The organization of the sensory and motor areas of cerebral cortex in the platypus (Ornithorhynchus anatinus)

TLDR: The organization of sensory and motor regions of the cerebral cortex has been studied in the platypus and a large single area of somatosensory representation in the posterior region of the hemisphere was found.

Abstract: The organization of sensory and motor regions of the cerebral cortex has been studied in the platypus (Ornithorhynchus anatinus), of the order Monotremata. Comparisons were made with the organization found in the other representative of this order, the echidna, and with primitive species of eutherian and metatherian mammals. Evoked potential and single neuron studies revealed a large single area of somatosensory representation in the posterior region of the hemisphere, extending from an approximately mid-sagittal position around to the region of the rhinal sulcus on the ventrolateral surface of the hemisphere. The mediolateral representation of contralateral body parts was consistent with the pattern in the primary somatosensory area of other mammalian species. No evidence of a second somatosensory area was found. Neurons with similar receptive fields were grouped in columns normal to the cortical surface and a highly ordered pattern of somatotopic representation was found. Within the large area of bill representation individual neurons had receptive fields which were often punctate and no more than 1 mm in diameter. They responded to dynamic components of tactile stimuli delivered to their receptive fields on the bill. Movements on the contralateral side of the body could be elicited by bipolar electrical stimulation over an area on the dorsal surface of the hemisphere which largely overlapped the somatosensory area, but extended further anteriorly towards the frontal pole of the hemisphere. Visual and auditory projection regions were found overlapping the somatosensory area in the posterior part of the hemisphere. The auditory area overlapped the visual area and appeared to be displaced posteromedially in relation to its position in other species, a displacement which may be a consequence of the large expanse of cortical area associated with the bill. The observation that a large proportion of cortical area is devoted to specific sensory and motor function in platypus corresponds with earlier findings in primitive eutherian and metatherian species. The platypus neocortex appears to represent a more primitive stage of cortical development than that found in the other member of the order Monotremata, the echidna.