Functional neuroimaging of umami taste: what makes umami pleasant?

The cortical processing of umami shows what makes it pleasant and appetitive. The pleasantness of umami reflects and is correlated with processing in the secondary taste cortex in the orbitofrontal cortex and tertiary taste cortex in the anterior cingulate cortex, whereas processing in the primary (insular) taste cortex reflects physical properties such as intensity. However, glutamate presented alone as a taste stimulus is not highly pleasant and does not act synergistically with other tastes (sweet, salt, bitter, and sour). When glutamate is given in combination with a consonant, savory odor (vegetable), the resulting flavor, formed by a convergence of the taste and olfactory pathways in the orbitofrontal cortex, can be much more pleasant. This pleasantness is shown by much greater activation of the medial orbitofrontal cortex and pregenual cingulate cortex than the sum of the activations by the taste and olfactory components presented separately. Furthermore, activations in these brain regions were correlated with the pleasantness and fullness of the flavor and with the consonance of the taste and olfactory components. The concept is proposed that umami can be thought of as a rich and delicious flavor that is produced by a combination of glutamate taste and a consonant savory odor. Glutamate is thus a flavor enhancer because of the way that it can combine supralinearly with consonant odors in cortical areas in which the taste and olfactory pathways converge far beyond the receptors. Cognitive and attentional modulation of the orbitofrontal cortex also contributes to the pleasantness and appetitive value of umami.