A visual field test can also tell the ophthalmologist and surgeon whether a lesion is located in front of, on, or behind the optic chiasm. Such information is used to help plan the patientâs surgery and to establish a vision baseline that can be referred to for comparison after brain surgery.he lighter the area, the more vision the patient has in that part of the eye. Figure 2. Visual field test of a patient with a meningioma shows vision loss (black areas) in half of the right eye, called hemaniopia.