[ Pobierz całość w formacie PDF ]
.This week will cover neural and molecularcorrelates of consciousness, anesthesia, and how/where quantummechanisms could beneficially operate in the brain.Layers of brain organization relevant to consciousnessAttempts to approach consciousness often view the brain as a hierarchicalsystem, comprised of layers of organization with bottom-up, as well as top-down feedback.In particular Alwyn Scott has elucidated such hierarchicalorganization, most notably in his book "Stairway to the Mind".In this view consciousness emerges as a novel property at an upper level of the hierarchyfrom nonlinear interactions among layers.The bottom level in Al Scott s scheme (and that of most others) is themembrane protein, implying ion channels and receptors as the fundamentalunits of information processing and representation.However this cutoff atthe level of neurons or membrane proteins is arbitrary.Since consciousnessis not understood we may need to go deeper.Furthermore the standarddogma in neuroscience---neurons or synapses as fundamental bits---is aimedat simplification to allow easier understanding, specifically to make the brainmore like something with which we are familiar (e.g.a classical computer).These prevalent models are based on unfounded assumptions, as discussedbelow.Figure 1 illustrates a more complete hierarchical scheme.Figure 1.Hierarchical levels of brain organization.Dotted line indicatescutoff for conventional approaches.Since we can t measure or directly observe consciousness, how can wedetermine which levels/structures/activities may be related to consciousness?We can ask in two ways: a) what are the levels/structures/activitiesseemingly most active in cognitive functions often associated withconsciousness (attention, learning, perception, volition etc,---the neuralcorrelate of consciousness ), and b) what structures/activities mediate theabsence of consciousness (e.g.anesthesia).At the systems level the current leading candidate for the "neural correlate"of consciousness involves neuronal circuits oscillating synchronously inthalamus and cerebral cortex.It has been known since the pioneering workof Herbert Jasper in the 1950 s that sensory input passes through thethalamus where it is "broadcast" to cortex (e.g.the lateral geniculate nucleusin thalamus mediates visual information from the optic nerves; theinformation is then conveyed to visual cortex).Some thalamic-corticalprojections carry specific sensory modalities whereas others are non-specific, but necessary for arousal and consciousness.The "reticularactivating system" also passes through thalamus.In recent decades numerous studies have also revealed extensive downwardprojections from cortex to thalamus.A consensus view has emerged inwhich reverberatory feedback ("recurrent loops") between thalamus andpyramidal cell neurons in cortex provides the "neural correlate of consciousness" (e.g.Bernie Baars "global workspace").Electrophysiologicalrecordings further reveal coherent firing of thalamo-cortical loops withfrequencies varying from slow EEG frequencies (2 - 12 Hz) to rapid gammaoscillations in the 40 Hz range and upward.Coherent gamma frequencythalamo-cortical oscillations (collectively known as "coherent 40 Hz") aresuggested to mediate temporal binding of conscious experience (e.g.Singeret al 1990; Crick and Koch, 1990; Joliot et al, 1994; Gray, 1998).Theproposals vary, for example as to whether coherence originates in thalamusor resonates in cortical networks, but reverberatory loops of "thalamo-cortical 40 Hz" activity stands as a prevalent view of the neural-levelsubstrate for consciousness (e.g.Baars, 1988).Figure 2.Left: Schematic of thalamocortical loops (from Newman, 1997).Right: Occurrence of high density of gap junctions in mammalian brain(Micevych and Abelson, 1991).Gap junctions are found throughout thebrain, but in particularly high density in thalamus and cortex.The prevalent view is that the neuronal network circuits involved incognition and consciousness (e.g.thalamo-cortical loops) are selected andmaintained through one particular type of neural-neural interaction: axonal-to-dendritic chemical synaptic transmission.The general idea is that theterminal axon of "Neuron A" releases neurotransmitter which then binds to apost-synaptic receptor on a dendritic spine on "Neuron B".This changesNeuron B s local dendritic membrane potential which interacts with those onneighboring spines and dendrites on Neuron B and can reach threshold totrigger axonal depolarizations ("firings", or "spikes") down Neuron B saxon [ Pobierz całość w formacie PDF ]
zanotowane.pl doc.pisz.pl pdf.pisz.pl milosnikstop.keep.pl
.This week will cover neural and molecularcorrelates of consciousness, anesthesia, and how/where quantummechanisms could beneficially operate in the brain.Layers of brain organization relevant to consciousnessAttempts to approach consciousness often view the brain as a hierarchicalsystem, comprised of layers of organization with bottom-up, as well as top-down feedback.In particular Alwyn Scott has elucidated such hierarchicalorganization, most notably in his book "Stairway to the Mind".In this view consciousness emerges as a novel property at an upper level of the hierarchyfrom nonlinear interactions among layers.The bottom level in Al Scott s scheme (and that of most others) is themembrane protein, implying ion channels and receptors as the fundamentalunits of information processing and representation.However this cutoff atthe level of neurons or membrane proteins is arbitrary.Since consciousnessis not understood we may need to go deeper.Furthermore the standarddogma in neuroscience---neurons or synapses as fundamental bits---is aimedat simplification to allow easier understanding, specifically to make the brainmore like something with which we are familiar (e.g.a classical computer).These prevalent models are based on unfounded assumptions, as discussedbelow.Figure 1 illustrates a more complete hierarchical scheme.Figure 1.Hierarchical levels of brain organization.Dotted line indicatescutoff for conventional approaches.Since we can t measure or directly observe consciousness, how can wedetermine which levels/structures/activities may be related to consciousness?We can ask in two ways: a) what are the levels/structures/activitiesseemingly most active in cognitive functions often associated withconsciousness (attention, learning, perception, volition etc,---the neuralcorrelate of consciousness ), and b) what structures/activities mediate theabsence of consciousness (e.g.anesthesia).At the systems level the current leading candidate for the "neural correlate"of consciousness involves neuronal circuits oscillating synchronously inthalamus and cerebral cortex.It has been known since the pioneering workof Herbert Jasper in the 1950 s that sensory input passes through thethalamus where it is "broadcast" to cortex (e.g.the lateral geniculate nucleusin thalamus mediates visual information from the optic nerves; theinformation is then conveyed to visual cortex).Some thalamic-corticalprojections carry specific sensory modalities whereas others are non-specific, but necessary for arousal and consciousness.The "reticularactivating system" also passes through thalamus.In recent decades numerous studies have also revealed extensive downwardprojections from cortex to thalamus.A consensus view has emerged inwhich reverberatory feedback ("recurrent loops") between thalamus andpyramidal cell neurons in cortex provides the "neural correlate of consciousness" (e.g.Bernie Baars "global workspace").Electrophysiologicalrecordings further reveal coherent firing of thalamo-cortical loops withfrequencies varying from slow EEG frequencies (2 - 12 Hz) to rapid gammaoscillations in the 40 Hz range and upward.Coherent gamma frequencythalamo-cortical oscillations (collectively known as "coherent 40 Hz") aresuggested to mediate temporal binding of conscious experience (e.g.Singeret al 1990; Crick and Koch, 1990; Joliot et al, 1994; Gray, 1998).Theproposals vary, for example as to whether coherence originates in thalamusor resonates in cortical networks, but reverberatory loops of "thalamo-cortical 40 Hz" activity stands as a prevalent view of the neural-levelsubstrate for consciousness (e.g.Baars, 1988).Figure 2.Left: Schematic of thalamocortical loops (from Newman, 1997).Right: Occurrence of high density of gap junctions in mammalian brain(Micevych and Abelson, 1991).Gap junctions are found throughout thebrain, but in particularly high density in thalamus and cortex.The prevalent view is that the neuronal network circuits involved incognition and consciousness (e.g.thalamo-cortical loops) are selected andmaintained through one particular type of neural-neural interaction: axonal-to-dendritic chemical synaptic transmission.The general idea is that theterminal axon of "Neuron A" releases neurotransmitter which then binds to apost-synaptic receptor on a dendritic spine on "Neuron B".This changesNeuron B s local dendritic membrane potential which interacts with those onneighboring spines and dendrites on Neuron B and can reach threshold totrigger axonal depolarizations ("firings", or "spikes") down Neuron B saxon [ Pobierz całość w formacie PDF ]