Examining Visual and Manual Inhibitory Processes across ADHD Subtypes

1. Examining Visual and Manual Inhibitory Processes across ADHD Subtypes Zachary W. Adams, Richard Milich, Mark T. Fillmore University of Kentucky, Lexington, KY, 40506 e-mail: zacharywadams@gmail.com Table1.Diagnostic Characteristics by Group. Abstract This study compared inhibitory functioning among ADHD subtypes on manual and visual versions of the stop task. Seventy-six children, identified as ADHD/I (n = 16), ADHD/C (n = 42), and comparison (n = 18), completed both tasks. On the manual task, individuals with ADHD/I responded more slowly than ADHD/C and comparison children. Both ADHD groups were slower to inhibit and more likely to omit responses than comparison peers, and children with ADHD/C but not ADHD/I committed more inhibitory failures than comparison children. On the visual task no subtype differences were observed. Children in both ADHD groups were more variable, slower in inhibiting, and less likely to inhibit responses than comparison children. These findings extend existing evidence highlighting the role of inhibitory problems in ADHD, and contrast recent work suggesting divergence between ADHD subtypes. Method Stop Task (Logan, 1994) This task requires participants to respond when a stimulus (go signal) appears on the screen, but to withhold responding when a stop signal tone is presented. Stimuli were small, white circles presented on a black background using a computer monitor. Manual responses were recorded via button-press, while visual responses were recorded using an Model 504 Eye Tracking System (Applied Science–Laboratory, Boston MA). Eye locations were sampled at 60 Hz and given an X/Y coordinate. Visual responses were defined as >100 ms periods when participants’ gaze was at least 1/2 the angle to the target. Data were collected over 128 trials for each task; 32 stop trials were randomly presented. 900 Hz audio stop signals were presented at 8 times each at 4 different delays after stimulus onset (50, 150, 250, 350 ms). SSRT was calculated according to Logan (1994). Participants Participants were 76 children (mean age = 10.8 yrs, SD = 1.1). ADHD groups (ADHD/I n = 16 ; ADHD/C n = 18) were recruited from an outpatient psychiatry clinic, whereas the comparison group (n = 42) was recruited via fliers and ads in the community. Groups were determined via screening medical records, clinical interview, and parent-report questionnaires. Groups were equivalent across age, race, gender, education level, parental education, and KBIT vocabulary. Note. Comp = Comparison. CBCL = Child Behavior Checklist Parent Report. * p < .05. ** p < .01. *** p < .001.a. Comparison significantly different from all other groups.b. ADHD/C significantly different from all other groups. c. Comparison significantly different from ADHD/I, and ADHD/I significantly different from ADHD/C. Background Children with attention-deficit/hyperactivity disorder (ADHD) are at an increased risk for a wide range of negative life outcomes, including academic and professional problems, relationship difficulties, and substance abuse. A comprehensive understanding of the causal mechanisms underlying the disorder contributes to the development of effective preventive efforts. The impairment of basic inhibitory processes is widely recognized as a central deficit in ADHD (Barkley, 1997; Nigg, 2001). Current ADHD research is largely focused on understanding these deficits and how they may influence or interact with other facets of executive functioning, which, in turn, influence behavioral symptoms (Barkley, 1997; Nigg, 2005). Research has historically been limited to the combined type of ADHD (ADHD/C), leaving theorists to question the generalizability of findings to individuals with the inattentive subtype (ADHD/I). Indeed, recent reviews highlight several important variables across which the subtypes differ, challenging current conceptualization of ADHD/I and ADHD/C as subtypes of the same disorder (Diamond, 2005; Milich, Balentine, & Lynam, 2001). Despite a growing body of research in this area, several questions remain. The primary goal of the present study was to investigate inhibitory performance between carefully defined ADHD subtype groups using the stop task. The stop task has been used extensively in ADHD research as a measure of inhibitory functioning, and performance deficits have been consistently observed in ADHD relative to controls (for reviews, see Lijffijt et al., 2005; Oosterlaan, Logan, & Sergeant, 1998) Specifically, children with ADHD are slower and more variable in inhibiting responses than non-ADHD controls. In addition to limited attention to subtype differences, task-related differences are also largely ignored in the literature. For instance, investigators are increasingly utilizing oculomotor variants of the stop task (e.g.,Hanisch et al., 2006; Logan & Irwin, 2000), but it remains unclear how such versions compare to the traditional, button-press task. Thus a secondary goal was to determine whether manual and visual versions of the stop task result in similar findings across groups. Results & Conclusions Visual Task: There were no variables for which the ADHD subtype groups performed differently from each other. Based on visual stop task performance, then, there do not appear to be any unique performance characteristics associated with ADHD/I or ADHD/C with respect to inhibitory functioning or attention. Although the visual task was not useful in discriminating between subtypes, it was a more sensitive tool than the manual task for distinguishing children with ADHD from comparison children. Manual Task: As with the visual task, the subtype groups were indistinguishable with respect to measures of inattention on the manual task. Overall, these findings depart from recent evidence suggesting divergent response styles between ADHD subtypes. • Conclusions • Inhibitory processes measured by the stop task seem to function in a comparable fashion across response modalities. • The stop task was not useful in discriminating between ADHD/C and ADHD/I. Other basic forms of inhibition or neurocognitive processes may be better at differentiating the subtypes. • Impairments in inhibitory processes may lead to similar negative outcomes in ADHD/I and should be further studied. • Future work should continue testing the validity of current models of ADHD in ADHD/I. Table 2. Stop Task Performance for Manual & Visual Tasks. Note. Comp = Comparison. Go RT = Go Reaction Time. SSRT = Stop Signal Reaction Time. a. ADHD/I is significantly greater than Comparison and ADHD/C, p < .05. b. Comparison is significantly less than both ADHD/I and ADHD/C, p < .05. c. ADHD/I is significantly greater than Comparison, p < .05. d. Comparison is significantly less than ADHD/C, p < .05. Acknowledgements This research was supported by the National Institute on Drug Abuse grants DA021027 and DA005312.