Basically what the study says is that women suffering from postpartum depression may have reduced brain activity in certain regions of their brains that are involved in emotional processing. So, depressed moms are not as in tune with others' emotions, nor are they as responsive to emotional cues as they should be. When her baby cries, a mom with PPD may not react or respond as she would if she weren't depressed.
I don't know why this would be a surprise to anyone, and I'm not really sure the point of the study. The author of the article asserts that the study will benefit future treatment of PPD, but I'm not so sure about that. For one thing, only 30 mothers participated in the study - 14 who were determined to be "depressed," and 16 who were "healthy." This doesn't seem like a large enough population sample.
Another thing that bothers me about this study is that the focus seemed to be on the mom-baby attachment, and when you get into this sticky area, you run the risk of blaming moms for a lack of attachment. And you wonder what the real goal is. Treatment shouldn't be focused on improving the level of attachment a mom has for her baby. The goal of treatment should be getting the mom healthy and free from PPD. Mom-baby bonding is but one of many aspects that will improve as she recovers and breaks free from PPD.
We need to focus more on avoiding PPD to begin with instead of treating just one of the issues connected with the disorder.