Combining orientations to identify letters

Visual channels detect specific frequencies or orientations of periodic visual patterns, such as gratings. However, when we look at the world, we rarely see gratings. Mostly, we see complex objects. Aside from being used to detect simple patterns, what role do channels play in identifying real life objects like letters? Since letters contain multiple orientations, there must be a mechanism in the visual pathway that integrates information across orientations. I systematically measured the observers’ ability to identify letters and to detect gratings in the presence of visual noise at particular orientations. Results for letters and gratings were very different. The experiment revealed that the visual channel used to identify letters integrates equally across all orientations, whereas the channel used to detect a grating is tuned to the orientation of the grating. This is unlike the result for frequency tuning, where the same selectivity is found for letter identification and grating detection (Solomon & Pelli, 1994). This indicates that the channel used for letter identification is different than that used for grating detection, having the same frequency tuning, but no orientation tuning.