http://www.thegreatonwardpress.com/9974/08/index25article1.html
We experience the world as a whole. Although myriad signals relentlessly bombard our senses, we somehow integrate them into unified moments of conscious experience that cohere together despite their diversity. Because of the apparent unity and coherence of our awareness, we can develop a sense of self that can gradually mature with our experiences of the world. This capacity lies at the heart of our ability to function as intelligent beings. The apparent unity and coherence of our experiences is all the more remarkable when we consider several properties of how the brain copes with the environmental events that it processes.
First and foremost, these events are highly context-sensitive. When we look at a complex picture or scene as a whole, we can often recognize its objects and its meaning at a glance, as in the picture of a familiar face. However, if we process the face piece-by-piece, as through a small aperture, then its significance may be greatly degraded. To cope with this context-sensitivity, the brain typically processes pictures and other sense data in parallel, as patterns of activation across a large number of feature-sensitive nerve cells, or neurons. The same is true for senses other than vision, such as audition. If the sound of the word GO is altered by clipping off the vowel O, then the consonant G may sound like a chirp, quite unlike its sound as part of GO.
During vision, all the signals from a scene typically reach the photosensitive retinas of the eyes at essentially the same time, so parallel processing of all the scene’s parts begins at the retina itself. During audition, each successive sound reaches the ear at a later time. Before an entire pattern of sounds, such as the word GO, can be processed as a whole, it needs to be recoded, at a later processing stage, into a simultaneously available spatial pattern of activation. Such a processing stage is often called a working memory, and the activations that it stores are often called short-term memory (STM) traces.
For example, when you hear an unfamiliar telephone number, you can temporarily store it in working memory while you walk over to the telephone and dial the number. In order to determine which of these patterns represents familiar events and which do not, the brain matches these patterns against stored representations of previous experiences that have been acquired through learning. Unlike the STM traces that are stored in a working memory, the learned experiences are stored in long-term memory (LTM) traces. One difference between STM and LTM traces concerns how they react to distractions. For example, if you are distracted by a loud noise before you dial a new telephone number, its STM representation can be rapidly reset so that you forget it. On the other hand, if you are distracted by a loud noise, you (hopefully) will not forget the LTM representation of your own name.
The problem of learning makes the unity of conscious experience particularly hard to understand, if only because we are able to rapidly learn such enormous amounts of new information, on our own, throughout life. For example, after seeing an exciting movie, we can tell our friends many details about it later on, even though the individual scenes flashed by very quickly. More generally, we can quickly learn about new environments, even if no one tells us how the rules of each environment differ. To a surprising degree, we can rapidly learn new facts without being forced to just as rapidly forget what we already know. As a result, we do not need to avoid going out into the world for fear that, in learning to recognize a new friend’s face, we will suddenly forget our parents’ faces.
I have called the problem whereby the brain learns quickly and stably without catastrophically forgetting its past knowledge the stability-plasticity dilemma. The stability-plasticity dilemma must be solved by every brain system that needs to rapidly and adaptively respond to the flood of signals that subserves even the most ordinary experiences. If the brain’s design is parsimonious, then we should expect to find similar design principles operating in all the brain systems that can stably learn an accumulating knowledge base in response to changing conditions throughout life. The discovery of such principles should clarify how the brain unifies diverse sources of information into coherent moments of conscious experience.
— Stephen Grossberg,
Brain Learning, Attention and Consciousness,
Chapter 61 in
Essential Sources in the Scientific Study of Consciousness,
ed. Bernard J. Baars et al.
Filed under: natural philosophy
