A search for odour encoding in the olfactory lobe

1. Studies were made of quality coding in the olfactory lobe of the insect by recording extracellular action potentials from single cells. 2. Listing cell spectra permits to distinguish two main groups of cells, namely, ‘odour specialist’ which respond very specifically to biologically important substances and ‘odour generalist’ which respond to a large variety of odorants (thirty‐two compounds) in an excitatory or inhibitory manner, or not responding at all. 3. Among more than fifty cells of the ‘odour generalists’, very few had similar, or identical, reaction spectra to an arbitrarily chosen set of thirty‐two odorants, while the ‘odour specialists’ are like each other in their response spectra. 4. There was an indication of a regional and layer differentiation of response in the lobe to the sex attractant. 5. ‘On’, ‘on—off’, and ‘off’ response types, as well as several variations on these response types, were found in single units during odour presentations. 6. Differences in patterning of excitation for each of the thirty‐two compounds can be readily detected by the comparison of the relative amounts of activity in each of the eighty‐one units tested. It is therefore concluded that the mechanism of odour encoding at the olfactory lobe may involve the linear combinations of every olfactory neurone's activity result ing in a unique across‐lobe pattern of discharges (‘odour code pattern’) for each particular odorant. 7. If it follows that odour discrimination by the lobe depends on such differences of ‘odour code patterns’, it would be possible then to distinguish very many odorants simply by having very many neurones possessing differential odour specificity.