Patterned response to odor in mammalian olfactory bulb: the influence of intensity

Responses of single neurons in the olfactory bulb of anesthetized hamsters were recorded extracellularly while odors of defined concentration and time course were delivered to the olfactory system at constant flow. Responses could be either excitatory or suppressive, as judged by the first distinguishable change in firing rate during odor delivery. However, when the time course of the response was examined in more detail, approximately one-third of all tests and one-half of the tests at high concentration resulted in complex temporal patterns of firing rate that involved both increases and decreases with respect to spontaneous activity. Approximately two-thirds of all tests produced responses where increased firing rate preceded any distinguishable suppression. Excitatory and suppressive responses were each classified into four groups according to their temporal patterns. Different patterns were not equally represented in the data and the proportions of patterns elicited by the same odor changed with stimulus intensity. Complex responses, where temporal patterns included periods of firing rate above and below spontaneous rate, were increasingly common and intensity was increased. Magnitude of response is difficult to define when a single response includes both increases and decreases of firing rate but more than half of the neurons that responded to more than one stimulus concentration clearly had nonmonotonic intensity-response functions. Forty-one out of 101 neurons were classified as output cells because they could be driven at short constant latency by lateral olfactory tract stimulation. Their responses were not clearly different from the remaining cells that could not be classified as output cells. The contribution of the inhibitory circuits of the olfactory bulb to the generation of patterned response and to changes in pattern with intensity are discussed. The lateral inhibitory circuits of the bulb appear to be sufficient to explain the data presented here.