Staph bacteria. Picture courtesy of Janice Carr/CDC via BBC.

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Is modifiable behaviour in bacteria evidence of learning?

Di Primio, Muller and Lengeler (2000, pp. 4, 7) cite cognitive scientists who regard the existence of indirect, modifiable stimulus-response coupling as evidence of cognition. In fact, as the authors point out, this coupling is a universal feature of (cellular) organisms:

R. 5 In any cellular organism, the reaction to a stimulus is always indirect and modifiable (through the addition or removal of other stimuli).

The coupling between stimulus and response in bacteria is indirect because when a sensor detects a chemical, it activates a chain of chemical reactions, each of which is reversible. The coupling is modifiable: if E. coli's sensors detect an attractant (e.g. galactose), and later sense another compound (e.g. glucose) that is more attractive than the first one, a "weighing" of the relative quality of the nutrients occurs, and the chain of reactions resulting in directed motion is amplified. The co-presence of attractants and repellents in solution generates an integration of the "run" and "tumble" responses, at the chemical level (so-called "conflict resolution").

I would suggest that the authors have been misled by an ambiguity in the word "modifiable", which may be understood in two ways: responsive to external (epigenetic) changes, or not governed by a rule or pattern which is fixed over time. The word "modifiable", understood in the former sense, has no mentalistic connotations. Only in the latter sense does it imply the kind of flexibility that might deserve to be called cognition. In fact, as Kilian and Muller (2001) point out, the way in which bacteria react to a chemical is utterly inflexible, at the molecular level:

In unicellulars, in each of the molecules of an information transfer path, regardless out of how many elements it is composed, both a functional specifity (sic) and a goal specifity (sic) can be discerned. Each molecule contacts its goal substrates and interacts with them according to its respective function. Both specifities (sic) are pregiven in the enzymatic active center on the molecular level (2001, p. 3).

In other words, the apparently complex behaviour of bacteria in response to multiple simultaneous stimuli (positive and/or negative) is merely the resultant of two or more inflexible existing action patterns. The rules governing the behaviour of bacteria do not change; all that changes are the external circumstances (i.e. the presence of a new attractant or repellent). The behaviour of the bacteria can be perfectly well described using a goal-centred intentional stance. E. coli bacteria have a built-in preference for one goal (glucose) over another (galactose), which explains their response to the new information that glucose is nearby. There is no need to invoke mental states here.

The philosophical distinctions that have been drawn allow us to formulate another conclusion:

S.7 Behaviour by an organism which conforms to a fixed pattern or rule is not a sufficient warrant for ascribing cognitive mental states to that organism, even if stimulus-response coupling is indirect and modifiable (by the addition or removal of other stimuli).

Fixed action patterns may, of course, be accompanied by mental states in an organism. Taken by themselves, however, they do not provide a warrant for ascribing mental states to that organism.

Definition - "fixed pattern"
We can mathematically represent a pattern of behaviour in an organism by an output variable (say, z). A fixed pattern can be defined as a pattern where the value of the output variable z remains the same, given the same values of the input variables. This definition may seem innocuous, but it has two surprising implications. First, the occurrence of different values of the output variable under different circumstances does not imply flexibility, despite the fact that some authors (Godfrey-Smith, 2001; Carruthers, 2004) refer to it as such (see Appendix).

Suppose that we can describe a piece of behaviour in an organism using a mathematical function F and some input variables (or parameters) x1, x2, x3, ... xN, where the value of the output variable z is F(x1, x2, x3, ... xN). The above definition entails that even when the values of x1, x2, x3, ... xN vary over time, the behaviour still conforms to a fixed pattern, so long as the function F remains the same.

Another surprising implication is that even in a fixed pattern, the value of the output variable z may be determined by two or more different functions, depending on the values of the inputs. A simple case would be the following program statement, written in Pascal code:

IF (x > 4) THEN z := F(x)
IF (x > -2) THEN z:= (F(x) + G(x))
ELSE z := G(x)

In the list of functions, we might define F(x) as, say, x + 3 and G(x) as (x - 5) / 2. Here we have two functions being invoked for different values of the input variable x, but the value of the output variable z remains the same for the same value(s) of the input(s). If the value of x changes from 5 to -7, the function changes, but because the program has not changed, we can describe the overall pattern as fixed.

In short: even behaviour that is categorised as "modifiable" may be the result of an underlying fixed action pattern. (Mathematically, adding or removing a stimulus can be represented as changing the value of an input variable for one of the functions determining the value of the output.)

Definition - "flexible behaviour"
If the program governing an organism's behaviour changes over time, such that the value of an output variable z is no longer the same for the same inputs, whether because of a change in the function(s) which define the value of z, or the parameters of the function(s), or the conditions in the program under which the function(s) are invoked, then the behaviour described by z is flexible.

In the case of the Pascal statement above, a change in
(i) the IF statement conditions (e.g. from (x > 4) to (x > 5)) or
(ii) the definition of the functions F or G, or
(iii) the number of parameters they require,
would qualify as an instance of flexible behaviour.

In other words, truly flexible behaviour requires not just new values of the output variable for different values of the inputs, but new patterns of output, new kinds of input, or new conditions under which the output patterns are generated.

N.11 Behaviour by an organism must be flexible before it can be regarded as a manifestation of a cognitive mental state. (Corollary of S.7.)

The importance of fixed patterns

The distinction drawn between fixed and flexible patterns of behaviour might suggest that we can divide the world into mindless individuals whose action patterns are utterly rigid, and individuals that are able to behave in a flexible manner, making them candidates for having minds. Such a dichotomy is both simplistic and profoundly mistaken, as it overlooks a more fundamental division between entities whose patterns of behaviour are internally regulated (by a master program of the kind described in chapter 1) and those whose patterns lack such internal regulation. Only entities of the former kind can be said to be alive, and hence to be eligible for mental states (see Conclusion N.4).

The point that needs to be made here is that flexible behaviour is built upon a supporting bedrock of fixed action patterns, which organisms require in order to survive:

Cognition, at least in Nature, can exist only in organisms that are able to live without it... All basic bodily functions are controlled automatically at the level of physiological reglation. Essential action patterns are innate... (Strube, 1998, pp. 2, 12).

Fixed action patterns, then, should not be seen as a "primitive" feature, but simply as a hallmark of organisms.

Why memory matters for having a mind

Conclusion S.7 above allows us to more clearly articulate the basis for our proposal (Conclusion N.10) that the existence of memory capacity in an organism was a necessary ground for ascribing cognitive mental states to it. An organism lacking a memory capacity could only exhibit fixed action patterns, which do not warrant the ascription of cognitive mental states. A goal-centred intentional stance is sufficient to explain the organism's behaviour.

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