2 A dynamic neural code may underlie multisensory integration and segregation in the primate superior colliculus

Prior work has suggested that much of human lateral frontal cortex (LFC) is part of a ‘multiple demand’ network, supporting a wide range of cognitive tasks and processes (e.g. Duncan 2010; Fedorenko et al. 2013). In contrast to this multiple-demand view, several groups have recently reported that portions of LFC show a preference for processes associated with a particular sensory modality (Michalka et al. 2015; Braga et al. 2016; Mayer et al. 2016). Michalka et al. demonstrated that selective spatial attention to visual or auditory stimuli recruits specific structures within LFC. Two bilateral regions are biased for visual attention, superior and inferior precentral sulcus (sPCS and iPCS), and two bilateral regions are biased for auditory attention, transverse gyrus intersecting precentral sulcus (tgPCS) and caudal inferior frontal sulcus (cIFS). Here, we used fMRI to replicate this finding in a substantially different task paradigm and to examine the ‘multiple demand’ responsiveness of these regions. We observed that visual and auditory 2-back working memory tasks recruited interleaved visualand auditory-biased structures that correspond to those reported by Michalka et al. These structures were mapped in each individual subject (n = 15). Within-subject reliability (n = 7) was high across the two tasks. We then tested the degree to which these sensory-biased structures demonstrated multiple demand behavior. First, we measured BOLD recruitment in the non-preferred memory task; visual-biased sPCS and iPCS showed significantly more recruitment during auditory WM than did tgPCS and cIFS during visual WM. Second, for each vertex within these structures we projected the activation in the two tasks into a 2D vector space; the multiple demand index derived from these values was significant for visual-biased but not for auditory-biased LFC structures. Both metrics indicate that the visual-biased LFC structures exhibit stronger multiple-demand responsiveness than do the auditory-biased structures, suggesting that the auditory processing network may be more specialized in its cognitive role. These results reconcile two competing theories of LFC’s contribution to human cognition by demonstrating the coexistence of sensory specialization and multiple demand behavior.