Coding of Spatial Parameters by Cat Visual Cortical Neurons: Influence of Stimulus Orientation, Length, Width, and Spatial Frequency

Abstract

Hubel and Wiesel (1959, 1962) introduced the use of elongated edges and light and dark bars as stimuli for visual cortical cells. Edges and bars are characterized by an orientation and a length (dimension parallel to orientation). Bars in addition have a width (dimension orthogonal to the orientation) (Fig. 7/1). Campbell et al. (1968) introduced gratings in visual cortical physiology. Gratings all have a given length and orientation. Gratings can have different luminance profiles. Sinusoidal gratings are mathematically the simplest. A sinusoidal grating is characterized by its spatial frequency (cycles/ degree) which is the inverse of the spatial period (sp), and its amplitude or modulation depth (contrast, see Chapter 6). Other gratings used are square wave gratings or rectangular gratings of different duty cycles (ratio of light and dark parts). Two-dimensional noise fields were introduced by Hammond and MacKay (1975) and Orban (1975). Although one can produce noise fields with different spatial frequency content, the influence of the spatial parameters (e.g. pixel size) describing this pattern have not yet been studied. One-dimensional noise has recently been used by Burr et al. (1981). The spatial parameters characterize a stimulus whether stationary or moving. When elongated stimuli move, it is customary that their axis of movement is orthogonal to their orientation. The influence of stimulus orientation, a parameter ranging over 180°, will be described first.