E.Roy John, New York University School of Medicine
Brain Research Reviews, 39, (2002) pp. 1-28
E. Roy John considers that there is a requirement for a mechanism to synchronise sensory elements in different modalities. He proposes that the phase-locked oscillatory 30-80 Hz gamma synchrony within the cortex, and also between the cortex and the thalamus could be critical for binding together different modalities and areas of the brain. It has been postulated that there is a resonance between the cortex and the thalamus (effectively a relay station for sensory inputs on their way to processing in the cortex), through activation of pyramidal cells in layer IV of the cortex, with the resonance returning to the thalamus via layer VI. These cortico-thalamic loops acquire coherent oscillations, and are proposed to bind together the otherwise fragmented sensory information from the external world. Perception is suggested to involve the integration of specialised brain areas. Even the perception of a self may arise from the integration of different brain areas.
Gamma oscillation is detected synchronously in different sensory modalities. The author suggests that synchronised discharges of neurons may bind representations in different modalities into a single perception. In particular, the gamma synchrony increases when there is selective attention to particular stimuli, and it has been suggested to be necessary for awareness. Similarly in sleep, there is no gamma synchrony during deep sleep, but it reappears during REM sleep, when more areas of the brain become active. Gamma waves in the cortex are influenced by the thalamus, the reticular formation and other areas. Gamma oscillations have also been observed as phase-locked between the prefrontal and the parietal. Activity in the gamma range has been suggested to underlie consciousness, and bind together the different modalities into the experience of a unified consciousness.
Studies of the reduction of brain activity produced by anaesthetic agents suggests that certain areas are most involved with consciousness, notably many areas of the prefrontal, the anterior cingulate, parietal association areas, the thalamus, the occipital cortex and parts of the temporal lobe. Loss of consciousness through anaesthetic agents involves a significant decrease in gamma and other coherence. The front and rear brain regions and the two hemispheres become disconnected. Gamma synchrony is also the first coherence to recover with return of consciousness. Loss of arousing inputs into the cortico-thalamic loops, and enhanced transmission of GABA, the inhibitory neurotransmitter, are seen as factors in loss of consciousness under anaesthesia.
The author contrasts the ground state of the brain with its more excited states. The ground state is defined here as maximum entropy. Excited states of the brain, which deviate from the ground state, give rise to synchrony, and the information in these areas is viewed as negative entropy. The fragmented information is bound into a coherent process in the thalamus, and the passage of signals from thalamus to cortex and back integrates the thalamus with the cortex. Each element of brain activity represents negative entropy across the active regions of the brain, and this negative entropy is identified with consciousness.
For a theory of consciousness, the author tends to favour the ‘dynamic core’ model postulated by Tononi and Edelman. According to their concept, there needs to be a set of spatially distributed thalamo-cortical elements that sustain consciousness, despite the constantly changing composition of neurons involved in any particular activity.
Conclusion: The paper produces a good deal of evidence for the gamma synchrony as a correlate of consciousness, and for the thalamo-cortical loops as an important aspect of that synchrony. However, correlation is not identity. In terms of classical physics, it is not clear how even a synchronised electrical potential could give rise to a property not detected in the rest of nature. For this reason, it seems more likely that the gamma synchrony and its activity in the thalamo-cortical circuits and other areas of the brain is a correlate of some underlying and possibly non-classical process. E. Roy John is himself rather less dismissive of quantum consciousness theories, such as Penrose’s, than are most other mainstream neuroscientists.Tags: gamma synchrony, modalities Posted by