event related potentials show primary visual consciousness

Posted comment on ´Cortical Neural Synchronisation Underlies Primary Visual Consciousness of Qualia: Evidence from Event-Related Potentials` by C.Babiloni, N. Marzona, A. Soricelli, S. Cordone, J.C. Millan-Calenti, C. Del Percio and A. Bujan and published in Front.Hum. Neurosci. 30th June 2016, doi.org/10.3389/fnhum.2016.00310


Babiloni and colleagues discuss in their article how primary visual consciousness (PVC) is linked to increased cortical neural synchronization in the case of three different types of visual stimuli. Their article begins with a general summary of views on consciousness and includes: a definition of PVC;  a description of the neural correlates where neuronal activity represents the mental representation of the visual features being experienced; the two opposing views of consciousness, that of ´globalism` and ´localization`; the binding problem of the conscious experience; and the link between brain rhythms and consciousness.

Babiloni and colleagues also describe in their article the limitations of some consciousness studies where only a few selected brain areas are investigated and the lack of technical capability to look at the real-time development of PVC from a temporal and spatial information perspective. This is why they used high resolution electroencephalography (EEG) and recorded event-related potentials (ERPs). ERPs represented here the coordinated neural activity of excitatory cortical pyramidal neurons and inhibitory interneurons in response to specific stimuli and Babiloni and colleagues developed a specific scientific program to investigate these in relation to PVC. They proposed that the ERP could synchronize millisecond by millisecond with the qualia of the PVC.

In their experiments, Babiloni and colleagues used three basic visual features (visuospatial, facial emotions and written words) and their stimulus paradigm was based on the following sequence of visual stimuli: the background masking stimulus (forward masking hiding the cue); the cue stimulus (the stimulus to self-report at the end of the trial); the background masking stimulus (backward masking hiding the cue); and  the target stimulus (the ´go` stimulus causing the hand motor response of pressing a mouse button). The durations of the cue stimuli were determined for each subject by a short preliminary test where he/she received the cue stimuli for different lengths of time on a computer monitor and then had to respond by pressing a computer button if the cue had been observed (ie. the correct ´seen` response). A verbal report followed after this hand response. The sequence of cue stimuli was planned to mix the cue stimuli with different durations in a random order to avoid the effect of learning. The ERP results were then analysed by a computer software program so that the computed percentages of ´seen` cue stimuli could be calculated and the cue stimulus duration of 50% ´seen` was taken as the test comparison point. In later experiments, EEG recordings were always carried out using this optimal duration time. The cue stimuli used by Babiloni and team were all visual based and the threshold times for the different experiments was varied, eg. 101ms in the visuospatial experiments, around 65 ms for the facial expression experiments and 37.2 ms for the written words experiments. They looked at the ERP peak latency and sources for the ´seen` and ´not seen` experiments and related to this either the presence or absence of PVC.

The results obtained showed that visual ERP showed a typical pattern independent of whether ´seen` or ´not seen`, but the brain area sources of firing were different. In the emotional face expressions experiment greater activity was observed in the parietal and frontal sources at about 180ms post stimulus, whereas in the written words/letters experiment higher activity occurred in the occipital, parietal and temporal areas at 230ms post stimulus. In the case of the visuospatial stimuli, higher activity was measured in the dorsal occipital and parietal areas later still at about 400ms post stimulus. The hypothesis advocated by the authors to explain their observations was that PVC is associated with greater cortical synchronicity of neuronal firing, but the sources were different and correlated to the cortical regions associated with PVC of that particular stimulus feature.

In the case of the results relating to ERP peak latency, no differences were found for the three different types of visual stimuli. Since a difference would indicate that there is specific timing of the neural correlates of the PVC for the features, the lack of difference indicated that there is no specific timing of neural correlates of the PVC. The authors therefore suggested similar timing of the cortical neural synchronization regardless of the PVC experienced.

The results of the visuospatial experiment showed a lack of latency in the three main components of the ERP (ie. the N1, P2 and P3) between the ´seen` and ´not seen` conditions. This indicated to the authors that timing and stages of cortical neural synchronization was the same whether PVC was present or not. However, a difference in reaction time of the physical response was observed with the response faster in the ´seen` trial. The authors explained this by suggesting enhanced information processing independent of PVC. Cortical neural synchronization also showed some difference in relation to PVC in the intraparietal P3 component peaking around 400 ms post-stimulus and this again was higher in the ´seen` trial compared to the ´not seen` trial. The sources appeared to be the extra-striate occipital and posterior parietal areas. Therefore, the authors suggested that the brain processes simple visuospatial stimuli with enhanced cortical neural synchronization around 400 ms post-stimulus in association with PVC. The effect was not related to the stimuli features themselves since the results of both trials were identical.

Therefore, Babiloni and colleagues concluded that in the case of visuospatial stimuli then the PVC and responses were linked to activity in the parietal and occipital areas which supported evidence from other research. The time of the ERP at 400ms confirmed that the PVC of visuospatial qualia occurs at the later stages of information processing although other experimenters had reported a P1 component at around 100ms or 120ms which they explained could have been spatial attentional processes instead. An experiment where subjects had to press on the opposite side of the screen to the presented cue also gave differences between ´seen` and ´not seen` trials in the P3 component at between 100ms and 400ms. The ERP activity in this case was located to the occipital, parietal and prefrontal cortical areas.

Babiloni and colleagues also discussed in their article cases of visuospatial PVC in subjects suffering from visuospatial neglect and visual extinction (ie. where there are deficits in spatial awareness for stimuli on the opposite side to the brain lesion, but the information of the extinguished stimuli is still processed). The authors found that reaction times were affected and that the extinguished stimuli were still processed by the same occipital and parietal areas of the dorsal stream exhibiting the same enhanced activity as in the ´seen` trials of healthy subjects. The authors said their findings that occipital and parietal ERP components were higher in ´seen` than ´not seen` trials and in the P3 supported work by others.

In the experiments on PVC and facial expressions, three emotional conditions (neutral,  happy and sad) were studied with tests based on emoticon recognition. The results obtained gave post-stimulus ERP waveforms with the highest amplitudes at the parietal midline electrodes and consisted of four main components, eg. the N100, N170, P200, and P300. No statistical difference was observed in latency between the ´seen` and ´not seen` trials. Therefore, the timing of the cortical neural synchronization was the same regardless of the presence or absence of PVC. The reaction time in this case was faster for the happy faces in the ´seen` trial and so the authors concluded that PVC is associated with enhanced information processing. Some differences in ERP components were observed between the two trials eg.the ERP component at the parietal N170 component peak at 180-200ms was higher. Therefore, Babiloni and colleagues concluded that the brain processes emotional face expressions with enhanced cortical neural synchronization around 200ms post-stimulus possibly in association with PVC.  The N170 peak latency occurred earlier with facial expression than with visuospatial information and the authors explained this by facial expression being processed faster as a result of biological and social salience. Source analysis showed that the N170 component had higher activity in the prefrontal, premotor, and posterior parietal areas for the sad face. Therefore, the authors concluded that the brain processes emotional face expressions giving PVC at an early 200ms post stimulus and enhanced cortical neural synchronization is observed in the parietal, temporal and frontal brain areas.

For the experiments on written words, the experimenters used 2 Italian words and 2 English words. ERP waveforms were found to be at their highest activity in the parietal and temporal electrodes and the ERP waveforms observed had 4 main components, the P1, N1, P2 and P3. Again there was no latency between peaks of the ´seen` and ´not seen` trials. Therefore, like the other visual stimuli neural cortical synchronization exhibits the same timing regardless of the presence or not of PVC. Also again reaction time was quicker in the ´seen` trials indicating that PVC is associated with enhanced information processing of visual stimuli. However, in this case the N1 peaked at 230 ms post-stimulus which was the one component with the highest difference between the ´seen` and ´not seen` trials and suggesting that the brain processes words around 230ms post-stimulus. Source analysis showed that brain activity associated with the N1 component was higher in the left parietal, occipital and temporal areas demonstrating that enhanced cortical neural synchronization occurs in the processing of words in the left dorsal and left ventral streams formed by the occipital, parietal and temporal areas. Different networks for the PVC for visuospatial information and facial expressions was hence, observed.

The authors discussed in their article stimulus expectancy in relation to written words. The ERP difference was observed in P3 when there was little stimulus expectancy and in P2 when expectancy was high. The P2 amplitude decreased as awareness of the stimulus increased. These observations were not expected, but the authors explained that the P2 component reflects the comparison of sensory inputs and stored memory. Therefore, a high amplitude of P2 is not observed due to a mismatch between high expectancy of the stimulus appearance and the missed stimulus detection. This effect was not observed in the written words experiment probably due to the negligible effect of learning (the experiment was so designed that learning was not an influence), stimulus expectancy and cognitive load (there was no fatigue and physical stimulus features remained fixed during the whole EEG session).

Therefore, in conclusion, Babiloni and colleagues state in the article that there are no differences in the ERP peak latencies between the ´seen` and ´not seen` trials which suggests that cortical neural synchronization timing is the same regardless of PVC. Analysis of the source of firing for the ERP show however differences between ´seen` and ´not seen` trials. For visuospatial stimuli, the PVC was related to higher activity in the dorsal occipital and parietal sources at about 400ms post-stimulus. For the emotional face expressions, greater activity was reported in the parietal and frontal sources at about 180 ms post-stimulus and for the written letters, there was higher activity in the left occipital, parietal, and temporal sources at about 230 ms post-stimulus. Therefore, Babiloni and colleagues suggested that PVC is associated with an increased cortical neural synchronization having entirely different spatiotemporal characteristics for different features of the visual stimuli studied ie. visuospatial, emotional facial expression and written words and letters and possibly, the corresponding qualia. Brain areas activated were specific with the dorsal visual stream synchronized in association with the PVC of visuospatial and emotional facial expression and both dorsal and ventral visual streams synchronized with the PVC of written words. The authors concluded that their cortical neural synchronization observations support the ´localisationist` theories of consciousness and that the cortical neural synchronization within specialized networks leads to PVC by what they termed a ´multidimensional palette` of each given feature and quale. Each element was shown to have its own specific timing. They also state that the PVC should not be considered as an instantaneous mental experience to be related to one peak of local neural responses, but should be regarded as a progressive ´build up` phenomenon. This explains PVC`s emergence over a period of time and the temporal synchronization of many different brain regions. The authors also state that the reliability of data from experiments could be subject to unknown factors and therefore, it would be better to use another procedure in for example the rating of the visibility of stimuli. The low spatial resolution of LORETA was also given as a limitation as well as that the analyses are limited to one process of consciousness, ie. Visual stimuli recognition. Babiloni and colleagues end their article by indicating future research areas which could include the influence of top-down processing on the PVC and ERPs, or the use of tDCS.


This article is interesting because it continues the exploration of the neural correlates of the conscious experience. The results confirm that the various elements of an event contribute to the overall conscious experience of that event with different post-stimulus timings. From an earlier article by Fairhill, Albi and Melcher discussed in this Blog in March 2015 the entire sensory integration of the final conscious experience relating to visual sequences appears to take place within 2-3 seconds of the stimulus and here Babiloni and colleagues show that individual visual processes, ie. those relating to facial expressions, written language and visuospatial events achieve awareness at different times within that overall period.

In the set of experiments described in this article visual processing is investigated until the point of conscious awareness at the level of primary visual consciousness (PVC) occurs. This means that the higher order consciousness associated with language, applied reasoning and decision-making is not considered. The level of Self relating to this lower level conscious experience in these experiments according to the definition of Damasio is that of photo Self and core Self, but not autobiographical Self since memories, views or reasoning are not needed for the task at hand. (However, it could be said that autobiographical Self actually does play a role since assessment of the incoming information is based on the memories stored of previous experiences. The Self allows perception to take place, but this analysis is carried out subconsciously and therefore, autobiographical Self in its truest sense of conscious application of personal knowledge is not involved.)

In their experiments on the timing of conscious awareness, Babiloni and colleagues measured the membrane potentials for three types of visual stimulus studied from the time of stimulus to the time of conscious awareness. Conscious awareness was assessed through a physical action, ie by the pressing of a button and by the common test for consciousness that of verbal report. Therefore, the neuronal firing occurring on stimulus does so unconsciously (ie termed here as ´not seen` by the subject) until the point when the stimulus evokes awareness (ie termed here ´seen` by the subject). Analysis of the ERPs recorded gave Babiloni and colleagues an average ERP when the stimulus was 50% of the time ´seen` by the subject. Since it is said that there are two types of information processing in the brain (fast, which is automatic, inflexible, effortless and dependent on context and slow, which is effortful, controlled flexible, requiring working memory and independent of context), the nature of the experiments means that fast unconscious processing occurs until the point of the ERP peak when PVC is said to be achieved. Therefore, Cleerman`s view of automatic behaviour in his Radical Plasticity Theory should be re-considered. Cleerman said that automatic behaviour is not truly unconscious behaviour rather behaviour where awareness is optional. Babiloni and colleagues experiments show that behaviour ie. the pressing of the button in this case is not instigated until PVC occurs. Therefore, unconscious processing must reach a certain level before the action can occur so that awareness is not ´optional` until after this point. This fits in with Franklin and Baars 2010 ´preconscious` and ´never conscious` descriptions of events. In this case, the time before the ERP represents the ´preconscious` processing of the stimulus and these reach conscious awareness at the point of PVC. However, there are definitely event characteristics that never reach consciousness (eg. the sound of the computer fan or the chair creaking) since the task demands that attention is focused on the stimuli required to complete the test. Therefore, these characteristics although processed to some extent never reach awareness and eventually fade.

The experimental results are said to support the´localisationist` hypotheses of consciousness rather than the ´globalist` ones. The authors explain that the  ´globalist` hypotheses like Baars original Global Workspace Theory then would provide no difference in timing with all features reaching conscious awareness at same time. They state that the fact that different times are observed for the three different types of visual stimuli means that the ´localisationist` theories are more likely to be appropriate. The theories given as examples are the Reentrant Dynamic Core Theory of Edelman and Tononi and Zeki`s Microconsciousness Theory and these indicate that consciousness arises from multiple neuronal groups firing with the mechanism of re-entry amongst distinct and distant neuron groups within the dynamic core of the thalamo-cortical connections binding the features together. The observations made here support the view of Marcel and slippage which he related to blink and tap elements of a sensory experience being separated. In the experiments described in this article firing of visual pathways (the dorsal WHAT and ventral WHERE pathways) represent the visual input, maintenance and recognition. The resulting PVC is seen through the brain areas firing at that time and these represent the areas critical for consciousness (Bor and Seth hypothesis) such as the cingulate cortex, prefrontal cortex and parietal cortex and the first modality areas such as the visual cortex V1 and secondly, the prefrontal parietal network (PPN) responsible for attention, working memory and the central executive. This is supported by work by Nog who said that visuoperceptual consciousness demands local activity in the visual cortex and global frontal parietal workspace activity with a 300 ms delay and strong temporal firing.

The firing observed by Babiloni and colleagues in their experiments is dependent on the type of stimuli used. Emotional face expression provoked high activity in the parietal and frontal brain areas, written letters in the occipital, parietal and temporal areas and visuospatial stimuli in the dorsal, occipital, and parietal areas. These areas represent different cognitive demands  ie visual and emotional memory for emotional face expression; visual pathways plus pathways for language and meaning for written letters and visual pathways and pathways for object recognition and location assessment for visuospatial stimuli. In all cases, the firing pathways associated with attention are activated. Although Bor and Seth maintain that attention is not consciousness and that the Baars original Global Workspace Theory did not address the matter of attention, attention plays an important role in conscious awareness. It is highly selective for task relevant visual events (Jacobs said attention in consciousness is top-down modulation to stop incoming visual information via inhibition at the early visual cortex level) and is sensitive to temporal order (Eimer) and is an important factor in any sensory input. That is why it is included in the later ´globalist` models and features in ´localisationist` consciousness models. Therefore, the experiments of Babiloni and colleagues explore the firing and connectivity of brain areas relating to the input and processing of visual stimuli with relation to time and the emergence of conscious awareness.

Three things can be said about the timing of the emergence of the conscious awareness observed by Babiloni and colleagues. The first concerns the nature of sensory experience. Fairhill, Albi and Melcher found that sensory information integrated over 2-3 seconds post-stimulus to form the visual experience. This was described by Zmigod as the temporal binding window (TBW). In this article, Babiloni and colleagues found that the three visual capabilities they examined reached consciousness at different times, but within this 2-3 second window. Owing to the types investigated, it is unlikely that the three capabilities are experienced within one event eg. facial expression and written words cannot be observed together and therefore, each stimulus has to be treated separately. Therefore, the unity of consciousness and how elements of sensory experiences come together cannot be demonstrated or investigated here. By changing and expanding the experiment eg. facial expression plus spoken word from a different location the unity of the conscious awareness could be looked at. It could be assumed that the presentation of the combined sensory event would lead to firing of the appropriate pathways and the emergence of conscious awareness of one ´draft` of the event if Dennett`s Multiple Draft theory of consciousness is the accepted tenet. The ´draft` experienced is the one fleeting version of the events occurring at the time and is reliant on the sensory neuronal cell firing and assembly formation and brain rhythms adapt accordingly. Alpha rhythms through the post-parietal and lateral occipital areas are required for event characteristic maintenance and gamma rhythms for assembly formation, feature and binding and holding. These would demonstrate the unity of the conscious experience even though the different features show Marcel`s slippage.

The purity or simplicity of the nature of the characteristics and the tasks demanded in the experiments also mean that there is no ´filling in` of event features occurring. For example, the subject is asked to judge whether a face is happy or sad. This is not open to interpretation since most people can judge such clear basic emotions without question or reflection. No assessment of questionable facial expressions such is showing regret or those requiring subjective opinions are asked of the subject and therefore, the exploration of the extent of the quale of the sensory event is relatively basic. Neither can the effect of processing on the event characteristics be explored. A number of researchers (eg. Windey, Gevers and Cleermans) support the Level of Processing Hypothesis which states that the transition of unconscious to conscious perception is influenced by the level of processing imposed by the task requirements. Subthreshold stimuli have bottom-up processing and a forward sweep of firing terminating in the somatosensory cortex preventing access to the conscious experience. This interruption is due to a predominance of inhibitory processing in this area. The increase in alpha rhythms and a disconnection from the somatosensory cortex area from the frontoparietal area are likely to correlate to the increased perception and are thought to serve as a gating mechanism for access to the conscious experience. In Babiloni and colleagues` experiments a high level of processing is not required and the experiment ends at the point of PVC. The forward sweep of the neuronal firing occurs in the areas observed by the researchers for each type of stimuli and these continue until the time when the ERP is measured. According to the Level of Processing Theory, the somatosensory cortex area inhibits access to the frontal parietal cortex area until the point when PVC occurs and this is likely to be related to the quantity of firing cells, not the quality ie. the threshold of conscious awareness is reached (ie quantified as phi). The speed at which this occurs may reflect the synchronicity of the relevant areas and the quantity of firing. For example, the inclusion of emotions as in emotional facial recognition means that the firing of the cerebellum and basal ganglia areas join the ´dynamic core` thalamo-cortical areas. The question as to why the conscious awareness of the written word comes before visuospatial information may be that visual and spatial firing and processing may require more coordinating pathways and information processing before phi is achieved.

The second observation about timing relates to the advantageous effect of priming of the subject due to the repeated experimental conditions. Priming through knowledge of the experiment means that the ERPs observed may be faster than normally expected since the subject knows what he is looking for and repetition gives him the practice. Prediction also leads to visual events achieving access to consciousness faster (Acer) and this too is reflected here by the repetition of the experimental condition. The effect of priming can be seen through the lack of latency in the three main components of the ERP (ie the N1, P2 and P3) between the ´seen` and ´not seen` trials. This means that there is the same timing and stages of cortical neural synchronization whether PVC is achieved or not. We know that the first 270 ms of neuronal activity for any stimulus is the same independent of later consciousness state, and effect increased to 750 ms for children. However, there is a difference in reaction time of the physical response and this is faster in the ´seen` than ´not seen` trials. The authors of the article explain this by enhanced information processing independent of conscious awareness.

The third point relating to timing is however aided by the experimental set-up. It has been reported that it is impossible to report the precise time of the conscious experience (Gray) and on looking back the timing always appears to be wrong with timing later than when it actually occurs (Paulignan). Therefore, the experimental set-up used by Babiloni and colleagues in their experiments which included the physical action of manually pushing a button and verbal reporting means that the time of conscious awareness is more accurate than those experiments relying on verbal report only. Through conditioning ie. repetition of the experiment, the physical actions become automatic and this unconscious processing means that the physical action can be started before the language capability kicks in. However, it does introduce an interesting question as to whether the unconscious processing which sets off the physical action simultaneously initiates the verbal response or whether the initiation of the verbal response actually is a result of subconscious beginnings of the hand movement.

Therefore, to summarise, Babiloni and colleagues experiments support the view that the conscious ´draft` for a visual event emerges over a period of time relying on firing of appropriate brain areas and binding. The speed at which this PVC occurs is dependent on the elements that make up the experience and also to some extent the subject`s own capabilities. The continuing development of faster and more accurate equipment and computer capability can only help the research into this very important area.

Since we`re talking about the topic…………………

….as stated above Babiloni and colleagues` experiments involve simple visual stimuli and therefore, would the use of more complicated stimuli using a combination of senses, or  using stimuli that provoke errors by including irrelevant material shift the awareness level from PVC to higher order consciousness so that a more in-depth observation of brain area firing and ERPs measurements can be made?

….can we assume that the use of more emotionally relevant material would shift the consciousness level from PVC to the higher order of consciousness and involve the autobiographical Self and hence a comparison of the two would confirm where conscious awareness of  more complicated emotional responses is?

…..the administration of ketamine increases the level of irrelevant information inputted because of the subject`s inability to ignore it. Would such an administration increase the level of errors in pushing the button and slow the verbal reporting in a repeat of Babiloni and colleagues` experiments?

…..since people with split personalities are reported to have two conscious awareness  with both hemispheres having special awareness (eg. right side – facial recognition, left side – language – Gazzaniga), what would happen to the ERPs if Babiloni and colleagues` experiments were repeated?

…..children are said to demonstrate a 750 ms delay in conscious awareness. Would they also demonstrate the same differences in ERP in the order of the stimuli? (The experiments may need to be adapted to take in the age of the child.)

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