ABSTRACT The development of the cochlear implant (CI) has revolutionized the treatment of severe hearing loss. Despite its overall success, not every patient who receives a CI will benefit from this technology. The limitation lies not within the technology itself, but primarily within the high level of diversity in the population of people who qualify for CI surgery. Each CI recipient has a distinct hearing-loss profile and clinical background, which imparts unique deafness-induced changes to each level of their auditory system, including the brain. Currently, it is unclear how individual differences within the cortical auditory system, specifically in speech-evoked brain activity, contribute to individual variability in CI speech-recognition outcomes. One reason for this gap in knowledge is that the electromagnetic signals emitted from a CI can disrupt traditional neuroimaging methods. A new optical neuroimaging tool, functional near-infrared spectroscopy (fNIRS), offers a CI-compatible imaging option that is non-invasive and unaffected by electromagnetic artifact. The knowledge gained from recent fNIRS investigations has not yet translated into clinical practice because results are often only reported at the group level; the reliability of fNIRS measurements remains somewhat poor on the single-subject level. The proposed research addresses this issue of reliability by controlling for the interference from systemic physiological signals in single-subject fNIRS recordings. In Aim 1, we will determine the contribution of naturally occurring, non-neural physiological signals to the within-subject variability in fNIRS recordings using systemic physiology augmented fNIRS (SPA-fNIRS). In Aim 2, we will determine the relationship between CI speech-recognition ability and speech-evoked brain activity measured via SPA-fNIRS using naturalistic connected speech signals. This investigation will use an individualized approach to examine cortical-speech processing in adult CI recipients. The findings from this proposal have the potential to reveal underlying sources of individual variability in CI recipients due to differences in their cortical processing of speech. The use of novel connected-speech passages to elicit brain activity in individual CI users will increase the ecological validity of our approach, which is an important step toward translating research findings into clinical practice. Our proposal aims to improve the tools used to objectively measure speech-evoked brain activity in single subjects so that personalized auditory-rehabilitation training can be prescribed to help individuals engage more effective speech-processing strategies.