Explain how illuminating a photoreceptor can lead to increased frequency of action potentials in ganglion cell?

When illuminated, the photoreceptor hyperpolarizes, stopping its release of glutamate. If bipolar cells that it synapses with are hyperpolarized by glutamate, they will depolarize in the light and release more neurotransmitter. If that neurotransmitter is excitatory, the ganglion cells they synapse with will produce action potentials at a higher frequency.