Animals are more likely to be considered sentient if
they can learn much, learn fast and make few errors
once they have learned. Classical conditioning and
operant conditioning can occur in animals with relatively simple nervous systems, such as the mollusc
Aplysia (Lorenzetti et al. 2006) and a headless locust
can learn aversive foot-shock conditioning (Carew &
Sahley 1986). Learning is not, in itself, evidence for
awareness but is an indicator that further investigation
of cognitive ability might reveal the existence of
awareness commensurate with sentience. Comparative studies of learning ability are not easy to conduct
because learning situations usually require that an
operant be performed, and animals may vary in their
physical ability to carry out the operant.