A Stereotaxic Modification for Retinal Neurophysiology

Kopf stereotaxic, thereby enabling it to be adapted for use in retinal physiology. A fundamental requirement for all visual physiology is an unobstructed visual field. This can be partially achieved using the standard optical investigation unit, in which the animal faces the rear of the instrument. However, in this configuration, the zero horizontal meridian is still somewhat obstructed. This can be remedied by using the Kopf optical investigation unit (875) in conjunction with the offset earbars and raised eyepieces (866), as shown in Fig. 1. The raised earbars place the zero horizontal meridian 35mm above the main support bars of the apparatus resulting in a visual field that is significantly less obstructed. The raised eyebars can be easily attached to the optical investigation assembly, and since they are also designed to raise the lower orbital margin by 35mm, the animal's head remains in stereotaxic orientation (even though proper stereotaxic orientation is not strictly required for retinal physiology, it is useful whenever it is necessary to place stimulating electrodes in central visual structures such as the optic tract, and visually important axes and planes of reference are commonly expressed in terms of standard Horsley-Clark planes (Bishop,_ Kozak and Vakkur, 1962)]. The mouth bar can also be inverted to ensure that the head is firmly held in the apparatus. It is convenient, when the animal faces the rear of the stereotaxic, to place the animal's body under the frame of the stereotaxic, with the body aligned at an angle of about 45 deg. to the sagittal plane of the skull. The height of the instrument can be easily adjusted to suit the size of the animal. In addition to greatly expanding the amount of unobstructed visual field, this arrangement has the advantage of making the eye itself freely accessible by raising it well above the main parts of the apparatus. The supports for the optical investigation unit can now be used to mount any additional apparatus that is required to support and position a recording electrode within the eye itself. Such devices are typically modified versions of the one originally described by Kuffler (1953). The particular device shown in Fig. 1 was designed and built by Dr. Jonathan Stone and Mr. David Solomons of the University of New South Wales, in Sydney, Australia, and incorporates the accumulated experiences and ideas of dozens of investigators from all over the world. It consists essentially of a 17gauge stainless steel hypodermic needle which can be inserted into the vitreous chamber of the eye and through which a microelectrode can be introduced (Fig. 2). A small brass ball, approximately 4mm in diameter has been fitted on to the needle and soldered in place about 5mm from its bevelled end.