Examining the neural correlates associated with the moment a stimulus enters or exits conscious awareness is one way to potentially identify the neural mechanisms that give rise to consciousness. In the present study, we examined neural activity using electroencephalogram (EEG) recordings while participants observed a bilateral shape-from-motion (SFM) display. While the display is in motion, the observer perceives an object that is immediately segregated from a noisy background. After the motion stops, the observer’s experience of the object remains momentarily in awareness, before it eventually fades out of consciousness back into the noisy background. Consistent with subjective reports of perceptual experience, we observed a prominent sustained posterior contralateral negativity (SPCN), but only in conditions associated with sustained awareness. Importantly, the amplitude of the SPCN was correlated with individual differences in visual awareness, suggesting that this activity plays a significant role in the maintenance of objects in consciousness.

Examining the neural correlates associated with the moment a stimulus enters or exits conscious awareness is one way to potentially identify the neural mechanisms that give rise to consciousness. In the present study, we examined neural activity using electroencephalogram (EEG) recordings while participants observed a bilateral shape-from-motion (SFM) display. While the display is in motion, the observer perceives an object that is immediately segregated from a noisy background. After the motion stops, the observer’s experience of the object remains momentarily in awareness, before it eventually fades out of consciousness back into the noisy background. Consistent with subjective reports of perceptual experience, we observed a prominent sustained posterior contralateral negativity (SPCN), but only in conditions associated with sustained awareness. Importantly, the amplitude of the SPCN was correlated with individual differences in visual awareness, suggesting that this activity plays a significant role in the maintenance of objects in consciousness.

Despite its great complexity, a great deal is known about the organization and information-processing properties of the visual system. However, the neural correlates of visual awareness are not yet understood. By studying patients with blindsight, the primary visual cortex (V1) has attracted a lot of attention recently. Although this brain area appears to be important for visual awareness, its exact role is still a matter of debate. Interactive models propose a direct role for V1 in generating visual awareness through recurrent processing. Hierarchal models instead propose that awareness is generated in later visual areas and that the role of V1 is limited to transmitting the necessary information to these areas. Interactive and hierarchical models make different predictions and the aim of this thesis is to review the evidence from lesions, perceptual suppression, and transcranial magnetic stimulation (TMS), along with data from internally generated visual awareness in dreams, hallucinations and imagery, this in order to see whether current evidence favor one type of model over the other. A review of the evidence suggests that feedback projections to V1 appear to be important in most cases for visual awareness to arise but it can arise even when V1 is absent.

A current controversy in the science of visual consciousness is whether conscious awareness of a stimulus emerges gradually or appears directly in an all-or-none fashion. Windey et al. (2013) observed that evidence supporting the gradual explanation often used low-level stimuli (simple shapes, colours), while evidence for a dichotomous divide of awareness used high-level stimuli (letters, numbers). Windey later constructed the level of processing (LoP) hypothesis, which states that different stimulus levels impact the transition to awareness, i.e. whether it occurs gradually or in an all-or-none fashion. Koivisto et al. (2017) further showed, using high-level stimulus, that different conceptualizations of awareness (detection and identification) impact different electrophysiological correlates of awareness. Specifically, the visual awareness negativity (VAN) only occurred for awareness based on detection, while the late positivity (LP) occurred for both detection and identification. The purpose of this thesis is to further investigate the VAN and LP for detection and identification, but with a low-level stimulus. Following the level of processing hypothesis, I expect that the mean amplitudes will be more negative in the VAN time range and more positive in the LP time range for identification trials compared to detection trials. Similarly, I expect VAN and LP to occur for both detection and identification. Unfortunately, the current results did not find any significant differences due to the low sample size. However, the results for the identification threshold hint towards a VAN but no LP.

The present dissertation aims to evaluate the phenomenon of visual masking as a tool for studying visual awareness focusing on two-alternative forced choice (2AFC) discrimination tasks. Two existing theories of masking - Bachmann's (1984) perceptual retouch theory and Marcel's (1983) recovery theory - are discussed along with the global neuronal workspace theory of awareness (Baars. 1989; Dehaene, Kerszberg, & Changeux, 1998). Performance accuracy on Semantic discrimination is compared to that on Orthographic discrimination as an indication of a potential difference between semantic and orthographic processing of masked word stimuli presented for 40 ms and 50 ms. This is further compared to an e-detection task previously used as an indicator of awareness in some masked priming experiments. Together, these tasks are further evaluated in terms of their relationship with participants' subjective reports collected in the form of confidence ratings. The implications and predictions drawn from the theories of masking and visual awareness as well as the notion of partial awareness (Kouider & Dupoux, 2001) are assessed taking into account the data obtained in the current experiments.The relevance of these data for masked priming is determined by performing a comparison between 2AFC discrimination and detection tasks, and the lexical decision task. An ERP study is also presented, in which Semantic and Orthographic discrimination as well as e-detection are paired with confidence ratings and electrophysiological recordings in search of an ERP component that can be correlated with both subjective (confidence) and objective (performance) measures of awareness. A binding account of visual awareness with special attention paid to visual masking is proposed and compared to the three existing theories.

Does visual recognition entail verifying an idea about what is perceived? This question was addressed in the three studies of this thesis. The main hypothesis underlying the investigation was that visual recognition is an active process involving hypothesis testing. Recognition of faces (Study 1), scenes (Study 2) and objects (Study 3) was investigated using eye movement registration as a window on the recognition process. In Study 1, a functional relationship between eye movements and face recognition was established. Restricting the eye movements reduced recognition performance. In addition, perceptual reinstatement as indicated by eye movement consistency across study and test was related to recollective experience at test. Specifically, explicit recollection was related to higher eye movement consistency than familiarity-based recognition and false rejections (Studies 1-2). Furthermore, valid expectations about a forthcoming stimulus scene produced eye movements which were more similar to those of an earlier study episode, compared to invalid expectations (Study 2). In Study 3 participants recognized fragmented objects embedded in nonsense fragments. Around 8 seconds prior to explicit recognition, participants began to fixate the object region rather than a similar control region in the stimulus pictures. Before participants’ indicated awareness of the object, they fixated it with an average of 9 consecutive fixations. Hence, participants were looking at the object as if they had recognized it before they became aware of its identity. Furthermore, prior object information affected eye movement sampling of the stimulus, suggesting that semantic memory was involved in guiding the eyes during object recognition even before the participants were aware of its presence. Collectively, the studies support the view that gaze control is instrumental to visual recognition performance and that visual recognition is an interactive process between memory representation and information sampling.

Visual conditions such as retinitis pigmentosa and Usher syndrome can gradually cause tunnel vision. Patients with these conditions usually face difficulties with navigation, avoiding obstacles, and performing visual search. Loss of mobility can affect patients' independence and quality of life. One of the rehabilitation strategies for patients with tunnel vision is the use of optical aids to enhance mobility performance. The main method used to evaluate the usefulness of optical aids is the patient’s subjective report after extended wear. In order to evaluate optical aid effectiveness in the clinic, a new test based on the visual search paradigm was designed to assess the patient's visual awareness. This was named the assessment of visual awareness (AVA) test. The main aim of this study was to develop the AVA test, establish its sensitivity, validity and repeatability, and then use it to investigate the efficacy of optical aids in this group of people. The AVA test consists of 32 peripheral targets presented at four different locations: 1st annulus (at 5° from the central fixation), 2nd annulus (10°), 3rd annulus (20°) and 4th annulus (30°)). In this study, the peripheral targets were presented singly against a spatial noise background in a presentation area of 81° H × 62° V. Participants were allowed to use head and eye movements and were asked to search for and locate each target. The detection time (DT) was recorded. A new, sensitive and easy to set up indoor mobility course was also designed and validated prior to its use in validating the AVA test. A total of 50 normally sighted participants with simulated tunnel vision (TV) (5° to 20°, in 5° steps) and 20 patients with TV (retained field 4° to 21°) were tested. The AVA test was found to be responsive to the change in field of view (FoV) and to the target locations in both groups of participants. In the simulated group, a significant relationship was found between FoV and DT at each annulus (r ranging from -0.55 to -0.77, p < 0.0001). A significant relationship was found between target location and DT within each FoV size (20°, 15°, 10° and 5°) (r ranging from 0.53 to 0.84, p < 0.0001). In the TV patients, a statistically significant relationship was found between FoV and DT at each annulus (r range from -0.40 to -0.60, p < 0.05). The target location was shown to have a significant relationship with the DT within each FoV size (r ranging from 0.50 to 0.60, p < 0.05). Finally, the AVA test was found to be significantly related to the simulated TV participants' performance on the indoor mobility course. The AVA test was used to assess the efficacy of three optical aids: the partial aperture prism (10 patients), the Tri-field prism (10 patients) and the reverse telescope (4 patients). The AVA test showed no significant improvement in DT with either of the prisms and the participants did not find these aids helpful. DT with the reverse telescope improved, but none of the participants were willing to use these on extended trial. The AVA test gave clear indications of the efficacy of each aid, a result which could affirm the importance of the AVA test. In conclusion, the AVA test was found to be sensitive, valid and repeatable. DT did not improve in either of the optical aids which were found to be unsuccessful, suggesting that the AVA could be a promising clinical test. However the aids which showed improved DT were not evaluated over the longer term, and therefore did not allow full evaluation of the AVA test.

Two decades of event-related potential (ERP) research have established that the most consistent correlates of the onset of visual consciousness are the early visual awareness negativity (VAN), a negative component in the N2 time range over posterior electrode sites, and the late positivity (LP), a positive component in the P3 time range over fronto-parietal electrode sites. A review by Koivisto &amp; Revonsuo (2010) had looked at 39 studies and concluded that the VAN is the earliest and most reliable correlate of visual phenomenal consciousness, whereas the LP probably reflects later processes associated with reflective/access consciousness. However, an “early” vs. “late” debate still persists. This thesis provides an update to that earlier review. All ERP and MEG studies that have appeared since 2010 and directly compared ERPs of aware and unaware conditions are considered. The result corroborates the view that VAN is the earliest and most consistent signature of visual phenomenal consciousness, and casts further doubt on the LP as an ERP correlate of consciousness. Important new methodological, empirical, and theoretical developments in the field are described, and the empirical results are related to the theoretical background debates.

Wir untersuchten die Verarbeitung sozial relevanter visueller Reize während “continuous flash suppression” (CFS), einer besonders wirkungsvollen Technik der interokularen Unterdrückung, die benutzt wird um Reize unsichtbar zu machen. In den Studien 1–6 maßen wir die Dauer der perzeptuellen Unterdrückung während CFS um zu testen, ob sozial relevante Reize bevorzugten Zugang zum visuellen Bewusstsein haben. Studie 1 zeigte, dass die Detektion von Gesichtern bei Erwachsenen durch Gesichtsmerkmale beeinflusst wird, welche auch Blickpräferenzen von Neugeborenen beeinflussen. Studie 2 zeigte, dass Gesichtsdetektion durch die Ethnie und Altersgruppe des zu detektierenden Gesichtes beeinflusst wird. In Studie 3 fanden wir größere Effekte der Inversion auf die Detektion von menschlichen Gesichtern und Körpern als auf andere vertraute Objekte. In Studie 4 fanden wir, dass Gesichter mit direktem Blick schneller detektiert werden als Gesichter mit abgewandtem Blick. Studie 5 deckte einen konfundieren Faktor in schematischen emotionalen Gesichtern auf. In Studie 6 fanden wir, dass die schnellere Detektion von furchtsamen im Vergleich zu neutralen Gesichtern auf hohen Raumfrequenzen beruht. Studie 7 zeigte, dass die Messung der visuellen Detektion während CFS keinen eindeutigen Nachweis für unbewusste Verarbeitung unter CFS erbringen kann. In den Studien 8 und 9 maßen wir deshalb Adaptations-Nacheffekte von Reizen, die durch CFS dauerhaft unsichtbar gemacht wurden. Studie 8 zeigte, dass lediglich monokulare Komponenten der Gesichtsform-Adaptation unbewusst ablaufen können, während komplexere Komponenten auf visuelles Bewusstsein angewiesen sind. Studie 9 zeigte, dass nur größenabhängige Komponenten von Blickrichtungen unbewusst repräsentiert werden können, während objektzentrierte Repräsentationen von Blickrichtungen visuelles Bewusstsein benötigen. / We studied the processing of socially relevant visual stimuli during continuous flash suppression (CFS), a potent interocular suppression technique that we used to render stimuli invisible. In Studies 1–6, we measured the duration of perceptual suppression during CFS to test whether socially relevant stimuli have privileged access to visual awareness. Study 1 demonstrated that face detection in adult observers is modulated by facial properties previously shown to modulate looking preferences in newborns. Study 2 revealed own-race and own-age biases in face detection, indicating that visual awareness of faces is shaped by visual experience with one’s own social group. In Study 3, we found larger effects of stimulus inversion on the detection of human faces and bodies than for other familiar objects, suggesting that detection mechanisms are preferentially tuned to conspecifics. Study 4 showed that faces with direct gaze are detected more quickly than faces with averted gaze. Study 5 revealed a confounding factor in schematic emotional faces that are considered to be well-controlled visual stimuli. In Study 6, we found that faster detection of fearful compared to neutral faces relies on high spatial frequencies, arguing against a functional role of a subcortical pathway to the amygdala. Study 7 showed that measures of visual detection during CFS cannot provide unequivocal evidence for unconscious processing under CFS. In Studies 8 and 9 we therefore measured adaptation aftereffects from stimuli rendered permanently invisible by CFS. In Study 8, we measured face shape aftereffects and found that only low-level monocular components of face shape adaptation can proceed unconsciously, whereas higher-level components depend on visual awareness. Study 9 revealed that only size-dependent low-level components of eye gaze can be represented unconsciously, while object-centered higher-level representations of eye gaze directions require visual awareness.

意識的議題古老而難解，但是近年來認知神經科學領域對此議題的探討已經熱烈展開，本研究之主要目的即在探索視覺意識與大腦功能性連結之間的關係。 根據一項人臉知覺的實驗結果，本研究依照線性對非線性、局部對整體等兩項條件所構成的四個取向，分別擬定用以反映視覺意識的腦電波指標。結果發現，線性的局部指標—即γ波的強度，以及線性的整體指標—即γ波的相位耦合程度，兩者皆無法有效反映視覺意識。然而，非線性的局部指標—即吸子的相關維度，在特定通道上可以反映視覺意識；至於非線性的整體指標—即廣義的同步化程度，乃為四者中最能穩定反映視覺意識的指標。 除了得到上述若干可以有效反映視覺意識的腦電波指標之外，本研究實質上整合了認知神經科學、非線性動力系統理論、小波轉換理論以及小世界理論等當代思維，因此文中亦做出大量而深入的理論探討，並且提出對現有相關研究在邏輯或方法上的改進與澄清。 / Consciousness is an ancient and puzzling mystery. Until recently, scientists have made little significant progress on it. This study is aimed to search for the neural correlates of visual awareness. / Based on empirical data from an experiment of face perception, this study explores linear vs. nonlinear and local vs. global human EEG indexes of visual awareness. The results indicate that neither linear local index, i.e. γ-band power, nor linear global index, i.e. γ-band phase coherence, can reveal the participant’s state of awareness validly. However, nonlinear local index, i.e. correlation dimension of attractor, can be a valid index of visual awareness, but only on specific channels. Last but not least, nonlinear global index, i.e. generalized synchrony, can be the most valid and efficient index of visual awareness. / In addition to the empirical findings listed above, this study, an interdisciplinary combination of cognitive neuroscience, chaos theory, wavelet transform and small-world theory, also presents numerous theoretical discussions and modifications to other related studies logically or methodologically.