One dimension of impulsivity is the inability to forego small, immediate rewards for larger, delayed rewards. This deficiency can be quantified in a behavioral task known as delay discounting. In general, the person or animal is given a choice between a response that produces an immediate, small reward, and a response that produces a larger reward after some temporal delay. If the delay is sufficiently short, normal subjects prefer the larger reward; as the delay increases, preference shifts to the small, immediate reward. People with psychiatric disorders such as attention deficit hyperactivity disorder (ADHD), drug addiction, and pathological gambling, which are associated with increased impulsivity, will discount a delay more readily than controls (i.e., they prefer the immediate reward) [32
]. Likewise, rats chronically treated with cocaine respond more impulsively than saline treated rats in a delay-discounting procedure for food reward [34
]. Interestingly, OFC lesioned rats, rather than behaving more impulsively, favor the larger, delayed reward at delays that make sham-lesioned rats switch their preference to the smaller reward [35
]. This result suggests that the role of the OFC may not be to inhibit impulsive behaviors, but may be to assess and update the value of an outcome under changing conditions. The inability to alter behavior despite a decrease in the value of the outcome is reminiscent of habit behavior (see discussion above), and suggests an alternative role for the OFC in guiding behavior.
In addition, impulsivity involves an inability to inhibit prepotent (well-established) motor responses. Differences in response inhibition can be measured using the 5-choice serial reaction time (5-CSRTT), go/no-go, or stop-signal tasks. Humans with impulsive disorders such as ADHD, trichotillomania (repetitive hair pulling), and drug addiction have been shown to have deficits in motor response inhibition in the go/no-go and stop-signal reaction time tasks [4
]. The 5-CSRTT allows for measurements of response accuracy, response omissions, premature responses or impulsivity, and perseverative responses. OFC lesioned rats have increased omissions, premature, and perseverative responses, suggesting that the OFC dysfunction plays a role in impulsivity, but also in response flexibility [36
]. OFC lesions in the rat also produce deficits in the stop-signal reaction time task, again implicating the OFC in the inhibition of prepotent motor responses [37
The most common finding in animals and humans with OFC lesions is a reversal-learning deficit. OFC lesions cause reversals to be acquired more slowly, and involve continued performance of the response that is no longer rewarded [38
]. This result has often been interpreted as a failure to inhibit prepotent responses, but several lines of evidence suggest that the OFC may actually be important for encoding the outcome of the response (i.e., the presence or absence of the reward) [39
]. Thus, the reversal-learning deficit observed with OFC dysfunction could be considered a failure to devalue the reinforcer (that is no longer presented), which could be interpreted as an increase in habitual responding.