Summary: The cortex is generally thought to be the seat of consciousness in human beings. A body of literature indicates that the cortex is not integral to many intuitively conscious behaviors in rats. This suggests that either rats don’t rely conscious experiences to guide their behavior in the way we do, or else that consciousness depends on distinct mechanisms in humans and rats. All told, we should think this body of literature is evidence against the hypothesis that mammals are generally conscious and non-mammalian vertebrates are not.

Epistemic status: I’m not an expert in much of the material surveyed here. Particularly the function of brain regions and lesioning studies in animals. These issues are important and my impression is that very few people have the comprehensive expertise to think well about these things.

Caveat: I’m wary of the decortication studies because they seem so out of line with what I would expect given the prevailing opinions and they are discussed very little in contemporary neuroscience. That said, I have a hard time seeing how they could be grossly inaccurate and I’ve never seen anyone challenge them.

Main ideas are bolded.

Intro

All vertebrate brains follow a similar plan in rough outline. At the highest level, they are generally divided into three parts: the fore, the mid, and hindbrain. In humans and other mammals, the forebrain consists in the cerebral cortex and several smaller structures.

The mammalian cerebral cortex houses a number of specialized sensory processors and muscle controls along with general association regions of less specific purpose. In humans and other apes, the cerebral cortex is also widely believed to be largely responsible for generating our conscious experiences. While structures in the mid or hindbrain control aspects of body regulation and arousal, our experiences are thought to be produced in the cortex. We have successfully mapped out sensory regions and have some understanding of how their activity correlates with conscious contents. Damage to regions of the cortex radically alters cognition and affects conscious experiences in a variety of ways. Moreover, severe damage to the human cerebral cortex destroys all signs of awareness and intentional activity. A human without a cerebral cortex is, for the most part, a living body with no mind.

Primates differ from many other animals in the central role their cortex plays in their cognitive lives. The effects of cortical loss in rats has been studied in depth in a series of neuroscientific experiments (Whishaw 1990)^[1], carried out mostly in the 1960s-80s. The results of these studies are striking. The behavior of decorticate rats is remarkably unaffected by extensive damage or removal of their cerebral cortex. The afflicted rats are impaired by the loss of cortical tissue, but not nearly as much as one would expect.

The obvious behavioral implications to decortication noted by experimenters are extremely benign relative to what I would have antecedently expected.

They include:

• A laundry list of small idiosyncratic changes, e.g.:
  • Impaired muscle control over the tongue and jaws
  • Postural changes while grooming
  • Increased use of forepaws during swimming
• Various social changes
  • Notably, inferior maternal care
• Difficulty with some complex cognitive tasks
  • Inability to reason abstractly about locations
  • Absence of food hoarding behavior

I suspect that there are a lot of complicated ways that decortication does change behavior, but which require subtle experimental designs to detect. Surely, the cortex must do a lot of important things to be worth its metabolic cost. However, losing the cortex does not radically affect rats’ ability to survive, navigate their environment, or interact with their peers. They can still find their way around landmarks, solve basic reasoning tasks, and learn to avoid painful stimuli.

This strongly suggests that the loss of the cerebral cortex does not have a major effect on the situational awareness, intentional activity, sensory processing, or learning capacity of rats. Whatever their cortex does, it isn’t necessary for that sort of processing, at least at a rudimentary level.

The extremely different effects of cortical damage tells us something about cognitive differences between humans and other animals, but it isn’t entirely clear what. The phenomenon has been cited in defense of theories that indicate both wide (Merker 2007b) and narrow (Key 2015) distributions of consciousness. I think it does both: the behavioral effects of decortication suggest that consciousness is either restricted to primates or else it is distributed fairly widely outside mammals. Both of these suggestions come at the expense of the hypothesis that consciousness is widely distributed within mammals and generally absent elsewhere. This is not to say that the decortication results show this to be false, but they seem to make it somewhat less likely.

Some Possibilities

I see four main possibilities.

1.) Conscious experiences are dependent on midbrain structures.

Bjorn Merker (2007a) argues that parts of the midbrain produce human experiences; the cortex plays only an ancillary role. While he accepts that the cortex contributes to the variety and nature of our experiences, he suggests that it does so through its effect on activity in the midbrain.

Merker draws evidence from a variety of sources, but it strikes me on a whole to be more evocative than substantial. The importance of midbrain to behavior in other mammals seems to be the strongest evidence, together with facts that we should expect to correlate with this evidence, such as the structural fitness for the midbrain to play such a role. The other most important evidence, in my mind, involves the fact that consciousness is hard to suppress from limited cortical damage alone, no matter where it occurs. However, this is consistent with a diffuse architecture where no single region is responsible. Such diffusion is supported by some neurological evaluations of the Global Workspace Theory (Deco, Vidaurre, and Kringelbach 2021; Mashour et al. 2020).

Merker’s proposal has not been embraced by the mainstream (though it hasn’t been entirely rejected, either). The majority of experts believe that human conscious experiences are primarily a product of the cortex. There is little evidence that conscious activity rests fundamentally on the midbrain. To be plausible, Merkers’ view requires that the particularities of consciousness are settled in the cortex, but only rise to the level of consciousness due to activity in the midbrain. This suggests a division of labor that demands far more evidence that Merker provides.

If Merker were right, however, that would suggest that consciousness does not depend much on the mammalian elaborations of the cortex that so clearly distinguish mammalian brains from the brains of other vertebrates. It seems more likely that consciousness exists in the midbrain machinery shared with rats as well. What’s most distinctive about the human brain isn’t the expansion and development of midbrain regions. So it is less likely that special faculties for human consciousness developed in the midbrain alongside the changes in the cortex.

Furthermore, given that the elaboration of the cortex has correlated significantly with the development of advanced capacities, we should also think it is more likely that animals with a complex midbrain and a comparatively simple forebrain, e.g. most vertebrates, are more likely to share the same mechanisms for consciousness.

2.) Conscious experiences are produced by different cognitive structures.

Many of the tasks that the human cortex performs are accomplished in midbrain structures of rats. This suggests a separate development of these capacities in the lineage leading to primate brains.^[2] Since our nearest common ancestors were likely to have brains that were more similar to rats, it seems that these tasks might have ether migrated from the midbrain or developed separately in the cortex.

The shift in responsibilities for visual perception from the superior colliculus to the cortex provides a notable comparison.^[3] Visual information in both humans and rats is carried from the eyes to the superior colliculus of the midbrain and to the occiptal lobe of the cortex. In humans, the large majority of connections terminate in the occiptal cortex, where layers of neurons parse and code for aspects of the visual scene. The human superior colliculus instead uses visual information for certain specific purposes, such as directing eyes to objects of interest. The function it serves in humans is generally thought to be unconscious, explaining how it can continue to function in people who claim to lack visual experiences (Schlag 2007).

In fish, the superior colliculus (labeled ‘tectum’ outside of mammals) has a much greater role, and is involved in organizing behaviors around pursuit and flight (Isa et al. 2021). It combines sensory inputs of multiple modalities and is connected directly to motor controls in the hindbrain, allowing it to efficiently manage responses to predators and prey.

Comparative evidence suggests that human ancestors relied on the superior colliculus for primary visual processing with some additional work occuring in the cortex. Gradually, the division of labor shifted so that the cortex became responsible for most visual processing and the superior colliculus was relegated to some tasks for which it remained better suited.

Something similar might have happened with the faculties responsible for consciousness (including, possibly, the superior colliculus). Faculties for consciousness might exist in the midbrains of most mammals, but the cortex has removed the need for these faculties in humans.

It is unlikely that any particular cellular structures migrated from the midbrain to the cortex. Brains evolve by increasing elaboration, not neural transportation. Further, the cortex is arranged in a different fashion from the midbrain. Instead, we should expect that increasing or changing demands caused an expansion of cortical tissue to accommodate the roles previously played by the midbrain. That expansion happened in our ancestors after the ancestral split. When the cortex expanded, some tasks no longer needed to be performed by the midbrain and so they fail to develop, or else have atrophied and disappeared.

This view suggests that consciousness is neurologically cheap. It would have evolved at least twice in our lineage. The second time it evolved it would have had to have been selected despite already functioning in the midbrain.

If consciousness arose twice in our lineage, it seems more likely that it is widely distributed among animals that have brains that are rather different from ours. This would suggest that something about consciousness provides value to faculties perception and action. As soon as the faculties primarily responsible for perception and action changed, consciousness developed in the new locale. We should expect it to occur widely among creatures with moderately complex behaviors, even if they lack regions comparable to either our cortex or our midbrain.

3.) Decorticate rats are not conscious.

There is a straightforward argument that rats are not conscious, even if they retain an intact cortex. The cortex is known to be essential to consciousness in humans, the only species we’re certain is conscious. The cortex has a critical role in producing many of the behaviors that we take as a clear sign its presence. In humans, it is responsible for the processing of sensory experiences, the weighing of pleasure and pain, producing inner and vocalized speech and most complex action planning. In contrast, the limited effects of decortication demonstrates that it plays a much more limited role in rats. It is harder to see how useful it could be if it doesn’t much help rats find food, navigate mazes, or learn to avoid pain.

Mechanisms of consciousness might have developed separately in the midbrain, but then they would have had to evolve twice, in two separate places. What good reason do we have to think that is so? The mild implausibility of the separate development of consciousness casts some doubt on the consciousness of creatures who do not rely on their cortex for behavioral guidance.

Setting the cortex aside, not everyone agrees that rats are likely to be conscious. The most plausible popular accounts that deny them consciousness (Carruthers 1989) take consciousness to involve higher-order representational states. The thought is conceptual rather than empirical (Lycan 2001): conscious states just are those states we are aware of having. Being aware of having a state is representing ourselves as having it. So something in our brains must represent any state that is conscious.

From an evolutionary perspective, higher-order representations are somewhat odd. What good does it do a brain to model itself?

For human beings, there is an answer I like; higher-order representations may help us to hone our mind-reading abilities. Mind reading is the ability to make predictions about other’s behavior on the basis of an understanding of their mental states. We’re quite good at it. We get better at it by observing our fellows’ behavior, but also by keeping track of our own behavior. We know what makes us angry and how we’re likely to act when we’re angry, what calms us down, etc. We can use that self-understanding to navigate our interactions with other friends and enemies. Introspection involving higher-order representations helps us to acquire this knowledge.

In contrast to humans, the vast majority of animals have no noteworthy experimentally verified mind-reading abilities. Even our closest primate ancestors are comparatively pretty terrible at it. They might predict what other creatures will do, but, as far as we can tell, it is not by understanding how their minds work and certainly not by projecting their self-understanding of their own behavior onto those minds.

If higher-order representations are necessary for consciousness, if they serve mainly to enhance mind-reading abilities, and if most animals have no ability to read minds, then this is a good reason to think that they are not conscious.

Mind reading is one potential use for higher-order representations. There may be others. Higher-order representations might also tend to form even without providing any evolutionary advantage – brains are predictive machines, so perhaps they over generate predictions, not only about the world around them but about themselves. That might be enough for higher-order representations to result. Higher-order consciousness theorists do not always deny consciousness to most animals.

The behavior of decorticate rats is perfectly consistent with the thought that few animals have conscious experiences. Higher-order representational states are typically thought to be cortical, and don’t have an obvious role in explaining rat behavior. If rats had higher-order representations that they lost with their cortex, we wouldn’t necessarily see their absence in any activities monitored in a lab. However, the cortex also isn’t particularly well set up to inspect the inner workings of the midbrain, so it is less likely that rats have cortical higher-order representations of what is occurring in their midbrains. If rats have higher-order faculties in their cortex, it is unlikely that those faculties are carefully representing activities of the midbrain.

Humans have a system that integrates perceptions and actions, produces consciousness, is housed in the cortex, and includes higher-order representational states. Rats have a different system for integrating perceptions and actions. It isn’t housed in the cortex. It might also have higher-order representations within it, but we have no independent reason to think that it does.

4.) Humans have two different sources of conscious experiences

Perhaps humans have some conscious experiences that are dependent on cortical structures and some conscious experiences that are dependent on midbrain structures. Rats share at least the latter.

Once we entertain the possibility that rats possess a conscious midbrain and that a cortical basis of consciousness developed separately in the lineage leading to primates, we should also consider the possibility that humans and other primates retain some conscious experiences in a vestigial part of the midbrain. Just because something took over a function doesn’t mean that it was completely lost in its prior location. It may be in the evolutionary process of atrophying, but it may not have finished that process yet.

The midbrain does continue to perform some sorts of activities that are reminiscent of consciousness, including action selection for saccades. Any conscious experiences produced in the midbrain may or may not be open to introspection. Their separation from the cortex could make them inaccessible from higher-order representations and memory, precluding us from being able to recognize and report their existence.

Conclusion

The surprisingly minor effects of decortication in rats seem to show that the mechanisms for consciousness most likely don’t exist solely in the cortex of both human and rat. If consciousness is generated entirely in the cortex in humans and decorticate rats are conscious, rats must have a different basis of consciousness from humans. It is also possible that rats, and probably most other vertebrates, are not conscious.

If rats have faculties for consciousness in their midbrain, then it is more likely that they share those faculties with other vertebrates. Rats, like other mammals, have a relatively large cortex. The fact that rats have substantially larger brains than reptiles or fish, and the fact that a significant part of the enlargement occurs in the cortex, might naively be taken to suggest that rats are more likely than fish to be conscious.^[4] But the fact that rats have a comparatively large cortex isn’t a significant reason to think that they are more likely to be conscious than fish if their faculties for consciousness reside elsewhere.

Therefore, the behavior of decorticate rats suggests that either consciousness is more restricted or else it is more widely spread than we might antecedently have thought.

Bibliography

Carruthers, Peter. 1989. “Brute Experience.” The Journal of Philosophy 86 (5): 258–69.

Deco, Gustavo, Diego Vidaurre, and Morten L Kringelbach. 2021. “Revisiting the Global Workspace Orchestrating the Hierarchical Organization of the Human Brain.” Nature Human Behaviour 5 (4): 497–511.

Herculano-Houzel, Suzana, Jon H Kaas, and Ricardo de Oliveira-Souza. 2016. “Corticalization of Motor Control in Humans Is a Consequence of Brain Scaling inCould functional neuroplasticity have allowed previously unconscious brain structures became conscious after some time? Primate Evolution.” Journal of Comparative Neurology 524 (3): 448–55.

Isa, Tadashi, Emmanuel Marquez-Legorreta, Sten Grillner, and Ethan K Scott. 2021. “The Tectum/Superior Colliculus as the Vertebrate Solution for Spatial Sensory Integration and Action.” Current Biology 31 (11): R741–R762.

Jerison, Harry J. 1973. “Evolution of the Brain and Intelligence.”

Key, Brian. 2015. “Fish Do Not Feel Pain and Its Implications for Understanding Phenomenal Consciousness.” Biology & Philosophy 30 (2): 149–65.

Lycan, William G. 2001. “A Simple Argument for a Higher-Order Representation Theory of Consciousness.” Analysis 61 (1): 3–4.

Mashour, George A, Pieter Roelfsema, Jean-Pierre Changeux, and Stanislas Dehaene. 2020. “Conscious Processing and the Global Neuronal Workspace Hypothesis.” Neuron 105 (5): 776–98.

Merker, Bjorn. 2007a. “Consciousness Without a Cerebral Cortex: A Challenge for Neuroscience and Medicine.” Behavioral and Brain Sciences 30 (1): 63–81.

———. 2007b. “Grounding Consciousness: The Mesodiencephalon as Thalamocortical Base.” Behavioral and Brain Sciences 30 (1): 110–20.

Schlag, John. 2007. “Should the Superficial Superior Colliculus Be Part of Merker’s Mesodiencephalic System?” Behavioral and Brain Sciences 30 (1): 105–6.

Whishaw, Ian Q. 1990. “The Decorticate Rat.” In The Cerebral Cortex of the Rat, edited by B. Kolb; R. C. Tees, 239–67. The MIT Press.