Abstract
Using textures mapped onto virtual nonsense objects, it has recently been shown that early visual cortex plays an important role in processing material properties. Here, we examined brain activation to photographs of materials, consisting of wood, stone, metal and fabric surfaces. These photographs were close-ups in the sense that the materials filled the image. In the first experiment, observers categorized the material in each image (i.e., wood, stone, metal, or fabric), while in an fMRI-scanner. We predicted the assigned material category using the obtained voxel patterns using a linear classifier. Region-of-interest and whole-brain analyses demonstrated material coding in the early visual regions, with lower accuracies for more anterior regions. There was little evidence for material coding in other brain regions. In the second experiment, we used an adaptation paradigm to reveal additional brain areas involved in the perception of material categories. Participants viewed images of wood, stone, metal, and fabric, presented in blocks with images of either different material categories (no adaptation) or images of different samples from the same material category (material adaptation). To measure baseline activation, blocks with the same material sample were presented (baseline adaptation). Material adaptation effects were found mainly in the parahippocampal gyrus, in agreement with fMRI-studies of texture perception. Our findings suggest that the parahippocampal gyrus, early visual cortex, and possibly the supramarginal gyrus are involved in the perception of material categories, but in different ways. The different outcomes from the two studies are likely due to inherent differences between the two paradigms. A third experiment suggested, based on anatomical overlap between activations, that spatial frequency information is important for within-category material discrimination.
Highlights
Perception research has focused primarily on the perception of lines and shapes, and on the processing of complex objects such as faces
The region-of-interest analyses in the right hemisphere yielded classification accuracies of 31% correct for V1 (p = 0.04, one-sided t-test), 30% for V2 (p = 0.008), 28% for V4 (p = 0.014), and 28% for the posterior parahippocampal gyrus (p = 0.09), compared to a chance level of 25% - analysis of permuted data indicated that the chance level was 25%
As texture information must be important for categorizing materials, the adaptation paradigm promises to reveal areas outside the visual cortex that are involved in processing materials
Summary
Perception research has focused primarily on the perception of lines and shapes, and on the processing of complex objects such as faces. An indication that material perception is different from shape or outline perception is provided by studies showing that certain brain areas are specialized in processing texture information and gloss (see below). A multi-voxel pattern classification approach is suitable for revealing a brain region’s responsiveness to manipulations that cannot be revealed with standard contrast approaches In such a classification approach, the pattern of voxel activations subsequent to an event such as the presentation of a certain stimulus is used to predict the nature of this stimulus. As texture information must be important for categorizing materials, the adaptation paradigm promises to reveal areas outside the visual cortex that are involved in processing materials. Different outcomes do occur for pattern analysis (as used in Experiment 1) and adaptation approaches (Epstein and Morgan, 2011), so the adaptation paradigm is a viable candidate for finding additional regions responding to material categories
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