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.
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 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