Cognitive Neuroscience Talks
Isolating the Top-Down Component of Perceptual Learning
Perceptual learning (i.e. an improvement in performance in a perceptual task following practice) is known to be driven by low-level, “bottom-up” processes (e.g. Crist, Li & Gilbert, 2001; Gold, Bennett & Sekuler, 1999, Karni & Sagi, 1991). It has even been shown to occur in the absence of high-level, presumably “top-down” factors such as awareness (Watanabe, Nañez & Sasaki, 2001, 2002). Several experiments, however, suggest that “top-down” processes can modulate perceptual learning to a certain extent (e.g. Shiu & Pashler, 1992; Ahissar & Hochstein, 1993; Ito, Westheimer & Gilbert, 1998). Here, we isolate “top-down” processes in perceptual learning. Four participants were submitted to 36 sessions of testing over a period of about two months. A session consisted in 250 trials; in addition, two 1/f2 target textures (T1 and T2) were presented twice per session (exposure to T1 and T2 was identical and minimal). On each trial, subjects had to indicate which one of two white Gaussian noise fields was more similar to either T1 or T2. Trials thus contained no “bottom-up” signal (Gosselin & Schyns, 2003). Crucially, the experiment comprised 24 times more T1- than T2-trials. The only difference between T1- and T2-trials was thus “top-down” practice. We compared the percentage of agreement between our human observers and an ideal observer over the logarithm of blocks of 10 successive trials. The slope of the best linear fit is significantly (p < .01) greater for T1 (R2 = 8.69%, slope = ..0145) than for T2 (R2 = 5.98%, slope = -.0148). Therefore, perceptual learning can occur in the absence of “bottom-up” signal.