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_at_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 ScienceLaboratory,
Boston MA). Eye locations were sampled at 60 Hz
and given an X/Y coordinate. Visual responses
were defined as gt100 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 lt .05. p lt .01.
p lt .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/hypera
ctivity 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 lt .05. b. Comparison is
significantly less than both ADHD/I and ADHD/C, p
lt .05. c. ADHD/I is significantly greater than
Comparison, p lt .05. d. Comparison is
significantly less than ADHD/C, p lt .05.
Acknowledgements This research was supported by
the National Institute on Drug Abuse grants
DA021027 and DA005312.