Effect on the taste cortex by attention to taste quality

Abstract

Recent neuroimaging studies have shown that attention on brain processing of tastes could increase the responsiveness of the insular cortex (putative primary taste cortex). However, how attention affects the identification of taste quality remains unclear. The aim of this study was to evaluate brain response during a taste quality identification task, particularly the identification of salty and umami tastes, in a functional magnetic resonance imaging (fMRI) experiment. The Human Experimentation Committee of Kyushu University approved all experimental procedures. Eighteen participants (9 males and 9 females; aged 22-33 years) participated in this study. Salty (0.1 M sodium chloride) and umami (0.1 M monosodium glutamate) solutions were used, and artificial saliva (25 mM potassium chloride with 2.5 mM sodium bicarbonate) was used as control. An originally developed taste delivery system was used to spread the taste solution on the surface of the tongue as widely as possible preventing swallowing. Before scanning, the participants were exposed to a mock test. In the mock test, an umami solution was administrated for 8 sec; during this period, a red (IDENTIFICATION condition) or black cross (PASSIVE condition) was presented on the screen set in front of the participant’s face, which was followed by a control period of 12 sec with a black cross. Before the mock test, we instructed the participants to identify a salty solution in the umami solution when a red cross was presented on the screen. In the IDENTIFICATION condition, a red cross was presented, and a salty solution was administrated with the umami solution. We administered varying amounts of the salty solution and identified the smallest amount that could be detected by each participant. Next, we instructed them to identify the smallest amount of the salty solution in the umami solution during fMRI scanning. In the actual experiment using the 3.0-T MRI scanner, the salty solution was not administrated. Therefore, we administered only the umami solution in the IDENTIFICATION and PASSIVE conditions. Each condition consisted of 12 trials, and the two conditions were presented pseudorandomly. To evaluate the effect of attention on taste identification, IDENTIFICATION-PASSIVE contrast was performed using SPM8 (Wellcome Department of Cognitive Neurology, London, UK). This analysis revealed activity in the left anterior insula (PFWE-corrected < 0.05). In conclusion, attention on identifying a taste quality resulted in the activation of the gustatory cortex (anterior insula). This finding may have an important implication in understanding the functional specialization of the taste cortex.