Spatial integration in the crustacean visual system: peripheral and central sources of non-linear summation

The spatial summing characteristics of the crustacean sustaining fiber were examined with concentric stimuli and observed to be highly dependent upon the stimulus intensity. In the dark adapted condition and with response criteria at or near the central threshold, summation is approximately linear. With moderate light adaptation or relatively large response criteria, spatial summation becomes markedly nonlinear. The nonlinearities appear as a diminishing sensitivity with decreasing stimulus diameters. These results are interpreted in terms of the nonlinear intensity-voltage transform of the retinula cell. An additional source of nonlinearity was observed under suprathreshold conditions with stimulus dia in excess of 10–12°. A two-point stimulus procedure indicated that these results are consistent with a lateral inhibitory mechanism operating within the excitatory field.