Marijtje L.A. Jongsma

An expanded omitted stimulus paradigm was investigated to determine whether expectancy would modulate the amplitude of the omission evoked potentials (OEPs). In addition, we examined the effects of musical expertise on OEPs. Trials started with 3–7 beats randomly and contained 5 omitted beats. Three types of trials (n=90) were presented with 1, 2, or 3 beats occurring between omissions. A tap response at the end of each trial was used to determine timing accuracy. Clear OEPs were observed over midline sites. We found main omission effects with respect to an N150 and a P400 OEPs component, such that peak amplitudes diminished whenever the occurrence of an omitted stimulus could be expected. In addition, an N600 OEPs component emerged in response to expectedly omitted stimuli toward the end of each trial within the group of musicians. Thus, musical training seems to lead to more efficient and more refined processing of auditory temporal patterns.

The general aim of this study was to investigate how rhythmic information is processed by the brain and how a mental representation of a rhythm leads to expectancies about events in the near future. We investigate this by means of EEG recordings from which evoked potentials (EPs), resulting from sensory and cognitive neural activity, are extracted.

We investigated whether evoked potentials to omitted stimuli could be measured in rats. Such an animal model would provide a measure of aspects of information processing concerned with expectancy and time estimation. In a first experiment, omission evoked potentials (OEPs) were elicited in rats by omitting stimuli (10%) from a train of tone pips with a fixed ISI (3 s). A control session consisted of omitting stimuli (10%) from a train of tone pips with a variable ISI (2.5–3.5 s). In a second experiment, OEPs were measured in rats that received 4 mg·kg⁻¹ diazepam or vehicle sc. In the first experiment, half of the animals showed OEPs that consisted of a late-latency positive wave, the other rats showed a slow negative drift. No OEPs were found in the control session. Rats showing OEPs consisting of a positive wave in Experiment 1 were passed to Experiment 2. In the vehicle condition of the second experiment, all rats showed OEPs. In the diazepam condition no OEPs were found. We found that OEPs can be measured in rats. In addition, OEPs are disrupted by diazepam. We propose that OEPs provide an elegant tool to elicit selectively endogenous EP components.

The effect of diazepam on sensory gating was studied in rats by measuring diazepam effects on auditory evoked potentials (AEPs) elicited in a ten-tone paradigm. Trains of 10 repetitive tone-pip stimuli were presented. Rats (n = 8) received 4 mg·kg^–1 diazepam subcutaneously or vehicle, counterbalanced over two sessions. Diazepam decreased the amplitude of the middle-latency P₃₀ component and increased the amplitudes of the late-latency N₆₀ and P₆₇ components. The increase in the late-latency components might be due to a diazepam-induced decrease in arousal. Stimulus repetition decreased the amplitudes of the middle-latency N₁₈ and P₃₀ components in both conditions. This suggests that automated neuronal recovery functions underlying sensory gating remain intact with diazepam. In the vehicle condition, the amplitude of the late-latency P₆₇ decreased with stimulus repetition, but not in the diazepam condition. This suggests a diazepam-induced decrease of behaviourally mediated habituation.