Joint modeling of performance and subjective reporting to assess sensitivity to drug-induced sleepiness

In an NASA ground study, two forms of cognitive tests were evaluated in terms of their sensitivity to sleepiness induced by the drug promethazine (PMZ). Performance for the two test modes (Y₁ and Y₂), PMZ concentration, and a self-reported sleepiness using the Karolinska Sleepiness Scale (KSS) were monitored for 12 h post dose. A problem arises when using KSS to establish an association between true sleepiness and performance because KSS scores are discrete and also because they tend to concentrate on certain values. Therefore, we define a latent sleepiness measure X^* as an unobserved continuous random variable describing a subject's actual state of sleepiness. Under the assumption that drug concentration affects X ^*, which then affects Y₁, Y₂, and KSS, we use Bayesian methods to estimate joint equations that permit unbiased comparison of the performance measures' sensitivity to X^*. The equations incorporate subject random effects and include a negativity constraint on subject-specific slopes of performance with respect to sleepiness.