THE SENSORY WORLD OF THE PLATYPUS

Vision, audition and somatic sensation in the platypus are reviewed. R ecent work on the eye and retinal ganglion cell layer of the platypus is presented that provides an estimate of visual acuity and suggests t hat platypus ancestors may have used vision, as well as the bill organ , for underwater predation. The combined electroreceptor and mechanore ceptor array in the bill is considered in detail, with special referen ce to the elaborate cortical structure, where inputs from these two se nsory arrays are integrated in a manner that is astonishingly similar to the stripe-like ocular dominance array in primate visual cortex, th at integrates input from the two eyes. A new hypothesis, along with su pporting data, is presented for this combined mechanoreceptive-electro receptive complex in platypus cortex. Bill mechanoreceptors are shown to be capable of detecting mechanical waves travelling through the wat er from moving prey. These mechanical waves arrive after the electrica l activity from the same prey, as a function of distance. Bimodal cort ical neurones, sensitive to combined mechanical and electrical stimula tion, with a delay, can thus signal directly the absolute distance of the prey Combined with the directional information provided by signal processing of the thousands of receptors on the bill surface, the stri pe-like cortical array enables the platypus to use two different senso ry systems in its bill to achieve a complete, three-dimensional 'fix' on its underwater prey.