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.
