Adaptation to chromatic contrast has provided an important tool for characterizing cortical color coding (Eskew,
2009; Webster,
1996). The losses in chromatic sensitivity following adaptation are strongest along the LvsM (stimulation of S cones varied, but L and M cone stimulation held constant) and SvsLM (stimulation of S cones varied, but L and M cone stimulation held constant) geniculate axes, and these aftereffects were the basis for defining the geniculate axes as the
cardinal directions of color coding (Krauskopf et al.,
1982). Subsequent work showed that this form of adaptation is also selective for multiple, intermediate directions and provided evidence for
higher-order chromatic mechanisms, each tuned to a different direction in the volume of color-luminance space (Krauskopf, Williams, Mandler, & Brown,
1986; Webster & Mollon,
1991,
1994). These higher-order mechanisms have been revealed in a variety of psychophysical tasks suggesting that they are not a consequence of the adaptation alone (Eskew,
2009; Krauskopf,
1999). They are also evident in physiological measurements showing a broader range of chromatic tuning preferences in cortical cells compared to cells in the lateral geniculate nucleus (LGN) (Conway & Livingstone,
2006; Horwitz, Chichilnisky, & Albright,
2007; Johnson, Hawken, & Shapley,
2001; Lennie, Krauskopf, & Sclar,
1990; Wachtler, Sejnowski, & Albright,
2003). Thus these higher-order mechanisms appear to reflect a general and important transformation of the representation of color in the cortex. However, the degree to which these mechanisms emerge and impact color coding remains uncertain. For example, both observers and tasks vary widely in the extent to which they appear to recruit or depend on mechanisms tuned to noncardinal directions (Eskew,
2009). The present study explored some of the potential factors underlying this observer variation as measured by the task of color contrast adaptation with the specific goal of examining age-related differences in cortical color coding. Little is known about the integrity of higher-order color mechanisms with aging or whether the processes of chromatic contrast adaptation that have revealed them might themselves change with age. Age-related visual losses (e.g., in intracortical connectivity) that compromise cortical visual processing might differentially affect the selectivity or adaptability of higher-order color mechanisms compared to mechanisms with selectivity to the cardinal axes that is derived from earlier stages. In the spatial domain, there is evidence that aging may differentially compromise visual coding and adaptation in central compared to precortical sites of the visual system. Significant age-related changes in the processing of stimuli defined by second-order characteristics, such as stereopsis (Laframboise, De Guise, & Faubert,
2006), bilateral symmetry (Herbert, Overbury, Singh, & Faubert,
2002), motion defined by contrast (Habak & Faubert,
2000) or percent coherence (Billino, Bremmer, & Gegenfurtner,
2008), and perceived shape defined by texture (Habak, Wilkinson, & Wilson,
2009) have been reported. Stimuli defined by second-order characteristics (Chubb & Sperling,
1988; Cavanagh & Mather,
1989) are thought to target cortical mechanisms separate from those underlying the initial processing of first-order stimuli (Baker & Mareschal,
2001).