As an example of learning, consider
the effect of injections of cholecystokinin
(CCK, see below) on the preference by
chickens for foods of different colours
(Covasa and Forbes, 1994). CCK given
intraperitoneally (i.p.) is known to depress
food intake, and the aim of the experiment,
the results of which are shown in Fig. 15.3,
was to determine whether this reduction
was due to a pleasantly satiating effect or
to an unpleasantly nauseating effect of
CCK. On days 1, 4 and 7, half of the birds
were injected with 100 μg of CCK i.p.,
sufficient to depress intake by about 20%
over the next 2 h; for 2 h after the injection,
the normal food was replaced with food of
the same composition but coloured green
for one-quarter of the birds and red for the
other quarter. The other half of the birds
were injected with saline and again a
quarter given green food, a quarter red for
the next 2 h. On days 2, 5 and 8, each bird
was given the alternate injection with the
alternate colour and on days 3, 6 and 9
were made mildly hungry by fasting for 1 h
and then individually given a choice
between green and red food. The colour
approached was noted and, as shown in
Fig. 15.3, they became progressively averse
to the colour of food associated with CCK;
it is clear that the effects of the hormone
are unpleasant and that the birds had
become conditioned to believe that their
discomfort was due to food of a particular
colour – which colour it was is not
important. For the next 11 days, no injec-
tions were made but the preference test
was repeated from time to time; the
aversion was lost within a few days. Then,
for each bird, injections were paired with
the opposite colours to those used in the
first phase of the experiment and aversion
rapidly developed, again followed by
return to no preference within a few days
of stopping the injections. Thus, colour
preference can be easily manipulated and
the same is true for the association between
food flavour and the consequences of
eating that is more readily evident in
mammals.
It is known that the intake depression
caused by giving CCK i.p. can be prevented322 J.M. Forbes
Fig. 15.3.Proportion of chicks showing preference for food of the colour paired with injections of CCK
(Covasa and Forbes, 1994). From days 1 to 9, injections of CCK were paired with one colour of food and
injections of saline with another colour; preference steadily moved away from the CCK-paired colour. From
days 10 to 20, no injections were given; the aversion quickly disappeared. From days 21 to 33, CCK and
saline injections were paired with the opposite colours to those used on days 1 to 9; aversion to the new
colour was established progressively. From day 34, no injections were given and the aversion gradually
disappeared.