Experiment 1

The experiment was a computer ‘game’ created in Multimedia Fusion 2 (Clickteam 1996–2011) and played on a touch screen monitor (Elo 1515 L; Tyco Electronics, Shanghai, China, 1280 × 1024 pixels, or 42.85 × 34.28 degrees subtended on the viewer’s eye) by human subjects. The achromatic target (90 × 45 pixels, or 3 × 1.5 deg/24 × 12 mm) started behind an occluding circle (diameter 179 pixels, 5.99 degrees) in the centre of the screen. The target then moved out in a random direction at a speed of 20.8 cm/s (approximately 26.7 degrees of visual angle per second) through a circular arena (diameter 1024 pixels, 34.28 degrees) before disappearing. The subjects’ task was to make a capture attempt before the target left the circular arena. The target did not change trajectory once it had started moving. After the subject touched the screen, a cross appeared on the screen with its centre in the position they had clicked. The colour of this cross provided feedback to the participant indicating whether they had hit or missed the target (green and red crosses respectively). The computer program recorded the coordinates of the capture attempt, the coordinates of the target at the time of the capture attempt, the time of the capture attempt and whether the subject had hit or missed the target. After a capture attempt (or after the target had left the screen) there was a short pause before the next target presentation began. The experiment used a block design: each of the six different target types was presented in a random order in one block, and the full experiment contained 20 blocks, meaning that each target type was presented 20 times throughout the experiment at approximately even intervals. Figure 6 shows an example screen shot from this experiment.

Fig. 6 An example screenshot of the general experimental set up of Experiment 1 Full size image

Targets were PNG images, created in Image J. Six different target types were used in this experiment (see Fig. 7). Three targets were created that had different orientations of stripes; either perpendicular to the direction of travel, parallel to the direction of travel or at 45 degrees oblique to the direction of travel. The width of the stripes was matched across targets, with stripes being 10 pixels (0.33 deg) across (and thus the spatial frequency was matched in terms of cycles/degree not cycles/object i.e. the perpendicular target had more cycles of stripes than the parallel target). This was done as it is known that spatial frequency in terms of cycles/degree can affect perceived speed of objects [36, 37], and the design of our stimuli was such that we could not keep both types of spatial frequency constant. Two control grey targets were also created; one with a luminance (perceived brightness) matched to the perceptual midpoint between the white and black stripes (RGB value = 95; see below for details of calibration and how the perceptual midpoint was determined) and one with a luminance matched to the average luminance of the striped targets (RGB value = 113). A white target was also used.

Fig. 7 Target types used in both experiments. Trial types from left to right are average background luminance matching grey, lighter grey, white, parallel stripe, perpendicular stripe and oblique stripe Full size image

The background exemplars used in this experiment were generated by taking ten photographs of various arrangements of artificial leaves from a fixed height. These images were converted to greyscale and luminance matched to the perceived midpoint between the white and black stripes of the striped targets (RGB value = 95) in MATLAB. This method was used to ensure that lighting conditions and scale were as similar as possible between images. The exact background exemplar presented on each trial was randomised.

Experiment 2

The target types and background types in Experiment 2 were the same as used in Experiment 1. However, the design of the trial was changed to address the question of whether increasing the number of targets on the screen during each trial would affect capture success differentially for different target types.

As in Experiment 1, the ‘game’ was created in Multimedia Fusion 2 (Clickteam 1996–2011). On each trial, a square arena with dimensions 1024 × 1024 pixels (or 34.28 × 34.28 degrees subtended on the viewer’s eye) was presented (see Fig. 2). Six targets of the same type were placed at separate random locations at the beginning of the trial (always at least 100 pixels away from the edge of the arena) and began moving in a randomly selected direction in a straight path (target speeds were equal and identical to those used in Experiment 1). When the target reached the edge of the arena, it would rotate, change direction and continue moving, to ensure that it remained inside the arena and the stripes remained at the same orientation relative to the direction of travel. Targets did not interact with each other (e.g. they slid over rather than ‘bouncing’ off each other). The subject’s task was to attempt to ‘catch’ all six targets as quickly as possible by touching them with their finger. If a target was successfully caught, it disappeared from the screen, and the target and capture positions and time of capture were recorded. The trial continued until all six targets had been caught. The total time taken for the whole trial and the total number of capture attempts (both hits and misses) were then recorded before continuing to the next trial. As in the previous experiments, the trials were presented in blocks, so that in each six trials, all target types were presented. Each participant completed six blocks. Fewer blocks were used in this experiment as each individual trial took longer to complete. 60 subjects took part in this experiment, and none of these had taken part in Experiment 1. An example screen shot of the experimental set up is shown in Fig. 8.

Fig. 8 An example screenshot of the general experimental set up for Experiment 2 Full size image

Monitor calibration

The display was calibrated for human luminance (perceived brightness) perception using a Minolta LS-110 luminance meter (Osaka, Japan) following previous work [1–3]. Images with grey values ranging from 0–255 on an 8 bit scale were displayed on the screen, and the luminance was measured in cd/m2 for each image at four different points on the screen and averaged. The grey value was then plotted against the average luminance to determine the value that would represent an intermediate grey between the black and white markings on a ratio scale, and this value was used in target and background creation. The display refreshed at 70Hz, which would equate to a frame by frame displacement of 0.57 degrees. The flicker of the striped targets was 41.6Hz (based on calculating the time taken for one complete cycle of white and black stripes), which was lower than the refresh rate of the display.

Subjects

60 participants were recruited to carry out each experiment, and each participant only took part in one of the experiments. Subjects were drawn from the undergraduate and graduate populations at the University of Cambridge, were naïve to the experimental aims and were only given enough information to be able to play the game. We did not collect individual age and gender data as they have not been shown to affect results in this type of experimental task, but subjects were predominantly aged between 18 and 25 and both datasets had an approximately even gender split. They gave written consent and the experimental methods were approved by the University of Cambridge Psychology Research Ethics Committee.Viewing distance was approximately constant at 45 cm, and the experiment was conducted in standard laboratory light conditions throughout the working day (lighting levels did not change with time of day as all windows were covered for the duration of the experiment). All subjects received 10 training target presentations first, where a black target was captured on a white background.

Statistical analysis

Due to the repeated measures design of the experiment, results were analysed using linear mixed models (LMMs) or generalised linear mixed models (GLMMs) [38, 39] using the lme4 package (version 1.1-7) [40] and the lmerTest package (version 2.0-6) [41] in R (version 3.1-0) [42].

For Experiment 1, a model was fitted using target type, trial number and position group (whether the capture attempt was ahead of or behind the midline of the target, as defined by its direction of travel; this factor was included as it greatly improved the model fit, as many more capture attempts were made behind the centre of the target, creating a bimodal distribution) as fixed factors. The initial model also contained all possible first order interactions with target type. The model was simplified based on their AIC weights and log likelihood to produce a best fit model [38, 39]. Analysis was run for Experiment 1 using a hit/miss dependent variable (binomial error structure) and using a time taken to capture measure (log normal error structure). Subject, trial direction, and trial number were included in the hit/miss model as random intercepts; a similar structure was also used for the time measure, but a random intercept of background exemplar was also included (although its variance and standard deviation in the final model was small, and thus we feel that treatment of the background as a random and not a fixed factor is appropriate). Collinearity of response variables was checked using the correlation of fixed effects. We calculated the overall main effects of the models using the Anova function from the car package (version 2.0-20) [43] and then analysed the effects of individual pattern types using planned contrast comparisons [44]. The luminance matched grey target was taken as the reference against which all other targets were compared.

As the design of Experiment 2 differed from that of Experiment 1 (multiple targets were present on the screen at once, instead of a single target on each trial), different measures of capture success were used; analysis focused on the number of attempts taken (log normal error structure) and the length of time taken to capture the targets, as calculated by taking the time since the previous target capture (log normal error structure). Initial models for the total number of hits measure were constructed using the fixed factors of target type and trial number and their interaction, and subject was included as a random intercept and trial number as a random slope. Initial models for the time taken to capture measure included the same fixed factors as the total number of hits model and also a factor indicating which target number (out of the total number of targets on a trial) was being captured and the interaction with target type, to consider whether it was easier to capture the targets when there were fewer of them on the screen, and whether this difficulty was modulated by target patterning. In addition to the random effect structure used for the total number of hits measure, target number was also included as a random factor with trial number as a random slope. Model simplification and inference for both measures was as in Experiment 1.