Cortical Integration: Possible Solutions to the Binding and Linking Problems in Perception, Reasoning and Long Term Memory
Nick Bostrom
Page 7
Source: http://www.nickbostrom.com/old/cortical.html
7. Discussion: the means of cortical integration
We have looked at various proposals for how cortical integration could be achieved, and discussed what evidence there is for each. It now time to put things together. The first point to make is that the proposals are not mutually exclusive. On the contrary, there is reason to believe that several of them are involved, in different regions and for various purposes in the brain.
To begin with early sensory processing, there can be no doubt that integration by convergence plays an important role here. There is also evidence that synchronization occurs and probably makes a functional contribution to contour extraction in visual processing and to spatial localization of the sound source in audition, though in the latter case not primarily as an integration mechanism.
It also possible that convergence is used in motor processing. Here it would not mean that a large number of stimuli or instances of more abstract tokens are subsumed under a unitary representation, but rather that such a unitary representation would constitute the precept to undertake a certain action, or sequence of actions. For example, it is possible that there could be in snake brains a cluster of cells whose activity indicated that the animal were about bite. Exactly which motor neurons were to be activated in order to carry out the bite, would be determined later on after co-ordinating the snakes present position and the position and movement of the target etc. So the cluster indicating "bite!" could be thought of as the point of convergence for all possible motoric outputs that would constitute an act of biting.
In view of the importance of sequenciation in motoric computing, it could seem that this would be the best place to look for integration by convolution, for example through RAAM-like structures. That is possible, but there are also other mechanisms that could be used to achieve sequencing. Integrated representations are not necessary for temporally structured responses.
Even less in known when we move away from the coastal areas of sensory and motor processing into the inland of cognition. When we investigate into the integration mechanism underlying abstract thought, it is necessary to consider both passive and active representations: both your knowledge of your first year in school, say, and the your knowledge and awareness of your present thoughts, which are hopefully about this paper and its subject matter. The vast majority of our knowledge is encoded in passive representations. Synchronization, the state of temporal coherence in neuronal spiking, can obviously not bind passive representations, since they are, per definition, not spiking. If "synchronization" has any role to play in the integration of passive representations, it must rather be the tendency to engage in synchronised spiking, rather than synchronised spiking itself.
How would such a tendency be synaptically encoded? Two representations that were to be bound by a tendency to synchronization would have to influence one another, presumably by sending excitation or disinhibition to one another. Thus they would first of all tend to activate one another, before there could be any possibility of synchronization. But that means that they would, in effect, form a complex attractor. Hence, it looks as if binding passive representations by means of a tendency to synchronization would already presuppose that that they were bound through annexation. One may therefore wonder what the synchronization tendency would contribute to the integration: wouldn't it merely wed a couple that was already married to each other?
Perhaps it could abolish the need of multiple concept instantiations? Suppose the patterns ABC and DBE are to be remembered. Instead of storing a symbol representing B twice, we could mold a complex attractor ABC and another complex attractor DBE overlapping the first one so that the B pattern appears only once. ABC would be distinguished from DBE by having a tendency to synchronize internally but not with the D and E parts of DBE; and vice versa.
There are problems with this suggestion. First, it is not clear how the synaptic weights would be configured to achieve this selective tendency to synchronization. Second, even it such a synaptic weight configuration could be specified, it is not easy to see how it could be learnt. Third, the specificity of the synchronization tendency for passive representations would have to be extreme. There are countless pieces of knowledge that involves concepts such as "whales", "Paris", "inflation" etc., all of which would have to have their own unique synchronization properties. But since the spacing between spikes of a neuron is often as short as 10 ms, and since the resolution of synchronization is about 2-3 ms, no more than about four propositions containing a given concept could be stored through synchronization tendency with a single concept instantiation. For these reasons it seems unlikely that the tendency to synchronised is a major part of the explanation of how declarative knowledge is stored in passive memory.
Less implausible is the suggestion that synchronization plays a role in the integration of working memory, the active representations of abstract thought. This would be merely actual synchronization; there would be no need to lay down tendencies to specific synchronised states in the synaptic weights, and this would simplify matters. Also, it is much more plausible to suppose that there are only a handful "propositions" in our immediate awareness than it is to postulate the same for our entire store of passive memories.
One outstanding feature of early visual processing is the prevalence of topographic maps. It is tempting to conclude that this accounts for the distinct phenomenological quality of spatial consciousness: the sort of peripheral presence that all details of the visual scene enjoy even when they are not the object of our attention, in contrast to abstract thoughts and beliefs, which do not seem to "be there" when we are not thinking them. Is it possible that the "peripheral presence" of shapes and colours in our visual field in absence of attention is due to the fact that they are represented by distributed neuronal activity on topographic maps; whereas abstract beliefs become conscious only when their neural representations are uploaded from passive memory onto a special stage that only has room for but a small number of active actors at a time?
Less metaphorically, there could be a sheet of reusable neurons whereunto active representation were pasted by the clamping mechanism discussed in the section about annexation. As soon as one of these propositions were dropped from our awareness, its place could be taken by another proposition. The unlearning may be no problem if only short term potentiation were used. The increased synaptic efficacies would simply decay away within a second if the pattern is inhibited.
This would hardly be a plausible model for anything other than explicit logical reasoning. Since explicit logical reasoning is not the only, or even the major, thing that goes on in higher cognition, there would have to be ways for passive representations to interact other than through being lifted up in the lime light of such a stage of active support. Interaction, or operations performed on neural representations, is not the primary topic of this paper; but such issues must nonetheless be taken into consideration when we search for potential forms of representations and integration in cortex; for whatever the mechanism that brings about cortical representations, it must surely be such as to facilitate the operations of reason. In fact, there are good grounds for believing that the greatest part of animal and human intelligence lies not in the quickness with which certain operations can be performed upon cortical representations, but rather in the way these representations themselves are organized: into a fairly coherent world view which narrows down enormously the search space when we seek a solution to an everyday problem.
We have focused on the integration of the basic constituents of our knowledge representations, but in order to get a realistic and workable system, there would have to be much more structure than that. Distinct representations of "propositions", if they exist in the brain at all, can only be the bricks of the architecture of mind. Ways must be found to achieve the coherence of organization that allows relevant considerations from variegated fields of experience to be brought to bear as constraints in the shaping of a plan or an opinion.
7. Conclusions
Convergence is indispensable for cortical integration, but not by itself sufficient for a flexible representation system. Convergence onto reverberating cell assemblies, in particular, is a necessary basis for working memory. It is unknown whether integration through convolution occurs in the brain. Synchronization may play a part in early sensory processing and possibly in active reasoning, but not in holding together representations that have been laid down in long-term memory. Annexation, on the other hand, has many virtues that makes it a strong candidate for the integration mechanism for declarative passive memory as well as working memory. It could be worth searching for annexation experimentally, though the task is made difficult by the fact that annexation could take on so disparate forms.
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