Reading 8: The neuroscience of consciousness Flashcards
Nagel’s view of consciousness
- Nagel uses this example to illustrate his argument that conscious experiences are only knowable to the individual experiencing them, and thus consciousness is not amenable to reductionistic objective analysis of things like action potentials, synapses, etc.
- We already know a lot about how bats use sonar to navigate the world, but still, we don’t know what it’s like to be a bat, and, according to Nagel, no matter how much we learn about the bat nervous system, we never will.
A Brief History of the Mind as a Subject of Scientific Study - descartes
- Jacques Vaucanson was an 18th century French inventor who was famous for building automata, complex mechanical devices that imitated living things. His most famous creation was the Digesting Duck (Canard Digérateur; Fig. 1), which dazzled the French public of his time.
- The 17th century French philosopher René Descartes attempted to solve this problem by dividing reality into the physical (res extensa) and the mental (res cogitans). Descartes proposed that the body was a mechanism – part of the physical world – and thus amenable to scientific investigation (Fig. 3). The mind on the other hand was a product of the soul and was thus the domain of religion.
What is the problem with descartes theories?
- The problem with Descartes dualism was that the brain is a physical thing, and it was only a matter of time before scientists began to study its properties and
their relation to the mind. - For example, the Russian physiologist Ivan Pavlov, around the turn of the 20th century, showed that dogs would learn to salivate in response to a tone that had been repeatedly associated with food. This simple form of learning followed predictable rules and was linked to body physiology.
Behaviourism
Two key tenets of behaviorism were:
1) even the most complex behaviors can be explained in terms of a few simple rules of learning;
2) the only thing about the workings of the brain that scientists can study is behavior.
What goes on between the sensory input and the behavioral output is a black box, closed off from and irrelevant to the study of psychology. The idea that “thinking” might be something worth studying was dismissed as unscientific.
Behaviorism made important contributions to psychology, but it was ultimately doomed by its limitations. For one thing, the simple kinds of learning that behaviorism studied, like classical and operant conditioning, could not account for most of human learning.
For example, Noam Chomsky, one of the founders of cognitive psychology, showed that children’s acquisition of language could not happen according to the simple learning rules proposed by behaviorists.
Second, it is obvious that people’s actions are motivated by their beliefs, their desires, and their understanding of the world, things that involve the mind and have little or nothing to do with the stimulus-response relationships that formed the basis of behaviorism.
What do we mean by consciousness?
- The philosopher David Chalmers has proposed a framework for thinking about consciousness which distinguishes between the “easy problem” and the “hard problem”.
- The easy problem concerns how the brain brings together sensory input, memory, and cognition to enable complex, flexible, intelligent behavior in response to novel situations. Clearly, this “easy” problem is not easy at all.
- It is, in fact, the subject of the vast field we call cognitive neuroscience. The easy problem is “easy” in the sense that we have a general idea what the solution ought to look like (it will involve networks of neurons carrying out computations, etc.) and how we can study it (by studying how the brain works).
The hard problem
- The hard problem is about subjectivity. Being you is like something and only you truly know what it’s like. Imagine a person who has been completely color blind their entire life and sees the world in shades of gray. Now imagine trying to explain “red” to that person. How would you do it? What would you say? “Red? Well, it has a reddish quality.” It would be sort of like a highly intelligent bat trying to explain to
you what it’s like for a bat to see with its ears. - The hard problem is hard because it’s not evident how
subjective first-person experiences, like the blue of the sky or the feel of the warm sun, could ever be explained by describing the behavior of brain neurons no matter how detailed the description.
Consciousness as a nexus of science and philosophy
Consciousness is a nexus of science and philosophy, and many modern philosophers have weighed in on the subject.
* On the one hand, philosophers such as Nagel and Chalmers argue that there will inevitably be something missing in any attempt to reduce consciousness to neurobiology.
* No matter how much we learn about action potentials and synapses, we will at best be describing correlations (e.g., when the subject sees a Mongolian gerbil, a specific group of neurons fire), without ever getting to the heart of consciousness.
* At the other extreme, philosophers like Daniel Dennett and Patricia Churchland contend that the whole idea of a hard problem is a red herring. Dennett makes the comparison with elan vital, the old idea that living things must be animated by some sort of life force. We now know there is no need for this concept.
* A living thing, such as a bacterium, is a vastly complex network of physical and biochemical processes. We haven’t discovered the life force; the concept has simply become irrelevant. By analogy, Dennett argues, if we knew enough about how the brain works, the hard problem of consciousness wouldn’t be solved, it would just no longer be seen as a problem.
Neuroscience of consciousness: content-specific NCC and full NCC.
Neuroscientists who study consciousness focus their research on the neural correlates of consciousness (NCC), the brain activity that is correlated with and required for consciousness. They make a further distinction between the content-specific NCC and the full NCC.
* The content-specific NCC is the brain activity that determines a specific conscious experience, for example, seeing a pink elephant.
* The full NCC refers to the brain activity responsible for consciousness as a whole, i.e., for being conscious as opposed to being in a coma, under general anesthesia, or in dreamless sleep
Why can consciousness only really be studied in humans?
- Consciousness can be truly studied only in humans because only humans can verbally report on their conscious experiences.
- Since humans are visual animals, much of the work on content-specific consciousness has involved visual perception. One strategy is to use functional brain imaging to measure changes in brain activity when a visual stimulus stays constant, but conscious perception changes.
- An example of this approach is binocular rivalry. In these experiments, the subjects wear goggles that present different images to the two eyes.
- For example, in Fig. 4A a red triangle is presented
to the right eye and a blue star is presented to the left eye. You might expect under these circumstances that the subject would see a merged image of the red triangle and blue star (Fig. 4B), but that’s not what happens. Instead, one of the two images is perceived for a while, then it fades out and the other image appears (Fig. 4C). - Conscious visual perception alternates between the two images for as long as the subject wears the goggles. So, the visual input stays constant, but conscious perception changes. These changes in conscious perception can be correlated with changes in brain activity to tease out the
activity associated specific perceptions.
Research to study consciousness.
Another approach is to have subjects look at an image, such as a face, that has been distorted to the threshold of detectability by introducing image noise (Fig. 5). The subjects press a button to indicate whether they see the image. By subtracting the trials in which they do see the image from those in which they do not, the researchers identify cortical regions involved in specific conscious perceptions
(Fig 5).
The full NCC
The full NCC has been investigated by taking the difference between functional images when subjects are awake versus when they are in dreamless sleep, or between dreamless sleep and dreaming sleepm(which is, after all, a conscious state). These studies suggest broad regions of the frontal, parietal, and temporal lobes are active during consciousness (Fig. 6).
How brain circuitry and physiology cause consciousness?
- One influential idea is that sustained activity reverberating within and between different cortical and subcortical networks binds together diverse elements (e.g., the different aspects of a visual scene) into a single unified conscious experience.
- Recall that many cortical areas are reciprocally connected by feedforward and feedback projections and that the cortex also forms loops of connections with the thalamus, including cortico-thalamo-cortical connections with association thalamic nuclei that interconnect high order cortical areas.
- Reverberating activity in cortico-cortical and corticothalamo-cortical loops may be involved in sustaining consciousness.
