The connection between smell and taste is sometimes obvious but where in the brain does it occur? In a paper titled ‘The Insular Taste Cortex Contributes to Odor Quality Coding’ (freely available here), Veldhuizen and colleagues investigate where the ‘sweetness’ of odours is located in the brain. There are two candidates for this. One is the piriform cortex. Activity in the piriform cortex is thought to encode the smells that we experience. The other is a small region within the insular cortex known as the insular taste cortex (check out this article for a closer look at the insular cortex). As its name suggests, activity in this region is thought to encode the quality of taste. So now that the researchers had narrowed down their options they chose to investigate their question using functional Magnetic Resonance Imaging. This imaging approach involves the application of strong magnetic fields to the brain. Without going into too much detail this aligns the molecules in the brain tissues and with the application of a pulsed radio frequency there is a realignment of the molecules with a corresponding release of energy. This energy is released in the form of photons and using some clever transformations, information about the tissues can then be deduced. A variation on this is the use of BOLD signals – that is blood oxygenation-level dependent signals. In essence this means that information about the blood flow in the brain can be deduced. Crudely speaking, the theory goes, if a region of the brain suddenly becomes active, it will become hungry for energy and will need more blood flow and will use up more oxygen. There are a number of reasons why it is not so simple but for the moment the argument holds – if a brain region is active, the researchers can tell this from the BOLD signal. That means that they can work out which part of the brain someone is using when they perform a certain task.
The researchers used a scanner with a strong field strength – 3 Tesla. Then their volunteers were asked to rate solutions and smells using measures including ‘edibility’, ‘familiarity’ and ‘pleasantness’. There were numerous subtleties to the design paradigm to ensure that other factors didn’t influence the test results and these are described in more detail in the paper. From the methodology section it looks as though the researchers created visual analogue scales for each of the psychological measures. In other words, they draw a line and ask the subjects to mark on it where their experience lies. So for instance the subject might be asked to rate the sweetness of a taste. The Visual Analogue Scale (VAS) would be minimal at one end and maximal at another end. This is a very flexible approach that can be used to give the researchers numbers that they can crunch. One drawback with this approach though is that sometimes it might not be as rigorous as the more thoroughly investigated scales that have been created to investigate these experiences.
When the researchers analysed the fMRI data, they factored in the ‘sniff volume’ and ‘tongue movements’ to ensure that the activity they were picking up didn’t represent these ‘movements’. They were of course interested only in the perception but movements themselves create activity in the brain. They then looked at an average activity for the ‘group’. In other words they looked at a ‘group brain’ representing the average activity amongst all subjects during a given response or task. Using this approach, they created a mask specifically for the strength of sweetness. So they discriminated between the perception of strong and weak tastes and then looked at where these occurred in the ‘group’ brain. Now in order to produce a map of activity in the brain, the researchers first of all divide up the brain into small cubes. These are known as voxels. In order to say a voxel is ‘active’ they have to set a threshold. It just so happens that if you take a large number of voxels, your bound to get some that will be firing above the threshold by chance when in fact they’re not really playing any role in the experience a person is having. This is where the statistics come in. Researchers have developed a strategy for dealing with this known as correcting for multiple comparisons (see this article for a very interesting debate on this subject). A very simple approach is to divide the probability used for the threshold calculation by the number of comparisons being made. In this manner, if more comparisons are being made then it means that it will be more difficult for an individual voxel to reach the threshold. There are many other ways to do this which involve varying degrees of sophistication.
Now that the researchers had the ‘mask’ that separated out regions of the brain associated with strong and weak sweet tastes, they combined the mask with the multiple comparison approach to select out regions of the ‘group’ brain that was likely to be associated with strong sweet tastes. The researchers also used a special approach to analysing the activity in the voxels. Since a previous study (mentioned above) by Vul and colleagues had discussed ways in which strong associations could be wrongly produced with certain methods of analysing voxels, the researchers used a very specific analysis of the voxels in which they looked at how strongly individual voxel firing activity correlated with the stimulus of interest (e.g smell or taste). This constrasts with another approach in which average activity in groups of voxels is considered first of all. The researchers argue that their approach would reduce the likelihood of identifying false connections between brain activity and perceptions.
The researchers recruited 19 subjects for the study, but the data from 15 subjects was used in their analysis. The subjects were presented with food and floral odours. Subjects are also presented with odorless stimuli and they will undergo a run of stimulus presentations. The researchers found that two regions in the insular cortex were more strongly activated by strong sweet tastes than weak sweet tastes. This evidence suggested that these regions of the insular cortex were thus associated with strong sweet tastes. The researchers also found that activity in the insular cortex was strongly correlated with the sweetness perception of food odours. This did not apply to floral odours suggesting that there may be a strong biological value for this association (i.e in evolutionary terms it provides a strong adaptive value to detecting potentially high energy nutrients). The researchers then looked at areas which were activated with both strong smells and strong tastes. They found that two brain regions – the insular cortex bilaterally (right and left side of the brain) and the operculum were activated in this manner. The other areas that they were interested in – the piriform cortex and the orbitofrontal cortex – didn’t fire strongly in association with sweetness perception of odours. More significantly still the researchers found that there was an overlap in the insular cortex between regions active in response to sweet tastes and sweet smells and the strength of the sweetness perception.
What this meant to the researchers was that a region within the insular cortex probably rates the sweetness of tastes and odours and links these perceptions together. Savouring the aroma of a food can make the mouth water and we can anticipate how that food might taste. More importantly if we or our ancestors were searching for sweet foods, a smell emanating from that food would allow an anticipation of how that food might taste. Linking the two in the brain would enable us to quickly home in food sources with the aid of our smell. Additionally the insular cortex is plugged into emotional networks in the brain and this rapid integration of our smell and taste might influence and be guided by our emotions. While in humans our sense of smell isn’t as sophisticated as our vision, our ancestors have a much more sophisticated sense of smell. Richard Dawkins in his book ‘The Ancestor’s Tale‘ writes about our common ancestor with lemurs which we shared some 63 million years ago! If the reader watches the video below they will no doubt be impressed by the prominent snout of our primate relatives which is a reminder of just how important the sense of smell was in our distant past. I have selected the clip below which shows the lemur’s use of both smell and taste in examining an object.
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