Latency effects in orientation popout

AbstractA target that differs in orientation from neighboring lines and “pops out” has been found to evoke larger responses in cortical V1 cells than lines in the uniform texture surround which do not popout (e.g., Journal of Neurophysiology 67 (1992) 961). If this is more than a coincidence of observations, physiological properties of contextual modulation should be reflected in the perception of salience. In particular, as the differential suppression from texture surround has been reported to be delayed, target salience may be affected by the history of surrounding lines, i.e. by their orientation before the target was presented. This was tested using a feature flicker paradigm in which target and background lines changed their orientations (Experiment 2). All subjects (N=4) indicated a benefit in target detection when target orientation was not previously present in the surround. A control experiment showed that this effect was not caused by the purely temporal aspects of asynchronous stimulus presentation (Experiment 3). To distinguish this effect from other sources of delayed processing, Experiment 1 compared the performance in target detection and target identification tasks, for single-lines and popout targets. All subjects required longer stimulus presentation time to identify the orientation of a single line than to detect the line itself, indicating that orientation coding needs longer processing than encoding stimulus onset. However, most subjects needed even longer presentations to detect popout, suggesting that the processing of orientation contrast adds to this delay. In an appendix, putative response variations of V1 cells to asynchronous flicker are computed

Distance versus hemifield costs in the identification of cued double targets

The paradigm of cued visual selection was used to measure the identification speed of single targets and target pairs in large item arrays. Four target categories were tested: oblique lines (orientation, Exp.1 and 5a), vertical bars with the upper and lower halves slightly displaced (Vernier's, Exp.2), T letters at four orientations (T's, Exp.3), and red or green oblique lines (conjunctions, Exp.4 and 5). In all experiments, performance with double targets in various distances from another was compared with that for single targets at the same presentation time. Despite reported differences in the need of attention for their discrimination, all four target types revealed similar performance characteristics in the task. The identification of double targets was strongly disturbed at near target distances (2.5 deg), slightly disturbed at medium distances (6.5 deg), and not or only little disturbed at far target distances (12 deg). Also the identification of individual targets in target pairs was usually worse than that of single targets, except at far distances. In later analysis, target pairs were also distinguished whether they had been located in same or different visual hemifields. It turned out that all near target pairs were located in same, all far target pairs in different hemifields. To disentangle hemifield from distance variations, a new set of target positions was tested (Exp.5), in which near and far distances occurred both within and across hemifields. The results revealed a clear predominance of hemifield effects. The identification of target pairs presented in same hemifields was notably worse than the identification of target pairs presented in different hemifields. Distance variations had almost no effect. In an aside finding, the study collected further evidence for an independent feature processing in conjunctions; the color of colored lines was always faster identified than their orientation (Exp.4)

Cued visual selection of conjunction targets – no evidence of additional attentional requirements for the binding of color and orientation

The technique of cued visual selection (CVS) was used to measure dynamic processes in the identification of combined color and orientation targets. It has been proposed that the different features in such items must be attentively linked together for correct identification. In arrays of red and green lines at different orientations, one line (which thus became the target) was cued and had to be identified. Like with onefeature identification tasks in CVS, in which color is generally faster identified than orientation, observers also identified the color of combined targets faster than their orientation. Even in conjunction targets thus, features are identified largely independent from each other. False conjunctions were not obtained from a lack of attention but because one or the other feature was not yet correctly identified. When the performance in (separate) one-feature identification tasks was taken to predict the performance in the (combined) conjunction task, orientation identification was found to be slightly accelerated compared to the predictions. An analogue effect in color was not seen or notably smaller and in the opposite direction. Detailed analysis however showed that the improvement of orientation identification in conjunction tasks was not achieved on the cost of simultaneous color identification, nor was iFigs.t explained by learning effects or possible luminance differences in the tasks. It rather seems to reflect a better encoding of orientation signals in color channels or a better utilization of attentional resources in conjunction than in pure orientation tasks. Altogether there is no evidence that the attentional resources needed for target identification were also used for the binding of target feature components. © Autho