In relation to our findings, several key questions emerge: Firstly, how is perceptual learning of contrast discrimination mediated in different visual areas? In our task, for example, when subjects were trained to make comparisons between stimuli of around 30% contrast, were their behavioral improvements attributable to changes in neuronal properties at the level of V1 and V2 (Bao, Yang, Rios, He, & Engel,
2010; Carmel & Carrasco,
2008; Ghose, Yang, & Maunsell,
2002; Li, Piëch, & Gilbert,
2004; Schoups, Vogels, Qian, & Orban,
2001; Yotsumoto et al.,
2009), the frontal cortex (Kahnt, Grueschow, Speck, & Haynes,
2011), attention-network-related parts of the parietal lobe (Mukai et al.,
2007), or some intermediate region in the visual and cognitive processing hierarchy such as V4 (Mukai et al.,
2007; Raiguel, Vogels, Mysore, & Orban
2006; Rainer, Lee, & Logothetis,
2004; Williford & Maunsell,
2006; Yang & Maunsell,
2004; Zivari Adab & Vogels,
2011)? Neuronal correlates of perceptual learning have been reported for areas V1 (Schoups et al.,
2001) and V4 (Raiguel et al.
2006; Yang & Maunsell,
2004; Zivari Adab & Vogels,
2011) in orientation discrimination tasks, but it remains to be seen whether this also holds true for contrast discrimination tasks in primates.