This study examined the relation between manual and oculomotor inhibitory control and domains of impulsivity in adults with and without ADHD. The major aim of this study was to determine whether the relation between inhibitory control and impulsivity differed according to the type of inhibitory control being assessed. Both a visual stopping task and a DORT assessed oculomotor inhibitory control by assessing participants' ability to inhibit a saccadic eye movement. A manual stopping task and a cued go/no go task were used to measure manual inhibitory control by assessing participants' ability to inhibit a manual response. The results revealed that visual SSRT was a stronger predictor of impulsivity than manual SSRT. This finding supports the notion that oculomotor inhibitory control is uniquely related to impulsivity. In addition, we sought to examine how the relation between behavioral and self-report measures differed between disinhibited and normal populations. The relations between oculomotor inhibitory control and several facets of impulsivity were most evident in those with ADHD and not controls. This suggests that oculomotor inhibitory control plays a role in the symptoms of impulsivity associated with ADHD.
The results of this study were supportive of our hypothesis that oculomotor inhibitory control tasks would be more closely related to self-reported impulsivity than manual inhibitory control. This is consistent with prior literature that has demonstrated this pattern of associations (Jacob et al., 2010
). Reasons why oculomotor inhibition showed a stronger relation to impulsivity are not entirely clear. It may be that oculomotor inhibitory control systems subserve general impulse control to a greater degree than manual inhibitory control. Indeed, oculomotor inhibition is critical for fundamental functions, such as the control and selection of attention to relevant stimuli and to the ability to effectively ignore irrelevant, distracting stimuli (Houghton & Tipper, 1994
). Norman and Shallice (2000)
reviewed experimental evidence implicating the control of attentional resources in action execution; these authors noted the importance of effective attentional control in tasks that “require the overcoming of a strong habitual response or resisting temptation” (p. 377). In the cognitive sciences, the premotor theory of attention posits a close link between attention allocation and saccadic eye movements, and experimental evidence has demonstrated a degree of interdependence between attention and manual response processes (Eimer et al., 2005
). Considering this, it is possible that disrupted inhibitory control of attention manifests as impulsivity.
Alternatively, the finding that oculomotor inhibitory control is more closely related to impulsivity than manual inhibitory control may relate to task characteristics. In addition to differences in response modality (i.e., ocular versus manual), these tasks differ in the type of response prepotency. Specifically, the oculomotor inhibitory control tasks require the inhibition of a reflexive response, whereas the to-be-inhibited response in the manual task is not reflexive in nature (Logan & Irwin, 2000
). The oculomotor inhibitory control tasks required participants to stymie a saccade towards a distracter stimulus. Visual orientation towards the abrupt presentation of a peripheral stimulus is an innate reflex that is present from early infancy (Johnson, 1995
). Conversely, the manual inhibitory control tasks required the participant to inhibit a button press. Although steps can be taken to increase response prepotency (e.g., response cueing, manipulating stop-trial frequencies, manipulating stimulus-onset asynchronies), these responses are prepotent inasmuch as the participant complies with the task instructions. Perhaps the executive override of an innate reflex better approximates impulse control in a natural setting than the inhibition of an artificial task demand. This notion might be tested with a novel inhibitory control task that requires the executive override of a reflexive manual response.
In sum, it appears that inhibitory control of oculomotor functioning bears a closer relation to self-report indicators of impulsivity than does inhibitory control of manual responses. However, it is important to recognize that despite this evidence for independence, the present study also demonstrated significant correlations between performance on the visual and manual stopping tasks that should be considered, as they demonstrate some degree of interdependence between the inhibitory control of manual and oculomotor responses.
There were also group differences in the degree to which measures of inhibitory control related to impulsivity. The tendency to display premature saccades on the DORT was more closely related to impulsivity in the ADHD group than the control group. This finding indicates that impairments of oculomotor inhibitory control are linked to impulsive behavior uniquely in adults with ADHD. This raises an important consideration for understanding the cognitive correlates of impulsivity in clinical populations. Even when appropriately decomposed into homogeneous personality constructs, there are likely to be multiple cognitive, behavioral, and affective factors that contribute to the traits measured by impulsivity inventories. Consider, for example, the basic psychological processes that are likely associated with lack of premeditation. One might speculate that effective planning requires the ability to hold a goal and various contingencies in working memory and inhibit responding until this information has been evaluated. Disruption in any of these cognitive processes may result in lack of premeditation.
Behavioral inhibitory control tasks, however, are designed to measure a relatively specific cognitive process (Dick et al., 2010
). The relative contribution of individual cognitive processes to complex personality traits may differ in populations characterized by psychopathology. It is possible that lack of premeditation is driven by impaired oculomotor inhibitory control uniquely in adults with ADHD, but not in other groups characterized by this type of impulsivity. For example, impaired premeditation in substance dependent individuals may relate to working memory deficits or reward sensitivity. Alternatively, the reported interactions could be interpreted as a restricted range of DORT performance in the control group. In either case, further work is needed to better understand how cognitive correlates of impulsivity differ in clinical groups.
Members of the ADHD group showed larger impairment on the oculomotor stopping task relative to the manual stopping task. This finding further highlights the dissociation between oculomotor and manual inhibitory control. Furthermore, it appears that oculomotor inhibitory control measures better capture the inhibitory deficits experienced by adults with ADHD. This is consistent with Adams and colleagues (2010)
who concluded that the oculomotor stopping task is more sensitive to the cognitive deficits observed in children with ADHD. In sum, adults with ADHD seem to have more difficulty inhibiting eye movements than manual movements, and this dysfunction in oculomotor inhibitory control relates to impulsivity— a core feature of the disorder. Taken together, this evidence suggests that oculomotor inhibitory control may be more important than manual inhibitory control in understanding neurological profile and functional impairment of adults with ADHD.
It is also interesting that among the variables of the cued go/no-go task, only the proportion of inhibitory failures in go cue/no-go target trials failed to show differences between groups. Prior work has shown that inhibitory failures during go cue/no-go target trials are elevated under conditions of disinhibition (e.g., alcohol intoxication; Marczinski & Fillmore, 2003
), whereas inhibitory failures during no-go cue/no-go target trials are often unaffected by such conditions. These findings can most likely be explained in terms of a speed-accuracy tradeoff (Fillmore, 2007
), because the ADHD group was slower to respond than the control group.
The findings of this study inform our understanding of the cognitive correlates of different facets of impulsivity. Most notable, premature saccades on the DORT correlated closely with scores on the UPPS: (lack of) Premeditation and the I7. These inventories similarly measure the lack of planning disposition discussed by Dick and others (2010)
. This overlap was confirmed in our data, as these two measures were correlated similarly with behavioral tasks. Individuals who are low in premeditation have difficulty planning out actions and anticipating consequences, and tend to act on previously rewarded behavior without reflecting on changing contingencies (Whiteside & Lynam, 2001
). Based on our data, it seems that lack of premeditation also relates to individuals' ability to inhibit reflexive behaviors based on contingencies (i.e., task instructions). Interestingly, oculomotor inhibitory control was not related to lack of perseverance. Inhibiting attentional shifts towards distractions has straightforward application to persevering in undesirable tasks. That this relation did not emerge empirically suggests that susceptibility to distracting stimuli may not be a determinant of task perseverance; perhaps susceptibility to endogenous sources of distraction better accounts for this trait (e.g., mind wandering; Gay et al., in press
The results of the current study contribute to our understanding of the relation between inhibitory control processes and impulsivity; however, there are some limitations. First, there were a large number of statistical tests conducted without corrections for inflated Type I error rate. As such, this research might be considered as preliminary and subject to replication. However, it is compelling that the pattern of findings were consistent across conceptually similar measures of impulsivity (i.e., I7, UPPS: [lack of] premeditation). Second, the use of a single ADHD group may have obscured differences between subtypes. It is possible that manual inhibitory control deficits are more central to the symptomatology of individuals with ADHD-combined type. Third, we limited participation to adults with ADHD who were currently taking prescription medication, and it may not be appropriate to generalize our findings to adults with ADHD who are not prescribed psychostimulant medication. Finally, there were several participants in the control group who scored in the positive range on one or more of the ADHD symptom self-report measures, and the inclusion of these participants may have attenuated between-group differences.
In sum, the present research provides new information supporting the distinction between manual and oculomotor inhibitory control processes. Researchers have recognized the complex nature of inhibitory control and the importance of decomposing the associated cognitive processes. Efforts have been made to identify which cognitive processes relate to impulsive traits; however, the distinction between manual and oculomotor inhibitory control has not been made in this context. By distinguishing between these types of inhibitory control the current study was able to show that oculomotor, but not manual, inhibitory control is related to specific facets of impulsive behavior. Furthermore, this study demonstrated the importance of considering clinical groups when examining the cognitive correlates of impulsivity.