A simple mechanism for detecting low curvatures

We measured thresholds for detecting the direction of curvature in slightly bent stimuli such as arcs and sinusoidal lines, and the direction of bend in chevrons and trapezoids in an initial experiment. Stimulus length varied over a wide range from 8.3 to 267 min arc. For sufficiently long stimuli, the angle between the stimulus and its chord was constant at 0.2-0.3 deg at detection threshold. The constant threshold implies that performance might be limited by the sensitivity of a single mechanism, in this case one that detects the orientation difference between the stimulus and its imaginary chord. In a second experiment, we investigated the two-dimensional extent of curvature sensitive units by measuring thresholds for detecting curvature of arcs that were flanked by straight lines on both sides. Thresholds remained unaffected at flank distances above 6 min arc but increased at lower distances independently of arc length, compatible with the assumption of a local, orientation sensitive mechanism. In a third experiment, we measured curvature detection with stimuli of different shapes to examine psychophysically the putative role of odd side margin receptive fields. We propose that low curvatures might be analysed by orientation sensitive units with a size around 20 x 8 min arc.