1. any of the retinal ganglion cells that are sensitive to contrast (i.e., light–dark borders). See also feature detector.
2. any structure, neural or theoretical, that is sensitive to contrast.
1. a person or object that substitutes for the role of an individual who has a significant position in a family or group. See father surrogate; mother surrogate.
2. more generally, any person or entity that substitutes for another.