Effects of stimulus duration and inter-letter spacing on letter-in-string identification

Effects of stimulus duration and inter-letter spacing were studied in a letter-in-string identification paradigm. Participants were shown strings of 5 random consonants (e.g., PGKDM) centered on fixation and were asked to identify the letter that had appeared at a post-cued location. Stimulus duration was manipulated in Experiment 1 (13 ms-91 ms), and inter-letter spacing manipulated in Experiment 2 (for a fixed stimulus duration of 26 ms). We contrasted performance to outer-letters (positions 1 and 5) with non-central inner letters (positions 2 and 4), the first-letter (position 1) with the final letter (position 5), and the central-letter (position 3) with the other inner letters (positions 2 and 4). The outer-letter advantage and the first-letter advantage were present throughout the entire range of exposure durations, whereas the central-letter advantage increased with longer exposures. On the other hand, increased spacing reduced both the outer-letter advantage and the first-letter advantage, whereas it led to a greater central-letter advantage. Changes in acuity and crowding as a function of stimulus exposure and inter-letter spacing, can account for this pattern of results.