(Warning: neuroscience weeds)
Some years ago, I reviewed Antonio Damasio’s theory of consciousness, based on his book, Self Comes to Mind. (He has a newer book, The Strange Order of Things, which I haven’t read yet, so this may not represent his most current views.) In that book, Damasio makes a distinction between two types of processing regions in the brain: sensory image forming and dispositions. In terms of evolution, he sees dispositions as being far more ancient. He concludes that we are sometimes conscious of the image content, but never of the dispositions.
This distinction between images and dispositions shows up in the writing of a number of other authors, perhaps influenced by Damasio. For instance, Todd Feinberg and Jon Mallatt follow it in their book, The Ancient Origins of Consciousness. Their take is that sensory or primary consciousness, which they equate with phenomenal consciousness, amounts to the presence and utilization of these images. So for them, the rise of image forming regions during the Cambrian explosion amounted to the rise of consciousness.
In my write up of Damasio’s views, I expressed some misgivings, that the sharp distinction he seemed to be making between the image forming regions and the dispositional ones was probably far messier then he was implying. Over time, with more reading, it’s progressively seemed even messier than I thought back then, to the extent that now I’m not sure any division between dispositions and images really makes sense. The division implies that there are sensory images, and things the brain does with those images. But that implies the images themselves are something passive.
To be sure, we have mental images. The question is how these map to the neural processing in the brain. I think conceiving them as localized neural image maps in sensory cortices, which is often the implication, is far too simple.
To illustrate the issue, let’s consider vision. The following brief summary is inspired by Richard Masland’s excellent book, We Know It When We See It, as well as a number of other sources. (I’m indebted to James Cross for recommending Masland, a top notch writer I was sad to discover passed away in 2019.)
Vision begins with photons striking photoreceptor cells in the retina lining the inside of the eye, including rod cells which detect levels of light and cone cells which are excited by different wavelengths of light. Cone cells are often described as red, green, or blue sensitive, but it’s more accurate to label them as L (long), M (medium), or S (short) wavelength sensitive, since actual color perception is a later conclusion of the brain. The patterns formed on the photoreceptors is the closest we come to a passive image in the visual system.
The retina is a multilayer nerve net, a neural network. The connections from multiple receptor cells in the first layer converge on ganglion cells in the final layer. In the fovea, the central region of the retina where visual acuity (resolution) is at its highest, the number of photoreceptor cells connecting to any one ganglion cell is small, but gets progressively higher as we move out toward the periphery, resulting in lower acuity. In all, about 126 million receptor cells connect to around a million ganglion cells.
There are many different types of ganglion cells (around 30 according to Masland), each of which have receptor fields of varying sizes. Some of these ganglion cells care about color, others light level, or changes in light level, whether there is an edge present, or whether there is movement in a particular direction. For many types, it’s not yet known what they care about. But each point in our field of vision has roughly 30 separate analyzers.
It’s often said that the connections from the retina are topographically preserved all the way to the visual cortex. And that’s true, but misleading. When I first read this years ago, I took it to mean that the pattern of photoreceptor firings get transmitted to the visual cortex. But that’s not right. It’s the topography from the ganglion cells that are preserved: the 30 different types of analyzers, their initial conclusions, interpretations, and dispositions about the stimuli coming in. In other words, the nervous system starts interpreting visual information right from the beginning, and it is that interpretation which gets passed to the brain.
The axons of the ganglion cells run up the optic nerve to the LGN region of the thalamus, a region in the central floor of the cerebrum. It’s not well understood how the signals might be modified there, although it’s thought that attention levels are the primary factor. The ganglion axons synapse to LGN neurons which have axons that run to the visual cortex.
The visual cortex is divided into many regions, with V1 feeding into V2, which feeds into V3, then V4, etc. That’s the feed forward flow, but it’s worth noting that all these cortical regions interconnect and feedback with each other. The neurons in V1 tend to get excited by edges in various orientations, allowing shapes at various spots to be detected. V2 becomes more selective, with the cells gaining some “positional independence”. V3 and V4 start caring about things like colors, movement, or depth.
There are broadly two “streams” in the visual system, the dorsal (top) one, which is the “where” stream, and the ventral (lower) one, which is the “what” stream. The “what” stream flows into the temporal lobe, with regions which light up for various types of objects. For example, a number of regions light up for faces. Moving anterior (forward) in the temporal lobe, the regions gradually become more abstract, more likely to light up for a particular face or recognized object, no matter which angle it’s being viewed at.
I’m obviously vastly oversimplifying here, and there are wide gaps in current knowledge, although what is known forms an outline of a vast nerve net, or network of interacting nerve nets, reaching conclusions about the visual stimulus. Initially these conclusions are simple ones, like brightness, movement, or color. (See these color illusions to see how our perception of color is a preconscious determination made by the nervous system.) As we move through the system, the conclusions progressively become more complex.
The question is, where in all these layers of processing is the image? As noted above, images are often described as being in the early sensory regions, but these are simply early conclusions about the sensory information. Since our introspective access only reaches so early into the process, it’s tempting to regard the image as being the conclusions on that introspective boundary.
But if you look at your current surroundings, the first thing you might perceive is a gestalt, a “gist” of the scene (office, living room, park, etc). If you then focus on a detail, you might notice the gestalt of that smaller scope, such as a desk, TV, or tree. If you focus on a particular surface, you might notice its color and texture, but this is itself arguably just another gestalt, a conclusion, albeit possibly getting closer to the most basic ones we have access to.
I think normal conscious visual perception flits between these various gestalts so fast that it gives us the impression we’re taking in the entire scene at once, but in reality we’re focusing on different aspects of it at a time. Those aspects are always there when we check. Of course, we can view it as the entire brain taking in the entire scene at a time with the focus of attention moving around.
On the neural image, there might be a few of ways to look at it. One is to regard the activations throughout the entire perceptual system for that modality as the neural image. For vision, that would include activity in the occipital lobe and much of the temporal and parietal lobes, a much broader scope than what is commonly understood. Another is to regard just the current focus of attention as the image. So when we’re focused on a high level gestalt, the image might be assemblies firing in the anterior temporal lobe. When focused on a particular color, it might be assemblies in V3 and V4, unless we’re thinking about the concept of the color, in which case we might be back in the temporal lobe, or some combination.
The key point is there is no sharp boundary, just a somewhat arbitrary one of convenience for us, a way of thinking about it. The brain itself doesn’t appear to make any distinction. It just continues the processing from lower order perception, to higher order perception, and then to thoughts we might not think of as perception, such as beliefs. So another way of looking at it, perhaps the most productive, is that there are no neural images, just vast distributed assemblies of conclusions, dispositions, which add up to the mental images we perceive. In this view, perception is disposition all the way down.
What does this say about Damasio’s theories, or Feinberg and Mallatt’s? In Damasio’s case, not much beyond the simple distinction he makes being problematic. I don’t recall the rest of his theories hinging much on that particular understanding of images. In Feinberg and Mallatt’s case, they could use one of the interpretations of image I described above, but it makes the simple criteria of images being present… not so simple. Although their conception of affect consciousness should be unaffected.
Overall though, I think it puts pressure on outlooks that insist that phenomenal consciousness is separate from access consciousness. I actually think the motivation to see simple versions of neural images is often driven by these outlooks. But data should trump philosophy.
Unless of course I’m missing something? Are there reasons to draw sharp boundaries between neural images and dispositions that I’m overlooking?