ABSTRACT
To dissociate the roles of small and large receptive fields in the processing of line orientation, we devised a sawtooth line stimulus where local tilt of the sawtooth segments could be varied independently of overall tilt of the line. Perceived orientation of the overall line varied with the tilt of the segments in a biphasic manner. For small segment tilts (peaking between 5–15°) overall orientation was shifted in the direction of the segment tilt, whereas for greater segment tilts (peaking between 30–45°) there was a larger perceived shift in the opposite direction. The results suggest an interaction between orientation-specific neurons with small receptive fields and those with large receptive fields.
To explain these effects, we developed a model which illustrates that peak orientation shift for a population of large receptive fields is proportional to the slope of the population response profile for small receptive fields. Within this framework, small receptive fields show both excitatory and inhibitory influences on larger receptive fields, and it is possible to estimate the magnitude and orientation selectivity of these influences. The model can account for two powerful illusions of orientation, the Fraser and the Zöllner illusions.
