Saccade-related sound pulses and phase-resetting contribute to eye movement-related eardrum oscillations (EMREOs)

This study reveals that eye movement-related eardrum oscillations (EMREOs) are generated by both saccade-associated sound pulses and phase resetting of ongoing oscillations, with phase resetting identified as the more prevalent mechanism across participants.

The Gist

The Big Idea: Your Ears "Click" When Your Eyes Move

Imagine you are looking at a bird on a tree branch. Suddenly, you spot a squirrel and your eyes dart quickly to the right to look at it. That quick jump of your eyes is called a saccade.

For a long time, scientists knew that when your eyes make this quick jump, your eardrums also vibrate in a specific way. They call this vibration an EMREO (Eye Movement-Related Eardrum Oscillation). It sounds like a low, rhythmic hum (about 30–40 times a second) that happens right when your eyes move.

But here was the big mystery: Why does the eardrum vibrate?

The researchers (Cynthia King and Jennifer Groh) wanted to know if this vibration was a brand new sound being created every time you move your eyes, or if it was something else entirely. They set up a study with 30 people to listen to their eardrums while they played a game of "look here, then look there."

The Two Theories: The "Drum Beat" vs. The "Conductor"

To explain their findings, imagine a room full of people clapping their hands.

Theory A: The "Drum Beat" (The Pulse)
Imagine that before you move your eyes, everyone in the room is sitting quietly. The moment you decide to look at the squirrel, everyone starts clapping loudly and rhythmically.

• What this means: The eye movement triggers a new burst of energy. The eardrum creates a fresh sound pulse every time.
• The result: If you average all the recordings together, you hear a loud, clear clap.

Theory B: The "Conductor" (Phase Resetting)
Now, imagine that everyone in the room is already clapping, but they are all out of sync. One person claps on the "one," another on the "two," and another on the "three." It sounds like a messy, random noise.
Suddenly, a conductor (the eye movement signal) waves a baton. Snap! Everyone stops their random clapping and instantly starts clapping on the exact same beat.

• What this means: The energy was there the whole time, but the eye movement "resets" the timing so everything lines up perfectly.
• The result: If you look at just one person, they were clapping the whole time. But if you record everyone and average it, it looks like a sudden, loud burst of clapping because they all started at the same time.

What They Found: It's Mostly the Conductor

The researchers analyzed the data from every single trial (every time a person moved their eyes) to see which theory was right.

1. Both happen, but one is more common: They found that for about half the people, the eardrum actually did create a new "pulse" of sound (Theory A).
2. The "Reset" is the winner: However, for the vast majority of people (about 50 out of 60 ears), the main event was Phase Resetting (Theory B). The eardrum was already vibrating, but the eye movement forced it to line up its rhythm perfectly.

Think of it like a choir. Sometimes the conductor makes the choir start singing a new song (a pulse). But usually, the choir was humming a tune the whole time, and the conductor just told them, "Okay, everyone, start singing the next note right now!" That sudden alignment makes the sound much louder and clearer to the audience.

Why Does This Matter?

Why would your body do this? Why reset the rhythm of your eardrum just because you looked at a squirrel?

The scientists have a few cool ideas:

• Tuning the Radio: Your brain needs to know where you are looking to understand what you are hearing. If you look left, the sound coming from the left is different than if you look right. By resetting the eardrum's rhythm, the ear might be "tuning" itself to better hear sounds coming from the direction you just looked at. It's like turning the volume knob on a radio to a specific station based on where you are pointing your antenna.
• Time-Stamping: When your eyes jump, the image on your retina changes instantly. The brain might use this "ear click" as a time-stamp. It's like saying, "Okay, the eyes just moved; everything I hear right now belongs to this new picture." This helps the brain mix up what you see and what you hear so they feel like they are happening at the same time.

The Bottom Line

This paper solves a puzzle about how our ears and eyes talk to each other. It turns out that when you move your eyes, your eardrums don't just randomly start making noise. Instead, they mostly act like a synchronized choir that gets a signal to "reset" their rhythm.

This synchronization helps your brain figure out where sounds are coming from and keeps your vision and hearing perfectly in step, even when your eyes are darting around the room.

Technical Summary

1. Problem Statement

Eye movement-related eardrum oscillations (EMREOs) are low-frequency (~30–45 Hz) acoustic signals detected in the ear canal that occur in conjunction with saccadic eye movements. While previous research established that EMREOs appear as distinct "pulses" in trial-averaged data, the underlying mechanism generating these signals remained ambiguous. Two competing hypotheses existed:

1. True Pulse Hypothesis: Saccades trigger a genuine increase in acoustic energy (a "pulse") at the EMREO frequency, meaning the signal is absent or flat during fixation and only appears during the movement.
2. Phase-Resetting Hypothesis: Ongoing oscillatory energy exists at the EMREO frequency even during steady fixation. The saccade does not generate new energy but rather "resets" the phase of this ongoing signal, aligning it across trials. When averaged, this alignment creates the illusion of a pulse.

The study aimed to distinguish between these possibilities by analyzing spectral power and phase at the individual trial level rather than relying solely on trial-averaged data.

2. Methodology

• Participants: 30 human subjects (19–54 years old) with clinically normal hearing.
• Task: A visually guided saccade task where participants fixated on a central target and made saccades to a grid of 24 peripheral targets.
• Data Acquisition:
  • Audio: Simultaneous recordings from both ear canals using ER10B+ microphones (sampled at 48 kHz, downsampled to 2000 Hz).
  • Eye Tracking: EyeLink 1000 system to record saccade onset and offset with high precision.
• Analysis Strategy:
  • Frequency Band: Spectral analysis focused on the 30–45 Hz band, capturing peaks observed in both ipsilateral (40 Hz) and contralateral (30 Hz) saccades.
  • Time Windows: Analysis was conducted in sliding 25 ms bins with 50% overlap, aligned to both saccade onset and offset.
  • Metrics: For each individual trial, the authors calculated spectral power (energy magnitude) and phase within the 30–45 Hz band.
  • Grouping: Data was categorized by saccade direction (ipsilateral vs. contralateral relative to the recording ear) and analyzed across 60 ears (30 subjects × 2 ears).

3. Key Contributions

• Individual Trial Analysis: The study moves beyond population averages to characterize the signal dynamics at the single-trial level, a necessary step to differentiate between energy generation and phase alignment.
• Mechanistic Differentiation: It provides empirical evidence that EMREOs are not a monolithic phenomenon but a composite of two distinct mechanisms: true energy pulses and phase resetting.
• Quantification of Prevalence: The paper quantifies the prevalence of these mechanisms across a population, revealing that phase resetting is the dominant contributor, while true pulses are variable.

4. Key Results

• Dual Mechanism Confirmation: Both mechanisms were observed, but their prevalence differed significantly:
  • Phase Resetting: Observed in the vast majority of ears (~50 out of 60 ears, or ~83%). In these cases, ongoing oscillatory energy was present during fixation but became temporally aligned (phase-locked) at saccade onset/offset. The phase pattern often exhibited a "W" shape, indicating a reset followed by sustained alignment.
  • True Pulses (Energy Increase): Observed in roughly half of the participants (26 out of 60 ears showed a clear increase in power). In these cases, the signal was relatively quiet during fixation and showed a distinct surge in energy coinciding with the saccade.
  • Variable/Flat: Some ears showed no clear power increase ("flat" pattern) or even a decrease, yet still exhibited measurable EMREOs in averaged data due to phase resetting.
• Temporal Dynamics:
  • The phase reset occurs at both saccade onset and offset.
  • The phase signal persists for approximately 50 ms after saccade offset.
  • Power increases (when present) are time-locked to the saccade window (-25 to +75 ms).
• Binaural and Directional Specificity: The phase of the reset signal was opposite for ipsilateral vs. contralateral saccades and between the left and right ears. This suggests the underlying mechanism generates a binaural difference cue dependent on gaze direction.

5. Significance and Implications

• Mechanistic Insight: The findings suggest that the primary driver of EMREOs is likely a mechanical shift (e.g., tension changes in the eardrum or middle ear muscles) that resets the phase of ongoing vibrations, rather than the generation of a new 30–40 Hz sound wave from scratch. This relaxes constraints on candidate mechanisms; for instance, it may be easier for middle ear muscles to induce a phase reset than to generate a high-amplitude low-frequency tone de novo.
• Functional Role:
  • Auditory-Visual Integration: The phase reset may serve as a "time stamp," aligning auditory processing with the arrival of a new visual image on the retina following a saccade.
  • Spatial Calibration: The binaural and directional nature of the signal suggests EMREOs may help the brain adjust interaural timing and level differences based on current eye position, facilitating the integration of auditory and visual spatial maps.
• Future Directions: The study highlights the need to investigate the specific actuators (middle ear muscles vs. outer hair cells) responsible for phase resetting and to explore how this signal modulates excitability in the auditory cortex.

In conclusion, the paper resolves a long-standing ambiguity in auditory neuroscience by demonstrating that EMREOs are primarily a result of phase resetting of ongoing oscillations, with energy pulses acting as a secondary, variable contributor. This distinction is crucial for modeling the physiological mechanisms of the auditory periphery and understanding multisensory integration.