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Subjectivity and the brainSubjectivity and the brainSUBJECTIVITY AND THE BRAIN Taste-olfactory convergence and the representation of the pleasantness of flavour in the human brain Ivan de Araujo, Edmund T. Rolls et al, University of Oxford European Journal of Neuroscience, Vol. 18, pp. 2059-2068, 2003 INTRODUCTION: This study is of importance in not only demonstrating that subjective assessments of pleasantness correlates to activity in specific parts of the orbitofrontal cortex, but further that these correlations have no relation to the intensity of the signal but only to the subjective pleasantness. This looks to point to a causal physical link between subjective experience and brain function. The orbitofrontal, amygdala, insula and anterior cingulate cortex are all activated by taste and smell signals. Signals for taste and smell are shown to converge on individual neurons in the orbitofrontal cortex. In particular, activity in the medial anterior orbitofrontal is correlated with the subjective pleasantness of taste and smell. The combination of taste and smell is defined as flavour and activity in this area thus correlates with the pleasantness of flavour. This study involves subjective assessments of three factors to do with taste and smell, firstly, intensity of the signal, secondly pleasantness, and thirdly, the degree of matching between the taste and smell signals. The study aimed to detect brain regions where activity correlated with pleasantness or degree of match. The subjects were also able to distinguish between degree of match and pleasantness, and some combinations of taste and smell that were rated as a good match could still be rated as unpleasant. The study showed distinct brain regions where activity was correlated with either pleasantness or degree of match. What is apparent is that subjectively rated values for pleasantness, on a scale of -2 to +2 in this study, were correlated with activity in specific brain regions, indicating that subjective experience is related to the activity of such brain regions. P. Further to this, where two pleasant tastes were mixed, for instance sucrose and strawberry flavour, there was a convergence of signals onto a single neuron or a particular brain area. The subjective rating of pleasantness was greater than the sum of the pleasantness rating of sucrose or strawberry by themselves. This looks to indicate not a simple addition of a signal for sucrose and a signal for strawberry, but more likely a subjectively calculated reward value for the combination. SUBJECTIVE PLEASANTNESS AND INTENSITY OF STIMULUS The medial orbitofrontal cortex is shown in this study to have activations correlated to the subjective rating of the pleasantness of taste and smell. It was demonstrated that the subjective assessment of pleasantness and the correlated orbitofrontal activity were not a function of the intensity of the stimulus, because this intensity was deliberately kept within a limited range during the study. The study also looked at the subjective assessment of matching between taste and smell and the increase in the blood oxygen level dependent (BOLD) signal, showing a straight correlation between an increased assessment of matching and an increase in the BOLD signal. Studies on macaques monkeys confirm that taste and smell signals converge on single neurons. The brain area involved in this extends from the insula cortex into the orbitofrontal cortex. The human orbitofrontal appears to be distinct from the macaque brain in having an area of the more anterior orbitofrontal than shows an activation in response to two stimuli such as sucrose and strawberry that is stronger than the sum of the activation produced by the individual stimuli. As a whole, the orbitofrontal is involved with encoding information about complex combinations of stimuli. he medial orbital frontal area that correlates to the pleasantness of flavour is close to other areas that correlate to the pleasantness of odour, taste, touch, and monetary reward. There is also an area activated by the unpleasantness of stimuli. Further to this, at the level of single neurons, there are also seen to be correlated activations for the reward value of taste, smell and visual stimuli. |
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