What activates memory better, sight or smell? Recently, 2 separate studies have been done to identify how effective both visual memory and smell are. In the first study, researchers showed a group of people images of over 2,500 objects for more than 5 hours. Afterward, they were shown the images again, along with similar images, and were asked to identify which ones they had seen before. The results (published in the Proceedings of the National Academy of Sciences (PNAS)) showed that 87% of the future images shown to the participants could be correctly identified as having been shown before or not. However, if asked to recant the image from memory, participants usually only have a very vague idea of what the image looked like.
When it comes to smell, it seems that it has a greater ability to spark personal experiences. In a study published in Psychonomic Bulletin and Review, the researchers found that memories associated with odor tend to be stronger than those associated with sight or language. Also, memories associated with smell tend to form earlier, as researchers found that memories associated with smell can form and stay before the age of 10, but memories associated with sight tend to only be forged strongly between the age of ten and twenty.
More studies may need to be done in these areas, but based on these two studies, there is already strong evidence that smell forms stronger memories than sight. Why is another question, and hopefully more researchers will look into that in future as well.