Abstract
Perception of complex auditory stimuli like speech requires the simultaneous processing of different fundamental acoustic parameters. The contribution of left and right auditory cortex (AC) in the processing of these parameters differs. In addition, activity within the AC can vary positively or negatively with task performance depending on the type of task. This might affect the allocation of processing to the left and right AC. Here we studied with functional magnetic resonance imaging the impact of task difficulty on the degree of involvement of the left and right AC in two tasks that have previously been shown to differ in hemispheric involvement: categorization and sequential comparison of the direction of frequency modulations (FM). Task difficulty was manipulated by changing the speed of modulation and by that the frequency range covered by the FM. To study the impact of task‐difficulty despite covarying the stimulus parameters, we utilized the contralateral noise procedure that allows comparing AC activation unconfounded by bottom‐up driven activity. The easiest conditions confirmed the known right AC involvement during the categorization task and the left AC involvement during the comparison task. The involvement of the right AC increased with increasing task difficulty for both tasks presumably due to the common task component of categorizing FM direction. The involvement of left AC varied with task difficulty depending on the task. Thus, task difficulty has a strong impact on lateralized processing in AC. This connection must be taken into account when interpreting future results on lateralized processing in the AC.
Highlights
We studied with functional magnetic resonance imaging the impact of task difficulty on the degree of involvement of the left and right auditory cortex (AC) in two tasks that have previously been shown to differ in hemispheric involvement: categorization and sequential comparison of the direction of frequency modulations (FM)
The results showed that the involvement of the left and right AC strongly depends on task difficulty for both categorization and comparison of FM direction
The involvement of the AC in categorization and comparison of FM direction strongly changes with variations in task difficulty
Summary
The perception of complex acoustic stimuli (e.g., speech, music) requires the simultaneous processing of different fundamental acoustic parameters such as frequency, duration, and intensity. Most studies that varied task difficulty did not explicitly address changes of lateralization of processing due to this variation (Binder, Liebenthal, Possing, Medler, & Ward, 2004; Harinen & Rinne, 2013; Rinne, Koistinen, Salonen, & Alho, 2009; Rinne, Koistinen, Talja, Wikman, & Salonen, 2012) except for Reiterer et al who directly compared parametric effects between the left and right hemisphere (Reiterer et al, 2005; Reiterer, Erb, Grodd, & Wildgruber, 2008) This variation is conceivable and many studies have shown that activity within the AC can vary positively or negatively with performance, depending on the type of task. We summarize such findings that directly relate to the AC in humans
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