Metabolism and Body Size Influence the Perception of Movement and Time

When we watch TV or a movie, we are essentially watching a series of still images presented in rapid succession... so rapid, in fact, that we perceive them to be a single moving image. The ability of movie-makers to convince us that still images are fluid in time is based on our physiology. Specifically, moving-pictures, as they were once called, rely on our critical flicker fusion frequency (CFF), the lowest speed at which we perceive a flashing light source to be a constant light. But our CFF didn't evolve so we could enjoy movies and TV; it came about from our evolutionary history.

The ability to identify and track moving objects is critically important for finding and catching prey, avoiding predators, and finding mates. It is these visual abilities that rely on an animal's CFF. An animal with a low CFF will miss many visual details, like watching your TV with a fast-forward function that jumps ahead 15 seconds at a time. An animal with a high CFF will see all the details that happen in between with a fine-time-scale resolution. But if having a high CFF conveys such an advantage, why don't all animals have a high CFF?

This figure from the Healy, et al., 2013 Animal Behaviour paper illustrates the difference between a ground squirrel's actual movement (in the left panel), what a second ground squirrel with a high CFF might see of that first squirrel's movement (in the middle panel), and what a short-eared owl with a low CFF might see of that first squirrel's movement (in the right panel).

The fact is that having a faster CFF does not come for free. An increase in CFF requires an increase in visual and neural processing, which in turn requires more energy. Thus, we would expect that faster CFFs should only evolve among species that 1) have a need for it and 2) generate a lot of energy. It is from this logic that Kevin Healy from Trinity College in Dublin, Ireland and his colleagues formed their hypothesis that smaller animals (that are more maneuverable) with higher metabolisms (that generate more energy) will have faster CFFs.

Healy and his colleagues scoured published papers for data on CFFs, body sizes and metabolic rates across species. But CFF could be influenced by more than just body size and metabolism. For example, animals that have evolved in low-light conditions may have very different visual systems than those that have evolved in high-light conditions. And metabolism is known to be strongly impacted by whether an animal generates its own body heat or not. Brain size also relates to metabolism because the brain is such an energy-hogging organ. And evolutionary relatedness among species could impact how a given species' CFF rates have evolved. So in addition to the CFF, body size and metabolic data, the scientists also collected published data on light environments, generation of body heat, brain size and evolutionary relationships. In the end, they included data from 34 species, including mammals, birds, reptiles, amphibians, fish and sharks.

After extensive mathematical modeling, the researchers found that small animals with high metabolic rates that evolved in high-light conditions had the fastest CFFs and the best ability to perceive visual information with fine-time-scale resolution. On the flip side, large animals with low metabolic rates that evolved in low-light conditions had the slowest CFFs. Brain size and evolutionary relationships did not affect these results.

This paper has some interesting and important implications. For one thing, it shows how large-scale physiological attributes such as body size and metabolism impact fine-scale physiological attributes such as our visual temporal resolution. For many animals, this could allow some species with fast CFFs to send fast visual signals to one another that perhaps their larger predators would not even perceive. And in humans, at least, our perception of the passage of time itself is also linked to our temporal resolution. This may lead us to wonder, does our metabolism impact our perception of time? I don't know, but I'm going to slow down and enjoy this New Year!

Further reading:
Healy, K., McNally, L., Ruxton, G.D., Cooper, N. and Jackson, A.L. Metabolic rate and body size are linked with perception of temporal information, Animal Behaviour, 86, 685-696 (2013). DOI: 10.1016/j.anbehav.2013.06.018.

Image Credits:

Zoetrope.jpg by Andrew Dunn at Wikimedia Commons.

Figure 1 from the Healy, et al. 2013 article in Animal Behaviour

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