The Internal Senses—Functions or Powers?: Part II

THE INTERNAL SENSES-FUNCTIONS OR POWERS? PART II We will now try to show that the internal senses have definite brain structures as sensoria and that a correlation of what our psychological analysis has shown about these senses with the knowledge derived from the results of neurological research can help us better to understand the functioning of the inner senses and also the functioning of the brain. The brain functions outlined here were arrived at by an extensive study of the available very recent research evidence. While we are presenting a theory of brain function, it is the only one available that integrates the reported facts in a consistent way. Indeed, it is the only theory available today. We will show that it fits in well with the traditional teaching of philosophy on the internal senses, which may be an additional reason for saying that it fits all known facts. According to St. Thomas, the sensus communis is a sensory power that has as its formal object the activity and content of the external senses. Now let us see what the sequence is that starts from the external senses and ends with the act of seeing objects. First, the sensory receptors are acted on by specific energies possessed by sense objects. This action has psychological as well as physical and physiological effects. For instance, light produces an optical image of the viewed object on the retina. The retinal cells react with action currents that travel along the optic nerve to the optic tract and the lateral geniculate bodies, the thalamic relay station for visual impulses . From there, relays go to the afferent plexus, one of six cortical layers in the visual area. Normally, the physiological activity up to this point disposes the visual sense to see. This disposition, the psychological effect of visual stimulation, is the visual species impressa. The act to which it disposes the visual sense is the actual visual sensation. If the physiological 15 16 MAGDA B. ARNOLD process stopped at this point, there would be an experience of light and color, but not of seeing things as we ordinarily know them. If we are to see objects, the afferent nerve impulses must connect with the cortical cells in the remaining five layers 1 of the sensory area. All these cells form an intricate network in which every cell is connected with several other cells. When these connections function the sensus communis apparently begins to operate; the effect of its act is the experience of seeing a visual object. Accordingly, the sensorium of the sensus communis seems to be the feltwork of cortical connections between the afferent and the efferent layers, both in the sensory and the adjoining association cortex. Though the sensus communis is one power, it is specified by its acts; and its acts are the intentional representations of the activity and content of different sense modalities. For this reason, we should not be surprised to find that the visual cortex is necessary for perceiving visual objects, the auditory cortex for perceiving direction and pattern of sound, the cortex of the somatosensory area for perceiving an object by touch, etc. The unity of the sensus communis is preserved by the connection of every cortical sensory area with every other such area, both via short and long association fibers. The primary sensory cortex seems to mediate the perception of objects, but the adjoining association cortex seems to make possible the retention of sense impressions. There are relays from the sensory thalamic nuclei distributed both to the primary sensory areas and to the adjoining association areas. For this reason, we are inclined to postulate two functions of the sensus communis: one of constructing its intentional image (mediated by the primary areas), the other that of retaining it (mediated by the association areas). St. Thomas ascribes the retention of the sensory construct produced by the sensus communis to the imagination, assum1 The neocortex, which includes sensory, motor and association cortex, has six layers of cells. The cells receiving afferent fibers are usually in the fourth (internal granular) layer, while the afferent fibers arise from the fifth (pyramidal) layer. INTERNAL SENSES-FUNCTIONS OR POWERS: PART II 17 ing that...