A unifying theory of general anesthetic-induced unconsciousness must explain the common mechanism through which various anesthetic agents produce unconsciousness. Functional-brain-imaging data obtained from 11 volunteers during general anesthesia showed specific suppression of regional thalamic and midbrain reticular formation activity across two different commonly used volatile agents. These findings are discussed in relation to findings from sleep neurophysiology and the implications of this work for consciousness research. It is hypothesized that the essential common neurophysiologic mechanism underlying anesthetic-induced unconsciousness is, as with (...) sleep-induced unconsciousness, a hyperpolarization block of thalamocortical neurons. A model of anesthetic-induced unconsciousness is introduced to explain how the plethora of effects anesthetics have on cellular functioning ultimately all converge on a single neuroanatomic/neurophysiologic system, thus providing for a unitary physiologic theory of narcosis related to consciousness. (shrink)

One method for investigating the neurobiology of consciousness is to experimentally manipulate consciousness as a variable and then visualize the resultant functional brain changes with advanced imaging techniques. To begin investigation into this area, healthy volunteers underwent positron emission tomography scanning while listening to randomized word lists in both conscious and unconscious conditions. Following anaesthesia, subjects had no explicit memories. Nonetheless, subjects demonstrated implicit memory on a forced-choice test . These subsequent memory scores were correlated with regional brain metabolism measured (...) during encoding. Conscious recall and unconscious recognition memory demonstrated considerable anatomic overlap in correlating significantly with relative glucose metabolism in auditory verbal memory areas. However, only conscious recall correlated with mediodorsal thalamic nucleus activity; unconscious recognition memory did not. These findings suggest that conscious versus unconscious memory distinctions may depend on the presence or absence of correlated thalamic activity, and directly implicate the thalamus in the neurobiology of consciousness. (shrink)

The neural correlates of rapid eye movements (REMs) in sleep are extraordinarily robust; including REM-locked activation in the retrosplenial cortex, the supplementary eye field and areas overlapping cholinergic basal nucleus. The phenomenology of REMs speaks to the notion that perceptual experience in both sleep and wakefulness is a constructive process – in which we generate predictions of sensory inputs and then test those predictions through actively sampling the sensorium with eye movements. On this view, REMs during sleep may index an (...) internalized active sampling or ‘scanning’ of self-generated visual constructs that are not constrained by visual input. If this view is correct, it renders REMs an ideal probe to study consciousness as “an exclusively internal affair” (Metzinger 2009). In other words, REMs offer a probe of active inference – in the sense of predictive coding – when the brain is isolated from the sensorium in virtue of the natural blockade of sensory input during REM sleep. REMs are temporally precise events that enable powerful inferences based on timeseries analyses. As a natural, task-free probe, [REMs] could be used in non-compliant subjects, including infants and animals. In short, REM constitutes a promising probe to study the ontogenetic and phylogenetic development of consciousness and perhaps its abnormal development in schizophrenia and autism, which have been considered in terms of aberrant predictive coding. (shrink)