Situacional de los Servicios de Telecomunicaciones

functional anatomy, physiology, and neural processes that underlie human actions attempting to move or hold still the fingertip. This has included, from shoulder to fingertip, a discussion of the human osseous skeleton (see Figure A-1) and contributing joint movements (see Figure 1.7); also, a brief introductory overview of the physiology of muscle contraction and receptor signaling, followed by a discussion of specific somatosensory receptor types that are involved in fingertip movement and touch (see Table 1.8); and, finally, a discussion of neural processes involved in fingertip movement and touch, culminating in the concepts of passive touch, active touch, and manipulation.

From these areas, we will now highlight and discuss a number of points that are fundamental for instrumental control of musical sound via the fingertips. Naturally, this also covers those forms of control characterized by unidirectional fingertip movement orthogonal to a surface, as defined in Section 1.2.5.

1.3.4.1 Anatomy An important anatomical fundamental is that in fingertip

instrumental control, the fingers are especially suited to flexing movements. This follows from the general anatomy of the human hand being especially suited to the execution of gripping movements enclosing objects.

The overall placement and relative lengths of the various bones of the hand are precisely related in ways that specifically enable various gripping movements [Tubiana et al. 1996]. Other anatomical factors further support the suitable combined orientation of the phalanges during the execution of gripping movements. These factors correspond to the joint movements involved not being straight, not happening around fixed rotational axes, not being identical across joints, and not happening orthogonally; issues which more generally have been highlighted in Section 1.3.1.3.

For example, in many gripping movements, the fingers flex together toward the palm. Here, the relative lengths of the phalanx bones of each finger, decreasing from proximal to distal, allow each finger as a whole to progressively wrap around ever smaller objects. Then, DIP joint flexion, occurring together with, but slower than PIP joint flexion, locks grip at the end of movement [Tubiana et al. 1996].

The small phalanx bones may then apply large forces to the objects enclosed in flexion, due to the projection of muscle tension via tendons. Here, it is telling that the two flexor tendons inside each finger can apply considerably greater muscle tension than the one extensor tendon [Tubiana et al. 1996]. Extension is further limited by the fibrous skeleton, at the articulations of the digits, where extension approaching dorsal flexion (“hyperextension”) is blocked.

The fingers are also better suited to flexion and gripping due to the type of skin on their palmar sides. Through its thickness, sweat glands, and friction ridges, palmar skin provides mechanical resistance against relative movements when making contact. It is further supported in this by the subcutaneous fat pads, also present on the palmar side

of the hand, which cause the hand to mold around objects during their enclosure, thereby increasing grip surface [Tubiana et al. 1996]. As has been discussed in Section 1.3.1.4, these palmar fat pads also include the pulp of the fingertip, supported dorsally by the distal phalanx bone and fingernail.

1.3.4.2 Neural processes Focusing on neural processes, it can first be said that

fingertip instrumental control is based on neural processes of sensorimotor integration, and is especially suited to these. This includes processes involving motor learning, recall and activation of motor programs, passive touch, active touch, and manipulation. Such neural processes are supported by detailed perception via the fingertips, simultaneous to precise and independent finger movements. The latter are possible because of the lumbrical and interosseous muscles inside the hand [Tubiana et al. 1996], which act under direct control of the motor cortex. There, the fingers have an exceptionally large representation in the motor homunculus. In the sensory homunculus, the fingers, and especially the skin of the fingertips, have a large cortical representation as well. As mentioned in Section 1.3.3.2, this representation may increase significantly over time as the result of instrumental control of musical sound. Detailed perception is further supported by the high density of small receptive fields in fingertip skin [Kahle 2001] [Goldstein 2002] [Wolters and Groenewegen 2004], which is reflected in the two-point threshold being lowest across the human body at the fingertips.

Then, of the neural processes of sensorimotor integration, those involving motor programs are crucial to fingertip instrumental control. As discussed in Section 1.3.3.3, spontaneously occurring neural processes of motor learning may result in fingertip instrumental control which largely relies on somatosensory transduction alone. This is associated with crucial advantages for instrumental control. One is faster movement execution through a reduced reliance on corrective sensory feedback. Another is the reduced or even absent claim on resources of attention and consciousness, once the execution of a movement has been initiated. In combination, these advantages may very well increase the number of changes that can be made intentionally and successfully to the sound-generating process over a given period of time. This is beneficial, in the sense of increasing the number of different possibilities for resulting musical sound. Also, the activation of motor programs may allow attention and consciousness to become more occupied by resulting musical experiences, which are the ultimate object of instrumental control. These advantages, realized, can be found e.g. in the study and practice of traditional acoustic instruments, which over time results in increasingly complex musical pieces becoming playable.

This also implies that when studying fingertip instrumental control, it is a pitfall to consider somatosensory transduction only as a means of feedback. As we have seen, somatosensory transduction informs the preparation of movement, consciously and unconsciously, and it underlies the feedforward of motor programs.

A second implication is that when studying fingertip instrumental control, it is a pitfall to consider only the consciously experienced aspects of sensorimotor integration. Consciously experienced processes of passive touch, active touch, and

manipulation might intuitively seem the most significant, simply because they are noticed. However, as we have just seen e.g. for motor programs, it may be the very absence from consciousness that indicates successfully realized benefits to instrumental control. The importance of both conscious and unconscious aspects of instrumental control is emphasized in the schematic overview shown in Figure 1.9.

Figure 1.9 Important types of human sensorimotor activity underlying instrumental

control of musical sound via the fingertips.

Finally, from the previous points, it follows that compared to other types of sensory transduction, somatosensory transduction is especially important to fingertip instrumental control. The neural processes of sensorimotor integration underlying fingertip instrumental control, involving motor programs, active touch, and manipulation, rely on somatosensory transduction.

Still, during instrumental control, it may very well be other types of sensory transduction, perhaps visual or auditory, which effectively convey to the human that a change has been made to the sound-generating process. For example, a touch screen being pressed may light up, or trigger a clicking sound, to unambiguously confirm its activation. Or, the heard musical sound itself may be what confirms that a change has been made. Also, besides forms of feedback, visual transduction may more generally provide information on the current state of the sound-generating process, and this may continuously guide fingertip instrumental control. (Examples of this include the GUI displays of sound-generating algorithms.) Visually transduced information may also specifically relate to the process of making changes itself, e.g. as when a pianist, for orientation between key presses, visually tracks hand position during large and rapid movements across the keyboard.

But it is an increased reliance in fingertip instrumental control on the feedforward of motor programs, with further guidance by somatosensory transduction alone, that will realize the crucial advantages discussed near the beginning of this section.

1.3.4.3 Fingertip instrumental control: Active touch or manipulation? In instrumental control via the fingertips, changes to the sound-generating process may be caused by processes that involve active touch as well as manipulation. However, of the two, only manipulation (as defined in Section 1.3.3.5) will, consciously or unconsciously, confirm to the human that a change has been effected through the perception of changed object properties.

Given a concrete example of instrumental control, such as playing the piano, are the changes made via the fingertips to the sound-generating process the result of active touch, or of manipulation? As we have discussed in Section 1.2.3, there are similarities in the manual operation of piano-type keyboards and computer keyboards, and pressing computer keyboard keys is sometimes regarded as an everyday example of active touch [Goldstein 2002]. This would seem to suggest active touch as the answer.

However, perhaps the answer may be both, depending on the time scale, and on how the invariant exterospecific components probed by active touch are defined. If we consider the piano keyboard on a time scale corresponding to probing before and after a performance, we might say the changes we heard were made due to active touch: The piano has not changed, its keyboard still feels the same to us. However, if we choose to probe on a smaller time scale, before and during a key press, we might say the changes we hear are being made due to manipulation: The depressed key presents a change in exterospecific components, and it, and the keyboard in general, do feel different to us.

In this text, we will opt for the latter interpretation of what constitutes manipulation.

1.3.4.4 Applicability to unidirectional fingertip movement orthogonal to a surface In

Section 1.2, we noted that unidirectional fingertip movement orthogonal to a surface can be regarded as a common component in many ancient and existing forms of fingertip instrumental control. We characterized it as fingertip motion which approximates a single path of movement, at right angles with a surface, and extending across at most a few centimeters. We noted some of the advantages of this for the control of musical sound.

The previous discussion of human fundamentals in Sections 1.3.4.1 to 1.3.4.3 now enables a fuller understanding of this type of movement. Summarizing, this is as a product of the human hand's predisposition toward gripping postures, while allowing independent and precise flexing movements by the fingers. The latter will be the result of conscious and unconscious processes of sensorimotor integration, possibly involving motor programs, passive touch, active touch, and manipulation. Naturally, each point discussed in the above sections relating to fingertip instrumental control in general will also apply in the cases where there is unidirectional fingertip movement orthogonal to a surface.