Considerable progress has been made in the treatment of hearing loss with
auditory implants. However, there are still many implanted patients that
experience hearing deficiencies, such as limited speech understanding or
vanishing perception with continuous stimulation (i.e., abnormal loudness
adaptation). The present study aims to identify specific patterns of cerebral
cortex activity involved with such deficiencies. We performed O-15-water
positron emission tomography (PET) in patients implanted with electrodes
within the cochlea, brainstem, or midbrain to investigate the pattern of
cortical activation in response to speech or continuous multi-tone stimuli
directly inputted into the implant processor that then delivered electrical
patterns through those electrodes. Statistical parametric mapping was
performed on a single subject basis. Better speech understanding was
correlated with a larger extent of bilateral auditory cortex activation. In
contrast to speech, the continuous multi-tone stimulus elicited mainly
unilateral auditory cortical activity in which greater loudness adaptation
corresponded to weaker activation and even deactivation. Interestingly,
greater loudness adaptation was correlated with stronger activity within the
ventral prefrontal cortex, which could be up-regulated to suppress the
irrelevant or aberrant signals into the auditory cortex. The ability to detect
these specific cortical patterns and differences across patients and stimuli
demonstrates the potential for using PET to diagnose auditory function or
dysfunction in implant patients, which in turn could guide the development of
appropriate stimulation strategies for improving hearing rehabilitation.
Beyond hearing restoration, our study also reveals a potential role of the
frontal cortex in suppressing irrelevant or aberrant activity within the
auditory cortex, and thus may be relevant for understanding and treating
tinnitus