DIRECTION TUNING OF INDIVIDUAL RETINAL INPUTS TO THE TURTLE ACCESSORYOPTIC-SYSTEM

Neurons in turtle accessory optic system [basal optic nucleus (BON)] w ere recorded to study convergence of retinal afferents, using whole-ce ll patch electrodes in a reduced in vitro brainstem preparation with t he eyes attached, BON cells primarily exhibit EPSPs from a contralater al retinal ganglion cell input and generate an output of action potent ials. Visual responses were evoked by different directions of either f ull-field or local moving patterns. Direction tuning of action potenti als was compared with that of EPSPs detected by passing the membrane v oltage through an AC amplifier and window discriminator. This rough me asure of retinal input indicated that the direction tuning of the full -field excitatory input from the retina matched that of the spike outp ut for the same BON cell. Using local patterns within the receptive fi elds of the BON cells, it was estimated that one to four adjacent reti nal inputs were being stimulated. The direction tuning of these inputs had preferred directions that were similar to that of the full-field spike output of the cell, irrespective of where the small window was p laced within the receptive field. Because more than one retinal input may have been stimulated by the small stimulus window, subsets of thos e EPSPs that may represent responses of a single retinal afferent were identified based on their amplitude and rise time, Again, the preferr ed direction of those putative single retinal afferents matched the di rection tuning of the spike output of the BON cell. These findings are discussed in terms of the formation of the retinal slip signal by the BON.