Rate at which monkeys sample the sensory environment
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The far wall of each chamber was made of ground Perspex. These walls were
back-illuminated by two slide projectors. The rest of the inside of the apparatus was
painted matt black. The colour of the illumination was determined by Kodak Wratten
filters in the projectors (red filter 25, blue filter 38A). The brightness level, measured
at the Perspex wall, was adjusted to 1 ∙ 2 log-footlamberts for the human eye (which
has approximately the same spectral sensitivity as that of the rhesus monkey).
Passage of the monkey through the tunnel was detected by capacity sensors in the
floor. Each bout was timed from the point at which the monkey entered the
chamber to the point when, having left it, it entered the other one. The distribution
of bout lengths was automatically recorded by solid-state logic modules in the form
of a ‘survivorship curve’, using 5 s bins. Since it took the monkeys on average 2∙0 s
to pass completely through the tunnel, the origin of the survivorship curves was
deemed to be not at 0 but at 2 s.
Two television cameras, mounted against fish-eye lenses in the walls, allowed the
experimenter to monitor the monkeys’ behaviour in the chambers.
At the beginning of each testing session the monkey, was introduced into one
chamber or the other through a door in the side wall. Recording began immediately
after the first move through the tunnel. The testing session lasted for 300 s.
The monkeys were tested in the morning and in the afternoon, 5 days a week.
In experiment 1 they were tested ten times with red light and ten times with blue;
in experiment 2 they were tested ten times with 60 dB noise and ten times with
90 dB noise. Stimulus conditions, time of day, and the chamber the monkeys
started in were arranged in a balanced design.
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Figure 2. Plan of the testing apparatus (to scale).
2.2 Subjects
The subjects were seven young male rhesus monkeys (Macaca mulatto). Four of
them had taken part previously in experiments with coloured light (Humphrey and
Keeble 1977) but they had had no experience which might have been expected to
bias the results of the present study. All the subjects were thoroughly familiar with
the testing situation, having taken part in a series of pilot studies involving similar
apparatus.