Effects of number-of-letters on eye movements during reading are independent from effects of spatial word length

Word length is an important determinant of eye movement behaviour in reading. The current study is the first attempt to disconfound a word's number of letters from its spatial extent. In a sentence-reading experiment using closely matched stimuli, clear differences were observed between target words that subtended the same visual angle but differed in number of letters: the more letters in the word, the more fixations made on the word, and the longer the duration of these fixations. Analyses of the full set of sentence words confirmed these results for a wider range of word lengths, and are consistent with a role for number-of-letters distinct from spatial extent. The most plausible explanation for these findings is that long words are subject to a reater degree of visual crowding, which is costly for both temporal and spatial eye movement systems.