The limits of listening: How cognitive load and cognitive capacity shape effort in speech perception

Speech recognition in adverse conditions varies greatly across individuals. Theoretical models suggest listening effort is shaped by the interplay of several factors, including cognitive capacity and task difficulty. However, the combined influence of cognitive capacity and cognitive load on speech perception remains underexplored. This study addressed this gap by measuring cognitive capacity, taxing it with a memory load, and evaluating their combined effects on listening effort, speech processing dynamics, and behavioral performance during spoken word recognition. Thirty-six young adults with normal hearing completed a speech recognition task in background noise under low (one-digit) and high (four-digit) memory load conditions. Working memory capacity was assessed using the Digit Span test. Eye movements and pupil dilation were recorded as indices of real-time word processing and listening effort. Increased cognitive load reduced behavioral accuracy and slowed word recognition, as reflected in delayed target fixations. Critically, these effects were moderated by working memory capacity. Under high load, higher-capacity individuals showed faster processing and maintained greater accuracy than lower-capacity individuals. Pupillometry revealed opposite effort patterns: under high load, low-capacity individuals demonstrated reduced pupil dilation, while high-capacity individuals exhibited increased dilation. This suggests individuals with sufficient available resources successfully mobilized additional effort to meet increased task demands, whereas low-capacity individuals may have reached a processing limit. Behavioral and physiological findings converge to suggest that individuals with higher working memory capacity are better able to maintain performance under increased cognitive load. These results highlight the importance of accounting for individual differences in cognitive capacity when evaluating speech processing in challenging environments.