**When perceptually ambiguous stimuli are presented intermittently, the percept on one presentation tends to be the same as that on the previous presentation. The role of short-term, acute biases in the production of this perceptual stability is relatively well understood. In addition, however, long-lasting, chronic bias may also contribute to stability. In this paper we develop indices for both biases and for stability, and show that stability can be expressed as a sum of contributions from the two types of bias. We then apply this analytical procedure to binocular rivalry, showing that adjustment of the monocular contrasts can alter the relative contributions of the two biases. Stability is mainly determined by chronic bias when the contrasts are equal, but acute bias dominates stability when right-eye contrast is set lower than left-eye contrast. Finally, we show that the right-eye bias persists in continuous binocular rivalry. Our findings reveal a previously unappreciated contribution of chronic bias to stable perception.**

*x*= 0.296,

*y*= 0.328) and background luminance was 41 cd/m

^{2}. Subjects used a stereoscope with front-surfaced mirrors to view the monitor. Stimuli for the left and right eyes were presented on the left and right sides, respectively, of the monitor. Subjects used the stereoscope's chinrest and forehead rest to stabilize their view, and the optical distance from eye to monitor was 1.14 m. Stimulus chromaticity was measured independently through each stereoscope eyepiece. Luminance differed between eyes by less than 3%, and measured contrast matched contrast setting to within 3% for each eye. Experiments were conducted in a darkened room so that the only light visible to the subject was from the monitor.

**Figure 1**

**Figure 1**

*p*as shown in Table 1.

_{ij}**Table 1**

*p*, the probability expected if the two percepts are independent:

_{ij}*p*=

_{ij}*p*. Thus Appropriately, this index is zero when successive percepts are independent. Third, chronic bias gives the probability that one percept occurs more often than the other,

_{i}p_{j}*p*, we can express stability in terms of acute bias and chronic bias. First, the indices are expressed in terms of

_{ij}*p*

_{22}and

*p*

_{2}, using Equations 1 to 4. To simplify acute bias, we first transform its first two terms, For the later terms, thus, Finally,

*p*

_{1}and

*p*

_{2}will increase the probability of repeating percepts, regardless of the direction of the imbalance. Acute bias can, however, increase or decrease stability depending on whether a percept increases or decreases the probability that the following percept is the same.

**Table 2**

**Figure 2**

**Figure 2**

*PSE*, was estimated as the contrast for which the fitted curve yields equal probabilities of right- and left-eye percepts. Chronic bias is then the displacement of the point of subjective equality from the point at which both monocular contrasts equal 0.5: The subscript,

*stim*, indicates that the measurement is made in stimulus terms rather than the probability terms used below. Figure 3B shows chronic bias for subjects in both the standard and large cohorts: chronic bias is significantly greater than zero (right-tailed sign test: sign = 48,

*α*= 0.05,

*p*= 3.8 × 10

^{−6}).

**Figure 3**

**Figure 3**

*correlation coefficient*= 0.89,

*α*= 0.05,

*p*= 0.00020). We conclude that the chronic bias toward the right eye's stimulus is due to ocular dominance rather than orientation preference. As described in the Discussion, right-eye dominance is a common finding in a variety of binocular rivalry experiments.

**Figure 4**

**Figure 4**

*Stability*is defined as

*Acute bias*refers to serial dependence between percepts and can be measured by subtracting out the contribution due to independence of percepts: The second term in this equation is the degree of stability expected if each percept is independent of the preceding percept, which is simply the product of the probabilities of the individual percepts. Finally,

*chronic bias*is a property of single percepts:

**Figure 5**

**Figure 5**

*p*

_{22}_{,}

*, is the observed probability that percept 2 is experienced on two consecutive trials. The second component,*

_{observed}*p*

_{22}_{,}

*, is the value of this probability when the two percepts are independent. Figure 6 shows both the probabilities as a function of right-eye contrast; values are means over 13 subjects. The vertical distance between the curves equals one quarter of the acute bias, and clearly varies with the contrast used. An alternative approach to measuring the index is also shown. The lateral distance between the two curves provides a value for the acute bias,*

_{independence}*a*, in terms of the stimulus rather than probability. This measure has the advantage that it is largely independent of the contrast used to measure it.

_{stim}**Figure 6**

**Figure 6**

**Figure 7**

**Figure 7**

*α*= 0.05,

*p*= 0.0041). We conclude that, just as with intermittent rivalry, there is a bias toward the right-eye's stimulus during continuous rivalry.

**Figure 8**

**Figure 8**

*α*= 0.05,

*p*= 1.0 × 10

^{−5}). Psychometric slope decreases as neural signals become noisier (Pelli, 1985), suggesting that noise plays a greater role during continuous rivalry. This is an issue taken further in the Discussion.

**Figure 9**

**Figure 9**

- Perceptual stability during ambiguous stimulation results from both chronic and acute biases.
- Stability can be expressed as a sum of contributions from chronic and acute bias.
- There is a pronounced chronic bias toward the right eye's stimulus during both intermittent and continuous binocular rivalry.
- Varying the relative strength of the rivaling stimuli varies the relative contributions of chronic and acute bias to perceptual stability.

*Binocular rivalry*. Cambridge, MA: MIT Press.

*Journal of Neurophysiology*, 87, 3102–3116.

*Nature*, 411 (6839), 798–801.

*PLoS One*, 3 (1), e1497.

*PLoS One*, 2 (4), e343.

*PLoS One*, 7 (1), e30595.

*Graefe's Archive for Clinical and Experimental Ophthalmology*, 243, 926–932.

*Attention, Perception, & Psychophysics*, 71, 174–182.

*Trends in Cognitive Sciences*, 3, 254–264.

*Nature Neuroscience*, 5, 605–609.

*British Journal of Psychology*, 57, 225–238.

*Psychological Bulletin*, 130, 748–768.

*American Journal of Psychology*, 43, 392–405.

*Optometry*, 72, 168–178.

*Perceptual and Motor Skills*, 17, 439–458.

*Trends in Cognitive Sciences*, 12, 334–341.

*Journal of the Optical Society of America A*, 2, 1508–1532.

*Nature Neuroscience*, 3 (11), 1153–1159.

*Psychological Bulletin*, 83, 880–897.

*Vision Research*, 30, 1–10.

*Proceedings of the National Academy of Sciences, USA*, 94, 3408–3413.

*PLoS One*, 6 (5), e18978.

*Trends in Cognitive Sciences*, 13, 310–318.

*A source book of Gestalt psychology*. London: Routledge & Kegan.

*Journal of Experimental Psychology*, 45, 205–217.

*Vision Research*, 47, 2741–2750.