Abstract
Neurons in the cat superior colliculus (SC) integrate information from different senses to enhance their responses to cross-modal stimuli. These multisensory SC neurons receive multiple converging unisensory inputs from many sources; those received from association cortex are critical for the manifestation of multisensory integration. The mechanisms underlying this characteristic property of SC neurons are not completely understood, but can be clarified with the use of mathematical models and computer simulations. Thus the objective of the current effort was to present a plausible model that can explain the main physiological features of multisensory integration based on the current neurological literature regarding the influences received by SC from cortical and subcortical sources. The model assumes the presence of competitive mechanisms between inputs, nonlinearities in NMDA receptor responses, and provides a priori synaptic weights to mimic the normal responses of SC neurons. As a result, it provides a basis for understanding the dependence of multisensory enhancement on an intact association cortex, and simulates the changes in the SC response that occur during NMDA receptor blockade. Finally, it makes testable predictions about why significant response differences are obtained in multisensory SC neurons when they are confronted with pairs of cross-modal and within-modal stimuli. By postulating plausible biological mechanisms to complement those that are already known, the model provides a basis for understanding how SC neurons are capable of engaging in this remarkable process.
Highlights
Superior colliculus (SC) neurons integrate inputs they receive from multiple sensory modalities, thereby enhancing and speeding their responses to spatiotemporally coincident cross-modal stimuli (Rowland et al, 2007b; Stein and Stanford, 2008)
The present network model is able to account for and provide a mechanistic explanation for the major physiological observations pertaining to SC multisensory integration
This includes multisensory enhancement and suppression, inverse effectiveness, the effects of selective cortical deactivation, NMDA blockade, and the differing responses and underlying computations that characterize responses to pairs of spatially disparate cross-modal and within-modal stimuli. This was achieved by including a limited number of biologically realistic mechanisms, some of which are known to be in place in this circuit and others of which require physiological verification
Summary
Superior colliculus (SC) neurons integrate inputs they receive from multiple sensory modalities, thereby enhancing and speeding their responses to spatiotemporally coincident cross-modal stimuli (Rowland et al, 2007b; Stein and Stanford, 2008). The SC is interesting because it receives converging, topographicallyaligned unisensory inputs from many subcortical and cortical areas (Edwards et al, 1979), but inputs from association cortex (AES and rLS in the cat) are requisite for the development (Jiang et al, 2007), maintenance (Wallace and Stein, 1994; Jiang et al, 2001, 2002; Jiang and Stein, 2003; Alvarado et al, 2007a), and expression (Wilkinson et al, 1996; Jiang et al, 2002, 2007) of multisensory integration. Our understanding of this highly adaptive capacity is limited by our ignorance of the specific biological mechanisms by which it operates
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