In reply to Subtle increases in interletter spacing facilitate the encoding of words during normal reading:
The authors, Manuel Perea and Pablo Gomez provide their explanation on page 5 of their text, just after the sentence I quoted, and down to the bottom of the first column of text on that page. In this section, difficulties in integrating the word as a whole when the spacing was somewhat large, are mentioned. I am guessing the authors mean: in what Pelli and Legge call the uncrowded window, though this isn’t clear. But a change in the parafoveal preview benefit isn’t ruled out.