Among psychologists and vision scientists,binocular rivalry has enjoyed sustainedinterest for decades dating back to the 19thcentury. In recent years, however, rivalry''saudience has expanded to includeneuroscientists who envision rivalry as a tool for exploring the neural concomitants ofconscious visual awareness and perceptualorganization. For rivalry''s potential to berealized, workers using this tool need toknow details of this fascinating phenomenon,and providing those details is the purpose ofthis article. After placing rivalry in ahistorical context, I summarize major findingsconcerning the spatial characteristics and thetemporal dynamics (...) of rivalry, discuss two majortheoretical accounts of rivalry ( eye vs stimulus rivalry) and speculate on possibleneural concomitants of binocular rivalry. (shrink)

The McCollough effect, an orientation-contingent color aftereffect, has been known for over 30 years and, like other aftereffects, has been taken as a means of probing the brain's operations psychophysically. In this paper, we review psychophysical, neuropsychological, and neuroimaging studies of the McCollough effect. Much of the evidence suggests that the McCollough effect depends on neural mechanisms that are located early in the cortical visual pathways, probably in V1. We also review evidence showing that the aftereffect can be induced without (...) conscious perception of the induction patterns. Based on these two lines of evidence, it is argued that our conscious visual experience of the world arises in the cortical visual system beyond V1. (shrink)

In addressing thescientific study of consciousness, Crick and Koch state, It is probable that at any moment some active neuronal processes in your head correlate with consciousness, while others do not: what is the difference between them? (1998, p. 97). Evidence from electrophysiological and brain-imaging studies of binocular rivalry supports the premise of this statement and answers to some extent, the question posed. I discuss these recent developments and outline the rationale and experimental evidence for the interhemispheric switch hypothesis of (...) perceptual rivalry. According to this model, the perceptual alternations of rivalry reflect hemispheric alternations, suggesting that visual consciousness of rivalling stimuli may be unihemispheric at any one time (Miller et al., 2000). However, in this paper, I suggest that interhemispheric switching could involve alternating unihemispheric attentional selection of neuronal processes for access to visual consciousness. On this view, visual consciousness during rivalry could be bi hemispheric because the processes constitutive of attentional selection may be distinct from those constitutive of visual consciousness. This is a special case of the important distinction between the neuronal correlates and constitution of visual consciousness. (shrink)

Many current neurophysiological, psychophysical, and psychological approaches to vision rest on the idea that when we see, the brain produces an internal representation of the world. The activation of this internal representation is assumed to give rise to the experience of seeing. The problem with this kind of approach is that it leaves unexplained how the existence of such a detailed internal representation might produce visual consciousness. An alternative proposal is made here. We propose that seeing is a way of (...) acting. It is a particular way of exploring the environment. Activity in internal representations does not generate the experience of seeing. The out- side world serves as its own, external, representation. The experience of seeing occurs when the organism masters what we call the gov- erning laws of sensorimotor contingency. The advantage of this approach is that it provides a natural and principled way of accounting for visual consciousness, and for the differences in the perceived quality of sensory experience in the different sensory modalities. Sev- eral lines of empirical evidence are brought forward in support of the theory, in particular: evidence from experiments in sensorimotor adaptation, visual “filling in,” visual stability despite eye movements, change blindness, sensory substitution, and color perception. (shrink)

he mystery of consciousness is among the most important questions pondered upon, not only in philosophy but also in the cognitive science, psychology, neurobiology and other sciences. The problem of consciousness has been traditionally dealt by philosophy, but its importance in explaining mental phenomena has made it a subject matter for other sciences that emerged later. Each philosopher and scientist has followed his own method in defining it, and arriving at a universal agreement on its definition has become difficult. In (...) philosophy, ‘consciousness’ does not have a definition in terms of genus and differentia, or the necessary and sufficient conditions. On the one hand, idealists put all their efforts into explaining consciousness as a separate entity, without stepping away from their traditional path, because they believe that thinking, experience, and free will are not the properties of the physical entity. On the other hand, the scientific world puts its efforts into reducing consciousness to electrical and neurochemical processes of the brain because they believes that the brain works like a biologically programmed computer. Our approach to consciousness varies between these two explanations. We would like to address the problem of consciousness at two levels: cosmic and individual. The basic thesis of this paper is: everything that we know is something that we experience and everything that we experience is a form arising within consciousness. (shrink)

Our visual system not only mediates information about the visual environment but is capable of generating pictures of nonexistent worlds: afterimages, illusions, phosphenes, etc. We are ''aware'' of these pictures just as we are aware of the images of natural, physical objects. This raises the question: is the neural correlate of consciousness (NCC) of such images the same as that of images of physical objects? Images of natural objects have some properties in common with afterimages (e.g., stability of verticality) but (...) there are also obvious differences (e.g., images maintain size constancy, whereas afterimages follow Emmert's Law: when seen while screens at different distances are observed, an afterimage looks larger, the greater the distance of the screen). The differences can be explained by differences in the retinal extent of images and afterimages, which favors the view that both have the same NCC. It seems reasonable to assume that before neural activity can produce awareness, all the computations necessary for a veridical representation of, e.g., an object, must be completed within the neural substrate and that information characteristic of a particular object must be available within the NCC. Given these assumptions, it can be shown that no retinotopic (in a strict sense) cortical areas can serve as the NCC, although some type of topographic representation is necessary. It seems also to be unlikely that neurons classified as cardinal cells alone can serve as NCC. (shrink)

The historical debate on representation in cognitive science and neuroscience construes representations as theoretical posits and discusses the degree to which we have reason to posit them. We reject the premise of that debate. We argue that experimental neuroscientists routinely observe and manipulate neural representations in their laboratory. Therefore, neural representations are as real as neurons, action potentials, or any other well-established entities in our ontology.

The interocular suppression technique termed continuous flash suppression (CFS) has become an immensely popular tool for investigating visual processing outside of awareness. The emerging picture from studies using CFS is that extensive processing of a visual stimulus, including its semantic and affective content, occurs despite suppression from awareness of that stimulus by CFS. However, the current implementation of CFS in many studies examining processing outside of awareness has several drawbacks that may be improved upon for future studies using CFS. In (...) this paper, we address some of those shortcomings, particularly ones that affect the assessment of unawareness during CFS, and ones to do with the use of ‘visible’ conditions that are often included as a comparison to a CFS condition. We also discuss potential biases in stimulus processing as a result of spatial attention and feature-selective suppression. We suggest practical guidelines that minimize the effects of those limitations in using CFS to study visual processing outside of awareness. (shrink)

Physiological studies of binocular rivalry have provided important clues to the relationship between neural activity in the brain and visual awareness.However, uncertainty about these insights has been raised by a recent study showing that the events underlying binocular rivalry occur earlier in the visual pathway than was previously thought.

Scientific concepts are defined by metaphors. These metaphors determine what atten- tion is and what count as adequate explanations of the phenomenon. The authors analyze these metaphors within 3 types of attention theories: (a) --cause-- theories, in which attention is presumed to modulate information processing (e.g., attention as a spotlight; attention as a limited resource); (b) --effect-- theories, in which attention is considered to be a by-product of information processing (e.g., the competition meta- phor); and (c) hybrid theories that combine (...) cause and effect aspects (e.g., biased- competition models). The present analysis reveals the crucial role of metaphors in cognitive psychology, neuroscience, and the efforts of scientists to find a resolution to the classic problem of cause versus effect interpretations. (shrink)

The mystery of consciousness is among the most important questions pondered upon, not only in philosophy but also in the cognitive science, psychology, neurobiology and other sciences. The problem of consciousness has been traditionally dealt by philosophy, but its importance in explaining mental phenomena has made it a subject matter for other sciences that emerged later. Each philosopher and scientist has followed his own method in defining it, and arriving at a universal agreement on its definition has become difficult. In (...) philosophy, ‘consciousness’ does not have a definition in terms of genus and differentia, or the necessary and sufficient conditions. On the one hand, idealists put all their efforts into explaining consciousness as a separate entity, without stepping away from their traditional path, because they believe that thinking, experience, and free will are not the properties of the physical entity. On the other hand, the scientific world puts its efforts into reducing consciousness to electrical and neurochemical processes of the brain because they believes that the brain works like a biologically programmed computer. Our approach to consciousness varies between these two explanations. We would like to address the problem of consciousness at two levels: cosmic and individual. The basic thesis of this paper is: everything that we know is something that we experience and everything that we experience is a form arising within consciousness. (shrink)

Logothetis, N.K.: Single units and conscious vision. Philosophical Transactions of the Royal Society of London Series B-Biological Sciences 353, 1801-1818 (1998) Abstract.

In the mid of nineteenth century, the hypothesis, “machine can think,” became very popular after Alan Turing’s article on “Computing Machinery and Intelligence.” This hypothesis, “machine can think,” established the foundations of machine intelligence and claimed that machines have a mind. It has the power to compete with human beings. In the first section, I shall explore the importance of Turing thesis, which has been conceptualized in the domain of machine intelligence. Turing presented a completely different view of the machine (...) itself. It addressed philosophers, rather than the mathematicians, and proclaimed that digital computers might be considered as intelligent machines. This approach was wholly new, both philosophically and technically, and started many philosophical debates which continue to this day. He proposed an imitation game, as a test, what is now referred to as a Turing test to evaluate if a machine thinks. That is to say that the main aim of machine intelligence not only constructs challenging programs to solve our day-to-day problems but also reproduce mentality in machines and claimed that all the mental qualities are ascribable to machines. What will be attempted in this paper is a critical evaluation of the arguments against the Turing test put forward by many philosophers. (shrink)

The processes whereby our brains continue to learn about a changing world in a stable fashion throughout life are proposed to lead to conscious experiences. These processes include the learning of top-down expectations, the matching of these expectations against bottom-up data, the focusing of attention upon the expected clusters of information, and the development of resonant states between bottom-up and top-down processes as they reach an attentive consensus between what is expected and what is there in the outside world. It (...) is suggested that all conscious states in the brain are resonant states and that these resonant states trigger learning of sensory and cognitive representations. The models which summarize these concepts are therefore called Adaptive Resonance Theory, or ART, models. Psychophysical and neurobiological data in support of ART are presented from early vision, visual object recognition, auditory streaming, variable-rate speech perception, somatosensory perception, and cognitive-emotional interactions, among others. It is noted that ART mechanisms seem to be operative at all levels of the visual system, and it is proposed how these mechanisms are realized by known laminar circuits of visual cortex. It is predicted that the same circuit realization of ART mechanisms will be found in the laminar circuits of all sensory and cognitive neocortex. Concepts and data are summarized concerning how some visual percepts may be visibly, or modally, perceived, whereas amodal percepts may be consciously recognized even though they are perceptually invisible. It is also suggested that sensory and cognitive processing in the What processing stream of the brain obey top-down matching and learning laws that are often complementary to those used for spatial and motor processing in the brain's Where processing stream. This enables our sensory and cognitive representations to maintain their stability as we learn more about the world, while allowing spatial and motor representations to forget learned maps and gains that are no longer appropriate as our bodies develop and grow from infanthood to adulthood. Procedural memories are proposed to be unconscious because the inhibitory matching process that supports these spatial and motor processes cannot lead to resonance. (shrink)

If successive, brief visual images are exposed for recognition or for psychophysical ratings, various effects and phenomena of fast dynamics of conscious perception such as mutual masking, metacontrast, proactive enhancement of contrast, proactive speed-up of the latency of subjective visual experience, the Fröhlich Effect, the Tandem Effect, attentional facilitation by visuospatial precuing, and some others have been found. The theory proposed to deal with these phenomena proceeds from the assumption that two types of brain processes are necessary in order to (...) consciously recognize visual stimuli: fast, specific processes of encoding that allocate and reactivate the stimulus representation which is based on the activity of selected cortical neurons and relatively slower processes of facilitation of the activity of this specific representation that are mediated by the excitatory modulation of the EPSPs of those selected cortical neurons by the ascending input from nonspecific thalamus. Theperceptual retouchconstruct is proposed in order to characterize and analyze the interaction of and . The neurophysiological characteristics of this bifunctional system of afference help to put forward several predictions that are found to be consistent with the empirical regularities of the above-described perceptual-attentional phenomena. These data form a body of converging evidence that is consistent with the predictions of the perceptual retouch approach. (shrink)

A midbrain neural basis for the perceptualoscillations of binocular rivalry is suggestedon the basis of fMRI studies of rivalry andinferences from the properties of rivalry thatcannot be explained from the known propertiesof primary visual cortical (V1) neurons. Therivalry switch is proposed to activatehomologous areas of each cerebral hemispherealternately, by means of a bistable oscillatorcircuit that straddles the midline of theventral tegmentum. This bistable oscillatoroperates at the same slow rate that ischaracteristic of perceptual rivalryalternations. Whilst attempting to divert thepresent preoccupation with (...) cortical mechanismsfor rivalry, the new proposal integrates manycortical areas, in keeping with recent evidencethat binocular rivalry involves widespreadareas of the hemispheres. By linking rivalry tointerhemispheric switching mechanisms in thisway, the new proposal for the switch makes theprediction that binocular rivalry will besubject to high level influences such as moodand motivation. These predictions are beingfulfilled, with rivalry playing an increasingrole in the diagnosis and understanding if mooddisorders, schizophrenia and other psychiatricconditions. (shrink)

What is art? What constitutes great art? Why do we value art so much and why has it been such a conspicuous feature of all human societies? These questions have been discussed at length though without satisfactory resolution. This is not surprising. Such discussions are usually held without reference to the brain, through which all art is conceived, executed and appreciated. Art has a biological basis. It is a human activity and, like all human activities, including morality, law and religion, (...) depends upon, and obeys, the laws of the brain. To understand the biological foundations of art, we must enquire into the biological foundations of knowledge, for art constitutes a form of knowledge; indeed is knowledge. We are still far from knowing the neural basis of the laws that dictate artistic creativity, achievement and appreciation, but spectacular advances in our knowledge of the visual brain allow us to make a beginning in trying to formulate neural laws of art and aesthetics; in short, to study neuroaesthetics. In this essay, I try to discuss the art of three Titanic figures in Western culture — Dante, Michelangelo and Wagner — in neurological terms. I try to show that we can trace the origins of their art to a fundamental characteristic of the brain, namely its capacity to form concepts. This capacity is itself the by-product of an essential characteristic of the brain. That characteristic is abstraction, and is imposed upon the brain by one of its chief functions, namely the acquisition of knowledge. (shrink)

Binocular rivalry provides a useful situation for studying the relation between the temporal flow of conscious experience and the temporal dynamics of neural activity. After proposing a phenomenological framework for understanding temporal aspects of consciousness, we review experimental research on multistable perception and binocular rivalry, singling out various methodological, theoretical, and empirical aspects of this research relevant to studying the flow of experience. We then review an experimental study from our group explicitly concerned with relating the temporal dynamics of rivalrous (...) experience to the temporal dynamics of cortical activity. Drawing attention to the importance of dealing with ongoing activity and its inherent changing nature at both phenomenological and neurodynamical levels, we argue that the notions of recurrence and variability are pertinent to understanding rivalry in particular and the flow of experience in general. (shrink)

The neural basis of binocular rivalry has beenthe subject of vigorous debate. Do discrepantmonocular patterns rival for awareness becauseof neural competition among patternrepresentations or monocular channels? In thisarticle, I briefly review psychophysical andneurophysiological evidence pertaining to boththeories and discuss important new neuroimagingdata which reveal that rivalry is fullyresolved in monocular visual cortex. These newfindings strongly suggest that interocularcompetition mediates binocular rivalry and thatV1 plays an important role in the selection ofconscious visual information. They furthersuggest that rivalry is not a unitaryphenomenon. (...) Interocular competition may fullyaccount for binocular rivalry whereas aseparate mechanism involving patterncompetition likely accounts for monocular andstimulus rivalry. (shrink)

Aftereffects induced by invisible stimuli constitute a powerful tool to investigate what type of neural information processing can occur in the absence of visual awareness. This approach has been successfully used to demonstrate that awareness of oriented gratings or translating stimuli is not necessary to obtain a robust orientation-specific or motion-specific aftereffect. We exploit motion-induced blindness to investigate the related question of the influence of visual awareness on the formation of negative afterimages. Our results show that MIB does not affect (...) the persistence and intensity of afterimages. Thus, there is no significant contribution to the formation of afterimages beyond the sites mediating MIB. (shrink)

The study of the neural correlates of awareness is nowadays an active research field in Neuroscience. This has been basically boosted by the study of neural correlates of conscious perception with single cell recordings in monkeys and voxel activities with human fMRI experiments. In this review, we discuss the main experiments with recording of single neurons and related evidence about the neural events underling visual perceptual awareness.

In this study, voluntary and involuntary visual attention focused on different interpretations of a bistable image, were investigated using magnetoencephalography. A Necker cube with sinusoidally modulated pixels' intensity in the front and rear faces with frequencies 6.67 Hz and 8.57 Hz, respectively, was presented to 12 healthy volunteers, who interpreted the cube as either left- or right-oriented. The tags of these frequencies and their second harmonics were identified in the average Fourier spectra of the MEG data recorded from the visual (...) cortex. In the first part of the experiment, the subjects were asked to voluntarily control their attention by interpreting the cube orientation as either being on the left or right. Accordingly, we observed the dominance of the corresponding spectral component, and voluntary attention performance was measured. In the second part of the experiment, the subjects were asked to focus their gaze on a red marker at the center of the cube image without putting forth effort in its interpretation. The alternation of the dominant spectral energies at the second harmonics of the stimulation frequencies was treated as changes in the cube orientation. Based on the results of the first experimental stage and using a wavelet analysis, we developed a method which allowed us to identify the currently perceived cube orientation. Finally, we characterized involuntary attention using the distribution of dominance times when focusing attention on one of the cube orientations, which was related to voluntary attention performance and brain noise. In particular, we confirmed our hypothesis that higher attention performance is associated with stronger brain noise. (shrink)