Development of a minimally-invasive magnetic system for high-quality wireless eye movement tracking in non-human primates

眼睛位置的精确跟踪在大脑功能的各种感觉、认知和运动控制研究中是至关重要的。在人类和非人类灵长类动物中，现在的大多数实验都依赖于基于视频的非侵入性眼动仪，在过去的几十年里，这种仪器的性能有了显著的提高。然而，对于微小的注视眼动，在认知和重新格式化进入视觉系统的图像的时空统计中发挥重要作用，基于视频的眼动仪仍然缺乏。例如，对于在扫视和微扫视之间连续发生的漂移性眼球运动，眼睛的位置变化约为1分弧度。像瞳孔直径变化（由于光线强度的变化或内部状态的变化）这样简单的因素，使得依靠瞳孔图像的追踪器无法测量如此小的眼球运动。这使得人类对眼球漂移的研究变得罕见。在非人类灵长类动物中，这是我们这里的重点，眼球追踪的黄金标准仍然是巩膜搜索线圈技术，在上个世纪中叶首次被描述。虽然这种技术适用于研究最微小的眼球运动，但它是一种高度侵入性的技术，需要在眼眶周围植入一圈金属丝，将这种金属丝在皮肤下穿过几厘米，与头骨上的连接器连接，并在连接器位置的头部皮肤上形成慢性“开放性伤口”。除了手术本身的困难之外，电线断裂的可能性（由于每天成千上万的弹道眼运动）和感染（由于连接处的慢性开放性伤口）导致需要进行额外的再植入手术。我们的目标是为非人类灵长类动物开发一种微创眼动追踪系统。这项技术利用无线感应随眼睛旋转的磁场，以前曾在小动物（如啮齿动物）身上得到过证明。它是微创的，因为它只需要一次植入手术，在眼眶周围植入微小的磁铁来产生磁场。一次性手术比巩膜搜索线圈所需的植入手术简单得多。通过广泛的理论和模拟初步实验，我们表明接近巩膜搜索线圈的质量是可能的。我们提出的研究的目的是证明成功的体内性能，并将其与眼动追踪光谱的两端进行详尽的基准测试：基于视频的和侵入式搜索线圈。我们的方法比基于视频和搜索线圈的技术都要便宜得多，我们将尽快将这种方法开放给更广泛的非人类灵长类动物群体。

Precise tracking of eye position is critical in a great variety of sensory, cognitive, and motor control investigations of brain function. In human and non-human primates, the majority of experiments nowadays rely on non-invasive video-based eye trackers, which have experienced significant improvements in performance during the last few decades. However, for tiny fixational eye movements, which are emerging to play important roles in cognition and in reformatting of the spatio-temporal statistics of images entering into the visual system, video-based eye trackers are still lacking. For example, for drift eye movements that continuously occur in between saccades and microsaccades, the position variation of the eye is on the order of approximately 1 minute of arc. Factors as simple as pupil diameter variations (due to varying light intensities or changes in internal state) make such small scales of eye movements simply unmeasurable with trackers that rely on pupil images. This has rendered human investigations of ocular drifts rare. In non-human primates, which constitute our focus here, the gold standard for eye tracking remains the scleral search coil technique, first described in the middle of the previous century. While this technique is suitable for studying even the tiniest of fixational eye movements, it is a highly invasive technique that requires surgical implantation of a coil of wire around the eye orbit, threading of such wire under the skin for several centimeters to a connector on the skull, and a chronic “open wound” in the skin of the head at the site of the connector. Besides the difficulty of the surgical procedure itself, potentials for wire breakage (due to hundreds of thousands of ballistic eye movements every day) and infections (due to a chronic open wound at the connector) result in needs for additional re-implantation surgeries. Our goal here is to develop a minimally-invasive eye tracking system for non-human primates. The technique uses wireless sensing of magnetic fields that rotate with the eye, and it has been demonstrated previously for small animals (e.g. rodents). It is minimally-invasive because it requires only a single implantation surgery, to implant tiny magnets around the eye orbit for creating the magnetic field. The 1-time surgery is much simpler than the implant surgery needed for scleral search coils. Using extensive theoretical and simulation preliminary experiments, we show that approaching the quality of scleral search coils is possible with this proposed technique. Our purpose in the proposed research is to demonstrate successful in vivo performance, and to exhaustively benchmark it against the two ends of the spectra of eye tracking: video-based and invasive search coils. We will make our method, which is much cheaper than both video-based and search coil techniques, openly available to the wider non-human primate community as soon as possible.