[From Bruce Abbott (2017.11.21.1805 EST)}
The hierarchy of perceptual control described by Bill Powers in B:CP and elsewhere is as much a hierarchy of perceptions as of control. At the bottom level of this hierarchy are, according to PCT, intensities, which arise directly from sensory endings. Bill notes that “any one first-order perceptual signal can vary only along one dimension – frequency. This means that however the associated input function is stimulated, only one dimension of that stimulation can affect the perceptual signal: the intensity of stimulation.” He goes on to note that “a first-order perceptual signal reflects only what happens at the sensory ending: the source of the stimulation is completely undefined and unsensed.” He states that the “information is one-dimensional, indicating only how much effect is occurring.” And from this he concludes that the only thing one can tell based on such a sensory input is its intensity; the nature of the input – whether visual, auditory, etc., is not communicated.
One could argue just as well that in PCT, all perceptual signals are one-dimensional, varying only in the amplitude of the neural currents that represent them. Nevertheless, we usually can and do distinguish them, even when they arise directly from the stimulation of sensory receptors. We are able to do this because of where the signals are analyzed in the brain. Auditory signals, for example, are routed from the cochlea of the ear (where the so-called hair cells respond to pressure variations within a certain range of frequencies and stimulate associated auditory neurons) to the cochlear nucleus in the brainstem, to the thalamus, and from there to the auditory cortex located in the temporal lobes. Our conscious experience of this stimulation arises there in the auditory cortex, where we perceive it as sound. Route the same stream of impulses to, say, the visual cortex, and it will be experienced as light.
According to Bill, it is only when two or more intensities are combined that the product is a definite sensation. I don’t see how the process of combination could yield an input identifiable as a specific kind of sensation (such as light or sound), because the resulting perception is just another neural signal varying in intensity.
To further complicate things, what our sensory receptors respond to is not necessarily simply the intensity of the stimulation. For example, receptors that respond to skin temperature respond more strongly to change in temperature than to temperature per se. The output of the retina of the eye depends strongly on change of intensity, so much so that stabilizing an image on the retina quickly produces a loss of vision (although fragments of the image may reappear and disappear over time). Thus, to suggest that the outputs of first-order sensors represent simple intensities seems oversimplified.
What I worry about is that the simple hierarchical structure imagine by Bill and colleagues doesn’t fit well with what we currently know about the perceptual systems. However, the evidence does support the view that perception is hierarchical. Retinal output via the ganglion cells is already the product of significant processing of the photoreceptor inputs, including contrast enhancement and the formation of color-opponent signals. Input to the visual cortex area V1 is analyzed to produce signals representing the angles of edges or lines passing across specific regions of each retina, and these signals are processed “upstream,” yielding the perception of objects and so on. Interestingly, movement within the visual field is analyzed by a specific cortical area; in the rare cases in which this area has ceased to function, individuals report seeing the world as a series of still images without motion. (One would think that perception of motion would depend only on “noticing” that the position of an object has changed, relative to others, but apparently not!)
It is worth noting that control systems do not need to “know” the nature of the sources of their inputs – they just compare the resulting neural signal to the reference signal and take action to correct the difference. Furthermore, control may not involve any consciousness of the stimulation. We have no conscious sensation of blood pressure, for example, yet there are “baroreceptors” located in the aorta leading into the heart, which provide inputs to the blood-pressure regulating systems. It is just as true, however, that we may consciously aware of what we are controlling, what the reference for that perception is, and what actions we are performing in order to exert that control.
Comments?
Bruce
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