Overview
A cochlear implant (CI) represents one of the most successful neuroprosthetic interventions in modern clinical medicine. Designed to restore functional auditory sensation in individuals with severe-to-profound sensorineural hearing loss, a cochlear implant bypasses non-functional hair cells within the organ of Corti. By directly stimulating the primary auditory afferent fibers of the spiral ganglion cells housed inside the modiolus, the device converts acoustic pressure waves captured by an external microphone into micro-electrode electrical current pulses.
The mammalian cochlea is organized tonotopically along its bony spiral canal, known as the scala tympani. Under normal anatomical conditions, high-frequency sound waves excite the basilar membrane near the stiff, narrow basal region, whereas low-frequency sounds travel deeper to excite the compliant, wide apex. The classical Greenwood position-frequency relation mathematically defines this tonotopic map across human basilar membrane geometry:
$$f = A \left(10^{a \cdot x} - K\right)$$
where $f$ represents the resonance frequency in Hertz, $x$ is the normalized distance along the organ of Corti ranging from $0$ at the apex to $1$ at the base, and empirical constants for humans are defined as $A = 165.4$, $a = 2.1$, and $K = 0.88$ (or $1.0$ for low-frequency boundary alignment). In a surgical cochlear implant installation, a flexible multi-electrode array is threaded through the round window into the scala tympani, spanning approximately 1.5 to 2.5 turns ($540^\circ$ to $720^\circ$) of the anatomical spiral.
This online interactive laboratory provides a real-time visualization of speech processor signal extraction. The simulation breaks incoming temporal sound streams into spectral bands using a Fast Fourier Transform (FFT) analysis, isolates channel envelope amplitudes, applies dynamic noise thresholding, and projects calculated current output amplitudes directly onto a simulated anatomical $2.5$-turn spiral electrode array.
How to Use
The interactive workspace is partitioned into two primary visual interfaces and an intuitive side diagnostic configuration panel. Below is a structured step-by-step workflow guide to operating the simulator:
- Start Demo Mode: Located at the top of the control panel, clicking
Start Demo initiates an automated multi-tone acoustic sweep across low, mid, and high formants. This mode allows you to observe dynamic array activation without enabling external microphone input. Any physical user touch, keypress, or slider movement instantly breaks demo mode and returns the system to baseline manual controls.
- Microphone Input Mode: Click the
Mic OFF / ON button to unlock Web Audio API microphone stream capture. Speak or play music near your device to witness real-time spectral decomposition and corresponding spiral electrode excitation.
- Reset Baseline: The
Reset Baseline button instantly returns all electrode densities, noise gate thresholds, vertical spectrogram scales, and visual palette choices back to default factory baseline settings.
- Array Geometry Toggle: Switch between a realistic 2.5-turn
Spiral array (mimicking anatomical insertion inside the scala tympani) and a Linear Array view (useful for visualizing electrode channel spacing without spatial curve overlap).
- Active Electrodes Slider: Adjust the total channel density from 10 up to 40 individual stimulation electrodes. Modifying this parameter updates the discrete frequency band distribution calculated by the filter bank.
- Color Map Palette: Choose between four visual themes (Default Emerald, Ocean Cyan, Sunrise Amber, and Spectral Rainbow) to alter the intensity contrast across energy nodes.
- Noise Gate & Threshold Calibration: Check the
Enable Noise Gate box and adjust the threshold slider from $0$ to $100$. Noise gate filtering zero-suppresses background ambient noise floor energy beneath the set cut-off value, yielding a crisp, highly defined electrode stimulation pattern.
Technical Details
The real-time signal processing engine underlying this application utilizes the Web Audio API AnalyserNode operating at an FFT resolution length of $N_{\text{FFT}} = 2048$ samples. The sampling frequency $f_s$ typically defaults to $44,100\text{ Hz}$ or $48,000\text{ Hz}$, yielding individual frequency bin widths of:
$$\Delta f = \frac{f_s}{N_{\text{FFT}}} \approx 21.53\text{ Hz/bin}$$
To map raw linear FFT spectral bins to $N$ discrete electrode channels, the engine performs a logarithmic logarithmic-frequency bin allocation algorithm. For a given total electrode count $N$, the logarithmic range across the spectral analysis buffer is partitioned into $N$ contiguous sub-bands. The energy $E_k$ assigned to channel $k \in \{0, 1, \dots, N-1\}$ is derived by calculating the root-mean-square (RMS) spectral power over the bin range $[l_k, m_k]$:
$$E_k = \sqrt{\frac{1}{m_k - l_k + 1} \sum_{j=l_k}^{m_k} |X(j)|^2}$$
When the noise gate is enabled with threshold $T_{\text{gate}}$, the effective stimulation intensity $I_k$ delivered to electrode contact point $k$ is calculated with a strict hard-thresholding step followed by linear normalization:
$$I_k = \begin{cases} \frac{E_k - T_{\text{gate}}}{255 - T_{\text{gate}}}, & \text{if } E_k \ge T_{\text{gate}} \\ 0, & \text{if } E_k < T_{\text{gate}} \end{cases}$$
For the graphical rendering pipeline, high-density device pixel ratio normalization ($DPR$) is applied on every canvas recalculation to eliminate blurriness on Retina displays without causing container layout shifts. The spiral geometry is rendered according to a polar logarithmic spiral coordinate curve spanning $5\pi$ radians ($2.5$ full rotations):
$$r(\theta) = R_{\text{max}} \cdot e^{-b \cdot \theta}, \quad \theta \in [0, 5\pi]$$
Electrode nodes are positioned along this logarithmic trajectory, with low-frequency channels positioned near the apical terminus ($\theta = 5\pi$) and high-frequency channels positioned near the basal entrance ($\theta = 0$). Outer radial glowing halos are rendered around each electrode node, with dynamic alpha blending and radius expansion directly proportional to the calculated activation value $I_k$.
Future Directions
While this web simulator provides an ideal educational overview of tonotopic bandpass filtering and channel allocation, actual modern sound processors employ advanced temporal coding strategies such as Continuous Interleaved Sampling (CIS), Advanced Combination Encoders (ACE), and Fine Structure Processing (FSP). Future development iterations of this sandbox aim to incorporate real-time acoustic envelope pulse-train vocoders, allowing users to listen to processed audio through simulated cochlear implant acoustic vocoding algorithms (e.g., sine-wave or noise-band vocoders).
Additionally, future enhancements will model electric current spread and spatial neural excitation patterns across spiral ganglion populations using Gaussian spread kernel models:
$$V(x) = V_0 \cdot \exp\left(-\frac{(x - x_0)^2}{2\sigma^2}\right)$$
where $\sigma$ models spatial tissue resistivity and current interaction between adjacent electrode contacts. Incorporating multipolar current steering and tripolar virtual channel generation will further enhance the fidelity of this interactive neuroprosthetic visualizer.