Effects of benzodiazepine receptor ligands on simultaneous visual discriminations of variable difficulty.

Benzodiazepine receptor (BZR) ligands have been demonstrated to affect the performance in tasks measuring attentional abilities. In such tasks, subjects typically are required to discriminate visual and/or auditory stimuli. The possibility that the effects of BZR ligands on the performance in tasks measuring attention are primarily due to effects on discriminative processes has not been tested systematically. Rats were trained to discriminate between simultaneously presented pairs of visual stimuli flashing either at 5 Hz versus 4.17, 3.75, 2.5, 1.67 or 1.25 Hz (group 1; FAST), or at 1.25, 1.46, 1.67, 2.5 or 3.33 Hz versus 5 Hz (group 2; SLOW) for 4.8 s (20 trials per discrimination type; sequence of pairs was randomized). In both groups, response accuracy depended significantly on the discriminability of the stimuli, with near perfect accuracy in response to most different pairs of stimuli and near chance-level accuracy in response to least different pairs of stimuli. Administration of the BZR full agonist chlordiazepoxide (1.56, 6.25, 9.38 mg/kg; i.p.) potently increased the number of errors of omission which, following the higher doses, confounded the effects on absolute numbers of correct and incorrect responses. However, the available data do not suggest that the agonist affected the animals' abilities to discriminate between the stimuli. Similarly, administration of the BZR ligands ZK 93 426 and MDL 26,479 (which negatively modulate GABAergic transmission) produced no systematic effects. These data suggest that the effects of BZR ligands on the performance in tasks measuring attentional abilities are not primarily due to effects on the animals' ability to discriminate sensory stimuli.