(A) The agent consists of an oriented point (black dot) from which the odour concentration is sampled, the grey arrow indicates its orientation and the red line indicates the motion trail. At each time step, the agent performs a rotation ( deg) on the spot alternating between left and right rotations (‘L’ and ‘R’) so as to set a new orientation (grey arrow), and then moves forward by 1 mm. (B) In the absence of stimulation, the extent of this rotation is set according to a constant baseline (typically =10 deg), which is alternated between L and R causing movement in a zig-zag fashion. In the presence of stimulation, the extent of the rotation (e.g., towards the right) is modulated by the change of stimulus intensity (blue line, ) perceived between the previous and current position. The modulation is simplified down to a linear constant gain ; so that the extent of the rotation (here towards the right) is: . As a result, when , an increase in stimulus intensity perceived () would decrease the extent of the rotation towards 0 degrees (i.e. the agent goes straight), whereas a decrease in stimulus intensity perceived (), would increase the rotation up towards 180 degrees (i.e. agent makes a U-turn). Effectively, a negative gain () yields attraction towards higher stimulus intensity. Inversely, a positive gain () yields aversion (D) ( results in neither). (C,D) Section path examples from agent simulation in an odour gradient, with line colours as in (A). The underlying dark continuous line indicates the overall path taken by the agent. The blue lines indicate the isoclines of the odour concentration.