The Impact of Stimulus Complexity and Frequency Swapping on Stabilization of Binocular Rivalry

Journal of Vision (2011) 11(2):6, 1–10 http://www.journalofvision.org/content/11/2/6 1

The impact of stimulus complexity and frequency swapping on stabilization of binocular rivalry

Cognitive Neuroscience Research Unit, Hammel Neurorehabilitation and Research Center, Kristian Sandberg MindLab, Aarhus University, Denmark

Institute of Cognitive Neuroscience, University College London, London, United Kingdom,& Wellcome Trust Centre for Neuroimaging, Bahador Bahrami University College London, London, United Kingdom

Cognitive Neuroscience Research Unit, Hammel Neurorehabilitation and Research Center, Jonas Kristoffer Lindeløv MindLab, Aarhus University, Denmark

Cognitive Neuroscience Research Unit, Hammel Neurorehabilitation and Research Center, MindLab, Aarhus University, Denmark,& Cognitive Neuroscience Research Unit, Department of Communication and Psychology, Morten Overgaard Aalborg University, Aalborg, Denmark

Institute of Cognitive Neuroscience, University College London, London, United Kingdom,& Wellcome Trust Centre for Neuroimaging, Geraint Rees University College London, London, United Kingdom

Binocular rivalry occurs when an image is presented to one eye while at the same time another, incongruent, image is presented to the other eye in the corresponding retinotopic location and conscious perception alternates spontaneously between the two monocular views. If a short blank period is inserted between intermittent presentations of rivaling stimuli, perception is stabilized and spontaneous alternations are drastically reduced. Whether the complexity of rivaling stimuli plays a role in stabilization is unknown. We replicated previous findings that swapping the stimuli between eyes across presentations abolishes stabilization for Gabors, but for more complex stimuli (a face and a house in our experiment), stabilization is eye-specific and not disrupted. Phase scrambling the rivaling face and house images did not change the stabilization pattern showing that the pattern can be observed without high-level perceptual content. We conclude that overlaps at low visual stages are the most likely cause of the eye-specific stabilization for both stimulus types. Additionally, we examined the impact of swapping the flicker frequency of the images and found a general impact on stabilization not specific to stimulus type. Taken together, the findings indicate that choice of stimulus features impact greatly on the results obtained in stabilization paradigms. Keywords: binocular vision, visual cortex, memory Citation: Sandberg, K., Bahrami, B., Lindeløv, J. K., Overgaard, M., & Rees, G. (2011). The impact of stimulus complexity and frequency swapping on stabilization of binocular rivalry. Journal of Vision, 11(2):6, 1–10, http://www.journalofvision.org/ content/11/2/6, doi:10.1167/11.2.6.

allow dissociation of changes in conscious perception from Introduction changes in physical stimulation (Andrews, Schluppeck, Homfray, Matthews, & Blakemore, 2002; Lumer, Friston, Bistable stimuli such as the Necker cube (Necker, 1832) & Rees, 1998). Binocular rivalry (BR; Breese, 1899)is and Rubin’s vase (Rubin, 1915) are visual stimuli that are one form of bistable perception that occurs when an image spontaneously perceived in two different ways and thus is presented to one eye while at the same time another,

doi: 10.1167/11.2.6 Received July 12, 2010; published February 8, 2011 ISSN 1534-7362 * ARVO

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incongruent, image is presented to the other eye. Percep- Recent models of BR propose that rivalry arises from tion alternates spontaneously between each monocular competition between neuronal populations at various view every few seconds. stages of the visual system in a multi-level process (Tong, If a short blank period (a stabilization interval) is inserted Meng, & Blake, 2006; Wilson, 2003). At early stagesVthe between intermittent presentation of a bistable stimulus, the lateral geniculate nucleus (LGN) and V1Vcompetition perceptual switch rate drops drastically (Leopold, Wilke, between monocular neurons is associated with BR Maier, & Logothetis, 2002; Orbach, Ehrlich, & Heath, between Gabor patches (Haynes, Deichmann, & Rees, 1963). Perception in consecutive trials stabilizes to one of 2005; Haynes & Rees, 2005) similar to those used by the two alternatives implying the existence of a perceptual Pearson and Clifford (2004). Swapping the stimuli memory across subsequent trials (Leopold et al., 2002). This between the eyes will clearly change which population of memory effect has been called percept stabilization, percept monocular neurons is stimulated. At higher levels of the maintenance, or simply stabilization (Leopold et al., 2002; visual system, however, processing of stimulus features Sterzer & Rees, 2008). It has recently been suggested that it occurs independently of the eye of origin (although of is mediated by accumulative changes in the sensitivity of course some monocular biases may persist in binocularly neural populations representing each alternative percept and driven cells in early extrastriate cortex), and BR between that these changes in sensitivity occur on multiple time scales images of complex objects (such as faces and houses) may (Brascamp, Pearson, Blake, & van den Berg, 2009; Brascamp result from competition between binocular neurons repre- et al., 2008;Noest,vanEe,Nijs,&vanWezel,2007). senting different stimulus features (Haynes & Rees, 2005; In the above-mentioned studies of stabilization, images Tong, Nakayama, Vaughan, & Kanwisher, 1998). Since are presented to the same eye throughout the experiment, the level of neuronal competition determining the course and it is thus impossible to determine whether the of binocular rivalry seems to depend on stimulus stabilization effect occurs due to facilitation of neurons characteristics, it is conceivable that stabilization of any selective for one of the stimuli or alternatively for one eye mnemonic trace during intermittent rivalry might thus also (i.e., whether interstimulus or interocular competition is occur in different levels of visual processing. modulated). Chen and He (2004) and Pearson and Clifford This hypothesis would suggest, in contrast to Chen and (2004) disentangled the two by swapping the two stimuli He (2004), that the mnemonic trace of a high-level stimulus between the two eyes across intermittent presentations. In will be independent of eye of origin. This difference between such a paradigm, perceptual stabilization and what might hypothesis and finding could be explained in two ways. One be called eye signal stabilization can thus be defined possibility is that an eye-specific feedback mechanism could separately: Perceptual stabilization occurs when an inhibit eye-specific neurons at early stages of the visual observer reports seeing the same image on two consec- system while stimulus processing proceeds at later stages. utive trials whereas eye signal stabilization occurs when Feedback operates during continuous BR (van Boxtel, Alais, he reports the stimulus associated with the same eye on & van Ee, 2008). Such a mechanism is consistent with the two consecutive trials. Chen and He reported that the eye long-lasting notion of BR suppression being eye-specific signal appeared to be stabilized and not the percept. (Asher, 1953;duTour,1760) and supported by exper- Similarly, based on their first experiment, Pearson and imental findings that test probes presented to an eye during Clifford conclude that eye of origin is the most important suppression are more difficult to detect than test probes factor for BR stabilization and make references to the presented during dominance (Fox & Check, 1972). Alter- similarity of the findings of the studies. However, Pearson natively, Chen and He’s findings might result from their and Clifford do not report complete stabilization of the broadband stimuli (which unlike those of Pearson & eye. When their results are plotted instead as eye signal Clifford, 2004 consisted of many different orientations) stabilization (Figure 1), a substantial difference between activating contiguous orientation columns and resulting in their findings and those on Chen and He can now be seen. greater overlap of monocularly biased neurons at early Whereas image swapping has no impact on stabilization in stages of the visual system, thus leaving a mnemonic trace Chen and He’s study, Pearson and Clifford find stabiliza- to impact perception on a trial-by-trial basis. tion to be reduced to chance levels when averaged across Thus, the results obtained by Pearson and Clifford might subjects. Thus, the conclusion of Chen and He that the reflect their stimuli being processed by non-overlapping direction of suppression between the eyes is stabilized monocularly biased neurons at an early visual stage, conflicts with the findings of Pearson and Clifford. Possible possibly V1. In contrast, the results obtained by Chen causes could be differences in the neural processing of the and He (2004) might be explained by their stimuli stimuli or simply differences in the experimental paradigm. activating overlapping monocular neurons or via an eye- Pearson and Clifford used Gabor patchesVlow-level specific feedback mechanism from downstream stages of stimuli processed principally in the primary visual cortex visual perception back to early visual areas. (V1)Vwhile Chen and He used radial and concentric Here we conducted three experiments to test these gratings, the processing of which are presumably per- hypotheses. To address the discrepancy between Chen and formed in V4 through the global pooling of local V1 He (2004) and Pearson and Clifford (2004) directly, we orientations (Wilkinson et al., 2000). present our results in terms of eye signal stabilization.

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EEG/MEG experiments in order to “tag” each image so they produce distinct neural responses (Pastor, Artieda, Arbizu, Valencia, & Masdeu, 2003). As mentioned, the flicker frequency of an image has not traditionally been considered a feature of the image as such, but rather a way of attaching a particular neural signature to a stimulus (see, e.g., Lansing, 1964). Such frequency tagging has previously been used in the majority of EEG experiments using BR (Brown & Norcia, 1997; Kamphuisen, Bauer, & van Ee, 2008; Lansing, 1964; Srinivasan, Russell, Edelman, & Tononi, 1999) and may be used in the future in BR stabilization experiments. If frequency swapping affects BR stabilization, however, this would be a potential confound in future EEG/MEG studies of stabilization. We examined all goals of the study in three experiments. In Experiment 1, we replicated the findings of Pearson and Clifford (2004) using Gabor patches. Addi- tionally, we conducted further analyses that indicated that our findings did indeed reflect a mnemonic trace in monocular neurons. In Experiment 2, we replicated the findings of Chen and He (2004) using higher level (face and house) stimuli. In Experiment 3, we tested whether phase scrambling the faces and houses while keeping the power spectra identical to Experiment 2 (thus removing the high-level meaningful content of rivaling images) modulates eye-specific stabilization. In Experiments 1–2, the role of frequency swapping was examined. In Experi- ment 3, only the impact of image swapping was examined and the two stimuli were flickering at the same frequency. Before reporting our findings, it should be noted that this classical type of BR for which the stabilization effect is found is distinct from other types of rivalry caused by different images being presented to each eye. For instance, swapping rivaling stimuli between the eyes every 300 ms or so does not cause perception to alternate rapidly, but instead produces regular rivalry alternations, i.e., stimulus rivalry (Logothetis, Leopold, & Sheinberg, 1996). More- Figure 1. Data reproduced approximately from Chen and He over, information from one half of an image presented to (2004) and Pearson and Clifford (2004), Experiment 1. Results one eye can be grouped with the other half of an image are plotted as eye signal stabilization instead of percept stabiliza- presented to the other eye and thus cause perception to tion. Average chance of eye signal stabilization across subjects alternate between the grouped images and not the eyes as a function of image swap is shown. (Kovaćs, Papathomas, Yang, & Fehe´r, 1996). The work- ings of these phenomena are distinct from conventional BR in that the eye-of-origin component has been However, as previous results have been reported in terms neutralized (Pearson & Clifford, 2004). An explanation of perceptual stabilization, we will refer to those as BR of these phenomena (and stabilization within such stabilization in general. When distinctions are needed, we paradigms) is beyond the scope of the present study. refer to eye signal stabilization or perceptual stabilization separately. In the literature, the phenomenon of BR stabilization is influenced by factors such as position in visual space, color, Experiments 1 and 2 and stimulus orientation (Chen & He, 2004; Pearson & Clifford, 2004). For this reason, we also sought to examine whether something not usually considered a stimulus Methods feature proper, the flicker frequency (frequency tagging) of the stimulus, can impact on BR stabilization. Research- Experiments 1 and 2 sought to replicate the findings of ers often use images flickering at different frequencies in previous experiments as well as conduct additional

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analyses and examine the role of frequency tagging on BR report option speciﬁc to the experiment. In Experiment 1, stabilization. the report options were left (counterclockwise)-tilted grating, right (clockwise)-tilted grating, and mixed perception. In Experiment 2, the options were face, house, Participants and mixed perception. Eight healthy young adults with normal or corrected-to- It is possible that BR stabilization in some cases is not a normal vision gave informed consent to participate in each result of perceptual memory but instead perceptual bias experiment. In Experiment 1, the age of the participants (Carter & Cavanagh, 2007). To minimize bias, we was between 22 and 36 years; four participants were increased the chances that participants would report each female. In Experiment 2, the age of the participants was percept equally often during the experiment by adjusting between 24 and 34 years; three participants were female. the relative luminance of the images for each participant The experiments were reviewed by the local ethics before each experiment. The starting luminance for each committee. image was maximum screen value, and one value was decreased until the participant reported seeing both Apparatus images equally often (T7%) during a 1-min-long continuous presentation. This was done for separately for each Stimuli were generated using the MATLAB toolbox eye. Cogent (www.vislab.ucl.ac.uk/Cogent/). They were dis- The experiments consisted of 32 blocks presented in a played on a CRT monitor (17W in Experiment 1,19W in pseudorandom order. Each block consisted of 20 trials. In Experiment 2) with a screen resolution of 1024 768 at a Experiment 1, each trial consisted of a stimulation period refresh rate of 60 Hz. Participants viewed the stimuli of approximately 700 ms followed by a blank screen (the through a mirror stereoscope positioned at approximately stabilization interval) lasting 1800 ms. The trial duration 50 cm from the monitor.

Stimuli In Experiment 1, stabilization of a green and a red Gabor patch (contrast = 100%, spatial frequency = 4 cycles/degree, standard deviation of the Gaussian enve- lope = 10 pixels) was examined (Figure 2). The green Gabor patch was tilted 45 degrees counterclockwise and the red Gabor patch was tilted 45 degrees clockwise. In Experiment 2, stabilization of a red face and a green house was examined. In both experiments, each stimulus was presented within an annulus (inner/outer r = 2/3 degrees of visual angle) consisting of randomly oriented lines. In the center of the circle was a small circular fixation dot. The luminance of the stimuli was set for each participant (see Procedure section). Flickering was applied by removing the stimulus from screen at a certain rate. For all participants, the first image flickered at a frequency of 6 Hz. For one half of the participants (selected at random), the second image flickered at 7.5 Hz; for the other half of the participants, it flickered at 15 Hz. No systematic difference was observed in the response pattern as an effect of fast or slow flicker, and the two subsets of the data were analyzed as one.

Procedure Participants looked into the mirror stereoscope while Figure 2. Binocular rivalry stimuli. Stimuli were dichoptically the ﬁxation circles around the stimuli were displayed, and presented to the eyes of the participant using a mirror stereo- the position of the circles as well as the mirrors of the scope. Stimuli were ﬂickering at a rate between 6 and 15 Hz. First stereoscope was calibrated until the circles fused. When row: Grating stimuli used in Experiment 1. Second row: Face/ stimuli were displayed, participants reported what they house stimuli used in Experiment 2. Third row: Phase-scrambled saw using one of three buttons, each corresponding to a face/house stimuli used in Experiment 3.

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was selected so that participants experienced a stable perceptual state, i.e., the percept had time to stabilize and did not switch during the stimulation period. As indicated by early findings that complex stimuli switch at a slower rate (Rogers, Rogers, & Tootle, 1977), we found that the face/house rivalry stabilized and switched slower than grating rivalry. For that reason, the stimulation period was slightly longer in Experiment 2. Stimulus durations of approximately 1500 ms and blank periods of 1500 ms were used. Between successive presentations of the BR stimuli separated by a stabilization interval, we swapped either the eye to which each image was presented, or the flicker frequency associated with one of the images, in order to determine whether this had an effect on BR stabilization. The experiment employed a factorial design with stabilization proportion as the dependent variable and frequency swap and image swap as independent variables (Figure 3). Hence, there were four experimental conditions. In the first, the image was always presented to the same eye with the same tagging frequency within each block (eye and frequency were counterbalanced across blocks). In the second condition, the image was always presented to the same eye within each block (counterbalanced across blocks), but the tagging frequency was swapped between eyes between trials. In the third condition, the image was swapped between eyes between trials, but the tagging frequency always displayed to the same eye (counterbalanced across blocks). In the fourth condition, both image and tagging frequency were swapped between eyes between trials.

Calculation of stabilization As mentioned above, the main predictor of stabilization is eye of origin. For this reason, we calculated stabilization from the perspective of the eye, not the percept. Participants were asked to report the identity of the image rather than the eye of origin as utrocular discrimination is rarely possible, and eye signal stabilization was thus calculated from the reported percept. For blocks in which the stimuli were not swapped, eye signal stabilization was identical to percept stabilization and was thus given by

stabilizationeye ¼ stabilizationpercept; ð1Þ Figure 3. The four experimental conditions employed in Experi- ments 1–3. In condition 1, the stimuli were displayed in the same where stabilizationpercept = 1 if the same percept was way on every trial; in condition 2, the flicker frequencies of the reported before and after the stabilization interval, and stimuli are swapped between eyes on consecutive trials; in stabilizationpercept = 0 if the reported percept changed condition 3, the images were swapped between eyes on consec- across such an interval. When the stimuli were swapped utive trials; in condition 4, both flicker frequency and image were between eyes between presentations, perception followed swapped between eyes on consecutive trials. In Experiment 3, either the eye or the percept; i.e., if the same percept was identical flicker frequencies were used for the two stimuli, thus reported, the reported image was presented to a different effectively leaving only two conditions: No image swap and image eye before and after the break in the presentation, and if a swap.

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different percept was reported, those stimuli were presented to the same eye before and after the break. For blocks in which the stimuli were swapped, eye signal stabilization was thus given by

stabilizationeye ¼ 1j stabilizationpercept: ð2Þ

Results

Average eye signal stabilization was calculated for each participant using only trials in which the participant reported a single clear percept, i.e., trials during which participants reported multiple or mixed percepts were excluded from the analysis. We excluded the possibility of perceptual bias causing the results by examining that participants reported the images equally often: Images 1 and 2 were reported on 41% vs. 59% of the trials in Experiment 1 and on 47% vs. 53% in Experiment 2. Importantly, all images were stabilized above chance level on j80% vs. 66% in Experiment 1 and 79% vs. 75% in Experiment 2. Data from Experiments 1 and 2 were then analyzed in a single mixed analysis of variance with eye signal stabilization as the dependent variable and image swap and frequency swap as related conditions and experiment as the unrelated condition (Figure 4). We found an effect of image swap (F(1,14) = 23.36, p G 0.0001) but also an interaction between image swap and experiment (F(1,14) = 27.67, p G 0.0001). An effect of frequency swap was also found (F(1,14) = 9.20, p G 0.01), and there was a just significant interaction between image swap and frequency swap (F(1,14) = 5.49, p G 0.05). There was no interaction between frequency swap and experiment (F(1,14) = 0.15, p = 0.71) or between image swap, frequency swap, and experiment (F(1,14) = 1.88, p = 0.19). There was no effect of experiment alone on stabilization (F(1,14) = 0.90, p = 0.36). Additional one-way ANOVAs revealed that for . rivalry between Gabor patches (Experiment 1), image swap Figure 4 Average chance of eye signal stabilization across disrupted BR stabilization highly significantly (F(1) = 38.9, subjects as a function of image swap and frequency swap. Eye p G 0.0001), whereas there was no such effect for face/ signal stabilization chance is the chance that the reported percept house stimuli (Experiment 2; F(1) = 0.11, p =0.74).Thus, was displayed to the same eye on two consecutive trials. The horizontal dashed line indicates chance perception. Error bars this analysis (1) replicated the findings that that image swap fi affects BR stabilization for Gabor patches (Pearson & represent 95% con dence intervals. (A) Results of Experiment 1 Clifford, 2004) but not for rivaling images with more (rivalry between Gabor patches). (B) Results of Experiment 2 complex structure (Chen & He, 2004)and(2)indicatedthat (rivalry between a face and a house). frequency swapping has an impact on eye signal stabilization independently of whether Gabor patches or face/house BR stabilization was different from chance for the two image stimuli are used. swap conditions of Experiment 1, the most important of BR stabilization of face/house stimuli thus followed the these being condition 4 in which both stimulus features eye of origin, as eye signal stabilization did not change as a (image identity and flicker frequency) were swapped function of image swap. In contrast, BR stabilization of between trials. We found that BR stabilization did not Gabor patches was disrupted and the reported percept differ from chance neither for condition 3 (t(7) = 0.94, p = appeared random when images were swapped between eyes 0.38) nor condition 4 (t(7) = 0.01, p = 0.99). In comparison, across presentations. In order to confirm that the reported BR stabilization in conditions 1 (t(7) = 26.37, p G 0.0001) percepts were indeed random, we tested the hypothesis that and 2 (t(7) = 13.20, p G 0.0001) of Experiment 1 clearly

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2, 4, 6, etc. If stabilization occurs in spite of interleaved stimuli for BRVas was observed for ambiguous ﬁgures/ motionVwe would thus expect a high degree of stabilization between the reported percept between every pair of consecutive odd and even presentations, i.e., stabilization should occur across trial 1, 3, 5, etc. and separately across trial 2, 4, 6, etc. As shown in Figure 5, this was indeed the case in our data. Eye signal stabilization did not vary across condition (F(3,28) = 0.30, p = 0.83), and the average degree of stabilization (93%) different highly signiﬁcantly from chance (t(31) = 39.7, p G 0.0001).

Discussion

We found that the effects of image swapping on BR stabilization differed substantially when comparing Gabor patches and face/house stimuli. For Gabor patches, swapping the images between presentations disrupted stabilization, with the reported percept being determined no more consistently than chance on consecutive trials. Analysis of stabilization on odd or even pairs of trials confirmed that eye signals were stabilized across every other trial. This suggests that a mnemonic trace was established. We propose that the most likely explanation Figure 5. Data from Experiment 1. Eye signal stabilization is for this pattern of stabilization is that non-overlapping calculated for every odd and every even pair of trials. Average populations of monocular early visual neurons carry such chance of eye signal stabilization across subjects as a function of a mnemonic signal. This interpretation is consistent with the four experimental conditions is shown. Note that although the recent findings that stabilization of Gabor patches conditions are labeled by what is usually swapped in that decreases as a function of distance in eccentricity and that condition, nothing is swapped across every other trial in any of perception is almost chance at eccentricities where no the conditions. Contrary to stabilization across every single trial, overlap is to expected in the receptive field of V1 neurons stabilization is not disrupted across every other trial in any (Knapen, Brascamp, Adams, & Graf, 2009). In contrast to condition, and it is thus unaffected by the intermediate presenta- the pattern observed for Gabor patches, image swapping tion of an image that activates a different subset of monocular had no impact on eye signal stabilization for face/house neurons. The finding is in line with our proposal that the memory stimuli. trace is situated in monocular neurons in the early stages of visual We found that swapping the tagging frequency of the processing. rivaling stimuli disrupted BR stabilization and that no difference was observed between experiments. Though the disruption was quite small, it is potentially important as it differed from chance, and it did so in all conditions of demonstrates that frequency tagging can, in fact, be Experiment 2 (condition 1: t(7) = 7.50, p = 0.0001, considered an object feature that influences the processing condition 2: t(7) = 6.81, p G 0.0005, condition 3: t(7) = of the stimuli. This is consistent with a previous finding 11.51, p G 0.0001, and condition 4: t(7) = 3.75, p G 0.01 (all showing that frequency tagging can bias perception in the tests uncorrected)). interocular switching paradigm (Silver & Logothetis, If the pattern observed in Experiment 1 was caused by 2007). Acknowledging this property of frequency tagging the stabilization memory trace being stored in monocular will be important in any future EEG and MEG studies neuronal populations, then swapping the images between examining electro- or magnetoencephalic correlates of BR eyes across trials would effectively be identical to stabilization, especially if image characteristics are swap- temporally interleaving two unrelated rivalry pairs as has ped between trials. Special notice should be paid to the been done by Maier, Wilke, Logothetis, and Leopold observation that in Experiment 2, frequency swapping was (2003) in a study of ambiguous figure/motion perception. the only factor impacting on eye signal stabilization. In their study, stabilization was found in spite of the The results of Experiment 2 replicated those of Chen interleaved stimuli. In the image swap conditions of our and He (2004). The results, however, do not allow us to experiments, the presented stimuli could similarly be distinguish between the hypotheses that BR stabilization considered trials with interleaved stimuli as one rivalry for the face/house stimuli is caused by mnemonic traces in pair was presented on trials 1, 3, 5, etc. and another on trials overlapping monocular populations of early visual areas

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or, alternatively, by an eye-specific feedback mechanism stabilize on each trial and prevent perceptual switches from later downstream stages of visual processing back within trials for most subjects. This need for slightly lower onto early visual neurons. In order to distinguish these stimulus durations for successful BR compared to Experi- possibilities, an additional experiment was conducted. ment 2 could be causes by decreased complexity of the stimuli due to phase scrambling. Flicker rates were always 7.5 Hz as the impact of frequency tagging was not examined in this experiment. Experiment 3 Results To identify whether feedback mechanisms working at later visual stages was necessary for eye-specific stabili- Experiment 3 had effectively only two conditions, “No zation (as observed in Experiment 2), we tested whether image swap” and “Image swap.” The results are plotted in meaningful content of rivaling is necessary or rather Figure 6. A paired t-test revealed no difference in eye having a complex (even though meaningless) image signal stabilization between conditions (t(8) = 0.65, p = structure should be sufficient for maintaining stabilization 0.53), and the average proportion of stabilized percepts as we saw in Experiment 2. We therefore removed all was above chance (t(17) = 5.16, p = 0.0001). The results high-level meaningful content from the images by phase of Experiment 2 were thus replicated. scrambling the rivaling images and conducted a follow-up experiment. Discussion

Methods Experiment 3 replicated the ﬁndings of Experiment 2 using phase-scrambled face/house images. We thus found Experiment 3 was identical to Experiment 2 with the that BR stabilization of eye of origin was not dependent following changes: 9 participants were tested, and instead on high-level processing but also occurs when stimuli lack of meaningful face/house stimuli, phase-scrambled ver- meaningful, high-level feature/object content. Overall, the sions of the same images were used (Figure 2). Stimulus experiments of the study support the notion that BR durations of 1200 ms were found to allow perception to stabilization reﬂects a memory trace in early visual areas and that this memory trace is located in non-overlapping populations of monocular neurons for rivalry between Gabor patches but in overlapping populations of monocular neurons for the other stimulus types tested.

Conclusions

We have identified a previously unnoticed difference in the stabilization patterns of Gabor patches and other stimuli. In two experiments using the same experimental design, we replicated the finding that swapping the stimuli between eyes across presentations caused chance perception for Gabors, but that for other stimuli (a face and a house in our case) stabilization was eye-specific. Further analyses revealed that perception was stabilized com- pletely on every odd pair and every even pair of trials for Gabor patches, consistent with a mnemonic signal associated with each Gabor being stored in non-over- Figure 6. Results of Experiment 3 (rivalry between phase- lapping populations of monocular neurons. Experiment 3 scrambled face/house images). Average chance of eye signal examined whether face/house stabilization was caused by stabilization across subjects as a function of image swap. Eye an eye-specific feedback mechanism. This was performed signal stabilization chance is the chance that the reported percept by phase scrambling the images thus obscuring high-level was displayed to the same eye on two consecutive trials. The features. In this experiment, we still found no difference horizontal dashed line indicates chance perception. Error bars in the stabilization pattern. We thus conclude that represent 95% confidence intervals. overlapping neuronal populations at early stages of visual

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