The neural pathways mediating color shifts induced by temporally varying light.

In natural viewing, an object's background often changes over time. Temporally varying backgrounds were investigated here with a steady test field within a time-varying surrounding chromaticity. With slow surround variation (below approximately 3 Hz), the color appearance of a steady test is also perceived to fluctuate. At somewhat higher temporal frequencies, however, temporal variation of the surround is visible but the test appears steady (R. L. De Valois, M. A. Webster, K. K. De Valois, & B. Lingelbach, 1986); also above approximately 3 Hz, temporal chromatic variation along the l- or s-axis of the MacLeod-Boynton space (symmetric about equal-energy-spectrum "white") shifts the steady appearance of the test field toward redness or yellowness, respectively (A. D. D'Antona & S. K. Shevell, 2006). In the study here, color shifts were measured with temporal surround modulation at 6 Hz or greater along axes intermediate to the l and s directions. Varying the relative phase of simultaneous surround variation in l and s should not change responses within independent l and s pathways but should differentially excite neural representations that combine l and s signals (so-called higher order chromatic mechanisms). Varying the phase of l and s showed that the induced color shifts were accounted for by neural responses both from nearly independent l and s pathways and from higher order chromatic mechanisms.