Directionally asymmetric (DA) units respond preferentially to one direction of image movement. If that preferred direction is independent of stimulus contrast then the DA unit is considered directionally selective (DS). We have analyzed receptive-field (RF) properties of striate units with these properties by presenting bar-shaped stimuli that are moved in a stepwise sequence. Short interstimulus durations for certain ranges of step size elicit DA responses similar to those from smooth movement, while still allowing identification of on- and off-components of the response. 2. We have been able to isolate three mechanisms underlying DA and DS. The simplest, superposition, explains the dependence of preferred direction on stimulus contrast found in some DA units. It relies completely on asymmetries in static RF regions to provide an advantage for one direction of image motion by means of the simultaneity of image elements leaving an apparently inhibitory region and entering an excitatory one. 3. For all DA and DS units we have encountered forward inhibition of otherwise excitatory influences that reduces the responsiveness in the antipreferred direction. The spatial specificity of inhibitory target RF regions and the nonlinearity of the effect suggest that lateral inhibition may be transmitted via sequence-detecting subunits. 4. Units that do not show superposition in the preferred direction exhibit forward facilitation of responses in a nonlinear and target-specific way which suggests that facilitation may also be transmitted via sequence-detecting subunits. 5. Each of these mechanisms depends on short-lived influences that are laterally transmitted between 0.125 and 0.5 degrees in visual space. These spatial and temporal values are appropriate for the analysis of smooth movement by the visual system. 6. Stepwise movement sequences using dark bars on a bright background demonstrate for some DA units exactly the same mechanisms as demonstrated using bright-bar sequences in other units or, in the case of DS units, in the same units. In such DS units, which do not normally exhibit strong stationary RF asymmetries, differential sensitivity of the nonlinear DS mechanisms to stimulus elements of either contrast will yield an effective preferred movement direction for complex stimuli.
