Cortical networks are thought to operate in a state of tightly balanced excitation and inhibition. A typical cortical neuron receives thousands of synaptic inputs from other cortical neurons. The majority of these inputs are excitatory and the network is stable only because strong recurrent excitation is balanced by similarly strong feedback inhibition. Theoretical models have shown that sparsely connected networks with strong, balanced excitation and inhibition exhibit chaotic activity that is consistent with the highly variable responses and large membrane potential fluctuations observed in cortex in vivo. In these models, and hypothetically in cortical networks, large noise is a consequence of large excitatory inputs being balanced by inhibitory inputs, keeping the mean membrane potential below threshold but leaving large fluctuations about this mean. Direct experimental evidence for this balanced state has been provided by measurements of synaptic conductances in vivo and in cortical slices. In chapter two we show that noisy fluctuations in membrane potential are an essential part of a mechanism by which neurons can modulate the gain of their responses. Gain modulation is common in cortical responses and is thought to play an important role in cortical computation.

Membrane potential fluctuations during spontaneous activity are not entirely random. In cat primary visual cortex spontaneous activity has been shown to exhibit spatial patterns similar to those evoked by a visual stimulus, in which neurons with similar preferred orientations are co-active. This suggests that, from unstructured input, cortical circuits selectively amplify activity patterns related to normal function. Current understanding of such amplification involves elongation of the lifetime of a neural pattern by mutual synaptic excitation among the neurons involved. In chapter three we describe a new mechanism by which recurrent networks with strong recurrent excitation and balancing feedback inhibition can amplify neural activity patterns without elongation of lifetime, which we call transient amplification. By better understanding the implications of balanced excitation and inhibition in cortical networks, we gain insight into the role of recurrent circuitry in cortical computation.

UV radiation exposure is one of the key modifiable risk factors for skin cancer. Hence, patient education regarding skin protection and sunscreen use is of tremendous importance to public health. To better understand patient practices regarding skin protection in a population level, we looked into the Internet search behavior of the US-based population. We investigated patient inquires on the United States Food and Drug Administration (FDA) announcements regarding sunscreen use by quantifying search terms such as “broad spectrum sunscreen”, “sunscreen” and “sunblock” with Google Trends, a novel methodology for understanding internet search practices. Our findings show that “broad spectrum sunscreen” searches were significantly increased post 2011 FDA announcements, which suggest increased public awareness regarding the importance of broad spectrum protection. It is encouraging these preliminary results indicate that skin protection practices are being increasingly investigated by the general public and may serve as a novel approach for identifying areas of improvement regarding patient education on the reduction of the risk for skin cancer.