Respiratory conditional reflex formation with CO2 and electric shock as unconditional stimuli

In 12 birds used in a CO₂ conditioning experiment there was no evidence of the formation of either a respiratory or a cardiac conditional reflex. Tone and light separately or together were used as conditional stimuli. CS-US intervals ranging from 0.5 to 10 seconds were used. CO₂ concentrations up to 7.4% were also employed. In the shock conditioning experiments six of seven birds showed a respiratory change to the conditional stimulus. In one pigeon perfect differentiation was obtained between the reinforced and nonreinforced tones. The marked difference in conditionability of sensory and chemical stimuli to the “respiratory center” is discussed in relation to the central nervous system connections of the respiratory control mechanism.