How does spatially focused neural stimulation affect the activity of local and distant neurons, possibly leading to alterations in behavior and changes in disease states? This question not only encompasses a fundamental concern in the field of neural circuit dynamics, but is also central to the quest for innovative treatments of neurological disease, rooted in better scientific understanding of the mechanisms of neural function and dysfunction. The ability to answer this and other related questions requires tools enabling selective neural stimulation coupled with simultaneous high-resolution spatiotemporal recording.
Neural stimulation by means of injecting electrical current through brain tissue has been a powerful tool in electrophysiology [17
] and clinical neuroscience [33
], in spite of the uncertainties associated with complex current paths, non-selective depolarization of axons, dendrites and neuronal cell bodies by the complex current pathways [10
]. Direct optical stimulation of neural cells in brain tissue genetically modified by expressing channelrhodopsin-2 (ChR2) has recently been achieved [11
] and applied in both in vitro
and in vivo
]. This new kind of ‘optogenetic’ stimulation technique can genetically be customized to target specific types of neurons with sub-millisecond temporal precision [2
]. Various light delivery schemes have been reported, to match the excitation spectrum of ChR2 in the blue, including band-filtered white light [11
], light emitting diodes (LED) [12
] or laser-coupled optics [39
] for in vitro
applications, as well as direct epicranial LED implantation [29
] for in vivo
applications. In broader terms of tools that have been applied by using optical fiber based endoscopic technologies in neuroscience, these have been so far mostly employed for structural imaging such as light delivery for fluorescence or 2-photon imaging [20
] and functional imaging such as optical coupling between light sources and photo-detectors in functional optical coherence tomography (fOCT) [31
]. The recent development of optogenetics methodology has opened avenues for optical fiber based neuronal stimulation including in vivo
]. So far, the spatial distribution of the stimulating light using these delivery schemes at the target brain tissue has been limited by the divergence of the light source itself or the numerical apertures of the optical instruments, further ‘blurred’ by scattering intrinsic to the tissue. Moreover, recording of the evoked neural activity has been limited to single patch or EEG/EMG [2
] recordings. A mechanically simple construct of an optical fiber glued to an extracellular electrode has recently been reported [23
] to monitor neuronal activity evoked by optical stimulation in ChR2 transfected mice. Elsewhere, a variety of techniques has been developed to record from a large population of neurons with high spatiotemporal resolution, ranging from multi-electrode arrays [25
] to optical imaging methods [24
]. The multi-electrode arrays do not easily allow for specific stimulation of neural electrical activity due to the presence of electrical artifacts etc. Meanwhile, the optical imaging methods such as intrinsic optical signal detection are technically challenging for detailed tracking of spiking activity in local intra-cortical microcircuits.
A paradigm case of extended neural circuit phenomena where space- and time-dependent excitation/recording is important is epileptic activity. Epileptiform activity in disinhibited brain tissue is characterized by synchronized population firing, and occurs predominantly in parts of the brain with high degrees of recurrent excitatory circuitry, such as the neocortex and hippocampus [52
]. The initiation and propagation of such activity is fairly well understood with a simple neuronal network model, utilizing the pattern of connectivity among these neural populations [49
]. One-dimensional recordings of the epileptiform wave in neocortical slices have been demonstrated previously by linear microwire arrays [14
], revealing the patterns of wave initiation and propagation across the columnar and laminar architectures in neocortex. It is thus interesting to study the two-dimensional or even three-dimensional spatiotemporal propagation patterns, including those under pharmacologically induced seizure conditions. Calcium imaging has been used to record single neuron activities but with a limited neural circuit recording area [5
]. In this paper we use the two-dimensional spatiotemporal propagation of epileptiform activity as an example of the application of a newly developed set of tools.
Below we describe a tapered coaxial optical waveguide construct, dubbed as the ‘optrode’, which, after a range of testing and characterization, is embedded within a 100-element ‘Utah’ intra-cortical multi-electrode recording array (MEA) [13
]. This dual-modality, hybrid, optrode–MEA device is capable of locally delivering light stimuli to neural tissue while simultaneously multisite recording extracellular activities from an area of approximately 4.0 mm × 4.0 mm around the stimulation focus (these are present dimensions—with extended MEA and optrode fabrication techniques any combinations and many geometries are possible). The optrode itself is a dual-device element providing simultaneous light delivery and electrical recording capabilities. As described below, we first tested the single optrode as a stand-alone unit in the intrinsically photosensitive mouse retina. Green laser light was delivered through the optrode to trigger retinal photoreceptors, and the light-induced change of spike activity was recorded from retinal ganglion cells via the integrated electrical pathway of the optrode. In a different validation and evaluation step, mouse brain slices with cortical neurons rendered light sensitive by viral transfection via ChR2 also showed direct blue light-triggered action potentials upon being stimulated by the stand-alone optrode. Next, and most important to this paper, the optrode–MEA dual function array device was used to study the 2D spatiotemporal propagation of optically induced epileptic waves in disinhibited ChR2 mouse cortical slices. Epileptic waves with robust propagation patterns were demonstrated.