Olfaction in rats with olfactory bulb lesions: A failure to find support for spatial localization of olfactory coding

To determine the extent to which reduction of topographical organization of the olfactory system affects the ability to detect and discriminate different odors, rats with unilateral bulbectomy plus removal of approximately 24%-100% of the remaining glomeruli were tested using olfactometric methods on amyl acetate, butanol, citral, and propionic acid vapor detection tasks, to discriminate these odors from cineole, and to discriminate mixtures of odors. The lesions were measured by the linear extent of the remaining glomeruli and expressed as a percentage of control values (glomerular savings score). In 31 experimental rats glomerular savings scores ranged from 0% to 76%. Rats were grouped with regard to regional bulbar savings (major savings in the medial, lateral, or ventral areas of the bulb) and the amount of savings. Those with glomerular savings of less than 10% of the controls were anosmic (failed all detection tasks). Most rats with higher glomerular saving scores were able to perform all or most tasks at accuracy levels similar to those of unilateral bulbectomized controls although rats with large lesions (glomerular savings scores of 11-20%) made, on average, more errors than controls. Nevertheless, several rats with glomerular savings scores of 14%-20% performed as well as controls on most or all problems. Correlations between glomerular savings scores and various measures of odor detection and discrimination performance were relatively low ($-$0.23 to $-$0.43). There were few differences among groups with regard to regional glomerular savings scores and there was no clear evidence for a specific anosmia within any group or for any individual rat. Contrary to the prediction from topographical theories of odor coding that restricted areas of the bulb may be essential for mediating specific odors, the present results indicate that small regions of the olfactory bulb are capable of mediating a wide variety of odor detection and discrimination tasks. These outcomes are in accord with theories of odor coding which do not rely on a topographical correspondence between the olfactory neuroepithelium and the olfactory bulb.