Recent findings suggest that the spatial receptive fields of mammalian retinal ganglion cells (RGCs) can exhibit peaks and valleys in sensitivity inconsistent with the classical dome-shaped model. Because these results were obtained from isolated retinas, it is difficult to exclude the possibility that the observed microstructure resulted from damage to photoreceptors during dissection. A strong test for this possibility is to record simultaneously from distinct cells with overlapping RFs. We performed multielectrode recordings on isolated guinea pig retina using a new 512-electrode recording system and automated spike sorting approach (Litke et al, SFN 2003). This hardware and software allowed simultaneous recording and isolation of spikes from several hundred RGCs, including cells with overlapping receptive fields, while the retina was stimulated with spatiotemporal white noise (i.e. a flickering checkerboard). Isolation of recorded cells was verified by the presence of a refractory period in the spike t...