Functional brain imaging of olfactory processing in monkeys

As a step toward bridging the gap between human and animal studies of olfactory brain systems, we report results from an fMRI study of olfaction in squirrel monkeys. High-resolution fMRI images at 3 T with 1.25 x 1.25 x 1.2 mm(3) voxels were obtained covering the whole brain using an 8-cm-diameter birdcage coil and a gradient-echo spiral pulse sequence. Data were acquired from six sedated adult males using a standard block design. All fMRI data were spatially normalized to a common template and analyzed at the individual and group levels with statistical parametric and nonparametric methods. Robust odorant-induced activations were detected in several brain regions previously implicated in conscious human olfactory processing, including the orbitofrontal cortex, cerebellum, and piriform cortex. Consistent with human data, no stimulus intensity effects were observed in any of these regions. Average signal changes in these regions exceeded 0.6%, more than three times the expected signal change based on human fMRI studies of olfaction adjusting for differences in voxel size. These results demonstrate the feasibility of studying olfaction in sedated monkeys with imaging techniques commonly used at 3 T in humans and help promote direct comparisons between humans and nonhuman primates. Our findings, for example, provide novel support for the hypothesis that the cerebellum is involved in sensory acquisition. More broadly, this study suggests that olfactory processing in sedated monkeys and nonsedated humans shares similar neural substrates both within and beyond the primary olfactory system.