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The relationship between memory and consciousnessEdit
There is a semantic problem that I have often run into when discussing memory and consciousness. People usually have distinct conceptualizations of "memory" and "consciousness" that do not overlap. In what follows I will try to bring memory and consciousness together. There are famous examples from history in which two apparently unrelated phenomena were found to be related. For example, Newton realized that the same force that holds people to the ground is what holds the Earth's Moon and the planets in their orbits.
Part of the problem is that memory has two sides to it: the storage side and the utilization side. In general, if I say "memory is at the center of the problem of consciousnes" people are going to think I mean that storage of memories is central to consciousness. As described below, I like Edelman's theory of consciousness and and in his theory the storage of memories is important to the construction of human consciousness. However, on a second-to-second basis, ongoing conscious experience is only dependent on access to existing memory stores. This idea shocks most people because they imagine that consciousness is made possible by some kind of memoryless system like a TV camera collecting input and sending it right to a TV screen for viewing.
We need to be clear about the nature of memory and consciousness. In the case of memory, we all agree that it is possible to have memories that are due to experiences from either long ago (say, years in the past) or from more recently (say, 30 minutes ago). Neurobiologists often make a distinction between short-term and long-term memory. Most people are fairly comfortable with the idea of memories that are years, days, or maybe 5 minutes old. Some Long-term memory mechanisms require relatively slow processes such as alterations in gene transcription and formation of new synapses. Some short-term memory processes seem to just involve quick changes to synapses such as alterations in protein phosphorylation. In general, there are ways of converting some shorter-term memories into longer-term memories. An interesting question is how the brain "decides" which experiences are important enough to be remembered.
Let's push things under 5 minutes. If I tell you a phone number you can repeat it back to me. Most neurobiologists are comfortable with the idea of "immediate memory" or "working memory". In the case of a phone number you might only remember it long enough to make a phone call, maybe 10 seconds after someone tells it to you. I have never read the book The Man Who Mistook His Wife For A Hat : And Other Clinical Tales by Oliver Sacks, but my guess is that most people who are described as having "lost the ability to form memories" still have "immediate memory" or "working memory".
Can we place memory in an even shorter time frame? What if I say a single number and ask you to repeat it back to me? Would you be willing to say that you are able to act on a memory of what you heard me say when you immediately repeat something I have just said? Most people start to get uneasy at this point. You might think, "No, this is not memory. I have simply held what you said in my conscious awareness and repeated what I heard you say. My act of repeating what you said did not involve the storage of a memory." We could adopt a definition of memory that includes the idea that something is only a memory if it can pass out of consciousness and then be brought back out of memory storage to again become part of our conscious thoughts, but I think this is an artificial limitation. As discussed below, some memory processes do not involve consciousness at all. In any case, I do not want to force upon you the idea that some forms of memory can be stored in a fraction of a second. My goal here is to get you to think about the idea that you can certainly access your memories very quickly.
Most people with normal brain structure and function are able to conceptualize three divisions of time: past, present, future. People usually think of consciousness as existing in the present. We sometimes describe the contents of consciousness as "present experience". This contrasts with what we think of as memory. We think of memory as being due to experiences that were in the past, not the present. But when we recall memories that act of recall CAN contribute to present experience. What happens if a person is in a sensory deprivation chamber? Is their consciousness generated by their memories? You might argue that there is some non-memory part of the brain that that generates consciousness in the absence of sensory input, but would anything that produces consciousness be independent of previously stored memories?
In Edelman's trilogy of books about the mind, the book about consciousness is called "The Remembered Present". The title of the book automatically forces us to wonder if it make sense to think of some form of memory as being what we experience as the present contents of our consciousness. Now, if you are dogmatically attached to the idea that memories are the domain of past experience while consciousness is the domain of the present, then you automatically reject the notion that memory can have anything fundamental to do with consciousness. You may also get mad at someone who tries to suggest such a thing. "You are just playing games, trying to redefine the meaning of the terms memory and consciousness! Stop it! We all KNOW that consciousness is not memory."
Simple neuronal networks are simple input-output devices. A sensory stimulus is used to trigger some output such as a muscle contraction. In a neuronal network with no memory, the same input to the network will always cause the same output. We can define memory storage as being a process that causes a change in the input-output relationship that a neuronal network expresses. Once a neuronal network has had its structure and function modified by the storage of a memory, all future activity of that network is to some degree an expression of that memory. Is consciousness the expression of certain types of memories?
An important division in memory types is the division between "explicit" and "procedural" memories. Sometimes these categories are called "declarative" and "implicit" memory, respectively. We can also call these two broad categories "conscious" and "unconscious" memory systems. Learning to ride a bike is a good example where "procedural" memories are formed. We are not conscious of all of the adjustments that our nervous system makes when we learn to ride a bike. Much of the learning of bike riding is accomplished by changes to neuronal networks that do not enter into our conscious awareness. We can conclude that there are neuronal networks in our brain that can store memories and that do not participate in consciousness while there are other networks that can store memories that can enter into conscious experience.
What is the relationship between unconscious brain processes and conscious brain processes? Are these two distinct sets of brain processes or is there some interaction and interdependence? I think it is clear that information can pass back and forth between those neuronal nets that can and do make our conscious experiences and those networks that are not directly involved in producing conscious experience..... but we often do not think about this because our unconscious brain processes are by definition outside of our conscious thoughts. People such as George Lakoff have struggled mightily to make the point that our unconscious brain processes heavily influence our consciousness. I like an analogy to an iceberg. Consciousness is the tip of the iceberg and its ability to rise up into the air of consciousness is totally dependent on the vast amount of unexperienced unconscious brain activity that supports it. In particular, a bit of introspection and thinking should convince people that their explicit memories depend heavily on unconscious memory processes. When a child learns an alphabet, neuronal networks are shaped in the brain and memories are stored that allow recognition of spoken words and that allow the child to say letters and words. When we speak, we are not consciously aware of the vast amount of neuronal network activity involved in how we recognize and understand the words we hear and how we make our bodies say words and what we want to say. If consciousness is an expression of memory utilization, it is only an expression of a subset of our memories. Only certain parts of the brain are expected to contribute directly to conscious awareness and do so because they store memories that are expressed within conscious experience.
Edelman's theory of consciousness makes a distinction between two types of consciousness; primary and higher-order. Primary consciousness is coupled to ongoing sensory inputs and perception, and in Edelman's model, normally depends heavily on the interaction of the perceptual input stream with "a special form of memory" that is located in cerebral cortex. In Edelman's model, higher-order consciousness is heavily dependent on the parts of the brain involved with language behavior and, like primary consciousness, is not independent of memory processes. If consciousness is dependent on memory processes then we need to be clear about the nature of that dependence. We can also ask: is the ongoing process of conscious experience is the SAME THING as the ongoing process of using existing memories and constructing new explicit memories? Is the purpose of consciousness to be a system for making use of past memories to control behavior and for controlling which new explicit memories to form?
This line of thinking leads us to the idea that memory and consciousness form a closed loop: our consciousness is made possible by what we have learned (depends on existing memories) and the purpose of consciousness is to control the formation of new memories. Does not such a closed loop introduce a chicken-or-the-egg problem? In Edelman's theory, this problem is solved by what Edelman calls the "primary repertoire" of neuronal networks that are able to be produced during embryonic development under the guidance of our genes and independent of interactions of the senses with an environment. In Edelman's model, we are genetically endowed with everything needed to make a basic neuronal network that can start storing new memories and start expanding our consciousness beyond the rudimentary form of consciousness that we have prior to interactions with an environment and prior to learning.
We can use the visual system as an example. If a mammal is prevented from being exposed to light during early life, the visual cortex never develops the complex neuronal networks required for normal visual processing and visual awareness. Such an animal may have a rudimentary consciousness of brightness and darkness, but not the sort of vivid visual awareness that is produced by a visual cortex that has learned how to wire and function because of past sensory inputs from a complex environment.
People almost always have a sensation that consciousness is unified in a single instant of the present. However, careful studies have been done which show that various sensory inputs arriving over a range of several tenths of a second can be experienced within a single perceptual moment. The brain processes that generate consciousness simply do not have a fine time resolution. What we experience as the present is actually a period of time that spreads out to include the effects of brain activity that extends into our immediate past. According to Edelman's model of consciousness, we have essentially no consciousness outside of what is generated by activation of neuronal networks that have in the past stored memories. What we experience as conscious awareness is almost entirely constrained to ongoing patterns of cortical activity that are memory in action. New sensory inputs and ongoing patterns of memory activation resonate with other existing stored memories to trigger yet more new patterns of memory activation. In this view, the contents of memory are the contents of consciousness. What we experience as consciousness is the remembered present. The remembered present is a construct generated by our memory and some triggers from new sensory inputs. In other words, utilization of neuronal networks that have been shaped by past experience (the circuits have stored memories) is what we experience as conscious awareness of the present. We literally remember what we experience as the present. In this model of consciousness, second-to-second conscious experiences are not dependent on the production of new long-term declarative memories, but consciousness does depend on access to previously stored memories.
So, back to the game in which I say a letter and you immediately repeat it. Assume that people with normal language function have a neuronal network that gives them the capacity to hear another person say a letter and then they can repeat it. How was that neuronal network formed? Is it a simple network that could have been genetically programmed and wired up during embryonic development? Or is this network something that has been heavily shaped and sculpted through past experience, learning, and the storage of memories? Some people seem to have the idea that consciousness is built into a brain like the ability to show an image is built into a computer monitor. Just turn it on and consciousness shines out of a brain. However, except for the rudimentary consciousness that is produced by our basic genetically-programmed brain structure, our consciousness is learned and is powered by the activation of neuronal circuits where certain memories are stored.
What about the purpose of consciousness and the idea that new declarative memory storage is guided by consciousness? We often think of the purpose of consciousness in terms of our ability to exercise our free will. If we are aware of things, then we can decide to behave in certain ways. An alternative view is that the purpose of consciousness is to allow a special kind of adaptive utilization of stored memories that is not possible for our procedural memory systems. Our unconscious memory systems produce adaptive behavior only when we engage in behavior. In contrast, our explicit memory system can produce adaptive behavior by functioning uncoupled from ongoing behavior. The explicit memory system can function in "virtual mode" by accessing and combining memories without immediately producing behavior. We use the explicit system to plan possible behaviors. When used in "virtual mode", what is produced is new explicit memory that can then be used as the basis for actual behavior.
I wonder if consciousness evolved as a way of playing with one's memories. The large bundles of neurons connected to the visual cortex from our eyes and the fact that our visual systems evolved to respond to the outside world means that our consciousness normally seems mostly concerned with sensory experience while our conscious experiences during sensory deprivation seem washed-out and pale in comparison. Our vivid visual awareness is due to the adaptive importance of paying close attention to visual sensory information. We usually say that we are easy to distract visually. Visual distractions can deflect our consciousness from "deep thought" and planning. This is adaptive. If danger is near we do not want to be "lost in thought".
If we are making a plan, (for example, how to get a piece of fruit before someone else does) we need to constantly update our plan depending on what we see. If we want to really think deep thoughts we either isolate ourselves from visual distractions or we create a set of visual cues that will guide our thinking in a particular direction.
What about the details of how conscious experience is generated? If the above view of dependence of consciousness on memory is correct, then we need to study the explicit memory circuits of the cerebral cortex. Once we understand the memory circuits, I suspect that the basis of conscious awareness will be a rather trivial and unavoidable outcome of how those memory circuits function.