Entries tagged with consciousness

Today was the second session of the Neuroinformatics 4 course that I'm taking. Each participant has been assigned some paper from this list, and we're all supposed to have a presentation summarizing the paper. We're also supposed to write a diary about each presentation and hand it in in the end, which is the reason why I'm typing this entry. I figure that if I'm going to keep a diary about this, I might as well make it public.

Session I: Global Workspace Theory. I held the first presentation, which covered Global Workspace Theory as explained by Baars (2002, 2004). You can read about it in those papers, but the general idea of GWT is that that which we experience as conscious thought is actually information that's being processed in a "global workspace", through which various parts of the brain communicate with each other.

Suppose that you see in front of you a delicious pie. Some image-processing system in your brain takes that information, processes it, and sends that information to the global workspace. Now some attentional system or something somehow (insert energetic waving of hands) decides whether that stimulus is something that you should become consciously aware of. If it is, then that stimulus becomes the active content of the global workspace, and information about it is broadcast to all the other systems that are connected to the global workspace. Our conscious thoughts are that information which is represented in the global workspace.

There exists some very nice experimental work which supports this theory. For instance, Dehaene (2001) showed experimental subjects various words for a very short while (29 milliseconds each). Then, for the next 71 milliseconds, the subjects either saw a blank screen (the "visible" condition) or a geometric shape (the "masking" condition). Previous research had shown that in such an experiment, the subjects will report seeing the "visible" words and can remember what they said, while they will fail to notice the "masked" words. That was also the case here. In addition, fMRI scans seemed to show that the "visible" words caused considerably wider activation in the brain than the "masked" words, which mainly just produced minor activation in area relating to visual processing. The GWT interpretation of these results would be that the "visible" words made their way to the global workspace and activated it. For the "masked" words there was no time for that to happen, since the sight of the masking shape "overwrote" the contents of the visual system before the sight of the word had had the time to activate the global workspace.

That's all fine and good, but Baars's papers were rather vague on a number of details, like "how is this implemented in practice"? If information is represented in the global workspace, what does that actually mean? Is there a single representation of the concept of a pie in the global workspace, which all the systems manipulate together? Or is information in the global workspace copied to all of the systems, so that they are all manipulating their own local copies and somehow synchronizing their changes through the global workspace? How can an abstract concept like "pie" be represented in such a way that systems as diverse as those for visual processing, motor control, memory, and the generation of speech (say) all understand it?

Session II: Global Neuronal Workspace. Today's presentation attempted to be a little more specific. Dehaene (2011) discusses the Global Neuronal Workspace model, based on Baars's Global Workspace model.

The main thing that I got out of today's presentation was that the brain is the idea of the brain being divisible into two parts. The processing network is a network of tightly integrated, specialized processing units that mostly carry out non-conscious computation. For instance, early processing stages of the visual system, carrying out things like edge detection, would be a part of the processing network. The "processors" of the processing network typically have "highly specific local or medium range connections" - in other words, the processors in a specific region mostly talk with their close neighbors and nobody else.

The various parts of the processing network are connected by the Global Neuronal Workspace, a set of cortical neurons with long-range axons. The impression I got was this is something akin to a set of highways between cities, or different branches of a post office. Or planets (processing network areas) joined together by a network of Hyperpulse Generators (the Global Neuronal Workspace). You get the idea. I believe that it's some sort of a small world network.

Note that contrary to intuition and folk psychology (but consistently with the hierarchical consciousness hypothesis), this means that there is no single brain center where conscious information is gathered and combined. Instead, as the paper states, there is "a brain-scale process of conscious synthesis achieved when multiple processors converge to a coherent metastable state". Which basically means that consciousness is created by various parts of the brain interacting and exchanging information with each other.

Another claim of GNW is that sensory information is basically processed in a two-stage manner. First, a sensory stimulus causes activation in the sensory regions and begins climbing up the processor hierarchy. Eventually it reaches a stage where it may somehow be selected to be consciously represented, with the criteria being "its adequacy to current goals and attention state" (more waving of hands). If it does, it becomes represented in the GNW. It "is amplified in a top-down manner and becomes maintained by sustained activity of a fraction of GNW neurons": this might re-activate the stimulus signal in the sensory regions, where its activation might have already been declining. Something akin to this model has apparently been verified in a number of computer simulations and brain imaging studies.

Which sounds interesting and promising, though this still leaves a number of questions unclear. For instance, the paper claims that only one thing at a time can be represented in the GNW. But apparently the thing that gets represented in the GNW is partially selected by conscious attention, and the paper that I previously posted about placed the attentional network in the prefrontal cortex (i.e. not in the entire brain). So doesn't the content in the sensory regions then need to first be delivered to the attentional networks (via the GNW) so that the attentional networks can decide whether that content should be put into the GNW? Either there's something wrong with this model, or I'm not understanding it correctly. I should probably dig into the references. And again, there's the question of just what kind of information is actually put into the GNW in such a manner that all of the different parts of the brain can understand it.

(Yes, I realize that my confusion may seem incongruent with the fact that I just co-authored a paper where we said that we "already have a fairly good understanding on how the cerebral cortex processes information and gives rise to the attentional processes underlying consciousness". My co-author's words, not mine: he was the neuroscience expert on that paper. I should probably ask him when I get the chance.)

http://www.cogsci.ucsd.edu/~pineda/COGS175/readings/Dietrich.pdf

It proposes that what we experience as consciousness is built up in a hierarchical process, with various parts of the brain doing further processing on the flow of information and contributing their own part to the "feel" of consciousness. It's possible to subtract various parts of the process, thereby leading to an altered state of consciousness, without consciousness itself disappearing.

The prefrontal cortex is usually associated with "higher-level" tasks, including emotional regulation, but the authors suggest that this is due to the prefrontal cortex refining the outputs of the earlier processing stages, rather than inhibiting them:

"In such a view, the prefrontal cortex does not represent a supervisory or control system. Rather, it actively implements higher cognitive functions. It is further suggested that the prefrontal cortex does not act as an inhibitory agent of older, more primitive brain structures. The prefrontal cortex restrains output from older structures not by suppressing their computational product directly but by elaborating on it to produce more sophisticated output. If the prefrontal cortex is lost, the person simply functions on the next highest layer that remains.The structures implementing these next highest layers are not disinhibited by the loss of the prefrontal cortex. Rather, their processing is unaffected except that no more sophistication is added to their processing before a motor output occurs."

Their theory is that several altered states of consciousness involve a reduction in the activity of the prefrontal cortex:

"It is proposed in this article that altered states of consciousness are due to transient prefrontal deregulation. Six conscious states that are considered putative altered states (dreaming, the runner's high, meditation, hypnosis, daydreaming, and various drug-induced states) are briefly examined. These altered states share characteristics whose proper function are regulated by the prefrontal cortex such as time distortions, disinhibition from social constraints, or a change in focused attention. It is further proposed that the phenomenological uniqueness of each state is the result of the differential viability of various [dorsolateral] circuits. To give one example, the sense of self is reported to be lost to a higher degree in meditation than in hypnosis; whereas, the opposite is often reported for cognitive flexibility and willed action, which are absent to a higher degree in hypnosis.The neutralization of specific prefrontal contributions to consciousness has been aptly called ‘‘phenomenological subtraction’’ by Allan Hobson (2001).The individual in such an altered state operates on what top layers remain. In altered states that cause severe prefrontal hypofunction, such as non-lucid dreaming or various drug states, the resulting phenomenological awareness is extraordinarily bizarre. In less dramatic altered states, such as long-distance running, the change is more subtle."

And about meditation in particular, they hypothesize that it involves a general lowered prefrontal activity, with the exception of increased activation in the prefrontal attentional network:

"It is evident that more research is needed to resolve the conflicting EEG and neuroimaging data. Reinterpreting and integrating the limited data from existing studies, it is proposed that meditation results in transient hypofrontality with the notable exception of the attentional network in the prefrontal cortex. The resulting conscious state is one of full alertness and a heightened sense of awareness, but without content. Since attention appears to be a rather global prefrontal function (e.g., Cabeza & Nyberg, 2000), PET, SPECT, and fMRI scans showed an overall increase in DL activity during the practice of meditation. However, the attentional network is likely to overlap spatially with modules subserving other prefrontal functions and an increase as measured by fMRI does not inevitably signify the activation of all of the region's modules. Humans appear to have a great deal of control over what they attend to (Atkinson & Shiffrin, 1968), and in meditation, attentional resources are used to actively amplify a particular event such as a mantra until it becomes the exclusive content in the working memory buffer. This intentional, concentrated effort selectively disengages all other cognitive capacities of the prefrontal cortex, accounting for the a-activity. Phenomenologically, meditators report a state that is consistent with decreased frontal function such as a sense of timelessness, denial of self, little if any self-reflection and analysis, little emotional content, little abstract thinking, no planning, and a sensation of unity. The highly focused attention is the most distinguishing feature of the meditative state, while other altered states of consciousness tend to be more characterized by aimless drifting."

They do not discuss permanent changes caused by meditation in the paper, but if the prefrontal cortex is involved with last-stage processing of incoming sensory data, then prefrontal regulation would fit together with meditators' reports of being able to experience sensory information in a more "raw", unprocessed form. Likewise, if the prefrontal cortex unifies and integrates information from earlier processing stages, then meditation revealing the unity of self to be an illusion would be consistent would reduced prefrontal activity.

Vipassana jhanas, or other forms of meditation aimed towards reaching enlightenment, would then somehow involve permanently reducing or at least changing the nature of prefrontal processing. Meditation practicioners speak of "the Dark Night", an intermediate stage during the search for enlightenment, which is experienced as strongly unpleasant and where "our dark stuff tends to come bubbling up to the surface with a volume and intensity that we may never have known before". This is achieved after making sufficient progress in meditation, and will continue until the practicioner makes enough progress to make it go away.

Under the model suggested by the paper, the Dark Night would then be an intermediate stage where the activity of the prefrontal cortex had been reduced/changed to such an extent that it was no longer capable of moderating the output of the various earlier emotional systems. Resolving the Dark Night would involve somehow finding a new balance where the outputs of any systems involved with negative emotions could be better handled again, but I have no idea of how that happens.

Neuroinformatics 4 seminar, session II

Interesting paper on the neuroscience of meditation