The interplay of stimulus modality and response latency in neural network organization for simple working memory tasks.

We used functional magnetic resonance imaging to examine how modality of input affects functional network organization beyond the sensory cortices for simple working memory tasks. The stimuli were auditory or visual bandpass-filtered white noise. On a given trial, three stimuli, each with differing center frequencies, were presented in succession. For temporal sequencing tasks, participants indicated when the stimulus with the highest frequency content appeared. For comparison tasks, participants indicated whether the frequency content of the last stimulus was lower, intermediate, or higher than the first two stimuli. Task difficulty was equated by establishing equivalent accuracy thresholds across subjects. We used behavioral spatiotemporal partial-least squares (ST-bPLS) analysis to identify neural patterns capturing the optimal association between brain images and reaction time. Because of statistical instabilities, subjects were divided into a SLOW group and a FAST group based on the median split of reaction times. ST-bPLS identified a significant interaction between stimulus modality and task demands for both groups, indicating that task-dependent brain-behavior correlations changed with stimulus modality. The large-scale activity pattern associated with this effect included prefrontal cortex and parietal cortex for the SLOW group and parietal cortex and cingulate for the FAST group. For the FAST group only, ST-bPLS also identified a significant main effect that differentiated tasks independent of modality. The pattern associated with this effect included prefrontal cortex and parietal cortex. These results confirm that modality of input affects network configuration even outside of the sensory cortices but that network configuration may vary with behavior.