Applications of visually evoked potentials in ocular diseases: A guided tour

Visually evoked potential (VEP) is an electrical signal generated by the brain (Occipital Cortex) in response to a visual stimuli. These responses are recorded non-invasively by placing the surface electrodes at the scalp, and observed as a reading on an electroencephalogram (EEG). This generated potential is smaller in amplitude compared to the EEG signal, which is in the range of 1 to 20µV compared to 1 to 100µV of the latter. The VEP signal has been widely used for the diagnostics of ocular diseases in patients having vision related problems. The main parameters that were considered for the diagnostics of these diseases are the amplitude and the latency values. This field of study is gaining interest from researches all over the world. Researches now are focusing on analyzing these electrical signals in time-frequency domain to extract more information. This paper is intended to give a brief review of the theory and techniques that has been used by the previous researches in analyzing VEP for the investigation of ocular diseases.

I. Introduction

Visually evoked potentials have been widely used as a reliable indicator for the diagnostics of ocular diseases. Although measured from the scalp, the VEP differs from the EEG signal in several ways [1]. First of all, the VEP corresponds to a specific response of occipital lobe, part of the brain which is involved in receiving and interpreting the visual signals, while the EEG signifies the ongoing activity of wide areas of the cortex [1],[2]. Another difference is that the VEP is smaller in amplitude (1–20μV) compared to that of EEG signals (1–100μV) [1]. The VEP is also more sensitive towards the changes in the stimulus conditions compared to the EEG. The commonly used visual stimuli is a black and white checkerboard pattern, and is presented to the patients in order to study the responses of VEP although other types of stimulus can also be used such as flash and line gratings depending on the type of diseases to be investigated. Doctors usually refer their patients to perform the VEP test if the patients are experiencing changes in vision that can be due to problems along the visual nerves pathways [2]. Analysis of the VEP would help to determine the nature of the diseases. Although VEP is particularly useful in detecting past optic neuritis, it can also be used to detect other ocular diseases such as glaucoma, multiple sclerosis, ocular hypertension, macular degeneration, colour blindness, Parkinson's disease, idiopathic intracranial hypertension and cataract [1]–[10]. The commonly extracted parameters for analysis from the VEP are the amplitude and latency values. The amplitudes examined are the N75, P100, and N125 where ‘N’ denotes the negative peak, ‘P’ for positive peak and the numbers (75,100, 125) indicating the time in microseconds. The time taken for a visual stimulus to travel from the eye to the occipital cortex is taken as the latency values [2],[4],[11]–[14]. Many researchers are now trying to analyze the VEP in the time-frequency domain in order to improve the diagnostics accuracy and to reduce time it takes.