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Schlagwörter:
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Zusammenfassung:
Introduction:
Accumulating evidence suggests that audiovisual interactions are not deferred to association areas, but start already at the primary cortical level. Moreover, visual stimuli induce fMRI deactivations in primary auditory areas as well as activations in the visual system, and vice versa for auditory stimuli. It is unknown whether these crossmodal deactivations are caused by top-down effects from association areas reflecting the withdrawal of attentional resources from the non-stimulated modality, or via direct connections between sensory areas. Previous neurophysiological studies in animals have demonstrated that visual and auditory stimuli induce activations in auditory cortices with distinct layer-dependent profiles (feedforward/granular vs. backward/infra+supragranular).
This study used 7T fMRI to resolve BOLD responses at different cortical depths and characterize the effects of sensory stimulation and modality-specific attention on the cortical layer-specific activation profiles.
Methods:
Thirteen participants took part in this fMRI experiment at 7T. In a 3 (stimulation modality) X 2 (modality-specific attention) design, they were presented with 30-second blocks of visual (concentric looming white circles on black background), auditory (looming frequency-modulated pure tones) or audio-visual stimuli. They attended and responded to either visual or auditory targets.
For anatomical reference, we acquired a whole-brain T1 map with a MP2RAGE sequence (spatial resolution: (0.7 mm)3; TR=5000 ms; TE=2.45 ms; TI1/2=900 ms/2750 ms; FA1/2=5°/3°; iPAT=2). During the functional experiments, we acquired 46 axial EPI slices with an axial coverage of 3.6 cm (spatial resolution: (0.75 mm)3; TR=3000 ms; TE=25 ms; FA=90; iPAT=4).
For cortical depth-dependent analysis, the cortex was segmented, upsampled to a (0.4 mm)3 resolution and automatically contoured into 6 laminae using the CBS Tools and MIPAV (Waehnert 2013).
Using SPM, the functional data were realigned, unwarped, coregistered to the up-sampled whole-brain T1 map and regridded to (0.4 mm)3 resolution. The first level subject-specific GLM analysis independently modelled the 3 blocks of the 6 conditions and the target events (i.e. 3*6+1=19 regressors). The parameter estimates for 6 conditions x 3 blocks x 4 sessions were averaged within 6 layers for each ROI. For each condition, the 6 layer-specific parameter estimates x 3 blocks x 4 sessions were entered into a second level subject-specific GLM that modelled the cortical profile across the 6 layers by a constant, linear and quadratic function across layers. The parameter estimates for the constant, linear and quadratic were entered in independent one sample t-tests at the third random effects level.
Results:
Behavioural results:
80 accuracy for target detection indicating that participant maintained modality-specific attention
fMRI results:
Auditory stimulation induced sustained deactivations in visual cortices that were most pronounced at superficial layers (i.e. significant linear effect in V1-5). No significant cross-modal deactivations were observed in A1.
Super-additive audio-visual interactions (i.e. AV≠A+V) were maximal at the cortical layer nearest the surface, in visual areas V1-V5 (i.e. significant linear effect in V1-5)
Visual attention, when compared with auditory attention, reduced activations in V1-2 similarly across all layers, and increased activations in visual areas V3-5, predominantly in the more superficial cortical layers (i.e. significant linear effect in V4v).
Conclusions:
Our results demonstrate that sensory BOLD activations and cross-modal deactivations are largest near the cortical surface. Likewise, audio-visual interactions are observed predominantly in superficial layers. Surprisingly, increased visual attention reduced visual-evoked BOLD activation in V1 and V2, irrespective of cortical depth, but produced increased activation in V4v, predominantly at the cortical surface. This may indicate distinct attentional mechanisms.
