In this study, the representation of taste in the orbitofrontal cortex was investigated to determine whether or not a pleasant and an aversive taste have distinct or overlapping representations in this region. The pleasant stimulus used was sweet taste (1 M glucose), and the unpleasant stimulus was salt taste (0.1 M NaCl). We used an on/off block design in a 3T fMRI scanner with a tasteless solution delivered in the offperiod to control for somatosensory or swallowing-related effects. It was found that parts of the orbitofrontal cortex were activated (P < 0.005 corrected) by glucose (in 6/7 subjects) and by salt (in 6/7 subjects). In the group analysis, separate areas of the orbitofrontal cortex were found to be activated by pleasant and aversive tastes. The involvement of the amygdala in the representation of pleasant as well as aversive tastes was also investigated. The amygdala was activated (region of interest analysis, P< 0.025 corrected) by the pleasant taste of glucose (5/7 subjects) as well as by the aversive taste of salt (4/7 subjects). Activation by both stimuli was also found in the frontal opercular/insular (primary) taste cortex. We conclude that the orbitofrontal cortex is involved in processing tastes that have both positive and negative affective valence and that different areas of the orbitofrontal cortex may be activated by pleasant and unpleasant tastes. We also conclude that the amygdala is activated not only by an affectively unpleasant taste, but also by a taste that is affectively pleasant, thus providing evidence that the amygdala is involved in effects produced by positively affective as well as by negatively affective stimuli.

The cortical areas that represent affectively positive and negative aspects of touch were investigated using functional magnetic resonance imaging (fMRI) by comparing activations produced by pleasant touch, painful touch produced by a stylus, and neutral touch, to the left hand. It was found that regions of the orbitofrontal cortex were activated more by pleasant touch and by painful stimuli than by neutral touch and that different areas of the orbitofrontal cortex were activated by the pleasant and painful touches. The orbitofrontal cortex activation was related to the affective aspects of the touch, in that the somatosensory cortex (SI) was less activated by the pleasant and painful stimuli than by the neutral stimuli. This dissociation was highly significant for both the pleasant touch (P < 0.006) and for the painful stimulus (P < 0.02). Further, it was found that a rostral part of the anterior cingulate cortex was activated by the pleasant stimulus and that a more posterior and dorsal part was activated by the painful stimulus. Regions of the somatosensory cortex, including SI and part of SII in the mid-insula, were activated more by the neutral touch than by the pleasant and painful stimuli. Part of the posterior insula was activated only in the pain condition and different parts of the brainstem, including the central grey, were activated in the pain, pleasant and neutral touch conditions. The results provide evidence that different areas of the human orbitofrontal cortex are involved in representing both pleasant touch and pain, and that dissociable parts of the cingulate cortex are involved in representing pleasant touch and pain.

When a food is eaten to satiety, its reward value decreases. This decrease is usually greater for the food eaten to satiety than for other foods, an effect termed sensory-specific satiety. In an fMRI investigation it was shown that for a region of the orbitofrontal cortex the activation produced by the odour of the food eaten to satiety decreased, whereas there was no similar decrease for the odour of a food not eaten in the meal. This effect was shown both by a voxel-wise SPM contrast (p < 0.05 corrected) and an ANOVA performed on the mean percentage change in BOLD signal in the identified clusters of voxels (p < 0.006). These results show that activation of a region of the human orbitofrontal cortex is related to olfactory sensory-specific satiety.

ALTHOUGH there has been much investigation of brain pathways involved in pain, little is known about the brain mechanisms involved in processing somatosensory stimuli which feel pleasant. Employing fMRI it was shown that pleasant touch to the hand with velvet produced stronger activation of the orbitofrontal cortex than affectively neutral touch of the hand with wood. In contrast, the affectively neutral but more intense touch produced more activation of the primary somatosensory cortex than the pleasant stimulus. This indicates that part of the orbitofrontal cortex is concerned with representing the positively affective aspects of somatosensory stimuli, and in further experiments it was shown that this orbitofrontal area is different from that activated by taste and smell. The finding that three different primary or unlearned types of reinforcer (touch, taste, and smell) are represented in the orbitofrontal cortex helps to provide a firm foundation for understanding the neural basis of emotions, which can be understood in terms of states elicited by stimuli which are rewarding or punishing.