Poster Session: Threshold versus intensity curves measured with a new high-brightness display system.

Classical threshold vs. intensity (tvi) curves were measured using optical systems and were generally limited to increment test stimuli and relatively simple spatial patterns. Modern displays provide more flexibility in terms of stimuli spatial profiles but are usually dim enough that there may be rod intrusion when measuring cone responses. Here we describe a high-brightness display system and present tvi's for increment and decrement achromatic tests. The system consists of a PROPixx three-chip DLP LED color projector (VPixx Technologies, Saint-Bruno, Canada) controlled via a Datapixx display driver, with 12-bit digital to analog conversion per RGB channel. Light from the projector is collected in a large diameter lens and focused on high gain rear projection screen. Retinal illuminance of the background may be varied in three ways: (a) varying the mean current supplied to the LEDs from the controller (adjustable in software); (b) using calibrated neutral density filters mounted near the eye; and (c) changing the midpoint of the RGB channels in software (e.g., making the white background as R=G=B=0.1 instead of 0.5). Method (c) is made easier by the fact that the PROPixx "gamma curve" is linear, which also means that no RGB bits are lost to gamma correction. We will show thresholds for achromatic tests on a white background varying from 0.56 to 4.03 log trolands, with preliminary results suggesting differences in the tvi curves between the increment and decrement tests.

Visual psychophysics: Luminance and color.

Human visual psychophysics is a mature field of research that employs specialized methods and has generated a large body of established findings, a few of which are summarized in this overview. The methods reviewed include those known as classical psychophysical methods, signal detection theory, and the efficient modern Bayesian adaptive methods. The covered results emphasize well-established findings in both rod and cone vision, including some effects of light adaptation, luminous efficiency and spectral sensitivity, color detection, and spatial and temporal contrast sensitivities.

Modeling individual variations in equiluminance settings.

Recently, we reported measurements of heterochromatic flicker photometry (HFP) in 22 young observers, with stimuli that (nominally) modulated only L- and M-cones and were kept at (approximately) a constant multiple of detection threshold. These equiluminance settings were represented as the angle in the (L, M) cone contrast plane, with the greenish peak of the flicker in quadrant II and the reddish peak in quadrant IV; equiluminance settings were reported as the greenish angle. The mean equiluminance angle was 116.3° (an M:L cone contrast ratio of -2 at equiluminance), but individual differences in the settings were substantial, with the variation across individuals almost five times larger than the within-subject precision in the settings. In the present study we sought to determine the degree to which we could account for our observers' HFP settings by plausible variations in the macular pigment optical density (MPOD), the lens pigment optical density (LPOD), the cone photopigment optical densities (PPOD), and serine/alanine polymorphism in L-cone opsin (λmax shift). Most of the range of our measured equiluminance angles could be accounted for by these factors, although the largest two angles (smallest |ΔM/M: ΔL/L| ratio at equiluminance) could not. Individual differences in HFP have sometimes been taken to indicate variations in the ratio of L:M cone number; our results suggest that most of the individual differences in HFP might be equally well ascribed to physiological factors other than cone number. Simple linear models allow predictions of equiluminance angle, cone adapting level, and artifactual S-cone contrast from the values of the four factors considered here.

Transfer and retention of oculomotor alignment rehabilitation training.

Ocular alignment defects such as strabismus affect around 5% of people and are associated with binocular vision impairments. Current nonsurgical treatments are controversial and have high levels of recidivism. In this study, we developed a rehabilitation method for ocular alignment training and examined the rate of learning, transfer to untrained alignments, and retention over time. Ocular alignment was controlled with a real-time dichoptic feedback paradigm where a static fixation target and white gaze-contingent ring were presented to the dominant eye and a black gaze-contingent ring with no fixation target was presented to the nondominant eye. Observers were required to move their eyes to center the rings on the target, with real-time feedback provided by the size of the rings. Offsetting the ring of the nondominant temporal or nasal visual field required convergent or divergent ocular deviation, respectively, to center the ring on the fixation target. Learning was quantified as the time taken to achieve target deviation of 2° (easy, E) or 4° (hard, H) for convergence (CE, CH) or divergence (DE, DH) over 40 trials. Thirty-two normally sighted observers completed two training sequences separated by one week. Subjects were randomly assigned to a training sequence: CE-CH-DE, CH-CE-DE, DE-DH-CE, or DH-DE-CE. The results showed that training was retained over the course of approximately one week across all conditions. Training on an easy deviation angle transferred to untrained hard angles within convergence or divergence but not between these directions. We conclude that oculomotor alignment can be rapidly trained, retained, and transferred with a feedback-based dichoptic paradigm. Feedback-based oculomotor training may therefore provide a noninvasive method for the rehabilitation of ocular alignment defects.

Methods for determining equiluminance in terms of L/M cone ratios.

Heterochromatic flicker photometry (HFP), minimum motion (MM), and minimally distinct border (MDB) settings have often been used to determine equiluminance, a relative intensity setting for two chromaticities that, in theory, eliminates the responses of a luminance or achromatic psychophysical mechanism. These settings have been taken to reflect the relative contribution of the long (L) and medium (M) wavelength cones to luminance, which varies widely across individuals. The present study compares HFP, MM, and MDB using stimuli that do not modulate the short (S) wavelength cones, in both practiced and naïve observers. MDB was performed with both flashed and steadily viewed stimuli. Results are represented in the (∆L/L, ∆M/M) plane of cone contrast space. Considering both practiced and naïve observers, both MM and HFP had excellent within-subject precision and high test-retest reliability, whereas HFP also had low between-subject variability. The MDB tasks were less reliable and less precise. The mean L:M contrast ratios at equiluminance were lower for the two temporal tasks (HFP and MM) compared to the spatial tasks (MDB), perhaps consistent with the existence of multiple luminance mechanisms. Overall, the results suggest that the best method for determining equiluminance is HFP, with MM being a close second.