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Marco J. H. Puts, Joel Pokorny, Vivianne C. Smith; Magnocellular and parvocellular mediated Vernier acuity. Journal of Vision 2004;4(8):64. doi: 10.1167/4.8.64.
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PURPOSE: In a Vernier acuity task, observers resolve a displacement between two objects. In such tasks, subjects can show hyperacuity, acuity higher than predicted by the cone mosaic. Lee, Wehrhahn, Westheimer & Kremers (1995, Vision Research, 35, 2743) showed that MC cell responses correlate to human psychophysical hyperacuity data. The goal of our study was to provide an independent psychophysical evaluation for MC-mediation of hyperacuity. METHODS: Two vertical bars (15 min by 10 min), one placed above the other, were presented on a 1 deg square pedestal for 40 ms. Horizontal displacement of the two bars ranged from 15 sec to 100 sec. The background was a 12 cd/m2 field; pedestals ranged from 8 to 17 cd/m2. In the Pulsed-Pedestal Paradigm, the pedestal was presented synchronously with the stimulus. The pulsed pedestal favors PC mediation (see Pokorny & Smith, 1997, JOSA A, 10, 545). In the Steady-Pedestal Paradigm, the pedestal was presented continuously. In this paradigm, PC as well as MC might mediate Vernier acuity. In both paradigms, the contrast threshold to resolve each Vernier displacement was measured as a function of pedestal contrast. RESULTS: The data showed a different signature for Vernier displacements smaller than 40 arcsec (i.e. hyperacuity) than for larger displacements. For the pulsed-pedestal, the threshold changed more dramatically as a function of pedestal threshold for smaller displacements than for larger displacements. For the steady-pedestal, only the thresholds for small displacements obeyed Weber's law. CONCLUSION: For hyperacuity displacements, the saturation of the MC pathway has a large effect, suggesting that hyperacuity is MC-mediated. High spatial acuity is mediated by the PC-pathway, since the steady pedestal thresholds above hyperacuity do not obey Weber's law.
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