Smooth or chunky? Like peanut butter, experience could have different granularities; in practice it seems the answer might be ‘both’. Herzog, Kammer and Scharnowski here propose a novel two-level model in which initial processing is done on a regular stream of fine-grained percepts. Here things get ‘labelled’ with initial colours, durations, and so on, but relatively little of this processing ever becomes conscious. Instead the results lurch into conscious awareness in irregular chunks of up to 400 milliseconds in duration. The result is nevertheless an apparently smooth and seamless flow of experience – the processing edits everything into coherence.
Why adopt such a complex model? What’s wrong with just supposing that percepts roll straight from the senses into the mind, in a continuous sequence? That is after all how things look. The two-level system is designed to resolve a conflict between two clear findings. On the one hand we do have quite fine-grained perception; we can certainly be aware of things that are much shorter than 400ms in duration. On the other, certain interesting effects very strongly suggest that some experiences only enter consciousness after 400ms.
If for example, we display a red circle and then a green one a short distance away, with a delay of 400ms, we do not experience two separate circles, but one that moves and changes colour. In the middle of the move the colour suddenly switches between red and green (see the animation – does that work for you?). But our brain could not have known the colour of the second circle until after it appeared, and so it could not have known half-way through that the circle needed to change. The experience can only have been fed to consciousness after the 400ms was up.
A comparable result is obtained with the intermittent presentation of verniers. These are pairs of lines offset laterally to the right or left. If two different verniers are rapidly alternated, we don’t see both, but a combined version in which the offset is the average of those in the two separate verniers. This effect persists for alternations up to 400ms. Again, since the brain cannot know the second offset until it has appeared, it cannot know what average version to present half-way through; ergo, the experience only becomes conscious after a delay of 400ms.
It seems that even verbal experience works the same way, with a word at the end of a sentence able to smoothly condition our understanding of an ambiguous word (‘mouse’ – rodent or computer peripheral?) if the delay is within 400ms; and there are other examples.
Curiously, the authors make no reference to the famous finding of Libet that our awareness of a decision occurs up to 500ms after it is really made. Libet’s research was about internal perception rather than percepts of external reality, but the similarity of the delay seems striking and surely strengthens the case for the two-level model; it also helps to suggest that we are dealing with an effect which arises from the construction of consciousness, not from the sensory organs or very early processes in the retina or elsewhere.
In general I think the case for a two-level process of some kind is clear and strong, and well set out here. We may reasonably be a little more doubtful about the details of the suggested labelling process; at one point the authors refer to percepts being assigned ‘numbers’; hang on to those quote marks would be my advice.
The authors are quite open about their uncertainty around consciousness itself. They think that the products of initial processing may enter consciousness when they arrive at attractor states, but the details of why and how are not really clear; nor is it clear whether we should think of the products being passed to consciousness (or relabelled as conscious?) when they hit attractor states or becoming conscious simply by virtue of being in an attractor state. We might go so far as to suppose that the second level, consciousness, has no actual location or consistent physical equivalent, merely being the sum of all resolved perceptual states in the brain at any one time.
That points to the wider issue of the Binding Problem, which the paper implicitly raises but does not quite tackle head on. The brain gets fed a very variable set of sensory inputs and manages to craft a beautifully smooth experience out of them (mostly); it looks as if an important part of this must be taking place in the first level processing, but it is a non-trivial task which goes a long way beyond interpolating colours and positions.
The authors do mention the Abhidharma Buddhist view of experience as a series of discrete moments within a flow; we’ve touched on this before in discussions of findings by Varea and others that the flow of consciousness seems to have a regular pulse; it would be intriguing and satisfactory if that pulse could be related to the first level of processing hypothesised here; we’re apparently talking about something in the 100ms range which seems a little on the long side for the time slices proposed; but perhaps a kind of synthesis is possible..?