Neuroscientifically speaking, our daily activities involve the recognition of surrounding objects, determination of their function, decision on how to use the objects, and execution of the action. The main topic in systems neuroscience today is to clarify the area and action of neurons in the human brain corresponding to each step of the above-mentioned process. For example, humans can recognize objects with a considerably wide field of view. When we analyze the neural activities in higher-order visual cortical areas in the brain, we can find that there are brain neurons responding to objects from various locations in the visual field. Therefore, it can be explained that these neurons facilitate the recognition of objects independent of their location in the visual field. However, this explanation is not easy to understand.
The determination of neuronal activities corresponding to object recognition and execution of action is only the first step toward a better understanding of the brain at the individual level. It is necessary to clarify the functions of neural circuits that elicit the specific responses of corresponding neurons. Let us return to the topic on neurons responsive to objects regardless of their location in the visual field. The eyes (retinas), which first receive visual stimuli from outside the body, have no responsiveness to variation in location. Hence, the response regardless of the location of objects is considered to be realized in specific neural circuits above the retinas to higher-order visual cortical areas. Unfortunately, however, the details of such neural circuits have remained unclear. In addition, neural circuits underlying the response regardless of the location of objects in the visual field have hardly been clarified although neuronal activities corresponding to recognition and execution of action have recently been reported in many papers. The purpose of this research project is to develop an effective method of clarifying the neural circuits that underlie the responses of neurons observed at the individual level.
It is difficult to identify a pair of connected neurons or neuronal groups from among tens of billions of brain cells, which in turn makes it difficult to clarify the neural circuits that underlie the responses of neurons. Our research group will attempt to identify these neurons or neuronal groups by the combined application of optogenetics and optical imaging. Optogenetics is a molecular biotechnological method that enables the expression of proteins that convert light energy to neuronal activity (e.g., channel rhodopsin) in neurons. Optical imaging is an engineering approach that is used to visualize neuronal activity by detecting optical signals, such as fluorescence, light absorption, and light scattering, which occur in association with neuronal activity. It will become possible to map a pair of connected neurons or neuronal groups by (1) activating neurons (or neuronal groups) that have expressed channel rhodopsin with light then (2) visualizing the neurons or (neuronal groups) that are activated by light stimulation. However, improvements of the optogenetic approach are required to induce the expression of photosensitive proteins in specific neurons. Moreover, the optical imaging approach must also be improved to detect weak optical signals that occur in association with neuronal activation. We will overcome each problem of the two approaches and clarify the type and area of signals received by cells that induce specific neuronal activities, enabling the realization of higher-level research on systems neuroscience.
