Perceiving and following an individual speaker in a crowded, noisy environment is a commonplace task for listeners with normal hearing. The underlying neurophysiology, however, is complex, and the task remains a struggle for people with peripheral and central auditory pathway disorders. The lack of a detailed neurobiological model of mechanisms and functions underlying robust speech perception has hindered our understanding of how these processes become impaired in the suffering population. In our innovative approach, we will record from high-density micro and macro electrode arrays surgically implanted on the superior temporal gyrus of epilepsy patients as part of their clinical evaluation. This method offers an exceptionally detailed perspective of cortical population activity. We will build upon two recent complementary findings where we identified a highly selective, spatially distributed neural representation of phonetic features (Mesgarani et. al. Science, 2014), which at the same time is highly dynamic and can change rapidly to reflect the perceptual bias of the listener (Mesgarani & Chang, Nature 2012). While significant, these studies revealed several gaps in our understanding of this process, which we intend to address in this proposal. Specifically, we will resolve the following unanswered questions: 1) what is the neural mechanism for joint encoding of both phonetic and speaker features? 2) How does attention modulate phonetic and speaker feature selectivity of neural responses? And 3) what computational mechanisms can account for dynamic feature selectivity of responses in STG? Answering these questions will significantly advance our understanding of a remarkable human ability, and will be of great interest to researchers from many areas including neurologists, and sensory and cognitive neuroscientists.