HISTORICAL REVIEW AND BIOLOGICAL APPLICATIONS

Late 1600s

R. Boyle questioned the extremely practically oriented chemical theory of his day and taught that the proper task of chemistry was to determine the composition of substances.

1750

A.-L. Lavoisier studied oxidation, and correctly understood the process. He demonstrated the quantitative similarity between chemical oxidation and respiration in animals. He is considered the father of modern chemistry.

Late 1700s

The work of J. Priestley, J. Ingenhousz, and J. Senebier established that photosynthesis is essentially the reverse of respiration.

1800s

The development of organic chemistry, despite the strong opposition of vitalists (who believed that transformations of substances in living organisms did not obey the rules of chemistry or physics but those of a vital force), led to the birth of biochemistry. In 1828, F. Wöhler performed the first laboratory synthesis of an organic molecule, urea. During the 1840s, J. V. Liebig established a firm basis for the study of organic chemistry and described the great chemical cycles in Nature. In 1869 a substance isolated from the nuclei of pus cells was named nucleic acid. O. Avery's experiments of 1944 on the transformation of pneumococcus strongly suggested that nucleic acid was the support of genetic information.

1860s

L. Pasteur is considered the father of bacteriology. He proved that microorganisms caused fermentation, putrefaction, and infectious disease, and developed chemical methods for their study. In 1877 Pasteur's “ferments” were named enzymes (from the Greek en “in” and zyme “leaven or yeast.”) In 1897 E. Buchner showed that fermentation could occur in a yeast preparation devoid of living cells.

1882

E. Fischer showed that proteins are very large molecules built of amino acid units; he also discovered the phenomenon of stereoisomerism in carbohydrates.

1926

Urease, the first enzyme to be crystallized in a pure form, was shown to be a protein by J. B. Sumner. In the 1960s, M. Perutz and J. Kendrew solved the molecular structures of hemoglobin and myoglobin, respectively, by crystallography, thus proving that proteins had well-defined structures. They were awarded the Nobel Prize.

HISTORICAL REVIEW

Mid-fifteenth century

The simple one-lens microscope was available at this time, with low-power magnifiers being used for the examination of insects. From these early beginnings, glass lenses of increased power were developed. A. Leeuwenhoek (1632–1723) produced remarkable microscopes. One of them had a resolving power of 1.35 μm, which was enough for basic cytology. Around the same time, the basic idea for the compound microscope, in which two or more lenses are arranged in such a way to form an enlarged image of an object, occurred independently to several people in the Netherlands (H. Jansen, his son Zacharias, and H. Lippershey). Subsequently, compound microscopes became widely available.

1830

The first achromatic microscope lenses appeared in the Netherlands at the end of the eighteenth century. In 1830, J. Lister published the first work in which the construction and design of achromatic lenses was placed within a theoretical framework. He showed how two separate achromatic lenses could be combined to act as a single lens so that the image rendered would be completely free from chromatic aberration and from distortion caused by curvature of the lenses. This discovery opened the way to the construction of high-power microscope lenses within the space of 50 years.

1834

Henry Fox Talbot built the first polarization microscope by equipping his light microscope with polarizers. It became an essential tool for mineralogists studying rock crystals.

Latter part of the nineteenth century

E. Abbe was the outstanding figure in development of the microscope. He placed the theory of microscopic image formation on a sound basis by emphasizing the importance of the light diffracted by the object in resolving its fine details. He introduced the concept of numerical aperture and developed oil-immersion lenses. Very soon this type of lens, with its much improved resolving power and high-quality images, was an essential part of the microscopist's equipment. Abbe showed that an increase of resolving power of the microscope was possible only by increasing the numerical aperture of the optical system or by using a shorter wavelength. Abbe proposed the diffraction limit of resolution power of a microscope was about 1/2 of the illuminating light's wavelength.

Early twentieth century

Developments of the microscope have included the introduction of larger stands with built-in illumination systems and higher standards of accuracy of construction.

The pitch helix

The pitch helix is a highly relevant concept in the psychology of music. Two nineteenth- century German mathematicians, Friedrich Wilhelm Opelt (1794 – 1863) and Moritz Wilhelm Drobisch (1802 – 1896), were the first to suggest a helical model to represent the sensation of pitch, octave equivalence and recurrence (Opelt, 1852; Drobisch, 1855).

Opelt started his career as a weaver, but later became a director of the Sächsisch- Bayrische Staatseisenbahn. Among astronomers he is still famous for his maps of the moon. Drobisch was a professor of mathematics and philosophy at the University of Leipzig and still has an enduring reputation in empirical psychology and logic.

The concept of a helical organization underlying the sensation of pitch can be related to neuronal mechanisms of temporal processing in the auditory system. Moreover, as the final chapter of this book will show, a helical organization is not restricted to mechanisms of hearing. Anatomical evidence for a variety of helical structures in non-auditory brain areas support a theory of harmonic processing of oscillatory brain signals even beyond the level of the hearing system.

But first we will have to understand the role of harmony for periodicity processing and pitch perception and the related concept of the pitch helix. It seems trivial that the pitches of two tones are perceived as very similar if their fundamental frequencies are nearly the same. Tones should sound similar, even if they do not activate the same neurons, provided they at least activate adjacent neurons in the pitch maps of the brain. However, two tones which differ in frequency by 100% (an octave) suddenly become similar again; in fact, they are often confused and may be quite difficult to distinguish if played, for example, by different instruments.

‘Grandmother cell’ and ‘cocktail-party problem’

In 1971 Otto Creutzfeldt, the director of the Department of Neurobiology of the Max- Planck Institute for Biophysical Chemistry in Göttingen, had sent Christoph von der Malsburg (Fig. 12.1c) and me – the two young physicists in his group – to the Sorbonne in Paris. We were supposed to join a neurophysiology course organized by the International Brain Research Organization. The Parisian atmosphere and the lively discussions about the functioning of the brain with a dozen young neuroscientists of various nationalities in the cafés of the Quartier Latin would certainly inspire our research in the following years. Back in Göttingen, both Christoph and I studied the temporal aspects of neuronal feature detection; I started to investigate the coding of amplitude modulations in the midbrain of guinea fowl, while Christoph worked on neuronal nets in the visual cortex, and also on a solution for the ‘cocktail-party problem’. In other words, he tried to answer the question of how our brain manages to isolate a particular voice under the noisy conditions of a cocktail party (see Chapter 6). But in his paper entitled ‘A neural cocktail-party processor’ the acoustic example of feature analysis served only as a pertinent example for the more general ‘ binding problem ’ (von der Malsburg and Schneider, 1986).

It is obvious that not only the auditory but all sensory systems have to coordinate features distributed over space and time – features like size, position, direction and colour in vision or loudness, position, pitch and timbre in hearing.

The telephone theory

Although a broadband harmonic sound, for example a human vowel (see Figs. 2.8, 2.9), activates many sensory cells along the basilar membrane, it is typically perceived as an entirety and has just a single pitch. We neither hear the rapid amplitude fluctuations, due to its periodic envelope, nor realize that it is composed of a fundamental frequency and perhaps dozens of harmonics, which all have quite different pitches when heard separately. Helmholtz was able to single out individual harmonics of tones by means of his spherical resonators (see Fig. 3.5), but he could not explain the fusion of these harmonics into single tones with just one pitch. This puzzle remained a challenge for scientists from the nineteenth century to the present time (Stumpf, 1890; Ebeling, 2008).

To overcome this, and other drawbacks of Helmholtz's resonance theory, the Scottish physiologist William Rutherford (1839–1899) proposed an alternative idea. In 1886, in a lecture to the British Association for the Advancement of Science in Birmingham, he postulated that the ear functions like a telephone, the pioneering device that had been invented only ten years earlier by Alexander Graham Bell (1847–1922). While Helmholtz had compared the basilar membrane to a piano, Rutherford, on the basis of his own anatomical investigations, had come to the conclusion that the basilar membrane must vibrate as a whole, just like the membrane of a telephone.

Consequently, the sensory cells of the inner ear should pick up the mechanical sound vibrations, transform them into electrical currents and transmit the resulting signal via ‘cables’ (the auditory nerve fibres) to the receiver (the brain). As a result, the essential auditory information would be coded exclusively in the time domain and the nerve fibres would have to transfer merely temporal signals to the brain for further temporal processing.

A potential problem with this theory, however, is that it would require a very high transmission rate to the brain.

Periodicity coding in the auditory nerve

Temporal coding

As we know from the previous chapter, a pure tone activates a certain place on the basilar membrane maximally, and hair cells at this place code the frequency of that signal by their position along the membrane. However, we also know that this labelled line coding, or ‘place information’, alone is not sufficient to explain the acuity of our perception and that the central auditory system obviously must make use of additional information that is supplied by the temporal firing patterns of the nerve fibres.

We also saw that the limited frequency resolution of our cochlea is actually advantageous for the temporal analysis of – for example – a harmonic sound. When its frequency range is sufficiently broad, neural responses in many frequency channels will be elicited by a waveform that is a superposition of harmonic components of the harmonic sound. As a consequence, hair cells signal the presence and intensity of the harmonics to which they are tuned to the central auditory system, but they also transmit temporal information about this superposition. Because all the components of a harmonic sound are integer multiples of its fundamental frequency, the periodicity of their superposition will correspond to that of the fundamental frequency (or a multiple thereof, if certain harmonics are missing). As a result, the fundamental of a harmonic sound is encoded by the temporal discharge patterns of auditory nerve fibres even if it is physically absent. This explains why, after a corresponding temporal analysis in the central auditory system, we are able to perceive the pitch of what Schouten called the ‘missing fundamental’.

If the fundamental can be coded in the auditory nerve, the question arises as to how the corresponding temporal information is decoded in the nervous system and how the properties of the nerve and the neurons in the brain contribute to this temporal analysis.

The sound of sirens

At the beginning of the nineteenth century, the field of experimental acoustics was floundering. One of the main problems at this time was a purely technical one – there was no easy method to vary the frequency of tones in a precise and reproducible way. Experimental research relied on the use of tuning forks or oscillating strings, or on simple musical instruments such as flutes and, in practice, these methods were imprecise and somewhat difficult to handle. Therefore no great progress could be made in understanding the essence and perception of tones.

A turning point for acoustical research came with the invention of the siren as a scientific instrument. Originally introduced by Charles Caignard de la Tour (1819), the siren made it possible to generate tones of a precise and quantifiable frequency. It was this new-found accuracy and reproducibility in tone production which opened the door for acoustics to become both an independent and an exact science.

Several prominent researchers invented their own versions of the siren for acoustical experimentation – one of them, a young German physicist named August Seebeck (Fig. 3.1), constructed a siren which incorporated a number of improvements over Caignard's original design (Fig. 3.2 shows a simple version). It allowed the creation of continuous, clear, steady tones over a large range of pitches. The tones were easily reproducible and could also be quantitatively defined by means of a mechanical counter. Moreover, since the tones were related to the geometric arrangement of the holes on a rotating disc, octaves and other harmonic intervals could be simultaneously generated with high precision.