The goal of the experiments in this section was to determine the features of line drawings of objects (or pseudo-objects) that were important for picture recognition by the pigeons. To make this determination, we adapted the classic approach for determining stimulus control, wherein stimuli that have established control over behavior are systematically altered to see how the changes in the stimuli produce changes in behavior (e.g., Lashley, 1938; Skinner, 1935; Tinbergen, 1951). As Skinner (1935) noted: "When a defining property has been decided upon, the stimuli that elicit responses possessing it are discovered by exploration. Subsequently the defining property of the stimulus is inferred from the part common to the different stimuli that are thus found to be effective" (pp. 48-49).

The analysis of stimulus control of complex objects was determined by first training pigeons to respond differentially in the presence of different line drawings of common objects using a four-key choice procedure (see below for details). In this procedure, the pigeon is shown a picture, such as a line drawing of a desk lamp, and then given four different choice keys. The pigeon's task is to peck the correct key for the desk lamp -- this might be the green key. The pigeon would be given four different objects, with each object associated with a different key. One can think of this procedure as the "name game" commonly employed to teach young children the proper verbal labels for the objects that they encounter. In effect, the four-key choice procedure teaches the pigeon to label the different line drawings by producing the correct response (here a key peck; in children a verbal report) in the presence of the object. 

Once the pigeon accurately discriminates the four objects by pecking the correct keys, then one can assess which features of the pictures were important in forming the discrimination. This is done by modifying the original training objects. Many different modifications were performed such as moving or deleting parts of an object, rotating or moving an entire object, or changing the object's size. The experiments that are listed below are organized so that a particular experiment contains tests that were aimed at assessing the contribution of a particular property of the object, such as the spatial organization of its components. 

Four-Key Choice Testing Procedure

Training Procedure. The initial phase of most of the experiments (except where noted otherwise) involved training with a four-key choice procedure. Each pigeon was trained to discriminate among line drawings of four different objects, such as a watering can, an iron, a desk lamp, and a sailboat. The training objects were displayed individually in the center of a video monitor on different trials. The pigeon had to initially peck at the object on the viewing screen in order to obtain access to four differently-colored choice keys. The choice keys were situated diagonally from each corner of the viewing screen. Each object was associated with a different choice key. For example, one pigeon might have to peck the red key in the presence of the watering can, the green key in the presence of the iron, the blue key in the presence of the desk lamp, and the violet key in the presence of the sailboat. Different birds received different object-choice key assignments. If the pigeon pecked the correct choice key, then food reinforcement was delivered to a food tray located on the back wall of the chamber. If the pigeon chose the incorrect key, then the trial was repeated until a correct choice was made, resulting in the delivery of food. Training sessions were conducted daily until the birds attained a high level of accuracy (e.g., 75% correct to each object).

Testing Procedure. Testing with different kinds of stimulus manipulations occurred following training on the original task. Test stimuli were presented in sessions along with normal training trials and their occurrence was relatively rare (e.g., 16% of the trials). On training trials, the normal contingencies were in place (correction trials for an incorrect response and food reward for a correct response).  On test trials, food reinforcement was delivered, regardless of the pigeon's initial choice response.  If performance on the training trials fell below a criterion (e.g., 75% correct on each key), then one or more retraining sessions were administered to re-establish accurate performance on the original discrimination. 

Next Section:  Spatial Organization of Components