Contents lists available at ScienceDirect Physiology & Behavior journal homepage: www.elsevier.com/locate/physbeh The effect of stimulus strength on binocular rivalry rate in healthy individuals: Implications for genetic, clinical and individual differences studies Phillip C.F. Law a,⁎ , Steven M. Miller a,b , Trung T. Ngo a,c,d a Monash Alfred Psychiatry Research Centre, Monash University Central Clinical School and The Alfred Hospital, Melbourne, Australia b School of Psychological Sciences, Monash University, Melbourne, Australia c Genetic Epidemiology Laboratory, QIMR Berghofer Medical Research Institute, Brisbane, Australia d Mater Research Institute-UQ, Neurosciences & Cognitive Health Program, Faculty of Medicine, University of Queensland, Brisbane, Australia ARTICLE INFO Keywords: Binocular rivalry rate endophenotype Mixed percepts Stimulus strength Drift speed Aperture size Individual differences Levelt ABSTRACT Binocular rivalry (BR) occurs when conflicting images concurrently presented to corresponding retinal locations of each eye stochastically alternate in perception. Anomalies of BR rate have been examined in a range of clinical psychiatric conditions. In particular, slow BR rate has been proposed as an endophenotype for bipolar disorder (BD) to improve power in large-scale genome-wide association studies. Examining the validity of BR rate as a BD endophenotype however requires large-scale datasets (n = 1000 s to 10,000 s), a standardized testing protocol, and optimization of stimulus parameters to maximize separation between BD and healthy groups. Such re- quirements are indeed relevant to all clinical psychiatric BR studies. Here we address the issue of stimulus optimization by examining the effect of stimulus parameter variation on BR rate and mixed-percept duration (MPD) in healthy individuals. We aimed to identify the stimulus parameters that induced the fastest BR rates with the least MPD. Employing a repeated-measures within-subjects design, 40 healthy adults completed four BR tasks using orthogonally drifting grating stimuli that varied in drift speed and aperture size. Pairwise compar- isons were performed to determine modulation of BR rate and MPD by these stimulus parameters, and individual variation of such modulation was also assessed. From amongst the stimulus parameters examined, we found that 8 cycles/s drift speed in a 1.5° aperture induced the fastest BR rate without increasing MPD, but that BR rate with this stimulus configuration was not substantially different to BR rate with stimulus parameters we have used in previous studies (i.e., 4 cycles/s drift speed in a 1.5° aperture). In addition to contributing to stimulus optimi- zation issues, the findings have implications for Levelt's Proposition IV of binocular rivalry dynamics and in- dividual differences in such dynamics. 1. Introduction Binocular rivalry (BR) is an intriguing visual phenomenon in which conflicting images presented to each eye are perceived in alternation rather than being superimposed. For example, simultaneously pre- senting a vertical grating to one eye, and a horizontal grating to the other eye, induces perception of the vertical grating for a few seconds, followed by perception of the horizontal grating for a few seconds, and so on (Fig. 1). BR and other perceptual rivalry types such as ambiguous figures have previously been examined, particularly with respect to alternation rate, in the context of clinical psychiatric disorders from the early to mid-20th Century (e.g., [14,17,20,21,25,27,37,51,58,74]). The modern clinical focus on BR emerged with reports from Australia that BR rate was slow in the heritable psychiatric condition, bipolar disorder (BD), relative to healthy individuals (e.g., [53,68]) — a finding that has since been independently replicated in populations from Japan [57], New Zealand [82] and China [87]. Following Pettigrew and Miller's [68] original study on BD, other clinical psychiatric conditions have been examined including schizo- phrenia and major depression [39,53], autism spectrum conditions [4,26,42,71–73], attention deficit hyperactivity disorder (e.g., [3,6]), and generalized social anxiety disorder [5]. Although some researchers (e.g., [82]) have attempted to use the same testing protocol as that of Pettigrew and Miller [68], so that data may be directly compared between clinical studies, other researchers have employed different test protocols (e.g., shorter viewing durations, different stimulus characteristics, different http://dx.doi.org/10.1016/j.physbeh.2017.08.023 Received 1 June 2017; Received in revised form 15 August 2017; Accepted 26 August 2017 ⁎ Corresponding author at: Level 4, 607 St Kilda Road, Melbourne, VIC 3004, Australia. E-mail address: Phillip.Law@monash.edu (P.C.F. Law). Physiology & Behavior 181 (2017) 127–136 Available online 30 August 2017 0031-9384/ © 2017 Elsevier Inc. All rights reserved. MARK