Response to Koch: Elaborations on the SCP hypothesis.

We appreciate the excellent questions raised by Koch [1] in response to our article [2] and the opportunity to elaborate on our ideas further here. Koch asked whether we propose that it is the existence of the SCP per se or the activity of associated pyramidal cells that is crucial for consciousness. We noted in the legend of Fig. 3a in the original paper: ‘We propose that long-lasting synaptic activities in superficial layers, manifesting as SCPs in surface recording or low-frequency current source density (CSD) activity in superficial layers, carry large-scale information integration in the brain and contribute directly to conscious awareness.’ So the SCP is only our measurement of the responsible neuronal activities. If clever ways are found to eliminate the SCP, while leaving all responsible neuronal communications intact, or even extract the responsible information transfer and integration relationships and implement those in artificial media [3], then the quality of consciousness would remain the same in our view. However, our interest here is in the physiological underpinnings of consciousness, starting with the human brain and the brains of those species that are most similar to ours. The SCP might be a marker of such physiological activities, as we tentatively propose. Second, Koch suggested that the information content of the SCP, and field potentials in general, is too low to support conscious experiences. ‘Think of every frame in every movie. . .each one corresponds to a discrete visual experience.’We suppose that if thatwere true, therewould never be any movies, the production of which depends precisely on the visual system’s ability to fuse adjacent stimuli into a coherent image. Existing evidence suggests that the time scale at which conscious experience seems to flow is at tens to hundreds of milliseconds [3]. Successive presentations of word stimuli get fused into a single percept if they are separated by less than 80 ms [4]. In fact, the slowness of consciousness is supported by many standard psychological paradigms, including backward masking ( 100 ms), attentional blink ( 500 ms), psychological refractory period ( 200 ms) and postdiction ( 80 ms) [5,6]. Still, the frequency range of the SCP (< 4 Hz) might seem too slow for the 80-ms-rate at which visual perception is updated. This, however, is not a problembecause negative SCP shifts in different neuronal groups can be integrated into conscious experience with temporal lags much shorter than a full SCP cycle, that is, one neuronal group can become activated and contribute to conscious perception before the activity in the previous neuronal group drops and returns to baseline. Viewed in a different way, the amount of information the brain can