A Gentle Introduction to..... Frith, Rees, and Friston's (1998) "Forward Model" of Self Derek J. SMITH High Tower Consultants Limited

PART 1 THE PROBLEM

BACKGROUND "Many of the symptoms of schizophrenia can be interpreted as reflecting abnormalities in the sense of self" (Frith, Rees, and Friston, 1998, p171). "We have been particularly interested in the passivity experiences in which the patient feels that he no longer controls his own actions (Frith, 1992). [..... T]he patient knows that he is acting, but experiences these actions as being determined by some external agency" (p171).

THE PROBLEM OF OUTPUT-RELATED INPUT In other words, Frith et al were interested in how motor behaviour initiated up here produced **anticipatable** feedback in the sensory channels down here..... which, being self- initiated, could be safely ignored up here.....

PART 2 THE FORWARD MODEL

THE FORWARD MODEL "Every time we perform an action, move our eyes, move a limb, or speak, there are perceptual consequences. [..... But h]ow can we distinguish these changes from sensory changes caused by independent events happening in the outside world? This distinction is crucial if the self is to be separated from the outside world" (p172). Frith et al suggest that the critical process is to set up a "forward model" of the likely effects of your own actions on the world, based on "efference copy". This will help you differentiate between what you are doing and what else might be going on.

THE FORWARD MODEL The efference copy concept was first described by Von Holst and Mittelstaedt (1950) [Technical background, if interested].Technical background "The forward model uses a 'copy' of the motor command (efference copy) to predict the sensory consequences of the resulting action" (p172; Figure 1 caption) Frith et al's explanatory diagram (Figure 1) now follows.....

THE FORWARD MODEL forward model motor system sensory system outside influences actual sensory feedback expected sensory feedback difference signal efference copy motor command

THE FORWARD MODEL The next three slides show how the different processing stages work together to keep track of self-generated sensory input in amongst the largely unpredictable other-generated input stream.....

THE FORWARD MODEL forward model motor system sensory system outside influences actual sensory feedback expected sensory feedback difference signal efference copy motor command Step 1a: You decide to carry out a movement, so you send instructions to your motor system Step 1b: At the same time to send a copy of those instructions to your forward modelling system

THE FORWARD MODEL forward model motor system sensory system outside influences actual sensory feedback expected sensory feedback difference signal efference copy motor command Step 2a: Your sensory system feels your body making the resulting movements. Step 2b: Your sensory system ALSO detects anything else which might be going on about you

THE FORWARD MODEL forward model motor system sensory system outside influences actual sensory feedback expected sensory feedback difference signal efference copy motor command Step 3: The comparator system then compares actual total input with expected, and only bothers to "perceive" the difference between the two

THE FORWARD MODEL Frith et al's approach only really shows its true worth "when forward modeling goes wrong" (p173). Here is the critical scenario..... "Discrepancies between expected and actual sensation might occur when there were no external influences. In this case the perceiver would erroneously conclude that external influences were causing his experiences. If his ability to distinguish between self-generated and other-generated sensations was generally impaired, then in the long run his ability to perceive himself as an agent would also become impaired" (p173).

THE FORWARD MODEL This loss of agency then has serious repercussions for the mental health of the subject. "We have suggested that many of the symptoms associated with schizophrenia can be explained in terms of a failure in a 'self-monitoring' system. Although this system has its origins in a fairly simple scheme for the control and learning of motor responses, it has features that give it a key role in making a distinction between the self and the outside world" (p177).

THE END

FURTHER READING Glossary entry on Friston, Karl J. [IMPORTANT]Friston, Karl J.

References: Frith, C., Rees, G., and Friston, K. (1998). Psychosis and the experience of self: Brain systems underlying self-monitoring. Annals of the New York Academy of Sciences, 843:

Copyright Notice: This material was written and published in Wales by Derek J. Smith (Chartered Engineer), at the time Senior Lecturer in Cognitive Science and Informatics at University of Wales Institute, Cardiff. It forms part of a multifile e-learning resource, and subject only to acknowledging Derek J. Smith's rights under international copyright law to be identified as author may be freely downloaded and printed off in single complete copies solely for the purposes of private study and/or review. Commercial exploitation rights are reserved. The remote hyperlinks have been selected for the academic appropriacy of their contents; they were free of offensive and litigious content when selected, and will be periodically checked to have remained so. Copyright © 2007 Derek J. Smith; Copyright © 2010, High Tower Consultants Limited.