  Electrons and photons offer complementary strengths for information
processing. Photons are excellent for communication, while electrons are
superior for computation and memory. Cognition requires distributed computation
to be communicated across the system for information integration. We present
reasoning from neuroscience, network theory, and device physics supporting the
conjecture that large-scale cognitive systems will benefit from electronic
devices performing synaptic, dendritic, and neuronal information processing
operating in conjunction with photonic communication. On the chip scale,
integrated dielectric waveguides enable fan-out to thousands of connections. On
the system scale, fiber and free-space optics can be employed. The largest
cognitive systems will be limited by the distance light can travel during the
period of a network oscillation. We calculate that optoelectronic networks the
area of a large data center ($10^5$\,m$^2$) will be capable of system-wide
information integration at $1$\,MHz. At frequencies of cortex-wide integration
in the human brain ($4$\,Hz, theta band), optoelectronic systems could
integrate information across the surface of the earth.
