They found that there was no change in the time needed to understand the happy sentences, which aligned with their hypotheses, since the Botox did not prevent the participants from smiling. But the subjects did take slightly but significantly more time to read the angry and sad sentences after Botox treatment. These data suggest that preventing the participants from frowning actually made it harder for them to interpret sadness and anger.
The scientist believe that by preventing frowns in their participants, they have blocked a feedback pathway between the brain and the face. Under normal circumstances, the brain sends the signal to frown, and in turn, once the face frowns, it sends signals back which reaffirm or enhance the brain's interpretation. Without the feedback, the brain gets a little confused or simply doesn't process the depth of the emotion as well.
I have to say that I find the implications of this to be impressive. Not only does this say that altering the faces ability to make an expression impacts your brain's ability to appropriately process the data, it strongly implies that our brain is reliant on our bodies to interpret. Sadness is not just serotonin uptake, sadness exists because the face tells the brain to uptake some serotonin. So our bodies are part and parcel of our brain. There was an author on the Daily Show, I can't remember his name, but he spoke to the blind side of neuroscience research. That analyzing and studying everything from a framework where the brain is just a processor of external stimuli fails to account for both what our brain projects into our environment and how a responding environment reacts to a projecting brain. Essentially, you can not reduce the system to its parts and gain true insight, you must try to see the whole system. We as a species seem particularly lacking in our ability to see the whole system and gain insight therein.