Necker Cube Research Articles

Impaired Reproduction of Three-Dimensional Objects by Cocaine-Dependent Subjects

This study employed a perceptual-motor task of figure copying in 27 cocaine-dependent, 26 marijuana-abusing or dependent, and 33 healthy subjects. Cocaine-dependent and healthy individuals did not differ in their scores on the copying of a two-dimensional diamond and a cross. In contrast, cocaine-dependent subjects displayed significantly poorer ability to copy a three-dimensional Necker cube, a smoking pipe, a hidden line elimination cube, a pyramid, and a dissected pyramid. Marijuana users' performance on all copied figures was comparable to that of the healthy comparison subjects. Considering that decreased three-dimensional copying ability has been found to be associated with fatal injuries, further studies are needed to investigate possible underlying mechanisms (e.g., parietal lobe damage) and their role in the pathophysiology of cocaine dependence.

Alternating perceptions of ambiguous scenes: what's out there?

When visualizing an ambiguous scene (such as the Necker cube) one may perceive ongoing random alternations between the possible interpretations. Dynamical models implement competition as reciprocal inhibition between neuronal populations; dominance alternates while slow negative feedback, adaptation, sets the basic time scale (seconds) for switching. When adaptation is strong enough it overcomes dominance and alternations occur intrinsically and periodically; noise perturbs the regularity. In a different framework, with attractor-based dynamics, adaptation is weak and switches are induced by noise operating on a bistable system. We find that statistics of the observed alternations provide constraints that favor an operating range near the transition zone between the parameter regimes for the two mechanisms. In some paradigms one can manipulate stimulus cues to bias the competition away from equal dominance. We have proposed that the percentage of time dominant is a measure for the likelihood of valid interpretation of the scene. from Seventeenth Annual Computational Neuroscience Meeting: CNS*2008 Portland, OR, USA. 19–24 July 2008

Perceptual multistability predicted by search model for Bayesian decisions

Perceptual multistability refers to the phenomenon of spontaneous perceptual switching between two or more likely interpretations of an image. Although frequently explained by processes of adaptation or hysteresis, we show that perceptual switching can arise as a natural byproduct of perceptual decision making based on probabilistic (Bayesian) inference, which interprets images by combining probabilistic models of image formation with knowledge of scene regularities. Empirically, we investigated the effect of introducing scene regularities on Necker cube bistability by flanking the Necker cube with fields of unambiguous cubes that are oriented to coincide with one of the Necker cube percepts. We show that background cubes increase the time spent in percepts most similar to the background. To characterize changes in the temporal dynamics of the perceptual alternations beyond percept durations, we introduce Markov Renewal Processes (MRPs). MRPs provide a general mathematical framework for describing probabilistic switching behavior in finite state processes. Additionally, we introduce a simple theoretical model consistent with Bayesian models of vision that involves searching for good interpretations of an image by sampling a posterior distribution coupled with a decay process that favors recent to old interpretations. The model has the same quantitative characteristics as our human data and variation in model parameters can capture between-subject variation. Because the model produces the same kind of stochastic process found in human perceptual behavior, we conclude that multistability may represent an unavoidable by-product of normal perceptual (Bayesian) decision making with ambiguous images.

Right Parietal Brain Activity Precedes Perceptual Alternation of Bistable Stimuli

Momentary fluctuations of baseline activity have been shown to influence responses to sensory stimulation both behaviorally and neurophysiologically. This suggests that perceptual awareness does not solely arise from physical stimulus properties. Here we studied whether the momentary state of the brain immediately before stimulus presentation indicates how a physically unique but perceptually ambiguous stimulus will be perceived. A complex Necker cube was intermittently presented and subjects indicated whether their perception changed with respect to the preceding presentation. EEG was recorded from 256 channels. The prestimulus brain-state was defined as the spatial configuration of the scalp potential map within the 50 ms before stimulus arrival, representing the sum of all momentary ongoing brain processes. Two maps were found that doubly dissociated perceptual reversals from perceptual stability. For EEG sweeps classified as either map, distributed inverse solutions were computed and statistically compared. This yielded activity confined to a region in right inferior parietal cortex that was significantly more active before a perceptual reversal. In contrast, no significant topographic differences of the evoked potentials elicited by stable vs. reversed Necker cubes were found. This indicates that prestimulus activity in right inferior parietal cortex is associated with the perceptual change.

Angle alignment evokes perceived depth and illusory surfaces.

There is a distinct visual process that triggers the perception of illusory surfaces and contours along the intersections of aligned, zigzag line patterns. Such illusory contours and surfaces are qualitatively different from illusory contours of the Kanizsa type. The illusory contours and surfaces in this case are not the product of occlusion and do not imply occlusion of one surface by another. Rather, the aligned angles in the patterns are combined by the visual system into the perception of a fold or a 3-D corner, as of stairs on a staircase or a wall ending on a floor. The depth impression is ambiguous and reversible like the Necker cube. Such patterns were used by American Indian artists of the Akimel O'odham (Pima) tribe in basketry, and also by modern European and American artists like Josef Albers, Bridget Riley, Victor Vasarely, and Frank Stella. Our research aims to find out what manipulations of the visual image affect perceived depth in such patterns in order to learn about the perceptual mechanisms. Using paired comparisons, we find that human observers perceive depth in such patterns if, and only if, lines in adjacent regions of the patterns join to form angles, and also if, and only if, the angles are aligned precisely to be consistent with a fold or 3-D corner. The amount of perceived depth is graded, depending on the steepness and the density of angles in the aligned-angle pattern. The required precision of the alignment implies that early retinotopic visual cortical areas may be involved in this perceptual behavior, but the linkage of form with perceived depth suggests involvement of higher cortical areas as well.

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.

Optimal spatio-temporal pooling of neural responses in area MT

Bistable visual stimuli such as Rubin's vase/face or the Necker cube refer to the phenomena of spontaneously alternating percepts while viewing the same visual image. The uncoupling between the stimulus and percept offers a means for understanding neural basis of visual perception. Here we asked whether pooling the responses of a large population of MT neurons over time and space could improve the predictability of perceptual decisions during ambiguous visual stimulation. Two well trained rhesus monkeys indicated the perceived direction of rotation of bistable structure-from-motion (SFM) stimuli by pushing one of two levers. During this task, multi-channel intracortical recordings including single-unit activity (SUA), multi-unit activity (MUA), and local field potentials (LFP) were collected from area MT. We sorted the neural data according to the monkeys' behavioral choices and employed statistical algorithms to classify brain states (i.e., the subjective interpretation of a bistable stimulus). Classification was performed with linear discriminant analysis with leave-one-out cross-validation. We found that SUA, MUA and LFP all had a rather modest capability of predicting the monkeys' perceptual report when considered in isolation. We developed dynamic models for spatio-temporal integration of distributed neural signals. We found that the discriminative information of neuronal population activity accumulates over time and the combination of simultaneously collected data greatly improved the prediction accuracy of each of the signals. The accuracy and statistical power of determining the monkeys' perception increased with the number of channels as well as with the types of neural signals used for analysis. Our results demonstrate that simultaneous collection of multiple neural responses in area MT can improve the determination of perceptual states.

How to Keep a Reversible Figure from Reversing: Teasing Out Top — Down and Bottom — Up Processes

The nature of processes underlying our perception of reversible figures was examined through two experiments investigating the effects of prior exposure conditions on an observer's report of figural reversal. In experiment 1, observers were adapted over several minutes to an unambiguous version of a rotating Necker cube prior to the presentation of the standard ambiguous figure. Results indicated that adaptation produced an immediate bias to perceive the ambiguous figure in the opposite configuration (ie reverse bias) and to reduce reports of reversal over the test period. The introduction of a brief delay between the adaptation and test periods revealed that this bias is a highly transient effect and is only clearly evident when the adaptation and test figures are matched in size. In experiment 2, observers were primed with an unambiguous figure for a few seconds prior to the presentation of the standard ambiguous figure. In this case, the obtained bias strongly favored the observer's reporting the ambiguous figure to be in the same configuration as the adapting figure (ie positive bias); and neither introducing a delay period nor changing figure size had any effect. We conclude that these experiments reveal the distinct roles of transient, retinally localized neural processes as well as more stable, global processes under specifiable conditions.

Competition between newly recruited and pre-existing visual cues during the construction of visual appearance

During perception, conflicting visual cues often trade against each other. Recent cue recruitment experiments show that the visual system can be conditioned to use artificial visual cues during the perception of a bistable stimulus. Does the visual system treat the new cue as an independent source of information, separate from the long-trusted cues that were used to train it? If so, presence of the long-trusted cue should not be sufficient to block the new cue’s effect. Here, we show that a newly recruited cue (stimulus location) and a long-trusted, pre-existing cue (binocular disparity) trade against each other: they contribute simultaneously to the direction of perceived 3D rotation of a Necker cube. We also show that the new position cue was based primarily on retinal position, so early visual areas may mediate the cue’s effect.

Electrophysiological correlates of perceptual reversals for three different types of multistable images

Electrophysiological recordings were made in 21 observers to investigate whether differences in signature components (P1, N1, selection negativity [SN]) would be revealed during perceptual reversals of three different multistable figures. Using a lattice of Necker cubes as a stimulus, J. Kornmeier and M. Bach (2004, 2005) reported differences in P1 amplitudes as well a broad reversal-related negativity occurring 200-400 ms poststimulus. The current study investigated whether these event-related potentials of Necker cube reversals represent general "perceptual switching" mechanisms and would, therefore, be common to other types of multistable figures. Three different types of multistable stimuli were utilized: a modified Rubin's face/vase, a modified Schröder's staircase, and a novel natural stimulus, Lemmo's cheetahs. Results revealed the broad reversal-related negativity for the face/vase and the reversible staircase but not for the cheetahs. This component is comparable to the SN in polarity, latency, and scalp topography. An effect of early visual spatial attention on figure reversals was suggested by an analysis of the occipital P1 and N1 components. The P1, N1, or both were enhanced for trials in which the observer reported perceptual reversals compared with trials in which no reversals were reported for the face/vase and reversible staircase stimuli. These results support a model of multistable perception in which changes in early spatial attention (indicated by P1 and N1 enhancement) modulate perceptual reversals (indicated by the reversal negativity or SN).

Discrete Stochastic Process of the Brain Estimated by Psychophysical Measurement of Ambiguous Figure Perception

Understanding perceptual consciousness is one of the most important problems of the contemporary neuroscience. Because sensory information is inevitably ambiguous, it is to be clarified how the brain selects and constructs a certain interpretation of the sensory information. Such typical phenomena are figure perception such as the Necker cube perception and binocular rivalry, both of which provide different conscious percepts during observation of the same visual stimulus and spontaneous alternations between the possible percepts. The present article explains the recent research progress on the brain regions associated with binocular rivalry and common dynamical properties to these kinds of ambiguous perception.

Time Frequency Characterization of Evoked Brain Activity In Multiple Electrode Recordings

This paper explores global time frequency approaches to EEG data analysis with the Wigner Distribution Function and the Symmetric Ambiguity Function. The task chosen was to characterize the activity profile of EEG signals in sample Frontal, Central and Occipital electrodes from human subjects, coincident with the perception of a reversal in the orientation of a bistable Necker cube figure. The result of this analysis has implications for blind signal processing as the goal was to identify an unknown input source eliciting the observed EEG signals. The methods demonstrate an internally initiated EEG signal source not tied to a regularly anticipated external source. The results demonstrate the general applicability of the methods for a wide variety of neural and biological signals and systems. The findings can be summarized as the observation of high energy activity patterns in terms of significant dissimilarities in the waveform, both in time and frequency, in the Frontal and Occipital electrodes, approximately 200-600 ms prior to the appearance of the premotor potentials in the medial electrodes.

PHENOMENOLOGY IN COMPUTATIONAL MODELS OF CONSCIOUSNESS

In this paper we argue that phenomenology needs to be supported by explicit mechanisms if one is to have computational models of consciousness. Computational work in this area is reviewed and a set of axioms that help to decompose being conscious into manageable concepts is evoked. This leads to a kernel architecture and a digital implementation which is shown to work in examples of visual illusions that are revealing of how the brain supports phenomenology. This model is used to address the unstable phenomenology related to observation of the ambiguous Necker cube.

We see the world the way we do because of how our brain activity moves

Cognitive neuroscience has been observing brain activity with increasing spatio-temporal resolution. This has enabled a shift in focus from localization to self-organization. As a result, there is increasing demand for a unifying perspective on the question: how do self-organized patterns of brain activity relate to information processing and conscious experience? To start with the latter, the philosopher Daniel Dennett referred to the standard model of conscious experience as the ‘‘Cartesian Theater’’. This is the location consciousness occupies, a place in the brain where all the information comes together. He rejected such a model in favor of ‘‘Multiple Drafts’’: there are multiple streams of information, partially conflicting and continuously changing. The simultaneity and unity of our experience thereby is no more than an illusion, albeit one that gives the notion of Cartesian Theater a seductive plausibility. A Cartesian Theater is not needed to enable a single representational medium for subjective simultaneity. Dynamic patterns of synchrony in brain activity may constitute the basis for the unity of experience. These intervals have a certain duration; the phenomenal counterpart of which is the psychological present (Stroud 1967). Similar intervals are manifested in visual perception. Let us consider the case of multi-stable figures. While watching Fig. 1 you will repeatedly experience spontaneous changes in the perceived groupings between its components. Certain groupings appear, and stay for a while, but none of them persists forever. A specific case is perceptual switching, the reversal of spatial orientation as observed with ambiguous figures, such as the Necker cube. We experience perceptual switching as instantaneous. The processes in the visual system that produce it, however, typically require approximately 0.5 s to complete (Ito et al. 2003; Nakatani and van Leeuwen

Perceptual exploration and intersensory merging in a visual–haptic Necker cube

Perceiving the three-dimensional structure of an object often involves merging vision and haptics over extended periods of exploration. An interesting feature of this process is that as exploration progresses, new information may require changing how the two sensory signals are treated. We tested whether (1) bimodal interactions during extended observation can adjust to changes in the quality of unimodal sensory signals; and (2) there are specific temporal constraints on these adjustments. When viewing a three-dimensional Necker cube with one eye, participants experience illusory reversals even while they feel the cube with their hands. This surprising property makes the visual–haptic Necker cube an excellent model to investigate bimodal processes when previously consistent bimodal signals change in quality over time or begin to conflict. Our participants 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 2 s after the transition and lasting at least another 1–2 s. These results suggest that intersensory merging is sensitive to changes in the quality of unimodal signals during exploration, and that adjustments caused by these changes are characterized by specific temporal dynamics.

Attentional control over either of the two competing percepts of ambiguous stimuli revealed by a two-parameter analysis: Means do not make the difference

We studied distributions of perceptual rivalry reversals, as defined by the two fitted parameters of the Gamma distribution. We did so for a variety of bi-stable stimuli and voluntary control exertion tasks. Subjects’ distributions differed from one another for a particular stimulus and control task in a systematic way that reflects a constraint on the describing parameters. We found a variety of two-parameter effects, the most important one being that distributions of subjects differ from one another in the same systematic way across different stimuli and control tasks (i.e., a fast switcher remains fast across all conditions in a parameter-specified way). The cardinal component of subject-dependent variation was not the conventionally used mean reversal rate, but a component that was oriented—for all stimuli and tasks—roughly perpendicular to the mean rate. For the Necker cube, we performed additional experiments employing specific variations in control exertion, suggesting that subjects have to a considerable extent independent control over the reversal rate of either of the two competing percepts.

Voluntary control of Necker cube reversals modulates the EEG delta- and gamma-band response

Reversible figures such as the Necker cube make up a well-known class of visual phenomena in which an invariant stimulus pattern gives rise to at least two different perceptual interpretations. Former EEG studies of our research group demonstrated a reversal-related delta response and a frontal enhancement of gamma activity during multistable perception which has been interpreted as signaling attentional top-down processes. The aim of the present study was to investigate the functional involvement of delta and gamma activity in top-down processes more explicitly by asking 21 healthy participants to bring the reversal rate under voluntary control during viewing of the Necker cube. Slowing down the reversal rate should be accomplished by focussing attention to the currently perceived alternative while speeding up should be accomplished by shifting attention as rapidly as possible from one perspective to the other. EEG was recorded from frontal, central, parietal, and occipital locations of both hemispheres. The data was analysed on the single-sweep level in the delta and gamma frequency range. The results showed that both delta response and gamma power were larger during slowing down than speeding up the reversal rate. These findings may indicate that more attentional resources have to be allocated by the cognitive system in order to prevent a reversal by means of focussed attention than to initiate a reversal by attentional shifts.

An Analysis of Inconsistent and Incomplete Necker Cubes

This paper aims to distinguish and classify sixteen versions of the Necker cube. In particular, it is shown how to describe inconsistent and incomplete theories which correspond in a systematic way to these sixteen diagrams. Concerning two of these sixteen cubes, there is a natural intuition that there is a sense in which they inconsistent. It is seen that this intuition is vindicated by an analysis in which their corresponding theories turn out to be globally inconsistent but not locally inconsistent, while various other cubes of the sixteen are merely locally inconsistent. The Routley functor is seen to be useful in classifying the relations between these diagrams.

Intermittent stimuli increase alternation of ambiguous figures

Using the repetitive appearance-disappearance stimuli of the Necker cube, we studied how perceptual alternations between two interpretations differed depending on whether the stimulus was shown intermittently or continuously. We found alternations were faster when stimuli were presented intermittently than when presented continuously…

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.