The Neural Control of Visually Guided Eye Movements

D. Temporal Factors in Visually Guided Saccadic Eye Movements

In 1982 Fischer and Boch reported that the distribution of saccadic latencies made to single targets is bimodal (see Figures 9 and 30). They termed the first mode of the bimodal distribution "express saccades" and the second mode "regular saccades." These findings have created considerable interest and consequently have generated numerous experiments that examined the rules of express saccade generation and the brain structures involved. Several theories have emerged, three of which will be considered in this section: (1) Express saccades reflect adaptive neural processes that have emerged in the course of evolution to enable organisms to cope expeditiously with suddenly appearing stimuli. (2) express saccades are a product of an artificial laboratory situation and involve predictive mechanisms, (3) express saccades arise in the course of learning that enables the organism to bypass some aspects of visuo-motor processing.

In the laboratory express saccades are typically obtained when stimuli are presented on a monitor that is otherwise homogeneously illuminated. First a fixation spot appears. When the subject looks at it for a brief time, the spot is extinguished and a target is presented at one of several locations. An example of the basic procedure appears in Figure 28. Click on the picture of the computer monitor to view the demo.