The Organization of Taste

Literature Review

The research developed in “A Gustotopic Map of Taste Qualities in the Mammalian Brain” (Chen X et al., 2011), builds an understanding of how the five different tastes are organized in the primary gustatory cortex and the functionality of this organization.

To understand the taste mapping in the brain, researchers used two-photon calcium imaging on mammals to examine neural activity in response to taste stimulation across the gustatory cortex. First, they confirmed the gustatory area in the thalamus by using extracellular electrodes. A sweet stimulus was then used to identify taste-responsive neurons. Cells around this area were infected with an AAV2/hu11-GFP virus to appear green. They stained the coronal section with TO-PRO-3 to appear red and the intersection between the RV and the MCA was injected with DiI to appear blue. The second layer of the gustatory cortex was inspected for taste responses and neurons showing patterns of taste responsive firing. A stimulus paradigm of artificial saliva, an exposure to a test tastant, and a post-stimulus wash were used to trigger and analyze responses in the brain. This was done for each of the five tastes: bitter, sweet, umami, salty, and sour.

Through these experiments, researchers were able to identify that the neuronal taste responses for specific tastes are spatially separated. These locations are distinct hot spots in which many neurons respond to taste stimuli. The two-photon calcium imaging made these hot spots evident as the taste-respondent neurons emit fluorescence when active. The only taste researchers could not locate a hot spot for was sour. They believe they were unable to locate the sour hot spot as it could be located elsewhere of their researched area or because sour perception involves other sensory pathways like pain and touch. Once hot spots were located, different tastes were tested to see if a specific taste hotstop would activate for another taste; demonstrating that each hot spot only activates for their specific taste. To further test the particularity of the taste-responsive neurons, the researchers used a specific bitter taste stimulus, cycloheximide, on knockout animals who were without the T2R5 receptor (the bitter receptor that detects cycloheximide) and animals with T2R5. This resulted in no responses to cycloheximide, although responses to other bitter chemicals in the knockout animals were displayed. Similarly, respective tests were performed for sweet tastes which had the same outcome. Through this research we have a better understanding of the taste system. It proves a distinction of taste respondent cells to their specific tastes and a gustotopic map which demonstrates the separation of taste hot spots in the insula.   

This research provides great information on taste response. However, it falls short in representing the sour taste response. There are further questions of how taste intensity is encoded and whether taste qualities with similar valence, i.e. emotional or hedonic association, connect to shared or similar targets. In order to gain an understanding of these topics, further experiments should be conducted with alterations. To locate the sour taste hot spot, more regions in the brain need to be included in a future experiment. To understand how taste intensity is encoded the experiment should include more stimuli that range in intensity and examine the differences for each taste and different intensities. To measure the associations between different taste qualities with similar valence, the experiment should target tastes based on valence and record the neuronal response in categories of tastes with similar valence. The acquisition for more knowledge is accredited to the extensive research done in “A Gustotopic Map of Taste Qualities in the Mammalian Brain” (Chen X et al., 2011) that provides far more than a starting point in understanding the organization of taste.