(Warning: neuroscience weeds)
Recently I noted that one of the current debates in cognitive science is between those who see phenomenal and access consciousness as separate things, and those who see them as different aspects of the same thing. Closely related, perhaps actually identical, is the debate between local and global theories of consciousness.
Local theories tend to see processing in sensory cortices (visual, auditory, etc) as sufficient for conscious perception of whatever sensory impression they’re processing. One example is micro-consciousness theory, which holds that processing equals perception. So formation of a neural image map in sensory cortices equals consciousness of that image.
(A lot of people seem to hold this processing equals perception idea intuitively. It fits with ideas from people like Jaak Panksepp or Bjorn Merker.)
Another more sophisticated version is Victor Lamme’s local recurrent processing theory, which adds the requirement of recurrent processing in the sensory cortex, processing that involves feed forward signalling as well as feed back signalling in loops. I discussed local recurrent theory a while back.
Global cognitive theories require that content have large scale effects throughout the brain to become conscious. Similar to Lamme’s theory, they often see recurrent processing as a prerequisite, but require that it span regions throughout the thalamo-cortical system, either to have wide scale causal effects (global workspace theories) or to reach certain regions (higher order thought theories).
I mentioned that the local vs global debate may be identical to the phenomenal vs access one. This is primarily because local theories only make sense if phenomenal consciousness is happening in the local sensory cortices independent of access consciousness. That processing is inherently pre-access, before the content is available for reasoning, action, and report.
The latest shot in this debate is a paper by Matthias Michel and Adrien Doerig, which will be appearing in Mind & Language: A new empirical challenge for local theories of consciousness.
Michel and Doerig look at a type of perception they call “long-lasting postdiction”. An example of postdiction is when subjects are shown a red disk followed in rapid succession by a green disk 20 ms (milliseconds) later, resulting in perceptual fusion, where the subject perceives a yellow disk. This is an example of short-lasting postdiction. In order for the fusion to occur, the red image needs to be processed, and then the green one, and then the two fused.
Short lasting postdiction could represent a problem for micro-consciousness theory, since it results in formation of image maps for each image in rapid succession but not in a way the leads to each being consciously perceived. (It’s not clear to me this is necessarily true. I can see maybe the two types of processing simply bleeding into each other. And see below for one possible response from the micro-consciousness camp.)
Short-lasting postdictions are less of a problem for local recurrent theory, because it takes more time for the recurrent processing to spin up. (The paper has a quick but fascinating discussion of the time it takes for information to get from the retina to the cortex and then to various regions, along with citations I may have to check out.)
It’s the long-lasting postdictions that are a problem for local recurrence. The paper discusses images of pairs of vernier lines that are shown to test subjects. The images are in various positions, changing every 50 ms or so across a period of 170-450 ms, resulting in a perception of the lines moving.
There are variations where one of the vernier lines are skewed slightly, but only on the first image, resulting in the subject perceiving the skew for the entire moving sequence, even though the verniers in the later images are aligned. Another variation has two images, one early in the sequence and one toward the end, skewed in opposite directions, resulting in the two skewed images being averaged together and the averaged shape being perceived throughout the sequence.
The main takeaway is that the conscious perception of the sequence appears to be formed after the sequence. Given the relatively lengthy sequence time of up to 450 ms, this is thought to exceed the time it takes for local recurrent processing to happen, and bleeds over into the ignition of global processing, representing a challenge for local recurrent theory.
The authors note that local theorists have two possible outs. One is to say that, for some reason, the relevant local processing actually doesn’t happen in these sequences. This would require identifying some other mechanism that unconsciously holds the intermediate images. The other is to say that the images are phenomenally experienced, but then subsequently replaced in the access stage. But this would result in phenomenal experience that has no chance of ever being reportable. (What type of consciousness are we now talking about?) And it would make local theories extremely difficult to test.
Interestingly, the authors note that the longer time scales may need to be reconciled with the ones in cognitive theories, such as the global neuronal workspace, which identifies 300 ms as a crucial point in conscious perception. In other words, while this is an issue for local theories, it could be one even for global theories.
All of this reminds me of the phi illusion Daniel Dennett discussed in his book Consciousness Explained. He describes this illusion in the build up to discussing his own multiple drafts model, a variation of global workspace theory. Dennett’s variation is that there’s no absolute moment when a perception becomes conscious. His interpretation of the phi illusion, which seems like another case of postdiction, is that there is no consciousness finish line. We only recognize that a perception is conscious retroactively, when it achieves “fame in the brain” and influences memory and report.
Anyway, I personally think the main flaw with local theories is that the processing in question is too isolated, too much a fragment of what we think of as an experience, which usually includes at least the sensory perception and a corresponding affective feeling. The affect part requires that regions far from the sensory cortices become stimulated. Even if the localists can find an answer to the postdiction issue, I think this broader one will remain.
Unless of course I’m missing something.