1. Mathematical Research Branch, NIDDK, National Institutes of Health, Bethesda, Maryland 20892, USA
2. Department of Zoology, University of Maryland, College Park, Maryland 20742, USA
3. Present address: New York University, Center for Neural Science and Courant Institute of Mathematical Sciences, New York 10003, USA.

Correspondence to: Catherine E. Carr² Correspondence and requests for materials should be addressed to C.E.C. (e-mail: Email: carr@zool.umd.edu).

Coincidence-detector neurons in the auditory brainstem of mammals and birds use interaural time differences to localize sounds¹,². Each neuron receives many narrow-band inputs from both ears and compares the time of arrival of the inputs with an accuracy of 10–100 s (refs 3–6). Neurons that receive low-frequency auditory inputs (up to about 2 kHz) have bipolar dendrites, and each dendrite receives inputs from only one ear⁷,⁸. Using a simple model that mimics the essence of the known electrophysiology and geometry of these cells, we show here that dendrites improve the coincidence-detection properties of the cells. The biophysical mechanism for this improvement is based on the nonlinear summation of excitatory inputs in each of the dendrites and the use of each dendrite as a current sink for inputs to the other dendrite. This is a rare case in which the contribution of dendrites to the known computation of a neuron may be understood. Our results show that, in these neurons, the cell morphology and the spatial distribution of the inputs enrich the computational power of these neurons beyond that expected from 'point neurons' (model neurons lacking dendrites).