Olfaction directs skilled forelimb reaching in the rat.

Rats were trained to reach for food pellets, which varied in size from 20 to 300 mg, from one of 21 locations on a shelf located just outside a test cage. After a barrier was lifted, giving access to a food pellet, latency to contact the food was timed, number of reaches to contact the food were counted, the distance of each misplaced reach from the food was measured and the success or failure of each attempt was noted. Control rats walked along the front of the cage sniffing to locate the food. Once they located a food pellet, they inserted their nose between the bars of the cage, sniffed the food for one to three respiratory cycles, raised their snout slightly, and then inserted a forepaw to grasp the food. They seldom required more than three reaches to contact the food and on about half of all trials they were successful in grasping the food. They were more successful in grasping medium-sized food pellets. Performance was unchanged on all measures after vision was occluded with eye patches. Following olfactory bulb removal, however, the rats no longer sniffed to locate the food or sniffed the food prior to initiating a reach. Rather they systematically moved along the length of the cage inserting their snout between the bars at each possible food location and without sniffing reached for the food 'as if blind'. Latency, number of reaches to contact the food and the distance of misplaced reaches increased whereas success rate remained unchanged. Substantial additional impairments were not produced when the bulbectomized rats were given visual occlusion. The study demonstrates that rats locate food and direct reaching using olfaction. The results are discussed in terms of the sensory control of skilled limb use and their relevance to the evolution of the sensory guidance of limb use.