The suggestion that FG perception is based on inhibitory competition raises the possibility that FG perception may change during healthy aging. Evidence from anatomical, physiological, and psychophysical studies suggests that aging may affect the balance of excitatory and inhibitory mechanisms in visual cortex. For example, a study of human visual cortex from older adults found that there were changes in several pre- and postsynaptic GABAergic markers (Pinto, Hornby, Jones, & Murphy,
2010), consistent with an aging-related reduction in the efficacy of the GABAergic system. A reduction in the efficacy of inhibitory mechanisms is thought to contribute to the age-related decline in orientation and directional selectivity of V1 neurons (Leventhal, Wang, Pu, Zhou, & Ma,
2003; Schmolesky, Wang, Pu, & Leventhal,
2000), and perhaps the reduction in selectivity of speed and directional MT neurons (Liang et al.,
2010; Yang et al.,
2009) in older monkeys. A decrease in the density of GABA-immunoreactive neurons also has been associated with neurophysiological changes in visual neurons in cat striate cortex (Hua, Kao, Sun, Li, & Zhou,
2008) consistent with an age-related decline in inhibitory function. The results of several psychophysical studies are consistent with the hypothesis that inhibitory cortical neural circuits are diminished by aging. For example, motion detection and discrimination thresholds are consistent with models of aging that incorporate broader directional tuning and increased internal noise (Bennett, Sekuler, & Sekuler,
2007), similar to the changes found in neurophysiological studies. Age-related changes in the ability to recognize faces across different viewpoints (Habak, Wilkinson, & Wilson,
2008) also are consistent with neural models that incorporate a reduction of inhibition with aging (Wilson, Mei, Habak, & Wilkinson,
2011). Age-related changes in inhibitory mechanisms, based on a model proposed by Lehky and Blake (
1991), also may play a role in older observers' increased suppression in binocular rivalry tasks (Beers, Bennett, & Sekuler,
2013; Norman, Norman, Pattison, Taylor, & Goforth,
2007). As well, spatial suppression in a motion discrimination task is significantly lower in older adults compared to younger adults (Betts, Sekuler, & Bennett,
2009, Betts, Taylor, Sekuler, & Bennett,
2005), a result consistent with the idea that aging alters the balance between excitatory and inhibitory mechanisms that encode motion direction (Betts et al.
2012; but also see Govenlock, Taylor, Sekuler, & Bennett,
2009,
2010; Karas & McKendrick,
2012,
2015; Rosen, Sekuler, & Bennett,
2013, for examples of tasks in which aging does not appear to be linked to decreased inhibition).