Dr. Patricia Sharp's work at the JP Scott Center for Mind, Brain and Behavior in the Department of Psychology, Bowling Green State University, was discussed in an earlier blog "Persistent meditative states - how? psychedelics - how? recent presos". At the Towards a Science of Consciousness Conference this spring we discussed how the brain maintains the persistent blissful awareness that occurs in persistent, deep meditative states.
Normally, pleasures "saturate" with time, typically a minute or two. How do the great pleasures of "thought-free" meditation, presumably operating by the same dopamine/opiod neurochemistry system that produces other pleasures, persist, and do not saturate. Patricia's paper developed the idea that our repetitive thought patterns are a form of addiction. If we reduce the internal narrative, the dopamine down-regulation/reduction apparently does not occur. Instead of an endlessly repetitive stream of thoughts, the brain is engaged in a dance of open awareness with its continually changing show, and all of its concomitant neurotransmitter-induced pleasure continues.
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| Donald O. Hebb |
This paper starts with discussion of the concept that suffering, which involves fears, aversions, compulsions, etc., is caused by perceptions and thoughts that are fabrications of our own minds caused by ignorance regarding the true nature of reality.
In Buddhist philosophy, these tendencies reside in "Store consciousness", which was formulated long before current cognitive neuroscience. Patricia suggests that "Store consciousness" corresponds to the neuroscientific understanding of Hebbian synaptic "weights". These weights are strong synaptic connections built-in through experience. Transforming these stored Hebbian synaptic weights reduces suffering.
The earlier blog "Painful Memories, - Meditative Approaches and Neuroscience", showed how Donald Hebb, in 1949 in "The Organization of Behavior", demonstrated that memories are formed when a stimulus activates the linkage from one neuron to a useful, functionally-related second neuron. As these neurons fire together, "Hebbian theory" operates, i.e. "neurons that fire together, wire together".
This theory also relates to "plasticity" at the synapses between neurons. The efficiency of transmission across the synapses increases with "repeated and persistent" stimulation. The more times these neurons fire together, or in near "synchrony", the stronger this firing pattern becomes. Neuroscientists call this "long term potentiation" (LTP); we call it "memory".
Patricia defines "attractor networks" which form around specific experiences, emotions, etc., as "various subsets of neurons are strongly interconnected, and these strong mutual connections lead to repeated co-activation of the involved neurons." These networks subsequently "attract" similar experiences, emotions, capabilities, etc., into firing together to regenerate the original experiences, emotions, capabilities, etc.
As Patricia points out "...as we look into ourselves to find the solid, inner rock of competence or successfulness, all we find is a series of repetitive self-referential thoughts. When we cling to the concept of self in this way, by repeating these thoughts, it causes rigidity in our actions as well as fear, jealousy and aggression toward others ".
There is, however, some good news. Bear, M. (2003) in "Bidirectional synaptic plasticity: from theory to reality", Philosophical Transactions of the Royal Society of London, and others, found that synapses that are active, but do not succeed in firing to create the link to the next neuron, will be weakened by a process called long-term depression (LTD).
Patricia has some very useful diagrams to explain this process, using visual stimuli and their action on the sensory cortex. A square is seen, for the first time, and it forms an image on the retina, which is then subsequently represented by the three neural elements in the sensory cortex.
Note that many of the initial neural elements and stimuli do not connect successfully to form an active attractor network and so are not fired in later repetition of the same/similar square. Thicker arrows in the "later repetition" demonstrate a strengthened "Hebbian" network as the "neurons that fire together, wire together".
Patricia further visualized the "attractor network" as being able to reproduce the square if only a part of the image was presented, as the representation of the whole image was "encoded" in the network. This happens "all the time" in everyday life. It is what we evolved to rapidly identify small fragments of images, like some brown hair in the bushes around the watering hole, as a lion.
The big step is how our brains tie the sensory input to emotions, reaction, "self", and craving. For this, Patricia uses a "higher level" conceptual network which processes more complex information related to concepts about the self, and emotions.
The model neurons in the "associative cortex" are connected to cells in the nucleus accumbens which generate dopamine to orchestrate the reward and craving at the behavioral and experiential levels. There is evidence that "connections between accumbens cells and other cognitive and sensory brain regions can be built up during experience, via "Hebbian" plasticity mechanisms." (Hyman, et al. (2006) "Neural mechanisms of addiction: The role of reward-related learning and memory", Annual Review of Neuroscience.)
Activation of the associative cortical cells which represent the autobiographical thoughts induces activity in the accumbens cells which produce some mix of pleasure and craving from dopamine to generate "intense behavioral activation and reward seeking".
All of this requires some way to synchronize these activities. The timing must be very precise, as for optimal Hebbian plasticity, there is a window of only a few milliseconds. Just does how does the brain do this? Well, that's where gamma-frequency oscillations produced in meditation come in. Gamma oscillations equal the 1 - 3 milliseconds required. This close synchrony across brain regions has been found in olfactory, auditory and somatosensory cortex, motor systems, and subcortical areas.
There is still the issue of how increased, enhanced awareness to new stimuli is maintained neurochemically while repetitive thought patterns are not occurring. Well, Giocomo, L.M. & Hasselmo, M.E. (2007) in "Neuromodulation by glutamate and acetylcholine can change circuit dynamics by regulating the relative influence of afferent input and excitatory feedback," in Molecular Neurobiology, demonstrated just that.
They found that acetylcholine consistently, throughout the cortex, serves to amplify the strength of the incoming sensory inputs, while dampening the strength of the recurrent, repeatedly firing, connections. When cortical acetylcholine levels are high, there is heightened responsiveness to ongoing sensory stimuli, with reduced interference from previously-learned memories/thoughts. This enhances both processing and learning of new incoming sensory inputs. Acetycholine levels, like gamma waves, are abnormally high during meditation.
Meditation is typically accompanied by strong oscillatory signals within each of the theta/alpha (4–9 hertz) and gamma ( >30 hertz) frequency ranges. This occurs as the fast gamma oscillations are nested within and "carried" by the slower theta/alpha oscillations for broader communication. (Lutz, et. al, (2004) "Long-term meditators self-induce high-amplitude gamma synchrony during mental practice", Proceedings of the National Academy of Science.)
As Patricia says "Hebbian connections responsible for conditioned craving, or perhaps those responsible for learned aversions and fears, could be transformed during meditation practice, so that these conditioned networks loosen their grip on our emotions and behavioural reactions...the combination of high acetylcholine and gamma set the conditions for neural plasticity (LTP and LTD)...allow for a gentle undoing of...some of the learned patterns..."
This is an interesting paper, and if you are so inclined, there is much discussion on Buddhist philosophy. Thanks Patricia.
The big step is how our brains tie the sensory input to emotions, reaction, "self", and craving. For this, Patricia uses a "higher level" conceptual network which processes more complex information related to concepts about the self, and emotions.
The model neurons in the "associative cortex" are connected to cells in the nucleus accumbens which generate dopamine to orchestrate the reward and craving at the behavioral and experiential levels. There is evidence that "connections between accumbens cells and other cognitive and sensory brain regions can be built up during experience, via "Hebbian" plasticity mechanisms." (Hyman, et al. (2006) "Neural mechanisms of addiction: The role of reward-related learning and memory", Annual Review of Neuroscience.)
Activation of the associative cortical cells which represent the autobiographical thoughts induces activity in the accumbens cells which produce some mix of pleasure and craving from dopamine to generate "intense behavioral activation and reward seeking".
All of this requires some way to synchronize these activities. The timing must be very precise, as for optimal Hebbian plasticity, there is a window of only a few milliseconds. Just does how does the brain do this? Well, that's where gamma-frequency oscillations produced in meditation come in. Gamma oscillations equal the 1 - 3 milliseconds required. This close synchrony across brain regions has been found in olfactory, auditory and somatosensory cortex, motor systems, and subcortical areas.
There is still the issue of how increased, enhanced awareness to new stimuli is maintained neurochemically while repetitive thought patterns are not occurring. Well, Giocomo, L.M. & Hasselmo, M.E. (2007) in "Neuromodulation by glutamate and acetylcholine can change circuit dynamics by regulating the relative influence of afferent input and excitatory feedback," in Molecular Neurobiology, demonstrated just that.
They found that acetylcholine consistently, throughout the cortex, serves to amplify the strength of the incoming sensory inputs, while dampening the strength of the recurrent, repeatedly firing, connections. When cortical acetylcholine levels are high, there is heightened responsiveness to ongoing sensory stimuli, with reduced interference from previously-learned memories/thoughts. This enhances both processing and learning of new incoming sensory inputs. Acetycholine levels, like gamma waves, are abnormally high during meditation.
Meditation is typically accompanied by strong oscillatory signals within each of the theta/alpha (4–9 hertz) and gamma ( >30 hertz) frequency ranges. This occurs as the fast gamma oscillations are nested within and "carried" by the slower theta/alpha oscillations for broader communication. (Lutz, et. al, (2004) "Long-term meditators self-induce high-amplitude gamma synchrony during mental practice", Proceedings of the National Academy of Science.)
As Patricia says "Hebbian connections responsible for conditioned craving, or perhaps those responsible for learned aversions and fears, could be transformed during meditation practice, so that these conditioned networks loosen their grip on our emotions and behavioural reactions...the combination of high acetylcholine and gamma set the conditions for neural plasticity (LTP and LTD)...allow for a gentle undoing of...some of the learned patterns..."
This is an interesting paper, and if you are so inclined, there is much discussion on Buddhist philosophy. Thanks Patricia.
wonderful work and very informative .... we call it bliss :-)
ReplyDeleteHi Unknown,
DeleteGreat that you found it so useful. Yes, "bliss" is the most frequently used word to describe it.
The Ribhu Gita, which is covered in my recent book, "Evolving Beyond Thought: Updating Your Brain's Software" states "Bliss is the primary measure".
FYI, EBT is available on Amazon and free download @ https://www.scribd.com/document/390031187/Evolving-Beyond-Thought-Final . Blogpost @ "Updating Your Brain's Software".
Trust this is useful.
stillness