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{"project":"2_test","denotations":[{"id":"23650525-15085132-95381910","span":{"begin":236,"end":238},"obj":"15085132"},{"id":"23650525-16306992-95381911","span":{"begin":242,"end":244},"obj":"16306992"}],"text":"Experimental Task and Stimuli\nTo measure differences in WM-capacity and the efficiency of excluding irrelevant items from memory between Age and Diagnosis groups, a VSWM task comparable to that used in earlier change detection studies [26], [28] was presented to the subjects. This task consisted of bilateral stimulus displays in which colored squares (0.76°×0.76°) or rectangles (1.15°×0.57°) were presented within two 4°×7.3° rectangular regions presented 3° to the left and right from of a central fixation cross; see Fig. 1). On each trial, the positions of the items were randomly distributed within upper and lower quadrants of the screen with the constraint that the distance between objects within a hemifield was at least 2° (centre to centre). The colour of squares and rectangles was randomly selected on each trial with limited replacement from a set of seven easily distinguished colours (red, blue, green, violet, yellow, black and white). The number of targets and distracters was always the same in both hemifields, only location and color of the stimuli could differ between hemifields. All stimuli were presented on a gray background.\n10.1371/journal.pone.0062673.g001 Figure 1 Example of distracters-present trial (T1D2) for the left hemifield. A trial started with the presentation of an arrow cue that indicated the hemifield that subjects should attend to on the following memory display. The subject’s task was to remember the locations and colors of the squares (targets: T) in this cued hemifield for a later test. A total number of 480 trials was presented. On half of the trials the squares were accompanied by distracters (D: colored rectangles) that had to be ignored. In total, there were four different types of memory displays differing in the number of targets and distracters. Either one or three targets (squares) were presented alone (T1D0 or T3D0; memory load of 1 or 3 items) or were accompanied by two distracters (T1D2 or T3D2). The T3D2 condition exceeded maximum capacity and was included to obtain equal numbers of trials with or without distracters. All memory displays were followed by a test display 900 ms later in which one colored square was presented at one of the locations in the preceding memory display within the upper or lower quadrant (to both hemifields). The subjects had to press a left button with the left index finger when the test stimulus shown at this location had the same color as that in the previous memory display (50% of all trials) or press right with the right index finger when it was different. A new memory display followed 500–700 ms after a response was given. See Fig. 1 for an example of a complete trial of the T1D2 condition with exact timing parameters.\nThe behavioral measures derived from the VSWM change detection task in T1D0, T1D2 and T3D0 were: 1) reaction times for correct detections (RT), 2) percentage correct responses (% Hits), and 3) K-scores (see below). Only reaction times to correct responses that fell within a response window from 250–4000 ms after the memory probe were included in the analysis. Cowan’s memory capacity measure K in T1D0 and T3D0 conditions was computed with a standard formula (Cowan, 2001): K = (H+CR –1) N, in which H is the hit rate, CR are the correct rejections in an array with N items. To derive a behavioral measure of filtering efficiency, following a study by (Lee et al., 2010) we also computed K in the distracter condition (T1D2) by filling in 1 for N since there was 1 target item; if distracters are perfectly filtered out K will be 1, in case of imperfect filtering K will be lower than 1. This K-T1D2 measure was subtracted from K-T1D0 to obtain an “unnecessary storage” measure."}

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NEUROSES

PMC:3641097 / 17841-21563 JSON TXT