Abstract
Functional magnetic resonance imaging (fMRI) was used to estimate neuronal activity in the primary somatosensory cortex of six participants undergoing cutaneous tactile stimulation on skin areas spread across the entire body. Differences between the accepted somatotopic maps derived from Penfield's work and those generated by this fMRI study were sought, including representational transpositions or replications across the cortex. MR-safe pneumatic devices mimicking the action of a Wartenberg wheel supplied touch stimuli in eight areas. Seven were on the left side of the body: foot, lower, and upper leg, trunk beneath ribcage, anterior forearm, middle fingertip, and neck above the collarbone. The eighth area was the glabella. Activation magnitude was estimated as the maximum cross-correlation coefficient at a certain phase shift between ideal time series and measured blood oxygen level dependent (BOLD) time courses on the cortical surface. Maximally correlated clusters associated with each cutaneous area were calculated, and cortical magnification factors were estimated. Activity correlated to lower limb stimulation was observed in the paracentral lobule and superomedial postcentral region. Correlations to upper extremity stimulation were observed in the postcentral area adjacent to the motor hand knob. Activity correlated to trunk, face and neck stimulation was localized in the superomedial one-third of the postcentral region, which differed from Penfield's cortical homunculus.
Highlights
Large portions of the mammalian cerebral cortex are devoted to processing sensory inputs, offering investigators a solid starting point for understanding functional organization in the brain (Krubitzer and Seelke, 2012)
Much of what is known about functional localization in the somatosensory cortex has been learned through lesion studies, electrical recording of neuronal activity through implanted electrodes (Dreyer et al, 1975; Beck et al, 1996; Manger et al, 1997; Lipton et al, 2010), and direct electrical stimulation of exposed brain tissue of conscious subjects (Penfield and Boldrey, 1937; Woolsey et al, 1979; Roux et al, 2018)
In the time since Wilder Penfield and his colleagues published the results of their intraoperative experiments in the 1930s, the cartoon of the cortical homunculus has become pervasive in the field of neuroscience, despite the seminal neuroscientists’ early warnings about reproducibility (Penfield and Boldrey, 1937; Snyder and Whitaker, 2013)
Summary
Large portions of the mammalian cerebral cortex are devoted to processing sensory inputs, offering investigators a solid starting point for understanding functional organization in the brain (Krubitzer and Seelke, 2012). A large body of work has been published that pertains to the evolution, structure, and function of the somatosensory nervous systems of mammals (Dreyer et al, 1975; Beck et al, 1996; Manger et al, 1997; Krubitzer and Seelke, 2012; Van Essen et al, 2019), including humans (Penfield and Boldrey, 1937; BrownSequard, 1968; Woolsey et al, 1979; Baumgartner et al, 1991; Engel et al, 1997; Arichi et al, 2010; Dall’Orso et al, 2018). The homunculus was revealed from experiments in which the input-output pathways were dramatically different from normal physiology, bypassing the entire peripheral nervous system The implications of these differences are not completely clear and cannot be explored until a method of routine, non-invasive measurement of brain function resulting from peripheral tactile stimulation is developed
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