Temporal resolution of colour vision in the honeybee

The temporal resolution of colour vision was measured in freely-flying honeybees by testing the performance of trained bees in discriminating between two stimuli, one of which presented a steady, homogeneous mixture of two colours, while the other offered a heterochromatic flicker between the two colours at various temporal frequencies. Pairwise combinations of the colours uv, blue and green were used, corresponding to the three receptor classes in the bee retina. For each colour combination, we determined a cutoff frequency beyond which discrimination no longer exists. For a given colour combination, the cutoff frequency depends upon the ratio of the intensities of the component colours, and is maximum at a particular ratio. The cutoff frequency at the optimum intensity ratio is approximately 100 Hz for each of the 3 colour combinations blue-green, green-uv and uv-blue, implying that colour computation requires ca. 10 ms. From the optimum intensity-ratios for the three colour combinations, we infer that the relative sensitivities of the green, blue and uv channels are approximately 1:1.3:24.

Visual scanning behaviour in honeybees

Freely flying bees were rewarded with sugar solution on a variety of black-and-white shapes as well as on coloured gratings in various training situations. In subsequent dual-choice tests, the bees' discrimination between the various shapes was measured. In addition, the bees were video-filmed while flying in front of the shapes. The scanning patterns thus obtained were then quantified in order to characterize scanning behaviour and its relationship to the geometrical parameters of the scanned shapes, investigate whether scanning plays a role in pattern discrimination and examine the influence of training on the characteristics of scanning. The scanning patterns clearly mirror the contours of the scanned shape in all cases, i.e. the bees fly along the contours contained in the shape. This behaviour does not depend on whether the scanned shape is one that was previously rewarded, or one that is completely novel to the bees. Comparison of the results of quantifying the scanning patterns with the results of dual-choice tests reveals that scanning behaviour is independent of discrimination performance. On the average, horizontal scanning directions occur more often than vertical directions. Variations of the training situation produce measurable differences in scanning behavior. However, except in the case of vertical scanning on a vertical grating, these differences are quite small, indicating that following contours is a largely stereotyped behaviour. Horizontal gratings are very well discriminated from vertical ones even if they offer contrast to only one receptor type, i.e. blue or green, demonstrating that the direction of contours is visible to the pattern recognition system even under these conditions. However, vertical and horizontal coloured gratings offering only blue-contrast do not elicit contour-following, whereas gratings offering only green-contrast do. Thus, the bees' scanning behaviour is colour-blind and most probably governed by the green receptors. We suggest that contour-following is the by-product of a behavioural mode which serves to prevent retinal image movement during flight in front of a contoured visual pattern.

Collateral pathways of the left gastric vein in portal hypertension

Since June 1974, 347 percutaneous transhepatic portal venographic studies were performed on 246 patients with portal hypertension who had had bleeding gastroesophageal varices. Of 234 patients in whom left gastric veins (LGV) (coronary) were demonstrated, 177 (75.6%) had a single LGV and 57 (24.4%) had multiple left gastric veins (21.8% had two LGVs, 2.1% had three LGVs, and 0.5% had five LGVs). Of 193 patients undergoing selective left gastric venography, spontaneous portosystemic communications to the left renal vein were found in 55, to the inferior vena cava in two, to the inferior pulmonary veins in five, to the pericardiophrenic vein in eight, to the right inferior phrenic vein in three, and to the left intercostal veins in one. Interportal communications with the left gastric vein and varices occurred from the left portal vein in 13, from the gastroepiploic vein in one, and from a superior mesenteric vein branch in one. The predominant drainage of esophageal varices was to the azygos vein in 78 of 155 patients, to the hemiazygos vein in 13, and to multiple small unnamed veins in the mediastinum in 57. Opacified varices did not extend above the level of the azygos vein arch in 71 of 130 patients; however, 59 continued cephalad to the azygos arch and drained through more superior veins of the thorax. Knowledge of the anatomy and incidence of each of these portosystemic or interportal venous communications is important to properly treat bleeding esophageal varices by surgery or angiographic embolization.

The value of postcardiac infarction exercise stress testing: identification of a group at high risk

We prospectively selected 154 patients who had survived acute myocardial infarction with electrical and/or mechanical complications to undergo an exercise stress test, four to six weeks after discharge from the Coronary Care Unit, Royal Melbourne Hospital. The patients exercised on a bicycle ergometer until they were unable to continue or had reached 70% of their predicted maximum heart rate for their age. Eighteen (11.7%) patients exhibited flat ST-depression of 1 mm or more on exercise. Six of these 18 patients later died (33.3%). Only seven (5.1%) of the 136 patients who exhibited no electrocardiographic change died during follow-up. We conclude that patients recording positive exercise stress test results after acute myocardial infarction, have a poorer prognosis and therefore should be considered for more intensive medical or surgical treatment.