Recent studies highlight the importance of the temporal domain in visual processing. Critical Flicker Frequency (CFF), the frequency at which a flickering light is perceived as continuous, is a widely used measure for evaluating visual temporal processing. Another important issue to investigate is the cortical interactions arising between the flicker stimuli of both eyes. This paper presents a robust and reliable dichoptic tool for evaluating the CFF threshold in both eyes. This system is based on an analog output device used to independently drive two LEDs through a custom-written MATLAB code (using a laptop PC) for eliciting sinusoidal flickering stimuli and for psychophysically measuring the perceived CFF threshold. The luminance and phases of each LED are individually controlled, enabling the investigation of the effect of phase and luminance differences on binocular summation in subjects with different ocular pathologies. Experiments were designed to evaluate the CFF threshold through a psychophysical test, based on a discrimination task with a stimulus duration of 1 s, based on a temporal alternative forced-choice paradigm. The target stimulus temporal features were modulated using the staircase method. Subjects were requested to discriminate between a target stimulus (a flickering light at various frequencies) and a flickering light at a frequency of 120 Hz, which is significantly higher than the CFF in humans; therefore, it is perceived as constant. One of the main advantages of the introduced dichoptic presentation system is that it enables the visual temporal performance to be measured under both monocular and binocular conditions where phenomena such as temporal binocular summation (BS) can be evaluated. Moreover, the system offers great flexibility by introducing a stimulus phase shift, which enables studying how stimulus timing affects the temporal function at millisecond scale resolution. Our results confirm that no crosstalk exists between the eyes and that the system can reliably separate the stimuli presented to the eyes. Using this set-up, we observed the binocular summation of CFF for low target luminance levels. The CFF was significantly (p = 0.011) higher (5.2%) under binocular compared with monocular viewing conditions. More importantly, introducing an inter - ocular phase shift reduced the binocular CFF in normally sighted subjects. Finally, in amblyopic subjects the amblyopic eye showed a decrease of 3.9 Hz (15%) in CFF, compared with the fellow eye (p = 0.001). The ability to assess binocular temporal performance using a dichoptic display can shed light on visual temporal performance in general, and on binocular temporal summation processes in particular, both for subjects with normal binocular vision and for subjects with impaired binocular vision (e.g., amblyopic subjects). Furthermore, such a presentation set-up may facilitate the development of training paradigms aimed at improving binocular vision performance. In this paper we describe the system and methods in detail and provide all necessary computer code and other details that will enable an easy and quick adaptation of the method by scientists interested in studying the temporal resolution of the visual system in general, and in studying inter-ocular differences or interactions in particular.
Keywords: Amblyopia; Binocular vision; Critical fusion frequency (CFF); Dichoptic vision; Temporal resolution.
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