Thanks to Daniel Wesson and his colleagues at Boston University, we now have an answer. Rats can discriminate between odors even faster than previously thought, in as little time as 140 milliseconds. The study is published this week in the online journal PLoS Biology.
The study authors used a variety of tasks to measure the rats' olfactory capabilities, including observation of the rodents' "exploratory sniffing" (we did this in college, too, but we never knew you could get graded on it). The researchers also used optical imaging to measure the responses of neurons in the olfactory bulb, where the nose's odor signals are processed. But very little actually happens at this stage, suggesting that odor discrimination actually takes place beforehand.
As the authors write: "In most trials, responses to a novel odorant began before the initial barrage of input had ceased and before spatial patterns of input to the bulb had fully developed. These results suggest a coding strategy in which the earliest inputs play a major role in the initial perception of odor quality and place constraints on coding schemes based on simple changes in firing rate."
There's just got to be an idea for a deodorant commercial in there, somewhere ...