Types of sensory consciousness

This is the third in a series of posts inspired by Todd Feinberg and Jon Mallatt’s new book, ‘The Ancient Origins of Consciousness: How the Brain Created Experience‘.  The first two were:

• What counts as consciousness?
• Predators and rise of sensory consciousness

With this post, we’re going to get into the different types of sensory consciousness that F&M (Feinberg and Mallatt) identify in their book.  They identify three types:

• exteroceptive
• interoceptive
• affective

The consciousness I discussed in the last two posts was exteroceptive consciousness, that is, consciousness about the outside world and its relation to the organism.  This type of consciousness requires distance senses such as vision, hearing, and smell, as well as more immediate ones like touch and taste, although these last two senses sit on the border between exteroceptive and interoceptive consciousness.

Exteroceptive consciousness involves building image maps of the environment and objects in it.  “Image” here doesn’t necessarily mean a visual image, although that type of image usually looms large in this type of awareness.  But it can also mean audio images, smell images, or touch images.  These images are formed from the signals coming in from their related senses.

From the initial images, image maps, or models, are constructed, eventually from information integrated across the senses.  These image maps are isomorphic with patterns in the environment, to varying degrees of success.  They are what form the inner world that conscious organisms live in.

Interoceptive consciousness is awareness of internal body states.  Similar to exteroceptive consciousness, it involves taking data from sensory input and constructing isomorphic image maps.  But in this case, it’s senses about the internal state of the body, covering things like the feeling of muscles, the stretch of tissues, the state of the stomach, etc.  The overall image constructed here is one of the body.

This resulting image map seems very similar to Antonio Damasio’s proto-self body image.  In fact, F&M cite Damasio approvingly, although noting that whereas Damasio’s approach was to study the human brain, theirs is broader, looking at the various architectures in the animal kingdom.

The last type of consciousness is affective or limbic consciousness.  The thing about the information in the other two types of awareness above, is that it has no valence, no value attached to it, no assessment of whether the sensory information is good or bad for the organism.  Affective consciousness is where this valence is introduced.

There are, broadly, two types of affects: positive and negative.  For positive, think pleasurable or attractive.  For negative, think aversion, fear, pain, etc.  All emotions and inclinations are essentially variations of these two affects.  F&M are cautious in using the word “emotion” here since it comes with so much baggage, with people meaning different things by that word.  But at their most basic level, emotions are affects.

My way of understanding this is that the emotion itself is a mental reflex, that earlier in evolution led to immediate action, but now surfaces as an affective feeling in conscious organisms, a mental state of preparation for a particular action, which may be overridden if there are other competing affects arising at the same time.  It is affective consciousness that provides the primal interpretation of sensory information in the exteroceptive and interoceptive models as either good or bad.

When we see something, such as a rainbow or an onrushing tiger, our knowledge of its appearance and perceived location comes from exteroceptive consciousness, but our appreciation of the beauty  of the rainbow or the danger of the tiger comes from affective consciousness.  Of course, we don’t subjectively experience these as separate systems, because the information processing happens below the level of our awareness, creating a subjectively unified experience.

When did these types of consciousness evolve?  Exteroceptive is the easy one here.  It’s evolution was heralded by the development of eyes during the Cambrian explosion.  As I mentioned in the first post of this series, high resolution eyes (as opposed to more primitive light sensors) imply mental imagery, which imply isomorphic modeling, models of the environment, a worldview.

There may be some debate about which distance sense evolved first, but eyes, unlike the other sense receptors, are part of the central nervous system.  In their earliest incarnations, they appear to be right next to the brain, if not actually part of it.  For this and other reasons, F&M see vision as being an early development, with possibly the neural hierarchies for vision being subsequently duplicated for the development of the other distance senses.

There are many theories about when affective consciousness developed.  Some put it at the very beginning of sensory consciousness in the Cambrian.  Others push it back to various stages of development such as the rise of amphibians, or of mammals, or even anatomically modern humans, giving a vast range of possible dates, from as early as 550 million ago to as late as 200,000 years ago (when more or less modern humans emerged).

Since affects are a crucial aspect of sentience, figuring out when it developed and which species have it is important.  F&M’s approach is to examine cases of behavior that require affective consciousness, such as learned responses to punishments and awards, behavioral trade-offs, frustration with insurmountable problems, and self-delivery of analgesics when in pain.  Having identified these behaviors, they then look at experiments testing various species to see if they exhibit these behaviors.

Of course, no one can test an extinct Cambrian species, and using modern animals as stand ins for earlier ones in evolutionary history is risky, but the morphological similarities between the simplest modern vertebrates and pre-vertebrates to fossil remains reduces the uncertainty.

In experiments, all vertebrates: fish, amphibians, reptiles, birds, and mammals, demonstrated behaviors that required affective consciousness.  But animals resembling pre-Cambrian forms: C-elegans, flatforms, and Chordates generally did not.  The evidence then, according to F&M, points to affective consciousness evolving more or less concurrently with exteroceptive consciousness, in other words, during the early Cambrian, 550-520 million years ago.

Interoceptive consciousness is a more difficult matter.  F&M found fragmentary evidence that the neurological pathways for it exist in all vertebrates, but were forced to admit that the evidence is sparse and uncertain.  In particular, the relevant pathways are poorly studied outside of mammalian species.

And there are surprising gaps, such as with pain.  Pain requires input from the affective system to actually be pain, but having an affect of pain requires certain types of interoceptive signals.  Pain seems like it would be one of the most fundamental aspects of feeling, but there appear to be good evidence that fish do not feel certain types of it.  They do seem to feel sharp pain, the pain of an injury immediately when it happens, but don’t appear to feel the long burning variety, the one that causes suffering.

F&M speculate that this might have something to do with their environment and feeding needs.  Fish often can’t heed continuous agonizing pain, but have to keep moving to survive.  However, for land animals, signalling that tells them they are damaged and need to find a place to hide and heal can be a survival advantage.

So the weight of the evidence is that exteroceptive and affective consciousness are ancient, along with aspects of the interoceptive variety, and therefore widely prevalent in vertebrates.  (F&M also discuss the possibility of consciousness in cephalopods and arthropods, including many insects, although they express reservations on whether insect brains have the necessary complexity.)

The next post will discuss how the image map models may be constructed.