Figure 4B shows an example of the spatial distribution of unimodal and bimodal cells, as defined by their calcium responses (see Experimental Procedures) within an optical plane. Examples of single trial and averaged calcium fluorescence changes in response to unimodal and bimodal stimulations are shown in Figure 4C. To address our question, we exploited the fact that many RL neurons are directionally selective to moving visual stimuli (Marshel et al., 2011). We used squared gratings drifting in either the rostro-to-caudal or caudo-to-rostral direction (Figure 4D and see Experimental Procedures). We found that many RL neurons

were selective for the direction of the stimulus: their direction-selectivity index, defined as (Pref − NonPref)/(Pref + NonPref)—where Pref and NonPref are the responses to the preferred and non preferred C59 wnt mouse direction, respectively, was on average 0.79 ± 0.33 (119 responsive cells from 7 mice), in line RO4929097 with a previous report (Marshel et al., 2011). The very same tactile stimulus (an air puff to the whisker pad directed rostrocaudally) was then presented simultaneously with either the preferred or the nonpreferred visual stimulus. On average, the tactile stimulus increased the response to the nonpreferred visual direction significantly more than the preferred

DNA ligase visual direction (Figure 4F; 119 neurons; average enhancement 52% versus 0%, paired Wilcoxon rank-sum test, p < 0.001). Hence, a given unimodal stimulus selectively enhances responses to the nonpreferred stimulus configuration of the other modality, in line with the so called “inverse effectiveness principle” described in other multisensory areas in the mammalian brain (Stein and Stanford, 2008). We next investigated the spatial distribution of unimodal and bimodal cells by means of population calcium imaging. Figure 5A shows the overlay of all imaged, responsive neurons (34%, 503/1480 labeled neurons from 10 mice),

where each cell is positioned along the rostrocaudal (S1-V1) axis with respect to the midline of RL. The mean positions of unimodal neurons were statistically different, with tactile cells (T cells) closer to S1 and visual cells (V cells) closer to V1 (Figure 5B; mean distances from midline: −4.8 ± 5.2 μm for T cells, 23.4 ± 5.5 μm for V cells and 4.6 ± 5.9 μm for multimodal cells (M cells), p < 0.01, one-way ANOVA, n = 165, 176, and 162, respectively; Tukey post-hoc: p < 0.01 for T and V cells, p = 0.08 for V and M cells, p = 0.53 for T and M cells). To investigate whether the positions of unimodal neurons follow a gradient along the V1-S1 axis, we divided the imaged area in three stripes orthogonal to the rostrocaudal axis.