The perception of visual objects is significantly affected by the nearby surroundings. This phenomenon is called the crowding-effect. We studied the influence of additional objects on recognition of the test visual objects in psychophysical experiments with aim to clarify the size of the crowding-effect zone.
The test objects were low-contrast Landolt rings having size 1.1, 1.5 and 2.3 deg the center of which was removed by 13.2 deg from the fixation point. The additional objects were similar Landolt rings or rings without gaps. The distance between the centers of the test and additional images varied from 2.2 to 13.2 deg.
Visual stimuli were synthesized on a computer and were displayed on a Mitsubishi Diamondtron 230 monitor with 1024x768 resolution, size 22", 0.24 mm aperture step and 100Hz frequency. With equal probability test images were randomly presented with different orientation of the gap and different distances between the test and additional Landolt rings. Time of presentation of stimuli was 40 ms, which does not allow the transfer of gaze from one area of the screen to another. In one experiment, the task of the observer was to identify both the test objects and surrounding images, in the other - only identification of test stimuli. Size of the crowding-effect zone was
determined by comparing the probability of distinguishing orientation test stimulus presented without additional image and with the environment, using the method of Chi-square and t-test.
In both experiments, for all distances between the test and additional images deterioration of identification test images was obtained, which was more significant when the observer carried out dual task. The presence of additional images contributed to the identification of the test, and vice versa, the test objects have affected the identification of additional images. As the distance between the test stimulus and the environment likely correct answer first rose and then fell. Apparently, the first at small distances affect proximity test stimulus, and then the identification is difficult due to the large distance between the two objects that need to be identified.
The data showed that the size of the crowding-effect area exceed the one at Bouma low (according to which the effects of the environment on the periphery extends a distance equal to half of the eccentricity of the test stimulus), and the impact of attention factor.