There are asymmetries in the way animals perceive numbers and time, and a recent experiment showed that pigeons underestimate the midpoint between two time intervals. In the experiment, pigeons were trained to tap one lever when a light flash was “short”, perhaps 1 second long, and another lever when the flash was “long”, say 16 seconds. When the birds then saw flashes of intermediate length, you would expect them to distinguish long from short around the mid-point of 8 or 9 seconds. But instead they switched at 4 seconds.
Pigeons might perceive time on a logarithmic scale on which higher values are increasingly compressed together. Alternatively, they might perceive time linearly but are confused by longer intervals. If pigeons use a log scale, they will correctly classify 9 and 10-second flashes more often than 7 and 8-second flashes, while if they use a linear model their accuracy should be similar. William Roberts from the University of Western Ontario in London, Canada, has now shown that six pigeons, tapping levers for 20 days, conformed neatly to the logarithmic model (Behavioural Processes, vol 72, p 207). The results may apply to humans, because brains have to prioritise the small numbers most relevant in life. It might be an evolutionary strategy to discriminate numbers like this, says Roberts.
June 6, 2006
Original web page at New Scientist