All computer-based tests were done with binocular viewing in a dark room. Each participant took part in four sessions. For the reading tests, participants were instructed to read the sentences aloud as accurately as possible when the stimuli were presented on the computer screen. Participants were allowed to complete their verbalization after the sentence disappeared from the display. If words in the sentence were reported out of order, for example, a correction made at the end of the sentence, credit was given for being correct. Participants were allowed to make eye movements during reading.
The first session was devoted to measuring the CPS, which is defined as the smallest print size that yields the maximum reading speed. The CPS was measured for each participant because we wanted to confirm that the two print sizes (0.08° and 0.15°) used in the main experiment straddled the participants' CPS values. We measured RSVP and flashcard reading speeds as a function of print size at the standard letter spacing. Six print sizes, in steps of √2, were used: 0.063°, 0.088°, 0.125°, 0.177°, 0.25°, and 0.35°, which covered the range of CPS measured by Chung (
2002) and Chung et al. (
1998). There were five exposure times per print size and four sentences (participant S1 did three sentences) per exposure time. Each participant was given a few minutes to practice before starting the test. Data from the practice trials are not included in this paper.
In sessions 2, 3, and 4, eight stimulus conditions (two print sizes × four letter spacings) were tested. For each of the eight stimulus conditions, performance was measured on three tasks—RSVP reading speed (measured for six exposure times and six sentences per exposure time), flashcard reading speed (measured for six exposure times and five sentences per exposure time), and trigram measurements to compile a visual-span profile (one profile was based on 234 trials, 13 positions, and 18 trials per position). There were 136 blocks of trials (48 blocks for the RSVP test, 40 blocks for the flashcard test, and 48 blocks for the trigram test). Fewer trials and blocks for flashcard reading were tested because we had fewer sentences in the flashcard sentence pool. The 136 blocks were divided into three sessions, and each session included 16 blocks of flashcard (8 blocks of flashcard in the first session), RSVP, and trigram trials. Every session was divided into halves. In the first half, the conditions with the larger print size were tested first, followed by the conditions with the smaller print size. For each print size, letter spacing was tested in a descending order, starting with the largest letter spacing. The order was reversed in the second half. The trial order in the first session was trigram (T), flashcard (F), RSVP (R), RSVP, and trigram. In sessions 2 and 3, the orders were FRTTRF and RTFFTR, respectively. All five participants were tested using the same sequence.
For both RSVP and flashcard reading, we computed reading speed from the exposure time that yielded 80% of the words read correctly on the fitted psychometric function. Reading speed was computed according to the following equation:
For flashcard reading, criterion exposure time for word equals criterion exposure time for sentence (in seconds) divided by 11.5, which is the average number of words per sentence across all the 411 flashcard sentences in the pool.
The trigram method was used to measure visual-span profiles of participants. A sample is shown in
Figure 2. During a trigram trial, participants were instructed to fixate between two vertically separated fixation dots. The participant was required to report all three letters from left to right following the presentation of a trigram. A letter was counted as correct only when it was reported in the correct position.
Visual-span profiles are plots of proportion correct letter recognition from the trigram trials as a function of horizontal position left and right of the midline. Only data from the central 11 letter positions (from position −5 to 5) were used in plotting. At each of these letter positions, there were 18 trials in which the letter presented belonged to the outer, middle, or inner letter of the trigram. Thus, each data point was based on 54 trials. As shown in
Figure 2, split Gaussians were used to fit the plot with three parameters: the amplitude, left-side standard deviation, and right-side standard deviation. The resulting curve is called a visual-span profile (Legge et al.,
2001).
We quantified the size of the visual span by calculating the bits of information transmitted by it (
Figure 2). Information transmitted at a given slot on the visual-span profile ranged from 0 bits (for chance accuracy of 3.8% correct) to 4.7 bits (for 100% accuracy). Proportion correct letter recognition was transformed to bits of information using letter-confusion matrices measured by Beckmann (
1998), who computed the mutual information associated with confusion matrices. A plot of mutual information versus proportion correct letter recognition was well fitted by a straight line (mutual information = −0.036996 + 4.6761 × proportion correct letter recognition) and was used to transform proportion correct letter recognition to bits of information. We quantify the size of the visual span by summing across the information transmitted by the 11 slots of the profile (similar to computing the area under the profile).