Evidence for separate, task dependent noise processes in orientation and size perception

Spatial acuity was estimated as the minimum difference in target spatial frequency which could act as a cue in a double concurrent, single interval orientation discrimination task. Experiments were conducted with vertical and oblique (45 deg) targets at three different spatial frequencies covering a range of two octaves. Frequency thresholds were found to be lower than those estimated by conventional methods by a factor of nearly 50%. They followed Weber's law for target frequency and were independent of target orientation. The orientation thresholds exhibited a normal oblique effect, and were not affected by the simultaneous judgements of target frequency. The data support the contention that the neural codes for image size and image orientation are segregated at an early stage of processing, and that the main source of noise that limits the precision of coding in these domains exists beyond the level of early mechanisms.