Form-motion suppressive interactions in normal and disabled readers

Detection of a low contrast static Gabor is strongly reduced by high contrast flankers whose spatial frequency (SF) is either equal or differed by ±1 and ±2 octaves to the target’s SF (Petrov, Carandini, & McKee, 2005). Suppression of a 0.5 c/deg drifting target occurs, with transient stimulation, with flankers SF < 2 octave but not ≥ 2 octave, suggesting that suppression occurs when both target and flankers stimulate the same (magnocellular) but not different systems (magnocellular and parvocellular). Based on the hypotheses of earlier development of parvocellular than magnocellular system, and of a magnocellular deficit in dyslexia, we compared the suppressive effect by flankers of SF ± 2 octave on the drifting target in adults and children either normo-reader or dyslexics. Children’s contrast threshold for the drifting isolated target did not differ from that of adults. However, both children’s groups had higher thresholds than adults when flankers SF was either lower or equal to that of the target. Moreover, only in dyslexics thresholds with +2 octave flankers are higher than with no flankers and higher than in adults. These results indicate stronger suppressive magnocellular lateral interactions in children than adults and, only in dyslexics, a motion-form stimulation imbalance.