Ictal stimulus processing during spike-wave discharges in genetic epileptic rats.

In the present experiment it was investigated whether and to what extent auditory information processing is possible during the presence of spike-wave discharges in rats. To that end, WAG/Rij rats which are an animal model for absence epilepsy, were provided with cortical electrodes for the registration of the electroencephalogram (EEG). The animals were first trained in an appetitively motivated conditioning paradigm to learn to discriminate between two auditory stimuli with equal duration and frequency but with different intensities. Next, the stimuli were presented in the test phase in pseudorandom order during spike-wave discharges. The reactivity of the ongoing EEG was analysed. It was found that the presentation of the reinforced stimulus induced a larger number of aborted spike-wave discharges than the non-reinforced stimulus, regardless of the intensity of the stimuli. This implies that during generalised spike-wave discharges the brain is still capable of evaluating the meaning of an ictally presented stimulus. It also shows that sensory, attentional and mnemonic processes are at least partially intact during the occurrence of a spike-wave discharge. The results of the present study are largely in agreement with results on human spike-wave activity-related cognitive disturbances. Moreover, they may lead to a refinement of the concept of epileptic consciousness and may emphasise the heuristic value of rodent models for studying both ictal and interictal information processing.

Serial and simultaneous feature positive discriminations in a human Skinner box.

In the present experiment participants were presented either a serial (F-->T+/T-) or a simultaneous (FT+/T-) feature positive discrimination using a Skinner box for humans. After the participants mastered the discrimination, the associative properties of F were examined using a transfer test and a counterconditioning manipulation. F affected responding to a transfer target less in the Serial than in the Simultaneous condition. However, counterconditioning of F did not affect initial discrimination performance in either condition. These results were discussed in the framework of occasion setting, elemental learning, configural learning, and a neural network model.

Contextual dependencies in a stimulus equivalence paradigm.

Two experiments with human subjects assessed contextual dependencies in a stimulus equivalence paradigm. Subjects learned to form two sets of stimuli in a matching-to-sample training procedure. Each set was presented against one of two different background colours, the contextual cues. At test, the influence of a context change-that is, presenting each set against the other context-was measured on baseline, symmetry, and equivalence trials. These three trial types reflect a difference in task complexity. It was predicted that the magnitude of context-dependent effects would be a positive function of task complexity. In Experiment 1, the context change was realized by switching the stimulus set at test while keeping the background colour constant. In Experiment 2, the stimulus set remained constant, and the background colour was switched. In both experiments, a change in context only resulted in an increase in response latency on equivalence trials; no effect was seen on symmetry and baseline trials. Results were discussed in the framework of switch costs, habituation to contextual stimuli, and a model based on Shea and Wright (1995) that explains the differential influence of a context switch on easy versus difficult tasks.

Acquisition of discriminations involving ambiguous or non-ambiguous features: an evaluation of two configural learning models.

The present rat experiment evaluated the validity of two formal accounts of configural learning in the framework of discrimination tasks involving the serial presentation of feature and target stimuli: Rescorla's (1973) modification of the Rescorla-Wagner model (1972) and the Pearce model (1987). The first, ambiguous feature task was of the form X-->A+, Y-->A-, X-->B-, Y-->B+, in which X and Y represent visual features, '-->' signifies a serial arrangement, A and B are auditory target stimuli, and '+' and '-' symbolise food-reinforcement and non-reinforcement, respectively. The second, non-ambiguous feature task was of the form: X-->A+, Y-->A-, X-->B+, Y-->B-. The former task was much more difficult to solve than was the latter task. The Rescorla model is able to account for the observed differences between the two tasks in learning rates and in the associative strength of feature X with more plausible parameter values than is the Pearce model. It is suggested that models acknowledging a role for both elemental and configural learning can better account for discrimination learning in discrimination tasks of the sort presented in this study than do models that exclusively allow for configural learning.

Effects of selective antagonism or depletion of the cholinergic system on visual discrimination performance in rats.

A two-lever simultaneous visual discrimination task was used to study the effects on performance in Long-Evans rats of the muscarinic antagonists scopolamine (0.0125, 0.05, 0.2 and 0.8mg/kg s.c.), the M(1) antagonist pirenzepine, the M(2) antagonist AF-DX 116, the M(3) antagonist UH-AH 37 (each 3.2, 10, 32µg/rat, i.c.v.), and the cholinergic depleting agent, hemicholinium-3 (0.04, 0.2, 1.0 and 5.0µg/rat i.c.v.). Scopolamine dose-dependently decreased accuracy, increased the number of trials on which the rats failed to respond, and significantly lengthened latency to respond. Only the highest doses of hemicholinium-3, pirenzepine and AF-DX 116 reduced accuracy and increased errors of omission as well as response latency. UH-AH 37 reduced overall task performance at 10 and 32µg, suggesting that antagonism of both M(3) and other muscarinic receptors (including M(1)) had a greater effect on performance than selective antagonism of the M(1) or M(2) receptors. These data indicate that the disruptive effects of cholinergic antagonism on attentionally demanding tasks are strengthened by activity at multiple subtypes of the receptor.