The Scheldt estuary revisited: occurrence and behaviour of particulate polychlorinated biphenyls and polycyclic aromatic hydrocarbons

Temporal and spatial variations in particulate PolyChlorinated Biphenyl (PCB) and Polycyclic Aromatic Hydrocarbons (PAH) concentrations were studied in the Scheldt estuary during a one-year period (May 1995-June 1996). This sampling campaign was arecurrence of a campaign held almost a decade ago. Both PCB and PAH concentrations in the Scheldt river were substantially lower in the 90's, but the Scheldt is still one of the most severely polluted rivers in the North Sea area. The annual input into the estuary was approximately 40 kg for total PCBs (sum of 8 congeners) and 3300 kg for total PAHs (19 compounds). The fraction transported to the North Sea is relatively small: 10-20 %.In the most recent sampling campaign the behaviour of particulate PCBs and PAHs found earlier was confirmed: as a result of the mixing of riverine with marine particulates the concentrations of the micropollutants generally decrease with increasing salinities, The quicker restoration of dissolved oxygen in the upper estuary gave rise to a more intense degradation of organic matter, and, probably as a result of cometabolism, degradation of low molecuiar weight PAH. Typical calculated half lives were 260 days for pyrene and 390 for fluoranthene

Goal-driven modulations as a function of time in saccadic target selection

Four experiments were performed to investigate the contribution of goal-driven modulation in saccadic target selection as a function of time. Observers were required to make an eye movement to a prespecified target that was concurrently presented with multiple nontargets and possibly one distractor. Target and distractor were defined in different dimensions (orientation dimension and colour dimension in Experiment 1), or were both defined in the same dimension (i.e., both defined in the orientation dimension in Experiment 2, or both defined in the colour dimension in Experiments 3 and 4). The identities of target and distractor were switched over conditions. Speed-accuracy functions were computed to examine the full time course of selection in each condition. There were three major results. First, the ability to exert goal-driven control increased as a function of response latency. Second, this ability depended on the specific target-distractor combination, yet was not a function of whether target and distractor were defined within or across dimensions. Third, goal-driven control was available earlier when target and distractor were dissimilar than when they were similar. It was concluded that the influence of goal-driven control in visual selection is not all or none, but is of a continuous nature. © 2007 The Experimental Psychology Society

Saccadic target selection as a function of time.

Recent evidence indicates that stimulus-driven and goal-directed control of visual selection operate independently and in different time windows (van Zoest et al., 2004). The present study further investigates how eye movements are affected by stimulus-driven and goal-directed control. Observers were presented with search displays consisting of one target, multiple non-targets and one distractor element. The task of observers was to make a fast eye movement to a target immediately following the offset of a central fixation point, an event that either co-occurred with or soon followed the presentation of the search display. Distractor saliency and target-distractor similarity were independently manipulated. The results demonstrated that the effect of distractor saliency was transient and only present for the fastest eye movements, whereas the effect of target-distractor similarity was sustained and present in all but the fastest eye movements. The results support an independent timing account of visual selection. © VSP 2006

The effects of saliency on manual reach trajectories and reach target selection

AbstractReaching trajectories curve toward salient distractors, reflecting the competing activation of reach plans toward target and distractor stimuli. We investigated whether the relative saliency of target and distractor influenced the curvature of the movement and the selection of the final endpoint of the reach. Participants were asked to reach a bar tilted to the right in a context of gray vertical bars. A bar tilted to the left served as distractor. Relative stimulus saliency was varied via color: either the distractor was red and the target was gray, or vice versa. Throughout, we observed that reach trajectories deviated toward the distractor. Surprisingly, relative saliency had no effect on the curvature of reach trajectories. Moreover, when we increased time pressure in separate experiments and analyzed the curvature as a function of reaction time, no influence of relative stimulus saliency was found, not even for the fastest reaction times. If anything, curvature decreased with strong time pressure. In contrast, reach target selection under strong time pressure was influenced by relative saliency: reaches with short reaction times were likely to go to the red distractor. The time course of reach target selection was comparable to saccadic target selection. Implications for the neural basis of trajectory deviations and target selection in manual and eye movements are discussed

Stimulus-salience and the timecourse of saccade trajectory deviations

The deviation of a saccade trajectory is a measure of the oculomotor competition evoked by a distractor. The aim of the present study was to investigate the impact of stimulus-salience on the time-course of saccade trajectory deviations to get a better insight into how stimulus-salience influences oculomotor competition over time. Two experiments were performed in which participants were required to make a vertical saccade to a target presented in an array of nontarget line elements and one additional distractor. The distractor varied in salience, where salience was defined by an orientation contrast relative to the surrounding nontargets. In Experiment 2, target-distractor similarity was additionally manipulated. In both Experiments 1 and 2, the results revealed that the eyes deviated towards the irrelevant distractor and did so more when the distractor was salient compared to when it was not salient. Critically, salience influenced performance only when people were fast to elicit an eye movement and had no effect when saccade latencies were long. Target-distractor similarity did not influence this pattern. These results show that the impact of salience in the visual system is transient. © 2012 ARVO