Figure 4. Ganzfeld-Electroretinogram of the right and left eyes of the patient BT189. The following abbreviations were used: Left eye (LE), right eye (RE), electroretinogram (ERG), visual-evoked potentials (VEP), positive peak (P1 and P2), negative peak (N1 and N2). The ERG and the VEP tests the function of the visual pathway from the retina (ERG) to the occipital cortex (VEP). These tests were conducted by placing a standard ERG device attached to the skin on 2 mm above the orbit. VEPs were recorded simultaneously from electrode attached to the occipital scalp 2 mm above the region on the midsaggital plane. An electrode placed on the fore head provided a ground. The results can be directly related to the part of a visual field that might be defective. This is based on the anatomical relationship of the retinal images and the visual field. After dark adaptation for 30 min, the doctor will place anesthetic drops in the patient's eye and place a contact lens on the surface of the eye. Once the contact lens is in place, a series of blue, red and white lights will be shown to the patient. The VEP is an evoked electrophysiological potential that can be extracted, using signal averaging, from the electroencephalographic activity recorded at the scalp. Both ERG and VEP were differentially amplified band pass filtred (0,1,30 Hz), recorded over 300 ms epochs, and signal average. 2 trials were given. The visual evoked potential to flash stimulation consists of a series of negative and positive waves. The earliest detectable response has a peak latency of approximately 30ms post-stimulus. For the flash VEP, the most robust components are the N2 and P2 peaks. Measurements of P2 amplitude should be made from the positive P2 peak at around 207.3 ms. The ERG recorded in BT189 showed an absence of responses. While the VEP showed a normal responses in both eyes. These traces confirm the evidence of a significant bilateral global retinal degeneration. Only cone flicker responses of less than 15% of the normal mean were recordable under photopic conditions while all other responses were below noise level, a typical finding for patients with retinitis pigmentosa.