ERP Studies Of The Neural Correlates Of Visual Consciousness

What is the biological basis of consciousness?An important way to answer this important question in the 21st century is to identify neural correlates of consciousness(NCCs).The neural correlate of consciousness is defined as "minimum neural mechanism jointly sufficient for any one specific conscious experience".Searching for the NCC has become a central endeavor in neuroscience of consciousness.Over the past 20 years,most of studies on NCC focused on visual processing.Using functional magnetic resonance imaging(fMRI),many researchers had found that visual awareness correlates with visual areas.But the temporal resolution of fMRI is low,and fMRI could not empirically capture the unfolding of neural events that correspond to subjective conscious experience.Using electro-encephalography(EEG),researchers can empirically capture the unfolding of neural events that correspond to subjective conscious experience.Through precise timing of electrophysiological events in response to stimulus,ERP recording provide a useful way in identifying NCC.In NCC studies,researchers commonly use contrastive experimental design comparing ERP elicited by physically identical stimuli of which participants are aware vs.unaware.By using such contrastive experimental designs,three candidate ERP components for NCCs have emerged:early positive correlates of consciousness(P1),visual awareness negativity(VAN)and late positivity(LP).However,more and more researchers realized that the result of NCC studies using contrastive experimental designs is not only NCC but also reflects pre-conscious and post-perceptual processing.NCC would be confounded by pre-conscious and post-perceptual processing in previous studies.Therefore,dissociating NCC from the pre-conscious and post-perceptual processing is one of the most challenging problems in neuroscience of consciousness.In order to address this problem,our ERP studies were to dissociate NCC from the pre-conscious and post-perceptual processing.Our dissertation was composed of four ERP experiments.The experiment 1 used reduced-contrast stimuli to explore the relationship between the participants’ confidence on reporting and LP.We used a forced-choice detection task.Following stimulus presentation,participants first indicated whether they had seen the stimulus—by pressing one of two designated keys—then they rated their confidence in their response.The ERP results revealed that LP was observed(significant difference in ERP within P3 time window between trials with "aware" response and trials with"unaware" response).In addition,LP was modulated by confidence.However,LP was found for trials with confidence ratings of "slightly confident" or with combined confidence ratings of "very confident" and "quite confident".The experiment 2 used reduced-contrast stimuli to explore the relationship between the requirement on reporting and LP.We used two task conditions that differed in their requirements on reporting.In immediate detection task,participants had to quickly report whether the stimulus was presented,whereas in delayed detection task,participants had to put off reporting whether the stimulus was presented.The results showed that the amplitude of LP over parietal,posterior temporal and occipital areas was influenced by the task manipulation during 650-850ms.The experiment 3 used masking to explore the relationship between the shape size of stimulate and VAN.We used two task conditions that differed in the shape size of stimulate.In the small shape of stimulate detection task,participants had to report whether the small shape of stimulate was presented,whereas in the big shape of stimulate detection task,participants had to report whether the big shape of stimulate was presented.The results showed that VAN was not observed in both detection tasks.However,P1 was observed in the small shape of stimulate detection task,but P1 was not observed in the big shape of stimulate detection task.In addition,LP was not influenced by task condition.The experiment 4 used masking to explore the relationship between the area size of stimulus and VAN.We used two task conditions that differed in the area size of stimulus.In the small area size of stimulus detection task,participants had to report whether the small area size of stimulus was presented,whereas in the big area size of stimulus detection task,participants had to report whether the big area size of stimulus was presented.When the duration of small area size of stimulus was longer compared to the big area size of stimulus,the results showed that VAN was observed in the small area size of stimulate detection task,but was not observed in the big area size of stimulate detection task.When the difference between the duration of small area size of stimulus and the duration of big area size of stimulus did not reach statistical significance,the results showed that VAN was not found in both task conditions.In summary,the ERP data suggested that LP did not reflect the participants’confidence on reporting,but LP reflected post-perceptual processes required in reporting perceptual awareness.In addition,P1 did not reflect the neural correlates of consciousness and might reflect pre-conscious.Importantly,The ERP studies adopting reduced-contrast stimuli supported that VAN was an early neural correlate of consciousness.