Assessing Receptive Vocabulary Knowledge in Individuals with Autism Using Implicit Measures

Background: Many individuals affected by autism fail to develop useful speech, and some never learn to express themselves in any functional way. An important question about nonverbal individuals with autism is whether their lack of expressive ability is necessarily accompanied by an equally severe deficit in receptive language.  Little research has addressed this question because of the difficulty of testing low-functioning participants and the insensitivity of most behavioral methodologies.  We have previously used eye movements (EMs), pupillary dilation (PD), and event-related potentials (ERPs) as measures of receptive vocabulary knowledge in normal adults and normally developing children, in whom self-report and behavioral accuracy served as measures of comparison.  Here, we use the same measures to assess receptive vocabulary knowledge in high-functioning and low-functioning individuals with autism.

Objectives: We hypothesize that eye movements, pupillary dilation, and the N400 component of ERPs could provide evidence of single-word comprehension in nonverbal individuals with autism, even in the absence of a behavioral response. We expect that eye movements will be faster to and fixate longer on pictures of known words, pupillary dilations will be greater when identifying unknown words, and an N400 congruency effect will be observed for known words, but not for unknown words.

Methods: Participants included lower-functioning, low-verbal individuals with autism and higher-functioning individuals with autism.  For the lower-functioning participants, caregivers completed checklists used to determine words expected to be known receptively by the participants; unknown stimuli were drawn from a pool of items developed for other subject populations.  Participants completed a) a forced choice recognition task (EM and PD), where four pictures were presented on a computer screen, along with an auditory token that named one of the pictured objects; and b) a congruity task (ERP), where single pictures were shown on the computer screen, accompanied by an auditory token that did or did not match the name of the pictured item. 

Results: Individuals with autism showed similar trends to that observed previously in normal adults and normally developing children for all three measures.  The eye movements of both high- and low-functioning individuals seemed to be faster and more accurate to the named picture in the known condition.  Changes in pupillary dilation also tended to be greater in the unknown condition for both groups.  For the low-functioning individuals tested to date, the amplitude of the N400 appeared to be greater in the incongruous condition, but only for known words.  This difference, to the extent that it was observed at all, tended to be smaller in higher-functioning individuals with autism. 

Conclusions: The three measures (EM, PD, ERP) differentiated known from unknown words, but in potentially different ways for the two participant groups.  Differences among the high- and low-functioning autism participants may reflect different developmental language trajectories among these participants.  Importantly, our implicit measures may prove to be useful measures of single-word comprehension in otherwise “nonverbal,” low-functioning individuals with autism, a group whose language abilities have been difficult to assess.