Incompatability between the pigeons' unconditioned response to shock and the conditioned key-peck response.
High-speed photography was used to compare the pigeon's response to unsignalled shock with the pigeon's key-peck response. During shock, pigeons flex their neck (i.e., the distance between their eyes and shoulders decreases). Following shock, the neck is extended. During key pecking, the neck remains extended and the head moves toward the key in a slight arc as though attached to a fixed fulcrum. Response topography during pecking and shock appear to be incompatible, and it is concluded that the difficulty in key-peck training pigeons to escape electric shock is due to interference from the unconditioned flexion response. This conclusion supports the species-specific defense theory of escape and avoidance behavior.