Optimising Stimulus Parameters for Recording the Photopic Negative Response

Abstract

Abstract: : Purpose:To establish the optimum conditions for recording the Photopic Negative Response (PhNR) in a clinical environment. Methods:Normal volunteers and patients with well documented uniocular retinal nerve fibre layer loss were tested. A standard pattern electroretinogram (PERG) was recorded. Pupils were dilated using Tropicamide 1% and Phenylephrine hydrochloride 2.5% and ISCEV standard photopic responses recorded, after which the PhNR recordings were taken. All recordings were performed monocularly. A single gold foil corneal electrode (GFE) on the stimulated eye served as the active electrode. This electrode was referred both to a further GFE on the unstimulated eye and to a standard ERG reference electrode on the ipsilateral outer canthus. The stimulus was a rectangular pulse of red (650nm) light superimposed upon a uniform blue background (470nm). The stimulus was generated using a light emitting diode (LED) based stimulator (CH Electronics) which back illuminated a 50mm hemisphere. The stimulus intensity was modified by controlling both the forward current though the LEDs and the pulse width, which was varied over a range of 100µs to 5000µs. Background intensity was controlled by current modulation only. Results:The contralateral corneal reference electrode montage gave a small increase in signal to noise ratio in some subjects. Stimuli of 0.20, 0.50 and 1.00 cd s m² and background intensities of 2.35 to 23.5 cd/m² elicited a discernible PhNR. The PhNR showed a similar degree of loss to the PERG N95 component. Because of the much higher amplitude of the signal, fewer repetitions (15) were needed to obtain a reproducible PhNR compared to the PERG (150). Conclusions:The small improvement in signal to noise ratio using a contralateral corneal reference electrode montage does not justify the increased test complexity. A bright stimulus (0.20, 0.50 and 1.00 cd s m²) and a background intensity of 2.35 to 23.5 cd/m² may be the most suitable conditions for PhNR recording in a clinical environment. The PhNR may have a place in assessing ganglion cell function based purely on ease of application of the test.

Keywords: electrophysiology: clinical