Color Changes From Nonlinear Processing in the Human Visual System

The appearance of a uniform region is strongly dependent on its spatial context.  A fixed light can be made to appear various different colors by changing its surround. When the surround varies slowly over time, the uniform region is perceived to vary in appearance.  This induced temporal variation may be useful for segmenting an object from its surroundings.  This perceived temporal variation of the uniform region, however, is strongly attenuated if the surround varies more rapidly (typically, faster than 3 cycles per second).  The classical explanation is neural temporal filtering at a cortical stage that removes temporal variation at higher frequencies.    This theory assumes neural responses are linear, so that only temporal frequencies in the stimulus can be represented in the neural response.  New experiments reveal that temporal frequencies much higher than ~3 Hz are capable of inducing temporal variation when 2 different frequencies in the surround are superimposed.  Further, the induced temporal variation in the uniform region is at the difference frequency of the two temporal frequencies in the surround, even though this  frequency is not physically present in the stimulus.  This rejects the classical linear filter theory.  Instead, the results are accounted for by a nonlinear neural process that precedes a linear temporal filter.  Neural nonlinearity causes temporal variation at the difference (or beat) frequency of the two frequencies in the surround.  This nonlinearity allows the visual system to exploit a larger range of temporal frequencies than previously thought to induce temporal variation and thus contribute to object segmentation.

09/04/07