Fetsch, Pouget, DeAngelis, and Angelaki Neural correlates of reliability-based cue weighting during multisensory integration

1 Single-cell neurometric (ROC) analysis To compliment the population decoding approach (Figs. 4, 6, main text), we also tested the ability of individual MSTd neurons to account for cue reweighting behavior. Specifically, we used receiver operating characteristic (ROC) analysis to quantify the behavior of an ideal observer performing the same task as the animal but using only the firing rate responses of a given neuron. Response distributions for each nonzero heading were compared to the distribution for heading = 0° (taken as a proxy for the internal reference) to compute the proportion of rightward choices made by the ideal observer. As with the psychophysical analysis (Fig. 1b-d), these ‘neurometric’ choice data were fit with cumulative Gaussian functions, and the neuronal PSE and neuronal threshold were taken as the mean and standard deviation of the fitted function, respectively. For the example neuron shown in Fig. 3 (replotted in Fig. S1a-c), neuronal thresholds in the single-cue trials were 6.7°, 13.6°, and 2.1° for the vestibular, low-coherence visual, and high-coherence visual conditions, respectively (Fig. S1d). These thresholds quantify the neuron’s sensitivity to small variations in heading for each modality, which is primarily dictated by the slopes of the respective tuning functions. Using Eq. 2, single-cue neuronal thresholds were used to generate optimal vestibular weights for the ideal observer (here, 0.80 and 0.08 for low and high coherence, respectively). These were compared to observed weights computed with Eq. 4, using the neurometric PSEs from the combined, cue-conflict trials (Fig. S1e,f). Note that the reference distribution (responses at heading = 0°) was taken from the Δ = 0° trials, regardless of the value of Δ. The use of non-conflict responses for the reference is a sensible strategy because the monkey cannot predict whether a trial will be a cueconflict trial, and may not even detect the conflict when present. Because the firing rates during cueconflict (green and blue traces in Fig. 3b,c and Fig. S1b,c) were compared with the non-conflict trials (orange traces), the shift of the tuning curves led to shifts of the neurometric functions, in a manner