N. J. Strausfeld

The present volume covers the physiology of the visual system beyond the optic nerve. It is a continuation of the two preceding parts on the photochemistry and the physiology of the eye, and forms a bridge from them to the fourth part on visual psychophysics. These fields have all developed as independent speciali­ ties and need integrating with each other. The processing of visual information in the brain cannot be understood without some knowledge of the preceding mechanisms in the photoreceptor organs. There are two fundamental reasons, ontogenetic and functional, why this is so: 1) the retina of the vertebrate eye has developed from a specialized part of the brain; 2) in processing their data the eyes follow physiological principles similar to the visual brain centres. Peripheral and central functions should also be discussed in context with their final synthesis in subjective experience, i. e. visual perception. Microphysiology and ultramicroscopy have brought new insights into the neuronal basis of vision. These investigations began in the periphery: HARTLINE'S pioneering experiments on single visual elements of Limulus in 1932 started a successful period of neuronal recordings which ascended from the retina to the highest centres in the visual brain. In the last two decades modern electron­ microscopic techniques and photochemical investigations of single photoreceptors further contributed to vision research.

Morphology and physiology are two fields which cannot be separated. This statement needs to be amplified: purely factual results of a morphological or physiological nature only have real value when they are gained in the context of certain guiding, embracing questions. By themselves they are mostly of little value, because only a guiding hypothesis or theory is of any importance. Equally, a physiological question will always raise questions as to the morphological substrate, and vice versa. Thus, Wiedemann's discovery, for instance, that the visual cells in each ommatidium of the dipterans have differing fields of vision has revived the question as to what the optical properties of individual visual cells, and the complete ommatidium, might be and how neighboring ommatidia interact. These questions in turn led to that of the morphological, neuronal wiring diagram of the visual cells in the optical ganglia. Within the realm of invertebrates, the morphological and physiological problems of visual perception have been resolved in very different ways on various levels, from the photo­ receptor to higher centers; despite many investigations, however, there remain unsolved problems. The first chapter of Vol. VII/6B deals with the neuroarchitecture in the visual system of two groups: crustaceans and insects. These systems are best known through recent investigations. The second chapter is devoted to the neural principles in the visual system of insects. It could not have been written without parallel research into morphology.