Control of Visually Guided Eye Movements
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.