  Reading saccades that occur within a single line of text are guided by the
size of letters. However, readers occasionally need to make longer saccades
(known as return-sweeps) that take their eyes from the end of one line of text
to the beginning of the next. In this study, we tested whether return-sweep
saccades are also guided by font size information and whether this guidance
depends on visual acuity of the return-sweep target area. To do this, we
manipulated the font size of letters (0.29 vs 0.39 deg. per character) and the
length of the first line of text (16 vs 26 deg.). The larger font resulted in
return-sweeps that landed further to the right of the line start and in a
reduction of under-sweeps compared to the smaller font. This suggests that font
size information is used when programming return-sweeps. Return-sweeps in the
longer line condition landed further to the right of the line start and the
proportion of under-sweeps increased compared to the short line condition. This
likely reflects an increase in saccadic undershoot error with the increase in
intended saccade size. Critically, there was no interaction between font size
and line length. This suggests that when programming return-sweeps, the use of
font size information does not depend on visual acuity at the saccade target.
Instead, it appears that readers rely on global typographic properties of the
text in order to maintain an optimal number of characters to the left of their
first fixation on a new line.
