Reactivity to light and development of classical cardiac conditioning in the kitten.

Kittens studied from 7 days onwards revealed that the earliest age at which cardiac conditioning can be established is 30 days, after some 30 light-electric shock associations. Older kittens showed both cardiac as well as motor conditioning. Animals younger than 21 days old had no well-defined cardiac responses to the clearly nociceptive unconditioned stimulus so that no cardiac conditioning was established; neither did they show signs of motor conditioning. This failure could be due to immaturity of the cardiac-emotional reactivity to nociceptive stimulation and immaturity of the neural structures involved in associative learning. Lack of reactivity to the light used as conditioned stimulus must be excluded because it already produced eye blink by 2 days of age. Moreover, a light-dark preference test done during the 1st postnatal week showed that kittens are able to discriminate light from dark by 8 days of age.