Biomedical subjects

Thomas Stephan

Publications and source records attributed to Thomas Stephan.

21 records · Page 2Linked to original sources

PubMed2002

Sensory system interactions during simultaneous vestibular and visual stimulation in PET.

The patterns of regional cerebral blood flow (rCBF) increases and decreases in PET were compared for unimodal vestibular, unimodal visual, and for simultaneous vestibular and visual stimulation. Thirteen healthy volunteers were exposed to a) caloric vestibular stimulation, b) small-field visual motion stimulation in roll, c) simultaneous caloric vestibular and visual pattern stimulation. Unimodal vestibular stimulation led to activations of vestibular cortex areas, in particular the parieto-insular vestibular cortex (PIVC), and concurrent deactivations of visual cortical areas [Brodmann area (BA) 17-19]. Unimodal visual motion stimulation led to activations of the striate visual cortex and the motion-sensitive area in the middle temporal/middle occipital gyri (BA 19/37) with concurrent deactivations in the PIVC. Simultaneous bimodal stimulation resulted in activations of the cortical representation of both sensory modalities. In the latter condition activations and deactivations were significantly smaller compared to unimodal stimulation. The findings are consistent with the concept of an inhibitory reciprocal vestibulo-visual interaction in all three stimulus conditions.

Adult↗

PubMed2002

Lid closure mimics head movement in fMRI.

Motion correction algorithms are commonly used to correct for involuntary head motions in fMRI. We found that lid closure increased signal intensity of the eyes by a factor of 1.6 to 2 in EPI-fMRI. If the eyes were included in the scanning volume, transitions from eyes open to eyes closed in darkness were regularly associated with erroneously detected head translations along the z-axis and head rotations around the x-axis in Talairach's coordinate system. These artefacts could be avoided by masking the eyes in the images during the estimation of movement parameters.

Adult↗

PubMed2002

Visual-vestibular and visuovisual cortical interaction: new insights from fMRI and pet.

PET and fMRI studies have revealed that excitation of the vestibular system by caloric or galvanic stimulation not only activates the parietoinsular vestibular cortex but also bilaterally deactivates the occipital visual cortex. Likewise, visual motion stimulation not only activates the visual cortex but also deactivates the parietoinsular vestibular cortex. These findings are functionally consistent with the hypothesis of an inhibitory reciprocal visual-vestibular interaction for spatial orientation and motion perception. Transcallosal visuovisual interaction between the two hemispheres was found by using half-field visual motion stimulation: activation of motion-sensitive areas hMT/V5 and deactivations of the primary visual cortex contralateral to the stimulated hemisphere. The functional significance of these inter- and intra-sensory interactions could be that they (A) allow a shift of the sensorial weight between two incongruent sensory inputs and (B) ensure a correspondence of the two hemispheres during evaluation of contradictory motion stimulation of the right and left hemifields. In terms of mathematical modeling, these findings may reflect the concepts of a sensory conflict mechanism or a mismatch between expected and actual sensory input.

Animals↗