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Abstract

Accurately recording eye movements is essential to understanding how an animal moves its eyes to establish vision. Rodents are a commonly used as a model for the mammalian visual system, but it is not known how they move their eyes during free movement. We describe here a custom-built ocular videography system light enough to be carried_in combination with a head-mount two-photon microscope (Sawinski et al., 2009)on the head of a freely moving rat. Each camera, complete with mounting arm and infrared (IR) illumination weighs 1.8 g, with outer dimensions of the camera about 2.5×1×1 cm³. Rats comfortably carry 2 cameras, one recording the movements of each eye. The off-the-shelf monochrome camera chips (Aptina) are capable of recording 752×480 pixel images at a maximum frame rate of 60 Hz, and have a wide wavelength range which allows IR illumination. Using a 45° IR reflector that is transparent to visible light allows the cameras to be positioned in a way that minimizes disturbance to the animal’s visual field. The optics consist of a plano-convex lens (focal length f=9 mm) and a visible-light reflector. The lens is mounted in reverse orientation favoring a more planar image plane. The image size is 1.3×0.9 cm² at a working distance of about 1 cm. Inbuilt illumination from an IR LED (850nm) provides consistent image quality during normal exploratory behaviors and jumping. Image quality and resolution is good enough to identify the fine detail of the edge of the iris, which can be used for the detection of ocular torsion (rotation of the eye around the optical axis). Cabling is minimal, as the camera chip can be controlled with a two-wire serial interface and is able to transmit image data over a twisted pair using low-voltage differential signaling (LVDS). To reduce rotational stiffness we have built 2 m long custom cables by twisting enameled 50 µm dia. copper wires. While the wire resistance is less critical for LVDS signaling (even though the impedance is lower than required due to small wire separation) the voltage-level-wise sensitive two-wire serial communication required galvanic separation of the ground connection of the mobile cameras power supply and the external signal decoding board. The signals are then decoded on a custom-built decoding board using a standard LVDS deserializer (12bit) and an additional two-wire serial bus buffer. Signals are then transmitted via a USB interface. In combination with the miniature two-photon microscope, the eye-cameras are deployed in combination with a fully optical head-orientation detection system consisting of 6 IR LEDs mounted on the animal’s head with the miniaturized cameras, and a set of 4 external overhead cameras.