Visual learning for a mid level pattern discrimination task

Our goal was to examine the plasticity of the human visual system at mid to high levels of visual processing. It is well understood that early stages of visual processing contain cells tuned for spatial frequency and orientation. However images of real-world objects contain a wide range of spatial frequencies and orientations. We were interested in how different spatial frequencies and orientations are combined. We used st pattern discrimination task - observers were asked to discriminate small changes in a "wicker-like" stimulus consisting of six superimposed sinusoidal gratings. Observers were asked to discriminate a 15% spatial frequency shift in two of these sinusoidal components, which were masked by four noise components. We found large amounts of perceptual learning for this task - over eight sessions of training observers' average percent correct increased by 31%, corresponding to their thresholds dropping to a third of their initial values. Further experiments suggest that learning was based on changes within a mid level stage of processing intermediate between low-level analyzers tuned for orientation and spatial frequency and high-level pattern matching or object tuned cells. This mid level stage seems to be "very roughly Fourier" and combines information from individual gratings using probability summation. This stage of processing is also remarkably plastic compared to earlier stages of processing.