1Calcium imaging is a powerful tool for capturing the simultaneous activity of large 2 populations of neurons. Here we determine the extent to which our inferences of neu-3 ral population activity, correlations, and coding depend on our choice of whether and 4 how we deconvolve the calcium time-series into spike-driven events. To this end, we 5 use a range of deconvolution algorithms to create nine versions of the same calcium 6 imaging data obtained from barrel cortex during a pole-detection task. Seeking suit-7 able values for the deconvolution algorithms' parameters, we optimise them against 8 ground-truth data, and find those parameters both vary by up to two orders of mag-9 nitude between neurons and are sensitive to small changes in their values. Applied to 10 the barrel cortex data, we show that a substantial fraction of the processing methods 11 fail to recover simple features of population activity in barrel cortex already estab-12 lished by electrophysiological recordings. Raw calcium time-series contain an order of 13 magnitude more neurons tuned to features of the pole task; yet there is also qualitative 14 disagreement between deconvolution methods on which neurons are tuned to the task. 15 Finally, we show that raw and processed calcium time-series qualitatively disagree on 16 the structure of correlations within the population and the dimensionality of its joint 17 activity. Collectively, our results show that properties of neural activity, correlations, 18 and coding inferred from calcium imaging are highly sensitive to the choice of if and 19 how spike-evoked events are recovered. We suggest that quantitative results obtained 20 from population calcium-imaging be verified across multiple processed forms of the 21 calcium time-series. 22 28 Pnevmatikakis et al., 2016; Friedrich et al., 2017; Keemink et al., 2018; Giovannucci et al., 29 2019). As somatic calcium is proportional to the release of spikes, so we wish to use these 30 fluorescence time-series as a proxy for spiking activity in large, identified populations of 31 neurons. But raw calcium fluorescence is slow on the time-scale of spikes, nonlinearly 32 related to spiking, and contains noise from a range of sources. 33 These issues have inspired a wide range of deconvolution algorithms (Theis et al., 2016; 34 Berens et al., 2018; Stringer and Pachitariu, 2018), which attempt to turn raw somatic cal-35 1 cium into something more closely approximating spikes. Deconvolution algorithms them-36 selves range in complexity from simple deconvolution with a fixed kernel of the calcium 37 response (Yaksi and Friedrich, 2006), through detecting spike-evoked calcium events (Jew-38 ell and Witten, 2018; Pachitariu et al., 2016), to directly inferring spike times (Vogelstein 39 et al., 2010; Lütcke et al., 2013; Deneux et al., 2016). This continuum of options raises 40 the further question of the extent to which we should process the raw calcium signals. We 41 address here the question facing any systems neuroscientist using...