Validation of periodic fMRI signals in response to wearable tactile stimulation

Abstract

To map cortical representations of the body, we recently developed a wearable technology for automatic tactile stimulation in human functional magnetic resonance imaging (fMRI) experiments. In a two-condition block design experiment, air puffs were delivered to the face and hands periodically. Surface-based regions of interest (S-ROIs) were initially identified by thresholding a linear statistical measure of signal-to-noise ratio of periodic response. Across subjects, S-ROIs were found in the frontal, primary sensorimotor, posterior parietal, insular, temporal, cingulate, and occipital cortices. To validate and differentiate these S-ROIs, we develop a measure of temporal stability of response based on the assumption that a periodic stimulation evokes stable (low-variance) periodic fMRI signals throughout the entire scan. Toward this end, we apply time-frequency analysis to fMRI time series and use circular statistics to characterize the distribution of phase angles for data selection. We then assess the temporal variability of a periodic signal by measuring the path length of its trajectory in the complex plane. Both within and outside the primary sensorimotor cortex, S-ROIs with high temporal variability and deviant phase angles are rejected. A surface-based probabilistic group-average map is constructed for spatial screening of S-ROIs with low to moderate temporal variability in non-sensorimotor regions. Areas commonly activated across subjects are also summarized in the group-average map. In summary, this study demonstrates that analyzing temporal characteristics of the entire fMRI time series is essential for second-level selection and interpretation of S-ROIs initially defined by an overall linear statistical measure.

Mapping the complex topological organization of the human parietal face area

“…Future high-resolution mapping studies of eccentricity representation in the parietal face area using wide-field phase-encoded visual stimuli will be needed to refine these observations. Finally, two-dimensional somatotopic mapping of the face using high-density tactile stimuli delivered via a wearable grid will be required to determine how visual eccentricity relates to face somatotopic ‘eccentricity’ in the parietal face area in future studies (Chen et al, 2017; Huang and Sereno, 2010; Huang et al, 2012; Moulton et al, 2009; Sereno and Huang, 2010). …”

“…To prevent occlusion and clear near-face space for a close-up screen, phase-encoded tactile stimuli (air puffs) were delivered to 12 locations around the face via plastic tubes embedded on a wearable mask custom molded for each subject (Fig. 1B and C; Chen et al, 2017; Huang et al, 2012). Each mask was made of X-Lite thermoplastic sheets with a grid of −0.5×0.5 cm.…”

“…The distribution of signal phase angles within each sROI is characterized using circular statistics (Chen et al, 2017). For a vertex v in an sROI r with V vertices corresponding to stimulus modality s , the phase angle, θ v,s , was first converted to the corresponding complex number, z v,s = exp( jθ v,s ).…”

“…While it is straightforward to acquire functional images at a higher spatial resolution (e.g., 2×2×2 mm or smaller voxels), the signal-to-noise ratio (SNR) in periodic fMRI signals drops significantly with smaller voxels when using standard head coils in 3 Tesla MRI scanners (Hoffmann et al, 2009). In phase-encoded mapping experiments, successful reconstruction of detailed retinotopic or somatotopic maps depends on precise estimation of the phase angles of periodic signals with high SNR in individual voxels (Chen et al, 2017). This makes mapping in small, higher-level cortical regions particularly challenging because the SNR there is generally lower than that in the early unisensory areas.…”

“…In the first experiment, an MR-compatible tactile stimulation device (the 12-channel air-puff “Dodecapus” manifold in Huang and Sereno, 2007) was mounted above the surface coils to deliver phase-encoded tactile stimuli around the face. In the second experiment, phase-encoded looming visual stimuli were projected onto a wide-field direct-view screen while phase-encoded tactile stimuli were delivered to the face via a wearable mask containing built-in air tubes and nozzles (Chen et al, 2017; Huang et al, 2012). These experiments clearly revealed one and often two or more subdivisions within the parietal face area across hemispheres and subjects.…”