The effect of opioid receptor blockade on the neural processing of thermal stimuli.

The endogenous opioid system represents one of the principal systems in the modulation of pain. This has been demonstrated in studies of placebo analgesia and stress-induced analgesia, where anti-nociceptive activity triggered by pain itself or by cognitive states is blocked by opioid antagonists. The aim of this study was to characterize the effect of opioid receptor blockade on the physiological processing of painful thermal stimulation in the absence of cognitive manipulation. We therefore measured BOLD (blood oxygen level dependent) signal responses and intensity ratings to non-painful and painful thermal stimuli in a double-blind, cross-over design using the opioid receptor antagonist naloxone. On the behavioral level, we observed an increase in intensity ratings under naloxone due mainly to a difference in the non-painful stimuli. On the neural level, painful thermal stimulation was associated with a negative BOLD signal within the pregenual anterior cingulate cortex, and this deactivation was abolished by naloxone.

Activation of the opioidergic descending pain control system underlies placebo analgesia.

Placebo analgesia involves the endogenous opioid system, as administration of the opioid antagonist naloxone decreases placebo analgesia. To investigate the opioidergic mechanisms that underlie placebo analgesia, we combined naloxone administration with functional magnetic resonance imaging. Naloxone reduced both behavioral and neural placebo effects as well as placebo-induced responses in pain-modulatory cortical structures, such as the rostral anterior cingulate cortex (rACC). In a brainstem-specific analysis, we observed a similar naloxone modulation of placebo-induced responses in key structures of the descending pain control system, including the hypothalamus, the periaqueductal gray (PAG), and the rostral ventromedial medulla (RVM). Most importantly, naloxone abolished placebo-induced coupling between rACC and PAG, which predicted both neural and behavioral placebo effects as well as activation of the RVM. These findings show that opioidergic signaling in pain-modulating areas and the projections to downstream effectors of the descending pain control system are crucially important for placebo analgesia.

The predictive value of white matter organization in posterior parietal cortex for spatial visualization ability.

Humans differ substantially in their ability to imagine spatial transformations of novel stimuli (i.e., mental rotation). Whereas "high-spatial" individuals are able to maintain high-quality representations even after complex mental transformations, "low-spatial" individuals often experience substantial degradation of the initial representation. Even though subdivisions of the posterior parietal cortex are known to instantiate the necessary spatial transformations, a direct demonstration of neuroanatomical differences predicting this behavioral variability is currently missing. Because recent evidence suggests that interindividual differences on the behavioral level might be related to regionally specific white matter organization, we addressed this question using diffusion tensor imaging in combination with well-established psychometric tests. As expected, behavioral results revealed a substantial disparity in mental rotation performance. Most importantly, despite controlling for differences in spatial short-term memory capacity, we observed a tight relationship between mental rotation proficiency and white matter organization near the anterior part of the intraparietal sulcus. Whereas high-level proficiency was paralleled by high fractional anisotropy (FA) values, the opposite pattern was observed in "low spatials". The present results strongly indicate that the efficiency of information transfer between posterior parietal regions involved in the mental transformation process could be one decisive factor in individual spatial visualization proficiency.

Changes in connectivity profiles as a mechanism for strategic control over interfering subliminal information.

Human behavior can be influenced by information that is not consciously perceived. Recent behavioral and electrophysiological evidence suggests, however, that the processing of subliminal stimuli is not completely beyond an observer's conscious control. The present study aimed to characterize the cortical network that implements strategic control over interfering subliminal information at multiple stages. Fourteen participants underwent functional magnetic resonance imaging (fMRI) scanning while performing a metacontrast masking paradigm. We systematically varied the amount of conflicting versus non-conflicting trials across experimental blocks, and behavioral performance demonstrated strategic effects whenever a high proportion of subliminal prime stimuli induced response competition. A psychophysiological interaction analysis revealed the pre-supplementary motor area (pre-SMA) to exhibit context-dependent covariation with activation in the lateral occipital complex (LOC) and the putamen. The pre-SMA thereby appears to fulfill a superordinate function in the control of processing subliminal information by simultaneously modulating perceptual analysis and motor selection.