In a study published Monday in iScience, researchers looked into the brain activity of dogs using MRI scanners. In the next round, they had 12 dogs look at photos of happy faces against photos indicating sadness, anger and fear. They also can use their keen sense of smell to sniff out chemical indicators of human emotion. Studies have shown that dogs can lower stress, anxiety and depression levels, while also helping with loneliness and overall human health. "So we should smile at the dogs — even at random dogs in the street."