The Strategies of Saccadic Planning: From Visual Inspection to Visual Search.

Chia-Chien Wu
Department of Psychology
Rutgers University


Saccadic eye movements play a crucial role in visual perception by 
bringing the line of sight to selected locations in visual scenes. 
The quality of visual performance depends on how efficiently saccades 
can be programmed. To understand the planning strategies of saccades, 
I conducted two studies, one dealing with rapid saccadic scanning,
and the other with visual search. The first study was an attempt to 
find out whether the classical speed-accuracy relationship, Fitts’s 
Law, applies to sequences of saccades. Subjects made saccades in 
sequence to four target circles located at the corners of an imaginary 
square. Performance of the sequences was in agreement with Fitts’s Law
in that the time to complete the sequence increased when the level of 
required landing precision, as well as the target eccentricity, 
increased. The increase in scanning time was not due to longer saccadic 
latencies, but to a higher frequency of secondary, corrective saccades 
when targets were small or far away. This suggests that there may be 
some processing advantages in making saccades at a high rate, relying 
on corrections, rather than delays, to clean up errors.

In the following study, saccade planning strategies were investigated 
with a more natural task. Subjects were asked to search through arrays 
of targets (thin circles containing oriented lines) embedded in non-
targets (thicker circles) either to estimate a statistical property of 
the targets (mean orientation of the lines), or to just look at the
targets (“look-only” task). Varying the visual similarity of targets 
and non-targets had large effects on the probability of landing on a 
target. Effects of target/non-target similarity on timing were 
noticeable only during the look-only task, when scanning was done at a 
fast rate. Despite the differences in timing, the probability of 
landing on a target was almost the same in the statistical estimation 
and look-only tasks.

Taken together, these results are consistent with the view that 
saccadic timing is controlled by the concurrent operations of at least 
two information accumulators, one analyzing foveal content and the 
other evaluating eccentric vision to select the target. The foveal 
timer may play a dominate role in the decision to trigger the saccade.