Ways to think about the brain: Space, time and memory
Neural computations are often compared to instrument-measured distance or duration, and such relationships are interpreted by a human observer. However, neural circuits do not depend on human-made instruments but perform computations relative to an internally defined rate-of-change. While neuronal correlations with external measures, such as distance or duration, can be observed in spike rates or other measures of neuronal activity, what matters for the brain is how such activity patterns are utilized by downstream neural observers. For example, episodic memory, supported by the hippocampus, is defined as “what happened to me where and when?” Accordingly, the recommended program in neuroscience is to uncover the neuronal mechanisms of the independent ‘what’, ‘where’ and ‘when’. According to this framework, numerous studies imply that the hippocampal system provides ’space coding’ and recent experiments suggest that the same neurons offer ’time coding’ as well. I will discuss the alternative option that hippocampal operations can be described by the sequential activity of neuronal assemblies and their internally defined rate of change without resorting to the concept of space or time. Brains make neither space or time nor do they sense them. Instead, one should try to understand how pre-existing evolving cell assemblies acquire ‘meaning’ (i.e., utility to the brain’s host) through action-based experience.