Valerie Gray Hardcastle:
My suggestion was that if we look at a higher level of analysis, the level of firing patterns across areas of cortex, then the problem of perceptual binding might disappear. It is now well-documented that there are oscillatory cycles above the areas of cortex responsible for perception that correspond to our "interpretations" of incoming stimuli....What I said was that the millions of "feedback loops [the brain] employs made a travesty of our efforts to establish Newtonian causal sequences...." Indeed, my intent was to suggest to the reader that nonlinear dynamics are characteristic of highly complex systems like the brain. But, Hardcastle's makes a number of points that merit consideration. It is very important not to confuse levels of description, or over-extend low level explanations to higher level cognitive processes. On the other hand, a good general theory of cosnciousness would integrate levels of explanation. In such a global model, lower-level binding would not so much "disappear" as fall into it appropriate, limited place.Here is an analogous way of looking at what I am trying to say. We are all familiar with the rolling pattern of boiling water. But if we trace the activity of a single water molecule as it breaks into its component pieces, we don't find a rolling pattern; we find random motions. If we trace the individual activity of many water molecules, we still only find lots of individual random patterns. So where does the rolling pattern come from? Answer: tracing the motion of individual atoms and molecules is looking at the wrong level of analysis for rolling. One needs to look at the cumulative effects of all the water at the same time. The only way to do that is to look at the water at a higher level of description and describe the motion at that level. The problem of where rolling patterns comes from disappears at the higher level, for it is just the pattern that emerges when water boils. Maybe binding works in the same way.
This is not to say that this higher level patterns are not "causal," as Jim Newman suggests ["the brain ... perniciously refuses to function in a linear, causal fashion"], but it is to say that the effects we see are nonlinear. It is causal; it is just very complicated and probably beyond our capacity to track at any lower level.
In Newman & Baars (1993) I postulate just such a higher-level theory of binding. It encompasses more than 40-Hz oscillations, and has some interesting parallels with Hardcastle's thinking. Here are selected passages from it:
The neural GW model proposes that an explicitly definable neural network subserves the functions of a global workspace. This network will [be] refer[red] to .. as a "Tangential Intracortical Network", or "TIN". Recent work by von der Malsburg and Singer (1988) on self-organization and pattern formation in cortical networks provides a considerably more detailed appreciation of the nature of this network.In subsequent paragraphs I build the argument that this TIN displays all of the basic dynamics of Baars (1988) Global Workspace for representing the stream of consciousness, but requires a "stimulus towards global organization" provided by a cortico-subcortical "attentional matrix" centered upon the thalamus. In a concluding section, "The Binding Problem Revisited", I elaborate,In conceptualizing the functions of this tangential network, Von der Malsburg and Singer (1988) have looked to examples of field interactions studied in physics and chemistry, where extremely large numbers of atoms or molecules interact with each other. They note that the cerebral cortex contains on the order of 10 to the 14 synapses. In physical systems of densely packed units, where the activity of adjacent units is mutually influencing, several principles of self-organization operate. These can be summarized as follows:
1) Local interactions tend to self-amplify. Synchronous or correlated interactions among coupled units strengthen and spread across ensembles of units, creating coherent activity patterns.
2) Developing activity patterns compete. The strongest (most coherent) patterns vigorously grow at the expense of others. This leads to the formation of activity "domains" of different self-amplifying patterns.
3) Domains of activity tend to cooperate. In spite of the overall competition in the system, domains of correlated activity will tend coalesce to form larger, coherent activity patterns. If there are no outside influences acting on the system, the activity patterns with the most internal cooperativ- ity and the least competition will win out.
(after von der Malsburg and Singer, 1988)
Von der Malsburg and Singer [50] note that a fundamental correlate of these three basic principles is that "global order can arise from local interactions .... ultimately leading to coherent behavior." [p. 71] In a large self-organizing network, a number of competing local domains can co-exist, but the tendency of the network is generally towards attaining a globally ordered state. Because of this, even a relatively weak stimulus towards global organization can decisively influence developing local patterns.
.... The key to the solution of the binding problem lies in an understanding of the functions of the nucleus reticularis thalami [nRt] as the heart of an extended cortical activation system. Only when the activities of the TIN are subjected to the global influences of the reticular core do the "local contingency matchings" of this cortical network coalesce into the unified representations that characterize conscious perception.At the time we wrote Newman & Baars (1993), I was not aware of Llinas and his colleagues' papers describing the 40-Hz scanning wave they found in human MEG studies, moving across the entire cortex from front-to-back very 12.5 msecs. If this is a replicable phenomenon, however, it makes testing our hypothesis that much more feasible. Of course, it may turn out that there are actually varieties of global scanning phenomenon at different frequencies and/or rates of movement. Nearly everything about the brain turns out to be more complicated than first thought. Yet, these seem like promsing hypotheis to me, if for no other reason than that they are testable.Thatcher and John [57] summarize a variety of EEG studies, employing multiple electrodes, which have shown that EEG rhythms are "in constant motion shifting across the cortical surface in complex recurring patterns." These scanning phenomenon have been studied in the greatest detail for alpha rhythms (7-13 Hz). In general, alpha waves are most prominent in the visual cortex, but are found in varying distributions throughout the cortex. Alpha is commonly thought to be an index of resting activity in an awake subject, when 70% of the EEG is at alpha frequencies. But what these EEG scanning studies indicate is that alpha activity sweeps periodically across large domains of the cortex in moving waves. The speed of scanning has been shown to vary as a function of general levels of EEG arousal, increasing as arousal levels increase. And although alpha activity, in itself, appears to be an index of a lack of cognitive processing, such period fluctuations in alpha may reflect filtering effects of the attentional matrix described above. Thatcher and John wrote,
The concept of scanning was originally introduced by Pitts and McCullough (1947) in relation to perception in humans. Their idea of a periodic subcortical scanning pulse that probes the momentary state of neural excitability was consistent with evoked potential recordings demonstrating excitability fluctuations coincident with background EEG. For instance, Bartley in 1942 demonstrated a periodic alteration in the excitability cycle of the sensory evoked potential coincident with the frequency and phase of the alpha rhythm. Similarly, Dustman and Beck (1965) and Morrell (1966) demonstrated that behavioral reaction time was minimal when the triggering stimuli occur on the rising phase of the cortical alpha rhythm. [57, p. 71]In their discussion of the genesis of rhythmic oscillations in the CNS, Thatcher and John [57] present evidence for alpha activity being generated at various levels of the CNS from "relatively discrete locus-to-locus thalamocortical interactions" to the cortical-wide synchronization of the EEG characteristic of "resting alpha". At all of these levels, however, thalamic circuits have an integral role. They characterize the thalamus as "a master synchronizer involved in determining information distribution and time parsing of information flow." [p. 70] The heart of this "master synchronizer" is the nucleus reticularis, whose inhibitory interneurons generate the synchronous bursts of activity that modulate both local and global distributions of alpha activity. Herkenham [37] and Steriade & Llinas [54] review more recent evidence confirming NR's "pacemaker" role.Corticothalamic pathways can effect oscillatory activity via their influence upon [nRt]. The firing rhythms of reciprocal thalamocortical projections can be suppressed or enhanced by converging cortical ... influences...
What controls the patterns of these rhythmic waves scanning periodically across the cortex? According to the present model, it could vary from a single ensemble of modular units in visual cortex, to a broad coalition of specialized processors distributed across the entire expanse of the cortex. In either case, it is [nRt] that modulates their combined effects upon the attentional matrix.....
It seems appropriate to conclude with a brief comment on a possible experimental approach for testing the model. The basic premise of the neural GW model is that oscillatory phenomenon play a central role in the global integration of neural systems mediating conscious processes. We have previously cited research ... implicating the tangential cortical network (TIN) in the integrated representation of global features of a visual stimulus via local, synchronous oscillations. This has been demonstrated, in primary visual cortex, for cortical columns separatedby as much as 7 mm. The researchers report, however, that electrode sites greater than 7 mm apart show no such oscillatory linkages (coherency matching).
How, then, might such linkages be made among widely separated cortical columns selectively responding to, say, the orientation, color and movement of the same object, as well as the sound its makes (e.g. a humming top)? Are there, indeed, "higher-order" oscillatory systems that accomplish these more global integrations?
An experiment to test the model could involve something as simple as presenting two commonly employed ERP stimuli, such as a light flash and click, simultaneously, and observing response patterns from arrays of electrodes placed in primary visual and auditory cortex. We would not predict that oscillations in such widely separated areas would be tightly phase-locked. Rather, if a thalamically-generated "scanning wave" does play a role in integrating perception across modalities, there would be a predictable latency corresponding to the time it takes the wave of activation to travel between the two areas (and speed of scanning should vary as a function of arousal levels).... It would be optimal, of course, to have electrodes recording from appropriate sites in the reticular nucleus at the same time. The research cited earlier ..., however, suggests that recordings from the superficial and deep layers of the cortex could provide evidence for such rhythmically integrative oscillations (layer IV neurons do no show "stimulus-linked" oscillations).
Jim Newman
newmanjb@aol.com
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