Neurophotonics

Optical imaging methods have historically been used to microscopically visualize neurons and neural tissue, and advances in diffuse optical tomography have enabled non-invasive tomographic imaging of the human brain.  With newly available methods in the expanding field of optogenetics, it is now possible to confer optical control of cell function using a variety of natural and genetically altered photoactivatable membrane ion channels, such as the well-known channelrhodopsins from the algal species Chlamydomonas reinhardtii.  In living organisms, visual systems play an essential role in survival, and many animals have not only developed elegant and sophisticated solutions to detect light and images, but have also evolved new ways of producing light, varying their optical appearance and signatures, and neurologically controlling the optical properties, pigmentation patterns, and texture of their skin. 

Neurophotonics is the science of how light interacts with or is utilized in natural and genetically modified neurological systems, across the scales of molecules, neurons, neural circuits, the brain, and living organisms.  While the majority of the research in neurophotonics has been driven by neuroscientists, we believe fundamental principles can be discovered and practical technologies can be developed by applying biophotonics, biomedical optics, laser technology, and optical science and engineering to the area of neurophotonics.  We are leveraging our expertise and technologies in the Biophotonics Imaging Laboratory to observe and study these systems using a different approach.

Prof. Boppart’s early graduate work involved the development of a flexible perforated microelectrode array for extended neural recordings from cell cultures, brain slices, retinal tissue cultures, and the in vivo cortex.  By microfabricating these arrays on flexible polymer substrates, the arrays can conform to convoluted tissue surfaces for optimal contact, while still maintaining their electrical properties for measurements of electrical activity.  By etching perforations in the array, fluid flow through the array extended the viability of the cells or tissue, enabling long-term recordings.

A flexible perforated microfabricated multielectrode array used for extended neural recordings from neuron cultures, brain slices, retinal tissue cultures, and the cortex.

A flexible perforated microfabricated multielectrode array used for extended neural recordings from neuron cultures, brain slices, retinal tissue cultures, and the cortex. 

Boppart SA, Wheeler BC, Wallace CS. A flexible perforated microelectrode array for extended neural recordings. IEEE Trans Biomed Engr 39(1):37-42, 1992.

 

A significant limitation of single-point micropipette electrodes, needle-based electrodes, and multi-electrode arrays is the limited number of sites and density from which electrical measurements can be made.  In addition, each electrode requires a separate pre-amplifier, amplifier, and signal conditioning and digitization hardware, making simultaneous or multiplexed measurements problematic.  A possible solution to this is to noninvasively record optical changes in neuron cultures or neural tissue that correspond to electrical activity.  While solutions exist with voltage-sensitive dyes or calcium level indicators, these frequently require to use of exogenous agents that limit the viability of the cells and tissues, and less attractive for use in vivo

Early work in the Biophotonics Imaging Lab first discovered that optical coherence tomography (OCT) was sensitive enough to detect subtle changes in the intrinsic optical scattering properties of neurons and nerves when they were electrically active, without the use of any exogenous agents or dyes.  This led to subsequent work in single neurons from the abdominal ganglion of Aplysia, where direct correlations were made between transient optical scattering changes and action potentials.  Subsequent work by other groups have shown this effect in the living brain, as well as in the retina.  While these intrinsic optical scattering changes are weak, phase-resolved interferometric detection using OCT offers the potential to replace electrophysiological recording methods with optophysiological recording methods across larger spatial scales, with higher temporal resolution, and simultaneously across wide fields-of-view.

Optical coherence tomography of the Aplysia abdominal ganglion showing dynamic depth-resolved changes in optical scattering that correlate to electrical activitiy.  (A) Photograph of exposed ganglion.  (B,D) Cross-sectional OCT images along the two red lines indicated in (A).  (C) M-mode image (fixed depth scan over time) through neuron 2 in (B, red line) showing transient scattering changes at various depths. 

Lazebnik M, Marks DL, Potgieter K, Gillette R, Boppart SA. Functional optical coherence tomography for detecting neural activity through scattering changes. Optics Letters, 28(14):1218-1220, 2003.

 

Transient optical scattering changes correlated to electrical activity in a single Aplysia neuron.  (Left) En face optical coherence microscopy based on intrinsic optical scattering properties in this single cell.  (Right) Optical scattering changes occurring over time at the site indicated by the red arrow.  The transient scattering intensity correlates with the electrical activity indicated by the blue trace.

Graf BW, Ralston TS, Ko H-J, Boppart SA. Detecting intrinsic scattering changes correlated to neuron action potentials using optical coherence imaging. Optics Express, 17(16):13447-13457, 2009.

 

Cuttlefish are amazing marine animals that are technically not fish.  They are cephalopods, in the same class as squid, octopuses, and nautiluses.  They are often called the chameleons of the sea because of their remarkable ability to dynamically alter the optical properties, patterns, and texture of their skin for camouflage, for defense, and to communicate.  A complex neurobiological control system is used in these animals, from the visual input of their surroundings to the precise neurological control of their skin, and understanding the biological structures, their dynamic optical properties, and their biomechanics is a scientific research area for our lab.  By considering the optical science and engineering behind these biological structures, we may learn how to design better visual displays, and even those with controllable texture for tactile interactions and feedback. 

We have developed an advanced and highly monitored marine environment for maintaining and investigating Sepia bandensis, a dwarf species of cuttlefish found naturally in the shallow coastal waters for the Philippines and Indonesia.  We have also utilized new optical imaging technology to visualize the optical properties of their skin, under both static and dynamic conditions.

Cuttlefish Passing Cloud Effect (video file, 26.8 MB).

 

Photographs of the cuttlefish Sepia bandensis in our marine environment.  By understanding their control over the optical properties, patterns, and textures of their skin, we can engineer new and better displays, optical sensors, and tactile interfaces.  (Left) Optical measurements from a free-swimming cuttlefish.  (Right) Cuttlefish over bottom features with different colors, patterns, and textures, and their varying responses on their skin.

Advanced integrated multimodal microscopy of cuttlefish skin revealing the intricate array of chromatophores (cells) and collagen that are used to regulate the optical properties, patterns, and textures of their skin.  TPEF: Two-photon excited fluorescence microscopy of autofluorescent cells.  OCM: Optical coherence microscopy of the intrinsic scattering properties of cells.  SHG: Second harmonic generation microscopy showing the collagen fiber network used to dynamically alter the textural features on the skin.  Composite image shows all three modalities overlaid, revealing their spatial relationship.  All images were captured simultaneously using our integrated microscope.

Graf BW, Chaney EJ, Marjanovic M, Adie SG, De Lisio M, Valero MC, Boppart MD, Boppart SA.  Long-term time-lapse multimodal intravital imaging of regeneration and bone-marrow-derived cell dynamics in skin.  Technology, 1:1-12, DOI:10.1142/S2339547813500027, 2013.

 

The Biophotonics Imaging Laboratory is also conducting research in the rapidly expanding field of optogenetics, but from the optical science and engineering perspective.  The ability to optically control not only the firing of a neuron, but also the feedback and dynamics of larger circuits of neurons in culture, in brain slices, or in vivo, is likely to have a significant impact on our understanding of the complex functions and systems-level control in the brain and our visual system.