TR: Most of your experiments have been done on monkeys. How did that begin to shape your view on the relationship between brain functions and human consciousness?
BN: We study motion perception. We train monkeys to look at a pattern of dots moving in a certain direction and to report the direction of the dots by moving their eyes in the same direction. If a monkey picks the correct answer, he gets a reward.
This simple behavior contains a world in terms of understanding how the nervous system performs intelligent behavior. Sensory information that comes into the brain through the eye must be coded into some neural language that represents the stimulus within the brain. Based on this neural representation, the monkey must then make a high-level judgment about what he is actually seeing. This “decision” in turn guides the selection of a motor response, to look to the left or the right.
TR: And you added a new level to this experimental setup by stimulating the monkey’s brain.
BN: We put an electrode in an area of the brain known as MT. The cells in this area respond selectively to a specific direction of motion. Some cells are active when the monkey looks at dots moving to the left, some cells are active when the monkey looks at dots moving to the right. People had suspected for a long time that MT was important for our ability to see motion. So we did an experiment where we stimulated these cells artificially with tiny pulses of electrical current – it changed what the monkeys reported seeing.
TR: So with the monkey experiments, you can stimulate the brain in very focused ways and change the way the monkey responds. But the monkey can’t tell you what he sees when you stimulate the brain.
BN: Yes. People can report what they see or hear or feel, but with monkeys, you can only look at their change in behavior. I can’t climb into a monkey’s head and see what the monkey really sees.
This gets to core of the current debate about the study of consciousness. What is the conscious experience that accompanies the stimulation and the monkey’s decision? Even if you knew everything about how the neurons encode and transmit information, you may not know what the monkey experiences when we stimulate his MT.
TR: People have shown that stimulating the human brain can do similar things too, right?
BN: Electrical stimulation of the brain is not new. Wilder Penfield, a neurosurgeon in Canada in 1930s and 40s, who pioneered the neurosurgical treatment of epilepsy, was the first to start stimulating the brains of conscious humans. He wanted to be able to identify the parts of the brain involved in speech and movement, before he took out the piece of brain he thought was responsible for disease, so he developed ways to make a hole in the skull and expose the brain in fully conscious humans.
While he was in there stimulating the brain for clinical purposes, he also stimulated other parts of brain. He showed that by stimulating visual cortex, you can get people to see stars or flashes of light. When he stimulated the auditory cortex, people could hear buzzing signals. When he went deeper into the brain, into the temporal cortex, he could elicit complex perceptions. A patient would say things like, ‘I’m sitting on the back porch of my mother’s house and she’s calling me to dinner.’
He did all of this in the 1930s, but the field never went anywhere because he knew nothing about the circuitry of the brain. Penfield was just stimulating neural tissue of an unknown nature. He could elicit conscious phenomena, but he gained no insight into how, exactly, the conscious phenomena are related to the [behavior] of the activated neurons.
Now we know about single cells, neural circuits, and their selective properties. So we can make better hypotheses about how cells might contribute to cognitive phenomena such as perception or memory or attention. We can tweak carefully targeted parts of the system and get a predictable response.