Behavioural correlates of negative BOLD signal changes in the somatosensory system

Background and Purpose: Functional magnetic resonance imaging (fMRI) hypothesis testing based on the blood oxygenation level dependent (BOLD) contrast mechanism typically involves a search for a positive effect during a specific task relative to a control state. However, aside from positive BOLD signal changes there is converging evidence that neuronal responses within various cortical areas also induce negative BOLD signals. Although it is commonly believed that these negative BOLD signal changes reflect suppression of neuronal activity direct evidence for this assumption is sparse. Since the somatosensory system offers the opportunity to quantitatively test sensory function during concomitant activation and has been well-characterized with fMRI in the past, the aim of this study was to determine the functional significance of ipsilateral negative BOLD changes during unilateral sensory stimulation. Methods: We recorded BOLD responses in the somatosensory system during unilateral electrical median nerve stimulation in 12 healthy, right-handed volunteers (9 males, 3 females, mean age±SD 27.6±4.4 years, range 22–36 years). To measure sensory function quantitatively during concomitant electrical median nerve stimulation, the perceptual threshold to electrical stimulation of the contralateral index finger (current perception threshold, CPT) was tested. Results: During electrical stimulation of either the right or left median nerve all subjects demonstrated significant increases in BOLD signal intensities within the primary and bilateral secondary somatosensory areas. In contrast to these areas with positive BOLD signal changes a decreased BOLD signal was observed in the ipsilateral primary somatosensory cortex. Before stimulation mean CPT values were 1.76±0.32 mA on the right side and 1.83±0.50 mA on the left side. During electrical median nerve stimulation ipsilateral CPT values significantly increased on both sides (2.1±0.38 mA on the right side and 2.22±0.44 mA on the left side, p