What pops out in positional priming of pop-out: insights from event-related EEG lateralizations

It is well established that, in visual pop-out search, reaction time (RT) performance is influenced by cross-trial repetitions versus changes of target-defining attributes. One instance of this is referred to as “positional priming of pop-out” (pPoP; Maljkovic and Nakayama, 1996). In positional PoP paradigms, the processing of the current target is examined depending on whether it occurs at the previous target or a previous distractor location, relative to a previously empty location (“neutral” baseline), permitting target facilitation and distractor inhibition to be dissociated. The present study combined RT measures with specific sensory- and motor-driven event-related lateralizations to track the time course of four distinct processing levels as a function of the target’s position across consecutive trials. The results showed that, relative to targets at previous target and “neutral” locations, the appearance of a target at a previous distractor location was associated with a delayed build-up of the posterior contralateral negativity wave, indicating that distractor positions are suppressed at early stages of visual processing. By contrast, presentation of a target at a previous target, relative to “neutral” and distractor locations, modulated the elicitation of the subsequent stimulus-locked lateralized readiness potential wave, indicating that post-selective response selection is facilitated if the target occurred at the same position as on the previous trial. Overall, the results of present study provide electrophysiological evidence for the idea that target location priming (RT benefits) does not originate from an enhanced coding of target saliency at repeated (target) locations; instead, they arise (near-) exclusively from processing levels subsequent to focal-attentional target selection

What are task-sets: a single, integrated representation or a collection of multiple control representations?

Performing two randomly alternating tasks typically results in higher reaction times (RTs) following a task switch, relative to a task repetition. These task switch costs (TSC) reflect processes of switching between control settings for different tasks. The present study investigated whether task sets operate as a single, integrated representation or as an agglomeration of relatively independent components. In a cued task switch paradigm, target detection (present/absent) and discrimination (blue/green/right-/left-tilted) tasks alternated randomly across trials. The target was either a color or an orientation singleton among homogeneous distractors. Across two trials, the task and target-defining dimension repeated or changed randomly. For task switch trials, agglomerated task sets predict a difference between dimension changes and repetitions: joint task and dimension switches require full task set reconfiguration, while dimension repetitions permit re-using some control settings from the previous trial. By contrast, integrated task sets always require full switches, predicting dimension repetition effects (DREs) to be absent across task switches. RT analyses showed significant DREs across task switches as well as repetitions supporting the notion of agglomerated task sets. Additionally, two event-related potentials (ERP) were analyzed: the Posterior-Contralateral-Negativity (PCN) indexing spatial selection dynamics, and the Sustained-Posterior-Contralateral-Negativity (SPCN) indexing post-selective perceptual/semantic analysis. Significant DREs across task switches were observed for both the PCN and SPCN components. Together, DREs across task switches for RTs and two functionally distinct ERP components suggest that re-using control settings across different tasks is possible. The results thus support the “agglomerated-task-set” hypothesis, and are inconsistent with “integrated task sets.

Analysis of the Sediment Filtering Action of Grassed Media

The movement of sediment in non-submerged flow through a rigid grass media was studied experimentally by simulating the media with cylindrical nails. Models of sediment movement were developed from probablistic reasoning and from the use of existing parameters describing total bed material in open channel flow. In the probability analysis, the percent sediment trapped was found to be a power function of the number of potential fall paths, Nf,a particle could make from the surface to the bed while traveling through the filter media. The percent trapped was also found to be an inverse power function of the Reynolds number ReT. The characteristic length used in the Reynolds number was a hydraulic radius calculated assuming rectangular open channel flow with a width equal to the spacing between elements and a depth equal to the depth of flow. This is defined as the spacing hydraulic radius, Rs. The percent trapped was finally related exponentially to a combined power function of Nf and ReT. Total bed material transport functions of Graf and Einstein were modified and evaluated as predictors of suspended and bed load. Bed shear was assumed to be equal to γRsS where γ is the weight density of water and S is channel slope. Both Graf\u27s and Einstein\u27s parameters were found to be good predictors of suspended and bed load. Based on the results of the study, procedures are proposed for analyzing the trapping capability of sediment by grass filters

Determination of Soil Contaminant Transport Parameters Using Time Domain Reflectometry

Proceedings of the 1993 Georgia Water Resources Conference, April 20-21, 1993, Athens, Georgia.A recent study of rural shallow drinking wells found that 4.6% of the wells in the Piedmont region of Georgia had nitrate levels above the EPA recommended level of 10 ppm nitrate nitrogen (Tyson and Issac, 1991). The most likely sources of this nitrate are septic systems, fertilizers, and manures. Nitrate transport models such as LEACHN (Wagenet and Hutson, 1989) can be used to investigate the contributions of these sources to groundwater contamination, but the models require soil transport parameters that are difficult to measure. Another problem is that we are interested in predicting nitrate transport at the field scale, but transport parameters are usually measured on a much smaller soil volume.Sponsored and Organized by: U.S. Geological Survey, Georgia Department of Natural Resources, The University of Georgia, Georgia State University, Georgia Institute of TechnologyThis book was published by the Institute of Natural Resources, The University of Georgia, Athens, Georgia 30602 with partial funding provided by the U.S. Department of Interior, Geological Survey, through the Georgia Water Research Institute as authorized by the Water Resources Research Act of 1984 (P.L. 98-242). The views and statements advanced in this publication are solely those of the authors and do not represent official views or policies of the University of Georgia or the U.S. Geological Survey or the conference sponsors

An Evaluation of Head-Mounted Displays in an Airborne Command and Control Simulation Environment

An examination of HMDs to ameliorate the problems associated with display clutter in an Air Battle Management environment was conducted. Information to complete tasks was given via two HMDs, on the primary display, or via paper. The results indicated that the paper condition engendered a higher percent of correct responses and faster response times in several of the tasks performed. The specific experimental results are presented and future experimental design propositions are discussed

Predictive distractor context facilitates attentional selection of high, but not intermediate and low, salience targets

It is well established that we can focally attend to a specific region in visual space without shifting our eyes, so as to extract action-relevant sensory information from covertly attended locations. The underlying mechanisms that determine how fast we engage our attentional spotlight in visual-search scenarios, however, remain controversial. One dominant view advocated by perceptual decision-making models holds that the times taken for focal-attentional selection are mediated by an internal template that biases perceptual coding and selection decisions exclusively through target-defining feature coding. This notion directly predicts that search times remain unaffected whether or not participants can anticipate the upcoming distractor context. Here we tested this hypothesis by employing an illusory-figure localization task that required participants to search for an invariant target amongst a variable distractor context, which gradually changed—either randomly or predictably—as a function of distractor-target similarity. We observed a graded decrease in internal focal-attentional selection times—correlated with external behavioral latencies—for distractor contexts of higher relative to lower similarity to the target. Critically, for low but not intermediate and high distractor-target similarity, these context-driven effects were cortically and behaviorally amplified when participants could reliably predict the type of distractors. This interactive pattern demonstrates that search guidance signals can integrate information about distractor, in addition to target, identities to optimize distractor-target competition for focal-attentional selection

Selective manipulation of target identification demands in visual search: the role of stimulus contrast in CDA activations

In classic visual pop-out search, response times are slowed remarkably when participants are required to precisely identify (e.g., vertical vs. horizontal orientation) as compared to simply localize (e.g., left vs. right position) a feature singleton target. This cost associated with stimulus identification has been recently proposed (Töllner, Rangelov, & Müller, 2012) to derive from the engagement of postselective recurrent processes that via feedback connections extract the information required for motor-response selection. Here, we examined whether the contralateral delay activity (CDA), an asymmetric neural marker generally assumed to reflect active maintenance of stimulus information in visual short-term memory (vSTM), may further index the degree of postselective processing requirements in visual search. Employing a compound-search task, we selectively manipulated the ease/difficulty of identifying the response-critical target orientation attribute (horizontal vs. vertical)—irrespective of the target-defining color feature (red vs. green)—by introducing different levels of stimulus-background contrast. As expected, we found a monotonic reaction time increase to be associated with gradually increasing CDA magnitudes as the stimulus contrast decreased. Thus, our findings provide direct evidence that CDA activations provide a useful tool to estimate the amount of postselective recurrent processing recruited to extract detailed object information from vSTM

How the speed of motor-response decisions, but not focal-attentional selection, differs as a function of task set and target prevalence

Over the last decades, the visual-search paradigm has provided a powerful test bed for competing theories of visual selective attention. However, the information required to decide upon the correct motor response differs fundamentally across experimental studies, being based, for example, on the presence, spatial location, or identity of the target item. This variability raises the question as to whether estimates of the time taken for (i) focal-attentional selection, (ii) deciding on the motor response, and (iii) response execution generalize across search studies or are specific to the demands of a particular task set. To examine this issue, we presented physically identical stimulus material in four different search task conditions, requiring target localization, detection, discrimination, or compound responses, and combined mental chronometry with two specific electroencephalographic brain responses that are directly linkable to either preattentive or postselective levels of visual processing. Behaviorally, reactions were fastest for localization, slowest for compound responses, and of intermediate speed for detection and discrimination responses. At the electroencephalographic level, this effect of task type manifested in the timing of the stimulus- and response-locked lateralized readiness potential (indexing motor-response decisions), but not posterior contralateral negativity (indexing focal-attentional selection), component. This result demonstrates that only the stage of preattentive visual coding generalizes across task settings, whereas processes that follow focal target selection are dependent on the nature of the task. Consequently, this task set-specific pattern has fundamental implications for all types of experimental paradigms, within and beyond visual search, that require humans to generate motor responses on the basis of external sensory stimulation