Effect of light on stochastic phase synchronization in the crayfish caudal photoreceptor

Phase synchronization between the firing of the crayfish caudal photoreceptor (CPR) and an applied periodic hydrodynamic stimulus is investigated. It is shown that the CPR firing synchronizes with a periodic stimulus over a range of frequencies corresponding to the known sensitivity range of the crayfish to hydrodynamic stimuli. This synchronization is quantified using previously developed measures of synchronization such as the synchronization index; multiple stimulus-response locking ratios occur in this system that are consistent with theoretical predictions based on the theory of synchronization of noisy oscillators. The maximal synchronization for various locking ratios is shifted to higher frequencies in the presence of light. Previous work (Pei et al. 1996a) has shown that the SNR of an applied low-amplitude, periodic, mechanical stimulus obtained from a power spectrum from delta pulses fit to CPR firing times is enhanced in light. In the present work, it is shown that an increase in the synchronization index parallels the increase in signal-to-noise ratio. This suggests that an increase in synchronization between the CPR firing and the mechanical stimulus is the mechanism by which the stimulus SNR is enhanced under light conditions; these results agree with theoretical predictions (Neiman et al. 1999c). Synchronization between the two photoreceptor cells is also investigated. It is found that while both cells synchronize their firing with a periodically applied stimulus, there is little, if any, statistically significant synchronization between the two photoreceptors. The mutual synchronization between the two CPRs is not affected by light. These results are consistent with previous experimental results indicating that there is no mutual excitation between the CPRs (Flood and Wilkens 1978); the results also indicate that the responses to hydrodynamic stimuli on the left side of the crayfish are relatively isolated from those on the right. The possible role of synchronization for information processing by the crayfish is discussed.