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)

Abstract -/- Before Descartes, middle age philosophers like Thomas Aquinas (1225-1274), Duns Scotus (1266-1308), and Francisco Suarez (1548-1617) used to discuss the distinction between essence and existence in three ways (of course, Ibn-Sina was the first who made this distinction to rehabilitate Aristotelian philosophy in the Islamic heritage). Descartes was aware of that, but discussed it according to the relation between mind and body. Yet, he told us many times that he was used to separate essence from existence in metaphysical (...) issues. The objective of this paper is first to rephrase his untold opinion about the relation between essence and existence on the basis of his conception of the problem of distinction according to mind and body, and second, to disclose that Descartes's conception of existence is twofold. At the end, it is claimed that his twofold notion of existence and his inability to overcome it, is the principal factor in his failure to explicate everything according to first principles, and so in his failure to defeat skepticism. (shrink)

Thus far, discarding such manuscripts as are copies of printed editions, we have arrived at the following grouping: 1. The group x, in which K is a copy of A, and B of C, while A and C are brothers, the former being only slightly superior to the latter. 2. SQq. Here Q is a copy of S, whose borrowings from Politian it incorporated. Q added some readings from C or K, and scholia and other readings from a manuscript of (...) the Ee stock; and after Q had received these additions, q was in turn copied from Q. 3. IID. Mr. Allen thinks it possible that, for the Homeric Hymns, these two are not direct copies of the same manuscript; and his conclusions may be accepted as indicating their mutual kinship in regard to Callimachus also. D's lost Callimachean portion gave some readings to Politian's text of Hymn V., and is in part represented by J. Lascaris' editio princeps of all six Hymns. 4.Ee, found to be brothers. 5. The group z. Here At appears to be a brother of F; and F stands apart from the z2 section, being nearer than they to xSTLDEe. Among the z2 manuscripts, Br is a copy of I, which is itself separated from GHA; this trio seem, on the imperfect evidence at hand, to be brothers. (shrink)