Sexually Monomorphic Maps and Dimorphic Responses in Rat Genital Cortex
Mammalian external genitals show sexual dimorphism [1, 2] and can change size and shape upon sexual arousal. Genitals feature prominently in the oldest pieces of figural art [3] and phallic depictions of penises informed psychoanalytic thought about sexuality [4, 5] Despite this longstanding interest, the neural representations of genitals are still poorly understood [6] In somatosensory cortex specifically, many studies did not detect any cortical representation of genitals [7 9] Studies in humans debate whether genitals are represented displaced below the foot of the cortical body map [10 12] or whether they are represented somatotopically [13 15] We wondered what a high resolution mapping of genital representations might tell us about the sexual differentiation of the mammalian brain. We identified genital responses in rat somatosensory cortex in a region previously assigned as arm/leg cortex. Genital responses were more common in males than in females. Despite such response dimorphism, we observed a stunning anatomical monomorphism of cortical penis and clitoris input maps revealed by cytochrome oxidase staining of cortical layer 4. Genital representations were somatotopic and bilaterally symmetric, and their relative size increased markedly during puberty. Size, shape, and erect posture give the cortical penis representation a phallic appearance pointing to a role in sexually aroused states. Cortical genital neurons showed unusual multi body part responses and sexually dimorphic receptive fields. Specifically, genital neurons were co activated by distant body regions, which are touched during mounting in the respective sex. Genital maps indicate a deep homology of penis and clitoris representations in line with a fundamentally bi sexual layout [16] of the vertebrate brain.
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