Nonlinear Asymmetric Blood Oxygenation Level Dependent Responses in Somatosensory Cortex.

Blood oxygenation level dependent (BOLD) responses in fMRI have previously been shown to be nonlinear with regard to changes in stimulus parameters, and as a result they may be asymmetric when comparing stimulus increases with decreases from an initial condition. We measured BOLD responses to varying vibrotactile stimuli of the hand digits in a monkey, including both increases and decreases in intensity and duration, relative to different levels of initial activation. Across variations in stimulus duration and intensity, positive and negative BOLD responses were asymmetric and nonlinear. Moreover, the asymmetry between positive and negative responses was manifest at different levels of baseline activation. The results confirm that the use of a common hemodynamic response function for increases and decreases in activity may underestimate the magnitude of decreases in activation. Electrophysiological recordings from multi-electrode arrays also revealed nonlinear and asymmetric features in multi-unit activities, linking neural firing properties to the nonlinear BOLD profiles.