(Warning: neuroscience weeds)
Earlier this year I discussed Victor Lamme’s theory of consciousness, that phenomenal experience is recurrent neural processing, that is, neural signalling that happens in loops, from lower layers to higher layers and back, or more broadly from region to region and back. In his papers, Lamme notes that recurrent processing is an aspect of global theories, but he couldn’t see any reason why consciousness should require global activity, and so argued that local recurrent processing in the sensory cortices should be sufficient for phenomenal conscious perception.
That post pointed out functional reasons to see the global activity as necessary, mainly related to affects, and that without them, it seemed like a fragment of an experience at best. I also saw issues with such a simple definition of consciousness, pointing out that it would require us to regard an artificial neural network with recurrent processing as conscious. Still, it’s an interesting theory.
Just to recap: the initial sensory stimulus that comes in is feed forward, first locally in the sensory region, then globally throughout the cortex. It’s well established that this causes unconscious perception. This may be followed by local recurrent processing (after about 100 ms) and later global recurrent processing. Global theories see the final stage as conscious. Lamme sees the local recurrent one as conscious.
A new paper in Neuroscience of Consciousness seems to provide empirical evidence that contradicts the theory: Causal manipulation of feed-forward and recurrent processing differentially affects measures of consciousness.
The study used TMS (transcranial magnetic stimulation) to disrupt processing in the visual cortex at various stages of a perception. Disrupting it early, in the initial local feedforward stage, had the expected effect of disrupting unconscious perception. But it also disrupted conscious perception. The authors note that this would actually be expected by an advocate of local recurrence theory, because the initial feed forward stages are still necessary prep work.
However, disrupting the later processing, in the local recurrent stages, didn’t have the expected effect on conscious perception. In other words, conscious perception seemed more dependent on the early processing than the later locally recurrent processing.
Although the later disruption did seem to have an effect on the subject’s judgment of whether they had consciously perceived the stimulus, which is interesting. The authors discuss how this might be consistent with predictive coding theories.
But the main takeaway seems to be that disruption later in the process, during local recurrent processing, didn’t have the effects on phenomenal experience that Lamme’s theory predicts. They also note that Ned Block’s association of an independent phenomenal consciousness with local recurrent processing is also not supported, although they acknowledge there may be interpretations that Block can take to reconcile with the findings.
I don’t find this particularly surprising. A simple equating of phenomenality with recurrent processing seems too simple. It seems to be a common strategy to try to equate consciousness with some aspect or effect of neural processing. I think trying to do so without an understanding of the causal role, if any, these phenomena play is like trying to understand a car motor by focusing on the timing of the spark plugs. It might be an important component of how it works, but only a fragment. At best, in and of themselves, these phenomena might be indicators of consciousness.
These results don’t seem to challenge global theories because the disruptions were only in the visual cortex. It seems clear the early feed forward signalling could have still ignited the necessary global processing (recurrent and otherwise).
But it seems like another strike against local theories. Unless of course I’m missing something.