Gardiner, Jayne M.
Talbot, Zoe N.
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
 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 . In somatosensory cortex specifically, many studies did not detect any
cortical representation of genitals [7-9]. Studies in humans debate, if genitals are
represented displaced below the foot of the cortical body map [10-12], or if 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-oxidasestaining
of cortical layer-4. Genital representations were somatotopic, 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 coactivated
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  of the vertebrate brain.