Optics in Neuroscience

In recent years there has been a strong trend to enhance tra-ditional research techniques in neuroscience with optical-based techniques. The trend is evident on every level of neu-roscience research, spatially spanning submicron tocentimeters and temporally spanning submillisecond to sec-onds. Optical techniques are attractive because they enableresearchers to study brain function from the smallest func-tional unit, ion channels in small patches of excitable mem-branes, to its largest unit, monitoring the correlates of cogni-tive activity in the human brain. Another major advantage ofoptical imaging methods is that in most cases using light as aprobe enables the researcher to study the neural tissue withoutany direct contact; being noninvasive to the tissue is espe-cially important in in vivo studies. Finally, most optical re-search is performed with compact, mobile, and affordableequipment that can be readily integrated in existing researchfacilities.The aim of this special section is to give the readers a state-of-the-art report on the current status of optical techniques inneuroscience and their applications. For various reasons, thegamut of optical techniques could not be represented hereand this section has a heavier representation of techniques fornoninvasive monitoring of human brain activity. Nevertheless,we hope that the readers will find their interest covered bythese papers.The first paper is a review by Demuro and Parker on opti-cal single channel recording. This is an exciting example forthe growing superiority of optical techniques. Single channelrecordings have been dominated in recent decades by the‘‘patch-clamp’’ recording technique, a very successful tech-nique that revolutionized the study of single channels and wasacknowledged by a Noble prize to its inventors. However,patch-clamp recording has its limitations: it can only probeone channel at a time, and the pipette used for recording hasto create a strong seal around the membrane, which can dam-age the probed tissue. Demuro and Parker demonstrate thatone can record from a channel (albeit with weaker temporalresolution) by using optical recording techniques withoutdamaging the tissue. Furthermore, their optical techniques en-able them to record from up to hundreds of channels simulta-neously, enabling the study of the spatiotemporal characteris-tics of many channels. In other words, optical techniquesenable research to move from a single channel focus to apopulation focus, providing obvious advantages.Electrodes are typically used to either record or stimulatethe neuronal tissue, and the move from electrodes or pipettesto optical methods is also evident in the next paper. Ko¨tteret al. describe how optical techniques can be successfullyused for precise stimulation of cortical tissue by using light torelease the caged neurotransmitter glutamate and by measur-ing the effects of the release within cortical networks. Theauthors use a combination of experimental results and mod-eling to highlight the advantages of this technique for eluci-dating functional networks in the cortex.Guiou et al., using a combination of intrinsic signal opticalimaging and field-potential measurements, report on the dis-ruption in the process of neurovascular coupling and cerebralblood volume related to the phenomenon of cortical spread-ing depression—a pronounced depolarization of neurons andglia that spreads slowly across the cortex followed by a periodof depressed electrophysiological activity. The use of intrinsicsignal optical imaging—an optical method that is based onmeasuring activity-induced reflection changes from the illumi-nated brain—enables the researchers not only to image pat-terns of cortical activity in vivowith high spatial and temporalresolutions, but also with the use of the right filters to extractadditional results on cerebral blood volume changes and neu-rovascular coupling. Quantification of the blood volume andhemoglobin concentration changes requires accurate knowl-edge of the photon path length through the cortical tissue.Yokoyama et al. describe a novel approach for estimating thespectral dependence of this path length factor from the tem-poral variance of the spectral measurements. As discussed byOkui and Okada, inaccurate knowledge of these path lengthfactors can cause cross-talk in the estimate of the hemoglobinconcentrations. These works are important steps towards ob-taining greater quantitative accuracy in the estimation of thephysiological parameters.The next two papers examine the use of novel technologiesfor diagnosing the diseased brain. Bizheva et al. utilize ultra-high-resolution optical coherence tomography to characterizethe signatures of healthy and diseased brain tissue. Their ini-tial results in ex vivotissue samples demonstrates that opticalcoherence tomography has the ability to visualize and identifymorphological features such as microcalcifications, enlargednuclei of tumor cells, small cysts, and blood vessels. This tech-nology could ultimately prove useful during brain surgery toidentify tumor boundaries for conservative resection. Skochet al. discuss the development of near-infrared fluorescenceimaging for detecting amyloid-bpeptide in the brain associ-ated with Alzheimer’s disease. The advancement of such amolecular imaging approach would serve an important role inthe diagnosis of Alzhemier’s disease as well as guide treat-ment.The remaining seven papers address the use of near-infrared spectroscopy (NIRS) for measuring brain function andcerebral oxygenation utilizing continuous-wave, frequency-domain, and time-domain methods. Gratton et al. provide areview of their recent results in noninvasively measuring neu-ronal signals and the resultant hemodynamic response. Theyconfirm the cerebral origin of the hemodynamic response withsimultaneous optical and function magnetic resonance imag-ing (fMRI).An exciting application for NIRS of brain function is in thestudy of the developing infant brain, since this population isnot easily studied with fMRI or positron emission tomography(PET). In addition, NIRS could complement electricallyevoked responses measured by electro-encephalography(EEG) that are currently being used to study the developingbrain. NIRS could give access to the hemodynamic responseto a stimulus and enable the design of behavioral experimen-tal paradigms that do not lend themselves well to EEG. Thepros and cons of NIRS for studying brain correlates of percep-tual, cognitive, and language development in human infants isreviewed by Aslin and Mehler. Wilcox et al. illustrate the po-tential for NIRS in this field of research by measuring hemo-dynamic responses associated with an infant’s ability to differ-entiate objects of different shapes and colors through theJournal of Biomedical Optics