From tongue to brainstem
As diagrammed in this image, the three taste nerves and the trigeminal nerve bring their messages to the brainstem, where they combine their signals in areas that are involved with arousal (for example from sleep), and are - according to Antonio Damasio in "The Feeling of What Happens" - the locus of our sense of core self.
Smell and the brain
As noted in the previous page, molecules of the food you are eating move through the back of the throat and reach olfactory nerve endings in the roof of the nose. The molecules bind to these nerve endings, which then signal the olfactory bulb to send smell messages directly to two critical parts of the brain:
The brain perceives flavor
As taste and trigeminal messages move further through the brain, they join up with smell messages to give the sensation of flavor, which feels as if it comes from the mouth. In fact, Dana Small and her colleagues (see sidebar) have demonstrated that when you smell something orthonasally (i.e. through your nostrils), the brain registers these sensations as coming from the nose, while smells perceived retronasally (i.e. through the back of the throat, as you would do if you were smelling food you have in your mouth) activate parts of the brain associated with signals from the mouth. This finding helps explain why we perceive flavor in the mouth, even when a large component of flavor is provided by smell in the nose. (Small DM, Gerber JC, Mak YE, Hummel T. Differential neural responses evoked by orthonasal versus retronasal odorant perception in humans. Neuron. 2005 Aug 18;47(4):593-605.)
We have to know what we are eating, so taste, trigeminal messages, and smell meet in a part of the brain called the anterior insula (not shown in the image above), which identifies what the flavor is.
But it is not enough knowing what the flavor is. We also need to react emotionally to what we are eating: do we like it? is it poison? should we enjoy it or spit it out? Therefore flavor messages also go to the emotional centers in the temporal lobe and the cingulate gyrus (see image above) involved in giving sensations an emotional coloring. Messages from all of these areas then reach parts of the orbitofrontal cortex in the frontal lobe of the brain, right above the eyes, in an area right next to the anterior insula. This part of the brain evaluates experience and chooses among alternatives - in the case of flavor, to eat or not to eat (see Dana Small and colleagues' paper "Experience-Dependent Neural Integration of Taste and Smell in the Human Brain" in the Journal of Neurophysiology, September 2004 Vol. 92 no. 3, 1892-1903.).
In this same article, Small and her colleagues also discuss their finding that whether we think a smell is familiar or not depends on pairing the smell with the taste that usually accompanies it. In their experiments, vanilla odor paired with a sweet taste made the vanilla odor familiar, but when it was paired with a salty taste, the same vanilla odor was unfamiliar. This finding helps to explain why we often say that certain odors are "sweet" - we have paired the odor with the sweet taste in our memory.
Finally, Small and her colleagues present strong evidence that the final common pathways of pleasant taste and smell go to the non-verbal right brain, which has implications for our ability to report thinking processes related to taste and smell; one of the reasons Titchener - and Wundt before him - only reported four tastes was probably that they had difficulty getting people to talk about taste and smell.