DIRECTIONAL SELECTIVITY IN A NONSPIKING INTERNEURON OF THE CRAYFISH OPTIC LOBE - EVALUATION OF A LINEAR-MODEL

1. Intracellular recordings, sine wave gratings, and paired flashes we re used to characterize the directional selectivity (DS) of the periph eral neurons of the crayfrsh visual pathway. DS was observed in nonspi king tangential (Tan1) neurons of the distal medulla externa and it is expressed by the amplitude of the modulated synaptic potential elicit ed with drifting gratings. 2. The directional mechanism was characteri zed by variations in the grating contrast, spatial frequency, and temp oral frequency. DS is both contrast and velocity dependent. 3. The vel ocity dependence of DS for fixed stimulus contrast can be described by a linear model including a delay and subtractive compare operation. T his mechanism operates over the entire useful range of spatial and tem poral frequencies. 4. The parameters of the linear model can be estima ted from the spatiotemporal structure of the Tan1 cell receptive field . The receptive field exhibits a spatially asymmetric inhibitory subfi eld that is offset from the excitatory subfield by 3-5 degrees (1-2 om matidia). The inhibition is delayed relative to excitation by 50-100 m s. 5. The contrast dependence of DS reflects an apparent nonlinearity in the mechanism that determines the null response amplitude. The pref erred response magnitude is approximately linear with variations in co ntrast. 6. The nonlinearity observed in the null direction can in prin ciple be attributed to either a tonic excitation at 0 contrast or a th reshold for inhibition. There is evidence for both processes in the Ta n1 cell visual response.