[From Bruce Abbott (960517.1445 EST)]
For the past couple of months I have been conducting a study in which
laboratory rats work for a portion of their daily meals during one-hour
sessions in an operant chamber. The rats must press a lever to receive 45
mg food pellets on what is termed a CRF (continuous reinforcement) schedule:
1 press per pellet. My students and I have been recording the number of
pellets earned in the chamber, the amount of the earned food actually
consumed, the amount of food subsequently consumed in the home cage, and the
rats' body weights, for each day since the study began. Our intention is to
develop a control-system model that will account for variations in food
consumption in the operant chamber as the amount of food available in the
home cage is systematically varied.
While conducting a review of the scientific literature on feeding and
weight-control, I have been surprised to discover that a control-systems
approach was quite in vogue during the late 1960s and early 1970s, and some
elegant models were constructed to account for the then-available data.
However, it seems that subsequent data did not appear to fit readily into a
control-systems framework, and before long a concensus seems to have
developed that the classical control system does not provide a useful or
compelling account of either feeding or the stability of body weight. No,
to develop an adequate account of these phenomena, one must turn to
external, environmental factors such as food availability, palatability, the
economics of supply and demand, competition among different activities, the
ecological niche occupied by the organism, things of that sort. Needless to
say, I found this conclusion rather discouraging. On what was it based?
Well, I looked for the answer to this question, and I found it. The main
problem turned out to be that pesky concept, the set point. Nobody could
find it.
The problem was highlighted by Wirtshafter and Davis in a 1977 paper
entitled "Set Points, Settling Points, and the Control of Body Weight."
Although there is more to this story than Wirtshafter and Davis's
contribution, I thought I'd focus on their paper as an example of the kind
of thinking that derailed control theory in the decade of the 70s.
Wirtshafter and Davis (1977) begin their paper by noting the growing
interest in applying control theory to the problem of body-weight stability.
They then briefly describe the basic control system:
The defining characteristic of such a system is that it contains a
reference point against which the current state of the organism is
compared. Deviations from the set point generate an error signal
which activates the system to correct the deviation. The effect of
such a system is to maintain a relatively constant value of the
controlled quantity in the face of disturbances imposed by the
environment. It is natural, therefore, when investigating homeo-
static systems which adjust to disturbances by making appropriate
corrections, to assume that the adjustments are made in response to
an error signal indicating the deviation from the desired state.
So far, so good. In fact, it's a rather nice description. But now comes
the caveat:
In spite of the popularity of this notion there is a real question
concerning its value in furthering our understanding of the control
of body weight. Postulating the existence of a neural set point in
order to account for the constancy of body weight does not explain
much; it merely assigns to the central nervous system (CNS) just
that property needed to account for body weight constancy.
Wirtshafter and Davis (1977) go on to note that such a concept has
difficulty explaining such facts as the increase in body weight that
precedes hibernation, or migration, or accompanies pregancy, or follows the
offering of a highly palatable diet, as observed in various species.
To explain these phenomena within the context of a set point model, one
would have to assume either that the set point is labile or that it
becomes inoperative under certain conditions such as the availability of
a high fat diet. Of course, either or both of these assumptions may be
correct, but skepticism begins to develop as the hypothetical set point
begins to acquire just that amount of variability which is necessary to
explain observed variability in body weight.
After describing some of the evidence used to support a control-systems
analysis, Wirtshafter and Davis present an alternative model that nicely
accounts for these data and also for data which they believe pose
difficulties for the control-systems model:
Let us now consider an alternative model for body weight control. Let
us suppose that an animal's feeding mechanism is activated by sensory
stimuli arising from available food, which we will represent by the
letter S, and inhibited by a feedback signal which is proportional to
body weight which we will represent by the letter W. A control system
incorporating these assumptions is shown in Fig. 1.
···
+
S----->[X]-------->[ G ]-------+------> W
- ^ |
> >
+----------[ H ]<------+
Fig. 1. Diagram of a simple body weight control
model which contains no set point.
This model is intended to deal with data showing that the body weight rats
defend varies with the palatability of the food. When the food is bitter,
rats eat less and lose weight, but will defend against disturbances to the
new, lower weight. A similar thing happens when food of high palatability
is offered, with weight increasing to a new level but defended at that
level. To describe the operation of this system, the authors introduce a
new term: "settling point."
The value which W assumes is the predicted steady state weight of an
organism whose body weight is controlled by the system shown in Fig. 1.
We call the value which W takes the settling point of the output of the
system, since any changes in the values of S, G, or H will alter the
value of W and cause it to settle to a new value. ... We prefer the
term settling point since this term is neutral with respect to the way
in which stability is achieved, which may or may not be by means of a
neural set point.
The authors illustrate how this model can be used to account for several
pieces of evidence, including the effect of food palatability and of lesions
to the rat's lateral or vertromedial hypothalamus. They note that changes
in the "settling point" W of the system can be produced by changes in (a)
the sensory stimuli arising from the food, (b) the forward gain G, or (c)
the feedback gain H, and that therefore, changes in body weight (settling
point) following a manipulation do not necessarily reflect changes in a set
point for body weight. In this assessment they are correct. Moreover:
This model suggests that the defense of body weight by animals may reflect
not the existence of an internal standard, but rather a constant magnitude
of stimuli which drive the intake mechanism.
This is also true of their model. Furthermore:
If body weight is regulated by an internal set point mechanism one
naturally wonders why it is singularly ineffective in maintaining a
constant weight in the face of altered dietary palatability. Rejection
of a set point concept of weight regulation would have the additional
advantage of resolving the problem of an animal's willingness to ingest
highly palatable substances in the absence of deprivation.
Wirtshafter and Davis admit that their model is far too simple to provide a
complete explanation, but they state that it does draw attention to several
points, among which is the following:
... the postulation of a neural set point is not necessary to account
for weight control. The fact that a given quantity is, under some
circumstances, maintained at a relatively constant value does not by
itself imply the existence of an internal reference value.
So there you have it: the concept of set point is unnecessary; body weight
may be determined more by external (stimulus) factors than by internal set
points.
But wait. Does Wirtshafter and Davis's argument make sense? Take a look at
their Figure 1. What role is S playing in the diagram? The neural
representation of the sensory quality of the food enters the comparator, as
does the neural representation of body weight. As S varies, W follows. In
fact, the Wirtshafter and Davis "alternative" to "set point" control systems
is nothing less than a "set point" control system, with S playing the role
of the set point. This is a _very_ peculiar sort of control system,
however, in that the reference level (set point) for _body weight_ is
determined by the magnitude of the sensory input representing _food
palatability_. Imagine that: the environment determining the reference
level! The animal's body weight is entirely at the mercy of the sensory
qualities of its food, and it will starve to death if presented with
perfectly good food having no taste, or a bad taste. It seems doubtful that
such an important variable would be left at the mercy of the environment.
But this is not what is observed; rats given quinine adulterated food
suppress feeding, but only until their body weights decline sufficiently
that they are willing to tolerate the bad taste. This fact strongly
suggests that weight (or a close correlate) is being referenced to an
internal standard, with increasingly severe deviations below reference
bringing about a greater and greater tendency to overlook the bitter taste
and consume the food anyway. And rats given quinine adulterated (bitter)
food injected directly into the stomach (eliminating gustatory stimuli) turn
out to regulate their nutrient intake quite well and do not lose weight.
[This latter bit of evidence was not available to Wirtshafter and Davis at
the time of their article was published.]
So, what are we to conclude? After suggesting that the notion of an
organism governed by control systems with internal set points is too
seductive, Wertshafter and Davis propose an organism governed by control
systems with _external_ set points whose levels are dermined by
environmental caprice, thus demonstrating that the concept of set-point
control is unnecessary? To paraphrase the character Fagan in the musical
"Oliver," I think they better think it out again.
Regards,
Bruce