Neural Basis of Haptic Object Processing, The

Canadian Journal of Experimental Psychology,  Sep 2007  by James, Thomas W,  Kim, Sunah,  Fisher, Jerry S

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Haptic processing of texture information has rarely been studied in isolation, but instead has been studied in the context of dissociating it from the processing of shape information. Findings from cortical ablation and single-unit recording studies in nonhuman primates show a remarkable amount of agreement on the functional division between the processing of macrogeometry (shape) and microgeometry (texture) in somatosensory brain areas. Neurons in Area 1 (of S1) are sensitive to object texture and ablation produces deficits in texture discrimination, but not angle discrimination (Randolph & Semmes, 1974). Ablation of Area 2 produces deficits in angle discrimination, but not texture discrimination (Randolph & Semmes, 1974). Similar to Area 1, neurons in SII are sensitive to object texture (Hsiao et al., 1993; Jiang, Tremblay, & Chapman, 1997; Pruett, Sinclair, & Burton, 2000, 2001; Randolph & Semmes, 1974), but ablation produces deficits in both shape and texture discrimination (Murray & Mishkin, 1984). These findings are largely consistent with a division of the somatosensory system into texture and shape processing pathways and are also consistent with a hierarchical system for texture processing. Texture information passes from Areas 3a and 3b to Area 1 and on to Area SII (Hsiao et al., 1993; Jiang et al., 1997).

The interpretation of findings from patients with tactile agnosia and from neuroimaging studies is less clear. As described above, one conclusive finding from studies of patients with tactile agnosia is that lesions to SII produce deficits in haptic object recognition that cannot be explained based on superficial sensory loss (Bohlhalter et al., 2002; Caselli, 1991; Reed & Caselli, 1994; Roland, 1987). But, whether or not the agnosia is due to deficits with shape processing, texture processing, the combination of shape and texture, or some other process involved in haptic object recognition, is less clear.

Several neuroimaging studies have investigated the role of SII in haptic object recognition, but the results have not been conclusive. Initial studies of tactile texture perception compared texture stimulation of the skin with no stimulation and found that SII was activated by texture stimulation (e.g., Burton, MacLeod, Videen, & Raichle, 1997). A more recent group of studies directly compared tasks that required texture discrimination with tasks that required shape discrimination. The first of these studies (Roland et al., 1998) found that SII produced more activation when the relevant characteristic was texture as opposed to shape. But, a second study (Servos et al., 2001) that used similar methods and stimuli found different results. Rather than separate pathways for shape and texture, that study found extensive overlap between the brain regions recruited for shape and texture processing, which were located along the postcentral gyrus. The same study found activation in SII for discriminations of hardness, but not for texture or shape. A third study (Bodegard et al., 2001) found that SII produced more activation with passive shape than texture discrimination. Even more recent is a study that parametrically varied the roughness of the texture stimulus used for discrimination (Kitada et al., 2005). That study found that activation in SII and the insula was correlated with the parametric variations in roughness. In sum, although three of the five studies reported here suggest that SII is involved in texture discrimination, the exact role of SII in texture processing remains controversial.