Neural activity in the olfactory bulb of mice responds to rapid changes in odor concentrations but not unchanged levels in consecutive sniff cycles. Fast sniffing helps the brain filter out irrelevant information, improving the animal's ability to locate an odor source more efficiently.
The hippocampus, a brain area important for spatial navigation, combines information about odors and movement through space to create a memory of a journey.
Mice can recognize odors as spatial landmarks and use them to improve navigation toward a goal location. Such landmarks increase the number of neurons in the hippocampus that act as "place cells," which show location-specific activity and are thought to help to create a cognitive map of space.
Many of the properties of odor molecules that are important for perceiving smells and how they are represented by circuits in the brain are poorly understood.
In mice, researchers mapped neuronal connections that convey odor information from the olfactory bulb to the rest of the brain. People have about different types of olfactory receptors. For comparison, dogs have about two times as many. Each receptor can be activated by many different odor molecules, and each odor molecule can activate several different types of receptors. Two of the keys are a perfect fit and open the door easily.
The complexity of receptors and their interactions with odor molecules are what allow us to detect a wide variety of smells. And what we think of as a single smell is actually a combination of many odor molecules acting on a variety of receptors, creating an intricate neural code that we can identify as the scent of a rose or freshly-cut grass. Once an odor molecule binds to a receptor, it initiates an electrical signal that travels from the sensory neurons to the olfactory bulb , a structure at the base of the forebrain that relays the signal to other brain areas for additional processing.
One of these areas is the piriform cortex, a collection of neurons located just behind the olfactory bulb that works to identify the smell. Smell information also goes to the thalamus, a structure that serves as a relay station for all of the sensory information coming into the brain. The thalamus transmits some of this smell information to the orbitofrontal cortex, where it can then be integrated with taste information. What we often attribute to the sense of taste is actually the result of this sensory integration.
This coupling of smell and taste explains why foods seem lackluster with a head cold. This happens because the thalamus sends smell information to the hippocampus and amygdala , key brain regions involved in learning and memory.
Although scientists used to think that the human nose could identify about 10, different smells, Vosshall and her colleagues have recently shown that people can identify far more scents. Starting with different odor molecules, they made random mixtures of 10, 20, and 30 odor molecules, so many that the smell produced was unrecognizable to participants.
When they changed which glomerulus was activated first, the mice demonstrated a 30 percent drop in the ability to sense the correct odor. When they changed the last one activated, there was only a 5 percent reduction in detection ability.
Rinberg likens smell perception to the melody of a song: The notes—in this case, representing activated glomeruli—are important. But without the right timing, the song, or the perceptual experience, falls apart.
Changing the seventh note of a melody might be unnoticeable. Swapping the first two might result in a new tune altogether. When we smell, it is not only about which glomeruli are activated but also what time sequence they follow.
Harvard University biology professor Venkatesh N. Murthy, who specializes in the neuroscience of olfaction and was not involved in the study, points out that there is a large body of evidence relating patterns of glomerular activation to smell perception.
Rinberg hopes to carry his research more deeply into the brain to see how other regions of the organ aid in perceiving odors and objects once they receive information from the olfactory bulb. The film features a world ceded to intelligent computers that relegate humans to a shared simulated reality created in their brains—similar to the way the researchers devised an artificial odor.
In the cortex, related odors led to more strongly clustered patterns of neural activity compared with patterns in the olfactory bulb. This observation held true across individual mice. Cortical representations of odor relationships were so well-correlated that they could be used to predict the identity of a held-out odor in one mouse based on measurements made in a different mouse.
Additional analyses identified a diverse array of chemical features, such as molecular weight and certain electrochemical properties, that were linked to patterns of neural activity. Information gleaned from these features was robust enough to predict cortical responses to an odor in one animal based on experiments with a separate set of odors in a different animal. The researchers also found that these neural representations were flexible. Mice were repeatedly given a mixture of two odors, and over time, the corresponding neural patterns of these odors in the cortex became more strongly correlated.
This occurred even when the two odors had dissimilar chemical structures. The ability of the cortex to adapt was generated in part by networks of neurons that selectively reshape odor relationships. When the normal activity of these networks was blocked, the cortex encoded smells more like the olfactory bulb. Part of the reason why things like lemon and lime smell alike, he added, is likely because animals of the same species have similar genomes and therefore similarities in smell perception.
0コメント