Participants in this study were 16 adults with ASD and 16 typical adults. Adults with ASD were recruited via advertisements on the National Autistic Society website and by contacting people in online ASD communities and on social networking websites.
Typical participants were recruited from undergraduate and postgraduate courses at University College London and through social networking websites. All participants in the ASD group had received a clinical diagnosis of autism (n
= 3) or Asperger syndrome (n
= 13) from a trained, independent clinician who used the criteria listed in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition
(American Psychiatric Association, 1994
). Diagnosis was then confirmed by assessment with Module 4 of the Autism Diagnostic Observational Schedule (Lord, Rutter, DiLavore, & Risi, 2002
). None of the participants had any other mental or neurological disorder. Two participants with ASD and two typical participants were excluded from the sample because of error rates that were greater than 2.5 SD
above their group mean. The resulting 14 in each group remained matched for chronological age and nonverbal IQ using the matrix reasoning subscale from the Wechsler Abbreviated Scale for Intelligence (WASI) (see ). Independent sample t
tests indicated that the groups did not differ on any of these measures (all p
Descriptive Statistics for Each Group
Apparatus and Stimuli
Microsoft Visual Basic (version 6) was used to create computer-based stimuli that were presented on a custom-built small-form desktop computer and displayed on a ProLite 15-in. flat LCD screen (1280 × 1024 pixel resolution, 2-ms response rate). Viewing distance was 60 cm.
The task involved a dual-task paradigm that was based on that of Macdonald and Lavie (2008)
that required participants to identify a target letter (X or N) and then indicate the presence or absence of a meaningless small gray character (the Critical Stimulus [CS]) that was presented outside of the ring. Participants were presented with six letters that were placed, equally spaced, around the circumference of an imaginary ring with a radius of 1.7° visual angles. In each trial, one of the letters was the target letter (X or N) and, depending on the condition, the other ring positions were occupied by a nontarget letter that was perceptually similar to the target (Z, H, K, Y, or V) or a small letter O
(easy to distinguish from the target-letter). Target and similar nontarget elements measured 0.6° × 0.6° visual angles. The dissimilar nontarget element (O
) measured 0.2° × 0.2°.
The perceptual load of the task was manipulated by changing the number of similar nontarget letters and Os in the ring to create trials with set size 1 (the target letter and five Os in the ring), 2 (target, one similar nontarget and four Os in the ring), 4 (target, three similar nontarget elements and two Os), and 6 (target and five similar nontarget letters).
In 50% of the trials, the CS, a meaningless gray squiggle, measuring 0.3° × 0.3° visual angles was presented, outside of the ring of letters, in one of six positions that were arranged on the circumference of an imaginary ring with radius 5.4°. The six CS positions were such that each one lay on an imaginary line that passed through the center of the ring and bisected two adjacent letter locations. The location of the target, its proximity to the CS, and, in set size 4, the target letter's location within the group of similar nontarget elements (edge/middle) were counterbalanced across all of the trials.
The letters in the ring were presented in black on a light gray background (red green blue [RGB] values: 204,204,204) and the CS was a darker gray (RGB: 153,153,153). After the presentation of the experimental stimulus, a black mesh pattern that covered the entire screen other than the central 9.5°- by 9.5°-square, was presented as a mask. The clear square in the center was to ensure that the ring of letters was not masked.
Seventy-two trials of each set size were created. These were presented as four blocks of each set size in which each block contained 18 trials of the same set size. Participants were permitted to take short breaks between each block. Blocks were presented such that all 72 trials of any one set size were performed before moving on to four blocks of a different set size. The first four blocks presented to each participant contained trials of set size 2, and the presentation order of the other set sizes was randomly determined by the computer program.
A control block of 72 trials was also created in which participants were told not to search for the target letter but only to indicate the presence or absence of the CS. Eighteen trials of each set size, 50% containing the CS, were used to create the control block.
After a fixation cross that was presented for 1000 ms, the experimental display was presented for only 100 ms to preclude voluntary eye movement. The mask was then presented for 500 ms, followed by a blank screen for 1.4 s, a question mark for 100 ms, and a final blank screen for 1.9 s. The duration of the two blank screen phases was such that there was a 2-s response window for the letter identification and the CS detection tasks. Both of these 2-s windows elapsed regardless of the response time. If no response was given within the time limit, the trial was recorded as “time out” and classed as an error (see ). The entire experiment took approximately 20 min to complete.
Example of an experimental trial with low perceptual load (set size 2) and critical stimulus present.
Participants were told that they would see a ring made up of letters in the center of the screen and that one of those letters would be either an X or an N. They were told to press the Z key if an X was present and the X key if an N was present—and to respond as quickly as they could after they saw the ring of letters. Participants were also told that they should look for a little gray squiggle that would appear outside of the ring in some of the trials. They were instructed to wait until they saw the question mark and then indicate with a key-press (N for present, M for absent) whether the CS was present or absent in that trial. Stickers were placed over the Z, X, N, and M keys to clarify which key corresponded to which response. All participants were right-handed and used their right hand to press the N/M buttons and their left hand for the Z/X buttons.
After a set of practice trials, the participants completed the 16 experimental blocks and were able to take breaks between blocks if required. No difference was noted in the number or duration of breaks taken by each group. An incorrect response on the letter detection task elicited a brief computer tone, and participants were informed that this indicated that an error had been made. The accuracy and response times for each trial were recorded by the computer program and subsequent comparison of CS detection rates at the various levels' set sizes allow for the effect of perceptual load on dual-task performance to be ascertained. After completing the experimental trials, participants performed the control block to ensure that all of the participants were able to detect the CS. This was vital to confirm that any failures to detect the CS during the experimental trials were due to the perceptual load of the central letter search task and not an underlying inability to recognize or perceive the squiggle.