Olfaction in rats with extensive lesions of the olfactory bulbs: implications for odor coding

Rats were initially trained on a series of odor detection tasks and then received a unilateral olfactory bulbectomy and removal of different parts of the contralateral bulb. After postoperative recovery they were tested for detection of different concentrations of four odors, on a series of odor discrimination tasks and for their ability to acquire a relatively easy and a more difficult odor mixture discrimination task. Groups were formed based on which region of the bulb was intact (regional savings score) and on amount of bulb intact (bulbar savings score). In general, only rats with bulbar savings scores of less than 21% had deficits in detection or discrimination tasks but most performed as well as controls in most tasks. Correlations between bulbar savings scores and error scores were relatively low across all rats but, within the subgroup with the largest lesions (bulbar savings scores <21%), high correlations between these variables were obtained. There was no evidence for a specific anosmia in any group or individual rat and, except for the more difficult odor mixture discrimination, no one task proved difficult for any subgroup. The present results demonstrate that rats with relatively small remnants of one olfactory bulb can perform a variety of odor detection and discrimination tasks as well or nearly as well as controls. These outcomes provide no support for localization of function within the olfactory bulb but are in accord with recent proposals that odors may be coded by a highly distributed pattern of bulbar input.