Morsella et al. write that conscious content is “sampled” only by the action systems in charge of controlling skeletal muscles (sect. 4.3, para. 3). They state that this has been known since at least the nineteenth century, but they do not discuss the evidence on which this knowledge is based. Here, I will discuss examples of conscious content being available to bodily effectors outside of the skeletal muscle system and suggest that the function of conscious content is not to serve a specific effector system, but rather to control adaptive behaviors in general.
One example of an output system other than the skeletal muscle system that has access to conscious content is: the glands responsible for salivary secretion. These glands are controlled exclusively by the autonomic nervous system (Proctor & Carpenter Reference Proctor and Carpenter2007). Although not controlled by skeletal muscles, salivary flow is responsive to conscious content. Simply forming a mental image of one's favorite dessert induces salivation. This widely shared experience, which also has been confirmed experimentally, shows that salivary glands have access to the content of consciousness (for a review, see Spence Reference Spence2011).
Just like fantasies about food can make the mouth water, erotic fantasies can result in sexual responses that are independent of the skeletal muscle system, for example, penile erections in men. Both in the case of imagery-induced salivation (White Reference White1978) and in the case of penile erection (Smith & Over Reference Smith and Over1987), subjects with more vivid imagery showed more pronounced effects, indicating that there is a close correlation between the strength of the conscious content and the strength of the bodily response outside of the skeletal muscle system. This correlation does not prove causation. The mental images of food and the increased salivation could both be caused by the same non-conscious neuronal processes and therefore be independent consequences of the same cause. However, this argument for the potential causal inertness of conscious content applies equally to cases in which the skeletal muscle system is the effector as to those in which there are other effectors.
According to Morsella and colleagues, when a food object is represented, the conscious content does not “know” whether the food will be eaten or used as a weapon. This is true, but it does not go far enough. The conscious content also does not “know” whether the representation of the food item will induce any muscle movement, or whether it will induce salivation, or both. When the brain integrates information, it does not differentiate between information that will ultimately be used to control skeletal muscles and information that will be used for other purposes. The conscious content is made available to any system to which it is of adaptive advantage to have it.
To illustrate the analogy between situations in which action selection conflicts concern skeletal muscles and situations in which action selection conflicts concern salivary glands, one can modify the authors' “creature in the cave” scenario so that it applies to both situations. In the “creature in the cave” scenario, an organism has to decide whether or not to leave a cave that is comfortably warm but slowly fills with (uncomfortable) smoke. A similar conflict can occur with salivation. Food stimuli or thinking about food increases salivation, whereas the thought of anxiety-inducing situations reduces salivary flow (Power & Thompson Reference Power and Thompson1970). A creature can therefore be in a situation in which these two tendencies are in conflict. Imagine a scavenger observing a predator feeding on its prey. The scavenger has to make a decision whether to approach the predator and its prey and expose itself to danger while feeding, or whether to stay hungry but safe. This is an action selection conflict in which the actions are mediated by the skeletal muscle system. However, the same action selection conflict is faced in this situation by the salivary glands, which can either increase or reduce salivary flow. The conflict is experienced consciously, and the conscious content is made available to both the skeletal muscles and the salivary glands.
In summary, the evolutionary function of conscious brain processes, like the evolutionary function of all brain processes, is to contribute to the selection of adaptive behaviors. However, the evidence that consciousness is a special adaptation to the processing of information that is exclusively used by the skeletomotor system is not conclusive. An alternative proposal is that consciousness is important to guide behaviors in situations in which the organism has to choose between many possible responses, regardless of the effector system concerned (Keller Reference Keller2014). The conceptual framework that Morsella et al. have developed will be a helpful tool for identifying the role of consciousness in the control of adaptive behavior.
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Morsella et al. write that conscious content is “sampled” only by the action systems in charge of controlling skeletal muscles (sect. 4.3, para. 3). They state that this has been known since at least the nineteenth century, but they do not discuss the evidence on which this knowledge is based. Here, I will discuss examples of conscious content being available to bodily effectors outside of the skeletal muscle system and suggest that the function of conscious content is not to serve a specific effector system, but rather to control adaptive behaviors in general.
One example of an output system other than the skeletal muscle system that has access to conscious content is: the glands responsible for salivary secretion. These glands are controlled exclusively by the autonomic nervous system (Proctor & Carpenter Reference Proctor and Carpenter2007). Although not controlled by skeletal muscles, salivary flow is responsive to conscious content. Simply forming a mental image of one's favorite dessert induces salivation. This widely shared experience, which also has been confirmed experimentally, shows that salivary glands have access to the content of consciousness (for a review, see Spence Reference Spence2011).
Just like fantasies about food can make the mouth water, erotic fantasies can result in sexual responses that are independent of the skeletal muscle system, for example, penile erections in men. Both in the case of imagery-induced salivation (White Reference White1978) and in the case of penile erection (Smith & Over Reference Smith and Over1987), subjects with more vivid imagery showed more pronounced effects, indicating that there is a close correlation between the strength of the conscious content and the strength of the bodily response outside of the skeletal muscle system. This correlation does not prove causation. The mental images of food and the increased salivation could both be caused by the same non-conscious neuronal processes and therefore be independent consequences of the same cause. However, this argument for the potential causal inertness of conscious content applies equally to cases in which the skeletal muscle system is the effector as to those in which there are other effectors.
According to Morsella and colleagues, when a food object is represented, the conscious content does not “know” whether the food will be eaten or used as a weapon. This is true, but it does not go far enough. The conscious content also does not “know” whether the representation of the food item will induce any muscle movement, or whether it will induce salivation, or both. When the brain integrates information, it does not differentiate between information that will ultimately be used to control skeletal muscles and information that will be used for other purposes. The conscious content is made available to any system to which it is of adaptive advantage to have it.
To illustrate the analogy between situations in which action selection conflicts concern skeletal muscles and situations in which action selection conflicts concern salivary glands, one can modify the authors' “creature in the cave” scenario so that it applies to both situations. In the “creature in the cave” scenario, an organism has to decide whether or not to leave a cave that is comfortably warm but slowly fills with (uncomfortable) smoke. A similar conflict can occur with salivation. Food stimuli or thinking about food increases salivation, whereas the thought of anxiety-inducing situations reduces salivary flow (Power & Thompson Reference Power and Thompson1970). A creature can therefore be in a situation in which these two tendencies are in conflict. Imagine a scavenger observing a predator feeding on its prey. The scavenger has to make a decision whether to approach the predator and its prey and expose itself to danger while feeding, or whether to stay hungry but safe. This is an action selection conflict in which the actions are mediated by the skeletal muscle system. However, the same action selection conflict is faced in this situation by the salivary glands, which can either increase or reduce salivary flow. The conflict is experienced consciously, and the conscious content is made available to both the skeletal muscles and the salivary glands.
In summary, the evolutionary function of conscious brain processes, like the evolutionary function of all brain processes, is to contribute to the selection of adaptive behaviors. However, the evidence that consciousness is a special adaptation to the processing of information that is exclusively used by the skeletomotor system is not conclusive. An alternative proposal is that consciousness is important to guide behaviors in situations in which the organism has to choose between many possible responses, regardless of the effector system concerned (Keller Reference Keller2014). The conceptual framework that Morsella et al. have developed will be a helpful tool for identifying the role of consciousness in the control of adaptive behavior.