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GOUTEUX, THINUS-BLANC, AND VAUCLAIR
longer walls, were present, but only one, because of technical problems,
permitted entry into the apparatus. Four opaque identical boxes (15 cm in
diameter x 25 cm in height) stood in the four corners of the room. Each
box could contain a food reward (Fruit Loops cereal), but the subjects
could not locate the reward unless they looked into the box. The room was
illuminated by a central overhead neon circular light (100 W). A video-
camera, suspended from the center of the room's ceiling, provided an
overhead view of the whole apparatus. At the center of the room, a
transparent Plexiglas cage (40 cm wide x 45 cm long x 50 cm high) was
fixed on a rotating support to allow the axial rotation of the cage. Above
that central cage, curtains were added to prevent the monkey from seeing
the experimental environment from inside the cage. The opening of the
central cage was externally controlled by a mechanical system made of
several pulleys and a cable that descended directly from the center of the
room's ceiling. A white-noise generator was located on the ceiling of the
apparatus, at the center, to prevent monkeys from maintaining their orien-
tation through the use of acoustical external cues. Thus, the apparatus was
symmetrical in its geometry.
Procedure. A training phase was conducted to familiarize the animals
with the experimental apparatus and the experimental conditions. Each
animal was daily captured and transported to the experimental room, using
an opaque transport cage. Each monkey was transferred in the central
transparent Plexiglas cage (darkened by black curtains) in such a way that
the animal could never see the extra-apparatus environment. After a 1-min
delay, the monkey was randomly released in front of one of the four walls
of the room by the external mechanical system. The animal could then
freely explore the experimental environment during a 10-min period and
catch food rewards that had been previously laid out on the floor. After the
10-min delay, the access hatch was opened, and the monkey could return
in the external opaque transport cage.
After 1 week of training (five sessions per day), the reward box was
placed at one corner of the experimental room. That box was systematically
rewarded, and the monkey was able to see the reward, which was being
hidden in that box (curtains opened). The same procedure mentioned above
was used: The monkey was released, and after a 10-min delay or imme-
diately after the subject had retrieved the reward, it was returned to the
outside opaque transport cage. The position of the corner reward box was
randomly chosen for each monkey but was maintained constant during the
experiment. Then, the three identical remaining boxes (not rewarded) were
progressively added to the experimental room. In the same manner, the
monkeys had to retrieve the reward previously indicated by the experi-
menter. The training phase ended when all monkeys systematically suc-
ceeded in retrieving the food reward. The delay to reach that level varied
for each subject (from 2 days for Subject Krill to 2 weeks for Subject
Crevet).
During Experiment 1, the monkeys were tested in the all-white condition
(see Figure 2), and the same procedure was used for all of the experiments.
As mentioned above, the subject was introduced to the central transparent
Plexiglas cage with the curtains closed. After the opening of the curtains,
the experimenter hid a reward in the same comer box that was rewarded
during the training phase. For a given subject, the reward was hidden in the
same location throughout the experiment to reduce extensive proactive
interference. Then, the curtains of the central transparent Plexiglas cage
were closed, and the monkey was disoriented by the experimenter (at
least 10 full rotations of the cage), who was walking around the cage at
varying speed to avoid serving as a landmark during disorientation. Then,
the cage rotation was stopped, its entrance randomly facing one of the four
walls of the rectangular chamber, and the experimenter continued walking
around slowly so as not to cue the monkey to any possible location. Finally,
the experimenter, without making any noise, left the experimental room
through the access door. After a 10-s delay, the door of the central
Plexiglas cage was opened from the outside, and the monkey could freely
search for the reward box.
When the food was found, the access hatch was opened, and the monkey
could return to the external opaque transport cage. Then, the subject was
transferred again to the experimental room to be tested again. Ten trials a
day during 5 consecutive days (50 trials) were conducted. A delay of 1 min
occurred between each test trial.
Dependent measures. All search patterns were videotaped to later
verify patterns of exploration made during the experiment. To count as a
search, the monkey had to look inside the box or at least touch the box with
its arm. A passage near the box or a look in its direction was not taken into
Figure 1. Schematic representation of the experimental environment (4.0 m long x 2.5 m high x 2.0 m wide).
Subjects were introduced into the central Plexiglas cage, fixed on a rotative support (gray central box), and were
released by an external mechanical command (full line drawn at the top of the ceiling), to retrieve the reward
hidden in one of the four identical comer boxes (gray circular boxes). An overhead camera, a white-noise
generator, and a circular light were affixed on the ceiling of the room (black rectangular box and black circle on
the inn of the fiaurel