Consciousness and the Brain, Stanislas Dehaene (2014)
Summary and review of the above book
INTRODUCTION: On the basis of the brain research of recent years, Dehaene describes both the extent and limitations of unconscious processing. Such processing can extend to sophisticated levels of cortical processing, such as the meaning of words. However, unconscious signals are transient and decay rapidly in the brain, while conscious signals can persist long after the original stimuli. Decision taking is argued to require such conscious stable images. The brain goes through what is described as a phase change to support these, involving substantial increases in activity, and massive pyramidal neurons in thickened areas of the cortex, some of which are specialised in the production of particular types of image.
The extent of the unconscious
The early sections of the book deal with the importance and extent of unconscious processing. Modern brain imaging has made it possible to trace the progress of subliminal or unconscious stimuli in the brain. It is argued that by determining how far unconscious processing can reach, it should be possible to define the remainder of the brain’s processing as being the exclusive role of consciousness. It has already become possible to determine which types of brain activity are only detected when they correlate with a reported conscious experience.
A very limited proportion of the stimuli available to the brain gain entry to consciousness. Experimentation indicates a definite threshold with particularly stimuli either in or out of consciousness. Visual stimuli become conscious if their duration is longer than about 50 ms. At this duration, an image is consciously experienced about half the time. Below 30 ms no subjects report a conscious image. Unconscious brain processing still responds to the emotional content of a shorter subliminal signal, but unconsciously processed stimuli do not seem to leave a trace in memory. When conscious processing is involved with particular stimuli or tasks other potentially conscious items are held in an unconscious ‘buffer’. Other stimuli, noise and distractions mean that there is a risk, increasing over time, that these signals are lost altogether. When a signal is above the threshold of consciousness there is a large change in the amount of information being processed, in addition to the appearance of subjective experience. Subjects are much better at naming, evaluating and memorising objects in conscious processing.
Prior subliminal presentation of a word speeds up its processing when the same word is subsequently brought into consciousness. This is called priming. A large part of processing in the ventral visual cortex, which is seen as being the conscious visual stream, in fact does its processing unconsciously. This includes areas involved in processing both shapes and words. An input can be unconsciously processed to an advanced level, reinterpreted in the light of past experience, amended for the blind spot and for head movements. What eventually comes into consciousness is an elaborate editing of the original stimuli.
Unattended data, such as background conversations, is still processed and can unconsciously bias a subject’s judgement. Even where participants could not consciously perceive a word, they could still unconsciously process as far as its meaning. However, when a word had to be processed according to its context, such as whether bank meant the bank of a river or a financial institution, conscious processing was required.
A brain wave called N400 because it has negative voltage and appears after 400 ms after a stimuli responds to words that are incongruous in a given context. This response occurs even for stimuli that are not consciously perceived, and the N400 wave is the same size whether the signal is conscious or unconscious. However, such unconscious processing is confined to a narrow range of the left temporal lobe. Consciousness comes into play when much larger networks including the frontal lobe become involved.
Attention is seen as determining whether unconscious stimuli are processed. The act of attending greatly enhances stimuli at the attended location or time. Unconscious stimuli are also enhanced if they happen to arrive at an attended time and place. Where there are two unconscious stimuli, there will be a preference for the one at the attended location. Some degree of evaluation, say between a value of a penny and a pound can also take place when the lower and higher value stimuli are unconscious. The ventral striatum, a region within the basal ganglia, is involved in this processing. Similarly, in trials of choice based on choosing between good and bad decks of cards unconscious responses preceded conscious awareness of which were the good and bad decks. This unconscious processing is related to activation in the ventromedial prefrontal cortex. It has also been shown that rational deliberation can sometimes lead to a more disadvantageous choice than unconscious choice. In a choice involving 12 aspects of a car purchase, subjects who were distracted from thinking much came up with a better choice than those who had been able to deliberate. It is suggested that too large a range of choices over loads the working memory. It is thus possible to compute a mix of positive and negative values unconsciously.
Neurons in the hippocampus and the cortex are active during non-dream sleep. Firing patterns are shown to replay those that happened earlier in wakefulness, for instance the action of running through a maze. The replay process is much faster than the original, and it is suggested that this speed of processing might allow for the detection of hidden regularities.
Consciousness and decision making
However, Dehaene does not see these unconscious capacities as justifying claims that everything is performed by the unconscious, and that therefore consciousness does not exist/has no role. He sees consciousness as something that evolved, and would not have been selected for if it had no function. Unconscious information is transient whereas conscious information is more stable. Consciousness can also compress a lot of data into symbols. Dehaene envisages a natural division of labour between unconscious and conscious processing. He provides the image of numerous labourers at the unconscious level and a small executive at the conscious level. This is related to the necessity when acting in the world to move from balances of probability in the unconscious to decision taking at the conscious level. Unconscious probabilities are suggested as being summarised in a single conscious choice. He quotes the fable of Buridan’s ass, which starved to death because it could not choose between two equidistant bales of hay.
Brain imaging suggests that consciousness is concentrated in the frontal regions of the brain rather than in the earlier stages of the sensory cortex. However the conscious processing feeds information back to the early stages of the sensory cortex. Consciousness is more important for some areas of the brain than others. Thus motor circuits can be activated without involving consciousness, while temporal, parietal and prefrontal areas are closely involved with consciousness.
Conscious perceptions involve Bayesian references in using past memories to interpret new visual stimuli. This processing happens at the unconscious level. Visual neurons are seen to effectively cast votes as to the interpretation of stimuli, and thus converge on a single outcome that is consciously perceived. With motion, for instance, unconscious processing takes up to 140 ms in the middle temporal area prior to conscious awareness of motion. One function of consciousness is argued to be create stable images to be the basis of decision taking. This involves working memory, a function of the dorsolateral prefrontal and areas connected to it. Neurons in these prefrontal areas can keep a stimuli in short term memory alive long after the original stimuli has ceased. The neuronal convergence process that leads to conscious perception disappears under anaesthesia, in which we can no longer integrate experience into a single whole.
Studies also indicate that subjects need to be conscious in order to lay down specific memory traces. While the coincidence of two things can be registered unconsciously, if there is even a short time gap between two occurrences, an intact prefrontal and hippocampus and an ability to report awareness of stimuli is regarded as essential for a memory trace to be formed in the brain. Thus it is argued that one evolutionary role of consciousness is the ability to learn from temporally separated events. Tests suggestion there is no comparable way for unconscious processing to be stretched across time.
Conscious images can having a lasting effect, whereas unconscious ones are transient, undergoing rapid exponential decay in the brain. In dealing with mathematical and logical problems, it is suggested that while particular steps can be performed unconsciously, strategies for dealing with problems in the most economic way need to be conscious. Consciousness is seen as having the evolutionary advantage of allowing a stimuli to be kept active in the brain after the original input has disappeared. The stimuli can therefore be evaluated, used to plan actions, and memorised for future use. The image is independent of the time and place at which it was originally perceived.
Unconscious stimuli propagate a long way into the brain, but are amplified when they move into consciousness, boosting activity in the parietal and frontal regions. Consciousness is identified with a late slow wave known as P3 (3 stands for 300 ms time lag from the initial stimuli). This is also related to high frequency oscillations and synchronisation across the brain.
Studies show that in the early visual areas of the cortex activity is unrelated to whether signals are conscious or unconscious. However, in the higher visual cortex there is a tight correlation between activity and consciousness. These regions are involved in producing images such as objects and faces. At this level, the image is likely to become conscious. Substantial changes in the higher visual areas were apparent whenever consciousness was reported. The level of activity could rise as much as twelve-fold. In contrast, a wide range of frontal and parietal areas remained inactive if the stimuli was masked and therefore unconscious. Masked/unconscious stimuli are clearly active in the early visual cortex, but lose strength as they progress through the cortex. On the other hand, conscious stimuli pick up strength as they progress. Synchrony was also much stronger for conscious stimuli. The same can be seen to be true of motor activity where movements may be unconsciously inhibited, but activity in control regions can be doubled by the signal becoming conscious. With inhibition of movement, it is shown that an unconscious signal can reach the anterior insula and the presupplementary motor area, but that when the same signal becomes conscious it activates additional regions in the parietal and prefrontal, and this is associated with voluntary control.
In one study, conscious and unconscious signals showed no processing difference in the early visual cortex, but after the first 200-300 ms of processing, unconscious activity faded while conscious stimuli progressed towards the front of the brain, and by 400 ms after the stimuli only these signals were apparent and were causing intense activity in the prefrontal, the parietal and the anterior cingulate. The P3 wave is a correlate of consciousness, and sweeps through these regions from about 270 ms, peaking between 300-350 ms. After half a second there comes a reverse wave back towards the earlier parts of the cortex. The conclusion is that only a small proportion of signals from the environment get into consciousness and those that do have at least a 300 ms time lag behind the environment. Unconscious information is seen as being confined to a narrow brain circuit, while conscious information permeates a much larger area.
Visual stimuli create a wave of gamma activity in the brain for the first 200 ms after the stimulus. Again, however gamma subsides after this in the case of unconscious stimuli, but continues for conscious stimuli. Widespread gamma activity from more than 300 ms after the stimuli is seen as another hallmark of consciousness. Gamma is related to both unconscious and conscious activity, but greatly amplified in the event of conscious activity.
Gamma synchrony is suggested to facilitate communication between different parts of the brain. Large scale synchronisation across the brain is viewed as a further hallmark of consciousness. Here again the main effect is about 300 ms after the initial stimuli with many different areas synchronising, but only if the stimuli is conscious. In contrast, unconscious signals produce only a temporary synchrony. Further to this from 300 ms after a conscious stimuli there is a large increase in bidirectional activity in the brain. The reverse signal from the front to the back of the brain is suggested to be possibly to do with direction of attention and/or a confirmation that the frontal interpretation of a signal is consistent with the original input.
Neuroscientists, Quiroga, Fried and Malach have studied neurons in the anterior temporal lobe that respond only when specific features, such as particular people or buildings, are consciously perceived. So the move into consciousness is not a general thing but relates to specific neurons. The pattern of active and inactive neurons in the anterior temporal is seen as a code for what is being perceived. The majority of anterior temporal neurons exhibit this selectivity. Imaging suggests that only a few hundred active neurons arranged in patches in the visual cortex that are specialised in the production of particular images. Groups of neurons are suggested to form into reproducible patterns or so-called attractor states. Memory recalls can activate these temporal neurons in addition to external stimuli. The anterior regions of the visual cortex dedicate specific neurons to particular inputs and then amplify their activity across much of the frontal part of the brain. Transcranial magnetic stimulation (TMS) can be used to disrupt neural processing so that consciousness of particular signals is lost, indicating the dependence of consciousness on high sensory/frontal and parietal processing.
The pyramidal neurons of the cortex have giant cell bodies necessary to support long axons as well as very large dendrites with abundant spines. It is also noted that these neurons are more branched and with more spines than comparable neurons in other primates. The Fox P2 genes are involved in the emergence of these neurons.
From the right inferior temporal lobe, long axons project to distant areas of the cortex including the opposite hemisphere. Such neurons tend to be concentrated in layers II and III, which are also involved in the callosal connection between the two hemispheres of the brain. These layers are much thicker in the consciousness-related prefrontal, cingulate, parietal and temporal than in other areas of the brain. Dehaene also suggests global assemblies utilising the hubs in the more frontal cortex. These are also referred to as convergence zones in that signals converge together in these areas. They are most common in the prefrontal but also present in the anterior temporal, inferior parietal and precuneus.
Multiple sensory inputs are suggested to converge on a single interpretation, which may also be sent back to the earlier regions of the cortex. This tends to relate to synchronisation and the convergent input of the various neurons is suggested to support consciousness over a few hundred milliseconds. Neurons which do not fire are also encoding information. In fact, many more neurons are inactivated than activated.
In the higher areas of the cortex, the tendency for neurons to communicate mainly with near neighbours is lost, and there is more long distance connectivity. Neurons with long distance axons are common in the prefrontal. There are connections particularly from the prefrontal to the inferior parietal, the middle and anterior temporal and both the posterior and anterior cingulate. These regions have recently been identified as the brain’s main hubs of interconnectivity. All are heavily connected to one another, and these regions also have strong connections to the thalamus, the basal ganglia and the hippocampus.
A good deal of brain processing is self activating rather than waiting passively for the brain to put something in. For example, the default network, which emerges whenever the brain is not primarily engaged with the external world, follows a pattern of loose associations or even random thoughts. This may allow the free generation of new plans of action. It is based on the natural excitability and occasional random spiking of cells or random release of neurotransmitters.
The author plays down the importance of the self. He sees the link between conscious perception and self as unnecessary, pointing out that in dealing with sensory inputs we are not constantly reminding ourselves that it is “I’ that is experiencing the particular sensory input. In other words, the self is part of the contents of consciousness rather than being consciousness itself.
Four signatures of consciousness
Dehaene identifies four signatures of consciousness, (1.) Intense activity leading to a surge of frontal and parietal activity (2.) The P3 slow wave about 300 ms after the initial stimuli (3) Late burst of gamma oscillations (4) Increase in bidirectional activity.
Some aspects seem missing from Dehaene’s description of unconscious and conscious processing. He effectively disposes of the idea that consciousness is something unimportant, and shows that it is distinct physical process related to particular areas of the brain, and responsible for stabilising images so that they can be used in decision taking. However, there is still no suggestion as to why the processes described in the more frontal areas are experienced subjectively as opposed to being just a different more powerful type of processing from that of earlier parts of the cortex.