Wednesday, August 8, 2007

Emergent Properties and Connectionism-by Francisco Varela, Evan Thompson, and Eleanor Rosch

The following is from THE EMBODIED MIND: COGNITIVE SCIENCE AND HUMAN EXPERIENCE (pp. 93-98) which combines insights from Cognitive Science and the Buddhist tradition. It illustrates the co-dependent arising---or emergence, or self-organization---of neural activity between different brain regions. It relates this to Buddhists insights that parse the arising of mental experience into five components or aggregates (forms, feelings/sensations, perceptions, dispositional formations, and consciousness). These may appear to be separate, but are really co-dependently arising aspects of experience. The previous entry, also from the same book, considered the Buddhist view of the co-dependent arising of both self and world. Reprinted with permission of the authors and MIT Press.

… [A]lthough neurons in the visual cortex do have distinct responses to specific features of the visual stimuli, these responses occur only in an anesthetized animal with a highly simplified internal and external environment. When more normal sensory surroundings are allowed and the animal is studied awake and behaving, it has become increasingly clear that stereotype neuronal responses become highly context sensitive. There are, for example, distinct effects produced by bodily tilt or auditory stimulation. Furthermore, the neuronal response characteristics depend directly on neurons localized far from their receptive fields. Even a change in posture, while preserving the same identical sensorial stimulation, alters the neuronal responses in the primary visual cortex, demonstrating that even the seemingly remote motorium is in resonance with the sensorium. (...)

It has, therefore, become increasingly clear to neurosciences that one needs to study neurons as members of large ensembles that are constantly disappearing and arising through their cooperative interactions and in which every neuron has multiple and changeable responses in a context-dependent manner. A rule for the constitution of the brain is that if the region (nucleus, layer) A connects to B, then B connects reciprocally back to A. This law of reciprocity has only two or three minor exceptions. The brain is thus a highly cooperative system: the dense interconnections among its components entail that eventually everything going on will be a function of what all the components are doing.

This kind of cooperativeness holds both locally and globally: it functions within subsystems of the brain and at the level of the connections among these subsystems. One can take the entire brain and divide it into subsections…. These subsections are made up of complex networks of cells, but they also relate to each other in a networked fashion. As a result the entire system acquires an internal coherence in intricate patterns, even if we cannot say exactly how this occurs. For example, if one artificially mobilizes the reticular system, an organism will change behaviorally from, say, being awake to being asleep. This change does not indicate, however, that the reticular system is the controller of wakefulness. That system is, rather, a form of architecture in the brain that permits certain internal coherence is to arise. But when these coherences arise, they are not simply due to any particular system. The reticular system is necessary but not sufficient for certain coherent states such as wakefulness and sleep. (...) In fact, there are many levels of resolution at which such neuronal emergences can be studied, from the level of cellular properties to entire brain regions….

Consider what happens in visual perception in its peripheral stages. (...) The optic nerve connects from the eye to a region in the thalamus called the lateral geniculate nucleus (LGN) and from there to the visual cortex. The standard information-processing description…is that information enters through the eyes and is relayed sequentially through the thalamus to the cortex where “further processing” is carried out. But if one looks closely at the way the whole system is put together one finds little to support this view of sequentiality. (....) It is evident that 80% of what any LGN cell listens to comes not from the retina but from the dense interconnectedness of other regions of the brain. Furthermore, one can see that there are many more fibers coming from the cortex down to the LGN than there are going in the reverse direction….

Thus even at the most peripheral end of the visual system, the influences that the brain receives from the eye are met by more activity that flows out from the cortex. The encounter of these two ensembles of neural activity is one moment in the emergence of a new coherent configuration, depending on a sort of resonance or active match-mismatch between the sensory activity and the internal setting at the primary cortex. The primary visual cortex is, however, but one of the partners in this particular neuronal local circuit at the LGN level. Other partners, such as the reticular formation, the fibers coming from the superior colliculus, or the corollary discharge of neurons that control eye movements, play an equally active role. (...)

What we have described for the LGN and vision is, of course, a uniform principle throughout the brain. Vision is useful as a case study since the details are better known than for most other nuclei and cortical area. An individual neuron participates in many such global patterns and bears little significance when taken individually. In this sense, the basic mechanism of recognition of a visual object or a visual attribute could be said to be the emergence of a global state, among resonating neuronal assemblies.

At this point we would like to return to topic of emerging biological processes and the five aggregates discussed in the previous chapter. We raised the issue there of whether the aggregates arise sequentially or simultaneously. (...) [C]oncern with the parsing of experience is one of the more remarkable points of convergence between cognitive science and mind full/awareness tradition. To take a sequential view of the aggregates seems similar to taking a sequential view of brain activity. Form would have to come first through some pre-attentive segmentation at the retinal and geniculate level, then sensations and perceptions would arise at the reticular and collicular input, whereas concepts and consciousness would be added at different stages of “higher” brain centers…. If, however, perception cannot be so simply analyzed into a straightforward sequence, then it becomes difficult to separate the “low” level of form from the ”higher” levels of, say, sensations and discernments [italics added to indicate the attributes]. The arising of form always involves some predisposition on the part of our structure. If we take the notion of a heap or a pile (Skandha) as a metaphor for the emerging configurations of a neural network, we will be led to think of the aggregates as resonant patterns in one moment of emergence. (...)

It is possible, then, to see the notion of a heap or pile as a metaphor for what we would now call a self-organizing process. The aggregates would arise as one moment of emergence, as in a resonating network where strictly speaking there is no all-or-none separation between simultaneous (since the emergence pattern itself arises as a whole) and sequential (since for the pattern to arise they must be a back-and-forth activity between participating components). Of course… the aggregates do not constitute an information processing theory. Nonetheless, the neuropsychological approach that we have just adumbrated seems compatible with the direct observation based on mindfulness/awareness meditation, thus making all the more remarkable the fact that this tradition has continued to verify the parsing of experience into coherent moments of emergence.