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Combining information across modalities can affect sensory performance. We studied how co-occurring sounds modulate behavioral visual detection sensitivity (d), and neural responses, for visual stimuli of higher or lower intensity. Co-occurrence of a sound enhanced human detection sensitivity for lower-but not higher-intensity visual targets. Functional magnetic resonance imaging (fMRI) linked this to boosts in activity-levels for sensory-specific visual and auditory cortex, plus multisensory superior temporal sulcus (STS), specifically for a lower-intensity visual event when paired with a sound. Thalamic structures in visual and auditory pathways, the lateral and medial geniculate bodies, respectively (LGB, MGB), showed a similar pattern. Subject-by-subject psychophysical benefits correlated with corresponding fMRI signals in visual, auditory, and multisensory regions. We also analyzed differential "coupling" patterns of LGB and MGB with other regions in the different experimental conditions. Effective-connectivity analyses showed enhanced coupling of sensoryspecific thalamic bodies with the affected cortical sites during enhanced detection of lower-intensity visual events paired with sounds. Coupling strength between visual and auditory thalamus with cortical regions, including STS, covaried parametrically with the psychophysical benefit for this specific multisensory context. Our results indicate that multisensory enhancement of detection sensitivity for low-contrast visual stimuli by co-occurring sounds reflects a brain network involving not only established multisensory STS and sensoryspecific cortex but also visual and auditory thalamus.

The auditory cortex of the Mongolian gerbil comprises several physiologically identified fields, including the primary (AI), anterior (AAF), dorsal (D), ventral (V), dorsoposterior (DP) and ventroposterior (VP) fields, as established previously with electrophysiological [Thomas et al. (1993) Eur. J. Neurosci., 5, 882] and functional metabolic techniques [Scheich et al. (1993) Eur. J. Neurosci., 5, 898]. Here we describe the cyto-, myelo- and chemoarchitecture and the corticocortical connections of the auditory cortex in this species. A central area of temporal cortex corresponding to AI and the rostrally adjacent AAF is distinguished from surrounding cortical areas by its koniocortical cytoarchitecture, by a higher density of myelinated fibres, predominantly in granular and infragranular layers, and by characteristic patterns of immunoreactivity for the calcium-binding protein parvalbumin (most intense staining in layers III/IV and VIa) and for the cytoskeletal neurofilament protein (antibody SMI-32; most intense staining in layers III, V and VI). Concerning the cortical connections, injections of the predominantly anterograde tracer biocytin into the four tonotopically organized fields AI, AAF, DP and VP yielded the following labelling patterns. (i) Labelled axons and terminals were seen within each injected field itself. (ii) Following injections into AI, labelled axons and terminals were also seen in the ipsilateral AAF, DP, VP, D and V, and in a hitherto undescribed possible auditory field, termed the ventromedial field (VM). Similarly, following injections into AAF, DP and VP, labelling was also seen in each of the noninjected fields, except in VM. (iii) Each field projects to its homotopic counterpart in the contralateral hemisphere. In addition, field AI projects to contralateral AAF, DP and VP, field DP to contralateral AI and VP, and field VP to contralateral AI and DP. (iv) Some retrogradely filled pyramidal neurons within the areas of terminal labelling indicate reciprocal connections between most fields, both ipsilateral and contralateral. (v) The labelled fibres within the injected and the target fields, both ipsilateral and contralateral, were arranged in continuous dorsoventral bands parallel to isofrequency contours. The more caudal the injection site in AI the more rostral was the label in AAF. This suggests divergent but frequency-specific connections within and, at least for AI and AAF, also across fields, both ipsilateral and contralateral. (vi) Projections to associative cortices (perirhinal, entorhinal, cingulate) and to other sensory cortices (olfactory, somatosensory, visual) from AAF, DP and VP appeared stronger than those from AI. These data support the differentiation of auditory cortical fields in the gerbil into at least 'core' (AI and AAF) and 'noncore' fields. They further reveal a complex pattern of interconnections within and between auditory cortical fields and other cortical areas, such that each field of auditory cortex has its unique set of connections.

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