**4. Tactile imagery**

Tactile imagery can be considered part of the haptic system, based on sensors in skin, muscles, tendons, and joints (Klatzky et al., 1991). Similar to other modalities, Craig & Rollman (1999) included the tactile sense as part of the working memory system, characterized by 3 different processing stages: retention up to 500 ms after stimulus offset; vivid recollections of uncategorized stimulus information, with interfering tasks affecting process until approximately 5 s after stimulus offset. The rehearsal mechanisms would last up to 30 seconds after stimulus offset (Burton & Sinclair, 2000).

Few fMRI studies were conducted to clarify the neural correlates of tactile imagery (Falgatter et al., 1997). Querleux et al. (1999) found that during the imagination of tactile stimulation, activation was mostly localized in the ipsilateral somato-sensory cortex (postparietal gyrus – BA 1/2/3). During a period of tactile perception, there was a strong activation in the contra-lateral cortex. Yoo et al. (2003) demonstrated that the left primary (post-central gyrus - BA 1/2/3) and left secondary somatosensory areas (frontal operculum, area 43) were activated when participants imagined a tactile stimulation on the back of the right hand, relative to resting condition. Although the left primary and secondary somatosensory areas were modulated by the mental imagery of tactile sensation, a significant portion of the activation occurred solely during the actual perception. In Yoo et al. (2003), tactile imagery also selectively activated the inferior parietal lobule. Olivetti Belardinelli et al. (2004a) found activation in the inferior parietal lobule, but not in the primary or secondary somatosensory cortex, during the imagination of tactile properties of objects when verbally cued (e.g., to touch something grainy), as compared to the abstract condition. Yet, Olivetti Belardinelli et al. (2009) demonstrated that the primary somatosensory cortex (right parietal post-central gyrus - BA 2), as well as the right postcentral gyrus (BA 5) were more activated in high-vivid participants compared to low-vivid participants. In other words, the level of vividness of tactile imagery can modulate the activation of primary somatosensory cortex. This study also revealed that the inferior occipital cortex (BA 18) was strongly activated in high-vivid participants.

#### **5. Motor imagery**

Motor (or kinaesthetic) imagery is the result of first-person kinaesthetic information processing, as people feel themselves executing a given action (Jeannerod, 1995). This experience is called internal imagery or first-person perspective, which is different from the

lyrics and instrumentals with no lyrics. Each piece of music was pre-rated by subjects as either familiar or unknown. Short sections of music (lasting for 2–5 s) were extracted at different points during the soundtrack and replaced with silent gaps. Participants were instructed to continue imaging the musical selection. Results revealed that imaging the continuation of familiar songs induced greater activation in auditory association areas than imaging the continuation of unknown songs (in both songs with lyrics and without lyrics). Moreover, when familiar songs contained no lyrics, cortical activity extended into the left primary auditory cortex. However, authors revealed that neural activation during lyrics were in the auditory association areas, whereas with instrumental music, neural activity

Tactile imagery can be considered part of the haptic system, based on sensors in skin, muscles, tendons, and joints (Klatzky et al., 1991). Similar to other modalities, Craig & Rollman (1999) included the tactile sense as part of the working memory system, characterized by 3 different processing stages: retention up to 500 ms after stimulus offset; vivid recollections of uncategorized stimulus information, with interfering tasks affecting process until approximately 5 s after stimulus offset. The rehearsal mechanisms would last

Few fMRI studies were conducted to clarify the neural correlates of tactile imagery (Falgatter et al., 1997). Querleux et al. (1999) found that during the imagination of tactile stimulation, activation was mostly localized in the ipsilateral somato-sensory cortex (postparietal gyrus – BA 1/2/3). During a period of tactile perception, there was a strong activation in the contra-lateral cortex. Yoo et al. (2003) demonstrated that the left primary (post-central gyrus - BA 1/2/3) and left secondary somatosensory areas (frontal operculum, area 43) were activated when participants imagined a tactile stimulation on the back of the right hand, relative to resting condition. Although the left primary and secondary somatosensory areas were modulated by the mental imagery of tactile sensation, a significant portion of the activation occurred solely during the actual perception. In Yoo et al. (2003), tactile imagery also selectively activated the inferior parietal lobule. Olivetti Belardinelli et al. (2004a) found activation in the inferior parietal lobule, but not in the primary or secondary somatosensory cortex, during the imagination of tactile properties of objects when verbally cued (e.g., to touch something grainy), as compared to the abstract condition. Yet, Olivetti Belardinelli et al. (2009) demonstrated that the primary somatosensory cortex (right parietal post-central gyrus - BA 2), as well as the right postcentral gyrus (BA 5) were more activated in high-vivid participants compared to low-vivid participants. In other words, the level of vividness of tactile imagery can modulate the activation of primary somatosensory cortex. This study also revealed that the inferior

extended to the primary auditory cortex.

up to 30 seconds after stimulus offset (Burton & Sinclair, 2000).

occipital cortex (BA 18) was strongly activated in high-vivid participants.

Motor (or kinaesthetic) imagery is the result of first-person kinaesthetic information processing, as people feel themselves executing a given action (Jeannerod, 1995). This experience is called internal imagery or first-person perspective, which is different from the

**4. Tactile imagery** 

**5. Motor imagery** 

representation of the external imagery or third person perspective, in which information processing involves the visualization of spatial components of the perceived world (Ruby & Decety, 2001).

Various studies using fMRI have yielded contrasting results regarding the involvement of primary motor cortex during motor imagery. On one hand, the first studies revealed bilateral supplementary motor area and pre-motor activations, without an increase in signal intensity in the primary motor cortex (pre-central gyrus – BA 4) or somatosensory cortex during self-paced complex finger movements (Rao et al., 1993) or sequential finger opposition movements (Tyszka et al., 1994). However, using stance and locomotion (walking and running) imagery condition (Jahn et al., 2004), complex imagery actions (e.g., running) (Olivetti Belardinelli et al., 2004a), mental training-related changes on a fingertapping task (Nyberg et al., 2006), and fingers or objects movements imagery task (Lorey et al., 2010), there was no reported activity in the primary motor cortex. Even when participants were instructed to imagine using a common tool, such as the brush for an action related to the hair, activity was observed in the pre-motor cortex, posterior part of the parietal cortex, and cerebellum, but not in the primary motor cortex (Higuchi et al., 2007). On the other hand, there was activity in the primary motor cortex more reduced for motor imagery than for actual performance (Fieldman et al., 1993). In particular, the primary motor cortex was activated during the mental execution of movements with either the left or right hand (Dechent et al., 2004; Lotze et al., 1999; Porro et al., 1996; Roth et al., 1996), when participants were instructed to imagine a right-hand self-paced button press sequence before (novel condition) and after (skilled condition) one week of intensive physical practice (Lacourse et al., 2005), when participants imagined dancing Tango after five training days (Sacco et al., 2006), when participants were instructed to imagine complex everyday movements (e.g., eating a meal, swimming) (Szameitat et al., 2007), or situations involving actions cued by appropriate motor phrases (e.g., to cut) (Tomasino et al., 2007). Sharma, Jones, Carpenter, & Baron (2008) likewise revealed that both the anterior and posterior primary motor area can be bilaterally activated when imaging a finger opposition sequence (2, 3, 4, 5; paced at 1 hz). According to these authors, the role of the primary motor area and its subdivisions may be non-executive, perhaps related to spatial encoding. Recently, Olivetti Belardinelli et al. (2009) demonstrated that primary motor cortex can be activated in high-vivid participants rather than in low-vivid participants, but did not confirm the same activation when motor imagery vividness was regressed onto the BOLD signal, showing activation for only the left pre-central gyrus (BA 6), the right medial frontal gyrus (BA 6), and the inferior parietal lobule. Nevertheless, even considering the studies showing the activation of the primary motor area during motor imagery, it should be noted that the majority of these experiments did not employ electrophysiological monitoring to exclude muscle contractions during scanning. To exclude this possible confounding factor, Takashi et al. (2003) employed electromyographic monitoring within the scanner while participants performed sequential finger-tapping movements in response to visually presented number stimuli in either a movement or an imagery mode of performance. Results revealed that the movement condition activated the primary sensory and motor areas, parietal operculum, anterior cerebellum, caudal pre-motor areas, and area 5 that had mild-to-moderate imageryrelated activity, whereas the motor imagery condition yielded the activation of the precentral sulcus at the level of middle frontal gyrus (BA 6/44), and the posterior superior parietal cortex/pre-cuneus (BA 7), and bilateral cerebellum. Moreover, activity of the

How fMRI Technology Contributes to the Advancement of Research in Mental Imagery: A Review 337

In the past few years, the role of the piriform cortex during olfactory imagery was reconsidered according to two points of view: retrieval-related processes and expertise. First, it was surmised that piriform cortex activation reflects retrieval-related olfactory "imagery" processes (Elmes, 1998; Bensafi et al., 2003). Because odour imagery was shown to elicit sniffing (Bensafi et al., 2003), which in turn can elicit activation in piriform cortex (Sobel et al., 1998), it is possible that the piriform activity indicates odour imagery associated with sniffing. In an fMRI study, Bensafi et al. (2007) evoked hedonic-specific activity in piriform cortex by asking participants to sniff during the perception and imagination of a pleasant odour (strawberry) and an unpleasant odour (rotten eggs). In particular, activity induced by imagining odours mimicked that induced by perceiving real odours, and for both real and imagined odours, unpleasant stimuli induced greater activity than pleasant stimuli in the left frontal portion of piriform cortex and left insula. Regarding the expertise issue, it was also surmised that experience with odours plays a key role for the reorganization of brain regions involved in olfactory imagery. In fact, Plailly et al. (2011) revealed that expertise plays a key role: olfactory imagery activated the primary olfactory (piriform) cortex, as well as the orbitofrontal cortex, and the hippocampus during the

Gustatory imagery refers to the ability to generate mental images of tastes. Although gustatory imagery is involved in food craving, or the "irresistible urge to consume" (Tiggemenn & Kemps, 2005), which in turn may have implications for clinical and nonclinical population, relatively little experimental research has been devoted to this imagery system. It is also uncertain whether proper gustatory mental imagery can be evoked, and to what extent images of tastes activate the primary gustatory cortex. In an fMRI study, Kobayashi et al. (2004) instructed their participants to perceive (water stimuli) and imagine several tastes (grapefruit, candy, pudding, coffee, lemon, banana, beer, sugar). Images were verbally cued by written words, by spoken language, and by using pictures. In general, results revealed that gustatory imagery can activate the primary gustatory cortex (anterior insula/frontal operuculm), especially the left side, sharing common parts of neural substrates with gustatory perception. Authors also clarified that the middle and superior frontal gyri were not activated by gustatory perception, but they participated in the generation of gustatory images, plausibly mediating the top-down control of retrieving gustatory information from the storage of long-term memories. The activation of the anterior insula and the middle frontal gyri during gustatory imagery (the spicy taste, the tart taste, etc) were confirmed also by Olivetti Belardinelli et al. (2004a) relative to an abstract condition, and by Olivetti Belardinelli et al. (2009) according to the level of vividness of participants. Finally, Kikuki et al. (2005) revealed that when participants concentrated on pickled plums (umeboshi), a traditional Japanese food with a strong and sour taste, activations were observed weakly in the right insula, but more strongly in the bilateral

creation of mental images of odours by professional perfumers.

opercula, the bilateral orbitofrontal cortices, and the left Broca's area.

Proprioceptive processing is part of the haptic system. In particular, proprioceptive imagery involves the ability to generate organic images based on body sensations, such as the

**7. Gustatory imagery** 

**8. Proprioceptive imagery** 

superior pre-central sulcus and intra-parietal sulcus areas, predominantly on the left, was associated with the accuracy of the imagery task performance.

However, the fMRI approach based on "effective connectivity" between network components, defined as the influence of one neural system over another, clarified the issue of the primary motor cortex activation during motor imagery. In particular, Solodikin et al. (2004) demonstrated the connectivity between the supplementary motor area and the primary motor cortex during motor imagery. By using structural equation modelling to estimate the effective connectivity networks underlying motor execution, visual mental imagery, and kinesthetic mental imagery with specified regions of interests, Solodikin et al., (2004) showed that the inputs from the supplementary motor area and lateral–dorsal premotor cortex to the primary motor cortex had a suppressing effect during motor imagery. These results suggest a physiological mechanism encompassing the prevention of overt movements. Using Dynamic Causal Modeling, Kasess et al. (2008) confirmed that the activity of the primary motor cortex was heavily suppressed by the supplementary motor area during motor imagery, namely imagine pressing buttons on a small panel, first with the index finger, then the middle finger, and again with the index finger, as rapidly as possible. Then, by using the Granger Causality Mapping method, Chen et al. (2009) found forward and backward connectivity between the supplementary motor area and the contra-lateral primary motor cortex during both the left- and right-hand motor imagery (finger tapping sequences cued by pictures). Gao et al. (2011) extended these results revealing the influence of the brain asymmetry of right-handedness on effective connectivity networks: left dorsal pre-motor cortex, inferior parietal lobule, and superior parietal lobule were identified as causal sources in both motor imagery and motor execution.

### **6. Olfactory imagery**

Olfactory mental images can be defined as short-term memory representations of olfactory events that give rise to the experience of smelling with the "mind's nose" (Rinck et al., 2009). However, experimental evidence about the existence of olfactory imagery is controversial given that it is not clear whether an olfactory mental image is semantically or perceptually mediated, or whether it reflects the influence of explicit knowledge of olfactory principles rather than a specific mode of operation of odour imagery (Elmes, 1998; Herz, 2000). Using fMRI, Levy et al. (1999), and Henkin & Levy (2002) found a substantial overlap in the areas activated by real and imagined stimuli (ripe banana and peppermint), although all activations were reduced in the imagery condition. However, given the lack of anatomical details, it is not possible to draw reliable conclusions from these two studies. Later, Olivetti Belardinelli et al. (2004a) found activation in the left insula, but not in the primary olfactory area, when the olfactory imagery condition was contrasted with the abstract condition. The lack of activity in the primary olfactory cortex was also observed when high-vivid participants were contrasted with low-vivid participants (Olivetti Belardinelli et al., 2009). In Olivetti et. al.'s studies, the olfactory-specific modality activations likely were not found because of the difficulty in generating vivid images of smells, especially when they are verbally cued (Herz, 2000). This is confirmed by Zelano et al. (2009), who found that remembering nameable odorants was reflected in sustained activity in prefrontal language areas, and remembering unnameable odorants was reflected in sustained activity in primary olfactory cortex. In other words, only smells dissociated from their verbal label were eligible to activate the primary olfactory cortex.

In the past few years, the role of the piriform cortex during olfactory imagery was reconsidered according to two points of view: retrieval-related processes and expertise. First, it was surmised that piriform cortex activation reflects retrieval-related olfactory "imagery" processes (Elmes, 1998; Bensafi et al., 2003). Because odour imagery was shown to elicit sniffing (Bensafi et al., 2003), which in turn can elicit activation in piriform cortex (Sobel et al., 1998), it is possible that the piriform activity indicates odour imagery associated with sniffing. In an fMRI study, Bensafi et al. (2007) evoked hedonic-specific activity in piriform cortex by asking participants to sniff during the perception and imagination of a pleasant odour (strawberry) and an unpleasant odour (rotten eggs). In particular, activity induced by imagining odours mimicked that induced by perceiving real odours, and for both real and imagined odours, unpleasant stimuli induced greater activity than pleasant stimuli in the left frontal portion of piriform cortex and left insula. Regarding the expertise issue, it was also surmised that experience with odours plays a key role for the reorganization of brain regions involved in olfactory imagery. In fact, Plailly et al. (2011) revealed that expertise plays a key role: olfactory imagery activated the primary olfactory (piriform) cortex, as well as the orbitofrontal cortex, and the hippocampus during the creation of mental images of odours by professional perfumers.
