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At the onset of bistable stimuli, the brain needs to choose which of the competing perceptual interpretations will first reach awareness. Stimulus manipulations and cognitive control both influence this choice process, but the underlying mechanisms and interactions remain poorly understood. Using intermittent presentation of bistable visual stimuli, we demonstrate that short interruptions cause perceptual reversals upon the next presentation, whereas longer interstimulus intervals stabilize the percept. Top-down voluntary control biases this process but does not override the timing dependencies. Extending a recently introduced low-level neural model, we demonstrate that percept-choice dynamics in bistable vision can be fully understood with interactions in early neural processing stages. Our model includes adaptive neural processing preceding a rivalry resolution stage with cross-inhibition, adaptation, and an interaction of the adaptation levels with a neural baseline. Most importantly, our findings suggest that top-down attentional control over bistable stimuli interacts with low-level mechanisms at early levels of sensory processing before perceptual conflicts are resolved and perceptual choices about bistable stimuli are made.

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Voluntary control and conscious perception seem to be related: when we are confronted with ambiguous images we are in some cases and to some extent able to voluntarily select a percept. However, to date voluntary control has not been used in neurophysiological studies on the correlates of conscious perception, presumably because the dynamic of perceptual reversals was not suitable. We exposed the visual system to four ambiguous stimuli that instigate bi-stable perception: slant rivalry, orthogonal grating rivalry, house-face rivalry, and Necker cube rivalry. In the preceding companion paper [van Ee, R. (2005). Dynamics of perceptual bi-stability for stereoscopic slant rivalry and a comparison with grating, house-face, and Necker cube rivalry. Vision Research] we focussed on the temporal dynamics of the perceptual reversals. Here we examined the role of voluntary control in the dynamics of perceptual reversals. We asked subjects to attempt to hold percepts and to speed-up the perceptual reversals. The investigations across the four stimuli revealed qualitative similarities concerning the influence of voluntary control on the temporal dynamics of perceptual reversals. We also found differences. In comparison to the other rivalry stimuli, slant rivalry exhibits: (1) relatively long percept durations; (2) a relatively clear role of voluntary control in modifying the percept durations. We advocate that these aspects, alongside with its metrical (quantitative) aspects, potentially make slant rivalry an interesting tool in studying the neural underpinnings of visual awareness.

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The use of the non-ionic, water-soluble contrast agent iopamidol for myelography in seven horses is described. Contrast columns of diagnostic quality were produced in all seven cases and the procedure did not invoke any adverse reactions in the five cases which were recovered from general anaesthesia. It is concluded that iopamidol is a safe and effective contrast agent for myelography in the horse.

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The Waters (to-and-fro) system is a commonly used anaesthetic system in small animal clinical practice. Although it may be used as a totally closed system, it has been recommended that it be used as a semi-closed system with relatively high fresh gas flow rates. The performance of the Waters system was studied with reference to wastage of neat, unused fresh gas flow. The results were compared with the performance of a modified Waters system. Using a fresh gas flow rate of 2 litres/minute considerable wastage of fresh gas flow and volatile agent was noted using the Waters system; the wastage was less with the modified system. Marked savings of fresh gas flow and volatile agent could result by reducing the fresh gas flow to 0.5 litres/minute. The findings are discussed in the light of current small animal clinical anaesthetic practice.

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A total of 103 anaesthetic inductions were performed in horses for a variety of elective procedures. All cases were premedicated with acepromazine maleate (0.02 to 0.05 mg/kg body weight [bwt] intramuscularly [im]). In 50 cases (Group A) anaesthesia was induced by a single intravenous (iv) bolus of thiopentone sodium (11.1 mg/kg bwt or 1 g/90 kg bwt) followed immediately by a bolus of suxamethonium chloride (0.1 mg/kg bwt). In 53 cases (Group B) anaesthesia was induced using iv guaiacol glycerine ether (GGE) (approximately 50 mg/kg bwt) followed by a bolus of thiopentone at half the usual dose rate (5.6 mg/kg bwt or 1 g/180 kg bwt). Induction of anaesthesia was uneventful in both groups although in Group B it was particularly smooth. Following endotracheal intubation anaesthesia was maintained with halothane in oxygen administered via a circle system. The duration of anaesthesia was comparable between the two groups; however, the mean (+/- sd) time to standing in Group B, 35 +/- 22 mins, was significantly shorter than in Group A, 48 +/- 25 mins. The use of the GGE/thiopentone technique is discussed.

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Oesophageal obstruction as a result of persistence of the right aortic arch was diagnosed in a nine--and-a-half week old kitten. Thoracotomy was performed and corrective surgery successfully carried out. This uncommon condition is discussed.

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Des-tyr1-gamma-endorphin (DT gamma E) was administered intramuscularly in a dose of 1 mg/day for 10 days to 18 neuroleptic-free schizophrenic patients in a double-blind crossover design. Six patients showed either a slight or no antipsychotic response; seven patients showed a moderate antipsychotic response; and the remaining five patients showed a marked antipsychotic response. DT gamma E led to a decrease of plasma prolactin levels in patients treated with DT gamma E in the first period of experimental treatment as compared to those treated with placebo. Neither plasma levels of growth hormone and cortisol nor cerebrospinal fluid concentrations of homovanillic acid, 5-hydroxyindoleacetic acid, and 3-methoxy-4-hydroxyphenylglycol were affected by DT gamma E. Patients suffering from a hebephrenic or paranoid type of schizophrenia and those presenting relatively fewer negative symptoms were most susceptible to treatment with DT gamma E. These data confirm and extend previous findings that DT gamma E has antipsychotic properties in a number of schizophrenic patients.

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Peptides related to the C-terminal part of cholecystokinin, including ceruletide, were found to be active in a number of behavioral test procedures in rats, predicting antipsychotic action. In a subsequent clinical trial ceruletide was administered intramuscularly in a dose of 40 micrograms twice for 2 consecutive weeks to 6 neuroleptic-treated schizophrenic patients following a single-blind design. In 3 patients, a pronounced long-lasting antipsychotic effect was observed, while in the other 3 the response was less marked. It is suggested that ceruletide may be a potential antipsychotic peptide.

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In a double-blind study we compared fluvoxamine, a new selective serotonin re-uptake inhibitor, with clomipramine. In 36 female inpatients with vital depression, the antidepressant properties of fluvoxamine and clomipramine were studied. Eight patients did not complete the study. During a 4-week treatment period the Hamilton-, Zung-, Clinical Global Impression- and Leyden ratings showed, apart from the last scale in the fluvoxamine group, significant improvement in both groups. In the latter scale, a statistically significant difference was found in favour of clomipramine. Additional anxiolytic-sedative medication was required equally in both groups. CSF data in 10 patients are discussed. Non-specific electrocardiographic (ECG) repolarization disturbances were observed in both groups. Anticholinergic side effects were more prominent with clomipramine than with fluvoxamine; gastrointestinal symptoms and agitation were more prominent with fluvoxamine than with clomipramine. Fluvoxamine did not show particular advantages or disadvantages over clomipramine.

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The effects of combining large doses of xylazine (1.1 mg per kg intravenously) with ketamine, methohexitone and thiopentone were studied in four experimental ponies. Onset of anaesthesia was more delayed after ketamine than after the barbiturates. Assessment of smoothness of induction and recovery indicated that all three combinations were effective and acceptable. Injection of xylazine was followed by atrioventricular (A-V) block which could be prevented by the prior administration of atropine. Blood pressure was well maintained with all three combinations of drugs. Arterial oxygen tension decreased as soon as the ponies became recumbent but there were no marked changes in arterial blood pH or carbon dioxide tension. Cardiac output was measured in one pony and was found to be least affected by ketamine. There was no great difference between the recumbency times after ketamine and methohexitone but thiopentone produced a significantly longer period of recumbency. In every instance the animals stood at the first attempt without struggling or excitement. The ability of the three drug combinations to produce surgically useful anaesthesia was not tested.

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