Information theoretic analysis of dynamical encoding by filiform mechanoreceptors in the cricket cercal system

J. C. Roddey and G. A. Jacobs Department of Molecular and Cell Biology, University of California, Berkeley 94720, USA. 1. The stimulus/response properties of 20 mechanosensory receptors in the cricket cercal sensory system were studied using electrophysiological techniques. These receptors innervated filiform hairs of various lengths and directional selectivities. Previous studies have characterized the sensitivity of such cells to the direction of air currents and to the amplitude of sinusoidal stimuli. In the experiments reported here, the quantity and quality of information encoded in the receptors' elicited responses about the dynamics of more complex air current waveforms were characterized. 2. Based on a white analysis of receptor response properties, the median frequency of each receptor's frequency tuning curve was found to be strongly correlated with the length of its associated mechanosensory hair. The receptors connected to hairs > 900 microns encoded frequencies below approximately 150 Hz very accurately and the receptors connected to shorter hairs encoded progressively higher bands of frequencies. These results were interpreted within the constraints imposed by the biomechanics of the air current-to-cercus boundary. 3. The encoding accuracy was expressed in the information theoretic units of bits/second, which characterizes the information transmission rate of a receptor. The information rates of the neuronal spike trains ranged from 75 to 220 bits/s. The information transmission rate was not correlated with the length of the mechanosensory hair. The average amount of information transmitted per action potential was negatively correlated with receptor hair length and ranged between 0.6 and 3.1 bits/spike. Decoding of the receptor responses was restricted to linear transformations of the spike trains. 4. The stimulus/response latencies of the different receptors ranged between 5 and 11 ms, and the integration time of the receptors ranged between 8 and 30 ms. The latency of a receptor was only weakly correlated with the length of its associated hair, and a receptor's integration time was correlated with hair length. 5. The stimulus/response phase difference for receptor cells that innervated hairs longer than approximately 800 microns increased with frequency > 50 Hz. The phase responses for receptor cells connected to hairs < 800 microns did not vary for frequencies > 50 Hz.