In 1637, Descartes framed the issue of how many senses unite into one mental experience as the mind-body problem. His answer was that sight, sound, smell, thoughts, and feelings all clear through the pituitary gland, and this lets an incorporeal mind observe it. The unification of brain functions is then done by the mind, like a little man watching a movie.
But by the same logic, that little man also needs another viewer in his head to integrate what he saw and so on, in an infinite regress (Dennett, 1991) (Figure 6.34). For this and other reasons, science rejected Descartes mind-body dualism to conclude that only the physical brain exists, so each neuron:
“… doesn’t ‘know’ it is creating you in the process, but there you are, emerging from its frantic activity almost magically.” (Hofstadter & Dennett, 1981), p352.
Yet that nerves with no ability to observe just act, then there you are as if by magic, makes even less sense than dualism, so the mind-body problem of centuries ago lives on today in neuroscience as the binding problem:
“One of the most famous continuing questions in computational neuroscience is called ‘The Binding Problem’. In its most general form, ‘The Binding Problem’ concerns how items that are encoded by distinct brain circuits can be combined for perception, decision, and action.” (Feldman, 2013), p1.
For example, the occipital lobes at the back of the cortex analyze visual data, while sound data is handled by the temporal lobes at the side, so how do vision and sound combine into one perception? This would need another area to process both outputs so the cortex should be wired like a computer motherboard, with many lines to a central processor, but it isn’t. Each neural hierarchy processes its own data but no central area binds them together, so how do sight and sound combine into one experience? The binding problem is that we recognize objects by sight, sound, touch, smell, memories, and thoughts but the neural circuits that generate them aren’t centrally connected. Different areas specialize in sight, smell, sound, thoughts, feelings, touch, and memory, but no area binds them all together.
This problem is compounded by the fact that brain hierarchies can’t talk to each as global workspace theory claims (6.1.6), because when a visual cortex nerve fires to report a line, it doesn’t say “I saw a line” like a little person. It just fires a yes-no response like any other neuron, so to bind that response to another feature like redness needs more processing in the same hierarchy. At each step in the hierarchy, a nerve can fire to trigger a motor response, but it can’t add to an experience because it doesn’t know why it fired. The six-layered visual cortex can process lines, shapes, colors, and textures but the last nerve to fire in a sequence knows no more than the first, so how features combine into a single experience is a mystery.
Different brain hierarchies are then encapsulated modules, so even if they could experience their content, they couldn’t exchange it with others:
“Because of the principle of encapsulation, conscious contents cannot influence each other either at the same time nor across time, which counters the everyday notion that one conscious thought can lead to another conscious thought … content generators cannot communicate the content they generate to another content generator. For example, the generator charged with generating the color orange cannot communicate ‘orange’ to any other content generator because only this generator (a perceptual module) can, in a sense, understand and instantiate ‘orange’.” (Morsella et al., 2016), p12.
The binding problem then isn’t just that brain activities combine in a way that its wiring doesn’t support, but that they do so in a way that no wiring could support, so the unification of sensations, feelings, thoughts, and actions we experience should be impossible.
For example, consider the fact that we see one visual field but two different areas generate it. The brain divides up the work of vision, as nerves from the right side of both eyes go to the left hemisphere, and from the left side go to the right, so each only analyzes half the visual field, but what puts it together? Apparently, the corpus callosum that connects the hemispheres does, as when it is cut, split-brain studies confirm that each hemisphere only responds to its half of the visual field. Do the nerves that connect the hemispheres then let them exchange data? Encapsulation doesn’t allow that but if it did, cutting those nerves should give a sense of loss, but it doesn’t:
“… despite the dramatic effects of callosotomy, W.J. and other patients never reported feeling anything less than whole. As Gazzaniga wrote many times: the hemispheres didn’t miss each other.” (Wolman, 2012).
Why don’t split-brain patients know that their hemispheres are disconnected? If an injury cuts the optic nerve, we know we are blind as no data comes from the eyes, and if it cuts the spinal cord, we know we are paralyzed as no data comes from the legs. But when the millions of nerves joining the hemispheres are cut, both carry on as if nothing had happened! If the verbal hemisphere normally sees the entire visual field using data from the other hemisphere, it should report being half blind, but it doesn’t. It follows that there is no sense of loss because the corpus callosum doesn’t transmit any data between the hemispheres.
The nerves connecting the hemispheres don’t exchange data, but cutting them does divide consciousness with unexpected results. For example, one patient couldn’t smoke because when the right hand put a lit cigarette in his mouth, the left hand removed it, and another found her left hand slapping her awake if she overslept (Dimond, 1980) p434. These conflicts made simple tasks take longer, as one patient found his left hand unbuttoning a shirt as the right tried to button it. Another found that when shopping, one hand put back on the shelf items the other had put in the basket. One patient struggled to walk home because one half of his body tried to visit his ex-wife, while the other wanted to walk home. These extraordinary but well documented cases show that a divided consciousness increases internal conflict, which reduces performance.
A united consciousness could then be how the brain solved the binding problem, as two hemispheres with one experience will together see one visual field. If consciousness is the ability to integrate information to yield adaptive action (Morsella, 2005), then it causes integration not the reverse, which is a clear evolutionary advantage for a decentralized brain. The next section explores the possibility that rather than a pointless epiphenomenon with no benefit, consciousness evolved to provide the unity that specialized functions need to act effectively.