Perception is traditionally categorized into five senses: hearing, vision, touch, taste and olfaction. These categories seem to reflect the organs of sense, rather than the sensory modalities themselves. For example, the sense of taste is generally (in the neuroscience literature) associated with the taste receptors in the tongue (sweet, salty etc). But what we refer to as taste in daily experience actually involves the tongue, including “taste” receptors (sweet, salty) but also “tactile” receptors (the texture of food), the nose (“olfactory” receptors), and in fact probably also the eyes (color) and the ears (chewing sounds). All these are involved in a unitary experience that seems to be perceptually localized in the mouth, or on the tongue – despite the fact the most informative stimuli, which are chemical, are actually captured in the nose. One may consider that taste is then a “multimodal” experience, but this is not a very good description. If you eat a crisp, you experience the taste of a crisp. But if you isolate any of the components that make this unitary experience, you will not experience taste. For example, imagine a crisp without any chemically active component and no salt: you experience touch with your tongue, and the crisp has “no taste”. If you only experience the smell, then you have an experience of smell, not of taste. This is another sensory modality, despite the fact that the same chemical elements are involved. If only the “taste” receptors on your tongue were stimulated, you would have an experience of “salty”, not of a crisp. So the modality of taste involves a variety of receptors, but that does not make it more multimodal than vision is multimodal because it involves many photoreceptors.
“Touch” is also very complex. There is touch as in touching something: you make contact with objects and you feel their texture or shape. There is also being touched. There is also the feeling of weight, which involves gravity, and also movement. There is the feeling of pain, which is related to touch, but not classically included in the 5 senses. Finally there is the feeling of temperature, which I will discuss now from an ecological point of view (in the way of Gibson).
The sense of temperature is not usually listed in the 5 senses. It is often associated with touch, because by touch you can feel that an object is hot or cold. But you can also feel that “it” (=the weather) is cold, in a way that is not well localized. Physically, it is a quantity that is not mechanical, and in this sense it is completely different from touch. But like touch, it is a proximal sense that involves the interface between the body and either the medium (air or water) or substances (object surfaces). The sense of temperature is much more interesting that it initially seems. First, there is of course “how hot it is”, the temperature of the medium. The image that comes to mind is that of the thermometer. But temperature can be experienced all over the body. So spatial gradients of temperature can be sensed. When touching an object, parts of the object can be more or less hot. So spatial gradients of temperature can potentially be sensed through an object, in the same way as the mechanical texture can be sensed. Are there temperature textures?
The most interesting and, as far as I know, underappreciated aspect of the temperature sense is its sensorimotor structure. The body produces heat. Objects react to heat by warming up. Some materials, like metal, conduct temperature well, others, like wood, don’t. So both the temporal changes in temperature when an object is touched, and the spatial gradient of temperature that develops, depends on the material and possibly specifies it. So it seems that the sense of temperature is rich enough to qualify as a modality in the same way as touch.