impact of neonatal low level PCB exposure on behavioral performance
by monkeys. Studies by Deborah Rice and Stephen Hayward.
three papers used experimental methodology from operant conditioning
(behavioral psychology) to explore low level impacts of postnatal
PCB exposure in monkeys (Macaca fascicularis). The
studies provide strong evidence that "exposure to an environmentally
relevant level of a PCB mixture for a short time postnatally results
in long-lasting behavioral effects."
monkeys were dosed from birth to 20 weeks of age with 7.5 µg/kg/day
of a PCB mixture representative of that typically found in human
milk. Three to five years after exposure, their performance on behavioral
tests was compared with a control group of monkeys.
PCB levels at the age of 20 weeks were 0.30-0.37 ppb for the controls
and 1.84-2.84 ppb for the treated monkeys. PCB levels in the fat
were 50-198 ppb and 1694-3560 ppb, respectively. The treated group's
contamination level in fat is within the middle of the range
of exposures noted for people, whereas the blood level levels
in the treated monkey group are at the low end of values seen in
has stated at scientific meetings that when the data for exposure
levels of the treated monkeys were first given to her, they were
mis-reported as being much higher than the levels actually found,
and that had she known how low these exposures really were, she
probably would not have conducted the experiments. That makes her
results even more striking.
DC. 1998. Effects of postnatal exposure of monkeys to a PCB mixture
on spatial discrimination reversal and DRL performance. Neurotoxicology
and Teratology 20(4):391-400.
set of tests was performed 4.5 to 5 years after exposure. Rice found
no difference between treated and control group monkeys in a "spatial
discrimination reversal" test. In this test, the monkeys were
challenged to respond adaptively to changes in the rules they needed
to follow to obtain a reward. Part of this test also involved showing
the monkeys cues that were irrelevant to solving the task, and seeing
if these irrelevant cues increased the error rates. Rice notes that
even though there were no differences between the groups, "there
was an indication of poorer performance in some treated individuals."
did, however, find significant differences between treated and controls
in a "differential reinforcement of low rate" test. In
essence, this test measured the ability of the monkeys to be patient.
Treated monkeys were much more likely to commit errors in this test,
i.e., to be less patient. Rice observed that the control group monkeys
in this test clearly "progressively adapted" to the test
procedures, whereas the treated group did not.
DC. 1997. Effects of postnatal exposure to a PCB mixture in monkeys
on multiple fixed interval-fixed ratio performance. Neurotoxicology
and Teratology 19:1-6.
reports that the exposed monkeys perform differently, compared to
controls, in basic behavioral tests. The tests she used were classic
approaches used by behavioral psychologists called "fixed ratio"
and "fixed interval" "schedules of reinforcement."
In these tests the monkeys were given reward of a squirt of apple
juice for meeting certain test criteria that were based on pushing
a button a certain number of times (fixed ratio) and pushing a button
once after a time interval had passed (fixed interval), a "waiting
period." In Rice's tests, she mixed these two schedules of
reinforcement in a standard way called a "multiple fixed interval
(FI)-fixed ratio (FR)" schedule of reinforcement.
monkeys responded to the tests differently compared to control monkeys.
The most interesting difference was the treated monkey's response
to the "fixed interval" part of the test. Typically, animals
and people develop a classic pattern of response to FI tests in
which during training they begin to pause after receiving the reward,
with the length of the pause over time getting close to the end
of the waiting period. Tested at 4 years of age, the treated monkeys
developed the pause much more slowly than the experimental monkeys.
Even at the end of the experiment, the treated group continued to
show a high number of short pauses. "If anything, this difference
appeared to be getting larger as the experiment progressed, as control
monkeys decreased their short IRTs across sessions and treated monkeys
did not. While behavioral psychologists balk at describing this
is simpler terms, in essence it is as if the treated monkeys showed
much less patience than the control monkeys."
DC and S Hayward 1997. Effects of postnatal exposure to a PCB
mixture in Monkeys on Nonspatial Discrimination Reversal and Delayed
Alternation Performance. Neurotoxicology 18:479-494.
experiments were performed 3 years after exposure. The authors state
in conclusion that their "results suggest that postnatal
exposure to PCBs produces cognitive deficits in monkeys."
and Hayward took the monkeys through two behavioral tests: a "nonspatial
discrimination reversal" (NSR) and a "spatial delayed
alternation" (SDA). These are standard tests in operant conditioning.
essence, the NSR test examines the subject's ability to respond
adaptively to changes in the rules that link behavior to reinforcement.
The experimenter periodically reverses the rules so that what had
been reinforced before no longer is, and vice-versa. Of interest
is the quickness with which the subject learns that the rules have
changed and the number of errors committed while adapting to the
SDA test involved requiring the monkeys to switch the behavior adaptively
and introducing delays into the test when errors were committed.
More errors then prolonged the delay, i.e., the length of time the
monkey had to wait before becoming eligible for another reinforcment.
the NSR test, Rice and Hayward found some differences between treated
and controls but many similarities. They observe; "If there
is a relationship between tissue levels of PCBs and performance
[on this test] it is a complex one."
contrast, the treated and control monkeys demonstrated some strong
differences on the SDA test. Treated monkeys required significantly
more sessions to "reach criterion on the acquisition"
[i.e. to perform well enough to "pass" the test]. The
treated monkeys made more errors and perservered more in counterproductive