Cortical processing and organization in the deaf following cochlear implant
Cochlear prosthetics have proven to be extremely useful tools to provide auditory sensations to profoundly deaf patients and have restored hearing to tens of thousands of individuals around the world. Although the electrical stimulation poorly represents cochlear place information, temporal information is reliably represented. This is obviously sufficient for frequency discrimination over lower frequencies and thus makes human speech comprehension possible. In congenitally deaf children, cochlear implants enable language development, but the outcome is critically dependent on the age of implantation. Congenitally deaf subjects implanted in adulthood show a markedly poorer performance in speech discrimination than subjects implanted in childhood. The optimum implantation age appears to be under five years of age.
Our long-term goal is to understand and define the capabilities of the auditory cortex to establish acoustic function following cochlear implant. By understanding how the cerebral cortex can adapt to process signals generated by a cochlear prosthetic, it will be possible to alter cochlear prosthetics to better serve the needs of the cerebrum. This endeavor requires four steps: 1) The elucidation of the essential contributions that primary and non-primary auditory cortex make to fundamental acoustically-guided behaviours in the hearing condition, 2) A determination of the acoustic abilities of congenitally deaf subjects following cochlear implant, 3) An assessment of the behavioural capabilities of primary and non-primary auditory cortex following cochlear implant in congenitally deaf subjects, and 4) An examination of the plastic changes that can be made by the auditory cortex when the age at time of cochlear implant is altered. We are presently examining the first three steps.
To that end, we are examining auditory cortical function in the hearing animal and in the congenitally-deaf (CD) animal following cochlear implant. Specifically, we will test the animals on a battery of tasks that require the subjects to: 1) discriminate temporal patterns of the same duration; 2) discriminate acoustic stimuli that differ only in their temporal duration; 3) discriminate different frequencies; 4) discriminate different natural vocalizations; and 5) detect the presence of an acoustic stimulus. Each auditory area is reversibly deactivated with cooling. As multiple bilateral pairs of cooling loops are implanted in the same animal, it is possible for double and even triple dissociations to be performed. The second aim in the study is to determine what acoustic abilities CD animals with cochlear prosthetics implanted early in development (2 months of age, correlating to 2-5 years old in humans) can establish. We hypothesize that the animals will be able to attain high performance levels on the tasks examined in Aim 1. The results of the first two aims are brought together in Aim 3, where we combine the CD animals with cochlear implants examined in Aim 2 with the reversible cooling deactivation technique used in Aim 1 to determine how the normal functional organization of primary and non-primary auditory cortex differs from that established following early cochlear implant. To accomplish this we reversibly deactivate each auditory area in the CD animals with cochlear implants while performing the battery of auditory tasks and compare the results with those from the intact subjects in Aim 1. The results from these studies are directly applicable to clinical studies presently investigating the functional outcomes of cochlear implants in young children.
These experiments are conducted in collaboration with Dr. Andrej Kral (University of Hannover).