(a) The development of a rapid photoperiodic response system. This enabled the research group to follow the neural and endocrine changes as photoinduction occurred in real time. The first overt change when quail are exposed to a single long day is a rise in LH secretion at about hour 20. This model was applied: to show definitively the circadian nature of the photoperiodic clock and its complex properties as an oscillator, to measure (with Russell Foster) the action spectrum for the non-retinal light receptors, and in many studies to determine the timed sequence of neural changes as induction occurred. Subsequently, Takashi Yoshimura in Japan used the quail to investigate these changes in molecular terms and was able to connect these into the separate discoveries that thyroid hormones play a critical role in the photoperiodic response (see below). Other groups have subsequently developed these ideas, mainly in mammals.