**How fMRI Technology Contributes to the Advancement of Research in Mental Imagery: A Review**

Marta Olivetti Belardinelli1,2, MassimilianoPalmiero3 and Rosalia Di Matteo2,4 *1Department of Psychology, "Sapienza" University of Rome 2ECONA, Interuniversity Centre for Research on Cognitive Processing in Natural and Artificial System 3Department of Internal Medicine and Public Health, University of L'Aquila* 

*4Department of Neuroscience & Imaging, "G. d'Annunzio" University, Chieti Italy* 

#### **1. Introduction**

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Mental imagery involves the generation of images using information stored in long-term memory, as opposed to the extemporaneous registration of information by our senses, giving rise to introspective experiences, such as 'seeing with the mind's eye', 'hearing with the mind's ear', 'smelling with the mind's nose'. In the past four decades, there has been much debate regarding the extent to which key elements of perception rely upon mental images versus propositional knowledge of sensory principles. Two contrasting approaches were developed i.e. perceptual and propositional theories. According to the perceptual approach, mental imagery is supported by mechanisms and processes involved in the actual perception. It functions as a modal analogue of that which is perceived by the senses (Kosslyn et al., 2006). Thus, mental images resemble perceptual information, e.g., visual images preserve both pictorial and spatial properties. According to the propositional approach, mental imagery is supported by abstract symbols of the sort used in a languagelike system. It functions as an a-modal description of the external world (Anderson & Bower, 1973; Pylyshyn, 1981, 2002, 2003). In particular, mental images rely on a code, structured by rules and relationships, rather than on mere verbal descriptions. Therefore, these mental images are epiphenomena of thought: instead of exhibiting the sensory aspects that determine their analogical nature, they are affected by "cognitive permeability," or the tacit knowledge of physical laws in the external world (Pylyshyn, 1981, 2002).

These perspectives have primarily been investigated using visual imagery as a reference modality. After many years of behavioral research, the debate reached an impasse, as empirical evidence could only be explained by considering one of the two competitive approaches at a time (Kosslyn, 1980). The advent of neuroimaging techniques, particularly fMRI, offered the scientific community a new opportunity to solve the imagery debate. Unlike previous neuroimaging techniques, fMRI is capable of isolating many simultaneous and coordinated brain events with high spatial resolution. This facilitates the delineation of

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Under specific circumstances visual mental images seem to involve neural mechanisms recruited by visual perception, as evidenced by the fact that the lateral geniculate nucleus is activated during visual imagery (Chen et al., 1998). Ganis et al. (2004) clarified, however, that when participants were instructed to either imagine or see faint drawings of simple objects, and then judge specific aspects of the drawings, such as 'taller than wider', visual imagery and visual perception recruited similar neural machinery, especially in frontal and parietal regions. Though the calcarine cortex was activated in both conditions, this spatial

The spatial overlap observed during imagery and perception does not necessarily imply that the corresponding representations are qualitatively similar to each other. Reddy et al. (2010) accordingly explored the extent to which it is possible to establish which item participants were imaging, as well as the comparability between representations evoked during imagery and visual perception. Participants were instructed to both imagine and see stimuli belonging to four object categories: food, tools, faces, buildings. By using pattern classification techniques, which test each classifier (perceptual or imagery) on the other condition, authors were capable to decode category information from ventro-temporal cortex in both imagery and perceptual conditions, but only during actual viewing from visual primary area. Using the same logic, Stokes et al. (2009) found similar results, revealing that imagery of the letter X versus the letter O could be decoded from the lateral

Intons-Peterson (1992, p. 46) defined auditory imagery as "the introspective persistence of an auditory experience, including one constructed from components drawn from long-term memory, in the absence of direct sensory instigation of that experience''. This type of imagery runs auditory traces from prior experiences (e.g., someone's voice, environmental sounds, melodies) through the "mind's ear". In this section, the fMRI studies on auditory verbal imagery will be presented first, followed by auditory imagery of environmental

Auditory verbal imagery can occur spontaneously or deliberately when recalling the sound of our own voice or someone else's voice. Within the frame of working memory, auditory verbal imagery has been associated with the operation of the phonological loop sub-system (Baddeley & Loogie, 1992). Two components have been discerned: a short-lived store that represents material in phonological form (inner ear), and an articulatory rehearsal process (inner voice) that is used to recode and refresh decaying representations into the

By using fMRI, Shergill et al. (2001) found that there is a lack of activation in the primary auditory cortex (Heschel gyrus – BA 41/42) during auditory verbal imagery. Relative to the baseline condition (listening to each word carefully), first person imagery (imagining sentences of the form "I like…" in ones' own voice) showed no activation in the auditory cortex, while second and third person imagery (imagining sentences of the form "You like…." and "He likes…." in voices that participants had heard on a tape) showed activation in the secondary auditory cortex (BA 22), namely in the superior temporal gyrus. Jancke &

overlap was neither complete nor uniform.

sounds, and musical imagery will be discussed last.

occipital complex.

**3. Auditory imagery** 

**3.1 Auditory verbal imagery** 

phonological store (Smith et al., 1995).

brain anatomy, which increases sensitivity while maintaining selectivity. In comparison to Positron Emission Tomography (PET), fMRI offers increased statistical power for two reasons: first, the activation maps from multiple individuals do not need to be averaged, and the spatial transformation is not necessary, providing an enormous advantage in terms of signal-to-noise ratios (Watson et al., 1993); second, scanning can easily be extended over time.

Because of these advantages, fMRI has been used extensively to identify brain structures uniquely involved in cognitive functions, included mental imagery. This accordingly makes fMRI more suitable than PET and Single Photon Emission Computer Tomography (SPECT) for examining the extent to which imagery and perception share overlapping cortical areas in the functioning of various sensory modalities. In exploring this issue, researchers aimed to clarify whether imagery involves the activation of primary sensory cortices in visual, auditory, tactile, olfactory, gustatory, motor, and proprioceptive modalities. The present review is therefore aimed at investigating the status of fMRI research in all imagery modalities, with separate sections for each imagery modality, followed by collective conclusions.
