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There are a few ways we perceive food, and not all of them are particularly well understood. We know that much of this happens in the olfactory bulb, a small lump of tissue between the eyes and behind the nose, but how the stimuli get to this part of the brain is still being worked out.
How these stimuli are processed in the brain plays an important role in our daily life. According to Fahmeed Hyder, understanding how our perception of food evolves is important, but it has been difficult to get a clear picture of what our brains are doing when we smell.
"Knowing which exact pathways are involved and teaching our brains to appreciate and recognize both modes of perception in understanding taste is part of our culture that we haven't fully exploited yet," he said. A better understanding of how smells enter our brains would not only tell us a lot about our eating habits, but potentially also help patients with certain diseases.
Hyder, Professor of Biomedical Engineering and Radiology and Biomedical Imaging, has done an in-depth study of the function of the olfactory bulb. It may not be one of the most talked about regions of the brain, but it helps us understand the outside world by taking in molecules from food – called volatile foods – and then sending those signals on into the brain. It plays a central role as a gateway for chemical stimuli to the rest of the brain – particularly the piriform cortex, amygdala, and hippocampus. To see exactly how to do this, Hyder and his team mapped the activity throughout the olfactory bulb. It is the first time this has been done for the two independent pathways of odor delivery – that is, the orthonasal and the retronasal pathways. The results were published in NeuroImage.
The orthonasal pathway – a path that smells take to the brain – is usually considered an odor when volatile food or odor molecules enter the nasal cavity through inhalation. The other – the retronasal route – is more associated with eating, when chewing releases volatile foods in the mouth and these odor molecules enter the nasal cavity. Both routes, together with the aromas that we ingest with the taste buds on our tongues, shape our perception of food. Professors Gordon Shepherd and Justus Verhagen, employees of Yale in this study, have previously worked on comparing these methods of odor emission.
There are two ways that smells can get to the brain: orthonasal (which we usually think of as smell) and retronasal (which we associate with food). Photo credit: Yale School of Engineering and Applied Science
Among other discoveries in their recent work, Hyder and his team found that responses to stimuli traveling the orthonasal route were much stronger than the retronasal route regardless of smell. And while the brain responses on the two pathways were similar in many ways, the orthonasal maps dominated in some parts of the lightbulb while the retronasal maps dominated in others. Scientists hadn't seen these differences before, largely due to the limitations of standard imaging tools for this type of research.
Hyder studied the olfactory bulb for years and worked with Yale researchers Gordon Shepherd and Robert Shulman on some of the earliest studies of this region of the brain. For the NeuroImage study, he and his team wanted to learn more about the different ways that smells lead to the light bulb. It is important, said Hyder, also because our perception of food is key to healthy lives and disease recovery. Certain diseases can affect taste and smell – the discovery that a temporary loss of these senses is a symptom in COVID-19 is a recent example.
"It has been shown that many diseases – especially in patients with later onset – affect smell much more than taste," he said. "This fact was not very much appreciated in the treatment of diseases, especially because smell was not seen as a major sensation in practiced medicine. But just like we see and hear, taste and smell are all crucial aspects of being human."
A detailed look at how these senses are processed could be crucial for certain patients. For example, a common side effect of chemotherapy is that it affects the patient's sense of taste. Knowing exactly how the brain reacts to food can educate patients to enjoy the taste of food in other ways. Conversely, dementia often takes away the patient's sense of smell.
"In these cases, people can be taught how to enjoy a taste with concentrated doses going the retronasal route," he said.
Different magnetic resonance imaging methods show the unique structure and function of the olfactory bulb. Photo credit: Yale School of Engineering and Applied Science
"Taste & # 39; versus & # 39; Taste": What is the difference?
"Taste" refers to the taste buds in the tongue to identify tastes such as sweet, sour, bitter, salty, and umami. "Taste" is a kind of umbrella term that takes into account the taste, but also the smell of the food and its texture. Culturally, Hyder said, taste has received the most attention between the two.
"When I ask what taste is, most people will say 'taste' – the taste of food and the pyramid of food that we have created in the western world are very much based on taste, not on the odor component, "he said. "But a big part of the taste is actually the other part of the chemosensation – the smell components. The smell is quite common – not only in humans, but also in animals – when we chew our food. When we chew the food, molecules become released and be in the air. "
One reason the retronasal and orthonasal routes are not fully understood is because of the limitations of the technology. To get a complete picture of brain activity, a technique is needed that can map both routes at the same time. Most previous examinations of the olfactory bulb have relied on optical imaging, with which only the dorsal and lateral regions of the bulb and only the superficial layers can be imaged. Hyder is the technical director for preclinical scanners at Yale's Magnetic Resonance Research Center. His team was able to map the entire olfactory bulb by mapping these routes with functional magnetic resonance imaging (fMRI), which measures brain activity by detecting changes in oxygen levels in red blood cells.
Hyder knows fMRI well. About 30 years ago, he pioneered its use in animal models for high-resolution neuroscientific research. For this study, they created the maps by contrasting images of brain activity in rats – some with smells and some without smells. Between these sets of cards, they were able to determine how the amount of oxygen supply was changed to support the activity of various synapses in the olfactory bulb.
Hyder said the study is also a starting point for finding new ways to study metabolism – another topic of interest in his laboratory. Typically, his metabolic research focused on the cerebrum, but work in the NeuroImage study paved a way for exploring it in the olfactory bulb. It's a promising path to study because the olfactory bulb is a well-organized region with layers that are easy to separate, like an onion. Since the anatomy in the fragrance has many layers and every single neuroanatomical make-up can be easily recognized, the localization of specific metabolic events can provide information about what is happening when and where. "Because of the nature of these separate layers in the olfactory bulb, studying is much easier," he said. "So we combine optical techniques to study specific cell types and we use fMRI techniques to study specific metabolic signals. By combining them, we can understand the physiology and chemistry of the neural code."
Your mouth helps you smell delicious food
Basavaraju G. Sanganahalli et al. Orthonasal versus retronasal glomerular activity in the rat olfactory bulb by fMRI, NeuroImage (2020). DOI: 10.1016 / j.neuroimage.2020.116664
Taste and its two paths to the brain (2021, February 18)
accessed on February 18, 2021
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