EVIDENCE FOR SEPARATE, TASK-DEPENDENT NOISE PROCESSES IN ORIENTATION AND SIZE PERCEPTION

Spatial acuity was estimated as the minimum difference in target spati al frequency which could act as a cue in a double concurrent, single i nterval orientation discrimination task. Experiments were conducted wi th vertical and oblique (45 deg) targets at three different spatial fr equencies covering a range of two octaves. Frequency thresholds were f ound to be lower than those estimated by conventional methods by a fac tor of nearly 50%. They followed Weber's law for target frequency and were independent of target orientation. The orientation thresholds exh ibited a normal oblique effect, and were not affected by the simultane ous judgements of target frequency. The data support the contention th at the neural codes for image size and image orientation are segregate d at an early stage of processing, and that the main source of noise t hat limits the precision of coding in these domains exists beyond the level of early mechanisms.