Are the unimodal cells we identified with calcium imaging in RL functionally distinct from those of primary areas, or is RL a selleck chemical transition area where unimodal “primary” visual and tactile neurons, possibly left over during cortical area parcellation, coexist? The latter possibility seems unlikely for several reasons. First, many neurons that appear unimodal at suprathreshold level receive synaptic inputs from the other sensory modality, accounting for the fact that they also show ME (see Figure S3), as also described in

cats (Allman and Meredith, 2007). Second, in primary cortices heteromodal inputs mostly give rise to inhibitory responses (Iurilli et al., 2012), that we failed to observe in RL. Third, we failed to find consistent labeling of specific thalamic nuclei (such as LGN or VPM) in our retrograde tracing studies, suggesting that unimodal neurons

in RL have a distinct connectivity compared to unimodal neurons in primary cortices. The functional responses of RL neurons appear to obey the “empirical principles of multisensory integration” (Stein and Stanford, 2008) such as evidence of significant ME, topographic alignment of the modality maps, and also adherence to the so-called “inverse effectiveness principle,” in which a tactile stimulus preferentially enhances responses to weak rather than strong visual stimuli (e.g., non-preferred versus preferred Z-VAD-FMK clinical trial direction of motion). This is line with the idea that one of the advantages of multisensory integration is to preferentially enhance sensory processing of weak or ambiguous sensory stimuli. The multisensory character of RL has interesting implications with regard to its possible behavioral role. It has been recently proposed that the visual association areas that surround V1 might be involved in different types of visual processing. Area RL could belong to the more “dorsal” stream involved in visual motion coding, as suggested by the presence of many direction-selective neurons in RL (Marshel et al., 2011). Our data indicate that this view could be reconsidered, because

RL has a clear multisensory (visuotactile) character, and because the visual direction selectivity could be disrupted by the arrival of a tactile stimulus TCL (i.e., a given tactile stimulus preferentially enhances the visual response to the non-preferred direction compared to the preferred direction). In this view, area RL is part of a circuit within the posterior parietal cortex of rodents that integrates visuotactile inputs in a behaviorally-relevant manner (Pinto-Hamuy et al., 1987). Area RL sends projections to motor areas related to whisker and eye movements (Wang et al., 2012). Also, area RL projects to other posterior parietal areas that are involved in path integration and spatial navigation, as shown by lesion (Whitlock et al., 2008) and imaging (Harvey et al., 2012) studies.