researcher Alvaro Pascual-Leone began to wonder about the speed at which these major brain changes can take place. He noted that aspiring instructors at a school for the blind were required to blindfold themselves for seven full days to gain firsthand understanding of their students’ living experiences. Most of the instructors become aware of enhanced skills with sounds—orienting to them, judging their distance, and identifying them:
Several describe becoming able to identify people quickly and accurately as they started talking or even as they simply walked by due to the cadence of their steps. Several learned to differentiate cars by the sounds of their motors, and one described the “joy of telling motorcycles apart by their sound.”38
This got Pascual-Leone and his colleagues considering what would happen if a sighted person were blindfolded in a laboratory setting for several days. They launched the experiment, and what they found was nothing short of remarkable. They discovered that neural reorganization—the same kind seen in blind subjects—also happens with temporary blindness of sighted subjects. Rapidly.
In one of their studies, sighted participants were blindfolded for five days, during which time they were put through an intensive Braille-training paradigm.39 At the end of five days, the subjects had become quite good at detecting subtle differences between Braille characters—much better than a control group of sighted participants who underwent the same training without a blindfold.
But especially striking was what happened to their brains, as measured in the scanner. Within five days, the blindfolded participants had recruited their occipital cortex when they were touching objects. Control subjects, not surprisingly, used only their somatosensory cortex. The blindfolded subjects also showed occipital responses to sounds and words.
When this new occipital lobe activity was intentionally disrupted in the laboratory by magnetic pulses, the Braille-reading advantage of the blindfolded subjects went away—indicating that the recruitment of this brain area was not an accidental side effect but a critical piece of the improved behavioral performance.
When the blindfold was removed, the response of the occipital cortex to touch or sound disappeared within a day. At that point, the participants’ brains returned to looking indistinguishable from every other sighted brain out there.
In another study, the visual areas of the brain were carefully mapped out using more powerful neuroimaging techniques. Participants were blindfolded, put in a scanner, and asked to perform a touching task that required fine discrimination with their fingers. In these conditions, investigators could detect activity emerging in the primary visual cortex after a blindfolding session of a mere forty to sixty minutes.40
The shock of these findings was their sheer speed. The shape shifting of brains is not like the glacial drifting of continental plates, but can instead be remarkably swift. In later chapters, we’ll see that visual deprivation causes the unmasking of already-existing nonvisual input into the occipital cortex, and we’ll come to understand how the brain is always sprung like a mousetrap to implement rapid change. But for now the important point is that the brain’s changes are more brisk than even the most optimistic neuroscientist would have dared to guess at the beginning of this century.
Let’s zoom back out to the bigger picture. Just as sharp teeth and fast legs are useful for survival, so is neural flexibility: it allows brains to optimize performance in a variety of environments.
But the competition in the brain has a potential downside as well. Whenever there’s an imbalance of activity in the senses, a potential takeover can happen, and it can happen rapidly. A redistribution of resources can be optimal when a limb or a sense has been permanently amputated or lost, but the rapid conquest of territory may have to be actively combated in other scenarios. And this consideration led me and my former student Don Vaughn to propose a new theory for what happens to brains in the dark of night.
WHAT DOES DREAMING HAVE TO DO WITH THE ROTATION OF THE PLANET?
One of the unsolved mysteries in neuroscience is why brains dream. What are these bizarre nighttime hallucinations about? Do they have meaning? Or are they simply random neural activity in search of a coherent narrative? And why are dreams so richly visual, igniting the occipital cortex every night into a conflagration of activity?
Consider the following: In the chronic and unforgiving competition for brain real estate, the visual system has a unique problem to deal with. Because of the rotation of the planet, it is cast into darkness for an average of twelve hours every cycle. (This refers to 99.9999 percent of our species’ evolutionary history, not to the current, electricity-blessed times.) We’ve already seen that sensory deprivation triggers neighboring territories to take over. So how does the visual system deal with this unfair disadvantage?
By keeping the occipital cortex active during the night.
We suggest that dreaming exists to keep the visual cortex from being taken over by neighboring areas. After all, the rotation of the planet does not affect anything about your ability to touch, hear, taste, or smell; only vision suffers in the dark. As a result, the visual cortex finds itself in danger every night of a takeover by the other senses. And given the startling rapidity with which changes in territory can happen (remember the forty to sixty minutes we just saw), the threat is formidable. Dreams are the means by which the visual cortex prevents takeover.
To better understand this, let’s zoom out. Although a sleeper looks as though he is relaxed and shut down, the brain is fully electrically active. During most of the night, there is no dreaming. But during REM (rapid eye movement) sleep, something special happens. The heart rate and breathing speed up, small muscles twitch, and the brain waves become smaller and faster. This is the stage of sleep in which dreaming occurs.41 REM sleep is triggered by a particular set of neurons in a brainstem structure called the pons. The increased activity in these neurons has two consequences. The first is that the major muscle groups become paralyzed. Elaborate neural circuitry keeps the body frozen during dreaming, and its elaborateness supports the biological importance of dream sleep; presumably, this circuitry would be unlikely to evolve without an important function behind it. The muscular shutdown allows the brain to simulate world experience without actually moving the body around.
The second consequence is the really important one: waves of spikes travel from the brainstem to the occipital cortex.42 When the spikes arrive there, the activity is experienced as visual. We see. This activity is why dreams are pictorial and filmic, instead of conceptual or abstract.
During dream sleep, waves of activity begin in the brainstem and end in the occipital cortex. We suggest this infusion of activity is necessitated by the rotation of the planet into darkness: the visual system needs special strategies to keep its territory intact.
This combination crafts the experience of dreaming: the invasion of the electrical waves into the occipital cortex makes the visual system active, while the muscular paralysis keeps the dreamer from acting on the experiences.
We theorize that the circuitry behind visual dreams is not accidental. Instead, to prevent takeover, the visual system is forced to fight for its territory by generating bursts of activity when the planet rotates into darkness.43 In the face of constant competition for sensory real estate, an occipital self-defense evolved. After all, vision carries mission-critical information, but it is stolen away for half of our hours. Dreams, therefore, may be the strange love child of neural plasticity and the rotation of the planet.
A key point to
