Digital holographic microscopy is an interferometric technique enabling the measurement of the quantitative phase shifts induced by cell bodies. We correlate the phase signal measured on neurons with calcium imaging measured by fluorescence on cells loaded with Fluo-4, to monitor responses to glutamate challenges, which provoke well-known calcium increases through activation of various membrane receptors. A very good correspondence can be identified between the two signals, showing the links between the phase signal, being a measure of the intracellular dilution, and the calcium concentration within cells. We then check cell viability by employing propidium iodide (PI), a fluorescent indicator relying on the cell membrane integrity loss to assess cell death. Strong intracellular calcium concentration is indeed known to induce excitotoxic effects, potentially inducing cell death. This enables showing that some cells cannot sustain the calcium saturation identified in our measurements, leading to subsequent cell death.