifield, whereas the normal hemisphere responded preferentially to stimuli in the sighted hemifield. As only one subject occasionally noticed the onset of stimulation in the impaired field, the unexpectedly strong sustained activity in ipsilesional dorsal and ventral cortical areas appears to be insufficient to generate conscious vision.

Adult↗

PubMed1998

The constructive nature of vision: direct evidence from functional magnetic resonance imaging studies of apparent motion and motion imagery.

Echoplanar functional magnetic resonance imaging was used to monitor activation changes of brain areas while subjects viewed apparent motion stimuli and while they were engaged in motion imagery. Human cortical areas MT (V5) and MST were the first areas of the 'dorsal' processing stream which responded with a clear increase in signal intensity to apparent motion stimuli as compared with flickering control conditions. Apparent motion of figures defined by illusory contours evoked greater activation in V2 and MT/MST than appropriate control conditions. Several areas of the dorsal pathway (V3A, MT/MST, areas in the inferior and superior parietal lobule) as well as prefrontal areas including FEF and BA 9/46 responded strongly when subjects merely imagined moving stimuli which they had seen several seconds before. The activation during motion imagery increased with the synaptic distance of an area from V1 along the dorsal processing stream. Area MT/MST was selectively activated during motion imagery but not during a static imagery control condition. The comparison between the results obtained with objective motion, apparent motion and imagined motion provides further insights into a complex cortical network of motion-sensitive areas driven by bottom-up and top-down neural processes.

Adult↗