Optimal olfactory discrimination in one theta cycle

Zach Mainen

Cold Spring Harbor

To investigate the significance of temporal coding in the mammalian olfactory system, we monitored the behavior of rats performing a two-alternative odor mixture discrimination task. This paradigm allowed us to obtain reliable psychometric discrimination functions in order to assay the relationship between accuracy and response time. As expected from a coding strategy involving temporal integration of an evolving sensory representation, mean odor sampling time increased significantly from the easiest pure odor discriminations to the most difficult near-threshold mixture discriminations. However, the amount of this increase was only ~50 msec (from 300 to 350 msec). Moreover, regardless of discrimination difficulty, performance levels saturated with only 200 msec combined odor sampling and decision time. Simultaneous measurement of sniffing during the behavior revealed that this time correspondeds to a single respiration cycle at theta frequency (7 Hz). These data place limits on the role of slow temporal patterning in olfactory coding and suggest that the formation of olfactory sensory images is constrained by a basic temporal cycle (theta) for sensorimotor integration.