RESPONSE DYNAMICS AND DIRECTIONAL PROPERTIES OF NONSPIKING LOCAL INTERNEURONS IN THE COCKROACH CERCAL SYSTEM

The response properties and directional receptive fields of nonspiking local interneurons in the cercal system of the cockroach are describe d. Wind-evoked responses were recorded intracellularly, and then analy zed by means of the Wiener kernel method in which a Gaussian white noi se signal was used as a stimulus. Cross-correlation between the respon se and the white noise signal produced first- (linear) and second-orde r (nonlinear) kernels that were used to define input-output characteri stics of the interneurons. Three sets of interneurons were distinguish ed on the basis of kernel analysis. First, responses in interneurons 1 01, 107, 111, and 203 were characterized predominantly by a differenti ating, first-order kernel, which suggests a linear relationship to the stimulus. The amplitude and waveform of the kernel changed with the c hange in stimulus angle, indicating that these four cells are directio nally sensitive. Second, responses in interneurons 102 and 103 were al so directionally sensitive but highly nonlinear. The first-order kerne l was biphasic, whereas the second-order kernel had an elongated depol arizing peak on the diagonal. The response dynamics were accounted for by a cascade of two filters, a linear band-pass filter and a static n onlinear filter, in which the nonlinearity is a signal compression (or a rectification). Third, responses in interneurons 104 and 201 consis t largely of the second-order nonlinear component. The second-order ke rnel, which had an elongated depolarizing peak or a hyperpolarizing va lley on the diagonal, did not show any directional preference. The sec ond-order nonlinearity was dynamic, and could be modeled by a band-pas s linear filter-static nonlinearity-low-pass linear filter cascade, wh ere the static nonlinearity is a full-wave rectification. The band-pas s filter would simply reflect the mechanical property of cercal hair s ensilla, whereas the low-pass filter represents the transfer at synaps es between the cercal afferents and the interneurons. The nonlinear re sponse thus explains the difference in the directional sensitivity whi le the differentiating first-order kernel explains the velocity sensit ivity of the interneurons. We show that 101 and 107 respond most prefe rentially to wind from the left versus right, whereas 102, 103, 111, a nd 203 respond to wind from the front versus rear. Thus, it is suggest ed that there are two subsystems responding maximally to wind displace ment along two coordinate directions, one for the longitudinal directi on and the other for the transverse direction. On the other hand, the full-wave-rectifier nonlinear interneurons are omnidirectional, and th us suggested to code simply the power of the wind displacement.