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*** SUMMARY of Conclusions reached
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Case study: action selection in cnidaria
The box jellyfish Chironex fleckeri. Courtesy www.barrierreefaustralia.com
Nervous system
The simplest phylum of "true" animals (Eumetazoa) is the cnidaria (commonly known as coelenterates, including animals such as jellyfish, sea anemones, corals and freshwater hydra). Their relationships to other animals, and their nervous systems and sensory capacities, are described in more detail in an Appendix.
Nervous systems and brains are unique to animals (World Biodiversity Database, 2000). Cnidaria have no brain, but they have a rudimentary nervous system, with neurons positioned regularly over the surface of the animal. Each neuron is in contact with its neighbours. The propagation of a nerve impulse is not transmitted along a linear chain of neurons, but radiates from its point of origin (Abramson, 1994, p. 176).
Although cnidaria do not possess a central nervous system, let alone a brain, their nerve net permits rapid communication between cells (in some cases taking only milliseconds), over relatively long distances. In "simpler" animals, which lack neurons, communication can only occur between neighbouring cells. Prescott (2001) considers the behaviour of cnidaria to be an important advance over that of sponges, which respond only to direct stimulation, at a very slow rate (about twenty minutes). By contrast, cnidaria exhibit "internally generated, rhythmic behavior, and co-ordinated patterns of motor response to complex sensory stimuli", allowing them to display an "integrated global response" to their environment (Prescott, 2001, pp. 5, 7). In some cnidaria, such as the hydrozoan jellyfish, the nerve net is arranged in a longitudinal circuit which supports fast attack, escape and defense reactions (Prescott, 2001, pp. 6-7).
Visual navigation in cnidaria
Cubozoans, or killer box jellyfish, are known to have complex eyes, similar in their basic design to those of vertebrates, despite the absence of a brain or central nervous system. The eyes connect into the neural network of the jellyfish, and there is evidence that they can see images. It has been suggested that for cubozoans, vision may play a role in feeding and reproductive behaviour:
Certain cubozoans are know to chase small fish and seize them with their tentacles. Further, many cubozoans exhibit complex sexual behaviors in which the males chase the females, grasp them with a tentacle, and subsequently inject packets of sperm into them. Vision may be important to the jellyfish for such complex behaviors (Martin, 2000).
Assuming that Martin's proposals prove to be correct, should we regard cubozoans as having mental states relating to events in their environment? For instance, are they aware of the world around them? I would argue that before we can impute mental states to these jellyfish, it needs to be shown that they are capable of internally generated flexible behaviour (Conclusion F.3). If they follow fixed action patterns when pursuing prey and mating, then the ascription of mental states is redundant. The relevant questions are: how do cnidaria act, and can they learn?
Agency in cnidaria?
Prescott considers the nervous system found in cnidaria to be a fundamental advance in the evolution of what he calls "action selection" or the problem of "resolving conflicts between competing behavioural alternatives" (2001, p. 1).
Action slection is an essential epistemic condition for the identification of mental states: an observer could never identify an organism as having a mind unless it possessed a repertoire of actions, combined with the ability to select the most appropriate one for the present circumstances.
A.9 An organism must have an action selection mechanism before it can be said to have cognitive mental states.
Unfortunately, this requirement does not take us very far. All organisms possess some ability to adjust to rapidly fluctuating environmental conditions: bacteria can adapt to changes in their food type and plants can respond in a sophisticated manner to changes in local lighting conditions (Godfrey-Smith, 2001, pp. 6-7). Prescott himself (2001, p. 1) acknowledges that action selection is part of a wider problem faced by all living creatures: behavioural integration, or the task of co-ordinating the activities of their parts and sub-systems.
A.10 All cellular organisms possess an action selection mechanism of some sort.
Nevertheless, he contends that the speed and co-ordination of the cnidarian response to stimuli represents a different kind of behaviour from that displayed by "simpler" animals such as sponges.
In some jellyfish, the nerve net is functionally divided into two relatively independent systems - one for feeding and the other for movement - which interact in neuron clusters. Others possess a single nerve net which can carry two different types of action potentials enabling either rapid escape swimming (to avoid predators), or, slow rhythmic swimming for feeding (Prescott, 2001, pp. 5-7).
Prescott likens this decentralised neural arrangement to the subsumption architecture described by Brooks (1986), which could be summed up in the adage, "You've got to crawl before you can walk". Rather than trying to build human intelligence into robots, Brooks designed a computer architecture that would display the range of functionality found in so-called "simple" life forms such as insects. Brooks argued that insects are "almost characterizable as deterministic machines". His architecture describes how an "agent" (be it a robot or a simple animal) can behave reliably in an unpredictable environment, despite having nothing more than cheap, small information processors, simple sensors and low memory. A Brooksian "agent" has no central control: it is hierarchically organised from the bottom up. Control is distributed between different components, making the "agent" better able to withstand damage (i.e. more robust). Behaviour patterns are hard-wired, and sensors and actuators (which produce movement) are closely coupled, to allow rapid response times. Co-ordination between the different components is ensured by built-in timers and by having behaviour modules that can inhibit one another. Simple behaviours combine to produce more complex patterns of behaviour (Laird, 1994). According to Prescott, the functional subdivision of the nerve net into two distinct circuits for feeding and movement, which is found in some jellyfish and sea anemones, resembles the Brooksian architecture proposed for some behaviour-based robots (2001, p. 6).
Can we regard an organism instantiating Brooks' architecture as having mental states? One could argue that some kind of primitive intelligence is required to co-ordinate the response of a motile, multicellular organism to a complex, unpredictable environment. This is the line of argument endorsed by Godfrey-Smith (2001), who has formulated his Environmental Complexity Thesis (ECT) to explain the evolution of cognition:
The function of cognition is to enable the agent to deal with environmental complexity.
The term "function" here is meant to be understood in a strong sense: the function of a trait is what it was selected for, in the course of its evolution. Godfrey-Smith defines "cognition" broadly, to include any kind of behaviour involving the coordination of actions with perceived environmental conditions. Naturally, he does not regard cognition as an on-off trait but as a trait that is found to some degree in all organisms - even those which are incapable of learning. However, we have argued that a mind-neutral intentional stance provides an adequate account of the adaptive behaviour found in bacteria, protoctista, plants and "simple" animals. The term "cognitive" is a superfluous here.
In any case, the specifications of Brooks' architecture make the ascription of mental states to a Brooksian "agent" redundant. A Brooksian "agent" has a very "low-tech" design. It has no internal model of the outside world - according to Brooks' design philosophy, such a system lacks the "computing" resources required to model a world which is dynamic and unpredictable. Additionally, a Brooksian "agent" does not engage in planning or learning of any kind. All of its behaviour is hard-wired and built-in, to ensure co-ordination and cope with unforeseen contingencies. In other words, its action patterns are fixed. If cnidaria do indeed behave like Brooksian "agents", then they cannot learn new ways of responding to unforeseen events. For reasons discussed above (Conclusion F.3), the ascription of cognitive mental states to cnidaria would then be redundant, as it would tell us nothing useful about their behaviour. A mind-neutral, goal-centred intentional stance would suffice.
Can cnidaria learn?
Habituation has been documented in cnidaria (Encyclopedia Britannica, 1989). As we saw above, habituation does not qualify as "true" learning (Conclusion L.4),
I have not been able to locate any reports of experiments testing whether cnidaria are capable of associative learning. Clearly more work needs to be done in this area. For instance, one could repeatedly pair an unconditioned stimulus which causes a rapid escape response in hydrozoan jellyfish with the presence of light (a conditioned stimulus), and attempt to induce a conditioned rapid escape response to the presence of light alone.
Certainly, the cnidaria possess biologically significant features that other creatures lacking minds do not - in particular, a nervous system which (in some cases) allows very fast signal conduction as well as (possibly) reflexes, and enables fast attack, escape, or defense reactions. It would be a mistake to equate the behavioural repertoire of cnidaria with that of bacteria or even plants. Unlike bacteria, cnidaria are multicellular creatures, which face the task of co-ordinating their entire bodies in response to sudden changes in their environment. For instance, swimming in jellyfish requires the "synchronous, simultaneous contraction of the entire perimeter of the bell" (Prescott, 2001, p. 7). And while plants, like cnidaria, are multicellular, plants are not motile. There is no plant analogue of a rapid escape response.
It may turn out to be the case that some cnidaria are capable of internally generated flexible behaviour, and have minds after all. However, the evidence to date suggests that the sensory capacities and behaviour modification observed in cnidaria can be adequately described using a mind-neutral intentional stance. There is no evidence that they possess an internal mechanism enabling them to modify their behaviour patterns and learn to do something new or different (Abramson, 1994, p. 38), so there is no warrant at present for ascribing mental states to them.
A.11 The fact that an organism has an action selection mechanism, sensors to guide navigation, and a nervous system with reflexes, does not provide a sufficient warrant for the ascription of mental states to it.
Case study: centralised action selection in flatworms
The relationships between the different groups (or phyla) of worms, and other animals, are discussed in further detail in an Appendix. Of special interest here are flatworms, which are believed to be the most "primitive" phylum of worms.
Prescott considers the appearance of the platyhelminthes in the fossil record (565 to 544 million years ago) to be a breakthrough in the evolution of action selection - i.e. the task of resolving conflicts between behavioural alternatives. He cites research by Raup and Seilacher (1969, cited in Prescott, 2001, pp. 12 - 13) showing that trace fossils of meandering foraging trails left by the earliest flatworms, which are described in detail in the Appendix, can be generated by combining four simple behaviour mechanisms: stay close to previously formed tracks; avoid crossing existing tracks; advance when the preceding conditions are not met; and make 180 degree U-turns at various intervals, which is achieved by swapping control from one side of the body to the other at various intervals. While the first two behaviour patterns are reflexive mechanisms associated with an animal's peripheral sensors, and the third mechanism (advance) is merely a default, the behavioural component that suppresses signals from one side of the body while the other is active, functions as a centralised conflict-preventing mechanism, of vital importance to an organism with a brain and bilateral symmetry.
Should the ability of bilaterally symmetric animals to engage in centralised action selection be envisaged in mentalistic terms, as a primitive form of intelligent behaviour? Or is there an alternative, neutral description of this behaviour? Robotics researchers such as Mataric (1990, cited in Prescott, 2001, p. 13) have pointed out that the first three mechanisms are the same as those used by a robotic wall-follower, while the U-turn behaviour can be generated simply by transferring control of staying close to the wall from one sensor arm to the other.
In keeping with the methodology I have advocated above, we should refrain from imputing mental processes to the simplest animals with brains, unless it helps us to understand and predict their behaviour better. The action selection mechanism described above does not appear to require such an explanation. Even if the foraging strategy being used is an optimal one, for a creature relying solely on physical contact (touch, taste and smell) to find food, the strategy may well be a fixed action pattern. It has been argued above (Conclusion L.5) that there can be no "true" learning (and hence no mental states) in an organism unless its behavioural response patterns are flexible. The fact that flatworms' action selection mechanisms (unlike those of cnidaria) are centralised does not imply that they are flexible.
Following this line of thinking, we should look for flexibility in flatworms' patterns of flexible behaviour (e.g. variations in their foraging strategies in response to environmental changes), before attempting to explain their foraging in mentalistic terms. For instance, if a flatworm could be conditioned to adjust its foraging strategy (e.g. by changing the length of its meander or the timing of its U-turns) in response to certain stimuli, we might interpret this as evidence of "thoughtful" foraging. I have not been able to find a discussion of any such experiments in the literature. This would be a fruitful area for further research.
A.12 The occurrence of centralised action selection, sensors and a central nervous system in an organism does not provide a sufficient warrant for the ascription of mental states to it.
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*** SUMMARY of Conclusions reached
References