The central projection of electrosensory information in the platypus

1. This is the first detailed description of the projection to the cerebral cortex of afferent information coming from electroreceptors in the bill of the platypus, Ornithorhynchus anatinus. 2. In animals anaesthetized with chloralose, with the bill immersed in tap water, applying a potential difference between plate electrodes on either side of the bill produced large amplitude potentials from the surface of a postero-lateral region of cerebral cortex. Response threshold was 300 microV cm-1, somewhat lower than threshold measured for single identified electroreceptors. Electroreceptor threshold was at least three orders of magnitude lower than threshold of mechanoreceptors to electrical stimuli (Gregory, Iggo, McIntyre & Proske, 1989a). 3. Monopolar stimulation of the bill revealed a crossed projection. The map on the cortical surface had the bill oriented dorso-laterally, its base towards the mid-line, the tip on the lateral edge, pointing slightly forwards. The edge of the bill faced backwards. Electrosensory information coming from the edge of the bill appeared to be much more strongly represented than input from the dorsal surface. 4. Weak electrical and mechanical stimuli applied to the bill both evoked large amplitude potentials from the same region of cortex indicating that there was complete overlap between the regions receiving tactile and electrosensory inputs. 5. Inserting microelectrodes into the deeper layers of cortex revealed burst discharges in single cells and groups of cells in response to weak electrical stimulation of the bill. Activity could be recorded over a range of depths from 0.3 to 4 mm, with the majority of responses coming from cells 1-3 mm deep. Histological examination of lesion sites made at 1.1 mm and at 3 mm suggested that cells in the pyramidal and ganglion layers were involved in generating the activity. 6. Some evidence was obtained for interactions at the level of the cerebral cortex between activity generated by tactile and electrosensory inputs. When electrical and mechanical stimuli were both applied to the bill with an interstimulus interval of less than 25 ms, cortical neuronal responses generated by one stimulus were completely suppressed by the other. However no evidence was obtained of a direct convergence at the level of the cortex between the two modalities. 7. Cortical activity could be evoked in response to rapidly changing voltage fields. This observation, together with our earlier finding of a high rate sensitivity of the receptors, emphasizes the high dynamic sensitivity of the system. 8. It is concluded that the electrosensory system of the platypus is closely associated with the sense of touch.(ABSTRACT TRUNCATED AT 400 WORDS)