Willpower and Conscious Percept: Volitional Switching in Binocular Rivalry

When dissimilar images are presented to the left and right eyes, awareness switches spontaneously between the two images, such that one of the images is suppressed from awareness while the other is perceptually dominant. For over 170 years, it has been accepted that even though the periods of dominance are subject to attentional processes, we have no inherent control over perceptual switching. Here, we revisit this issue in response to evidence that top-down attention can target perceptually suppressed ‘vision for action’ representations in the dorsal stream. We investigated volitional control over rivalry between apparent motion (AM), drifting (DM) and stationary (ST) grating pairs. Observers demonstrated a remarkable ability to generate intentional switches in the AM and D conditions, but not in the ST condition. Corresponding switches in the pursuit direction of optokinetic nystagmus verified this finding objectively. We showed it is unlikely that intentional perceptual switches were triggered by saccadic eye movements, because their frequency was reduced substantially in the volitional condition and did not change around the time of perceptual switches. Hence, we propose that synergy between dorsal and ventral stream representations provides the missing link in establishing volitional control over rivalrous conscious percepts

The Effects of Motion on Binocular Rivalry between Simple and Complex Images

The term binocular rivalry refers the perceptual alternations that occur when a different image is presented to each eye. There is an ongoing debate as to whether competition between two eyes or the two perceptual interpretations instigate this bistability. The thesis investigated theses mechanisms by comparing the effects of continuous stimulus movement on simple checkerboard and complex face/house pairs of rival stimuli. Data was collected from a sample of 7 male and 18 female participants. As predicted, motion increased perceptual dominance durations, spatial coherence and suppression depth during rivalry between simple stimuli, yet not during rivalry between complex stimuli. It was concluded that mechanisms underlying binocular rivalry act upon the level at which stimuli are represented in the visual system

The Relationship between Pursuit Eye Movements and Perception during Binocular Rivalry

It has recently been shown that action-percept congruency plays a role in binocular rivalry, a form of bistable perception that occurs when incompatible images are presented to the two eyes. Here, we investigated the degree to which smooth-pursuit eye movements can bias perceptual competition. In the first experiment, observers pursued a horizontally oscillating dot that was superimposed on rivalrous, leftward and rightward drifting gratings. Perceptual dominance was consistently biased in the direction of smooth-pursuit and tended to switch when pursuit direction switched. The strength of this relationship increased with speed, especially when the pursuit speed matched the grating speed. In a second experiment, we investigated the interaction between pursuit and intentional control on rivalry dynamics. Relative to non-volitional viewing, percept-pursuit coupling was weakened when observers were instructed to selectively maintain one percept or to mismatch their percept with pursuit. However, instructions to match percept with pursuit did not further increase the strength of coupling. Our results contribute to converging evidence that self-generated actions can influence perception. We have provided insight into how this affects binocular rivalry, by showing that percept-pursuit coupling can be suppressed at the observer's will

'Vision for action' enables volitional control of binocular rivalry

Abstract not available

Illustration of the stimuli and raw data.

<p>(A) The apparent motion (AM) stimulus was created by presenting a red/green grating on alternate frames to a luminance-defined yellow grating, with gratings displaced by a quarter of a cycle in each frame (see Method Section). When viewed through red/green glasses, observers experience binocular rivalry between the red, rightward and green, leftward apparent motions. (B) The drifting (DM) and stationary (ST) gratings were matched with the apparent motion gratings for mean luminance and spatial frequency. During monocular (M) presentation, stimuli were exogenously switched upon each cue, so that one eye received a drifting grating and the other a blank field. (C) This segment of raw data illustrates voluntary control over binocular rivalry between the apparent motion gratings. The solid red and green bars represent auditory commands to switch to the red, leftward and green, rightward stimuli respectively. The red and green shadings illustrate subjective reports of leftward and rightward perception and the black trace illustrates corresponding switches in the direction of optokinetic nystagmus (OKN), our objective measure of perceptual state.</p

Proportion of coherent rivalry under passive and volitional viewing conditions.

<p><i>N</i> = 4.</p

Latency and probability of volitional switches.

<p>One observer (LH) was instructed on tone to make a perceptual switch either immediately, or after mentally counting one, two or three seconds. The tone was delivered 1 s into each trial. There were 20×6 s trials for each instructional condition, with data rejected if the first percept was not reported (or if OKN did not commence) prior to the tone. (A) This panel shows the average time it took to switch from the initially dominant, ‘percept A’ to the initially unseen, ‘percept B’ during rivalry between leftward and rightward moving AM gratings. The light bars represent subjective reports and the dark bars represent changes in OKN direction. Error bars denote ±1 s.e. (B) This panel displays the probability of the observer reporting percept B as a function of time and panel (C) displays the probability of OKN ‘pursuit direction B’. The strong match between OKN and subjective traces indicates that LH reported perceptual switches accurately. As it was rare for Percept B to occur prior to the desired time, these results indicate that the tone itself did not trigger perceptual switches, but rather the observer could use willpower to decide when to switch between percepts.</p

Saccade occurrence versus time for natural and volitional perceptual switches.

<p>The top panels display 1 s epochs of raw eye movement signal exhibiting OKN when the rightward drifting AM grating was perceptually dominant, under passive (A) and volitionally controlled (B) binocular rivalry conditions. Note that the pursuit phases tended to be slightly longer during volitionally controlled binocular rivalry. The bottom panels display event-related analyses of saccade occurrence for the passive (C) and volitional (D) rivalry conditions. Z-score deviations in saccade occurrence were calculated relative to baseline occurrence and plotted in the time surrounding perceptual switches. On the x-axis, t = 0 corresponds to when observers reported the onset of perceptual switches. The grey bars are an estimate of when the switch actually occurred, based on reaction times to exogenously switching monocular gratings. <i>N</i> = 4.</p

Temporal structure of human magnetic evoked fields

Nonlinear analysis of the multifocal cortical visual evoked potential has allowed the identification of neural generation of higher-order nonlinear components by magnocellular and parvocellular neural streams. However, the location of individual brain sources that make such contributions to these evoked responses has not been studied. Thus, an m-sequence pseudorandom stimulus system was developed for use in magnetoencephalographic (MEG) studies. Five normal young adults were recorded using an Elekta TRIUX MEG with 306 sensors. Visual stimuli comprised a nine-patch dartboard stimulus, and each patch fluctuated between two luminance levels with separate recordings carried out at low (24 %) and high (96 %) temporal contrast. Sensor-space analysis of MEG evoked fields identified components of the first- and second-order Wiener kernel decomposition that showed qualitative similarity with EEG-based cortical VEP recordings. The first slice of the second-order response (K2.1) was already saturated at 24 % contrast, while the major waveform of the second slice of the second-order response (K2.2) grew strongly with contrast, consistent with properties of the magnocellular and parvocellular neurons. Minimum norm estimates of cortical source localization showed almost simultaneous activation of V1 and MT+ activations with latencies only a little greater that those reported for first neural spikes in primate single cell studies. Time-frequency analysis of the kernel responses from five minimum norm estimate scout sources shows contributions from higher-frequency bands for the first compared with the second slice response, consistent with the proposed neural sources. In support of this magno/parvo break-up, the onset latencies of the K2.2 responses were delayed by approximately 30 ms compared with K2.1 responses