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Adam Kane, Anna Ma-Wyatt; Comparing chromatic and luminance information in online correction of rapid reaching. Journal of Vision 2010;10(7):1067. doi: https://doi.org/10.1167/10.7.1067.
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Humans update goal-directed reaches online. There are additional delays in integrating location changes for chromatic (parvocellular or koniocellular) targets compared to luminance (magnocellular) targets. This may reflect the chromatic pathways' slower conduction velocities. But the chromatic information may also take a longer route to the parietal cortex. Integration times increase with stimulus intensity. Comparing different stimulus types directly is problematic. Circumventing this problem in different ways has produced different results. Veerman et. al. (2008) found additional chromatic delays of ∼50ms, while White et. al. (2007) found a negligible difference. We compared integration times for for pointing chromatic and luminance targets of equal intensity. We used identical stimuli in setting subjective equiluminance and detection thresholds, and in the final experiment. The stimuli were gaussian blobs (SD ∼.5o) on a grey background. Participants first adjusted red, green, yellow and blue blobs (defined in DKL color space) to be equiluminant to the background. Next, we measured detection thresholds for these chromatic stimuli and for two luminance contrast blobs for each participant. This produced stimuli of equal salience that primarily stimulated the parvocellular, magnocellular or koniocellular geniculate pathways. Finally, participants made fast pointing responses towards a fixation cross on a touchscreen. In one-in-three trials, the blob appeared 6o left of the cross, 12-106ms after movement onset. Participants were instructed to touch the blob in under 410 ms or the trial was repeated.‘50% Integration time’ (IT50) is the ‘threshold’ time the blob was present during the reach for participants to correct more than half way towards it. Generally, IT50 was shortest for magnocellular blobs and longest for koniocellular blobs. IT50 for parvocellular targets varied between participants. The small additional integration delays for chromatic stimuli are best explained by slower conduction velocities.
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