Propositions Versus Images

Even in our initial description of the nature of imagery, it was hard not to mention the idea that there are propositional aspects to imagery. Some years ago, this conflict between propositions and images became the subject of considerable debate (see Anderson, 1978; Bannon, 1981; Pylyshyn, 1973, 1979, 1981, 1984). We will not rake over the embers of this debate here (see Eysenck & Keane, 1995, Chapter 9, for details; also Kosslyn, 1994). The upshot of this has been that images are a distinct representational format with distinct functional significance over and above propositional representations (later, in Kosslyn's theory

PANEL 9.2:

PAIVIO'S DUAL-CODING THEORY

• Two basic independent but interconnected coding or symbolic systems underlie human cognition: a non-verbal system and a verbal system.

• Both systems are specialised for encoding, organising, storing, and retrieving distinct types of information.

• The non-verbal (or imagery) system is specialised for processing non-verbal objects and events (i.e. processing spatial and synchronous information) and thus enters into tasks like the analysis of scenes and the generation of mental images.

• The verbal system is specialised for dealing with linguistic information and is largely implicated in the processing of language; because of the serial nature of language it is specialised for sequential processing.

• Both systems are further sub-divided into several sensorimotor sub-systems (visual, auditory, and haptic).

• Both systems have basic representational units: logogens for the verbal system and imagens for the non-verbal system that come in modality-specific versions in each of the sensorimotor subsystems.

• The two symbolic systems are interconnected by referential links between logogens and imagens.

of imagery, we will see how the two might relate). In this section, we will present an empirically driven account for the argument, made by Allan Paivio, that propositions and images are distinct coding systems.

Paivio's dual-coding theory

Allan Paivio's dual-coding theory (see Paivio, 1971, 1979, 1983, 1986, 1991) is devoted to determining the minimal basic differences between imagistic and propositional representations, grounded in empirical data from a large corpus of experiments. The basic proposals of the theory are shown in Panel 9.2.

Stated simply, the essence of dual-coding theory is that there are two distinct systems for the representation and processing of information. A verbal system deals with linguistic information and stores it in an appropriate verbal form. A separate non-verbal system carries out image-based processing and representation (see Figure 9.10). Each of these systems is further divided into subsystems that process either verbal or non-verbal information in the different modalities (i.e., vision, audition, tactile, taste, smell).

However, it should be noted that there are no corresponding representations for taste and smell in the verbal system (see Table 9.5).

Within a particular sub-system when, for example, a spoken word is processed it is identified by a logogen for the auditory sound of the word. The concept of a logogen comes from Morton's (1969, 1979) theories of word recognition. Paivio (1986) characterises a logogen as a modality-specific unit that "can function as an integrated, informational structure or as a response generator" (p. 66): for example, there may be logogens for the word "snow". Logogens are modalityspecific, in the sense that there are separate logogens for identifying the spoken sound "snow" and its visual form (i.e., the letters "s-n-o-w"). The parallel to logogens in the non-verbal system are imagens. Imagens are basic units that identify and represent images, in the different sensorimotor modalities. The important point to note about logogens and imagens is that they allow the theorist to posit a processing unit that identifies or represents a particular item (i.e., an image of a dog or a particular word) without having to specify the internal workings of this processing unit or the detailed representation of the item being processed. This lack of specification is one criticism of Paivio's work, although it is a deficit that is compensated for by later computational theories (like Kosslyn's, 1980, 1994).

The verbal and non-verbal systems communicate in a functional fashion via relations between imagens and logogens. The simplest case of such a relation is the referential link between an object and its name. That is, if you see a visual object (e.g., a dog runs by) it would be recognised by an imagen and a link between this imagen and an auditory logogen for the word "dog" may activate the word "dog". Thus, the links between these basic units constitute the fundamental ways in which the sub-parts of the two symbolic systems are interconnected.

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