How visual information reaches the goldfish Mauthner neuron: from seeing to a fast-start decision in 35 ms

The two huge Mauthner neurons (MNs) form the center of the fast-start network in the hindbrain of fish. Their activation initiates a rapid turning maneuver, the so-called C-start, within milliseconds. Here, we recorded intracellularly from the MNs of goldfish to quantify the spatiotemporal flow of visual information from the two eyes, the optic nerves, and the ipsilateral and contralateral hemispheres of the optic tectum (OT) to each MN. Strong light flashes delivered to one eye induced postsynaptic potentials (PSPs) in both MNs after a delay of 35 ms. By directly stimulating the optic nerve, we show that most of this time (80%) is needed for transduction and processing in the retina. Visual stimulation of the eye and direct electrical stimulation of the optic nerve was effective regardless of which eye or which optic nerve was stimulated. Stimulating any region of the ipsilateral or contralateral OT caused PSPs in both MNs. Remarkably, the PSPs induced by our brief visual and electrical stimuli were of remarkably complex structure and long duration. The connectivity pattern and PSP durations we describe here suggest an arrangement that supports a high degree of flexibility in C-start directionality and the integration of sensory information that arrives with different delays in a natural encounter.