Babies and PCT

[From Bruce Gregory (970905.1030 EDT)]

My wife told we of a TV program she watched last night dealing
with things that make people happy. "You'll love this
example," she said. An experiment was set up in which babies
were able to view pictures of human faces by pulling a string.
When they carried out this activity the babies demonstrated
signs of infant happiness. When the string no longer produced
the pictures, but the babies were exposed to pictures of human
faces for the same fraction of time as they had produced in the
first condition, the babies were unhappy. (I will refrain from
saying anything about reinforcers...)

Can anyone point me to a written description of these studies?