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T. Saito, K. Mori, Y. Umino, R. Barlow; Visual Processing in the Limulus Brain . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4536.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Examine responses of brain cells to optic nerve signals from the Limulus lateral eye. Methods: The brain was exposed through an access hole cut in the dorsal surface of the prosoma. The surrounding connective tissue was removed and Lucifer yellow–filled glass microelectrodes were advanced into the primary visual regions of the protocerebrum. We analyzed a recorded cell first by plotting its receptive field using to a point–source probe light and second by determining its response properties to visual stimuli located in the center of its receptive field. Then we stained the cell by passing current through the electrode, removed the brain for anatomical analysis with a confocal microscope. Results: We injected 119 cells and recovered the anatomy of 49. We found most recovered cells (38) were optic nerve fibers that branch in the lamina (first synaptic layer) and traverse to the medulla (second synaptic layer). They have small receptive fields (60) and excitatory responses (spikes) with no depolarizing prepotentials. The other 63 cells had the following properties: "transient ON"(3), "sustained ON"(13), "transient OFF"(2), "sustained OFF"(22), "silent"(5), and "mixed"(18). Most of these responses exhibited slow depolarizations, both pacemaker and synaptic–type potentials. Conclusions: Limulus brain cells exhibit responses similar to those recorded from retinal ganglion cells in vertebrates. This is not surprising because in vertebrates the second synaptic layer (IPL) that shapes ganglion cell responses is in the retina; whereas, in Limulus the second synaptic layer is in the brain. The synaptic mechanisms underlying neuronal responses in the Limulus brain are not yet known.
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