Abstract
Identification tasks are commonly used when assessing visual acuity (VA), constraining observers to categorical responses by only allowing them to report the item closest in appearance to an available response option. Thus, information about target appearance is lost. Here, we investigated the appearance of ten Snellen symbols, a set of high contrast symbols comprised of regular elements where each stroke width is a fifth of the optotype’s full height. Snellen’s symbols were presented to the fovea of 8 observers for unlimited viewing time. Symbol size was set to participant’s VA threshold measured prior to the experiment, as well as sizes below and above threshold (0.5, 0.67 and 1.5 times the threshold, respectively). In Experiment 1, participants replicated the appearance of each target on a 5x5 square grid interface. In Experiment 2, the responses on the grid were simultaneously also depicted in the same size as the target (at a different location on the screen), allowing for direct comparisons of target and response. Results showed that response-target similarity generally increased with increasing target size. Considering all connected target squares as individual elements (e.g., each bar in a target of three separated black bars), we found that deviations from the target such as different locations of elements and fusions of separate elements were particularly pronounced at 0.67, 1 and 1.5 times the threshold size in both experiments. By contrast, at the smallest size, responses were often reduced to a single square, especially when there was no response stimulus for comparison. In summary, we introduce a new method to capture the appearance of visual stimuli that are difficult to discern, and reveal how perceptual structure emerges from an undifferentiated point below threshold. Our results show details of visual appearance important to understand visual processing around threshold, going beyond typical performance measures.