Solving the problem of consciousness remains one of the biggest challenges in modern science. One key step towards understanding consciousness is to empirically narrow down neural processes associated with the subjective experience of a particular content. To unravel these neural correlates of consciousness (NCC) a common scientific strategy is to compare perceptual conditions in which consciousness of a particular content is present with those in which it is absent, and to determine differences in measures of brain activity (the so called "contrastive analysis"). However, this comparison appears not to reveal exclusively the NCC, as the NCC proper can be confounded with prerequisites for and consequences of conscious processing of the particular content. This implies that previous results cannot be unequivocally interpreted as reflecting the neural correlates of conscious experience. Here we review evidence supporting this conjecture and suggest experimental strategies to untangle the NCC from the prerequisites and consequences of conscious experience in order to further develop the otherwise valid and valuable contrastive methodology.

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One fundamental feature of consciousness is that the contents of consciousness depend on the state of consciousness. Here, we propose an answer to why this is so: both the state and the contents of consciousness depend on the activity of cortical layer 5 pyramidal (L5p) neurons. These neurons affect both cortical and thalamic processing, hence coupling the cortico-cortical and thalamo-cortical loops with each other. Functionally this coupling corresponds to the coupling between the state and the contents of consciousness. Together the cortico-cortical and thalamo-cortical loops form a thalamo-cortical broadcasting system, where the L5p cells are the central elements. This perspective makes one quite specific prediction: cortical processing that does not include L5p neurons will be unconscious. More generally, the present perspective suggests that L5p neurons have a central role in the mechanisms underlying consciousness.

on the notion of interaction of the specific and nonspecific afferent visual systems is presented. Discussionis focused on nonmonotonicmasking functions. In particular, it is proposed that in order for visual information (patterns, forms, etc.) to be consciously perceived,both specificretina-genicula-striate impulses and nonspecificretina-reticulacortical impulses should converge in the same cortical space. Nonspecific activity is shown to be necessary for subjective awareness. This activity is shown to be of longer latency than specificactivity. It is concluded that trailing conscious-experience-generating impulses are elicited by collateral activity from the specific information received from the first stimulus. These impulses reach the cortex at the same moment as the specific activity of the second (masking)stimulus as coded,whichhas a relatively higher signal-to-noise ratio in the given retinotopically specJfied cortical space. Consequently, subjects consciously perceive the second stimulus. This operation of awareness generation is termed perceptual retouch and is considered as a special psychologicalmechanism worthy of psychophysical study.Classical works in the physiology of arousal and nonspecific sensory systems have proved convincingly that the neurophysiological substrate necessary for energizing the brain and providing sufficient activity for the manifestation of conscious experience (perceptual awareness) is located subcortically and consists of the brainstem reticular formation and nonspecific thalamic activating system (Dixon, 1971; Jasper, Proctor, Knighton, Noshay, & Costello, 1958;Magoun, 1958;Riklan& Levita, 1969;Smirnov, Muchnik, & Shandurina, 1978; Worden, Swazey, & Adelman, 1975).The importance of energetic, rather than purely structural or algoristic, processes in visual masking and information processing is rarely stressed. It has been shown, however, that percepts evolve and accumulate over time (Eriksen & Schultz, 1978). This perceptogenetic or microgenetic process takes a relatively long time (see Bachmann, 1977Bachmann, , 1980Flavell & Draguns, 1957;Kahneman & Norman, 1964;Kragh & Smith, 1970; Lange, 1893;Nikitin, 1905;Vekker, 1974). Furthermore, the time for specific impulses to reach the highest levels of the nervous system is much shorter than the whole microgenetic process. Thus, we have reason to believe that, first, much of this evolution of subjective experience ofThe author wishes to convey his sincerest thanks to Professor Charles W. Eriksen for his most generous help in making this article more idiomatic and succinct. The author's mailing address is: Department of Psychology, Tartu State University, 78 Tiigi Street, Tartu, Estonian SSR, 202400 U.S.S.R. the presented actual input consists of heterarchic cyclic activity rather than purely of afference, and, second, the temporally trailing nonspecific sensory activities playa crucial role in this process.The present article will argue that, in visual masking, the interaction of specific sensory systems with nonspecific "ene...

Research on neural correlates of consciousness has been conducted and carried out mostly from within two relatively autonomous paradigmatic traditions – studying the specific contents of conscious experience and their brain-process correlates and studying the level of consciousness. In the present paper we offer a theoretical integration suggesting that an emphasis has to be put on understanding the mechanisms of consciousness (and not a mere correlates) and in doing this, the two paradigmatic traditions must be combined. We argue that consciousness emerges as a result of interaction of brain mechanisms specialized for representing the specific contents of perception/cognition – the data – and mechanisms specialized for regulating the level of activity of whatever data the content-carrying specific mechanisms happen to represent. Each of these mechanisms are necessary because without the contents there is no conscious experience and without the required level of activity the processed contents remain unconscious. Together the two mechanisms, when activated up to a necessary degree each, provide conditions sufficient for conscious experience to emerge. This proposal is related to pertinent experimental evidence.

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Paris of geometric forms of equal area were presented, one form after another, with interstimulus intervals t ranging from 0 to 250 ms. The subject's task in experiment 1 was to recognise both stimuli. Identification of the form presented first (backward masking) across all values of t was of a nonmonotonic nature, with greatest impairment at values of t from 30 to 60 ms. Identification of the second form presented (forward masking) increased monotonically with increasing t. Different forms were recognised with different effectiveness across all values of t where masking took place. The results are interpreted on the basis of a multiple-stage processing model which assumes interaction between stimuli at different levels at different values of t. In experiment 2 the subject's task was to detect the presence or absence of a predesignated form. This condition yielded monotonic functions for both backward and forward masking. An explanation in terms of attention-dependent masking is given.

In the McGurk effect (McGurk and MacDonald, 1976 Nature 264 746-748), illusory auditory perception is produced if the visual information from lip movements is discrepant from the auditory information from the voice. A study is reported of the tolerance of the effect to varying levels of spatial degradation (videotaped images of a speaker's face were quantised by a mosaic transform). The illusory effect systematically decreased with an increase in the coarseness of the spatial quantisation. However, even with the coarsest level (11.2 pixels/face) the illusion did not completely disappear. In addition, those participants who did not experience the illusion nevertheless showed the effects of auditory-visual interaction in their clarity ratings of the auditory stimulus. It is concluded that auditory-visual interaction in visible speech perception is based on relatively coarse-spatial-scale information.