Reading units that include interword spaces: filling spaces around a letter can facilitate letter detection.

College students searched for the letter "a" in prose passages typed normally, with an asterisk (Experiments 1 and 2) or the letter "x" (Experiment 3) replacing every interword space, or with asterisks replacing only some of the interword spaces (Experiment 2). Contrary to predictions based on masking through lateral interference but consistent with predictions based on studies of eye movement monitoring and unitization, asterisks or instances of the letter "x" surrounding the word "a" actually made the letter "a" easier to detect in that word, but generally not in other words in the text. It is concluded that for very common words, reading units may extend beyond the word boundary to include the surrounding interword spaces.