Space and Time in the Brain

 

Science  27 Oct 2017: Vol. 358, Issue 6362, pp. 482-485

Space and time in the brain

György Buzsáki, Rodolfo Llinás

Neuroscience Institute, Departments of Physiology, Neurology, and Psychiatry, and Center for Neural Science, New York University, New York, NY 10016, USA.

[paraphrase]

Episodic memory: Mental travel in space and time

Navigation and memory are deeply connected. Analogous to map- and path-based navigation, there are two forms of hippocampal system–dependent memories:    memorized facts (or semantic memory) and one’s personal experiences (episodic memory). To reexperience egocentric episodes, we project ourselves back in space and time (episodic recall), whereas traveling into the imagined future represents planning (prediction). The neuronal mechanisms used to create and recall episodic memory are analogous to those evolved for computing first-order (neighborhood) and higher-order (e.g., shortcuts, detours) distances to explore the physical world via path-based navigation. Similarly, neural algorithms that support map-based navigation are consonant with those needed to create and remember semantic knowledge. Like the formation of allocentric maps from egocentric exploration, semantic knowledge emerges gradually after repeated encounters with the same thing or event by the episodic memory system

Storing and remembering what happened to us, where and when, defines our personal episodic memories, distinguishing us from others. This definition of episodic memory requires neuronal mechanisms to support both the “where” and “when” axes. Many investigators have postulated the existence of neuronal clocks in the brain and assumed that these clocks make time needed for various computational purposes. During wheel or treadmill running in a memory task, the ordinal sequences of neurons faithfully track elapsed duration (termed “time cells”) from the beginning of the run on subsequent trials in both the hippocampus and the entorhinal cortex. Because place cell firing frequency varies as a function of running velocity, the elapsed time (and distance) can be continuously derived from the combined knowledge of velocity and sequential firing of neuronal assemblies. These experiments demonstrate that sequential neuronal activity can be tightly correlated with instrument-measured units. However, they do not show that these neuronal circuits are dedicated to compute duration per se.

The hippocampal-entorhinal system has a topographically organized bidirectional communication with the large neocortex. During the course of mammalian evolution and the corresponding disproportional enlargement of the neocortex, hippocampal inputs shifted from largely sensory and motor representations in the rodent to interactions mainly with other higher-order cortical areas in primates. Hippocampal circuits are largely “blind” regarding the modality and nature of cortical inputs. They process the sent messages the same way, regardless of their origin. Given the many possible routes, the answers to the question “What is the function of this circuit?” will be very different, depending on the routes the investigator tests in a given experiment. The function may appear to be space, time, sound frequency, odor-sound sequence, memory, or something else, even though the hippocampus responds to each case by generating ordinal cell assembly sequences relevant to the particular situation. These considerations generalize to other brain regions as well.

Relating sensory inputs to brain activity can provide important yet limited clues to the function of neuronal circuits, and there is no doubt that the terms “space” and “time,” as well as other mental constructs, will be part of research for years to come. These concepts are part of our everyday lives. Regarding sequential order as space or time is an attempt to supplement the “outside-in” strategy with a brain-centered, “inside-out” approach. It is such inferential, model-building, explanation-seeking brain mechanisms that should lead our quest for clarifying these fundamental concepts.

[end of paraphrase]

 

Return to — Human-type Consciousness