Objective thresholding for the detection of immunoreactive objects. A, Stack projection of 10 raw confocal laser scanning microscopy 97 × 97 μm images from a PV (red) and GluA2 (green) double stain in layer IV of macaque V1. B, Projection of the same image stack after automatic thresholding for size and intensity (see Materials and Methods). C, The number of individual, unconnected objects (>100 voxels) in the red (PV) channel, as a function of luminance threshold. I, A low threshold results in the detection of a few contorted objects. II, The optimal threshold leads to the detection of many well defined objects surrounded by empty space. III, A high threshold results in the detection of a few small objects. D, Distribution of object sizes of all objects exceeding the size threshold (100 voxels) in the red (PV) channel. Note the logarithmic x-axis and the bimodal distribution with a small second peak of very large objects, which correspond to the cell bodies.