Philosophy of Mind and Psychology Reading Group -- The Predictive Mind chapter 3

Susanna Siegel

Welcome to the third post of the online reading group in the Philosophy of Mind and Psychology hosted by the Philosophy@Birmingham blog. This month, Professor Susanna Siegel (Harvard and Birmingham) presents chapter 3 of The Predictive Mind by Jakob Hohwy (OUP 2013).

Update: on 4th May we posted a reply to this post from Jakob Hohwy that can be seen here.

Chapter 3 - Prediction Error, Context, and Precision
Presented by Susanna Siegel

Chapters 1 and 2 sketch a picture on which the brain generates perceptual experiences and judgment by relying on learned expectations to help interpret sensory signals. The need for interpretation arises initially because the signals are informationally impoverished, compared to the contents of the perceptual experiences and judgments that we end up with. The main point of Chapter 3 is that in addition filling in missing about the external world that’s missing from the initial sensory signal, there is a second dimension along which the brain has to respond to the sensory signal. It has to assess whether any given signal itself is ‘noisy’, where this means that it is not the result of a mechanism that systematically relates the subject to the distal stimulus that the perceptual experience purports to characterize. A signal is noise if it results from a random fluctuation, or some other process that isn’t systematically connected to any properties or objects in the world.

There are thus two sources of uncertainty that each generate a need to interpret sensory signals: The first-order impoverishment of the initial sensory signals themselves, and the second-order uncertainty about whether to ‘trust’ whatever information is given by the sensory signals, however paltry that information may be.  

Hohwy describes what’s needed to address the problem of second-order uncertainty in several ways. What’s needed is “second-order perceptual inference”, “engaging in second-order statistics that optimize precision expectations”, a “need to not only assess the central tendency of the distributions, such as the mean, but the variation about the mean”, a need to “modulate the way prediction errors are processed in the perceptual hierarchy”. Suppose I’m collecting signals and they form a trend. Now the n^th signal comes in. The trend predicts that the n^th signal will say “Yellow”. But instead the signal says “Grey”. Suppose I have little confidence in this signal. My second-order verdict on whether to trust it is that it’s probably unreliable.

Philosophy of Mind and Psychology Reading Group -- The Predictive Mind chapter 2

Welcome to the second post of the online reading group in the Philosophy of Mind and Psychology hosted by the Philosophy@Birmingham blog. This month, Alastair Wilson (Birmingham Fellow in Philosophy specialising in Metaphysics and Philosophy of Physics) presents chapter 2 of The Predictive Mind by Jakob Hohwy (OUP 2013).

Alastair Wilson

Chapter 2 - Prediction Error Minimization
Presented by Alastair Wilson

According to the perceptual error minimization (PEM) model, perception is inferential. Chapter 2 addresses a crucial question for any inferential approach - where do the priors come from? - and argues that PEM offers a new answer to this question.

A statistical illustration Translating talk of Bayesian priors into curve-fitting for illustrative purposes, what we want is a mechanism which will optimize the trade-off between accuracy and over-fitting. This means that there is a need to factor in expected noise levels in the incoming data when determining how low to force the prediction error. PEM implements this via the following feedback loop:

Use prior beliefs harnessed to an internal model to generate predictions of the sensory input	→	Revise models prediction or change sensory input to minimize prediction error subject to expectations of noise.
	←

Reconceiving the Relation to the World  PEM goes beyond the analogy with trial-and-error procedures. The primary representational content of perception is encoded in the downwards/backwards connections between levels in the hierarchy rather than in the upwards/forwards connections. That is, perceptual content primarily consists in the predictions that higher levels are making about lower levels rather than in any top-down interpretation of the signals that higher levels receive from lower levels. "The functional role of the bottom-up signal from the world is then to be feedback on the internal models of the world." (47)

Philosophy of Mind and Psychology Reading Group -- The Predictive Mind chapter 1

The Predictive Mind
by Jakob Hohwy

Welcome to the online reading group in the Philosophy of Mind and Psychology hosted by the Philosophy@Birmingham blog. Our first book is Jakob Hohwy’s The Predictive Mind (OUP, 2013). We have a new post on the last Friday of each month. Who is “we”? Many of the people involved are affiliated with the Philosophy Department at the University of Birmingham, but everyone is welcome to participate. To present a chapter by writing a 500-word post on it, please contact Lisa Bortolotti. Comments will be moderated and so there may be a delay between submitting them and seeing them published on the blog.

Chapter 1 – Perception as Causal Inference
Presented by Lisa Bortolotti

In the book, Jakob Hohwy presents and defends the theory that “the brain is a sophisticated hypothesis-testing mechanism, which is constantly involved in minimizing the error of its predictions of the sensory input it receives from the world.” According to Hohwy, the theory is supported by philosophical argument and empirical evidence. Its greatest appeal is its unifying power. The basic idea is that the mind “arises in, and is shaped by, prediction.”