ADHD has a genetic link to Autism Spectrum Disorder (ASD), and the 2 conditions frequently coexist. Both were potentially associated with abnormal glutamate and GABA neurotransmission, which can be modeled using an electroretinogram (ERG) to measure synaptic activity in the retina. Retinal responses were reduced in ASD, but little was known about retinal activity in ADHD. For a study, researchers compared the light-adapted ERGs of people with ADHD, ASD, and healthy to see if retinal responses differed between these neurodevelopmental conditions. Full-field light-adapted ERGs were collected from 15 ADHD, 57 ASD (without ADHD), and 59 control participants ranging from 5.4 to 27.3 years. A Troland protocol was used, with 9 flash strengths ranging from −0.367 to 1.204 log photopic cd.s.m−2. Using generalized estimating equations, the time-to-peak and amplitude of the a-and b-waves and the parameters of the Photopic Negative Response (PhNR) parameters were compared among the 3 groups of participants. When compared to the control and ASD groups, those with ADHD had statistically significant increases in ERG b-wave amplitudes, PhNR responses, and faster b-wave time-to-peak timing. The greatest increase in b-wave amplitudes associated with ADHD was observed at 1.204 logs photopic cd.s.m−2 flash strength (P<.0001) when the b-wave amplitude in ASD was significantly lower than in controls. With an area under the curve of 0.88, ADHD could be distinguished from ASD using the measure. The amplitude of the ERG b-wave appeared to be a distinguishing differential feature for both ADHD and ASD, resulting in a reversed pattern of b-wave responses. These findings suggested that glutamate and GABA neurotransmission, which primarily regulate the b-wave formation, were out of balance. The b-wave amplitude abnormalities could serve as a biomarker for both neurodevelopmental conditions.