Effects of sensory modality and task relevance on omitted stimulus potentials.

Omitted stimulus potentials (OSPs) occur when a sensory stimulus is unexpectedly omitted. They are thought to reflect predictions about upcoming sensory events. The present study examined how OSPs differ across the sensory modalities of predicted stimuli. Twenty-nine university students were asked to press a mouse button at a regular interval of 1-2 s, which was immediately followed by either a visual or auditory stimulus in different blocks. The stimuli were sometimes omitted (p = 0.2), to which event-related potentials (ERPs) were recorded. The results showed that stimulus omissions in both modalities elicited ERP waveforms consisting of three components, oN1, oN2, and oP3. The peak latencies of these components were shorter in the auditory modality than in the visual modality. The amplitudes of OSPs were larger when participants were told that the omission indicated their poor performance (i.e., they pressed a button at an irregular interval) than when it was irrelevant to their performance. These findings suggest that OSPs occur from around 100 ms in a modality-specific manner and increase in amplitude depending on the task relevance of stimulus omissions.

A case of poisoning in a man who drank a nutrition supplement containing methomyl, a carbamate pesticide.

A 50-year-old man was admitted to the emergency room complaining oppression on his chest, sweating and vomiting. He had drunk a 30 ml volume nutrition supplement 60 minutes before. As myosis and decrease of serum choline esterase activity were observed on admission examination, poisoning was suspected and toxicological analyses were carried out on the heeltap of the drink. Drug screening by gas chromatography-mass spectrometry (GC/MS) revealed the presence of methomyl and the concentration of methomyl in the heeltap determined by liquid chromatography was 2.1 mg/ml. Methomyl concentrations in the serum and urine were determined after converting methomyl to its oxime form followed by derivatization and GC/MS. Methomyl concentration in the serum collected 6 hours after ingestion was 0.63 microg/ml, and that in the urine collected 7-20 hours after ingestion was 0.10 microg/ml. Based on these values and reported data, the amount of methomyl contaminated to the drink was considered to be a toxic dose.