Multisensory dysfunction accompanies crossmodal plasticity following adult hearing impairment.

Until now, cortical crossmodal plasticity has largely been regarded as the effect of early and complete sensory loss. Recently, massive crossmodal cortical reorganization was demonstrated to result from profound hearing loss in adult ferrets (Allman et al., 2009a). Moderate adult hearing loss, on the other hand, induced not just crossmodal reorganization, but also merged new crossmodal inputs with residual auditory function to generate multisensory neurons. Because multisensory convergence can lead to dramatic levels of response integration when stimuli from more than one modality are present (and thereby potentially interfere with residual auditory processing), the present investigation sought to evaluate the multisensory properties of auditory cortical neurons in partially deafened adult ferrets. When compared with hearing controls, partially-deaf animals revealed elevated spontaneous levels and a dramatic increase (∼2 times) in the proportion of multisensory cortical neurons, but few of which showed multisensory integration. Moreover, a large proportion (68%) of neurons with somatosensory and/or visual inputs was vigorously active in core auditory cortex in the absence of auditory stimulation. Collectively, these results not only demonstrate multisensory dysfunction in core auditory cortical neurons from hearing impaired adults but also reveal a potential cortical substrate for maladaptive perceptual effects such as tinnitus.

Connectional parameters determine multisensory processing in a spiking network model of multisensory convergence.

For the brain to synthesize information from different sensory modalities, connections from different sensory systems must converge onto individual neurons. However, despite being the definitive, first step in the multisensory process, little is known about multisensory convergence at the neuronal level. This lack of knowledge may be due to the difficulty for biological experiments to manipulate and test the connectional parameters that define convergence. Therefore, the present study used a computational network of spiking neurons to measure the influence of convergence from two separate projection areas on the responses of neurons in a convergent area. Systematic changes in the proportion of extrinsic projections, the proportion of intrinsic connections, or the amount of local inhibitory contacts affected the multisensory properties of neurons in the convergent area by influencing (1) the proportion of multisensory neurons generated, (2) the proportion of neurons that generate integrated multisensory responses, and (3) the magnitude of multisensory integration. These simulations provide insight into the connectional parameters of convergence that contribute to the generation of populations of multisensory neurons in different neural regions as well as indicate that the simple effect of multisensory convergence is sufficient to generate multisensory properties like those of biological multisensory neurons.

Somatosensory and multisensory properties of the medial bank of the ferret rostral suprasylvian sulcus.

In ferret cortex, the rostral portion of the suprasylvian sulcus separates primary somatosensory cortex (SI) from the anterior auditory fields. The boundary of the SI extends to this sulcus, but the adjoining medial sulcal bank has been described as "unresponsive." Given its location between the representations of two different sensory modalities, it seems possible that the medial bank of the rostral suprasylvian sulcus (MRSS) might be multisensory in nature and contains neurons responsive to stimuli not examined by previous studies. The aim of this investigation was to determine if the MRSS contained tactile, auditory and/or multisensory neurons and to evaluate if its anatomical connections were consistent with these properties. The MRSS was found to be primarily responsive to low-threshold cutaneous stimulation, with regions of the head, neck and upper trunk represented somatotopically that were primarily connected with the SI face representation. Unlike the adjoining SI, the MRSS exhibited a different cytoarchitecture, its cutaneous representation was largely bilateral, and it contained a mixture of somatosensory, auditory and multisensory neurons. Despite the presence of multisensory neurons, however, auditory inputs exerted only modest effects on tactile processing in MRSS neurons and showed no influence on the averaged population response. These results identify the MRSS as a distinct, higher order somatosensory region as well as demonstrate that an area containing multisensory neurons may not necessarily exhibit activity indicative of multisensory processing at the population level.