The following three papers appeared in the British Journal of Psychology in 1927, forming a symposium on "the relevance of visual imagery to the process of thinking". Long before the Kosslyn/Pylyshyn debate about whether mental representations are depictive vs. descriptive, there was quite some insight that this depended on the individual*: T. H. Pear clearly had dominantly depictive representations, F. Aveling clearly had dominantly descriptive representations, and F. C. Bartlett proposed that most people (including himself) probably use mixed or alternating representations as task demands or preference dictates. This symposium, and the research that contributed to it, formed the theoretical basis for the objective measurement of individual differences in mental imagery.

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The relevance of visual imagery to the process of thinking: I (click for pdf)

T. H. Pear

British Journal of Psychology, 1927

Summary: Pear proposes that any experiment of imagery ability needs to take certain points into account: that there are many dimensions of imagery; that it can refer to experiences at different levels of consciousness (e.g., dream imagery); that people may have different mental representation strategies that are not related to imagery ability; that it is important to consider whether imagery is required or useful for a given task; and  that many tasks can be performed without imagery, even if the subject is a visualizer. Particularly, many scientists attempting to measure imagery in that era used tasks requiring manipulation of - or memory for - geometric patterns. These, the author pointed out, are quite abstract, and subjects may use abstract strategies to perform them even if instructed to activate a mental image. Voluntary vs. involuntary imagery is briefly touched on. 

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The relevance of visual imagery to the process of thinking: II (click for pdf)

F. Aveling

British Journal of Psychology, 1927

Summary: Aveling states that introspection is necessary, but not sufficient, to elucidate the functional significance of mental imagery, and calls for the use of objective measurements in determining the "presence" or "absence" of mental pictures. He points out that the vividness of a mental image is judged compared to an original experience, which is itself, in that moment, now a thought and not a percept. One study that required subjects to describe and draw their mental images, found that descriptions did not often match drawings, even in visualizers - this led the author to conclude that imagery is not needed to draw objects from memory. Aveling concludes that if one sets out to measure mental imagery, they should design a task that differentiates conditions in which the subject experiences imagery vs. no imagery.

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The relevance of visual imagery to the process of thinking: III (click for pdf)

F. C. Bartlett

British Journal of Psychology, 1927

Summary: Bartlett proposes that all thoughts are expressed symbolically, and mental imagery is a type of symbol that can be used in thought. He also points out that a sense of "knowing" something does not require any symbol or image. He refers to 15 years of research he has conducted on this topic; although no study is specifically referenced there are a few from this time that seem relevant, including: Bartlett, 1916 (click for a pdf) and Bartlett 1921 (click for a pdf), presenting a series of experiments in which he asked subjects to introspect about the clarity of percepts and mental images using geometric stimuli of different complexity (potentially the source of Pear's criticisms).

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*This symposium was already part of a second wave of discussions on individual differences in mental imagery -- the first wave goes at least as far back as 1880 with Francis Galton's article Statistics of Mental Imagery (click for pdf), in which 100 men reported on the clarity of their mental imagery of their breakfast tables. In addition to providing insight into the lavish meals these men enjoyed, this article aptly summarizes a breadth of individual differences in mental imagery abilities: some reported imagery as clear as real life that could be manipulated and projected into the external environment; others reported only abstract thoughts about what they had for breakfast. Galton briefly discusses the possibility that imagery ability can diminish over the lifespan due to lack of use in favor of more efficient (abstract) mental representation formats.