Many scientists ask us “What is the meaning of consciousness?”
That is an historical question, because none of the empirical sciences started with adequate definitions.
Scientific concepts like “heat,” “force” and “momentum” evolved over long periods of time, inductively, step by step, based on useful and reliable observations. A failure to understand this plain and obvious point about the history of science makes it impossible to pursue a rational approach to the unknowns of biological consciousness. We cannot skip the long process of inductive information gathering and simply leap to an answer by faith.
As pointed out, the thermodynamic definition of heat only emerged in the late 19th century. Beginning with the Renaissance, scientists from Galileo to Fahrenheit developed increasingly precise ways to measure “heat.” But without 19th century discoveries in the physics of molecular motion and the Kelvin scale, a clear theoretical definition was out of reach.
This basic point about inductive science is often misunderstood. Empirical science commonly starts with observational definitions that seem roughly correct. Refined theory tends to come much later.
Part IV of “On Consciousness: Science & Subjectivity” shows more than twenty reliable findings about the state of consciousness and its reportable contents. Each of these facts could be used as observational definitions.
In practice, we generally start with a specific behavior — the ability to accurately report stimuli or evoked mental events under careful experimental conditions. (see 1.4 below)
Today we know many “neural correlates” of consciousness. None of them are complete, but they give useful starting points.
(Emerging theoretical definitions. Note that theoretical definitions of the conscious state and its contents have been proposed. See W.J. Freeman, M. Steriade, B.J. Baars, G. Tononi, G.M. Edelman, and others.)
“Accurate Report” as an Observable Index
The most widely used behavioral index of conscious events is “accurate, voluntary report” under optimal conditions, such as minimal distraction and time delay. Reportability corresponds well with our everyday understanding. Indeed, the words “accurately reportable” can often be used instead of “conscious.” However, a purely behavioral index would cause us to miss something essential, the fact that accurate reports refer to a rich Umwelt of experiences that we all share.
In the sensory sciences accurate report has long been fruitful, beginning with Newton’s prism experiments, and still in general use today. The study of color perception would have been impossible without accurate report. Clinical examinations of vision and hearing continue to rely on accurate report.
Failures of accurate report are just as important, as in the case of various types of color blindness. People who lack one, two or three types of retinal color receptors selectively confuse three-color with two-color pictures, and two-color pictures with grayscale copies. Just as perceptual difference reports are fundamental in psychophysics, equivalence reports or confusability between two physically different but subjectively identical stimuli has long been important in perception and cognition.
Improving Report Measures
Observational definitions do not remain static. Improved measures often reveal unexpected facts, which may require a basic rethinking of the proposed concept. The history of science is filled with examples. Our understanding of conscious states and contents is evolving in precisely that way.
L. Jacoby and colleagues pioneered the use of “process dissociation,” as a measure of conscious control. In process dissociation, subjects are instructed to try to stop automatic (over-learned) mental events, such as “word reading” rather than “color naming” in the classical Stroop task. Since subjects are skilled readers, the novel task of colornaming requires voluntary effort, to overcome automatic habits of reading for sound and meaning. Such interference effects occur whenever an overlearned skill can be pitted against a new task. Errors and delays in performing the new task are thought to reflect a drop in “consciously-mediated” control. Process dissociation bears on a major theoretical question, the interplay of conscious experiences with voluntary control.
The most dramatic empirical improvements have come with the brain imaging revolution. Recording techniques continue to improve year by year, with significant advances in our understanding of the conscious brain.
Contrastive Analysis
There are two ways to study conscious experiences experimentally. One is to compare them to each other, as in the sensory sciences. Content comparisons are used routinely in perception, recognition memory, mental imagery, short-term memory, and the like.
A more recent approach is to compare conscious events with closely matched unconscious analogues. This approach dates back several decades, when researchers discovered convincing evidence for unconscious but “intelligent” brain events, including sensory processing, memory maintenance, automaticity of complex skills, etc.
A simple example is to say a word mentally, and then let it fade; for about ten seconds afterwards, it can still be recalled. (The reader is encouraged to try this several times.) Our ability to retrieve the word after fading suggests that an unconscious memory of the word must have been preserved. This informal experiment provides conscious/unconscious comparison conditions, even for a single word. Now we can try to answer the question, “What is the brain effect of our being conscious of a word?”
In effect, we have a controlled experiment using conscious access as an independent variable. Brain recordings show clear differences. Numerous experiments like this have been published, over a range of phenomena. They give the most relevant body of evidence about conscious experience “as such.”
For an anatomical example, the human cerebellum, which has about the same number of neurons as cortex, does not seem to support conscious contents directly. Patients with damage to the cerebellum are still conscious of the same range of contents as before.
However, very local damage to sensory cortex can produce very specific deficits in conscious experience, such as cortical color blindness and face blindness — the inability to recognize a visual pattern as a face, in spite of being able to name eyes, noses, mouths, chins, and so on. Specific kinds of cortical blindness can be localized to specific regions of cortex, notably V3/V4 for color perception, and area IT (inferotemporal) for face and object identity. These phenomena have been replicated in the rhesus macaque, which has a strikingly similar visual brain to ours. Depending on the specific aspect of consciousness being studied, an experimental comparison may show two or more values; it is often maintained that conscious events have high underlying dimensionality.
The classical dimensions of color perception are one example, but visual perception in the natural world must have much higher dimensionality.
These are testable questions.
A Growing Set of Brain Correlates
Part 4 of “On Consciousness: Science & Subjectivity” shows 22 empirical correlates of conscious states and their specific contents. These continue to be refined, clarified and expanded.
Historically, scalp EEG of the waking state was described as “irregular, low in amplitude, and fast.” By “irregular” one meant something close to random. When scalp EEG was averaged over multiple samples it reliably added up to zero volts. Yet scalp EEG is still a useful first index for defining waking consciousness in medicine and physiology.
Nevertheless, direct cortical recording improves the signal-to-noise ratio by a factor of 1,000. These “intracranial” recordings reveal a very different picture of cortical signaling, between specific arrays of neurons signaling via neurons spike firing, or by population oscillations ranging from <.1 to 200 Hz.
Visual cortex has more than 40 “visuotopical” arrays, ranging from V1, which resembles a pixelated screen with high spatial resolution, to area IT (inferotemporal) with much less spatial resolution and much higher gestalt organization. In V1 the retinal input from a human face is simply a 2D distribution of colored and grayscale pixels. About forty maps later, in IT, that input is seen in terms of facial features — mouth, nose, eyes, ears, facial expressions and individual faces.
To a first approximation, the cortex is a vast collection of spatially organized neuronal arrays with six histologically different layers. Cortical arrays begin in sensory two-dimensional receptor arrays like the retina, and are ultimately converted to motor signals that trigger arrays of muscle cells. Signaling between arrays are often “point to point” so that a foveal neuron (x1 , y1) in the retina corresponds well to a similarly located thalamic neuron (x2 , y2) in LGN, followed by some 40 corresponding cells in cortical arrays V1 – V4 and ultimately areas IT and MTL. Each layered array sends axons to both higher and lower cells in other arrays. This bidirectional signaling gives rise to resonant excitatory activity, the typical signaling style of cortex.
The broad preservation of visuotopical and spatiotopical patterning across more than 40 visual maps is called labeled line coding. Signaling between these broadly similar arrays involves both neuronal spike firing and population oscillations, from <0.1 to 200 Hz. In general, cortical “close-up” recordings look radically different from traditional scalp EEG.
In retrospect, scalp EEG is misleading for detailed studies of cortex, since it suffers a thousand-fold loss of signal voltage compared to direct brain recordings. Direct recordings also solve potential artifacts of scalp EEG, such as scalp muscle activity, eye movement artifacts and poor source localizability. Such methods have yielded a harvest of new insights.
Independent variables are also much improved. Binocular rivalry is a classical method for comparing closely matched conscious vs un conscious retinal input. Thanks to three decades of rivalry studies in the macaque, we can now compare the fate of two visual input streams, one that is reportable, while the other is not. These studies show that the conscious stream shows significantly higher cortical activation, more oscillatory phase-linking, and wider spatial propagation than closely matched uncs stimuli.
Conscious stimulus processing propagates not only within the visual cortex, but also to prefrontal and plausibly hippocampal regions. Direct brain recordings converge well with other techniques like fMRI, PET, MEG, etc. Compared to the conscious processing stream, matched unconscious events are not voluntarily reportable; they evoke less neuronal activity; show less oscillatory phase-linking; and tend to decay locally in visual cortex.
Several other experimental methods have been used, like visual backward masking, the attentional blink, inattentional blindness, and selective listening. There is reasonable agreement between stimulation and recording methods.
[Excerpted from Part I – Observational Definitions of Consciousness in “On Consciousness: Science & Subjectivity.”]
Global Workspace Theory (GWT) began with this question: “How does a serial, integrated and very limited stream of consciousness emerge from a nervous system that is mostly unconscious, distributed, parallel and of enormous capacity?”
GWT is a widely used framework for the role of conscious and unconscious experiences in the functioning of the brain, as Baars first suggested in 1983.
A set of explicit assumptions that can be tested, as many of them have been. These updated works by Bernie Baars, the recipient of the 2019 Hermann von Helmholtz Life Contribution Award by International Neural Network Society form a coherent effort to organize a large and growing body of scientific evidence about conscious brains.
