Illuminating hydrological processes at the soil-vegetation-atmosphere interface with water stable isotopes

Funded by DFG research project “From Catchments as Organised Systems to Models based on Functional Units” (FOR 1Peer reviewedPublisher PDFPublisher PD

Evaporation fractionation in a peatland drainage network affects stream water isotope composition

There is increasing interest in improving understanding of evaporation within a catchment for an enhanced representation of dominant processes in hydrological models. We used a dual‐isotope approach within a nested experimental design in a boreal catchment in the Scottish Highlands (Bruntland Burn) to quantify the spatiotemporal dynamics of evaporation fractionation in a peatland drainage network and its effect on stream water isotopes. We conducted spatially distributed water sampling within the saturated peatland under different wetness conditions. We used the lc‐excess—which describes the offset of a water sample from the local meteoric water line in the dual‐isotope space—to understand the development of kinetic fractionation during runoff in a peatland network. The evaporation fractionation signal correlated positively with the potential evapotranspiration and negatively with the discharge. The variability of the isotopic enrichment within the peatland drainage network was higher with higher potential evapotranspiration and lower with higher discharge. We found an increased evaporation fractionation toward the center of the peatland, while groundwater seepage from minerogenic soils influenced the isotopic signal at the edge of the peatland. The evaporation signal was imprinted on the stream water, as the discharge from a peatland dominated subcatchment showed a more intense deviation from the local meteoric water line than the discharge from the Bruntland Burn. The findings underline that evaporation fractionation within peatland drainage networks affects the isotopic signal of headwater catchments, which questions the common assumption in hydrological modeling that the isotopic composition of stream waters did not undergo fractionation processes

The Prospects of Utilizing Geometrical Visual Illusions as Tools for Neuroscience

Geometrical visual illusions have long been used as tools in neuroscience. Most commonly, researchers have taken illusions as a given and attempted to explain phenomenal impressions in terms of known neural mechanisms. In a psychophysical approach to this topic, it is customary to modify stimuli until conditions for which illusions are enhanced, attenuated, or annihilated have been found. Additionally, the focus is not exclusively on response bias but equally on sensitivity, because observers may fall prey to an illusion but at the same time be able to discriminate between stimuli perfectly. For the T-figure, the length of the undivided line is usually overestimated relative to the length of the divided line, and evidence has accrued that suggests that the illusion may be due to the processing of the figure as a coherent unit (a “T-schema”). Dissecting the T or tilting its lines influenced the amount of illusion, suggesting that interactions between orientation-sensitive and end-inhibited neurons are at work. Examples of cognate research with the Ponzo, Ebbinghaus, and Müller-Lyer illusions are also discussed

The ⊥-Illusion Is Not a T-Illusion

Variants of the capital Latin letter T were prepared with the straight strokes replaced by J-, C-, or S-curves, mimicking handwritten Ts. These were used to test the hypothesis that the overestimation of the length of the T’s undivided line, relative to the length of its divided line may be understood as an adaptation to a corresponding letter schema. The illusion was greater for S-curve Ts than for C- and J-curve Ts, suggesting that approximate bilateral mirror symmetry is more important for the illusion to occur than letterness. Despite the illusion, observers were quite sensitive to the different lengths of the Ts’ curves

Geometrical Visual Illusions Revisited: The Curse of Symmetry, the Cure of Sighting, and Taxing Task Demands

For nine popular geometrical visual-illusion figures, a mathematical analysis is provided along with a characterization of the figures’ psychological effectiveness. Supported by graphical illustrations, for the L and the T, it is shown how mathematical singularities of these figures can be isolated, and the illusions annihilated. For the Poggendorff, the Hering, and the Zöllner figures, building on observations from Kennedy and Portal (1990), sighting the figures from specific vantage points at a shallow angle is proposed as a means to overcome these illusions. For the T, the Oppel–Kundt, the Müller–Lyer, and the Ebbinghaus figures, a new experiment demonstrated that observers were able to find a slant of the stimuli at which the illusory impressions vanished. Task demands on part of the beholders comprise discrimination and identification. The observed independence of response bias and sensitivity in psychometric functions can possibly be explained by the intrusion of identifying responses into discrimination tasks

Visual Discrimination of the 17 Plane Symmetry Groups

Within most of the 17 plane symmetry groups, individual symmetry operations act in multiple, nonequivalent ways. This, and the fact that many groups can be realized on the basis of different unit cells and generating regions, poses difficulties for visual discrimination and identification. Because of inherent confounds, only few of the groups can be studied by traditional experimental methodology. The use of an oddity paradigm and specific tiling patterns that camouflage groups in complex textures are recommended as partial remedy to this impasse. In order to prepare readers for an appreciation of the aforementioned issues and to provide a rationale for their investigation, the reporting of experiments and the discussion of methodological problems is preceded by a brief overview of the role which symmetry has played in the visual arts

Effects of distance and eye-height on time-to-contact estimates

When making time-to-contact estimations, observers often factor misleading information into their judgments, maybe sacrificing accuracy for the sake of decision speed. We investigated to what extent judgments are vulnerable to misleading information caused by changes in observer perspective. Five experiments tested effects of distance and eye-height on predictions of object-object collisions seen against the background of a regular, an irregular, or a null ground-surface texture. By selectively keeping either viewing time, time-to-collision, or velocity constant, we unconfounded these variables. A total of 62 observers responded relatively earlier at greater distances, lower eye-heights, and longer times-to-collision. Ground-surface texture was practically ineffective. Results are discussed with regard to the controversies regarding the effects of irrelevant cues on the ability to extract the information relevant for making reliable estimates

Visual discrimination thresholds for time to arrival

In a seminal article, Todd (Journal of Experimental Psychology: Human Perception and Performance 7:795-810, 1981) reported a difference threshold of about 50 ms to discriminate the times of arrival of two differently sized objects that simultaneously approached head-on at constant but different velocities. Subsequent investigators, however, have often found much higher thresholds. We did one complete replication of Todd's experiment, and then modified his stimuli and experimental regime, which we hypothesized may have been responsible for some of the discrepancies reported in the literature. Unlike Todd and most other researchers, we exclusively used untrained observers. Several of our participants performed almost as well as the trained observers used by Todd and others, but the performance of most of our participants fell short of this standard. Furthermore, thresholds were affected by the experimental regimes, with large differences between objects' sizes and speeds compromising performance. Analyses of the response patterns revealed that the responses were driven mainly by the objects' relative apparent sizes