Time Distortion Due to Visual Flicker

Time pervades our understanding of the world - we use it to coordinate our movements, to perceive motion, to plan our behaviors, and perhaps even to understand causality.

But it is an under-appreciated factor in cognition. Even in the domain of the well-understood visual system, few realize that neurons in visual cortex are tuned not only for sensitivity to visual input of particular orientations, but also tuned also to time - in terms of temporal contrast.

Johnston, Arnold and Nishida were able to manipulate this temporal tuning with a relatively simple method. The authors presented a sine-wave grating which appeared to "drift" in one direction or another (at 5 or 20 Hz) on either side of a central fixation point. This was presented for 15 seconds, but the direction of drift changed every 2s to prevent motion aftereffects. Next, the authors presented a a similar grating on the same side of fixation for 600 ms, which drifted at a rate of 10 times per second. Finally, the authors presented a similar grating on the opposite side of fixation for a variable amount of time (between 300 and 1200ms) and subjects indicated whether it was presented for longer or shorter than the prior grating.

This simple manipulation distorted people's perception of time: the first 10Hz stimulus was perceived as lasting less than 600ms (or, alternatively, that the second 10Hz stimulus was perceived as lasting longer than it did), but this effect was found only for adaptation to the 20Hz stimulus (the 5hz stimulus had a similar, but nonsignificant effect). A control experiment showed that this manipulation did not affect the perceived onset or offset of the stimulus, suggesting that the processes of time estimation themselves were affected and that simple "fatigue" explanations based on neuronal accomodation are not sufficient.

Another experiment showed that this same adaptation also held for estimates of the apparent frequency of motion - that is, subjects perceived the first 10Hz stimulus as moving more slowly (or alternatively, the second 10Hz stimulus as moving more quickly).

Based on several additional control experiments, the authors concluded that:

1) the time distortions demonstrated here cannot reflect "neural signal persistence, attention, and the density of stimulus changes."

2) these results argue against the use of a central "brain clock" and instead support a more distributed and explicit encoding of time.

3) the authors suggest these effects are due to influences at the level of decisions, rather than more peripheral or sensory processes, and propose that subjects extract a "temporal rate signal" from stimuli in their environment and somehow "scale" it to provide an estimate of duration.