Location of nonlinear processes within the pupillary pathway

The pupil oscillates in synchrony with a slow beat produced by adding two lights flickering at slightly different frequencies. This behavior shows that nonlinear processes are present within the pathway. To localize these processes, a light of one frequency was presented to one eye, while a light of a different frequency was presented to the other. No pupil oscillations were seen in response to the beat in these dichoptic conditions, but when both lights were superimposed and presented to the same eye a powerful pupillary response was produced. We conclude that the nonlinearity giving rise to the pupil's beat response occurs before the pathways from the two eyes join and hence is neural rather than muscular in origin. Furthermore, the lack of a pupil response in the dichoptic conditions suggests that the signals from the two eyes may combine linearly.