Reduced Monocular Luminance Increases Monocular Temporal Synchrony Threshold in Human Adults

Abstract

Purpose: The purpose of this study was to present our investigation of the influence of reduced monocular luminance on monocular and dichoptic temporal synchrony processing in healthy adults.

Methods: Ten adults with normal or corrected to normal visual acuity participated in our psychophysical study. The temporal synchrony threshold in dichoptic (experiment 1), monocular (experiment 2), and binocular (experiment 3) viewing configurations was obtained from each observer. Four flickering Gaussian dots (one synchronous and one asynchronous pair of two dots) were displayed, from which the observers were asked to identify the asynchronous pair. The temporal phase lag in the signal pair (asynchronous) but not in the reference pair (synchronous) was varied. In addition, a neutral density (ND) filter of various intensities (1.3 and 2.0 log units) was placed before the dominant eye throughout the behavioral measurement. In the end, dichoptic, monocular, and binocular thresholds were measured for each observer.

Results: With decreasing monocular luminance, the dichoptic threshold (2 ND vs. 0 ND, P < 0.001; 2 ND vs. 1.3 ND P = 0.001) and monocular threshold (2 ND vs. 0 ND, P < 0.001; 2 ND vs. 1.3 ND, P = 0.003) increased; however, the bincoular threshold remained unaffected (P = 0.576).

Conclusions: Reduced luminance induces delay and disturbs the discrimination of temporal synchrony. Our findings have clinical implications in visual disorders.

Figure 1.

Illustration of the experimental design. The stimuli consisted of two pairs of Gaussian dots each flickering at 1 Hz presented at either above or below the fixation cross. A pair of two dots that flickered synchronously (phase-locked) is the reference pair (e.g. the two white dots above the fixation in this illustration), and a pair of two dots that flickered asynchronously (with different phases) is the signal pair (e.g. the two black dots in different shades below the fixation in this figure). Contrast jitters were added to the four dots across the trials, so that there was no phase-locked perception between the two pairs. The participants were asked to indicate which of the two pairs flickered asynchronously. (a) Experiment 1 - Dichoptic configurations. Dichoptic nondominant eye viewing configuration (D2ND), in which the signal pair was presented to the nondominant eye and the reference pair to the dominant eye; dichoptic dominant eye viewing configuration (D2D), in which the signal pair was presented to the dominant eye and the reference pair to the nondominant eye; pure dichoptic viewing configuration (Di), in which the two dots further away from the fixation cross were presented to one eye, and the remaining two closer to the fixation cross to the other eye. The order for these configurations was randomized in each luminance condition. (b) Experiment 2 - Monocular configurations. Monocular nondominant eye viewing configuration (MND), in which the two pair of the dots were both presented to the nondominant eye; monocular dominant eye viewing configuration (MD), in which the two pair of the dots were both presented to the dominant eye. The order for these configurations was randomized in each luminance condition. (c) Experiment 3 - Binocular configuration. Binocular viewing configuration (Bi), in which the signal pair and the reference pair were presented to both eyes. (d) The mean luminance in the nondominant eye was fixed, whereas the mean luminance of the dominant eye was varied by applying a neutral density (nominal ND) filter of various intensities: 100% (0 ND), 5% (1.3 ND), and 1% (2 ND).

Figure 2.

Dichoptic configurations results. (a) Mean psychometric functions. The proportion correct of all responses is plotted against the temporal lag. The different shades of grey represent different luminance conditions (black, dark grey, and light grey for 2 ND, 1.3 ND, and 0 ND, respectively). The psychometric functions were fitted with Palamedes 1.8.1. (b) Mean dichoptic thresholds across observers. The temporal synchrony thresholds under different configurations are illustrated. The different shade of grey represents different luminance conditions (black, dark grey, and light grey for 2 ND, 1.3 ND, and 0 ND, respectively). Each solid circle represents the dichoptic threshold of one observer. Error bars represent the standard errors. ** P < 0.01; *** P < 0.001.

Figure 3.

Monocular configurations results. (a) Mean psychometric functions. The proportion correct of all responses is plotted against the temporal lag. The different shadse of grey represent each luminance condition (black, dark grey, and light grey for 2 ND, 1.3 ND, and 0 ND, respectively). The psychometric functions were fitted with Palamedes 1.8.1. (b) Mean monocular thresholds across observers. The temporal synchrony threshold under different configurations is plotted. The different shades of grey represent each luminance condition (black, dark grey, and light grey for 2 ND, 1.3 ND, and 0 ND, respectively). Each solid circle represents the monocular threshold of one observer. Error bars represent the standard errors. ** P < 0.01; *** P < 0.001.

Figure 4.

Binocular configurations results. (a) Mean psychometric functions. The proportion correct of all responses is plotted against the temporal lag. The different shades of grey represent each luminance condition (black, dark grey, and light grey for 2 ND, 1.3 ND, and 0 ND, respectively). The psychometric functions were fitted with Palamedes 1.8.1. (b) Mean binocular thresholds across observers. The different shades of grey represent each luminance condition (black, dark grey, and light grey for 2 ND, 1.3 ND, and 0 ND, respectively). Each solid circle represents the binocular threshold of one observer. Error bars represent the standard errors.

Figure 5.

The effect of reduced monocular luminance under the Di viewing configuration as a function of effect under the MD viewing configuration. The effect of reduced monocular luminance was quantified by the threshold ratio between the 2 ND and 0 ND conditions. Each orange dot represents the result of one participant. The red square represents the average results of all participants. Error bars represent standard errors.