Participants. Sixteen students (9 males; age range = 19–35 years, M = 27.75, SD = 4.81) from the University of Haifa participated in this study, either in exchange of course credit or payment of 50 shekels (around 14$) per hour. Based on previous literature, we estimated that a sample size of 12 participants was required to detect a crowding effect with 95% power, given a .05 alpha 23 . However, we collected data from four additional participants to account for possible dropouts or technical difficulties. All participants ignored the research question and reported normal or corrected to normal vision, and no attention deficits. An informed written consent was obtained from all observers before starting the study. All methods, practices, and procedures were performed in accordance with the Declaration of Helsinki and were approved by the University Committee on Activities Involving Human Subjects at the University of Haifa (No. 226/20).
Apparatus. Stimuli were presented using Matlab software (The MathWorks, Inc., Natick, MA) and the Psychophysics Toolbox, and displayed on a gamma-corrected 21 inch CRT monitor (with 1280 × 960 resolution and 85-Hz refresh rate). Eyelink 1000 (SR Research), an infrared eye tracker, was used to monitor and record eye movement, and a SpectroCAL MKII spectroradiometer (Cambridge Research Systems, UK) was utilized to calibrate brightness and color. Participants were individually tested in a dimly lit room and prompted to use a keyboard to generate responses. Finally, a chin-rest was used to ensure all participant were 57 cm away from the computer monitor.
Stimuli and procedure. Figure 1 illustrates the present experiment’s paradigm. All stimuli were colored black (luminance 0.0073 cd/m2) and presented on a gray background (53 cd/m2). Firstly, participants were asked to fixate their gaze on the location of the fixation mark. This fixation mark was a centered black dot (subtending 0.24° of visual angle), which appeared on the screen for 500ms and continued to appear until the observer maintain fixation for 300 ms. Following observer fixation a cue appeared on the screen for 50ms. The cue was a black ring (1 px pen width) subtending 1º of diameter. In the neutral cue condition, the cue circle appeared at the center of the screen. In the valid cued condition, the cue appeared 5.9º away from the center of the screen, on the horizontal meridian, in the same hemisphere as the target. An interstimulus interval (ISI) of 50 ms followed the cue, and the target display appeared next, for 100ms. In crowded display trials, three letter shapes (each subtended 0.75° of visual angle) appeared on the screen: one target and two flankers.
The target was a “T” shape, oriented either upright (0º), inverted (180º), or tilted to the left (270º) or the right (90º), and it was presented at an eccentricity of 7º on the horizontal meridian either to the right or to the left of the fixation mark. Note that on valid trials the cue was inner to the target and at 1.1º center to center distance from the target. Flankers were two “H” shapes, either upright or tilted 90º. On half of the crowded display trials, the flankers were positioned radially: one to the right and one to the left of the target. On the other half of the trials, the flankers were positioned tangentially: one above and one below the target. In each crowded display trial, both flankers were equally spaced from the target. Target-flanker center-to-center spacing was either: 1.1°, 2°, 3°,4°, 5°, 6°, 8° or uncrowded (target alone). Target and flankers were always black. After 500 ms, the response period began, and the monitor displayed a blank screen.
Participants were instructed to report the target's orientation by pressing on one of four designated keys on the keyboard (each key representing one of the 4 possible target orientations). Subjects could take as long as needed to respond. The orientation of both target and flankers, as well as display hemifield, were randomly selected in each trial, There were 40 trial for each combination of cue condition (neutral vs. valid), target-flanker spacing (1.1º ,2º ,3º ,4º ,5º ,6º ,8º and uncrowded), and display arrangement (tangential vs radial). Trial order was unpredictable (quasi-randomized). In total, the experiment consisted of 1280 trials, which were divided into two sessions of 640 trials each. Participants rested for half an hour between the two sessions. Each session was further divided into ten blocks. Following each response, a high or low-pitched tone played to indicate a correct or incorrect response, respectively. Note that participants completed 40 practice trials prior to starting the actual experiment.
Analysis. A three-way analysis of variance (ANOVA: cue condition × target-flanker spacing × stimuli arrangement (radial vs. tangential)) with repeated measures was performed on the accuracy data, excluding the trials where the target appeared without flankers (uncrowded). Additionally, individual data was fitted to an exponential curve using the Weibull function 24 with the goal to compute critical spacing thresholds, per condition (cue and neutral cue). Critical spacing thresholds were defined as the Weibull function coordinate corresponding to 75% of correct trials. Next, using the critical spacing data, we conducted a 2x2 repeated measures ANOVA to explore the relation between cue condition, display arrangement, and critical spacing. Follow-up repeated measures t-tests were performed to further parse out condition differences in critical spacing.