Perceptual Properties: Sensory Effects and the Representational Structure of Perception

According to von Helmholtz ([1867] 1925, ill: 2off.), every property of natural things is an effect that is brought about by the interaction of physical objects or bodies, given their mutual relations due to the forces that objects exert on one another. He considers mechanical, chemical, optical, electrical, and magnetic properties as descriptions of the effects of the forces that physical bodies exert on one another by contact, pressure, attraction or reaction. Hence, the prop­erty ascribed to an object does not denote one of the peculiar intrinsic fea­tures of the individual object, rather it implies a relation to at least one other object. Von Helmholtz claims that for logical as well as material reasons, the characteristics of the effect depend on the nature of the interacting bodies: that which causes the effect and that upon which the effect is forced.

Perceptual properties are a subset of properties in general. They are the effects of forces that the physical bodies exert on our nervous systems. Percep­tual properties are not features of things, nor do they denote their qualities, rather they are sensory effects that the bodies induce on the sensory organs by means of those physical forces that trigger the physiological mechanisms underlying perception. The properties that are ascribed to things, for exam­ple color, touch, taste, smell, cold or warmth, and sounds, are in reality the manifestation of the interaction between the bodies in the physical world and the sensory organs. As chemical properties are produced when two sub­stances react with each other, so perceptual properties are produced when the physical object and the nervous system interact with each other. Therefore, the ascription of perceptual properties to something always implies the re­lation caused by the interaction between one physical body and the sensory organs. Nevertheless, as von Helmholtz remarks, when we speak of the per­ceived properties of things, we don’t mention the conditions in which they occur, in contrast to when we speak of the properties of external bodies pro­duced by physical interactions. We don’t merely say that lead is soluble; we mention the body or physical substance with respect to which this property exists, specifying what is obtained, for example, with nitric and sulfuric ac­ids. Instead, if we say that vermillion belongs to the surface of a red thing or is due to the light reflected from this object, we do not mention the nervous system as the reagent whose reaction allows that particular color to appear as a sensory effect.

However, von Helmholtz claims that perception cannot be simply narrowed down to an elementary sensory phenomenon. Nor can it be downsized to nervous system activity, which consists in recording the values of some param­eter of the physical world. Sensations themselves do not merely reproduce the physical signals or forces. Like effects in general, sensations depend substan­tially on the nature of the nervous system, that is, the reagent that perceivers are endowed with by their natural constitution. Von Helmholtz acknowledges as a matter of fact that different physical stimuli bring about sensations of the same type if they impinge on the same nervous sensory fibre, while the same physical stimulus brings about different sensations if it impinges on dif­ferent nervous sensory fibres. Light sensations are brought about by energy radiations that are transmitted through air vibrations, by electrical currents or by mechanical forces, such as pressure on the eyeball, provided only that they stimulate the visual nerve endings. On the other hand, the same air vibrations can be perceptually felt as light sensations by the eyes, heat sensations by the skin and sounds by the ears.

Von Helmholtz (1878) contends that sensations can be classified accord­ing to differences of modality and quality. According to modality, sensations are grouped into the same class if relations of similarity hold among them. Thus yellow is judged to be more similar to orange-red than to blue. It is pos­sible to pass from blue to scarlet through violet and carmine by comparing their appearances. Taste and color do not fall in the same class, for it makes no sense to ask if the sweet is more similar to red than to blue or if it is possible to pass from sweet to red or blue in a given direction. According to quality, the sensations of the same modality are ordered as more or less similar over one or many dimensions, such as hue and brightness for colors or pitch and loudness for tones. Von Helmholtz contends that in this respect the sensations are co-determined by physical causes. Nonetheless, their dependence on the nature of the nervous fibres is still essential. For example, colors and sounds can be ordered according to the qualities of hue and pitch in relation to the frequency of oscillations, yet this physical cause gives rise to orders of differ­ent ranges. If the ratio of oscillations is used as a yardstick and the names of musical intervals are applied as notches to measure the range of hue and pitch distinctions, it is found that auditory sensations span ten octaves, whereas color sensations span about a sixth. Furthermore, colors are ordered starting from three basic elements (red, green and violet-blue), whose combination gives rise to all the other color sensations in such a way that no alternation of the elements is distinguished. For example, white is equivalent to the mixtures of red and spectral teal-blue, yellow and ultramarine, green-yellow and violet, or even to two or all these mixtures added together. Instead, this perceptual equivalence does not hold for any two chords. The sounds are not ordered like colors, because the auditory nervous system allows for many tonal elements that are distinguished by pitch. If the sounds were ordered according to the same qualitative range as colors, the tone combinations C-F, D-G, E-A would be perceived as equivalent consonances to one another or to the combination C-D-E-F-G-B. Therefore, the classification and ordering of sensations depends on their characteristics as effects – hence as regards the modality, on what derives from the nervous reagent, and as regards the quality, on what derives from the physical causes.

On the grounds of this theory von Helmholtz claims that sensations fulfil the epistemological function of perception by making the external cause known to perceivers as a sort of notification. Sensations do not resemble objects in the outer world, rather they make them and their relations known as signs do with their referents. Unlike language, sensations are not arbitrary signs ([1867] 1925, ill: 19). A sensation is a meaningful sign if the same stimulus in the same circumstances always gives rise to it, while different signs are likely to concur with different stimulations (1868: 319; [1878] 1971: 185b). Yet this condition is not sufficient. Von Helmholtz (1878) claims that sensory signs are equivalent to samples of external objects drawn by the nervous system from the avail­able physical stimulation. Given the specific nature of the different nervous fibres, each sensory sign is drawn as an independent sample from the various dimensions of stimulation. Although not inherently conjoined, the sensations can only be efficient signs of what seem to be the things of the outer world if tied together. Von Helmholtz suggests that the sensations are tied according to the rate of their association on the grounds of the movements that subjects carry out for them to arise. For example, the sensory signs of shape, color, size, spatial direction and location of objects are tied with one another if they fre­quently occur together on the grounds of the movements by which the eyes are adjusted to distinguish the relevant sensations as accurately as possible. In par­ticular, eye movements promptly and smoothly steer each eye’s fovea to fixate the significant points of objects, while head and body positions either preserve the optimal axis of rotation of the eyes to have an appropriate view of objects or obtain distinct views of the object to minimize the change of the reference system upon which the eyes map points and directions of objects. In general, the more the rate of association of sensations increases for given movements, the more sensations play the role of symbolic shorthand for the manageable and efficient perception of the external things.

Indeed, von Helmholtz regards perception as a complex function. It in­cludes an aggregate of sensory signs for coordination with given movements. It also requires non-sensory components to interpret the meaning of the signs: a knowledge base and an inferential process. The knowledge base does not consist of explicit representations of cases and laws as, for instance, the knowledge that astronomers have on the basis of the laws of optics. It is an implicit knowledge of how things have looked in past experiences and of the “normal conditions” in which certain movements have enabled the sense or­gans to sample sensations effectively. This knowledge is learned by means of the successful repetition of movements and aggregates of sensations. It may be available as representations stored in memory or as a set of dispositions to perform determinate body motions in given circumstances. The inferen­tial process is like scientific induction, but unlike scientific induction it is an unconscious reasoning ([1867] 1925, iii: 3off.) that is needed to infer from sensations what the external bodies that caused them look like (1855: 101-102). Then, the general form of the inferential process that constitutes perception is the following. The major premise is the available learned knowledge base. The minor premise is the present aggregate of sensations. The conclusion is the property ascribed to external things as the cause of the perceptual experi­ence. Therefore, perception is theoretically decomposable into aggregates of sensations, a knowledge base of representations or dispositions, an inferential process of inductive conclusion.

Source: Calì Carmelo (2017), Phenomenology of Perception: Theories and Experimental Evidence, Brill.