Notes from Yost: Chapter 1 “The world we hear, An introduction” pp 1-7. [15 January 2004]

 

Yost provides a quick and easy introduction to the domain of the text and its treatment of audition, stressing the point that he is going to concentrate on the neural coding of the simple physical characteristics of sound waves, and on their sensory consequences. He also justifies his treatment dealing almost entirely on “normal” human hearing so that students (who may become practitioners of the various sorts who deal with auditory function) will have a good understanding of basic phenomena on which to found their concern with the hearing impaired. (The last chapter in the book is concerned with forms of hearing impairment and we will spend 5 - 10% of the lecture time on various hearing problems.) His last two topics include the very brief history of auditory research and the basics of scientific research.

 

Yost introduces the problem of auditory science by its biological function -- to provide information about the identity and location of sound sources: “... we need to know that an object exists, what is it, where it is, if it is moving, and so on.” (Page 1). But for many animals it does more than this: for humans hearing and speech together are certainly the most significant communicative channel and thus provides us with the greatest contact with social and cultural forces. This may be true of many other animals as well, most evidently the avian species, but also many mammals who interact with others of their species at great distances – whales and elephants, for example.

 

               One apparent difficulty in understanding how audition works is the fact that unlike vision, in which “objects” occupy different parts of the retina and hence “visual space”, auditory stimuli overlap on the same receptor surface -- note in Figure 1.1 that sounds combine in a “complex sound field” to affect the ear, yet we are able to pull out of that complex a single auditory source. Yost does not yet show how all of these sound sources overlap with each other, but they do: so a sound source has to be somehow identified by something other than receptor location. Receptor activity, about which we now know a great deal, is translated into a neural code, about which quite a bit is known, and then it is the processing of this neural code that yields for us the perception of the sound source: it is this last step of processing that remains to a large extent mysterious. And then, of course, the fact of “perception” requires integration with other senses and with memory processes as well, which is even more mysterious. All of this chain of circumstance, from the complex sound wave to the occurrence of a response (and perhaps, of our “experiencing” something, is outlined schematically in Figure 1.2). Out of this figure comes the plan of the book: the discussion of the three (just 3!) characteristics of a sound wave, namely, frequency, intensity, and time (= phase) and the mechanical and electronic devices that produce and process sound; neural coding, given in the anatomy and physiology of the auditory system; the sensory aspects of the three simple attributes of sound, including the source location; and then complex perceptions.

 

               Yost speaks briefly of the professions concerned with the practice of hearing science (MDs and PhDs, otologists and audiologists, speech therapists, and so on); and the variety of backgrounds of people who do research in hearing science -- which can include those from all of the basic and applied sciences, physicists and physicians, chemists, biologists, different sorts of engineers, computer scientists, neuroscientists, psychologists, and more. He points out that most applied practice now is concerned with the neural encoding of sound at the early periphery of the auditory system (the ear) rather than the brain -- this is because most problems seem to arise at this point, and also, most is know about this part of the system: as a result Yost will concentrate most on the physiology of the periphery in his text. Two more modern developments are becoming apparent, that are not well described in this text (or, unfortunately any other text). One is that ideas about possible genetic engineering to remedy/prevent hearing disorders are appearing, and beginning to have an inpact at the level of peripheral mechanisms. The second is that more is becoming known about the contributions of central auditory mechanisms to complex auditory perceptions, and as more becomes known about the neural problems that may appear in these mechanisms, then it becomes possible to imagine central manipulations, pharmacological manipulations perhaps, that may benefit people who suffer from subtle perceptual problems, such as dyslexia and presbycusis.

 

               Yost brings in the concept of a “communication system” at this stage, defined as consisting of three parts: as a “sender” a “message” and a “receiver”: in other words, “speech,” “language”, and “hearing.” By concentrating on hearing, one of Yost’s goals is to provide a better understanding of at least part of the communication system. (And, of course, hearing will shape both speech and language: probably no one of them is independent of the others.) He also introduces the value of animal research here. The auditory system is much the same across mammals, and its evolution across the animal kingdom indicates that much of it has been conserved, particularly at the periphery. Much of what we know about the sensorineural bases of hearing result from animal work.

 

               A brief history of work in hearing science is presented in the next section, going back to the mathematical work on music by Pythagoras, showing the relationship between string length and the pitch of tones produced by stringed instruments (about 500 BC), but he then jumps to the mid 19th century in one sentence. (A slightly longer and certainly more scholarly account of the early development of hearing science in the early 19th century is presented in Boring, A History of Experimental Psychology, 1950, pp 106 - 110). Many contributions were made by biologists and by physicists who tried to work out theories of how we heard the attributes of sound waves: the ideas of Helmholtz on pitch perception (1863) that pitch was determined by the place on the basilar membrane remain influential, though they were clearly wrong in detail. And then in the early part of this century there was a rapid advancement of experimental work aided by access to increasingly capable electronic apparatus for measuring and producing sounds, and considerable research was supported by the need to improve telephone systems. Advances in physiological techniques beginning in the 1950s allowed the measurement of neural activity from auditory centers, while advances in computer technology since the 1960s has fostered better control over experimental conditions and the analysis of data of greater quantity and complexity. As a result the basic understanding of auditory function is rapidly increasing.

 

               Yost concludes the chapter with a smidgen of scientific methodology, and the concept of “empirical methods”, “functional relationships”, and “confounding variables” involving dependent and independent variables, and variables that covary with the independent variables; displaying the results of experiments in graphic form, with the y-axis as the ordinate and and the x-axis as the abscissa; and the concepts of prediction, hypotheses, laws, models, and theories. I hope this is all “old hat” to you.