Apparatuses and methods for determining whether a test compound solution induces cell activity, an embodiment of the method of the present invention comprising placing a test compound solution in contact with a cell suspension media containing cells, diffusing the test compound solution into the cell suspension from a point source, and detecting activity in the cells with respect to their distance from the point source. Detecting activity in the cells can involve detecting activity in a first group of the cells proximate to the point source, and detecting activity in a second group of the cells farther from the point source than the first group.