Academic journal article Attention, Perception and Psychophysics

Somatotopic or Spatiotopic? Frame of Reference for Localizing Thermal Sensations under Thermo-Tactile Interactions

Academic journal article Attention, Perception and Psychophysics

Somatotopic or Spatiotopic? Frame of Reference for Localizing Thermal Sensations under Thermo-Tactile Interactions

Article excerpt

The thermal sense is diffuse and incapable of providing precise spatial information. From a phenomenon known as thermal referral, we know that touch influences the localization of cold or warmth, leading to our perceiving illusory thermal sensations at a thermally neutral site. This study investigated the frame of reference for localizing thermal sensations under thermal referral in order to shed light on how thermal and tactile modalities coordinate to process localization information. One thermally neutral tactile stimulator and two cold (warm) stimulators were presented to different sets of three fingers of both hands. The location of the neutral tactile stimulator varied, and the strength of the referral was estimated from participants' performance in localizing the neutral tactile stimulator. By manipulating the somatotopic and spatiotopic distances between the stimulated sites, we found that the somatotopic distance-more specifically, the distance between and among the sites being defined in cortical topography-determines the strength of thermal referral. Our findings suggest that localization of thermal sensations under thermo-tactile interactions is processed with respect to the somatotopic frame of reference and that this cross-modal processing resides in early cortical areas whose organization conserves topographic information.

The thermal sense has been shown to be diffuse and incapable of providing precise spatial information. Thermal sensations are mediated by the small-fiber spinothalamic system, and the neurons on which the spinothalamic fibers terminate have huge receptive fields (Bowsher, 2005; Craig, Chen, Bandy, & Reiman, 2000; Han, Zhang, & Craig, 1998; Mountcastle, 1961; Mountcastle & Powell, 1959; Rose & Mountcastle, 1959). In addition, the thermal sense summates spatially separate inputs, such that the perceived thermal intensity is a function of the total area of the stimulation, and our capacity to distinguish spatially separate inputs is limited (Cain, 1973; Marks & Stevens, 1973). As a consequence, when a noncontact thermal stimulus, such as radiant heat, is applied to the skin, the spatial features of the thermal stimulus, such as its location, area, and shape, are poorly resolved (Cain, 1973; Marks & Stevens, 1973; Stone, 1937; Taus, Stevens, & Marks, 1975).

It is commonly believed that concurrent tactile inputs facilitate thermal localization. For example, when the hand makes contact with an object, the change in skin temperature and the deformation of the skin activate thermoreceptors and mechanoreceptors located in the skin, and the firing of these receptors transmits the thermal and tactile1 inputs to the central nervous system (Kenshalo, 1976). The cross-modal processing of these thermal and tactile inputs facilitates the localization of thermal sensations and is indicated as well by the thermal referral phenomenon, in which the concurrent tactile inputs modify the thermal localization. When observers touched three stimulators simultaneously with the middle three fingers (D2, D3, and D4) but only the outer two stimulators were cooled or heated, the central (neutral) stimulator was also perceived to be cold or warm (Green, 1977). Interestingly, this illusory thermal sensation disappeared when D3 was withdrawn from the central (neutral) stimulator, clearly indicating that this mislocalization of thermal sensation was induced by the concurrent tactile input received from D3.

In this study, we investigated the cross-modal processing of the thermal and tactile inputs under thermal referral; more specifically, we sought to determine whether the localization under thermo-tactile interaction is processed somatotopically or spatiotopically. Tactile inputs have been shown to be coded somatotopically at early stages of processing and, subsequently, to be processed in spatiotopic space (Azañón & Soto-Faraco, 2008). The influences from the somatotopic distance have been reported in simple temporal tasks, such as tactile simultaneity judgment (Clark & Geffen, 1990; Kuroki, Watanabe, Kawakami, Tachi, & Nishida, 2010) and tactile apparent motion detection (Kuroki et al. …

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