Is the b-wave positive or negative in an ERG?

• A. Negative
• B. Neutral
• C. Cannot be determined
• D. Positive

Answer: (D)

In an electroretinogram, the a-wave reflects photoreceptor activity and appears as a negative deflection, while the b-wave is a positive deflection produced by the inner retina, mainly the ON-bipolar cells (and supporting Muller cells). When light hits the retina, photoreceptors release less glutamate, which depolarizes ON-bipolar cells via the sign-inverting pathway. This inner-retinal signaling creates the large positive bump we call the b-wave. Because of this inner-retinal origin and the way the signals flow in the standard recording setup, the b-wave is positive. The other possibilities don’t fit normal ERG physiology: a negative or neutral b-wave would imply an atypical recording condition or abnormal retinal processing.