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Thursday, June 21, 2018

Consciousness Explanation. Part II


In an earlier post, “Where Neuroscience Stands in Understanding Consciousness,” I presented a summary of the progress occurring in neuroscientific understanding of consciousness (https://www.psychologytoday.com/us/blog/memory-medic/201804/where-neuroscience-stands-in-understanding-consciousness).

Now a recent report in the May issue of Science adds to a growing understanding of how the brain generates conscious recognition. The study examined neural impulse discharge responses of monkey brain to visual stimuli. Electrodes were implanted in the four visual cortex areas that are sequentially activated by visual stimuli. The stimulus was a circular spot of varying contrast in the lower left area of the visual field. Monkeys were cued when a stimulus was delivered, though whether they saw it or not varied with the spot’s contrast against the visual background. Monkeys were trained to report when they knew they saw the spot by shifting  their gaze from a central fixation point to the spot’s prior location some 450 msecs earlier. Monkeys reported unrecognized spots by shifting the gaze to the right of the default fixation point. Investigators imposed the delay for reporting to eliminate the response being a simple reflex saccade. A longer delay would have been more convincing, but it might have taxed the monkey’s working memory and easy distractibility.

As expected, spots of sufficient contrast evoked impulse discharge in each of the four visual cortex regions. Whether or not the monkey reported actually seeing the spot depended on whether there was also increased impulse discharge in the region of frontal cortex that had implanted electrodes. No doubt, other non-monitored frontal areas might also have been activated under conditions where recognition was reported. The point is that conscious recognition requires activation of widely separated brain areas at the same time.

Not demonstrated here is how the frontal activity is interacting with activity in the visual cortex areas, but that certainly could be predicted from studies in my lab, reported in 2000. We showed that conscious realization of alternate perceptions of ambiguous figures in humans occurred when brain electrical activity (EEG) over the visual areas of scalp became highly synchronized, over a wide range of frequencies with multiple frontal areas, both in the same and even opposite hemisphere. Figure 1 shows the topography of coherence change at the moment of realization for the upper frequency band of 25-50Hz.




Figure 1. Topographic summary of p<0.01-level coherence increases across all 10 ambiguous figures, all subjects, in the 25-50 Hz band. Each square matches a given electrode and shows how activity at that location became more coherent with activity at other locations at the instant the subject consciously realized the alternate perception in any of 10 ambiguous figures. From Klemm et al. (2000). Widespread coherence increases were also seen in the band below 25 Hz.

Thus, it seems that a meaningful detectable signal, which need not be limited to vision, not only activates its immediate neural targets, but those target cells can trigger feed-forward to trigger activity in more frontal areas. Feedback from those frontal areas can set up time-locked oscillatory coupling across wide expanses of cortex that is apparently necessary for conscious recognition. The time locking probably amplifies the signals to the threshold for conscious realization.

The distributed signal processing does not necessarily mean consciousness requires huge expanses of neural tissue. Recall from the split-brain studies in Roger Sperry’s lab that even half a brain can be fully conscious of the stimuli it can receive. The magic of consciousness seems to lie in the qualitative nature of data sharing, not in the volume of tissue involved.
Thus, the major issue is how oscillatory coupling of otherwise isolated circuitry amplifies signals to become consciously recognized. “Amplify” may be a misleading word, inasmuch as there is no compelling evidence as yet that consciousness is related to having more nerve impulses per unit of time. The impulses certainly don’t get bigger, because their voltage magnitude is constrained by concentration and electrostatic gradients. Rather, the secret may lay in the controlled timing of impulses. A likely form of amplification results from the reverberation of activity among coherent neuronal ensembles, which could have the effect of sustaining the stimulus long enough to be consciously detected, that is, for the brain to “see” what the eyes were looking at.

Consciousness may also simply be a matter of improving the signal-to-noise ratio. Time-locked, reverberating activity should be more isolated and protected from random activity which is unreliably associated with a given stimulus. Intuitively, that is what we sense in daily experiences. When we look at a tree, the cognitive noise of the multitude of tree signals may obscure our seeing the bird in the tree until, by accident or intent, we are able to see the bird.

This still leaves us with an incomplete answer. What is it about amplifying or reducing background noise that makes stimuli consciously recognizable? Where is the “who” in the brain that does the realizing? When my brain sees or hears something, it is “I” who consciously see or hear it. How does my brain create my “I” and where in my brain is my “I?” One possibility is that the unconscious brain can release a set of unique network activity that operates much like an avatar, giving brain a functionality it otherwise would not have. I elaborated this idea in my post, “The Avatar Theory of Consciousness” (https://www.psychologytoday.com/us/blog/memory-medic/201506/the-avatar-theory-consciousness).

How does this avatar “I” find a stimulus that it recognizes? Is it searching for it, like a searchlight scanning across the cortex for stimulus-induced activity? Or maybe it is not “looking for” sensation but rather is triggered into temporary existence when a stimulus acquires the needed threshold to launch consciousness. The monkey experiments support the latter option. However a stimulus becomes recognized, the awareness may outlast the triggering. We often consciously think about the meaning of a momentary stimulus, integrate it with memories, and develop beliefs, intentions, and responses, either cognitive or behavioral or both.

One more thing needs mention. In the monkey experiments, it was clear that the monkeys were continuously awake, even when they were not detecting presented stimuli. Thus, being awake is not the same as being conscious. We know this also from human experiments on inattentional blindness, which reveal that consciousness depends on selective attention. Wakefulness is a necessary condition for consciousness but not, by itself, sufficient.

For more explanation of brain function, see my book:

Sources

Van Vugt, Bram, et al. (2018). The threshold for conscious report: signal loss and response bias in visual and frontal cortex. Science. 360 (6388), 537-542.

Klemm, W. R., Li, T. H., and Hernandez, J. L. (2000). Coherent EEG indicators of cognitive binding during ambiguous figure tasks. Consciousness & Cognition. 9, 66-85.

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