How does the brain express the information people know and manipulate? One answer to this central biological question is by "ensemble coding". This idea postulates that the brain operates like a light-matrix display. Each light bulb in the display is analogous to a single neuron and the brightness of a bulb is analogous to the electrical activity of that neuron. A display can express many messages, similarly, an ensemble neural code can express anything by specific and dynamic patterns of active and inactive neurons. No single neuron is essential for any message and each neuron participates in many messages. If the same bulbs in a display are used to express two messages at once, the overlapping patterns merge causing catastrophic information loss. In contrast, brains are good at expressing multiple, even conflicting ideas, but it is unknown how this is achieved. How the catastrophe is avoided when two messages are expressed in an ensemble code is being investigated electrophysiologically in this project, by recording from many neurons in the hippocampus of rats on a slowly-rotating disk. Conditions are arranged so that the rats must know where they are both on the rotating disk and in the stationary room. The activity of some neurons signals rotating-disk locations and the activity of other neurons signals stationary-room locations. Disk-coding and room-coding activity is rarely observed in the same recordings, indicating hippocampal activity is functionally grouped according to the two types of information. The project will determine whether this functional grouping occurs because disk-coding neurons are anatomically separated from room-coding neurons, or alternatively, because the activity of neurons in one functional group suppresses the activity of neurons in the other group. It will also be determined whether this functional grouping is learned, in a sense reflecting the organization of cognitive experience. The answers will help identify how the brain solves a fundamental challenge to the ensemble coding idea. In the process of this project students will be offered training in electrophysiological recording and behavioral analyses, and will be expected to present their findings through publications and national conference presentations.
