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Let me explain the increasingly bold commitments involved in accepting this conjecture.

First, the things categorised in categorical perception of expressions of emotion are events rather than configurations or anything static. (Note that this is consistent the fact that static stimuli can trigger categorical perception; after all, static stimuli can also trigger motor representations of things like grasping \citep{borghi:2007_are}.)

Second, these events are not mere physiological reactions (as we might intuitively take blushing to be) but things like frowning and smiling, whose performance involves motor expertise. \footnote{ To emphasise, one consequence of this is that not everything which might intuitively be labelled as an expression of emotion is relevant to understanding what is categorised by perceptual processes. %For example, in the right context a blush may signal emotion without requiring motor expertise. }

Third, these events are perceptually categorised by the outcomes to which they are directed. That is, outcomes represented motorically in performing these actions are things by which these events are categorised in categorical perception.

Should we accept the wild conjecture? It goes well beyond the available evidence and currently lacks any reputable endorsement. In fact, we lack direct evidence for even the first of the increasingly bold commitments just mentioned (namely, the claim that the things categorically perceived are events). A further problem is that we know relatively little about the actions which, according to the wild conjecture, are the things categorical perception is supposed to categorise (\citealp[p.\ 47]{scherer:2013_understanding}; see also \citealp{scherer:2007_are} and \citealp{fernandez-dols:2013_advances}). However, the wild conjecture is less wild than the only published responses to the problems that motivate it.% % (which, admittedly, are wilder than an acre of snakes).% \footnote{ See \citet[p.\ 15]{motley:1988_facial}: ‘particular emotions simply cannot be identified from psychophysiological responses’; and \citet[p.\ 289]{barrett:2011_context}: ‘scientists have created an artifact’. } And, as I shall now explain, several considerations make the wild conjecture seem at least worth testing.

Consider again the procedure used in testing for categorical perception. Each experiment begins with a system for categorising the stimuli (expressions). This initial system is either specified by the experimenters or, in some cases, by having the participants first divide stimuli into categories using verbal labels or occasionally using non-verbal decisions. The experiment then seeks to measure whether this initial system of categories predicts patterns in discrimination. But what determines which category each stimulus is assigned to in the initial system of categories? You might guess that it is a matter of how likely people think it is that each stimulus---a particular facial configuration, say---would be associated with a particular emotion. In fact this is wrong. Instead, each stimulus is categorised in the initial system according to how suitable people think such an expression would be to express a given emotion: this is true whether the stimuli are facial \citep{horstmann:2002_facial} or vocal \citep{laukka:2011_exploring} expressions of emotion (see also \citealp[pp.\ 98--9]{parkinson:2013_contextualizing}). To repeat, in explicitly assigning an expression to a category of emotion, people are not making a judgement about the probability of someone with that expression having that emotion: they are making a judgement about which category of emotion the expression is most suited to expressing. Why is this relevant to understanding what perceptual processes categorise? The most straightforward way of interpreting the experiments on categorical perception is to suppose that they are testing whether perceptual processes categorise stimuli in the same ways as the initial system of categories does. But we have just seen that the initial system categorises stimuli according to the emotions they would be best suited to expressing. So on the most straightforward interpretation, the experiments on categorical perception of expressions of emotion are testing whether there are perceptual processes whose function is to categorise actions of a certain type by the outcomes to which they are directed. So the wild conjecture is needed for the most straightforward interpretation of these experiments. This doesn't make it true but it does make it worth testing.

So far I have focussed on evidence for categorical perception from experiments using faces as stimuli. However, there is also evidence that perceptual processes categorise vocal and facial expressions alike (\citealp{grandjean:2005_voices,laukka:2005_categorical}; see also \citealp{jaywant:2012_categorical}). We also know that %judgements about which emotion an observed person is expressing in a photograph can depend on the posture of the whole body and not only the face \citep{aviezer:2012_body}, and that various contextual factors can affect how even rapidly occurring perceptual processes discriminate expressions of emotion \citep{righart:2008_rapid}. There is even indirect evidence that categorical perception may concern whole bodies rather than just faces or voices \citep{aviezer:2008_angry,aviezer:2011_automaticity}. In short, categorical perception of expressions of emotion plausibly resembles categorical perception of speech in being a multimodal phenomenon which concerns the whole body and is affected by several types of contextual feature. This is consistent with the wild conjecture we are considering. The conjecture generates the further prediction that the effects of context on categorical perception of expressions of emotion will resemble the myriad effects of context on categorical perception of speech so that `every potential cue ... is an actual cue' (\citealp[p.\ 11]{Liberman:1985bn}; for evidence of context effects see in categorical perception of speech, for example, \citealp{Repp:1987xo}; \citealp{Nygaard:1995po} pp.\ 72--5; \citealp{Jusczyk:1997lz}, p.\ 44).

... it’s also that the goals in this case involve emotions. Since emotions aren’t represented motorically, how could they be represented?

\citep{adolphs:2003_dissociable} reported that ‘Patient (B.) had bilateral damage to the insula: he was able to retrieve knowledge about all basic emotions except disgust when viewing facial expressions or hearing descriptions of emotionally related actions. Strikingly, when the experimenter acted out behaviors typically associated with disgust (e.g. spitting out of food), the patient remained indifferent or even indicated that the food was ‘delicious’’ (Rizzolatti and Sinigaglia, in preparation).

*modified by me: they make it specific to the face; although faces are clearly special, I think this may be a mistake for reasons given in the last lecture.