The identification of single units in central visual pathways

There have been many single-unit studies on central visual pathways in recent years and several of these studies have been concerned with unit activity in or near the lateral geniculate nucleus (Tasaki, Polley & Orrego, 1954; Freygang, 1958; Lennox, 1958a, b; De Valois, Smith & Kitai, 1959; Erulkar & Fillenz, 1960; Griisser-Cornehls & Griisser, 1960; Hubel, 1960; Widen & Marsan, 1960; Hubel & Wiesel, 1961). In some cases the identification of the nature of the unit (i.e. presynaptic axon, cell-body, postsynaptic axon, etc.) has been made on a single criterion, e.g. latency. While this may suffice for many units, it is obviously desirable to improve the accuracy of identification by using as many criteria as possible. In connexion with our studies on the visual system we have found it essential to be able to distinguish between optic-tract axons, geniculate cells and radiation axons. The present paper is concerned with the criteria which we have found useful in this respect when recording extracellularly from central visual pathways. The criteria have been established for tract and radiation axons by recording directly from these pathways after having inserted the electrode under stereotaxic control. The information obtained in this way could then be applied to recordings obtained from the lateral geniculate nucleus (LGN) or its vicinity, situations in which a knowledge of the position of the electrode did not necessarily assist in the identification. In this way the three groups mentioned could be distinguished from one another. The possibility of other groups occurring will be discussed later. The wave form of the extracellularly recorded response, its attenuation with distance from the unit and other features are critically dependent on the type of electrode used; hence our findings in this respect cannot be applied indiscriminately to situations where the electrodes are of different shape, material or resistance. These points are discussed below. Probably for this reason, our results are not in entire agreement with those of previous investigators in this field. Some of the material presented here has appeared in earlier shorter communications (Bishop, Burke & Davis, 1958, 1959; Bishop, Burke, Davis & Hayhow, 1958). The antidromic