Reprinted from: Waiter, D.O., Rogers, L. and Finzi-Friied, J.M. (eds.).
BIS Conference Report #42 (Conference on Human Brain Function) Univ. of Calif., Los Angeles, 1976
- It was pointed out by the philosopher, L.J. Rather, with respect to the ontologic argument between dualists and monists that, "The problem is not likely to be resolved within the set of presuppositions that helped to generate it" (Rather, 1965).
- The controversy between holists and topists in neurology is hardly as ancient as that argument; but the controversy is of sufficient age, and continues with such unabated heat, that we might easily wonder if it could only be resolved by finding a new context within which to consider the evidence adduced by both sides.
- In fact, there has been available for over a century a unifying view which is consonant with most of the evidence. This view, originating with Hughlings Jackson (1931), is quite ingenious and, considering the time of its origin, remarkably prescient.
- Jackson began by considering two facts, equally true and seemingly contradictory. He wrote: "The fact that the symptoms are local implies, I hold, that there is of necessity a local lesion. I submit that one-sided spasm, or spasm beginning in one side, implies local change in the central nervous system ... where the fits always start on one side and always in the very same fingers, it is simply incredible that there is no persistent local lesion. The fact that the patient is seemingly quite well betwixt the paroxysms does not negative this view in the least ... [On the other hand] no fact is better recognized than that a large part of one cerebral hemisphere may be destroyed when there are no obvious symptoms of any kind
- "If a small number,. or, let us say, a square inch, of convolutions were cut away by the knife, there would be no loss of power, no paralysis. This is admitted. How then can discharge of this square inch produce violent convulsions? If lack of the part lead to no loss of function, how can discharge of that part lead to excessive function?"
- Jackson's solution to this paradox was to suppose that, in any arbitrary piece of cortex the face, hand, and leg were all represented, but unequally. If the lesion (for example, a small tumor) were located in an area where the representation of the hand was greater than of the other parts, the fit would start in the hand. But if the same area were ablated, no paralysis of the hand would ensue, since the hand was also represented in many other places.
- More generally, we can say that everything is represented everywhere, but in markedly different degrees. There is here a certain resemblance to another entity, the optical hologram, in which each small piece contains a representation of the entire object. Before elaborating on Jackson's scheme, including a consideration of how it is related to holograms, it may be helpful to digress briefly into the nature of brain modeling, from a historical point of view.
- As pointed out by Jaynes (1970), Descartes' model of the brain was a result of his having been impressed by the remarkable hydraulic machinery of his time. This is but one example of what seems to be a general rule: that the workings of the brain are often explained by the natural philosophers of each age by analogizing with the most advanced artifacts of that age. Before Descartes, an analogy was often made with clockwork; after his hydraulic model was abandoned, the Voltaic pile had a vogue. And (as noted by John Marshall) there was a time when railroading inspired neurologists (as witness the "switching" mechanisms postulated by Rudolf Magnus as late as 1924). Telegraphy is still with us (in the form of "relay stations") while telephonic "circuitry" still dominates the thinking of many neurophysiologists. Almost indispensable for most of us is an idea ("feedback") from servomechanisms. More recently, computer language has swamped the field. In this last respect, I recently submitted to a neurobiology journal a paper which was returned by the editor for some corrections, including the advice that the phrase "genetically determined reaction patterns" should be changed to "motor programs ". (Actually this change is worse than gratuitous, since it confuses "hardwiring" with "software".)
- The latest artifact to serve as a model is the optical hologram. But such a model is flawed from the start, since holograms presently available are essentially homogeneous, each sample (if large enough to contain a diffraction pattern) containing much the same information as every other (Gabor, 1972). This is in contrast with Jackson's model, each sample of which is (in general) different from every other. That his model accommodates more facts is probably attributable to his having derived it from observation, rather than out of admiring imitation of some technologic achievement currently the rage. This may suggest that those who are hoping to learn more about the brain from artifacts are less likely to succeed than those who are hoping to develop more sophisticated artifacts by learning more about the brain. In any event, so long as we find it useful to describe people in terms of machines, we must aspire to construct machines which are more like ourselves. Perhaps we come closer and closer to understanding ourselves as we are better able to build machines which are more and more like ourselves, these in turn providing better metaphors and thus a better basis for our self-description.
- Jackson's heterogeneous hologram can be seen as a generalization of which the optical hologram and a system of compartmentalized "centers" are boundary (that is, oppositely extreme) cases. Because it is more general, it can be expected to accommodate a much greater variety of facts. What it cannot do is to tell us what it is that is represented in the heterogram. Jackson suggested that "movements" were represented rather than body parts. Some individuals have supposed that individual muscles are represented in the cortex, whereas a great many have hypothesized the existence of "word images" and the like, having specific location. An impressive lack of progress in this historical debate suggests that this problem, too, requires a certain housecleaning of the verbiage with which the holism-topism controversy has been carried on. For example, the term "equipotentiality" refers to a hypothetical situation in accord with practically no facts.
- The term "cortical center" also refers to a hypothetical entity. A "cortical center", in the sense of a circumscribed area having definite boundaries within which a "function" is wholly contained, does not exist. The term should be consigned to the theoretical limbo in which already reside such outmoded concepts as "phlogiston" and "the ether".*
*Professor S. Chapin has called my attention to the fact that the "luminiferous ether" has yet some advocates (Chappell, 1965).
- In the same vein, the word "localization" applies to lesions but not to functions. We should not speak of functions as being "localized", but rather "represented". Indeed, we may find it desirable to abandon the idea of a "function" in some abstract or Platonic sense. This means that we would no longer speak of hypothesized cerebral functions (for example, gnosis, apperception) which are inferred to underlie those things which we can actually see or measure. Rather, we could try to restrict ourselves as much as possible to descriptions of observable output. Such an emphasis on observables, although not yet given a mathematical treatment suitable to this context, does put us closer philosophically to the quantum mechanics approach to the brain which has recently been urged by Waiter (1971a; 1971b). This may be clearer if I give a few examples of how an observable behavior might be represented in terms of ablation data. (The same principles apply to data from stimulation and evocation.)
- In the present approach, the amount of deficit expectable from an ablation of some arbitrary size (say 1 cm2) would be greatest at some point (which can be called the "max") on the brain surface. That is, the max (possibly multiple) is the location of maximum representation. In general, the expectable deficit per unit ablation would fall off the farther away the ablation is from the max.
- We would have (if we ignore for the moment, at our peril, the depth of the lesion from the surface) a two-dimensional probability distribution over the brain surface. The probability distribution will vary with each observable. For example, with the Similarities subtest of the WAIS, the max is probably left parasylvian and the probability density falls to zero toward all three poles (frontal, temporal, and occipital) and it is essentially zero over the entire right hemisphere.* The Block Design subtest has a high right parietal max as well as a somewhat smaller left parietal max; in both hemispheres the likelihood of deficit falls off in all three polar directions, although it is still consequential at the occipital poles on each side.
*In other words, the area of likeliest Similarities deficit approximates the area most commonly involved in aphasia. Exclusion of aphasics necessarily gives other results. For example, Aaron Smith (personal communication) found that in tumor patients, excluding aphasics, the Similarities max is left frontal. The type of lesion (such as tumor versus infarct) is undoubtedly relevant. The Block Design maxima indicated here are from McFie (1969). Smith also found parietal maxima but he found them to be equal, right and left. This finding (as he forcefully points out) is possibly attributable to the confounding effects of diaschisis, an especially important consideration in progressive lesions such as tumors. It is worth emphasizing, also, that the approach suggested here requires that the locality data be given numerically (possibly on a grid such as that suggested many years ago by von Economo) rather than labeled merely "frontal", "parietal", etc.
- The Block Design representation is apt to be more variable between persons than the Similarities representation since the available solution strategies appear to be more numerous (for example, relatively few persons rely heavily on visualization with the Similarities test, whereas many persons seem to use a verbal, analytic approach to the Block Designs).
- Given the extent (that is, the boundaries) of the lesion we could then do a double integration over the lesion area to obtain the amount of deficit expected. However, such a simple summation may be inadequate since the deficit with larger lesions is likely not a linear sum of the deficits assignable to each of the individual units making up the total lesion.
- Some other complications should be mentioned. For example, since the behavioral loss per unit of damage is less in a slower lesion, the formula in any real case would require the inclusion of a momentum function.
- This approach, it should be noted, can also accommodate cases in which further ablation produces an increase in function (Bogen, 1974), merely by letting the probability density assume negative values at appropriate places.
- One difficulty with this approach is that it seemingly ignores cytoarchitectonic and myelogenetic subdivisions as well as a good deal of patho-anatomical information laboriously acquired over the past century (Meyer, 1974). Nor does it distinguish observable deficit attributable to fibers of passage rather than integrative ganglia (Geschwind, 1970). Nor is it so readily integrated with the entire connectionist, circuitry interpretation for which so much experimental evidence (mainly subcortical in amphibians) was amassed by Sperry (1951). Although this may not be a practical problem, so far as prediction or management is concerned, it leaves a great deal to be desired from the theoretical point of view. But, in the beginning at least, we can be reassured by the considerable evidence showing the non-relevance of certain architectonic information. For example, Rasmussen (personal communication) reported that the sharp functional boundaries (for evoking either movement or sensation with stimulation of the exposed cortex of the waking human) are at the posterior edge of the postrolandic gyrus and at the anterior edge of the prerolandic gyrus. This is in spite of the fact that the sharpest cytoarchitectonic boundary in the brain is halfway between these two, that is, exactly at the rolandic fissure.
- Determination of the probability functions and the relevant constants will require a massive amount of data, but the easy availability of brain scans makes this more feasible than previously. We may even be able to include the influence of depth, now that the computerized tomographic (EMI) scan is available.
- The practical difficulties in the accumulation of the quantitative data required by this approach are awesome, but they are not insuperable. And surely we should not shrink from a more sophisticated model which can accommodate (indeed, requires) large amounts of data which have often been conveniently ignored by the more simplistic traditional alternatives.