Poster Session: A direct measure of adaptation and visual salience.

One hypothesized function of adaptation is to increase the salience of novel targets by discounting the properties of the ambient environment. Previous studies have suggested this by finding faster search times for novel targets when searching on backgrounds observers are currently adapted to. However, this provides only an indirect measure of salience. Here, we developed a more direct measure of the impact of adaptation on feature salience. Backgrounds were oriented 1/f noise images with power confined within 15 deg of horizontal or vertical. Targets were 5 c/deg Gabor patches centered on the 8 deg backgrounds. Observers simultaneously adapted to the horizontal or vertical backgrounds shown on the left or right of fixation. A 250ms test probe then showed the Gabor patch on the same background (horizontal or vertical) on both sides. The target orientation was adjusted on one side until it appeared as conspicuous as a fixed target on the other side. Settings were made for fixed targets ranging from 10 to 45 deg from the backgrounds. For most conditions/observers, the salience matches required a smaller orientation offset on the same- vs. different-adapt background. These results support a functional role of adaptation in highlighting novelty by potentially "unmasking" the target from its background, and emphasize the importance of considering adaptation aftereffects not only for isolated targets but within the stimulus contexts they are embedded in.

Color perception and compensation in color deficiencies assessed with hue scaling.

Anomalous trichromats have three classes of cone receptors but with smaller separation in the spectral sensitivities of their longer-wave (L or M) cones compared to normal trichromats. As a result, the differences in the responses of the longer-wave cones are smaller, resulting in a weaker input to opponent mechanisms that compare the LvsM responses. Despite this, previous studies have found that their color percepts are more similar to normal trichromats than the smaller LvsM differences predict, suggesting that post-receptoral processes might amplify their responses to compensate for the weaker opponent inputs. We evaluated the degree and form of compensation using a hue-scaling task, in which the appearance of different hues is described by the perceived proportions of red-green or blue-yellow primary colors. The scaling functions were modeled to estimate the relative salience of the red-green to blue-yellow components. The red-green amplitudes of the 10 anomalous observers were 1.5 times weaker than for a group of 26 normal controls. However, their relative sensitivity at threshold for detecting LvsM chromatic contrast was on average 6 times higher, consistent with a 4-fold gain in the suprathreshold hue-scaling responses. Within-observer variability in the settings was similar for the two groups, suggesting that the suprathreshold gain did not similarly amplify the noise, at least for the dimension of hue. While the compensation was pronounced it was nevertheless partial, and anomalous observers differed systematically from the controls in the shapes of the hue-scaling functions and the corresponding loci of their color categories. Factor analyses further revealed different patterns of individual differences between the groups. We discuss the implications of these results for understanding both the processes of compensation for a color deficiency and the limits of these processes.

The effect of refractive surgery on blur thresholds.

PURPOSE: The aim of this study was to measure blur thresholds before and after refractive surgery. METHODS: In this prospective cohort study conducted in a tertiary eye hospital in South India. Blur thresholds were measured for 30 young adult myopic patients 1 month prior to and after refractive surgery. Patients were asked to report three stages of blur, namely Detectable Blur (DB), Bothersome Blur (BB), and Non-resolvable Blur (NB). Blur was created by adding plus lenses (in steps of 0.12D) over their optimal subjective refraction. The blur judgments were made both monocularly and binocularly when looking through a 3 mm artificial pupil at one line above the best-corrected visual acuity. RESULTS: A total of 30 participants were included in this study (mean age = 25.5 ± 3.8 (20-36) years; 77% female). The mean binocular preoperative blur of this group was: DB = 0.39 ± 0.26D, BB = 0.74 ± 0.28D and NB = 1.04 ± 0.42D. The corresponding mean binocular blur one-month post-operatively was DB = 0.46 ± 0.28D, BB = 0.83 ± 0.35D, and NB = 1.21 ± 0.44D. Although there was a marginal increase in the blur thresholds postoperatively, the difference was not statistically significant (DB: P = 0.320; BB: P = 0.229; NB: P = 0.054). CONCLUSION: All three blur thresholds showed an insignificant minimal increase at 1 month post-operatively suggesting that patients adapt to the induced blur following refractive surgery. A longer follow up would reveal how the adaptation to blur would change with time.

Deviation in the Position of Foveal Bulge from Foveal Center in Normal Subjects Measured Using Spectral-Domain OCT.

PURPOSE: To report a deviation in the position of the foveal bulge (FB) from the foveal center and to analyze its relationship with the foveal shape in normal subjects using spectral-domain (SD) OCT. DESIGN: Cross-sectional study. PARTICIPANTS: A total of 146 clinically normal subjects. METHODS: Macular Cube 512 × 128 was used to obtain the cross-sectional images of the macula in Cirrus HD-OCT (Carl Zeiss Meditec, Inc, Dublin, CA). The foveal center was identified manually as the deepest point within the foveal dip, which was termed the "manually identified foveal center" (MFC). The position of the FB with respect to the MFC was noted, and the radial distance between them was measured. The foveal center detected by the inbuilt auto-fovea finder algorithm was considered as the automatically detected foveal center (AFC), and the distance between the AFC and the FB was measured. The radial distances of the FB from the MFC and the AFC were compared, and its direction of deviation from the foveal center was noted. All OCT measurements were adjusted for the ocular magnification factor. The amount of deviation was correlated with the foveal shape parameters, such as foveal radius. MAIN OUTCOME MEASURES: Radial distance between FB and foveal center and its direction of deviation. RESULTS: Mean age of the subjects was 43.9±14.4 years. The position of the FB was deviated from the foveal center in 125 subjects (85.6%). Median radial distance between the FB and the MFC was 58.6 µm (35.16-75.04 µm) with the maximum separation of 222.68 µm. From the AFC, the FB was separated by a median of 58.6 µm (46.88-84.51 µm), maximum separation being 181.94 µm. The direction of deviation of the FB from foveal center was nasal in a majority of subjects (n = 75, 51% in MFC). The separation between the FB and the foveal center was not associated with age (P = 0.149), gender (P = 0.762), or axial length (P = 0.25). CONCLUSIONS: The position of the FB did not coincide with the foveal center and was deviated in the direction of the optic disc in a majority of normal subjects. Further studies are required to correlate this anatomic deviation with the fixation locus of the individuals.