Historical overview of inner speech investigations

1.2.1.1 From introspection to experimental psychology

The question of the relation and intertwinement of thought and language is one of the most enduring philosophical question. Most notable reflections can be traced back to Plato’s Theaetetus, in which Plato defines thinking as “the conversation which the soul holds with itself in considering anything”. For Plato, the definition of thinking is taken to correspond to “word[s]

spoken in silence”. Sokolov (1972) notes that ancient thinkers, by noticing a relation between thoughts and words, and between words and breathing, used to think that thoughts and words originated in the lungs. For instance, Socrates, in Plato’sPhaedrus, said that “his chest is full of thoughts” (as quoted in Sokolov, 1972, p. 14). In another context, by noting the progressive internalisation of external speech into inner speech during normal development, Egger (1881) wonders whether the phylogeny (the evolution of the species) followed this same course of development. In support of that idea, Egger (1881), citing Maspero (1875), reports the existence of an ancient Egyptian ideogram, representing a crouched man, with the right hand close to the mouth. Egger (1881) explains that this ideogram was meant to represent undistinctly the ideas of eating, drinking, screaming, talking, meditating, knowing or judging, suggesting that thought, during the Egyptian Ancient Empire, was considered to be associated with the mouth organ (p. 84).

Somehow consistently with that idea, Stricker (1880) reported (based on his own introspections) that he was not able to mentally produce speech sounds without contracting his articulators. He also reported not being able to produce two different speech sounds, or to produce speech sounds that were incongruent with movement of the articulators. To give a reproducible example of his intuition, Stricker (1880) suggested the following experiment:

open your mouth and try to pronounce a word including labials or dentals, such as “bubble”

or “toddle”. Ask yourself whether the image of the word (your inner speech) is clear or distinct?

According to Stricker, most people would find it very difficult to imagine cleary these words with the mouth being open. Instead, the image of the work is rather imprecise and sounds like we were trying to produce (overtly) the word while keeping the mouth open. This sensation was already nicely described and analysed by Bain (1855):

“When we recall the impression of a word or sentence, if we do not speak it out, we feel the twitter of the organs just about to come to that point. The articulating parts –the larynx, the tongue, the lips,– are all sensibly excited; a suppressed articulation

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is in fact the material of our recollection, the intellectual manifestation, the idea of speech.”

James (1890) then notes that Stricker, “Like most psychologists, however, […] makes of his personal peculiarities a rule, and says that verbal thinking is normally and universally an exclusively motor representation.” Indeed, Paulhan (1886) replied to Stricker that he was able to produce overtly the phoneme /a/ while simultaneously being able to get and maintain the mental image of any other vowel. He also reported that he was able to imagine the sound of any vowel without motor actions or feelings (images). On a similar note, Egger (1881) believed inner speech to exist independently of motor phenomena and to be based predominantly on auditory representations. He noticed that although inner speech may be accompanied by vivid auditory imagery, inner speech is also very different from overt (external) speech, with inner speech being usually shorter and less grammatically structured than overt speech (we will come back to that observation later when discussing the development of inner speech).

In an attempt to reconcile the view of Stricker (1880) for whom inner speech was purely motor with the view of Paulhan (1886) and Egger (1881), Ballet (1886) suggested (as James, 1890), that these authors probably generalised to the population what they observed on themselves. Ballet then asserted that the predominance of motor over sensory representations (or the reverse) might be a question of individual differences. We might add that the relative predominance of motor or sensory representations during inner speech might also be due to individual differences in the phenomenological sensitivity to some specific representation (some might be very acute in discriminating similar auditory images while not being able to discriminate similar visual images) and to contextual differences.⁶ Nonetheless, for many authors, this debate highlighted the limitations of the introspective method (e.g., Reed, 1916).

To be able to decide between different individual experiences and interpretations, some researchers therefore tried to find more objective methods to assess inner speech, or as put by Reed (1916), to go beyond introspection and to start looking for “the stamp of objective certainty”. With this ambitious goal in mind, Reed (1916) described the apparatus he used to examine tongue movements (see Figure 1.2). Reed then reported the results of an experiment aiming to examine the involvement of inner speech (and speech motor processes) in thinking.

Reed (1916) observed that while reading, his participants were moving their tongue and lips (and were sometimes whispering). These observations, in addition to the behaviourist revolution in Psychology paved the way for new lines of research. The initial suggestion of Watson (1913) that “thought processes are really motor habits in the larynx” led to a fruitful line of research about the muscular bases and/or correlates of thought and inner speech. Sokolov (1972) gives an overview of the experiments carried out at the beginning of the XXth century in that perspective. For instance, Dodge (1896) anesthetised his lips and tongue and realised

6Indeed, it is plausible that the predominance of some sort of representation over other forms might be contingent on contextual demands. In other words, depending on the task to be realised, the motoric and sensory aspects of inner speech might be weighted differently.

Figure 1.2. Figure 1 & 2 from Reed (1916) describing the apparatus used to record tongue movements during thinking and inner speech.

that it did not have any impact on his inner speech. Curtis (1900) and Courten (1902) recorded laryngeal movements using a pneumatic drum and a kymograph while their participants recited verses or were reading. They observed that laryngeal movements were not always present and depended on what was being read and/or produced, as well as on the “degree of understanding” of the participant (for further references, see Sokolov, 1972, pp. 43–45).

Using a galvanometer and electrodes inserted in the tip of the tongue, in the cheek, or under the lip, Jacobson (1931) recorded muscular action potential while participants were asked to produce verbal content covertly (e.g., counting or reciting a poem), but not during relaxation. Interestingly, Jacobson (1931) adds that “the series of vibrations during the mental activity occur in patterns evidently corresponding with those present during actual speech.”

More precisely, the pattern of muscular of activity recorded during inner speech production was similar to the pattern of muscular activity recorded during overt speech production, but of lesser amplitude.

Throughout the present section, we briefly reviewed the history of ideas and methods used to describe inner speech in the second part of the XIXth century and at the beginning of the XXth century. In the next section, we make a brief pause in our historical tour to discuss the developmental trajectory of inner speech. How and when do we (humans) acquire the ability to talk to ourselves silently? Is it even acquired? To answer these questions, we will briefly review Vygotsky’s theory of inner speech development and some of its more recent refinements.

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Moreover, by examining how inner speech develops, we might gain new insights about the characteristics of inner speech in the adult mind.

1.2.1.2 Interlude: the development of inner speech

The developmental course of inner speech was possibly the most investigated issue related to inner speech in the first part of the XXth century. Among many, Watson, Piaget, Luria, Leontiev, and most famously Vygotsky confronted this question. Watson (1919) suggested that thought was rooted in (overt) speech, with maturation leading from speech to thought (where thought is to be understood as a synonym to inner speech, in Watson’s terminology). This hypothesis also applied to reading, with the novice reader reading overtly and progressively shifting to silent reading. For Vygotsky, the study of inner speech in the mature (i.e., adult) brain could only be understood from a developmental perspective. In the last chapter of his bookThought and Language, Vygotsky analyses the relationship between thought and word in the mature mind. The central idea of this chapter is stated as follows:

“The relationship between thought and language is not a thing, but a process, a continual movement back and forth from thought to word and from word to thought. Viewed in the light of a psychological analysis, this relation is a process that passes through a series of phases and stages, during which its essential features undergo changes that may be called development in the strict sense.

Of course, this is a functional development, not development in the sense of aging; but the path traversed by thinking as a process from thought to word is development nonetheless.”

Fundamentally, Vygostky believed that language was a psychological tool and that its development during childhood interacts with the development of abstract thinking. Vygotsky observed, as Piaget before him, that children tend to speak (aloud) to themselves while playing.

Piaget characterised this form of speech as “egocentric speech” because in this form of speech, according to Piaget, the child does not try to take the perspective of the listener. Piaget thought this form of speech to disappear at the age of seven or eight. In contrast, Vygotsky thought that the so-called egocentric speech (or private speech) continues but that it becomes more and more internalised, until reaching the status of “inner” speech. For Vygostky, this internalisation process starts with social speech, that is speech addressed to others. During development, this form of speech evolves to either communicative speech (speech addressed to others) and so-called egocentric speech (speech addressed overtly to oneself ). This form of speech appears naturally in children in situations when they are being faced with a problem to solve, but also in adults faced with difficult problems. This egocentric speech would then became internalised, resulting in what we call inner speech. This led Vygostky to claim a functional equivalence between egocentric speech and internal speech, the later resulting from a progressive internalisation of the former.

However and importantly, this internalisation process does not only entail a movement from the outside to the inside but also entails a transformation of speech, or, as put by Vygotsky, an “internal reconstruction of an external operation”. Therefore, for Vygostky, it follows that the passage from inner speech to overt speech consists not in simply “vocalising” inner speech but inrestructuring inner speech (e.g., retrieving a syntax proper to overt speech, retrieving the phonetic structure, etc). According to Vygotsky (1934/2012), inner speech is described by some essential properties such as: i)abbreviation: the phonetic aspect is “diminished”, reduced: “In inner speech we do not need to pronounce a word in its entirety. We understand, by virtue of our very intention, what word we wanted to say […] Strictly speaking, inner speech is almost wordless”, ii)predicativeness, “Psychologically, inner speech consists of predicates only”; “the subject of our inner reason is always present in our thought”; it is always implicitly understood, iii) it has a semantic structure of its own: predominance ofsenseovermeaning,⁷it is idiomatic, agglutination of semantic units (several words can be “merged” into a single word), andinfusion of senseinto a word (a word in inner speech becomes “loaded” with more associations than in conventional use).

Interestingly, Vygotsky rejected both the verbal memory view of inner speech (i.e., inner speech is simply the retrieval of acoustic, optic or motor images of words) and the behaviourist view of inner speech as merely a soundless form of external speech (à la Watson). For Vygotsy, the most determining factors of inner speech are its semantic (psychological) features, as expressed by his famous dictum:Thought is not expressed in words; it comes into existence through them.

More recently, Fernyhough (2004) proposed an extension of Vygotsky’s three-level model of inner speech development (i.e., external speech, egocentric speech, inner speech) to a four-level model, from external dialogue to private speech, expanded inner speech and condensed inner speech (see Figure 1.3). Fernyhough (2004) notes that this model describes stages in the development of inner speech (during childhood) but also movements “between the levels at any given point in time”. Indeed, it is possible to “move” between levels under certain conditions. For instance, in cognitively demanding conditions, we can observe transitions between levels, with condensed inner speech being transformed to expanded inner speech and even private speech through a process of “re-expansion”. This idea is supported by many studies showing an progressive externalisation of inner speech under cognitively demanding situations (e.g., Sokolov, 1972).

To sum up, it is suggested that inner speech (in the adult mind) is the result of a progressive internalisation process. This internalisation process covers different stages or expressions of speech from social speech, self-addressed speech (private speech or egocentric speech) to inner speech (first in a fully expanded form and then in a more condensed form). Being an internalised version of private speech, inner speech is hypothesised to be attached with the

7Referring to Paulhan’s distinction between the dictionarymeaningof a word on one hand, and the individual senseof a word which is acquired by usage, on the other hand.

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The result of these syntactic and semantic changes is that inner speech loses most of the acoustic and structural qualities of external speech, and becomes a process of

‘thinking in pure meanings’ (Vygotsky, 1934/1987). In elaborating on Vygotsky’s ideas in this regard, it is possible to sketch out a four-level developmental scheme for the development of inner speech (seeFig. 1). At Level 1 (external dialogue), children and caregivers engage in overt dialogue which displays the characteristic give-and-take of conversation. At Level 2 (private speech), children conduct these dialogues in their own overt (and then gradually subvocalised) private speech. At Level 3 (expanded inner speech), private speech is fully internalised and covert, but the give-and-take of normal conversation is still manifested internally as the process of talking silently to oneself. At Level 4 (condensed inner speech), the syntactic and semantic transformations of internalisation ensure that inner speech retains few if any of the accoutrements of external language, and has become an dialogic interplay between alternate perspectives on reality (Fernyhough, 1996)—that is, the stage of

‘thinking in pure meanings’ described by Vygotsky.

Importantly, this model does not merely describe the stage-like developmental process through which the child internalises dialogue. It also allows for movement between the levels at any given point in development. Of particular importance for