Signal processing by octopus cells for acoustic and electrical hearing : a modelling study

Abstract

A computational model of a single octopus cell as well as a population of octopus cells was developed. The models were used to investigate the ability of octopus cells to compensate for the travelling wave delay, remove jitter from the neural activity and encode pitch for normal hearing. Furthermore the response of octopus cells to cochlear implant (CI) stimulation with the ACE strategy was investigated to determine whether pitch can be extracted from CI stimulation in the same way as from acoustic stimulation. Their ability to extract the pulse rate from single-electrode stimulation was also investigated. The response of the octopus cells to single-electrode stimulation at different pulse rates was used to predict pulse rate difference limens, which were compared to psychoacoustic measurements found in literature. It was found that octopus cells are sensitive to the delay of synaptic inputs on their dendrites but are broadly tuned to this delay. By evaluating the jitter together with the travelling wave delay present in the activity of auditory nerve fibres (ANFs), it was determined that octopus cells may rather act as coincidence detectors, which extract common interspike intervals (ISIs) from many ANFs. The octopus cell model was found to encode the frequency of pure tones in their ISIs for pure tone acoustic stimulation. They were also found to encode the pitch of a vowel in their ISIs, which was the same as the fundamental frequency extracted from the vowel with a speech processing algorithm. The octopus cell model responded to the pulse rate of the CI stimulation and could therefore not extract the frequency of pure tones from CI stimulation in the same way as from acoustic stimulation. The entrainment of the modelled octopus cell population decreased when the pulse rate of a single electrode increased beyond 300 pps. Pulse rate difference limens were predicted from the standard deviation of the ISIs of the octopus cell population response to single electrode stimulation. The predicted difference limens were in the same range as measured values, which suggests that octopus cells may play a role in the measured perceptual limit at 300 pps. From the findings of this study it is suggested that CI stimulation strategies should be developed to encode pitch in the periodicity of their stimulation to enable octopus cells to extract pitch information from CI stimulation.

Afrikaans: ’n Model van ’n enkelseekatsel (octopus cell) sowel as ’n populasie van seekatselle is ontwikkel. Die modelle is gebruik om die vermoë van seekatselle te ondersoek om te kompenseer vir die loopgolfvertraging, om bibber te verwyder uit die neurale kode en om toonhoogte te enkodeer vir normale gehoor. Verder is die respons van seekatselle vir kogleêre inplantingstimulasie met die ACE-strategie ondersoek om te bepaal of toonhoogte op dieselfde manier vanuit kogleêre inplantingstimulasie onttrek kan word as vanuit akoestiese stimulasie. Hulle vermoë om pulstempo van enkel-elektrodestimulasie te onttrek is ook ondersoek. Die respons van seekatselle op enkel-elektrodestimulasie is gebruik om die kleinste waarneembare verskil in pulstempo te voorspel, wat vergelyk is met psigoakoestiese metings vanuit die literatuur. Een bevinding is dat seekatselle sensitief is vir die vertraging van sinapsinsette op hul dendriete, maar hulle is wyd ingestem vir hierdie vertraging. Deur die bibber op die gehoorsenuwee-aktiwiteit saam met die loopgolfvertraging in hul aktiwiteit te evalueer, is bevind dat seekatselle waarskynlik eerder saamvaldeteksie doen, wat gemeenskaplike interpulsintervalle vanaf die gehoorsenuwee onttrek. Daar is bevind dat die seekatselmodel die frekwensie van suiwer tone in hul gemeenskaplike interpulsintervalle enkodeer vir akoestiese stimulasie met suiwer tone. Verder is bevind dat seekatselle die toonhoogte van ’n vokaal in hul gemeenskaplike interpulsintervalle enkodeer, wat dieselfde is as die grondtoon wat vanaf die vokaal onttrek is deur ’n spraakverwerkingsalgoritme. Die seekatselmodel reageer op die pulstempo van kogleêre inplantingstimulasie en kan daarom nie die frekwensie van suiwer tone op dieselfde manier vanuit kogleêre inplantingstimulasie as vanuit akoestiese stimulasie onttrek nie. Die sinchronisasie van die stimulus van die gemodelleerde seekatselpopulasie neem af wanneer die pulstempo van ’n enkel-elektrode toeneem bo 300 pps. Die kleinste waarneembare verskil in pulstempo is voorspel vanuit die standaardafwyking van die gemeenskaplike interpulsintervalle van die seekatselpopulasie se respons op enkel-elektrodestimulasie teen verskillende pulstempo’s. Die voorspelde kleinste waarneembare verskille is in dieselfde bereik as gemete waardes, wat aandui dat seekatselle moontlik ’n rol speel in die gemete persepsielimiet by 300 pps. Op grond van die bevindinge van hierdie studie word voorgestel dat kogleêre inplantingstimulasiestrategieë ontwikkel word wat toonhoogte enkodeer in die periodisiteit van hul stimulasie om dit moontlik te maak vir seekatselle om toonhoogte-inligting vanuit kogleêre inplantingstimulasie te onttrek.