by Thad Polk

Christians believe that there are aspects of human experience that are supernatural and that can’t be explained by natural laws. In contrast, most neuroscientists are materialists and assume that every aspect of human experience is ultimately reducible to physics and chemistry. Put bluntly, they believe that human beings are meat machines—amazingly complex and sophisticated machines to be sure—but meat machines nonetheless.

Well, I’m a Christian AND a neuroscientist. As a neuroscientist, I’ve seen first-hand how the scientific study of the human brain can lead to fascinating insights into the physical mechanisms underlying perception, memory, language, and thinking. But as a Christian, I also believe there are aspects of human experience that science can’t explain. In fact, I think that the more progress science makes in explaining human experience, the clearer the limits of scientific explanation become.

One domain in which both the power and limitations of neuroscience become very clear is in the scientific study of consciousness. It turns out that neuroscientists have made significant progress in understanding the neural basis of consciousness over the past 10-20 years, but there’s still a lot we don’t know—and I would argue may never know. 

But as a Christian, I also believe there are aspects of human experience that science can’t explain.

As an example of the power of neuroscientific studies, consider the study of split-brain patients. Patients with severe and intractable epilepsy occasionally undergo a procedure called a corpus callosotomy, in which the major band of nerve fibers (the corpus callosum) connecting the brain’s left and right hemispheres is surgically severed. This so-called split-brain surgery isolates the hemispheres from each other and can therefore prevent out-of-control seizure activity from spreading from one side of the brain to the other. 

But split-brain surgery also provides a unique opportunity to see into the amazing relationship between the mind and the brain and even into the nature of consciousness. For example, most people report experiencing a single, unitary consciousness, but what about split-brain patients? Do they experience two? Does a split brain also imply a split mind?

Amazingly, the answer appears to be yes. For example, consider briefly flashing a picture to the patient’s left and asking them to name it. Visual information from the left side of space actually gets processed by the brain’s right hemisphere (reflecting what is sometimes called the brain’s contralateral organization). But language is typically controlled by the left hemisphere. So before the object can be named, information needs to be communicated from the right hemisphere to the left. That’s a problem for split-brain patients and so they typically won’t be able to name objects presented in their left visual field. In fact, the speaking left hemisphere will even claim that it didn’t see anything.

Now here’s the amazing thing: These same patients who say they didn’t see anything can still draw a picture of the object with their left hand! You see, the left hand is controlled by the right hemisphere and the right hemisphere did recognize the object; and it can prove it by drawing a picture of it.

The bottom line is that the split-brain patient’s right hemisphere is consciously aware of the object even though the same patient’s left hemisphere isn’t. Apparently, these patients do not have a single, unitary consciousness. They have at least two. 

Another example of neuroscientific progress on consciousness comes from experiments using neuroimaging. With techniques called Positron Emission Tomography (PET) and functional Magnetic Resonance Imaging (fMRI), neuroscientists can now watch the human brain at work and gain insight into which parts of the brain are active under different circumstances. 

The bottom line is that the split-brain patient’s right hemisphere is consciously aware of the object even though the same patient’s left hemisphere isn’t.

For example, a number of neuroimaging studies have compared neural activity when people are awake and conscious vs. anesthetized and unconscious. Other studies have compared brain activity during waking and sleeping. And both sets of studies have found similar results: there is a particular brain structure called the thalamus whose activity seems particularly important in determining our level of consciousness (conscious vs. unconscious, awake vs. asleep).

As these examples illustrate, neuroscience can, and regularly does, answer important questions about the neural basis of consciousness. In fact, it seems inevitable that scientists will continue to make progress until we eventually reach a point that we can literally read a person’s mind based on their neural activity. I would go so far as to say that we’ll even be able manipulate conscious experience by stimulating the brain in specific ways (brain stimulation during neurosurgery actually already does that to a limited degree).

But will science then have “solved” the mystery of consciousness? Many neuroscientists and philosophers believe the answer is a resounding no. For example, the philosopher David Chalmers makes the point that science still hasn’t scratched the surface of the most fundamental question about consciousness: How does physical matter (the brain) give rise to subjective experience? This is what Chalmers refers to as the hard problem of consciousness. His point is that even if we completely understand the neural correlates that are associated with a particular subjective experience (the answers to all the “easy” problems), that still doesn’t explain how those neural correlates give rise to that subjective experience.

But will science then have “solved” the mystery of consciousness? Many neuroscientists and philosophers believe the answer is a resounding no.

To illustrate the problem, consider the following thought experiment devised by the Australian philosopher Frank Jackson. Imagine that Mary is a 23rd century vision scientist who completely understands the neural correlates of color vision. She knows what every cell in the brain’s visual system does. She can analyze patterns of neural activity and tell you exactly what color someone is looking at. She can build computer simulations of the brain that exactly mimic the neural activity observed when people are viewing different colors. In fact, she can even stimulate the brain in specific ways in order to make someone feel like they’re seeing a specific color even if they’re not.

But here’s the kicker. Imagine that Mary has lived in a black and white room her entire life and has never actually seen any colors herself. So even though Mary understands everything there is to know about the neural correlates of color vision, she still doesn’t know what it’s like to see red. In Chalmers’ terms, Mary knows the answers to all the easy problems about consciousness, but there’s still more to know.

Other famous thought experiments similarly illustrate the limitations of science in being able to explain subjective experience. Is my experience of the color blue the same as yours? What if my experience of blue is actually like your experience of red, but we use different labels? Would there be any way for science to tell the difference? Likewise, is there any way for a scientist to prove that other people really have subjective experience?

matrix-podWhat if a drug was invented that could eradicate subjective experience, but that left a person otherwise completely indistinguishable (e.g., the person would still act like they were conscious and would adamantly claim to be conscious even though they weren’t)? Would the scientist be able to distinguish people who took the drug from those who didn’t? Or consider the brain-in-the-vat argument: Is there a way to prove that we’re not brains sitting in vats being stimulated to experience what we experience? For those who have seen the movie “The Matrix,” think of the humans in the pods.

What all these thought experiments have in common is that they illustrate the inability of science to explain first-person subjective experiencewhat it’s like to be you. Science depends on objective, measurable third-person data that other scientists can observe themselves. But of course, first-person subjective experience is by definition not observable to anyone else; only you know what it’s like inside your head. It’s therefore difficult to see how science could ever make progress in answering the hard question of consciousness.

And of course, the same argument applies to all the other aspects of human experience that are subjective and first-person. Will science ever be able to explain why we feel love, or compassion, or hope, or faith? I would argue that the answer is no. Although science will continue to make progress in understanding the physical correlates of human experience, I have my doubts that it will ever be able to explain the deepest mysteries of what it means to be human. Those questions may simply be outside the scope of scientific investigation. 

Dr. Polk will be speaking to the 55 Alive group on Monday, May 19, at 11:30 a.m. in the West Wing of St. Luke-Ann Arbor.