Neural processing of surface qualities, particularly the glossiness that can be named as “GLOSSINESS PERCEPTION” is discussed in this blog. Last week I wrote about the basics of Japanese concept “shitsukan”. To build a background for those who have missed my last blog I am enclosing a short introduction of this marvelous concept again. During our daily life, we came across many sensations. The sensation or feeling of touch is the one we encounter mostly and very clearly. This sensation helps us recognize the texture (rough or smooth) of the material we are touching immediately and after that our brain tells us the type of material based on its stored sensation vocabulary. This judgement about material perception is called “Shitsukan” In Japan, “which means that how human brains make sense of material quality. Shitsukan (質感) is a Japanese word whose literal meaning is the sense (kan, 感) of quality (shitsu, 質), and it is commonly used to cover the wide range of topics to which material perception in a broad sense is assigned. The material quality, mean the properties of materials, such as whether they are gloss or matt; wet or dusty; metal or ceramic and so on”. This science or this behavior of human brain can be the basis of incorporation of artificial intelligence in textiles. This science can be applied to any material type however in this blog I would try to cover this topic with special focus on textile materials, so that my readers may have a basic knowledge of the relationship between science and textile materials.
In this blog, I wrote about what is currently known about the neural mechanisms underlying material perception, or shitsukan perception with special focus on “GLOSSINESS PERCEPTION” which plays important roles in our perception of material properties, recognition of the conditions of objects, decision making about preference/avoidance of objects, and motor control of our actions toward objects. Neuro-physio-logical evidences including single neuron recordings and neuroimaging on human and non-human primates collectively provide a view that visual information about materials and surface qualities are processed and represented mainly through a hierarchy of the ventral visual pathway. The representation of higher areas in the hierarchy tends to more reflect perception, although early and intermediate stages also play key functions. We have concentrated mainly on vision, but other sensory modalities, such as audition and tactile sensation, also play important roles. Recent studies on the cross-modal aspects of material perception give a new perspective that the representation of non-visual material properties emerge within the visual areas in the ventral pathway.
What kind of information is involved in material perception?
The interaction of light with objects having specific surface meso-structures and optical properties produces various structures within the retinal images of objects, and these structures are the source of generation of a variety of surface qualities. In addition to this, other sensory modalities are also involved in material perception. For instance, when we see a sweater made of fine wool, we can perceive that it will be soft and warm, or we can sense that a metal cup will be cold and hard to the touch. Therefore, the information obtained through different sensory modalities is closely linked with each other in material perception. In this blog I will review cross-modal aspects of material perception including following.
- Neural processing of surface qualities, particularly the glossiness that can be named as “GLOSSINESS PERCEPTION”.
- Neural processing of surface meso-structures, that is another important property and is closely linked to material perception named as “NATURAL TEXTURE PERCEPTION”.
- Neural processes involved in categorization of materials and recognition of various material properties, such as roughness and hardness, based on information about surface glossiness, natural textures, and so on. It can be called as “RECOGNITION OF MATERIAL CATEGORY AND ITS PROPERTIES”.
Now I will elaborate all of the modality of “GLOSSINESS PERCEPTION” in the subsequent section.
1. Glossiness perception (Surface reflection and glossiness)
Different materials can be discriminated because of the difference in visual features contained in their retinal images. Differences in the visual features of materials are the product of differences in the patterns of the light rays reflected from the surface of the materials. Another important factor related to this is the optical property for example, because light does not penetrate a metallic object, the light is strongly reflected. On the other hand, materials like glass transmit most of the light. In many materials, a portion of the incident light is reflected at the object surface, while some is transmitted. Therefore, it can be stated that even if an object has a smooth homogeneous surface, a variety of retinal images of objects are generated due to differences in the optical properties.Moreover, a precise description of surface reflection requires measurement of the reflected light made with combinations of incident and reflected light coming in various directions. These measurements yield what is called the bi-directional-reflectance-distribution-function. The surface reflection of many common materials is the sum of two components: Specular reflection and Diffuse reflection. Specular reflection is the component in which incident light is reflected at the surface without entering the interior of an object. It has strong directionality, and the light is reflected at the same angle θ but in the opposite direction of the incident light with respect to the normal direction of surface. Because of this strong directionality, the strength of the reflected light changes greatly depending on the relationship between the direction of the light source and the viewing direction. When these two directions have a specific relationship, strongly reflected light will be observed at some surface regions. On the other hand, diffuse reflection is a component in which incident light penetrates the object slightly and then re-emerges after being refracted or absorbed by pigments within the object. This complex process at the surface causes directionality to be lost, and the light is reflected uniformly in all directions. As a consequence, the strength of the reflected light does not depend on the relationship between the direction of the incident light and the viewing direction. In other words, the apparent brightness of the object surface is constant irrespective of the viewing direction. The large difference in the directionality between the specular and diffuse reflections greatly influences object images and can lead to marked differences in the appearance or surface quality of objects. The smoothness or roughness of the object surface also affects the glossiness. Generally, for an object with completely smooth surface just like a mirror, the light is reflected in a single direction. In most cases, however, an object’s surface is not completely smooth; instead, there is a degree of unevenness on a microscopic scale or roughness. This causes variation in the direction of the specular reflection.
In my next blogs I will elaborate more about other modalities including “NATURAL TEXTURE PERCEPTION” and “RECOGNITION OF MATERIAL CATEGORY AND ITS PROPERTIES”.
- Komatsu H, Goda N. Neural mechanisms of material perception: Quest on Shitsukan. Neuroscience. 2018 Nov 10;392:329-47.