Evidence for subtle verbal fluency deficits in occasional stimulant users: quick to play loose with verbal rules

Abstract

Psychostimulants like cocaine and amphetamine are commonly abused by young adults who often state that they take these drugs to increase social or cognitive performance. The current study tested the hypothesis that individuals at early stages of occasional stimulant use show subtle executive dysfunctions such as verbal fluency deficits. 155 young (age 18-25), non-dependent occasional users of stimulants and 49 stimulant naïve comparison subjects performed the Delis-Kaplan Verbal Fluency test. Correlation and median split analyses were conducted to account for stimulant history and co-drug use. Compared to stimulant naïve subjects, occasional stimulant users generated significantly more responses on an over-learned verbal fluency task (Category Fluency), but at the expense of increased error rates (Set Loss and Repetition Errors). These performance differences were not related to lifetime uses of stimulants or marijuana. Taken together, these results support the hypothesis that individuals who are using stimulants occasionally exhibit subtle executive dysfunctions when required to generate verbal sets under time pressure. In particular, occasional stimulant users apply quickly but inaccurately verbal rules, which may represent a mix of diminished cognitive flexibility along with increased rigidity and impulsivity. This specific executive dysfunction may help to identify individuals at risk for stimulant use or dependence.

Figure 1

While outperforming comparison subjects on the over-learned capacity to recall words from a semantic group (Category Fluency), stimulant users produced more Set Loss and Repetition Errors than comparison subjects, reflecting a tendency to disrespect rules which is known to be associated with frontal lobe impairments.

Figure 2

To investigate the effect of lifetime stimulant use on verbal fluency performance, stimulant users were divided into below and above median users (median_lnSTIM = 3.2581). Stimulant users with higher lifetime stimulant use significantly outperformed comparison subjects on the category fluency task (p=0.001).

Figure 3

Evidence for an effect of stimulant type on verbal performance: Stimulant users with a clear preference for recreational stimulants outperform comparison subjects while only a strong trend (p=0.052) was observed for prescription stimulant users.