Teadvuse neuronaalsete korrelaatide uurimismetoodika edasiarendusi

Väitekirja elektrooniline versioon ei sisalda publikatsioone.Küsimust selle kohta, mis on teadvus, peetakse kaasaegse teaduse viimaseks tõeliselt suureks probleemiks, sest senini pole mitte keegi suutnud selgitada, miks ja kuidas aju töö on seotud teadvuselamustega. Tänapäeval on üldiselt aktsepteeritud, et teadvuse mõistmiseks on esimese sammuna tarvis täpsemalt määratleda ja tundma õppida teadvuse neuronaalseid korrelaate – neid ajuprotsesse, mis on minimaalselt tarvilikud mingi kindla teadvuselamuse jaoks. Antud väitekirjas on sellele eesmärgile kaasa aidatud nägemistajuga kaasnevate teadvuselamuste neuronaalsete korrelaatide ning nende markerite uurimise kaudu. Kuna on tõenäoline, et varasemate uurimuste tulemused ei peegelda mitte ainult teadvuse neurokorrelaate, vaid ka neile süstemaatiliselt eelnevate või järgnevate protsesside korrelaate, püüti käesolevas töös neid kaasnevaid protsesse täpsemalt uurida ning osades katsetes metodoloogilistel eesmärkidel hoopis vältida. Selleks kasutati varasemast hoolikamalt planeeritud katseparadigmasid, tulemuste töötlemise uusi võtteid ning mitmekesisemat loomulikku pildimaterjali. Lisaks võeti arvesse neurovõrgustike pidevalt muutuvat dünaamilist seisundit ja selle mõju teadvustamise protsessidele. Kokkuvõtvalt võib öelda, et väitekirjas kajastatud uurimustööde tulemuste järgi hakkavad visuaalsete teadvuselamuste vahetud neuromarkerid ilmnema umbes 200 ms jooksul pärast visuaalse stimulatsiooni jõudmist võrkkestale. Samas võib teadvuselamuste kujunemine aga olla hoopis järkjärguline protsess, sest teadvusega korreleerub usaldusväärselt ka umbes 100 ms hilisem neuromarker. Lisaks viitavad tulemused sellele, et visuaalse süsteemi keeruline dünaamika tuleb kõige paremini esile realistliku pildimaterjaliga, mitte lihtsate ja kunstlike geomeetriliste kujunditega. Inimajus on talletunud ohtralt eelteadmisi reaalse visuaalse maailma ning selle reeglipärasuste kohta. See informatsioon mõjutab juba väga varajases faasis nägemistaju ja teadvustamise protsesse, neid täiendades ning parandades.Consciousness is the biggest unsolved problem of modern science because no one has successfully explained how the concerted firing of brain cells is able to produce our subjective experience of the world. It has been argued that in order to understand this phenomenon better we must first identify the neural correlates of consciousness – those neural events which are jointly sufficient for producing a specific conscious experience. The present thesis was set to contribute to this research effort by investigating the neural correlates and markers of conscious visual perception. It is likely that previous studies have failed to identify the true correlates of consciousness because their results also contain processes that systematically precede or follow conscious experience, but do not directly reflect consciousness itself. Thus, the current aim was to study these additional processes in more detail or to even avoid their contribution is some studies. To that end more carefully designed experimental paradigms and more realistic stimulus material was employed. Furthermore, the ongoing state of more global neural networks and its influence on conscious perception was taken into account. Together the results demonstrate that neural markers of conscious visual perception begin to arise about 200 ms after the visual image reaches our eyes. It is however possible that conscious perception is a gradual phenomenon proceeding step-by-step, because another marker also reliably correlates with conscious visual perception around 100 ms later. Furthermore, some results indicate that the complex dynamics of our visual system are best observable with realistic images and not to the same extent with simple and artificial figures. The brain retains a lot of prior knowledge about the natural visual world and its regularities. This information influences the processes of conscious perception early on by complementing and correcting them

General markers of conscious visual perception and their timing

The goal of the present investigation was to identify reliable markers of conscious visual perception and to characterize their onset latency and its variability. To that end many visual stimuli from different categories were presented at near-threshold contrast and contrastive analyses were carried out on 100 balanced subsets of the data. N200 and P300 were the two reliable markers of conscious perception common to all perceived stimuli and absent for all nonperceived stimuli. The estimated mean onset latency for both markers was shortly after 200 ms. However, the onset latency of both of these markers of conscious perception showed considerable variability depending on which subsets of the data were considered. Some of this variability could be attributed to noise, but it was first and foremost the amplitude fluctuation in the condition without conscious perception that explained the variability in onset latencies of the markers of conscious perception. The present results help to understand why different studies have observed different onset times for the neural correlates of conscious perception. Moreover, the consciousness markers explored here have more generality as stimulus specificity was reduced

Individual differences in the effects of priors: a multi-paradigm approach

The present study investigated individual differences in how much subjects rely on prior information, such as expectations or knowledge, when faced with perceptual ambiguity. The behavioural performance of forty-four participants was measured on four different visual paradigms (Mooney face recognition, illusory contours, blur detection and representational momentum) in which priors have been shown to affect perception. In addition, questionnaires were used to measure autistic and schizotypal traits in the non-clinical population. We hypothesized that someone who in the face of ambiguous or noisy perceptual input relies heavily on priors, would exhibit this tendency across a variety of tasks. This general pattern would then be reflected in high pairwise correlations between the behavioural measures and an emerging common factor. On the contrary, our results imply that there is no single factor that explains the individual differences present in the aforementioned tasks, as further evidenced by the overall lack of robust correlations between the separate paradigms. Instead, a two-factor structure reflecting differences in the hierarchy of perceptual processing was the best fit for explaining the individual variance in these tasks. This lends support to the notion that mechanisms underlying the effects of priors likely originate from several independent sources and that it is important to consider the role of specific tasks and stimuli more carefully when reporting effects of priors on perception

No validity without a theory : a critical look at subjective measures of consciousness

Michel (The mismeasure of consciousness: a problem of coordination for the perceptual awareness scale. Philos Sci 2019;86:1239-49) claims that the Perceptual Awareness Scale (PAS) faces the problem of coordination (also known as validity). We argue that his claim holds only under certain theoretical assumptions which need to be made explicit as these are likely not in line with the PAS proponents’ standpoint. We also call for terminological clarity, an example being the usage of ‘levels’ of consciousness. Precise terminology combined with an explicit reference to the chosen theoretical perspective is necessary conditions for making progress in consciousness research and the development of consciousness theories