Neural coding of salt taste quality in the blowflyCalliphora vicina

Summary

• 1. Previous work revealed that blowfliesCalliphora vicina can discriminate between alkali chlorides on the basis of salt taste quality, using their labellar taste organ (Maes and Bijpost 1979). This paper and the subsequent one (Maes and Ruifrok 1986) are concerned with the neural coding of salt taste quality in the blowfly. The present paper addresses the possibility of temporal coding. Two modes of temporal coding are considered: salt taste quality may be encoded in the over-all time course of the response, or in its fine-grain temporal pattern. The methodology of analyzing temporal characteristics receives special attention.

• 2. Data obtained are responses of salt receptor cells in 89 labellar taste hairs to a reference stimulus (lM KCl) and to a doubling concentration series of a test salt (LiCl, NaCl, KCl, RbCl or CsCl). Times of occurrence of salt receptor spikes were measured in the first 500 ms after stimulus onset, using an ‘electronic ruler’ at 0.2 ms resolution.

• 3. Analysis of over-all time course. Responses were converted into adaptation curves, i.e., curves of momentary firing frequency (reciprocal of interspike interval) vs time after stimulus onset. Mean adaptation curves for the reference stimulus fitted neatly into the array of curves for KCl, RbCl and CsCl as test salt, but seemed to adapt somewhat slower than the curves for LiCl and NaCl (Fig. 2). The significance of these deviations could not be assessed, however.

• 4. We then tried to quantify adaptation curve shapes by iterative curve fitting of nine mathematical functions. This attempt was not very successful, because either the functions did not fit well to all curves, or the parameters attained inconsistent values (Tables 1, 2).

• 5. A more robust quantification of the shape of the adaptation curve was provided by the ‘shape factor’ SF (ratio of spike rates in the 0–125 ms and 125–500 ms intervals), together with the mean spike rate SR in the 0–500 ms interval. In the SF-SR plane, the point representing the reference was on the trajectories for KCl, RbCl and CsCl, but significantly outside those for LiCl and NaCl (Fig. 4). The differences presumably arise from the different diffusion coefficients of these salts.

• 6. Analysis of fine-grain temporal patterning was restricted to the pseudo-stationary tonic response phase (150–500 ms). Interspike interval distributions and serial correlations of orders 1 to 10 were obtained. No salt-specific differences were found (Figs. 5–7).