The contributions of transient and sustained responses to audio visual integration of dynamic information

Transient and sustained responses have been shown to play distinct functional roles in auditory processing: Transient responses may subserve rapid stimulus detection, sustained responses contribute to a more detailed sound characterization. While numerous fMRI studies have reported audiovisual interactions at multiple levels of the cortical hierarchy, they were not able to dissociate transient and sustained responses. This fMRI study optimized the design to disentangle the contributions of sustained, onset and offset responses to superadditive and subadditive interactions and localize the effects within the visual and auditory processing hierarchies. Seventeen subjects participated in this fMRI study (Siemens TimTrio 3T scanner, GE-EPI, TE = 40 ms, 38 axial slices, TR = 3.08 s). While engaged in a target detection task, they were presented with 1 s, 10 s, 20 s, 30 s blocks of (i) video clips of an expanding radial star-field, (ii) auditory pink noise or (iii) both. The velocity of the star-field and the sound amplitude were jointly modulated according to 0.1 Hz sine-wave function. The regressors of the general linear model were formed by convolving (i) delta functions encoding the onset and offset of each block and (ii) box car functions adjusted for block length with the hemodynamic response functions. Blocks of 1 s duration were modeled only as onsets. In addition, the model included targets and parametric modulators encoding the amplitude / velocity modulation. To allow for a random-effects analysis (SPM5), contrast images for each subject were entered into second level one sample t-tests. We tested for superadditive and subadditive interactions separately for onset, offset and sustained block responses. Results are reported at p