  The mammalian brain is a densely interconnected network that consists of
millions to billions of neurons. Decoding how information is represented and
processed by this neural circuitry requires the ability to capture and
manipulate the dynamics of large populations at high speed and resolution over
a large area of the brain. While there has been a rapid increase in use of
optical approaches in the neuroscience community over the last two decades,
most microscopy approaches lack the ability to record the activity of all
neurons comprising a functional network across the mammalian brain at relevant
temporal and spatial resolution. In this review, we survey the recent
development in the optical calcium imaging technologies in this regard and
provide an overview of the strengths and limitations of each modality and their
potential for scalability. We provide a guidance from a biological user
perspective that is driven by the typical biological applications and sample
conditions. We also discuss the potential for future advances and synergies
that could be obtained through hybrid approaches or other modalities.
