When we sniff an odor, it is first detected by a set of specific odorant receptors expressed by olfactory receptor neurons in the nose. Olfactory receptor neurons then activate the brain area called the olfactory bulb. Interestingly, neurons in the olfactory bulb are activated with an odor-specific sequence, with some neurons activated early during a sniff and some later. Whether these temporal patterns, in addition to which neurons are activated, underlie odor perception is hotly debated.
To address this question, we activated olfactory receptor neurons with light instead of odors. This has become possible with a specific transgenic mice that express light-gated ion channel, channelrhodopsin-2, in olfactory receptor neurons. We projected light patterns onto the surface of the olfactory bulb using a computer projector (see “Sensory Deception: Lab Mice Can “Smell” Light”). This allowed us to change relative timing of activations in sets of inputs to the olfactory bulb without changing the total number of spikes generated by them. We then recorded the neural activities in neurons downstream to the olfactory bulb (i.e. olfactory cortex). We found that many olfactory cortex neurons were sensitive to the order of the olfactory bulb activation. For example, some neurons responded only when a set of neurons was activated few tens of milliseconds after the other set of neurons. However, they did not respond when the order of activation was reversed, or if the lag between two activations was longer or shorter. Furthermore, the posterior part of the cortex showed higher sensitivity than the anterior part, while neurons in the olfactory bulb rarely showed such sensitivity, suggesting specific neural circuits in the cortex generate order sensitivity.
These results show that olfactory cortex neurons can act as sequence or lag detectors. These results provide a strong support for the idea that temporal patterns in the olfactory bulb can be deciphered by downstream neurons. We hope out experiment provides a critical insight into the functional relevance of a temporal code, a hotly-debated issue in many sensory systems.
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