In humans! James Swain of Yale scanned the brains of new mothers two to four weeks after delivery. He found their brains were less responsive to their babies' crying. From the article in News Medical.
Many people think that the pulses of oxytocin released by the brain to start contractions during natural childbirth also prime the brain's emotional centers to love and care for the baby. By contrast, the steady intravenous drip of pitocin used to jumpstart labor in the majority of U.S. hospital births may not allow for the emotional circuits to respond -- especially since the contractions caused by pitocin can be much more painful than natural ones.
Because oxytocin can act as a mood elevator and is synergistic with serotonin, it's also possible that c-section might increase a mother's risk of postpartum depression.
A possible attachment gap created by c-section has also been shown in survey-type studies. Swain's work shows that it happens at the level of brain activity, possibly preventing important and permanent changes in the brain.