In the previous three Chapters, we have been using an identical setup for all experiments. However, this setup is not suited for the visual search experiments in this and the next two Chapters. In this Section, we discuss the new experimental equipment, setup and
methodology used for the visual search experiments. Exact details of the stimuli used are discussed in the individual Experimental Sections.
Here, we present a brief overview of the visual search experiments. In these visual search experiments, we measure the time it takes for a participant to find a target placed at a random location in a RDS that covers the majority of each half of the screen. The longer it takes the participant to find the target, the better camouflaged the target. In order to test the effect of different manipulations of target shape and size on the target’s camouflage, we use several different target conditions – either adjustments of the target’s disparity or it’s smoothness (profile in depth). Each experiment is blocked in to sections, in each
experimental block, we choose to display a certain number of repeats of each condition, and interleave these between the trials, displaying only one target of one condition in each trial.
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The visual search experiments we use follow a simple procedure: first the observer is presented with a black screen with a central fixation cross. After one second, this proceeds to the RDS. Within this RDS is the target object, defined by either luminance or disparity or a combination of these two cues. The participants’ task is to press a button as soon as they spot the target, then indicate on a response screen (discussed in Section 7.2.2) where they saw the target.
7.2.1 Experimental equipment
In order to use as large a display as possible, the visual search experiments used a mac monitor which was placed at 1.1m away from the participant. The monitor has a resolution of 2560 by 1600, and a size of 32 by 19 degrees in visual angle. Pixels are at 0.747 arcmin across (making the individual 2x2 pixel square dots of the RDS have a side length of 1.49 arcmin). A random dot stereogram and Wheatstone stereoscope (Section 3.1) are still used. Maximum luminance (white) for this screen is 268.8 cd m-2, and minimum luminance (black) is 0.4 cd m-2.
We replaced the keyboard input method with the mouse, for reasons explained in the next Section.
7.2.2 Experimental methodology
The visual search experiments require a different procedure to the previous experiments as they are asking different questions. Participants search in a RDS for an object, with no limits to the length of time they can spend searching. A participant who is fast at the task will therefore progress through trials faster than a slow participant (unlike in the previous experiments, where each trial was set to take 2s). We allow participants to proceed through blocks of the experiment at their own rate, completing as many blocks as they can during the one-hour session. In order to ensure a reasonable degree of precision, we require that the participant completes a minimum number of blocks (three in Experiment 8).
We reduce the number of trials in each block (to fifty in Experiment 8), aiming at an average of 10 minutes per block for each participant. As different conditions are interleaved within each block, we wish to avoid participants stopping partway through a block at the end of the session, and then having completed a different number of trials for each experimental condition. These 10-minute blocks should allow us to fit as many blocks into the one hour as possible without breaking blocks partway though at the end of the one-hour session. Participants are required to correctly complete at least the sixth panel in the TNO test. Additionally, participants complete a visual search demo to familiarise themselves with the task before undergoing the experiment. Participants were informed that their task was to find a square-shaped object in the field of random dots. The demo consisted of 12 trials (each condition was displayed to the participant 3 times), and were identical to the experimental blocks – the demo was for practice and to ensure the participants had
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correctly understood the task. Participants unable to complete the demo with at least 80% success rate at identifying the target objects were rejected from further study.
7.2.3 Experimental procedure
The procedure of the experiment is significantly changed from the previous three Chapters. At the beginning of the experiment, the participant is shown an alignment screen consisting of a box around the area that will contain the visual search stimuli. Participants are asked to ensure that they can see the entire box in each eye individually, and only one box with comfortable fusion when both eyes are open. If needed, they are shown how to adjust the angle of the stereoscope mirrors to achieve this.
The experiment then starts with a mouse click, initiating the display of the visual search stimulus. During the presentation, the mouse pointer is hidden, and the participant is given as long as they wish to spot the target – as soon as the participant identifies the target they click the mouse button. The time taken between the mouse clicks is measured by the program as their reaction time. In order to ensure that the participant correctly identified the location of the target, the participant is then presented with a response screen containing a set of white square outlines as in Figure 7.2, one square corresponding to the location of the target object. The mouse pointer is revealed at the fixation cross, and the participant requested to click which of the squares corresponded to the location of the target. No two squares were allowed to be within 74.7 arcmin of each other, called the padding, to ensure that participants could not get confused about the location of the target with respect to the response patches. The participant moves the mouse to the square where they thought the target was. This square will turn white to indicate the participant’s selection – the participant presses the button to confirm and move onto the next trial. The participant can mouse over as many selections as they wish before making a selection. Once a square is selected, the fixation cross was displayed for one second before the next
stimulus onset. No feedback was given.
We switched from a keyboard input in the previous three Chapters to a mouse in these visual search Chapters because there is no clear way of selecting between the five response boxes using a keyboard. Using a mouse, selecting the correct box is intuitive for the
participant.
Figure 7.2: Mock up response screen. The mouse is initially at the fixation
cross, with all possible response positions as hollow white squares.
When the participant hovers the mouse over a square, it is highlighted
white to indicate the participants’ selection. The participant clicks the
square they think was at the target location.
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To ensure that search times were not dictated by the initial fixation position the position of the target was pseudo-random. The target was not allowed to appear within 74.7 arcmin of the fixation cross to ensure that participants had to search the stimulus to locate the target. Additionally, the target could not appear within 74.7 arcmin of the edge of the random dot stereogram.
Randomised position within these constraints was obtained by using a matrix, each element of the matrix corresponded to one pixel on the screen where the target object could be placed. Every element of the matrix was set to zero, to indicate that it was an allowed position, as in Figure 7.3a. The top left corner of the target was calculated by a randomised (x, y) coordinate, and all elements that would be covered by the target and the banned region around it (which stops objects being too close to each other) were assigned a ‘1’ as in Figure 7.3b. A second random (x, y) coordinate was then generated as the top left of the first distractor. If any of the elements within the area of the second object are ones (as in Figure 7.3c), then this position is rejected and a new one generated until the object contains only zeros as in Figure 7.3d (this is only attempted 100 times before the entire matrix is reset). All elements covered by the new object and the area around it are assigned to one, as in Figure 7.3e. As can be seen in Figure 7.3e, there is only one randomised top left position left that could result in the placement of an object at (4,1) – once an object is placed here we cannot fit any more in. This typically takes place at around 55% coverage (Brosilow, Ziff, & Vigil, 1991). This not an issue with our stimuli however, as they only have approximately 1% coverage, so all objects can be reliably positioned randomly in the scene.
a. Initial matrix b. Target added c. Distractor first attempt d. Distractor second attempt 5. Distractor added A matrix corrisonding to all available positions was set to 0. Top LHS of target is randomised as (1,1). Target is located in the box. Trial position of a distractor at (3,2) is shown by the dotted square. This contains 1’s and is rejected. Second trial position at (3,4) does not contain 1’s and is accepted. Position of the distractor and surroundings are set to 1’s to indicate they are occupied.
Figure 7.3: An example small matrix of how the screen was populated with objects. The objects in this demonstration are 2 by 2, with a padding of 1. Solid lines indicate the position
of stored objects, dotted lines the attempted positioning of objects.
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 1 1 1 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 1 1 1 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 1 1 1 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 1 1 1 0 0 1 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 1 1 1 1
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