Given that subjective experiences can accompany almost any sensory, cognitive, emotional, and behavioural function of the brain, phenomenal consciousness turns out to be an extremely complex and multi-dimensional process. Nevertheless, introspection shows that despite its qualitative richness, phenomenal consciousness appears to us as a unified and coherent model of the external and internal environment (Bayne 2010; Revonsuo 2006). The continuity in the diversity of subjective experiences is famously referred to as the stream of consciousness (James 1890). This metaphor points to the unification of experiences occurring at different points in time and space, which is achieved through temporal and spatial binding.
At the cognitive level of description, temporal binding, i.e., integration of subjective experiences over time, is realised through the perception of simultaneity, duration, and successiveness (Kiverstein 2010; Pöppel 1997). Perception of simultaneity may integrate several experiences, e.g., seeing a cat and hearing a birdsong in the park, as occurring at the same time. Perception of duration of selected experiences may extend them in time, e.g., the birdsong might seem to last for a certain period of time. Finally, perception of successiveness may signal the end of one temporally-extended experience, and the beginning of another one, e.g., as the cat reaches the bush and the birdsong ceases, we may notice a cone under the bush. Notably, the change does not typically involve all experiences, and as we are aware of some changing contents, some other experiences continue to endure in time, e.g., we still see the same bush. In addition to the timing-specific functions, temporal binding seems to depend on the iconic memory that contains the just-experienced contents of consciousness, and on the anticipation of subsequent ones, forming the temporally-extended phenomenal experience of now, sometimes referred to as the specious present (Dainton 2006; Kelly 1882). Temporal extension of subjective experiences have a simple, yet very important and often overlooked implication for NCC research: if there is a single neural mechanism generating phenomenal consciousness, it should be present as long as we are conscious of at least one single content. Given that our experiences do not cease at a fixed rate, i.e., some are shorter and some longer, and that the change does not happen abruptly for all experiences at once, the NCC should persist for the duration of the stream of consciousness. Thus, a temporary-confined correlate of awareness, such as a negative Event Related Potential (ERP) waveform briefly peaking at about 200ms from the onset of visual stimuli (Railo et al. 2011), cannot be a sufficient correlate of consciousness, as awareness of visual contents lasts for a much longer period of time.
Spatial binding, i.e., integration of subjective experiences in space, is realised through several complimentary processes, through which each subjective experience occupies a specific location in relation to other experiences, which is sometimes referred to as location binding (Treisman 1996). In the baseline state of consciousness, one experience never occurs in isolation from other experiences; and when some experiences cease, we do not experience emptiness, because other experiences fill in their place. Furthermore, individual experiences are spatially integrated not only with respect to each other in phenomenally external space, but also with respect to the common egocentric reference point (Revonsuo 2006). The reference point is typically located in the phenomenal head or chest, and all other experiences are realised in the space as taking a certain distance and angle from this point. While typical phenomenological analysis of 3D space considers visual and auditory experiences, it has recently been shown that emotions and feelings are also experienced as taking certain location with respect to our body parts (Nummenmaa et al. 2014). For instance, anger is overrepresented in hands and arms when compared to sadness. Arguably, even thoughts, which are often regarded as non-spatial entities (Clarke 1995), are usually experienced as occurring within the head rather than somewhere else. Given that the whole phenomenal space is bound together, the NCC should also represent awareness of the whole space rather than, for instance, selected regions on the computer screen. That is, a promising candidate for an NCC should not cease when a specific experience vanishes as long as the spatial and temporal unity of the stream of consciousness is maintained.
So, what type of neural processes should we be looking for when searching for the NCC? If we take the unity of consciousness seriously, we should be looking for a neural process that steadily represents the whole phenomenal space, and sustains its activity over periods of time longer than the existence of a single experience. Arguably, the neural correlate of unified consciousness cannot be discovered by studying and contrasting only isolated contents of consciousness, as the unity of spatiotemporal interactions simply cannot be derived from solitary experiences. Thus, while continuing to search for the neural mechanisms of the contents of consciousness, the NCC program should be extended by carrying out systematic contrasts between unconscious, baseline, and/or altered states, which would consider the whole stream of consciousness. A possible objection to this proposal is that the unity of consciousness is not fundamental in the strong form of fundamentalism, i.e., some forms of consciousness may still exist despite the possible disintegration of its unity, which seems to happen in states like schizophrenia, sleep onset, or a minimally conscious state. For instance, the stream of consciousness may occasionally undergo a sudden, unpredictable alteration in terms of inner speech and imagery (Noreika et al. 2014). Nevertheless, if one aims to explain the neural mechanisms of normal waking consciousness, the unity thesis, with its NCC-related implications, cannot be ignored. With these considerations in mind, let us examine now the proposal of global gamma synchronisation as the NCC (Singer this collection).