Does noise or hard grammar make listening feel and look effortful?
Background noise mainly made listening feel harder, while complex sentence structure mainly made the pupils dilate, suggesting that self-reported effort and pupil size capture different kinds of listening effort.
Source
Impact of Background Noise and Sentence Complexity on Processing Demands during Sentence Comprehension
Study at a glance
- Design
- Human experiment — 2 x 2 within-subject design (noise level +12 vs -6 dB SNR x simple SVO vs complex OVS Danish sentences) in a sentence-picture matching task with pupillometry, effort ratings and span tests
- N
- N=20 · 20 normal-hearing adults tested; one was dropped from the pupil analysis because more than half of their trials needed blink interpolation
- Population
- Normal-hearing Danish-speaking adults aged 19-36 (11 women, 9 men)
- Outcome
- Picture-matching accuracy, subjective effort ratings, and pupil dilation in three time windows; correlations with digit span and reading span
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What they did
Twenty normal-hearing adults saw a picture of two animals doing something, then heard a Danish sentence in steady speech-shaped noise and judged whether it matched the picture. Sentences were either simple (subject-verb-object) or complex (object-verb-subject), and noise was either mild (+12 dB signal-to-noise ratio) or loud (-6 dB). Pupil size was recorded throughout, listeners rated difficulty after each trial, and they completed digit span and reading span tests of working memory.
What they found
Louder noise produced a large increase in rated difficulty and a small pupil increase early in the sentence, even though accuracy did not differ between noise levels. Complex sentences produced larger, longer pupil dilations after the disambiguating word and during the retention period, but only a small effect on ratings; accuracy on complex sentences was only 57.2-58.1% versus 87.8-93.1% for simple ones. People with higher backward digit span showed larger pupil dilations but rated simple sentences in loud noise as less effortful, fitting the resource hypothesis for pupils and the efficiency hypothesis for ratings.
The limits
What it doesn't show
With only 20 young normal-hearing adults, the individual-difference correlations are fragile and were significant only in one condition; they cannot be generalised to older or hearing-impaired listeners, who are the main target of listening-effort research. Accuracy on complex sentences was close to chance, so some complex-sentence pupil effects may reflect confusion or guessing rather than successful effortful processing. Motivation, which could drive both pupil size and ratings, was not measured, and the reading span task used an older, less sensitive procedure, which the authors note may explain its null correlations.
Key terms
- Listening effort
- The mental resources a listener deploys to understand speech, which can rise even when speech is fully intelligible.
- Signal-to-noise ratio (SNR)
- How loud the speech is relative to the background noise, in decibels; lower or negative values mean noisier listening.
- Pupillometry
- Measuring changes in pupil size, which dilates with cognitive load and is used as a physiological index of processing effort.
- Resource hypothesis
- The idea that people with more cognitive capacity mobilise more resources on hard tasks, producing larger pupil responses.
- Efficiency hypothesis
- The idea that people with more cognitive capacity process more efficiently and so experience tasks as less effortful.
- Object-verb-subject (OVS) sentence
- A non-canonical word order in which the object comes first, making it harder to work out who did what to whom.
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Quiz yourself
Which manipulation mainly increased subjective effort ratings?
Common questions
If accuracy was the same in both noise levels, how can noise have increased effort?
Listeners could still understand the sentences, but they rated the noisy ones as much harder and their pupils dilated more early on, showing that extra effort can be spent to keep performance up.
Why do the authors conclude that ratings and pupils measure different things?
Noise mainly moved the ratings, complexity mainly moved the pupils, and working memory related positively to pupil size but negatively to rated effort, so the two measures did not track each other.
Does this study tell us about hearing-aid users?
Not directly. All participants had normal hearing; the authors suggest the paradigm could be useful for hearing-impaired listeners, but that remains to be tested.
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