we have previously described the procedures for obtaining pieces of ground squirrel retina . in brief , after euthanasia , eyes were enucleated under dim red light and hemisected around the anterior pole . following vitrectomy ,
a piece of superior retina with attached pigmented epithelium was separated from the sclera and placed on the electrode array ganglion cell side down .
experimental use of animals was approved by the institutional animal care and use committees at the university of california , santa cruz . during the experiment , the retina was superfused with carboxygenated ames solution ( sigma - aldrich , st .
l - ap4 and ly341495 were obtained from tocris bioscience ( minneapolis , mn ) .
application of the ly341495/l - ap4 mixture resulted in a roughly 40% drop in spike rate in the m - dominated on - center cells in the presence of the visual stimulus , consistent with either no change or a small hyperpolarization in the membrane potential of the on - bipolar cells . for comparison ,
application of l - ap4 alone led to almost complete silencing of these cells , consistent with a strong hyperpolarization of on - bipolar cells .
the image from a computer controlled color crt monitor was focused on the photoreceptor layer through the transparent electrode array and the retina .
spatiotemporal white noise , in which the three monitor primaries varied independently , had a 9090 m pixel size and a refresh rate of 60 hz .
the mean photon absorption rate by s(m)-cones was equivalent to the one caused by a monochromatic light of 440(500 ) nm wavelength and 5400(10000 ) photons-m - s .
transformation from the measured response to the green and blue monitor primaries into the s - cone and m - cone responses was calculated using the emission spectrum of the monitor phosphors and s - cone and m - cone spectral sensitivity curves ( supplementary fig .
the response to the red monitor primary color was negligible and was not taken into account in the cone response calculations .
the transformation resulted in cone responses consistent with the ones obtained through s- and m - cone isolating stimulus ( verified in one preparation ) .
the time - course of a ganglion cell s sta response was calculated from regions within the receptive field center that exceeded background intensity at any time during a response by more than 3 standard deviations .
2b ) were obtained from the first ( closest to the spike ) peak in the sta time - course .
a two dimensional gaussian surface was fitted to the sta frame that contained the peak response .
receptive field size was calculated as the diameter of a circle with the area equivalent to the area of the 1 contour .
the center point of the gaussian fit was used to calculate nearest neighbor distance ( nnd ) . a theoretical prediction for the nnd distribution arising from random cell placement
was calculated according to the probability function p(r ) = 2r exp(r ) , where r is the distance between cells and  is the measured cell density .
we used a single point kolmogorov - smirnov test to assess the difference between the observed and random distributions .
we used the unique pattern of electrical activity detected on the electrode array as each ganglion cell produces a spike to track each ganglion cell through different conditions ( supplementary fig .
the ei of a retinal ganglion cell consisted of voltage traces recorded on all 512 electrodes at the time of the cell s spike , and then averaged over all the spikes produced by that retinal ganglion cell in the recording .
a typical ei contains large amplitude somatic signals on electrodes close to cell body location and smaller , time delayed , axonal signals on electrodes along the axonal trajectory of the neuron . in ei plots ,
a filled circle assigned to the location of each electrode in the array corresponds in diameter to the relative amplitude of the signal at that electrode .
we have previously described the procedures for obtaining pieces of ground squirrel retina . in brief , after euthanasia , eyes were enucleated under dim red light and hemisected around the anterior pole . following vitrectomy ,
a piece of superior retina with attached pigmented epithelium was separated from the sclera and placed on the electrode array ganglion cell side down .
experimental use of animals was approved by the institutional animal care and use committees at the university of california , santa cruz . during the experiment , the retina was superfused with carboxygenated ames solution ( sigma - aldrich , st .
l - ap4 and ly341495 were obtained from tocris bioscience ( minneapolis , mn ) .
application of the ly341495/l - ap4 mixture resulted in a roughly 40% drop in spike rate in the m - dominated on - center cells in the presence of the visual stimulus , consistent with either no change or a small hyperpolarization in the membrane potential of the on - bipolar cells . for comparison ,
application of l - ap4 alone led to almost complete silencing of these cells , consistent with a strong hyperpolarization of on - bipolar cells .
the image from a computer controlled color crt monitor was focused on the photoreceptor layer through the transparent electrode array and the retina .
spatiotemporal white noise , in which the three monitor primaries varied independently , had a 9090 m pixel size and a refresh rate of 60 hz .
the mean photon absorption rate by s(m)-cones was equivalent to the one caused by a monochromatic light of 440(500 ) nm wavelength and 5400(10000 ) photons-m - s .
transformation from the measured response to the green and blue monitor primaries into the s - cone and m - cone responses was calculated using the emission spectrum of the monitor phosphors and s - cone and m - cone spectral sensitivity curves ( supplementary fig .
the response to the red monitor primary color was negligible and was not taken into account in the cone response calculations .
the transformation resulted in cone responses consistent with the ones obtained through s- and m - cone isolating stimulus ( verified in one preparation ) .
the time - course of a ganglion cell s sta response was calculated from regions within the receptive field center that exceeded background intensity at any time during a response by more than 3 standard deviations .
2b ) were obtained from the first ( closest to the spike ) peak in the sta time - course .
a two dimensional gaussian surface was fitted to the sta frame that contained the peak response .
receptive field size was calculated as the diameter of a circle with the area equivalent to the area of the 1 contour .
the center point of the gaussian fit was used to calculate nearest neighbor distance ( nnd ) . a theoretical prediction for the nnd distribution arising from random cell placement
was calculated according to the probability function p(r ) = 2r exp(r ) , where r is the distance between cells and  is the measured cell density .
we used a single point kolmogorov - smirnov test to assess the difference between the observed and random distributions .
we used the unique pattern of electrical activity detected on the electrode array as each ganglion cell produces a spike to track each ganglion cell through different conditions ( supplementary fig .
the ei of a retinal ganglion cell consisted of voltage traces recorded on all 512 electrodes at the time of the cell s spike , and then averaged over all the spikes produced by that retinal ganglion cell in the recording .
a typical ei contains large amplitude somatic signals on electrodes close to cell body location and smaller , time delayed , axonal signals on electrodes along the axonal trajectory of the neuron . in ei plots ,
a filled circle assigned to the location of each electrode in the array corresponds in diameter to the relative amplitude of the signal at that electrode .
 the dynamic range of visual coding is extended by having separate ganglion cell types that respond to light increments and decrements . while the primordial color vision system in mammals contains a well - characterized s - on center ganglion cell , less 
 is known about s - off counterparts . 
 we identified a regular mosaic of s - off ganglion cells in the ground squirrel . 
 contrary to convention , s - off center responses come from an s - on bipolar and inverting amacrine cell . 
Methods
Tissue preparation and recording conditions
Light stimulation and data analysis
The Electrophysiological Image (EI) of a ganglion cell
Supplementary Material