Visual perception is easy to experience but difficult to measure. The psychophysical tools available for measuring perception have evolved steadily since Fechner's seminal work (Fechner,
1860/
1966; Kingdom & Prins,
2010; Thurstone,
1959) but remain somewhat primitive. One useful approach is to use ambiguous stimuli, that is, patterns that support two interpretations. Viewing ambiguous stimuli evokes a bistable percept, with an irregular alternation between the two competing percepts: Conscious awareness alternates even though the sensory inputs are constant (Blake & Logothetis,
2002). In binocular rivalry, for example, the stimulus presented to one eye is incompatible with that presented to the other eye and perception switches between the monocular stimuli every few seconds in a never-ending cycle (Levelt,
1966). The seen and unseen stimuli are termed dominant and suppressed, respectively. The loss of visibility during suppression is not absolute, and visual sensitivity in the suppressed eye can be measured using a monocular test stimulus to compare sensitivity during suppression relative to dominance. The drop in sensitivity when the tested eye's image is invisible (Alais, Cass, O'Shea, & Blake,
2010; Fox & Check,
1966; Nguyen, Freeman, & Alais,
2003) is called suppression depth and provides a useful metric for quantifying how much attenuation is needed to suppress an image from conscious awareness.
The standard method for measuring suppression depth requires the subject to continuously report which rivalry stimulus is currently visible so that the experimenter can deliver the test stimulus in the appropriate rivalry state—either dominance or suppression, depending on condition. When in the desired perceptual state, a test stimulus is triggered and the subject makes a forced-choice response about the stimulus (typically a contrast increment threshold task). This method of measuring suppression depth brings a host of problems. First, the experiment is complicated by a dual-task design, as the percept tracking is required in parallel with the forced-choice sensitivity task. Second, transitions between dominance and suppression are often difficult to define (Blake, O'Shea, & Mueller,
1992), and percept tracking therefore introduces a degree of response bias. The transition period may elicit mixed percepts in which parts of each eye's stimulus are visible, further complicating percept categorization. A third problem is that attention to the percept alters the dynamics of the system being measured (Lack,
1974; Meng & Tong,
2004; Paffen, Alais, & Verstraten,
2006).
Together, these problems add unnecessary variability to the data and even question the reliability of sensitivity measures in rivalry. Here we introduce a new method for measuring binocular rivalry suppression that overcomes these limitations: The subject responds to a test stimulus delivered at random times and with random contrast. Our new approach simplifies the traditional dual-task design to a single forced-choice task with a more objective response. Moreover, it allows us to measure visual sensitivity during rivalry dominance and suppression without asking subjects to report their subjective perceptual fluctuations. This overcomes the fundamental subjectivity of the standard approach and also avoids the problem of response bias. The work described here has been previously published in abstract form (Alais, Keetels, & Freeman,
2011).