Eye-movement recording in freely moving animals Fernando Rodrõ Âguez a, *, Cosme Salas a , Juan Pedro Vargas a , Blas Torres b a Laboratorio de Psicobiologõ Âa, Facultad de Psicologõ Âa, Universidad de Sevilla, Avda. San Francisco Javier, s/n, 41005 Sevilla, Spain b Laboratorio de Neurobiologõ Âa de Vertebrados, Facultad de Biologõ Âa, Universidad de Sevilla, Avda. Reina Mercedes, 6, 41012 Sevilla, Spain Received 10 September 1999; received in revised form 1 May 2000; accepted 5 June 2000 Abstract A new method is described for precise recording of eye movements in freely moving animals using Hall-effect devices. This inexpensive system, of small size and low weight, allows the analysis of horizontal and vertical components of saccadic eye movements, optokinetic nystagmus, slow tracking movements, eye vergence, etc., in unrestrained animals. A set of Hall-effect devices mounted in the skull is used to sense variations in the position of high-power miniature magnets fixed to the eye sclera. The output of the Hall-effect devices is amplified by operational amplifiers and collected through an analog-to-digital converter to be displayed on-line in a personal computer and stored for later analysis by specific software. Some examples of simultaneous body- and eye-movement recordings obtained in freely moving goldfish in different experimental situations are presented. This method would be useful in the recording of eye and gaze movements under natural conditions and for behavioural studies in freely moving animals. D 2001 Elsevier Science Inc. All rights reserved. Keywords: Eye-movement recording; Hall-effect device; Freely moving animals; Goldfish 1. Introduction The study of the oculomotor system is a major issue in different neurobiological, psychobiological, and psycholo- gical areas, both experimentally and clinically. Various methods are available to record eye movements in restrained animals [9,12,16,20], but these methods present disadvan- tages Ð or cannot be used at all Ð in recording eye movements in freely moving animals. For example, the scleral search coil technique [12], one of the most wide- spread methods for precise recording of eye movements, requires the animal to be restrained in the centre of the magnetic field generated by the coil frame. The electro- oculographic method [19,20] can be used to record eye movement in freely moving animals, but presents limita- tions, such as low stability and contamination by non-ocular events. The present paper describes an inexpensive system for recording eye movements in unrestrained animals using Hall-effect sensors. The Hall effect consists of the genera- tion of a voltage across an electrical conductor carrying current when it is placed in a magnetic field. Thus, Hall- effect sensors produce an output voltage that changes in proportion to the intensity of the surrounding magnetic field. Hall-effect transducers have been proven useful in different applications, such as accelerometers [8], head- [10] or jaw- [1] movement recorders, and eye-lid closure [3,4]. In the present application, a small magnet attached to the eye sclera provides the magnetic field. The system records the horizontal and vertical components of eye movements such as saccades, optokinetic nystagmus, slow tracking move- ments, eye vergence, etc., in unrestrained animals. It allows a degree of precision, resolution, and dynamic range similar to that of the most precise and widespread methods used [16], while having small size and low weight. The system described herein allows the recording of eye movements during simultaneous recording of neural activity or brain stimulation because it does not produce electric interference with electrophysiological equipment. Moreover, the ani- mal's position and displacement in space can be recorded simultaneously by the synchronization of the eye-movement recording system with a video-tracking or high-speed cine- matography system. The present eye-movement recording system has been used in our laboratory in conjunction with a video-tracking system to record simultaneously eye and body movements elicited by intracranial electrical micro- stimulation in unrestrained goldfish [13]. The extension of * Corresponding author. Fax: +34-95-4557752. E-mail address: fernanr@cica.es (F. Rodrõ Âguez). 0031-9384/01/$ ± see front matter D 2001 Elsevier Science Inc. All rights reserved. PII:S0031-9384(00)00314-0 Physiology & Behavior 72 (2001) 455 ± 460