Their idealised version of this question was to ask how two signal chemicals, starting from opposite ends of an embryo, could result in a pattern that looked like the French flag (ie, three different coloured vertical stripes).
Its reaction with the odour molecules changes a detector's electrical resistance, and this signal is fed into a neural net (the closest that computer technology has got to imitating the pattern-recognising abilities of networks of nerve cells) to work out what is being smelt.
However, when we notice that a new five-wave pattern on the way up has surpassed the fourth wave on the way down, we recognize it as a powerful signal that the trend is changing.