Chapter 6 - Is predicting seeing?
Presented by Rachel Gunn
In Chapter 6 Hohwy asks the reader “does what we believe to some degree determine what we perceive?” (p.118). My initial reaction to this is – yes, of course it does. I believe that many perceptual experiences are cognitively penetrable. It seems straightforward that different people often see different things depending on prior beliefs. A person who believes in ghosts will see their dead mother in the reflection on a darkened window whereas the person who does not believe in ghosts will just see a weird reflection. This may be a similar finding to Hohwy’s reference to people who believe in extra sensory perception seeing more meaningful patterns than others in ‘noisy’ images (p.134).
I’m not clear whether this example is really a change in perception or simply a change in judgement but it did seem as if the object was one thing and then it looked like a different thing. I am aware that I was confused by the object I was looking at and may have changed how I was looking at it. However, it does seem that for practical purposes I saw something different when I gained the ‘new’ knowledge. Reading on, it seems that this might fit with Hohwy’s proposal that cognitive penetrability occurs when uncertainty is high (p.137) – although I am still not sure that my example is the kind of thing he’s getting at. Hohwy also cites the duck/rabbit, old/young woman and kangaroo/whale as possible cases of cognitive penetrability (p.130).
As we have no way of knowing how information is weighted* at a neuronal level this neither supports nor undermines the PEM model. If neuronal mechanisms (or systems) that give us this kind of outcome cannot be altered by person level information all this tells us, if PEM is true, is that the perceptual inference that the line on the left is longer than the line on the right has a probability of1 (in Bayesian terms) so to all practical intents and purposes there’s no room for, or possibility of, revision. The popular explanation cited in Hohwy (p.125) which relates to an accumulation of priors that gives us this perceptual experience could be said to be evidence for PEM. As I understand it Hohwy proposes that in the Muller-Lyer case prediction error is supressed. The wrong kind of input (lacking the correct fineness of spatiotemporal grain) would have no impact on the Bayesian framework. I’m not sure if we need any explanation, or even if an explanation is possible (for the reason cited above*).
I don’t suppose for a second that there is any such thing as an ideal Bayesian brain (I’m not even sure what that would mean). As I understand it, Prediction Error Minimisation can only be about making the best attempt at minimising the prediction error given previous best attempts, which give us the priors. How we can ever get to any understanding of the ‘subjective’ probability of anything (at a neuronal level) is beyond me…. But perhaps that says more about my limited understanding of the topic than the possibilities of this project.
In some cases, we can alter our perception by acquiring person level knowledge and in other cases this does not happen – this is not a problem for PEM. So, as Hohwy proposes, we can accommodate cognitive penetrability and cognitive impenetrability within the PEM theory.