The basic idea is shown in Figure 2. No preattentive process can identify black vertical conjunctions but a color process can mark all black items and an orientation process can mark all vertical items. However, the eventual goal of a model of visual search must be to handle real world images like those that interested Enoch in the 50's. GS had ignored two factors central to Enoch's interests. First, the eyes move during real-world visual search.
Most searches involves an interplay of covert attentional movements and overt eye movements.
Second, visual processing is much more detailed at the fovea. Presumably, eye movements in search exist to direct the fovea toward regions of interest. Enoch found that viewers spent much more time looking at the center of an image. The simulation of GS3 starts with a Gray Scale Image as its input. Next the image is processed by an array of On and Off-Center Units, approximating retinal ganglion cell response.