The Ravenous Brain: How the New Science of Consciousness Explains Our Insatiable Search for Meaning (32 page)

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Authors: Daniel Bor

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Patients may also only shave the right half of their face, eat from only the right half of their plates, and, if asked to draw a clock face, would write all the numbers squashed up on the right half of the circle, or just leave the numbers 7 to 11 out of their drawing entirely. Similarly, if asked to draw a house, they’d merely draw the right half, with lines unfinished, half the windows missing, only half a door present, and a precarious right half of roof looking as if it could collapse at any moment. In every case, they are completely oblivious to the striking flaws in their behavior.
The condition extends to every aspect of the neglect patient’s world. These patients will ignore sounds or touches on the left. And the disorder can even reach into their imaginations: In a simple yet striking experiment, Edoardo Bisiach and Claudio Luzzatti asked hemispatial-neglect patients to imagine they were in a square that was very familiar to them, the Piazza del Duomo, in Milan. They were first told to imagine that they were facing the cathedral from the opposite side and to describe all the buildings they could see. The patients reported all the buildings in their mind’s eye, which were only those on the right of the square, from that perspective. They were then told to imagine they were on the opposite side of the square, facing outward from the cathedral, and describe all the buildings they could then see. They proceeded to fail to imagine or mention all the buildings they’d just listed a few minutes before, as these were now on their left in their mind’s eye, but did describe all the buildings they’d previously failed to notice! So even in imagination, these patients were completely ignoring the left half of space.
The fact that you can either describe this condition as one of a profound attentional deficit or as an absence of awareness provides more ammunition for the view that attention is a critical component of awareness, functioning as its directional gatekeeper.
To reinforce the point that hemispatial neglect is not purely a visual impairment, Margarita Sarri and colleagues carried out an experiment on neglect patients in which she would touch either their right or left index finger while they were in the fMRI scanner. Half the time the patient wasn’t aware of the touch on the left. The region of the patient’s cortex responsible for finger sensations was activated regardless of whether the patient was aware of the touch or not, but if the patient
was
aware of the touch, then the intact portions of the prefrontal parietal network lit up.
CONSCIOUSNESS SHRINKING TO A SMALL POINT
 
Habitually failing to be aware of half of your world is a profound enough impairment of consciousness, and yet there is an obvious question here—why aren’t these patients more impaired, if these regions are so critical for consciousness? Why, for instance, can they occasionally still be largely aware of the left half of space? The answer lies in how much of the collective prefrontal parietal cortex is damaged in these patients.
In all the neurological cases I’ve described so far in this chapter, the damage has only included a small proportion of the total volume of the prefrontal and parietal cortices in both hemispheres. Most of the time the damage was only in one hemisphere, so at most would have only affected half of the volume of this network of regions. It’s also likely that these brain areas, so flexible in function normally, would be more capable than any other region at adapting to the brain damage, with the intact parts of the network merely taking on more of the burden of processing. So what happens if the whole prefrontal and parietal network is damaged? Is such a person still conscious? Well, in humans I know of no case studies that have included such a profound loss of cortex—probably because such extensive damage would lead to death. However, there are rare reports of near complete damage to both prefrontal cortices or both posterior parietal cortices. Are these patients any more impaired than those I’ve described above? Unfortunately, they most certainly are.
The posterior parietal cortex, although in function highly similar to the prefrontal cortex, is marginally more linked to spatial processes. Patients who have extensive damage to both sides of the top portion of their posterior parietal cortex have a rare condition known as Bálint’s syndrome, which completely knocks out their sense of space. Their experience of the spatial extent of the entire outside world seems to disappear, and patients report that “there is no there, there.” One feature of this syndrome is that patients are unable to perceive more than one object at a time. Even with this single object, they cannot locate it, nor can they tell if it’s moving toward or away from them. If they are looking at a crowded scene of multiple objects, once they recognize one object the others will become invisible. Sometimes, even perception of a single object is impaired. One patient, known as KE, could recognize colors and read words, but couldn’t do both—so if he read a word, he was unable to identify its color. Although this condition is skewed toward the spatial domain, it nevertheless reflects the importance of the posterior parietal cortex for working memory. KE, and patients like him, have their spatial working memory effectively reduced to one—and in some cases not even a single complete object can be held in these patients’ working memory—it will be stripped into its component parts, and only one of those parts will make its way into consciousness.
The prefrontal cortex is in some ways the most abstract general higher-order region that humans possess. Penfield’s sister had most of her prefrontal cortex removed, but only in one hemisphere, with the left prefrontal cortex intact and able to take on functions that both prefrontal cortices had previously carried out. But if you lose both prefrontal cortices, what happens? Such cases are rare, possibly because the patient is far too ill to take part in any scientific research. But one neurologist, Bob Knight, has come across such a patient. The patient was awake, but otherwise looked disturbingly like a zombie. He had no motivation of his own, just constantly sat immobile in a chair as he stared into space, and tragically, it seemed as if any meaningful awareness was totally absent.
It seems clear from all these patients with varying degrees of damage to the prefrontal parietal network that such regions carry out a combination of attention and working memory processes, and that when such functions are severely impaired, consciousness consequently shrinks to a very marked degree.
BRAIN-SCANNING THE PREFRONTAL PARIETAL NETWORK
 
But what is the consensus from brain-scanning studies exploring the role of the prefrontal parietal network? The main thrust of the research has indeed centered on working memory and attention, with the prefrontal parietal network closely associated with both.
For instance, the prefrontal parietal network will increase in activity if you increase the number of letters you place in working memory, the number of abstract relations between items of an IQ task you hold in working memory, and the number of spatial locations you have to remember. Likewise for attention. The prefrontal parietal network increases in activity, for instance, if you switch attention between tasks, or if you attend to visual changes on a screen.
In fact, as this is our most abstract, higher-order brain network, a common finding is that these areas are activated whenever we perform a complex or novel form of
any task whatsoever
, whether it involves short-term memory, long-term memory, mental arithmetic, or any other potentially challenging cognitive process. For this reason, these regions are also most closely linked with IQ.
This pattern of results implies two vital features of our thoughts and consciousness: first, the intermingling of cognitive processes, and second, how this amalgam of advanced mental functions is inextricably linked with awareness.
If you read a psychology textbook, even now, it will probably make neat distinctions between working memory, attention, long-term memory, mental arithmetic, reasoning, and so on, as though these were all independent processes. Increasingly, though, researchers are eroding the spaces between almost all types of thought and memory, and building broad links between them.
Aside from the fact that all such thoughts will activate the prefrontal parietal network, there are many other connections. For instance, it’s clear that working memory is intimately linked with attention, with working memory effectively being the output of attentional filtering. Manipulations in working memory, especially of some current goal, can influence which attentional filters are set. And if you fill up working memory with items, your attentional ability suffers. To demonstrate this, Nikki Pratt and colleagues recently gave volunteers a classic attentional task that involved responding to the direction of a central arrow while ignoring a distracting group of surrounding arrows pointing the wrong way—a surprisingly difficult test requiring firm attentional control. On some of the trials, volunteers also had to keep a set of letters in working memory prior to stating which direction the central arrow was pointing. This extra working memory load caused the volunteers to guess the arrow’s direction more slowly and less accurately—in other words, it reduced their attentional resources for the arrows. Pratt also found, using EEG, that brain-wave markers of attention for the arrows were weakened when working memory items were stored, further reinforcing the links between the two processes. Results such as these have led the most prominent current theories of attention to subsume working memory within an attentional framework.
Thus, this traditional picture that there are a group of largely independent brain areas and corresponding types of thought—say, one for attention, another for working memory, another for long-term memory encoding, and even another for consciousness—is one that increasingly clashes with the evidence. Instead, the most parsimonious way to describe the data is in terms of another distinction, that between static, automatic, unconscious processes, on the one hand, and highly dynamic, flexible, conscious ones, on the other. The automatic processes are those we’ve stored in our specialized memory and motor areas as overlearned habits and goals, and are usually the product of our roaming consciousness. And consciousness is an inextricably interlocked collection of processes carried out in the main by the prefrontal parietal network, with attention and working memory being the most prominent two functions. This consciousness machine is designed to kick in whenever the task cannot be achieved by our instincts or bank of unconscious automatic habits. It analyzes and manipulates the contents of our working memory in logical ways, drawing in further information from our specialist systems if necessary, and brings to bear this substantial portion of general-purpose cortex to solve complex or novel tasks, ideally by creating new automatic habits that the next time won’t require consciousness.
THE PREFRONTAL PARIETAL NETWORK, CONSCIOUSNESS, AND CHUNKING
 
But what of chunking and spotting patterns? While it is indeed a near universal finding that the prefrontal parietal network will be increasingly activated as the demands of a task increase, there is one clear and important set of results with the exact opposite picture.
My colleagues and I at Cambridge carried out one of the first experiments explicitly to demonstrate this exception. Volunteers in the fMRI scanner would view an array of 16 red squares arranged in a 4 by 4 matrix. Four of these red squares would blink blue in a sequence, and a few seconds following this, the participants would try to correctly recall this sequence of locations. This is so far a standard spatial working memory test. The twist is in the fact that we actually gave volunteers two different types of sequences—ones that looked entirely random, just like the conventional spatial working memory tests, and another that took advantage of the 4 by 4 structured array to make the sequence form squares, triangles, or other symmetrical and regular paths (see
Figure 8
). This made the sequences easily chunkable. Subjects consciously detected these chunks and talked after the experiment about how these sequences were easier because of the patterns.
If difficulty in every single situation drives the prefrontal parietal network, then the unstructured, more difficult sequences should generate more activation in these regions. But if instead there is something special about chunking processes, then perhaps these easier structured sequences will drive the prefrontal parietal network more than the harder, unstructured alternatives.
In fact, very robustly, the easier patterned path trials did light up the prefrontal parietal network more brightly compared with the more difficult, unstructured sequences. So in some cases, at least, task demands don’t drive this network—at least when chunking is involved.
Because this was a rather unexpected result, we repeated the same experiment, except this time with digits: Subjects heard 8 single digits in the fMRI scanner, and then after a few seconds had to say back the sequence in the order that they’d heard it. Some sequences were distinctly structured, such as 8 6 4 2 9 7 5 3 (descending even numbers, then descending odd numbers), while other sequences were deliberately made to be as random as possible. Just as with the spatial structured and unstructured trials, these structured digit sequences were easier because subjects could chunk them—and they, too, activated the prefrontal parietal network more.
One outstanding question we had with this work was whether it was simply memory for preexisting chunks, such as squares or single-digit, even-number sequences, to which we are all exposed from early childhood—that was driving the participants’ responses, or if participants were instead noticing the chunks on the fly, spotting the pattern as a powerful new rule to apply on each trial. So we devised a new experiment, sticking with verbal working memory and digits, but this time moving on to sequences of double digits. This way we could give volunteers sequences they were very unlikely to have seen before, to make sure we were dealing with mathematical and novel pattern spotting. For instance, subjects might have to remember the sequence 57 68 79 90 over a few seconds (each number going up by 11 each time). We also had a standard, totally unstructured sequence that subjects couldn’t apply any chunking strategies to (say, 31 24 89 65). And there was also a memory-based chunking condition, to see if that would also activate the prefrontal parietal network, and if so, how strongly. To enable a memory-based chunking condition, I trained my volunteers for at least 4 hours each to memorize 20 different unstructured 4-digit chunks before I scanned them. I told them they had to imagine they were playing a game where they were a new receptionist at a medium-sized company, and as part of their induction they had to memorize the faces, names, and phone extension numbers of 20 key members of staff. So, after a series of graded training exercises, they each had a relatively naturalistic set of 4-digit numbers cemented in their memories. When it came to the scanning part of the study, they might have seen the novel sequence 21 05 81 63 to recall in working memory, where 2105 would be one phone extension number they’d comprehensively learned during the previous week, and 8163 would be another. So we had three different fMRI digit-based working memory conditions—one involving mathematical structure, a second involving mnemonic structure (made up of the phone extension numbers they had learned so thoroughly), and a third with no structure (so subjects just had to rely on their working memory and not chunking). With these three, we could distinguish the brain regions activated for mathematical chunking against those for memory-based chunking. In fact, we also had two controls—one to match the memory content and a second to match the mental arithmetic content, but in each case without the additional digit sequences task, and thus the opportunity to benefit from chunking.

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