**4. Conclusions**

326 Neuroimaging – Cognitive and Clinical Neuroscience

cylinder with pearls stuck on it). The results revealed crossmodal recognition of texture in

The findings suggest that for the property of texture, exchanges between the sensory modalities are bi-directional. Complete cross-modal transfer occurs with texture but not shape. However, this is true if only the object is volumetric and not flat, because newborns do not use the *"lateral motion"* exploratory procedure to detect differences between the textures of flat objects (Sann and Streri, 2008b). Cross-modal transfer between hands also reveals differences between shape and texture properties, and suggests that establishing representations of object shape is difficult for newborns. How should these results be explained? Human infants are particularly immature at birth, and brain maturation is protracted until adulthood. Almost no neuroimaging data is available because non-invasive techniques are difficult to apply in healthy infants. For example, newborns and young infants are often asleep (however, see Fransson et al., 2010 for a review on the functional architecture of the infant brain). Adult neuroimaging data, in contrast, offer some insights

On the basis of these findings, two main questions emerge: First, why is bi-directional intermodal transfer observed for texture and not for shape? Second, how is haptic input translated into a visual format in newborns, i.e. by an organism that has never both seen and

On the basis of animal and human studies, Hsiao (2008) claimed that 3D shape processing involves the integration of both proprioceptive and cutaneous inputs from the hand. As the hand explores objects, different combinations of neurons are activated, and object recognition occurs as these 3D spatial views of the object are integrated. Cutaneous inputs related to 2D stimulus form and texture properties do not need such integration and may be processed differently than 3D shape in cortex. Cutaneous inputs stemming from the form and texture of 2D stimuli are processed in area 3b of SI cortex, whereas the sensitivity of neurons in area 2 to cutaneous inputs depends on hand conformation and its changes. Moreover, according to Hsiao (1998), the mechanisms underlying the early stages of 2D form processing are similar for vision and touch. Newborns' exploration of objects is very weak, and they may not be able to establish the 3D representations needed to perform tactile recognition after visual exploration of the object. Since texture and 2D form are similar in vision and touch, this data could explain why in 2-month-olds intermodal transfer from visual 2D object to haptic 3D objects is found, but not transfer from visual 3D objects to haptic 3D objects. Similarly, this data could explain the bi-directional transfer of texture

Moreover, neuroimaging data from human adults suggests a functional separation in the cortical processing of micro- and macrogeometric cues (Roland et al. 1988). In this study, adults had to discriminate the length, shape, and roughness of objects with their right hand. Discrimination of object roughness activated lateral parietal opercular cortex significantly more than length or shape discrimination. Shape and length discrimination activated the anterior part of the intraparietal sulcus (IPA) more than roughness discrimination. More recently, Merabet *et al.* (2004) confirmed the existence of this functional separation and suggested that occipital (visual) cortex is functionally involved in tactile tasks requiring fine spatial judgments in normally sighted individuals. More specifically, a transient disruption

both directions.

on how the brain processes cross-modal tasks.

between touch and vision observed in newborns.

**3.4 Neuroimaging data** 

felt a 3D object?

We recognize, understand, and interact with objects through both vision and touch (cf. Hatwell, Streri and Gentaz, 2003; Gentaz, 2009). In infancy, despite the various discrepancies between the haptic and visual modalities—such as asynchrony in the maturation and development of the different senses, distal vs. proximal inputs, and the contrast between the parallel character of vision and the sequential nature of the haptic modality—both systems detect regularities and irregularities when they are in contact with different objects, from birth onward. Conceivably, these two sensory systems may encode object properties such as shape and texture in similar ways. Behavioral evidence in newborns has revealed the involvement of different levels of abstraction in different types of transfer. Intermanual transfer of shape and texture seems to be bi-directional from birth. When newborns hold an object in one hand, left or right, its shape and texture are recognized by the other hand despite the immaturity of the corpus callosum. The maturity of the haptic sense is sufficient for gathering and processing information in a way that makes symmetrical correspondences between hands possible. This intermanual transfer may involve a low level of abstraction, because it does not require a change of representational format, since the steps involved, habituation and recognition, occur entirely within one modality—despite the fact that the transmission runs through the corpus callosum, which is immature at birth. Cross-modal transfers between vision and touch require a change of format and seem to be more difficult for newborns because of the higher level of abstraction involved.

Studies on crossmodal transfer tasks have revealed some links between the haptic and visual modalities at birth. Newborns are able to visually recognize a held object (Streri and

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Gentaz 2003). This neonatal ability is independent of learning or the influence of the environment. However, by means of bi-directional crossmodal transfer tasks, Streri and colleagues have provided evidence on the perceptual mechanisms present at birth that constrain or limit the exchange of information between the sensory modalities. Newborns visually recognize the shape of a felt object, but are unable to recognize the shape of a seen object with their hands (Sann and Streri 2007). The link is obtained from the simplest information gathered, i.e. tactile information. Moreover, it is observed only with the newborn's right hand and not with the left (Streri and Gentaz 2004). A third striking result is that crossmodal transfer depends on object properties, being bidirectional with texture but not with shape (Sann and Streri 2007)—although this finding holds if, and only if, the felt textured object is volumetric, and not flat (Sann and Streri 2008b). For shape, just as for texture, the newborn's exploratory procedures are limited to the grasping reflex, which makes effective exploration of object properties impossible. All of these findings suggest that at birth, the links between the senses are specific to individual modalities and are not yet or entirely a general property of the brain.

Asymmetries in cross-modal transfer tasks continue to be found throughout the course of development. Several studies have also revealed that the links between the haptic and visual modalities are fragile, often not bi-directional, and representation of objects is never complete: this holds not only in infancy (Rose and Orlian 1991; Streri 2007; Streri and Pêcheux 1986b), but in children (Gori *et al.* 2008) and adults (Kawashima *et al.* 2002). Crossmodal transfer of information is rarely reversible, and is generally asymmetrical even when it is bi-directional (see Hatwell, Gentaz and Streri, 2003 for a review). The links between sensory modalities for object shape over the course of development appear to be flexible rather than immutable.

Why does cross-modal integration of spatial information develop in an asymmetrical manner? Several explanations may be offered. Sensory systems are not mature at birth, but become increasingly refined as children develop. Sometimes seen objects are observed to be well-recognized by touch, and more often, felt objects are well-recognized by vision. One possibility is that the sensory systems involved in spatial perception need to be continuously recalibrated during development, to take into account physical growth, such as changes in digit length (which affect haptic judgments), interocular separation, and eyeball length (affecting visual judgments). However, from birth, the links between the senses are more often effective when they begin with the hands rather than the eyes. Animal and adult neuroimaging studies also highlight asymmetries in cross-modal transfer tasks. Another suggestion would be that the links from eyes to hands are more effective for reaching and grasping objects than for cross-modal recognition. When we see an object, usually we take in information for some other purpose: e.g., transporting it to the mouth or somewhere else. In infancy, the hands are used as instruments to transport objects to the eyes or mouth, and the acquisition of this new ability develops to the detriment of the hands' perceptual function. Sensorimotor coordination triggered by the sight of an object is present from birth even though this ability mainly starts to be effective at about 4/5 months, at the beginning of prehension-vision. This ability may be better understood as the counterpart of cross-modal transfer from touch to vision. In both cases, perception and action are strongly linked. It is therefore important to note that sensory integration problems have often been observed in developmental disorders such as autism, dyslexia, and attention deficit disorder: understanding how incoming sensory information is transformed into outgoing motor commands is crucial for the diagnosis of such disorders (see Stein et al., 2009).
