Multi-channel metallic electrode for threshold stimulation of frog's retina.

A simple method for making a stimulating multi-channel metallic electrode is proposed. The method uses furnace-shaped multiple fine glass tubes as channel guides for inserted tungsten wires that are pulled as a unit to produce multi-channel metallic electrode with a tapered tip. By shortening the tip, the desired inter-channels distances (from tens to hundreds of micrometers) can be established with a high degree of accuracy. Such an electrode was used to stimulate the layer of ganglion cells in a frog's retina. The responses to stimuli were recorded extracellularly in the contralateral tectum from the terminal arborizations of the excited ganglion cells' axons. A larger area of the retina may be excited when several channels are used as a cathode and several others as an anode. By varying the stimulating pairs of the channels it is possible to change the site of excitation without displacing the electrode. Separate ganglion cells may be stimulated, provided the appropriate pair of channels is selected. In this case 'all-or-none' responses to the threshold stimuli are obtained in the tectum. The artefact of the stimulus is diminished effectively by joining additional channels to the anodic one.