C. Example Result
XIII. Summary and Future Directions
showed complex response specificities to odorants of differ- ent functional groups and molecular size. Maps of receptor neuron input were chemotopically organized at near-thresh- old concentrations but, at moderate concentrations, they involved many widely distributed glomeruli (Fig. 11D).
These results suggest a high degree of complexity in odorant representations at the level of input to the olfactory bulb.
XII. Intrinsic Imaging and Fluorescence
embryonic chick and lamprey spinal cords (Tsau et al., 1996). An identified cell class (motoneurons) was selectively stained. While spike signals from individual neurons were sometimes measured in lamprey experiments, further efforts at optimizing this staining procedure are needed. The second approach is based on the use of cell-type-specific staining developed for fluorescein by Nirenberg and Cepko (1993). It might be possible to use similar techniques to selectively stain cells with voltage-sensitive or ion-sensitive dyes. Third, Siegel and Isacoff (1997) constructed a genetically encoded combination of a potassium channel and green fluorescent protein. When introduced into a frog oocyte, this molecule had a (relatively slow) voltage-dependent signal with a frac- tional fluorescence change of 5%. More recently, Sakai et al.
(2001) and Ataka and Pieribone (2001) have developed similar constructs with very rapid kinetics. Neuron-type- specific staining would make it possible to determine the role of specific neuron types in generating the input–output func- tion of a brain region.
Optical recordings already provide unique insights into brain activity and organization. Clearly, improvements in sensitivity or selectivity would make these methods more powerful.
Acknowledgments
The authors are indebted to their collaborators Vicencio Davila, Amiram Grinvald, Kohtaro Kamino, David Kleinfeld, Les Loew, Bill Ross, Guy Salama, Brian Salzberg, Alan Waggoner, Jian-young Wu, Joe Wuskell, and Dejan Zecevic for numerous discussions about optical methods. This work was supported by NIH Grant NS08437-DC05259, NSF Grant IBN- 9812301, a Brown-Coxe fellowship from the Yale University School of Medicine, and an NRSA fellowship, DC 00378.
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5
Optical Imaging Based on Intrinsic Signals
Nader Pouratian and Arthur W. Toga
Laboratory of Neuro Imaging, UCLA Department of Neurology, Los Angeles, California 90095-1769
I. Introduction
Several functional brain mapping techniques have been developed over the past 3 decades which have revolution- ized our ability to map activity in the living brain, including positron emission tomography (PET), functional magnetic resonance imaging (f MRI), optical imaging, and, more recently, near-infrared spectroscopy (NIRS) and transcranial magnetic stimulation (all are discussed in other chapters of this book). Each modality offers distinct information about functional brain activity and has certain advantages and limitations. In choosing a functional imaging modality for experiments, one should consider a modality’s spatial and temporal resolution, the etiology of its brain mapping signal, the practicality of the imaging methodology, as well as the cost of implementation. In this chapter, the methodological details of optical imaging of intrinsic signals are explored, with special attention to the considerations listed above.
Optical imaging of intrinsic signals maps the brain by measuring intrinsic activity-related changes in tissue reflect- ance. Functional physiological changes, such as increases in blood volume, hemoglobin oxymetry changes, and light scattering changes, result in intrinsic tissue reflectance
97 I. Introduction
II. Sources of Intrinsic Signals and Wavelength Dependency
III. Preparation of an Animal for Optical Imaging
IV. The Apparatus V. Data Acquisition
VI. Data Analysis for Mapping Functional Architecture
VII. Chronic Optical Imaging
VIII. Optical Imaging of the Human Neocortex IX. Combining Optical Imaging with Other
Techniques X. Applications
XI. Comparison of Intrinsic Optical Imaging with Other Imaging Techniques
XII. Conclusions and Outlook References
▼ ▼
Brain Mapping: The Methods, 2nd edition
Copyright © 2002 by Elsevier Science (USA) All rights reserved.
changes that are exploited to map functional brain activity.
This offers a distinct advantage over extrinsic signal imaging, such as dye imaging, which may cause phototoxi- city, especially in in vivo preparations, and thereby alter the normal physiology of the sample. It is unclear how normal physiology may be affected by the addition of dyes and radioisotopes or electrode insertion. By not requiring any contact with the tissue of interest whatsoever, optical imaging of intrinsic signals is ideally suited to studying chronic preparations, in which an investigator may wish to image a sample over a period of days, weeks, or months, and for intraoperative mapping of the human cortex during neurosurgery (Mazziotta et al., 2000).
Although activity-related intrinsic optical changes in tissue reflectance associated with electrical activity or metabolism were first observed over 50 years ago (Hill and Keynes, 1949), it was not until the 1980s that these intrinsic optical changes were used to map cortical activity in vivo (Grinvald et al., 1986). Since this initial report, intrinsic optical changes have been reported in rodents (Masino et al., 1993; Narayan et al., 1994), cats (Frostig et al., 1990;
Bonhoeffer and Grinvald, 1991), monkeys (Ts’o et al., 1990; Grinvald et al., 1991), and humans (Haglund et al., 1992; Toga et al., 1995a). The increasing popularity of optical imaging of intrinsic signals is largely because this technique offers both high spatial and high temporal resolu- tion simultaneously. The spatial resolution of intrinsic imaging is unparalleled among in vivo imaging techniques (on the order of micrometers), making it ideal for studying the fine functional organization of sensory cortices as well as the physiology of neurovascular coupling at the level of the arteriole, venule, and even capillaries. Although the tem- poral resolution of optical imaging is not as great as with electrophysiological techniques, imaging is commonly performed at video frame rates (30 Hz). This is more than sufficient for imaging the slowly evolving perfusion-related responses, which peak 3 to 4 s after stimulus onset.
Because of these advantages, the number of studies using optical imaging of intrinsic signals has been growing rapidly (especially now that optical imaging systems are commer- cially available). This chapter provides a detailed methodo- logy for investigators to design their own imaging system, with special attention to the limitations of certain approaches and different strategies that have been devised by various groups to overcome them. Understanding the various limita- tions and strategies will give investigators greater versatility in designing their systems and experiments and avoid making the commercially available optical imaging systems “black boxes” that merely produce functional maps.
This chapter surveys a wide array of optical imaging techniques and applications, including discussions of how optical spectroscopy has significantly advanced our under- standing of intrinsic signal etiology, advantages and disad- vantages of the different species used for optical imaging,
different approaches to cortical immobilization, advances in detector technology, recent advances in both single-wave- length and spectroscopic analysis, baseline vasomotion and how it complicates data analysis, recent advances in optical imaging in humans, and integrating optical imaging with other functional imaging techniques to better understand the etiology of functional brain mapping signals.