A method was developed to investigate transfer properties of neurons in the visual system using pictures of complex visual stimuli. The picture is moved over the receptive field of a neuron so that it can scan it along programmed lines. The activity of the neuron during the scanning procedure is presented in a two-dimensional dot display on scale with the original picture. By superposition of the stimulus and the transfer pattern, one can find out to which detail of a stimulus the neuron responds. Neurons in the first intracerebral relay of the visual system, the lateral geniculate body, reduce a complex stimulus, such as a photograph of a natural environment, to its contours. Cortical cells only respond to contours either of a limited or of a wider range of orientations (simple and complex cells, respectively). But the course of contours is only described by a continuous representation of these contours in the cortical map of the visual field. This is done by the simple cells, which have small receptive fields and thus a higher resolving power, whereas complex cells with their large receptive fields monitor the approximate location of a moving stimulus. The function of these two classes of neurons is discussed in terms of visual behavior, i.e., for fixation, hold, and binocular vergence movements (simple cells), and for detection of moving objects and motor command signals towards these objects (complex cells). These functions are an important condition for foveal vision which is the basis of perception in primates. An important function of orientation sensitivity of simple cells may be the binocular alignment of contours in binocular fusion and stereoscopic vision.