We tried everything we could think of in the perceptual realm. For example, we tried to see if V.P. could compare two simple wave patterns, one presented to each side of the visual midline. Nope, no deal. She couldn’t cross-compare two nonnameable symbols, a very simple task. J.W. couldn’t do this kind of task, either. And P.S. had failed to compare words or pictures many, many times.
To begin to understand the likely explanation, it should be kept in mind that the ability of split-brain patients to look as if they were integrated develops over time. Immediately after surgery and for some time, this seeming ability to transfer information across their disconnected brains is clearly not present. The effects only come from the highly practiced patients after years of testing—or living with someone for years, as I have already pointed out. When this kind of yoking happens between two people, no one is surprised or mystified.
Now, imagine conjoined twins, say, conjoined at the neck. Yes, two completely wonderful human beings coming out of the same neck, like two flowering roses coming off the same stem. Such cases exist and have been well documented.11 The twins Abigail and Brittany Hensel, now in their twenties, who grew up on a farm in Minnesota and graduated college with teaching degrees in 2012, have two quite different personalities. They are unquestionably two separate mental entities, but also reveal the myriad of ways they cross-communicate to keep purposeful actions of their shared body, such as playing softball, integrated.12
Let’s say the conjoining is at a higher level. Basically, the split-brain surgery has disjoined one mental system and made it two. One of the systems, the left brain, is very bright and creative; the other also has a set of skills, but they are different. Nonetheless, two distinct mental systems that were formerly joined are going to have to learn how to get along without having the direct neural communicative networks that they previously enjoyed. They are going to have to learn a lot about cueing, about nonverbal communications, about how, in fact, most humans live their days, tipping off others as to their desires, frustrations, and impending actions by subtle, very subtle, cueing. There is not too much argument about this reality.
Our argument was that this cueing skill markedly improves over time, making it appear as if the split-brain patients were reconnecting after many years of looking so different. For lack of a better phrase, we came to call these strategies part of a “readiness response.”13 Thus the positive interhemispheric results that are observed when performing value comparisons between numbers shown to one hemisphere and dot arrays of same or different amounts shown to the other can be explained by a cueing system: Each hemisphere independently, and without any knowledge of the stimulus presented to the other, has a disposition to respond in a manner that is determined by the magnitude of the digit presented to it. The hemisphere more disposed to respond then initiates the motor output. In other words, if each hemisphere decides to act if the number is high, it is possible to obtain 78 percent accuracy by simply applying a strategy based on the digit in a single visual field (that is, if the digit is <4, guess that the other side is higher; if it is >6, guess that this side is higher; and if it is 5 simply guess). We ran this test on J.W. and he hit this level of accuracy and then told us he was using this exact strategy! No communication between the hemispheres, just a cooperative strategy.
Endless variations of these experiments exist, but the point is clear: Two mental systems forced to share the same resources, work it out. It took two very bright and talented young scientists to get it all straight, Seymour and Reuter-Lorenz.
BRAIN MECHANISMS OF ATTENTION
Just as colleagues were ready, willing, and able to visit Hanover to carry out studies on our patients, we, scientists and patients, were ready to travel to far-off places for testing. This was especially true for places like Steve Hillyard’s lab at the University of California, San Diego, in La Jolla, with its unique natural setting.
As I mentioned earlier, Hillyard used event-related potentials, the complex brain imaging procedure that allows one to study both the timing and, to some extent, the actual place in the brain that generates particular brain waves.14 When he first arrived at UCSD, Steve was housed in the Scripps Institution of Oceanography, on the beach. As the years rolled by, the university built a more traditional building up on the bluffs and Steve lost his coveted space. It is there that I met his hotshot student, Steve Luck. Luck describes his first testing session with J.W., who’d accompanied us out west.
My very first experience with a split-brain patient was with J.W. in an experiment on visual search. For reasons that I still don’t understand, people in the Hillyard lab thought I was fully competent to test J.W. with absolutely no prior experience. I brought him into the lab, sat him in the chamber, and explained the task.
I said something like this: “In this task, the target is a rectangle formed by a red square on top of a blue square. The distracters have a blue square on top of a red square. Your job is to press the left button if you see the target on the left side of the screen and to press the right button if you see the target on the right side of the screen. In other words, press with the left hand if you see red-on-top/blue-on-bottom on the left side and press with the right hand if you see red-on-top/blue-on-bottom on the right side.”
I then asked if he understood, and he said “sure.” He was obviously a pro, so I figured he understood perfectly. Well, his left hemisphere—which was the hemisphere that was talking to me—did understand the task, but this was way too syntactically complex for his right hemisphere. So when I started up the task, his right hand pressed the button every time the target appeared on the right side, but he didn’t make any button presses with his left hand.
I stopped the task and went back into the chamber. I explained the task again, but all the while he protested that he understood the task. I left the chamber, started the task again, and J.W. again did perfectly with his right hand/left hemisphere but made no responses with his left hand/right hemisphere.
I stopped the task and tried explaining again. He again said he understood, and was starting to get a little peeved that this kid was trying to explain something that he, as a professional subject, clearly understood. But again his left hand did not respond.
And then I suddenly realized that I was trying to explain this complicated task verbally to a hemisphere that had limited language abilities.
I went back into the chamber, and I said “Please be patient with me while I try this one more time.” I started up the task, and every time the target appeared on the left, I pointed at the target and at his left hand, saying “blue-top, left-hand, blue-top, left-hand.” He kept protesting that he understood, and then suddenly a funny “aha” look came over his face. He then said, “OK. I’m sure I understand now.”
I left the chamber, started up the task, and both hemispheres did perfectly from that point onward. We got some great data that led to a Nature paper, and I learned how to explain a task to the right hemisphere of a split-brain patient—a hemisphere with poor syntactic ability.15
Steve was just getting his feet wet in science, but it was already clear he would be a star. In fact, most of Hillyard’s students have become scientific leaders. His standards were impeccably severe, and his insights were frequent. Before Steve Luck entered Hillyard’s lab, Marta Kutas, Ron Mangun, Marty Woldorff, Bob Knight, Helen Neville, and others—all household names in neuroscience today—had come under his tutelage. Collaborating with any of them always led to solid research. Still, each was a novice when it came to studying patients. All had cut their teeth on examining normal college undergraduates, who had allowed one to talk as Luck described. It took experience to learn how to describe what you wanted to two very different disconnected hemispheres. The only way to learn was by doing it—trial by fire. As you may be coming to understand, our patients were patient, exceedingly.
Understanding human attention is one of the grand challenges of modern cognitive neuroscience. In many ways, the best and brightest researchers have
been committed to various aspects of the problem. The field was beginning to understand how attention could be directed to particular points in space in order to enhance the sensory moment or how it could be directed away from one conversation to hear another conversation. Attention was thought of as a beacon of light swinging through the rich landscape of our sensory experience, focusing on the specifics of the scene we are currently engaged with. It was the great enhancer to both perception and cognition. Naturally, we began to wonder: Does each hemisphere of a split-brain patient have its own attentional system, or is it shared? Could one hemisphere attend to the left while, at the same time, the other could attend to the right side of space? If you have an intact callosum, you cannot do this at all.
Once again it was Jeff Holtzman who had laid the groundwork. In many ways, the problem of attention seemed mercurial. Either half brain of a split-brain patient was able to direct attention to places in its sensory world. What surprised us was that each half could also direct attention to specific places in the part of the sensory world it did not have direct access to, places that were in the other half brain’s bailiwick. This exception to the rule, that spatial attention could flow across the disconnected brain, seemed weird.16 So, we wondered if it were possible for each half brain to direct its attention to a different place at the same time. Would that be a nonstarter? Was it like, say, a tight end being asked to be at two different places at the same time? Apparently it was.
Patti Reuter-Lorenz nailed this important idea at Dartmouth.17 The attentional system was unifocal. In short, the two disconnected hemispheres could not prepare for events in two spatially disparate locations. Something was still glued together in the split brain. There seemed to be some sharing going on of a common resource—for lack of a better word, let’s call that resource “oomph,” energy—the stuff that is drawn upon to do anything. This idea led to a further refinement of the different kinds of attention the brain calls upon to do its work.
In an earlier study that Jeff and I did at Cornell, we showed that J.W. would react faster from one hemisphere if the other hemisphere was working on an easy problem instead of a hard problem. We supposed that in order to solve a hard problem, more resources would be drawn off than would be to solve an easy problem. When the hard problem was being shown, the opposite hemisphere would, as a consequence, be slower to respond to the different task it was being asked to solve at the same time. Somehow, resources were common to both hemispheres.18 We thought we had it confirmed.
There was this nagging feeling, however, that we hadn’t fully characterized what was going on. From my Caltech days on, I had been showing that split-brain monkeys seemed able to respond accurately to more information presented in a brief flash than normal monkeys were. At one level, it seemed like the animals’ resources had been expanded and improved, not lessened. Jeff and I had found a similar result with human patients. What, indeed, was going on?
This is the test we’d run: Imagine looking at a point in space or even better, a point on your laptop (Video 11). On each side of the fixated point there is a box divided into 9 cells, 3 up, 3 across, like a tic-tac-toe grid. Now imagine the experimenter is about to present to you a sequence of four X’s, distributed one after the other in 4 of the 9 cells, which you are to remember. Further, this memory test will be presented in both visual fields at the same time. I am kidding you, right? No, that is what we did, and we did it in an easy way and a hard way. In the easy way, the nine-cell box in each visual field had the same sequential pattern presented, so we called that the redundant condition. In the hard condition, the box in each visual field got a different pattern of sequences. Trust me, that is a hard condition. After either the easy or the hard stimulus sequence had been presented, another pattern of four X’s—a probe—came on, which either matched the pattern in the field that had just been observed or did not match the pattern in that field. All the subjects had to do was hit a button marked “yes” or “no,” meaning yes the probe was identical to what it had just seen, or no it was not.
Nonsplit subjects whipped through the easy trials. They were fast and accurate. Even though there were 8 different X’s coming on quickly, 4 in each visual field, they were easy to apprehend because the X’s were coming on in the same sequence and in the same pattern in each visual field. They were redundant. Because of that, it is easy to do. J.W. found it easy to do, too.
The hard trials were a different matter. It stopped even smarty-pants undergraduates in their tracks. It was too much information presented in a brief time to grasp. Robert Bazell, the distinguished NBC science reporter, was visiting during one of our tests and exclaimed, after seeing the flurry of stimuli, “What on earth was that?” Clearly, the normal memory system couldn’t handle it, and accurate responses fell to chance.
Not with J.W. When the mixed trials came along, with each hemisphere receiving 4 different X’s at 4 different positions in the nine-cell box, J.W. held on to the information and kept on getting the correct answers. It was like he had two independent processors, which made for better scores when combined.19 It looked like the common unifocal attentional system, which we thought we had captured in our previous studies, couldn’t explain this remarkable increased capacity. The cool thing about science is that explanatory models, which have been proposed to explain mechanisms, can change, to the disappointment or enthusiasm of the researcher. As a scientist, you need to be flexible. If new data disprove your belief, you have to change your belief. The scientific field of attention is peppered with truly great and congenial researchers. They are happy to adjust their ideas to fit new data. Holtzman, Reuter-Lorenz, Luck, Mangun, and Kutas were about to change split-brain research.
Having these young experts and, of course, one of the doyens of attention research, Hillyard, on the case was, as they say in Texas, high cotton. From another angle, it was plenty as well. When running a broad-gauged lab, with only so many slots, only so much money, and with so many issues to be studied, it becomes necessary to limit who else joins in the effort. Those are the plans and that is the management theory. Then Alan Kingstone walked into my life, a Canadian student of another famous attention researcher, Ray Klein at Dalhousie University in Nova Scotia. Great, I mumbled to myself. I need this like the proverbial hole in the head. Kingstone warmly tells the story like this:
. . . Michael Posner pointed me towards Michael Gazzaniga. . . .
So in the blissful ignorance bestowed upon the very young, and those that hold Ph.D.s, I picked up the telephone, used my dime, and called Michael Gazzaniga. He answered, and deduced in about one millisecond that I didn’t know anything about the brain and its relation to human cognition. The conversation went something like this:
Mike: Do you know anything about the brain? [This, I was to learn, is classic Michael; he cuts straight to the heart of an issue, or as was often the case where I was concerned, the weakness of an issue.]
Alan: No. [This was not going the way I had hoped!]
Mike: Don’t you think that’s a problem?
Alan: No. I’ll learn.
Mike: Come on down. Let’s see what you’ve got to offer.
Truly. That was it. A short time later I found myself flying to Montreal, and then catching a train on a beautiful spring morning through the magical countryside of Vermont down into White River Junction. From there it’s just a ten-minute cab ride to Dartmouth College, in Hanover, New Hampshire. And by the time I stepped out of the cab and onto that remarkable Ivy League Dartmouth Campus—a campus that manages to blend the old and new together in such a seamless manner—I was completely and absolutely sold. And I began to suspect the truth—that my life had already begun to change forever, and for the better.
The next day I set off for Mike’s lab. At that time, in the early 1990s, Mike and his team were conducting their research in a white clapboard, side-gabled house that had been built by Mrs. A. Pike in 1874. There at Pike House I met several of the future stars in cognitive neuroscience: people like Patti Re
uter-Lorenz and Ron Mangun, and of course, Michael Gazzaniga himself. He had me give a little talk to his group, and then whisked me off to an elegant French restaurant, where he offered me a spot in his lab. “Say yes, and come do your thing” was his offer. I accepted of course. We shook hands, and that was that.
It really only took about three minutes to decide to hire him and it only took him two seconds to accept the offer. He had that glint in his eyes, the energy level of a buzz saw, and the smarts of the rest of the group. He also had a thirst for new problems, new angles, new adventures. So I changed my theory on the spot and decided we needed to dig into attention issues more deeply.
ATTENTION REDUX
Steve Luck was busy in San Diego puzzling about what split-brain patients were capable of doing and how they did it. He carried out an amazingly clever experiment that, at one level, confirmed the idea that they were capable of enhanced information-processing capacity. How could that be? Luck went after the problem by applying a well-established test from the experimental attention literature. He took an array of blue and red squares that were stuck together with the blue square on top and spread a bunch of them out on a computer display screen. Each time he presented the array of squares, he snuck in one square pair that was different. It had the red square on top while the yoked blue square was on the bottom. The squares in the array were called the distracters, and the single red/blue square was called the target. The task was simple: Find the target.
When neurologically intact subjects do this task, an interesting and consistent behavior occurs. As more distracters are added, it takes longer to find the target. In fact, our response time goes up in a reliable way. Every time two more distracter squares are added to the distracter array, it takes another 70 milliseconds to respond. The distracters slow down our search to find the one target. This happens like clockwork. It also doesn’t matter where in the left or right visual fields the added distracters appear.
Tales from Both Sides of the Brain : A Life in Neuroscience (9780062228819) Page 23
