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Department of Neurobiology, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany.

1. We have measured responses of macaque ganglion cells to moving borders under conditions designed to simulate the minimally distinct border (MDB) task. 2. Extending previous results, we show that minimization of responses of phasic ganglion cells of the magnocellular (MC)-pathway obey the photometric laws of transitivity and additivity. 3. To equal luminance borders, a residual response was present in MC-pathway cells analogous to the second harmonic response seen in these neurones with temporal chromatic modulation. It was proportional to the tritanopic purity difference (magnitude of delta Pt, the rectified middle- to long-wavelength cone opponent signal) between the two colours on either side of the border. For a delta Pt of one, the mean residual response was equivalent to the response evoked by achromatic borders of about 14% luminance contrast. Both these properties of the MC-pathway closely resemble psychophysical estimates as to the distinctness of equal luminance borders. 4. We show how MC-pathway cell responses could be used centrally to support the MDB task. It was difficult to generate a model from responses of tonic ganglion cells of the parvocellular (PC)-pathway which would support the task. 5. The MDB task is still possible psychophysically after blurring the retinal image. Although blurring the border spatially smeared the responses of MC-pathway ganglion cells and reduced their amplitude, responses still went through a minimum close to equal luminance. Thus, blurring the image did not affect the ability of MC-pathway cells to support the task. Blurring the retinal image decreased the 'sharpness' of the border response of tonic, PC-pathway ganglion cells, but response amplitude was unaffected. Response features indicative of centre-surround organization were attenuated. A central mechanism reliant on centre-surround field structure of PC-pathway cells would thus not be able to support the task after blurring. 6. Taken together, these results strongly suggest that the MC-pathway forms the sole physiological substrate of the MDB task, and any contribution of the PC-pathway is, indeed, minimal.

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