Hearing sounds when the eyes move: A case study implicating the tensor tympani in eye movement-related peripheral auditory activity

This case study demonstrates that eye movement signals reach the tensor tympani muscle and can produce audible sounds (gaze-evoked tinnitus) in a patient with tensor tympani myoclonus, confirming that these signals have functional consequences for auditory perception.

The Gist

Imagine your brain is a highly sophisticated orchestra conductor. Its job is to make sure what you see and what you hear happen in the same place, even though your eyes are constantly darting around like hyperactive hummingbirds. Every time you look left or right, your brain has to instantly recalculate where sounds are coming from so you don't get confused.

Usually, this process is invisible and silent. You move your eyes, and the world stays stable. But for a woman named S98, this silent process has a very loud, very strange side effect: When she looks to the far left, she hears a fluttering sound in her left ear.

Here is the story of how scientists figured out why this happens, explained simply.

The Mystery of the "Flutter"

S98, a 67-year-old woman, contacted researchers because she had a unique condition. She was diagnosed with tensor tympani myoclonus.

• The Tensor Tympani: Think of this as a tiny, automatic "shock absorber" muscle inside your ear. Its normal job is to tighten up when loud noises happen to protect your inner ear (like a reflex).
• The Glitch: In S98's case, this muscle spasms (twitches) not just from loud noises, but when she moves her eyes to the extreme left.

When she does this, the muscle spasms, causing her eardrum to vibrate. It's like a tiny drum being hit inside her ear, creating a sound she can actually hear.

The Experiment: Catching the Ghost

The researchers invited S98 into a quiet soundproof booth to catch this sound on tape.

1. The Setup: They put a sensitive microphone right inside her ear canal (like a stethoscope for the eardrum).
2. The Task: They asked her to look at a dot on a screen, then suddenly look as far left as she could.
3. The Result: Every time she made that big, extreme jump to the left, the microphone picked up a loud "fluttering" noise.
4. The Timing: The sound didn't happen during the eye movement itself (which is very fast, like a blink). Instead, the sound started right after her eyes stopped and held that far-left position. It lasted for about a second.

It was as if her brain sent a "look left" signal, but the signal got "stuck" in her ear muscle, causing it to twitch and make noise.

The "Normal" Ear vs. The "Loud" Ear

Here is the fascinating part: Everyone has this connection between eye movements and ear muscles, but we usually don't notice it.

Scientists have discovered that in people with normal hearing, moving your eyes does cause tiny vibrations in the eardrum (called EMREOs). However, these vibrations are so faint—like a whisper in a hurricane—that your brain filters them out completely. You never hear them.

But S98 is different:

• The Volume Knob: In S98's ear, the "volume knob" on these eye-movement signals is turned way up. Her muscle spasms are so strong that the tiny vibrations become loud enough to hear.
• The Comparison: When the researchers compared her ear to 30 people with normal hearing, S98's ear vibrations were 50% to 300% louder than average. Even when she moved her eyes normally (not just to the extreme left), her ear was making much more noise than anyone else's, she just couldn't hear it unless the movement was extreme.

Why Does This Matter?

This case study is like finding a leak in a dam that reveals how the whole wall is built.

1. Proof of Connection: It proves that the brain really does send "eye movement" signals all the way down to the tiny muscles in the ear. It's not just a brain thing; it's a whole-body thing.
2. The "Silencer" Theory: The researchers suspect that in a healthy ear, the tensor tympani muscle acts as a silencer. It usually dampens these eye-movement vibrations so we don't hear them. In S98, because of her condition, the silencer is broken or overactive, letting the "noise" of her eye movements leak through.
3. New Clues for Tinnitus: This helps explain a rare type of tinnitus (ringing in the ears) that some people get after brain surgery or tumors. It suggests that sometimes, the brain gets confused and mistakes "eye movement signals" for "actual sounds."

The Takeaway

Think of your brain as a master editor. It usually edits out the "static" caused by your own eye movements so you can enjoy the movie of your life without distraction. For S98, the editor accidentally left the "eye movement static" on the track, and it's now playing loud enough for her to hear.

This paper shows us that our senses are deeply interconnected, and sometimes, a glitch in one tiny muscle can reveal a massive, hidden truth about how our brains keep the world steady.

Technical Summary

1. Problem Statement

The brain must dynamically map visual space (anchored to the retina) to auditory space (anchored to the ears) to maintain perceptual stability during frequent eye movements (saccades). Previous research has identified Eye Movement-Related Eardrum Oscillations (EMREOs)—faint mechanical sounds generated in the ear canal coincident with saccades. These are thought to be a peripheral signature of the brain's mechanism for compensating for eye movements.

However, a critical gap remains: while EMREOs are measurable in healthy humans, they are generally inaudible. Consequently, it is unclear if these peripheral signals have direct perceptual consequences or if they are merely epiphenomena of a successful computational process that usually goes unnoticed. The study aims to bridge this gap by investigating a pathological case where eye movements produce audible sounds, thereby establishing a direct link between peripheral auditory mechanics and conscious perception.

2. Methodology

The study utilized a mixed-method approach combining a detailed case study of a single participant (S98) with a comparative analysis against a normative control group.

• Subject Profile:

  • S98: A 67-year-old female diagnosed with tensor tympani myoclonus (spasms of the middle ear muscle). She reported hearing a "fluttering" sound in her left ear specifically during large leftward eye movements. She has normal hearing thresholds and no vestibular deficits.
  • Controls: A group of 30 neurotypical participants (ages 19–54) with normal hearing.

• Experimental Tasks:

  1. Self-Directed Saccade Task (S98 only): S98 performed voluntary, extreme saccades (from +30° right to -32° left/up) to elicit her specific sound percept.
     • Recording: An ER-10b+ microphone was placed in the left ear canal. A second microphone on the desk recorded the subject tapping when she heard the sound.
     • Eye Tracking: Due to the extreme range of motion, the standard EyeLink 1000 software could not quantitatively track the full saccade. Researchers recorded the EyeLink monitor screen via an iPhone camera to manually align video timestamps with audio recordings (estimated alignment accuracy: ~50 ms).
  2. Visually Guided Grid Task (S98 + Controls): Both groups performed standard saccades (±18° horizontal, ±12° vertical) to compare EMREO waveforms.
     • Analysis: EMREOs were analyzed for leftward saccades in the left ear. Waveforms were aligned to saccade onset and offset.

• Data Analysis:

  • Comparison of EMREO amplitude, latency, and duration between S98 and the control population (median, 25th–75th percentile ranges).
  • Temporal alignment of audible sound bursts with saccade kinematics and eccentric fixation periods.

3. Key Results

A. Characterization of the Audible Phenomenon

• Trigger: The sound is elicited only by large, extreme leftward saccades (approx. 62°) and is not perceived during smaller, standard saccades or central fixation.
• Timing: Contrary to the brief nature of typical saccades (150–300 ms), the audible sound lasts approximately 1 second.
• Phase: The sound does not occur solely during the saccade movement. Analysis indicates the sound begins near the end of the saccade but persists primarily during steady eccentric fixation at the extreme leftward position.
• Mechanism: The sound is a "fluttering" noise, consistent with a spasm of the tensor tympani muscle, recorded clearly in the left ear canal.

B. Comparison of EMREOs (S98 vs. Controls)

• Amplitude: S98's EMREOs were significantly larger than the control population average (approx. 50% to 300% larger, depending on the metric). Her peak-to-trough amplitude exceeded the 75th percentile of the control group.
• Latency: S98's EMREO waveforms began earlier relative to saccade onset compared to controls.
• Duration: The EMREO signal in S98 persisted longer after saccade offset (deviations from zero continued for >50 ms post-offset) compared to controls.
• Perceptibility: Despite having larger EMREOs during the standard grid task, S98 did not report hearing sounds during this task, suggesting a threshold effect where the signal must reach a specific magnitude or duration to become audible.

4. Key Contributions

1. Perceptual Validation of EMREOs: This is the first study to demonstrate that eye movement-related signals reaching the ear can produce conscious auditory percepts. It confirms that the peripheral auditory system receives and processes eye movement signals.
2. Identification of the Tensor Tympani: The study provides strong evidence that the tensor tympani muscle is a specific neural target for eye movement signals. The patient's myoclonus (spasm) transforms these signals into audible noise.
3. Functional Role of the Muscle: The findings suggest a dual role for the tensor tympani:
   • It is involved in the generation/modulation of EMREOs.
   • In a healthy state, it likely acts as a dampener, reducing the amplitude of EMREOs to prevent them from becoming audible (a "noise floor" regulation). Dysfunction removes this dampening, leading to perception.
4. Expansion of Gaze-Evoked Tinnitus: The study expands the clinical understanding of gaze-evoked tinnitus. While previously associated primarily with acoustic neuromas (where the brain misinterprets eye signals as sound due to input loss), this case shows that peripheral muscle dysfunction (tensor tympani myoclonus) can also cause the same phenomenon.

5. Significance

• Neuroscience: The findings support the hypothesis that the brain uses peripheral auditory mechanisms (specifically middle ear muscle activity) to dynamically link visual and auditory spatial maps. The fact that these signals can be "heard" when the dampening mechanism fails proves they are robust physiological signals, not just theoretical constructs.
• Clinical Implications: This provides a new diagnostic and mechanistic framework for patients suffering from gaze-evoked tinnitus. It suggests that treatments targeting the tensor tympani (e.g., for myoclonus) could alleviate symptoms in patients who do not have acoustic neuromas.
• Methodological: The study successfully overcomes the challenge of tracking extreme eye movements to correlate them with high-fidelity ear canal audio, establishing a protocol for studying rare oculomotor-auditory interactions.

Conclusion: The paper establishes that eye movement signals reach the middle ear and modulate the tensor tympani. While normally suppressed to maintain auditory silence, abnormalities in this muscle can unmask these signals as audible tinnitus, confirming the functional integration of oculomotor and auditory systems at the peripheral level.