Suprathreshold contrast response in normal and anomalous trichromats

Maximum Likelihood Difference Scaling was used to measure suprathreshold contrast response difference scales for low-frequency Gabor patterns modulated along luminance and L-M color directions in normal, protanomalous, and deuteranomalous observers. Based on a signal-detection model, perceptual scale values, parameterized as d′, were estimated by maximum likelihood. The difference scales were well fit by a Michaelis-Menten model, permitting estimates of response and contrast gain parameters for each subject. Anomalous observers showed no significant differences in response or contrast gain from normal observers for luminance contrast. For chromatic modulations, however, anomalous observers displayed higher contrast and lower response gain compared to normal observers. These effects cannot be explained by simple pigment shift models and support a compensation mechanism to optimize the mapping of the input contrast range to the neural response range. A linear relation between response and contrast gain suggests a neural trade-off between them.

The elementary soil areas of the Mykolaiv-Horodok Opillia

During the investigation of soil structure of Mykoliv-Horodok Opillia we used the methods of detection (natural-cartometric, relief sculpture, quality-genetic) and interpretation (statistical cartometric, functional and analytical) of the soil structure. To characterize elementary soil areas of the key area the natural-cartometric method was used, it makes it possible to determine the parameters of the soil structure and obtain quantitative indicators for the key elementary soil areas (ESA) parameters, which are elementary soil areas composition and area ratio, complexity, fractionality, breakdown, heterogeneity and contrast range. The article presents results of a study of qualitative and quantitative characteristics of elementary soil areas of the Mykolaiv-Horodok Opillia. The degree of variability of ESA areas was identified and analysed with coefficient of the differentiation of the soil outlines and methods of variation statistics. Form, degree of tortuosity and elongation of area limits using the coefficient of the dismemberment were studied. The character of outlines of elementary soil areas within the area of research was investigated. The basic features of ESA borders depending on various environmental factors are identified. Certain laws of changing of the basic parameters of elementary soil areas, which form the soil structure of Mykolaiv-Horodok Opillia were traced. Key words: elementary soil area, structure of the soil cover, coefficient of variability of soil outlines, coefficient of the differentiation of the soil outlines, coefficient of the dismemberment of the soil boundaries.

Concealed by conspicuousness: distractive prey markings and backgrounds

High-contrast markings, called distractive or dazzle markings, have been suggested to draw and hold the attention of a viewer, thus hindering detection or recognition of revealing prey characteristics, such as the body outline. We tested this hypothesis in a predation experiment with blue tits ( Cyanistes caeruleus ) and artificial prey. We also tested whether this idea can be extrapolated to the background appearance and whether high-contrast markings in the background would improve prey concealment. We compared search times for a high-contrast range prey (HC-P) and a low-contrast range prey (LC-P) in a high-contrast range background (HC-B) and a low-contrast range background (LC-B). The HC-P was more difficult to detect in both backgrounds, although it did not match the LC-B. Also, both prey types were more difficult to find in the HC-B than in the LC-B, in spite of the mismatch of the LC-P. In addition, the HC-P was more difficult to detect, in both backgrounds, when compared with a generalist prey, not mismatching either background. Thus, we conclude that distractive prey pattern markings and selection of microhabitats with distractive features may provide an effective way to improve camouflage. Importantly, high-contrast markings, both as part of the prey coloration and in the background, can indeed increase prey concealment.

Amplitude of the transient visual evoked potential (tVEP) as a function of achromatic and chromatic contrast: Contribution of different visual pathways

We investigated how the stimulation mode influences transient visual evoked potentials (tVEP) amplitude as a function of contrast of achromatic and isoluminant chromatic gratings. The chromatic stimulation probed only responses to the red-green axis. Visual stimuli were monocularly presented in a 5° diameter circle, achromatic and chromatic horizontal gratings, 1 Hz pattern reversal stimulation, and achromatic and chromatic gratings, 300 ms onset per 700 ms offset stimulation. For the achromatic pattern reversal stimulation, a double slope function describes how the P100 amplitude varied as a function of log contrast which had a limb at low-to-medium contrasts and another limb at high contrasts. For the achromatic onset/offset stimulation, C2 amplitude saturated at the highest contrast tested and a single straight line described how it changed along most of the contrast range. Both presentation modes for chromatic gratings resulted in amplitude versus log contrast relations which were well described by single straight lines along most of the contrast range. The results may be interpreted as if at 2 cpd, achromatic pattern reversal stimulation evoked the activity of at least two visual pathways with high and low contrast sensitivity, respectively, while achromatic onset/offset stimulation favored the activity of a pathway with high contrast sensitivity. The neural activity in the M pathway is the best candidate to be the high contrast mechanism detected with pattern reversal and pattern onset/offset VEPs. The activity of color opponent pathways such as the P and K pathways either combined or in isolation seems to be responsible for VEPs obtained with isoluminant chromatic gratings at both presentation modes. When the amplitudes of chromatic VEPs were plotted in the same contrast scale as used for achromatic VEPs, chromatic contrast thresholds had similar values to those of the achromatic mechanism with high contrast sensitivity.