Current theories of reading eye movements claim that reading saccades are programmed
primarily on the basis of information about the length of the upcoming word, determined
by low-level visual processes that detect spaces to the right of fixation. Many studies
attempted to test this claim by filling spaces between words with various non-space
symbols (fillers). This manipulation, however, confounds the effect of inserting extraneous
characters into text with the effect of obscuring word boundaries by filling spaces.
We performed the control conditions necessary to unconfound these effects. Skilled
readers read continuous stories aloud and silently. Three factors were varied: (i)
position of the fillers in the text (at the beginning, the end, or surrounding each
word); (ii) the presence or absence of spaces in the text; and (iii) the effect of
the type of filler on word recognition (from greatest effect to least effect: Latin
letters, Greek letters, digits and shaded boxes). The effect of fillers on reading
depended more on the type of filler than on the presence of spaces. The greater effect
the fillers had on word recognition, the more they showed reading. Surrounding each
word with digits or Greek letters slowed reading as much as filling spaces with these
symbols. Surrounding each word with randomly chosen letters, while preserving spaces,
slowed reading by 44-75%--as much as, or more than, removing spaces from normal text.
Removing spaces from text with Latin-letter fillers slowed reading by only 10-20%
more. We conclude that fillers in text disrupt reading by affecting word recognition
directly, without necessarily affecting the eye movement pattern.