Abstract
Purpose: :
When normally-sighted people adapt to high-pass filtered (sharpened) images using their central vision, subsequent images appear to be blurred. Similarly, after looking to low-pass filtered (blurred) images, subsequent images appear sharpened. We investigated whether this effect persists in the periphery of normally-sighted observers and whether patients with central visual field loss (CFL) who use a peripheral retinal locus (PRL) report the same adaptation.
Methods: :
The characteristic of adaptation curves were evaluated in a group of normally sighted observers using a peripheral viewing area, and a group of patients with CFL due to macular degeneration. Initial screening included determination of the PRL and best correction for the task’s viewing distance. The psychophysical procedures were modified from Webster et al (2002). Observers adapted (initial 30s, top-up 3sec) to images that were digitally blurred or sharpened by varying the slope of the spatial spectrum up to ±0.75 relative to the natural slope. Observers were asked to decide whether a briefly presented (500ms) test image was perceived to be "too blurred or too sharp compared to what you think is normal". The point of subjective neutrality (PSN - the spectral slope of the image that appears normal) was determined for at least one scene at a minimum of 7 adaptation levels. The PSN for each adaptation level was used to compute individual adaptation curves.
Results: :
Adaptation curves were characterized by the slope of the sigmoid fitted curves (representing the gain of adaptation) and the asymptotes (saturation levels). Inter-observer variability in the gain of adaptation and adaptation saturation levels was found. Normally sighted observers showed similar adaptation curves peripherally and centrally, both with the sigmoid-shaped function crossing at zero as previously reported for central viewing (Webster et al., 2002; Vera-Diaz et al., 2008). Most patients with CFL showed a shift in their PSN adaptation curves, choosing sharper images when adapted to the original image.
Conclusions: :
Adaptation to blurred and sharpened images is also experienced at the PRL of patients with CFL and in the periphery of normally-sighted individuals. The shift in PSN found in CFL patients suggest a continuous state of adaptation to blur and may have implications to image enhancement rehabilitation approaches.
Keywords: adaptation: blur • low vision • perception