Functional asymmetries in cockroach ON and OFF olfactory receptor neurons.

The ON and OFF olfactory receptor neurons (ORNs) on the antenna of the American cockroach respond to the same changes in the concentration of the odor of lemon oil, but in the opposite direction. The same jump in concentration raises impulse frequency in the ON and lowers it in the OFF ORN and, conversely, the same concentration drop raises impulse frequency in the OFF and lowers it in the ON ORN. When the new concentration level is maintained, it becomes a background concentration and affects the responses of the ON and OFF ORNs to superimposed changes. Raising the background concentration decreases both the ON-ORN's response to concentration jumps and the OFF-ORN's response to concentration drops. In addition, the slopes of the functions approximating the relationship of impulse frequency to concentration changes become flatter for both types of ORNs as the background concentration rises. The progressively compressed scaling optimizes the detection of concentration changes in the low concentration range. The loss of information caused by the lower differential sensitivity in the high concentration range is partially compensated by the higher discharge rates of the OFF ORNs. The functional asymmetry of the ON and OFF ORNs, which reflects nonlinearity in the detection of changes in the concentration of the lemon oil odor, improves information transfer for decrements in the high concentration range.