Hartline investigated the electrical responses of the retinas of certain arthropods, vertebrates, and mollusks, because their visual systems are much simpler than those of humans and thus easier to study.
Hartline meneliti respon listrik retina arthropoda, vertebrata, dan moluska tertentu karena sistem penglihatannya lebih sederhana daripada manusia sehingga lebih mudah dipelajari.