Fig. 5.

Hypothetical schematics of the patterns of sensory inputs that generate different forms of sensory/multisensory processing. The figures at the top represent the current model of multisensory processing. For a given neuron (grey colored body and dendrite), afferent axons and terminals (black) from one sensory modality (e.g., A) or another (B) provide excitatory drive via the priority (location) or weight (number) of their inputs. In the case of unimodal neurons (top right), inputs from modality ‘A’ generate suprathreshold responses to that modality while there is no effect of modality ‘B’ alone or in combination with ‘A.’ In contrast, for bimodal neurons (top-left), inputs from either modality ‘A’ and ‘B’ are sufficient to activate the neuron alone, and their combination produces an integration of the responses to ‘A’ and ‘B.’ A combined ‘A + B’ stimulus generates a integrated multisensory signal in bimodal neurons, but a distinct unisensory response from unimodal neurons. The results of the present investigation suggest that there is a range of convergence patterns that occur between the bimodal and unisensory extremes. By reducing the priority and/or weighting of inputs from modality ‘B,’ a neuron may loose its suprathreshold response to that modality while still being significantly affected by it when combined with inputs from modality ‘A.’ These forms of multisensory neurons have been termed ‘subthreshold’ (e.g., Dehner et al. 2004; Allman and Meredith 2007; Meredith and Allman 2009). In addition, further reduction of input priority/weighting of modality ‘B’ further reduces its effectiveness of influencing responses to modality ‘A,’ such that its effect can be seen at population levels or when local inhibitory circuits are pharmacologically blocked (Allman et al. 2008a). In this way, combination of modality ‘A + B’ produces a response within a mixed population of neurons whose levels of activity generate a smoothed, or continuous distribution between the two extremes. The pattern of multisensory convergence is not likely to be limited to these schematics; nor do these figures intend to represent any specific anatomical relationships