The brain’s relativity: no absolute simultaneity, no pure present

In 1933, physiologist Hudson Hoagland asked his feverish wife to count at one number per second. The hotter she became, the less wall-clock time she needed to reach sixty. Hoagland fit counting rate with a temperature-dependent chemical relation and proposed that the process setting an internal pace might itself speed with temperature.

Decades later, optical clocks separated vertically by only 33 cm were shown to tick at detectably different rates because of gravitational potential. These are not the same phenomenon. One is a physical change in clock frequency; the other is behavior changing with bodily state. They belong together only as a methodological warning: whenever the measuring device changes, measurement must disclose its conditions.

This essay borrows four tools from relativity, clock rate, simultaneity, event order, and reference frame, to ask better questions about neural time. The analogy is not a claim that cognition obeys Einstein’s equations. Its value is to stop us from treating subjective time as a transparent copy of physical time.

Clock rate is a state variable

Temperature, dopamine, arousal, attention, and movement all alter timing behavior. Classical internal-clock theories describe these effects as changes in pulse rate: faster accumulation makes an interval seem longer. But identical behavior can arise if memory, decision boundaries, or motor output change. “The clock sped up” is a model claim, not a direct observation.

The distinction matters clinically and pharmacologically. A manipulation may change temporal sensitivity, bias, or variability without changing an internal rate. To identify clock speed, one needs multiple tasks whose predicted patterns separate encoding, retention, comparison, and response.

Falling for two seconds: not faster then, longer afterward

People who experience a frightening fall often report that time slowed. In a controlled free-fall experiment, participants judged other events as longer after the fall, yet showed no improved temporal resolution during it. If perception literally ran at a higher sampling rate, rapidly changing visual information should become easier to resolve. It did not.

The event may instead become longer in retrospective memory. Arousal creates dense, distinctive encoding, and a memory containing more change can later be interpreted as having occupied more time. “Time slowed” then describes the final experience, but not necessarily an online clock.

This separates two questions that introspection merges: how fast information was processed during an event, and how long the event appears when reconstructed afterward.

Simultaneity is calibrated

Light and sound from the same event reach the senses at different times and are processed at different speeds. The brain does not observe a universal “now.” It learns a temporal binding window within which signals are treated as one event.

Adaptation can move the point of subjective simultaneity. Repeatedly presenting one modality before another makes a smaller version of that lag feel simultaneous. The nervous system recalibrates because stable transmission delays are more likely than a world whose causes repeatedly split apart.

Simultaneity is therefore an inference under a reference condition. A button press and flash, a voice and face, or a tool action and consequence can each acquire their own expected delay. Asking whether two neural events are simultaneous without specifying the sensory pathway, adaptation history, and decision rule is incomplete.

Across a saccade, order can reverse

Several times each second, the eyes jump. During a saccade, retinal input is disrupted, spatial coordinates are remapped, and perception must join samples from before and after the movement into one stable world.

Stimuli flashed around a saccade can be mislocalized. Under some conditions their reported temporal order can also reverse. A physically later flash may be experienced as earlier because spatial remapping and temporal reconstruction interact.

This is more than poor temporal precision. Random noise would make order uncertain. Systematic reversal suggests that the brain reconstructs sequence within a coordinate transformation. The reference frame is not only where an event occurred, but where the eyes were about to point.

Serial dependence is measurable gravity

The present is attracted toward the recent past. A line’s orientation, a face, a location, or a duration is often judged as more similar to what appeared on the preceding trial. This serial dependence stabilizes perception in a world that usually changes smoothly. It can also bias judgment when successive events are independent.

The metaphor of gravity is useful because the attraction is local and measurable. Its strength depends on similarity, delay, uncertainty, attention, and whether the previous item was encoded as relevant. Yet even here one must separate levels. A previous stimulus can alter sensory representation, shift a decision criterion, prime a response, or change the prior for the next trial.

There is no pure present because a present estimate is computed from a current measurement and a history-dependent model of what is likely now.

Three measurement problems, one possible experiment

The examples create three different measurement problems.

First, the instrument’s rate changes with physiological state. Second, the zero point of simultaneity is calibrated from recent cross-sensory statistics. Third, event order is reconstructed across changing sensor coordinates. Serial dependence adds a fourth: the current measurement inherits the previous estimate.

A unified experiment should manipulate bodily state, audiovisual lag, saccade timing, and sequence statistics while fitting a model with distinct latent components. It should measure discrimination, reproduction, confidence, and retrospective duration rather than relying on one response. The critical question is whether one change in inferred temporal precision explains several phenomena, or whether the apparent family resemblance hides separate mechanisms.

The unified theory does not yet exist. Calling these effects a neural relativity is a research program, not a result. It asks us to state the clock, calibration, event definition, and reference condition before saying what the brain’s time is.

Physics taught us that time coordinates require an operational procedure. Cognitive neuroscience needs the same discipline. Subjective time is not unreal because it varies. It is a measurement produced by a biological instrument whose rate, alignment, and memory are part of what is being measured.

References

Hoagland, H. (1933). The physiological control of judgments of duration. Journal of General Psychology.

Stetson, C., Fiesta, M. P., & Eagleman, D. M. (2007). Does time really slow down during a frightening event? PLoS ONE, 2, e1295.

Fujisaki, W., Shimojo, S., Kashino, M., & Nishida, S. (2004). Recalibration of audiovisual simultaneity. Nature Neuroscience, 7, 773-778.