Perceptual dominance time distributions in multistable visual perception

Cerebral processing of spontaneous reversals of the rotating Necker cube

The cerebral processing of spontaneous perceptive reversals of the rotating Necker cube was studied in humans by combined functional MRI and electroencephalography. These reversals prefer certain positions of the Necker cube and can be studied without external reference of the perception. Functional MRI revealed six bilaterally active regions in the visual, parietal, and premotor cortex. A new method determined phase-locked electroencephalography-activations in the regions of interest and showed a significant stimulus-locked activity that started in the left Brodmann area 18. This activity started 38 ms after passing the symmetric position of the Necker cube and spread along the dorsal stream. We suggest that a further portion of the event-related potential signal reflects additional top-down processing, dependent on the position of the Necker cube.

A Model for Perceptual Averaging and Stochastic Bistable Behavior and the Role of Voluntary Control

We combine population coding, winner-take-all competition, and differentiated inhibitory feedback to model the process by which information from different, continuously variable signals is integrated for perceptual awareness. We focus on “slant rivalry,” where binocular disparity is in conflict with monocular perspective in specifying surface slant. Using a robust single parameter set, our model successfully replicates three key experimental results: (1) transition from signal averaging to bistability with increasing signal conflict, (2) change in perceptual reversal rates as a function of signal conflict, and (3) a shift in the distribution of percept durations through voluntary control exertion. Voluntary control is implemented through the use of a single top-down bias input. The transition from signal averaging to bistability arises as a natural consequence of combining population coding and wide receptive fields, common to higher cortical areas. The model architecture does not contain any assumption that would limit it to this particular example of stimulus rivalry. An emergent physiological interpretation is that differentiated inhibitory feedback may play an important role for increasing percept stability without reducing sensitivity to large stimulus changes, which for bistable conditions leads to increased alternation rate as a function of signal conflict.

Temporal Dynamics of Auditory and Visual Bistability Reveal Common Principles of Perceptual Organization

When dealing with natural scenes, sensory systems have to process an often messy and ambiguous flow of information. A stable perceptual organization nevertheless has to be achieved in order to guide behavior. The neural mechanisms involved can be highlighted by intrinsically ambiguous situations. In such cases, bistable perception occurs: distinct interpretations of the unchanging stimulus alternate spontaneously in the mind of the observer. Bistable stimuli have been used extensively for more than two centuries to study visual perception. Here we demonstrate that bistable perception also occurs in the auditory modality. We compared the temporal dynamics of percept alternations observed during auditory streaming with those observed for visual plaids and the susceptibilities of both modalities to volitional control. Strong similarities indicate that auditory and visual alternations share common principles of perceptual bistability. The absence of correlation across modalities for subject-specific biases, however, suggests that these common principles are implemented at least partly independently across sensory modalities. We propose that visual and auditory perceptual organization could rely on distributed but functionally similar neural competition mechanisms aimed at resolving sensory ambiguities.

Transient synchrony of distant brain areas and perceptual switching in ambiguous figures

We studied the relationship between perceptual switching in the Necker cube and long-distance transient phase synchronization in EEG. Transient periods of response related synchrony between parietal and frontal areas were observed. They start 800-600, ms prior to the switch response and occur in pairs. Four types of pairs could be distinguished, two of which are accompanied by transient alpha band activity in the occipital area. The results indicate that perceptual switching processes involve parietal and frontal areas; these are the ones that are normally associated with various cognitive processes. Sensory information in the visual areas is involved in some, but not in all, of switching processes. The intrinsic variability, as well as the participating areas, points to the role of strategic cognitive processes in perceptual switching.

A visual-haptic Necker cube reveals temporal constraints on intersensory merging during perceptual exploration

When viewing a three-dimensional Necker cube with one eye, participants can experience illusory reversals even while they feel the cube with their hands. This surprising property of the visual-haptic Necker cube affords a unique opportunity to investigate temporal constraints on interactions between vision and touch during extended observation of a three-dimensional object. Our observers reported reversals while they viewed the cube and, at the same time, they either held it with two-finger grips, felt it with while their hands remained stationary, or actively explored it by moving one hand. Consistent with a multisensory approach to three-dimensional form perception, touch had a clear effect on both the number and the duration of illusory percepts. Additionally, when observers alternated between stationary and moving periods during exploration, transitions from stationary to moving-hand haptics played a crucial role in inhibiting illusory reversals. A temporal analysis of the probability of first reversals occurring after different types of motor transition revealed a "vetoing window" initiating approximately 2s after the transition and lasting at least another 1-2s. Implications for multisensory processes during exploration are discussed.

Inertia and memory in ambiguous visual perception

Perceptual multistability during ambiguous visual perception is an important clue to neural dynamics. We examined perceptual switching during ambiguous depth perception using a Necker cube stimulus, and also during binocular rivalry. Analysis of perceptual switching time series using variance-sample size analysis, spectral analysis and time series shuffling shows that switching times behave as a 1/f noise and possess very long range correlations. The long memory feature contrasts sharply with the traditional satiation models of multistability, where the memory is not incorporated, as well as with recently published models of multistability and neural processing, where memory is excluded. On the other hand, the long memory feature favors the concept of "dynamic core" or coalition of neurons, where neurons form transient coalitions. Perceptual switching then corresponds to replacement of one coalition of neurons by another. The inertia and memory measures the stability of a coalition: a strong and stable coalition has to be won over by another similarly strong and stable coalition, resulting in long switching times. The complicated transient dynamics of competing coalitions of neurons may be addressable using a combination of functional imaging, measurement of frequency-tagged magnetoencephalography and frequency-tagged encephalography, simultaneous recordings of groups of neurons in many areas of the brain, and concepts from statistical mechanics and nonlinear dynamics theory.

Emotional task-dependent low-frequency fluctuations and methylphenidate: Wavelet scaling analysis of 1/f-type fluctuations in fMRI of the cerebellar vermis

Ion channel currents, neural firing patterns, and brain BOLD signals display 1/f-type fluctuations or fractal properties in time. By design, fMRI methods attempt to minimize the contribution of variance from low-frequency physiological 1/f-noise. New fMRI methods are described to visualize and measure 1/f-type BOLD fluctuations in volunteers recalling affectively neutral or emotional memories or meditating (i.e., attending to breathing) then retrospectively rating emotional content. A wavelet scaling exponent (alpha) was used to characterize signals from 0.015625 to 0.5Hz in cerebellar lobules VIII to X of the vermis (posterior inferior vermis; PIV), a region coordinating balance, eye tracking, locomotion, and vascular tone, and a possible site of pathology in attention deficit hyperactivity disorder (ADHD).

RESULTS

Changes in alpha and emotional measures were correlated in PIV voxels (r = 0.622, d.f .= 14, P < 0.0005), but not other regions examined. In contrast, conventional means and standard deviations of PIV voxels were unchanged. Methylphenidate, shown to decrease slow oscillations in rodent basal ganglia [Ruskin DN, Bergstrom DA, Shenker A, Freeman LE, Baek D, Walters JR. Drugs used in the treatment of attention-deficit/hyperactivity disorder affect postsynaptic firing rate and oscillation without preferential dopamine autoreceptor action. Biol Psychiatry 2001;49:340-50.], abolished task-dependent alpha changes in the PIV of an adult with ADHD. Wavelet analysis of long BOLD time series appears well suited to fractal physiology and studies of pharmacologically modulated cerebellar-thalamic-cortical function in ADHD or other psychiatric disorders.

Individual differences in perceptual switching rates; the role of occipital alpha and frontal theta band activity

Prolonged presentation of visually ambiguous figures leads to perceptual switching. Individual switching rates show great variability. The present study compares individuals with high versus low switching rates by investigating human scalp electroencephalogram and blink rates. Eight subjects viewed the Necker cube continuously and responded to perceptual switching by pressing a button. Frequent switchers showed characteristic occipital alpha and frontal theta band activity prior to a switch, whereas infrequent switchers did not. The alpha activity was specific to switching, the theta activity was generic to perceptual processing conditions. A negative correlation was observed between perceptual switching and blink rates. These results suggest that the ability to concentrate attentional effort on the task is responsible for the differences in perceptual switching rates.

Binocular rivalry is partly resolved at early processing stages with steady and with flickering presentation: a human event-related brain potential study

When their two eyes are presented with radically different images, observers report a fluctuation in perception between those images. This phenomenon, i.e. that only one image dominates the percept at a time, is termed binocular rivalry. Consequently, when rivalrous stimulation is changed into non-rivalrous stimulation, it depends on the image that has been currently dominant whether a change in perception occurs (incompatible change) or not (compatible change). That is, the perceptual experience differs although the physical input is the same, namely the stimulus of one eye has been changed. In the event-related brain potential (ERP) to compatible and incompatible changes we found differences as early as in the P1-N1-range, suggesting that in humans the binocular rivalry phenomenon is already (partly) resolved in extrastriate visual areas latest. Moreover, similar results were obtained with steady and flickering stimulus presentation showing that the processes involved in dealing with competing visual input do not depend critically on stimulus presentation mode.