March 20, 2018
One of the central questions in neuroscience is how neural activity creates diverse brain functions and resultant behaviours. To answer this question, neural activity must be recorded during behaviour, to establish its causal role. This is covered in more detail in the first chapter of this Fundamentals series.
Each subsequent chapter will focus on a different method used for optical recordings and stimulation, where we will briefly address the following topics:
- Introduce the basic principles behind the method.
- List some of its main benefits and restrictions (with reference back to the challenges mentioned in the first chapter).
- Refer to existing commercially available systems and
- Look at the latest developments, and possible future trends.
Where the second chapter discussed fiber photometry, this third chapter will focus on a variation on this method – the optical fiberscope.
Optical fiberscopes are a variation of the basic fiber photometry technique, with more complex optical components.
The main difference between the two techniques is that fiberscopes work by utilizing fiber-optic bundles, which consist of up to 100,000 individual micro-fibers (~3µm diameter each) in a closely packed arrangement, enable imaging with spatial resolution. These bundles act as illumination and detection channels (see existing technologies section for more details), and the combination of the bundles on the detection path enables the user to discern spatial information.
Fiber-optic imaging/recording systems- How an “image” is formed
Because fiber-optic bundles do not produce a focal plane, they mainly collect light at lower levels, where tissues may be damaged. This problem can be solved with a graded index (GRIN) lens (Figure 1; 1), a cylinder like an optical fiber that creates a focal-plane with its radial or axial variation in a refraction index while a general lens produces a focal-plane depending on its curvature (Figure 1A). The combined use of different-NA GRIN lenses can up- or down-scale images (Figure 1B). By connecting the GRIN lenses and fiber-optic bundles, this image is transmitted to the objective lenses (Figure 1C).
Figure 1. GRIN lenses. (A) The numerical aperture (NA) of the GRIN lens changes in a radial direction. The light path changes gradually along the GRIN lens to create a focal plane. (B) The magnification rate is defined by M = D1/D2 = H1/H2. D1: distance between the image and lens 1. D2: distance between the object and lens 2. For conventional lenses, the combination of low and high NA GRIN lenses can change the magnification ratio. (C) A bundle-fiber transmits the image produced by the GRIN lenses. (D) Light from a single-core fiber can be condensed by the GRIN lenses.
1. Benefits of Fiberscopes
Since fiberscopes are a variation of fiber photometry, many of the benefits and restrictions of the two techniques can be merged (see the second chapter for more details). However, there are some key benefits of fiberscopes, in addition to the benefits of fiber photometry:
1.1. Modular, Scalable & Reconfigurable
This important feature allows the researcher to ask different biological questions and adjust the focus of the project, with relatively minor modifications of the system. For instance:
- Simultaneous calcium imaging and optogenetic manipulation within a specific brain region can be executed with an optical fiberscope; all while the animal is freely behaving.
- Reproducible imaging of deep brain regions can be accessed without being overly invasive and at the same time, because the imaging and light is delivered through a fiber, which is very light (~0.7g), the length of behavioural experiments can be extended, allowing for natural animal behaviour to be observed and analysed. The end of the fiber can be split to record simultaneously from different brain areas or from two animals at the same time.
- One can use different light sources with different wavelengths and insert different optical filters into the light paths. This allows for dual-colour imaging to be performed.
- The fiberscope is compatible with high-end scientific cameras (including Cameras fast and sensitive enough to use for detecting GEVIs), allowing for very high signal-to-noise ratios
1.2. Implant Weight
Both the imaging and stimulation are delivered through a flexible fiber, and the weight of the head-mounted fixture is low (~0.7g).
1.3. Targeted Optogenetic Manipulation
Individual micro-fibers within the imaging fiber can be independently activated by light, offering precise single-cell targeting (when combined with Mightex’s Polygon400 spatial illuminator) and/or widefield optogenetic stimulation.
2. Restrictions of Fiberscopes
2.1. Spatial Resolution
Fiber bundles have limited spatial resolution compared to two photon imaging, because it is dictated by the inter-core spacing within the bundle. The inter-core spacing is in turn limited by the need to minimise cross talk. Therefore, there is a trade-off between the diameter of the bundle and the number of pixels in the image.
2.2. Spectral Cross-Talk
Light for photostimulation may disrupt Ca2+ imaging data because the stimulation wavelength slightly overlaps with the excitation wavelength for imaging.
2.3. Commercial Options
One company manufacturing such fiberscopes for life sciences is Mightex’s OASIS implant, which allows simultaneous imaging and photo-manipulation of neurons within the region of interest in freely behaving animals. The fiber, when coupled to a GRIN lens, provides the researcher with both single-cell resolution imaging capabilities as well as the option to target individual neurons with light.
Within the clinical setting, Mauna Kea Technologies manufactures the Cellvizio, which is a probed-based confocal laser endomicroscope that can take images and biopsies within deep tissue of the human body.
Fiberscopes typically have separate illumination and imaging channels. However, to increase resolution and number of pixels, without impacting on the overall size, a single bundle can be used for both illumination and detection. The illumination channel is typically coupled to a bright external white-light source and or Mightex’s Polygon400 spatial illuminator. In this configuration, the researcher can address each micro-fiber individually using commercially available software, in order to manipulate the region of interest with light.
3. Latest & Potential Future Developments in this Technnology
Fiber-optic imaging has recently undergone great advancements in terms of its spatial resolution for visualizing single cells (2, 3). However, devices still have room for improvement in terms of their ability to image single dendrites or axons in freely moving animals during naturalistic behaviour.
Apart from hardware improvements, including the fibers themselves, other areas for enhancement include improving genetically encoded fluorescent indicators for faster response times, inducing brighter fluorescence, thus creating higher signal-to-noise ratios.
Therefore, at present, fiber-optic imaging/recording and manipulation systems are useful for revealing the relationship between behaviours and neural activity across various brain regions. We expect further developments in cellular resolution or sub-cellular (e.g., dendrites or axons) illumination methods to better understand their respective roles within single cells or local circuits during behaviour.