Bistable Percepts in the Brain: fMRI Contrasts Monocular Pattern Rivalry and Binocular Rivalry

The neural correlates of binocular rivalry have been actively debated in recent years, and are of considerable interest as they may shed light on mechanisms of conscious awareness. In a related phenomenon, monocular rivalry, a composite image is shown to both eyes. The subject experiences perceptual alternations in which the two stimulus components alternate in clarity or salience. The experience is similar to perceptual alternations in binocular rivalry, although the reduction in visibility of the suppressed component is greater for binocular rivalry, especially at higher stimulus contrasts. We used fMRI at 3T to image activity in visual cortex while subjects perceived either monocular or binocular rivalry, or a matched non-rivalrous control condition. The stimulus patterns were left/right oblique gratings with the luminance contrast set at 9%, 18% or 36%. Compared to a blank screen, both binocular and monocular rivalry showed a U-shaped function of activation as a function of stimulus contrast, i.e. higher activity for most areas at 9% and 36%. The sites of cortical activation for monocular rivalry included occipital pole (V1, V2, V3), ventral temporal, and superior parietal cortex. The additional areas for binocular rivalry included lateral occipital regions, as well as inferior parietal cortex close to the temporoparietal junction (TPJ). In particular, higher-tier areas MT+ and V3A were more active for binocular than monocular rivalry for all contrasts. In comparison, activation in V2 and V3 was reduced for binocular compared to monocular rivalry at the higher contrasts that evoked stronger binocular perceptual suppression, indicating that the effects of suppression are not limited to interocular suppression in V1

How Simultaneous is the Perception of Binocular Depth and Rivalry in Plaid Stimuli?

Psychophysical experiments have demonstrated that it is possible to perceive both binocular depth and rivalry in plaids (Buckthought and Wilson 2007, Vision Research 47 2543–2556). In a recent study, we investigated the neural substrates for depth and rivalry processing with these plaid patterns, when either a depth or rivalry task was performed (Buckthought and Mendola 2011, Journal of Vision 11 1–15). However, the extent to which perception of the two stimulus aspects was truly simultaneous remained somewhat unclear. In the present study, we introduced a new task in which subjects were instructed to perform both depth and rivalry tasks concurrently. Subjects were clearly able to perform both tasks at the same time, but with a modest, symmetric drop in performance when compared to either task carried out alone. Subjects were also able to raise performance levels for either task by performing it with a higher priority, with a decline in performance for the other task. The symmetric declines in performance are consistent with the interpretation that the two tasks are equally demanding of attention (Braun and Julesz 1998, Perception & Psychophysics 60 1–23). The results demonstrate the impressive combination of binocular features that supports coincident depth and rivalry in surface perception, within the constraints of presumed orientation and spatial frequency channels

Binocular rivalry minus monocular rivalry.

<p>(A) Areas in which the activation for binocular rivalry exceeded that for monocular rivalry, shown for the three contrasts (9%, 18% and 36%). There was greater activation for binocular rivalry in occipital pole regions at the lowest contrast, but this reversed at higher contrasts. Note that the occipital pole (OP) is circled in white on a lateral and ventral view. Generally there was greater activation for binocular rivalry in superior parietal cortex (SP), inferior parietal cortex (IP) close to the temporoparietal junction, supplementary motor area (SMA), ventral temporal areas (VT) and lateral occipital (LO) areas including MT+ and lateral occipital complex. (B) Region of interest analysis. Binocular rivalry minus monocular rivalry in percent signal change (average of six subjects). The analysis for V1, V2 and V3 was carried out only in the foveal part of each area (0–2.9 deg eccentricity). Generally, the results did not differ between the left and right hemisphere, and have been averaged, except for area V1, for which the results are shown separately. There was greater activation for binocular rivalry in areas V2 and V3 at the lowest contrast, but this reversed at higher contrasts.</p

Correlations between activation for (A) monocular rivalry or (B) binocular rivalry and alternation rates.

<p>Correlations were performed between activation levels (% signal change minus baseline blank condition) and alternation rates for the six subjects obtained during the fMRI scan sessions. The correlations shown for monocular rivalry are for 36% contrast gratings, while the correlations for binocular rivalry are for 18% contrast gratings. Correlations are shown for areas V2 and V3 (average of left and right hemisphere), and in all four cases were statistically significant with correlation coefficients (r-values) of 0.82 and greater (p<0.05). For monocular rivalry, the activation levels increased with faster alternations, while the opposite effect occurred with binocular rivalry.</p

Binocular rivalry minus blank baseline.

<p>Figure follows the same format as <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0020367#pone-0020367-g003" target="_blank">Figure 3</a>, but results are for binocular rivalry (BR) minus the blank baseline condition. (A) As with monocular rivalry, binocular rivalry showed a U-shaped function of activation as a function of contrast; there was higher activation in a number of different areas at 9% and 36%. Abbreviations: inferior frontal cortex (IF); lateral occipital cortex (LO); middle frontal cortex (MF); premotor cortex (PM); superior parietal cortex (SP); supplementary motor area (SMA); temporoparietal junction (TPJ). (B) The region of interest analysis also confirmed that binocular rivalry showed a U-shaped function of activation as a function of contrast in many areas, particularly V2 and V3.</p

Correlations between binocular or monocular rivalry activation (% signal change minus baseline blank condition) and alternation rates for the six subjects obtained during the fMRI scan sessions.

<p>The Pearson correlation coefficients (r-values) are shown with positive or negative values, to indicate that activation levels increased (positive) or decreased (negative) with faster alternation rates. The correlation coefficients are shown for V1 (left hemisphere), V1 (right hemisphere), and other areas averaged for the left and right hemisphere (V2, V3, V3A, MT+). The statistically significant (p<0.05) correlations are shown in bold typeface.</p

Stimuli used in the fMRI and psychophysics experiments.

<p>(A–B) Left and right oblique gratings used for dichoptic presentation in binocular rivalry. (C) Composite grating stimulus presented to both eyes for monocular rivalry. (D) Baseline blank condition.</p

Binocular rivalry minus replay.

<p>(A) Figure follows the same format as <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0020367#pone-0020367-g006" target="_blank">Figure 6</a>, but shows areas in which the activation for binocular rivalry exceeded rivalry replay. There was greater activation for binocular rivalry in superior (SP) and inferior parietal (IP) cortex, supplementary motor area (SMA), ventral temporal (VT) areas and lateral occipital (LO) areas, including MT+ and lateral occipital complex. (B) Region of interest analysis, as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0020367#pone-0020367-g006" target="_blank">Figure 6</a>, but for the subtraction of binocular rivalry minus replay condition in percent signal change. With the replay condition subtracted, binocular rivalry continued to show a U-shaped function of activation as a function of contrast, with higher activation at 9% and 36%.</p

Psychophysical data averaged across all six subjects.

<p>(A) Alternation rates for binocular or monocular rivalry with grating stimuli. There was a slight tendency for alternations to be slower for monocular than binocular rivalry, but this was not statistically significant. (B) Data from the suppression test for binocular or monocular rivalry with grating stimuli. There was a greater change in visibility with alternations for binocular than monocular rivalry, especially at higher contrasts. Error bars are ±1 s.e.</p