Space and time in the brain

Abstract

Nothing is more intuitive, yet more complex, than the concepts of space and time. In contrast to spacetime in physics, space and time in neuroscience remain separate coordinates to which we attach our observations. Investigators of navigation and memory relate neuronal activity to position, distance, time point, and duration and compare these parameters to units of measuring instruments. Although spatial-temporal sequences of brain activity often correlate with distance and duration measures, these correlations may not correspond to neuronal representations of space or time. Neither instruments nor brains sense space or time. Neuronal activity can be described as a succession of events without resorting to the concepts of space or time. Instead of searching for brain representations of our preconceived ideas, we suggest investigating how brain mechanisms give rise to inferential, model-building explanations.

Fig. 1. Cell assembly sequences can track distance and duration

(A) During physical travel, successive assemblies of neurons (1 to n) respond sequentially because of the changing constellation of environmental landmarks and/or proprioceptive information from the body (left). During mental travel, sequential activation is supported by self-organized patterning (right). (B) Sequential activation of neuronal assemblies in an episodic memory task. (Middle) A rat was required to run in a running wheel during the delay between choosing either the left or right arms of the maze and to remember the last corridor choice. The rat obtained a water reward if it chose the arm opposite of the previous choice. Color-coded dots represent spike occurrences of simultaneously recorded hippocampal neurons. (Left) Normalized firing-rate profiles of neurons during wheel running, ordered by the latency of their peak firing rates during left trials (each line represents a single cell). (Right) Normalized firing rates of the same neurons during right trials. Note that an observer can infer the run duration (and distance) in the wheel from the sequential firing patterns of the neurons. Modified from (23).

Fig. 2. Displacement-duration conversion by velocity in the hippocampus

(A) Trajectories of a rat through a place field on two trials with different speeds. (B) Spikes of one place cell and the corresponding local field potential (LFP) theta rhythm of the same two trials as in (A). Horizontal arrows indicate the time it took for the rat to run through the place field. (C) The number of spikes within the neuron’s place field is similar on slow- and fast-run trials. Trials are sorted by velocity. (D) Spiking activity of two place cells (blue and green ticks) and LFP theta rhythm in a single run. Temporal duration (T) is the time needed for the rat to run the distance between the peaks of the two place fields (behavioral time scale). τ, time offset between the two neurons within the theta cycle (theta time scale). (E) Three idealized place cells with identical theta oscillation frequency, illustrating the relationship between T and τ. (F) Correlation between the distances of place field peaks and theta time scale lags (τ) for many pairs of neurons. Solid curve, sigmoid fit to the values; dashed line, line of equity. Above and right: histograms of distance and time lag, respectively. Modified from (34).

Fig. 3. The core deficit in patients with hippocampal lesions is their inability to narrate events in the order in which they occurred

(A) Map of 11 events that occurred during a guided campus walk. Sidewalks, gray; buildings, blue. Arrows indicate the path taken during the walk. (B) Events from the walk, described during 6-min narratives. The control group (Con; blue squares) tended to describe all 11 events in the order in which they occurred. The order in which the patients (Pat; open triangles) described events was unrelated to the order in which the events occurred. n, number of individuals in each group. Reproduced from (55).