CONTRIBUTION OF T.D. LAMB

The theories of Rushton and Barlow were very much alive in the 1960s and 1970s and dominated the discussion about visual adaptation. The most convincing defence of Barlow's view was published by Lamb (1981). He admitted that Rushton's theory had received stronger support from the scientific community, but argued that Barlow's theory could more easily be reconciled with known properties of the visual system. He supported his view by presenting three strong arguments against Rushton's theory: (1) intracellular electrical recording from rods and cones under conditions of light adaptation had shown that photoreceptors could become desensitized in a way approximating the Weber law, (2) Rushton's theory implied that the photoreceptor could generate two quite different signals: an incident light intensity signal and a signal of the fraction of bleached photopigment, but no distinguishing mechanism located in the receptor had been found, and (3) the uniform desensitizing effect found by Rushton and Westheimer (1962) and Rushton (1965a) could not be repeated in an experiment performed by Barlow and Andrews (1973). In fact, these authors found a marked difference between sensitivity in the dark and bleached areas.

Lamb (1981) then went on to propose his own specification of Barlow's theory. By analyzing a series of rod dark-adaptation curves of a trichromat previously published by Pugh (1975), he came to the conclusion that the long-term dark-adaptation curve of rods could best be described by three straight lines, each with a different slope.

After a comprehensive review of the anatomical and histological research on the organs of vision from the classical antiquity to his own time, Polyak (1941) concluded that this research had played a relatively insignificant role in forming hypotheses and theories of the function of the visual system. A major reason was that the retina of humans and other primates had never been examined in any detail by analytical histological methods such as those of Golgi and Ehrlich. These methods, although very time consuming, were preferable, since they permitted the staining of only a few nerve cells at a time from the bewildering mass of tissues. Indeed, in successfully treated preparations, the individual neurons could be stained completely, revealing the whole nerve cell with all its extensions and branches extending as far as the finest terminals. Furthermore, he held that a disclosure of the structural details of the retina of primates would make possible interpretations related to the many psychophysical functions already established in humans.

Polyak, therefore, completed a very impressive histological investigation on the retina of macaques and chimpanzees, using the Golgi method. He also occasionally used retinas from humans. His comprehensive and seminal research work (summarized in 1941; second impression,1948) has long been recognized as a classic contribution to our knowledge of the retinal structure and function in primates.

PSYCHOPHYSICAL EVIDENCE

The close relationship between amount of bleached photopigment and sensitivity obtained during long-term dark adaptation strongly indicated that the sensitivity regulation mechanisms were mainly located in the receptors. Both Hecht and Wald had accepted this basic assumption. Rushton (1965a, b), however, strongly opposed this view. Thus, he held that both the signals from bleached photopigments and the signals generated by background light regulated sensitivity centrally to the receptors in what he termed ‘an automatic gain control-summation pool’ (AGC pool). He based his position on two different lines of evidence.

Firstly, he found that a background light so weak that less than 10% of the rods could have caught a single quantum of light raised the threshold three-fold. Apparently, the 10% of the rod receptors that had received a single photon hit had markedly reduced the sensitivity of the 90% that were quite unaffected by the adapting light.

Secondly, Rushton (1965a) could provide strong supporting evidence in favour of his view by both light- and dark-adaptation experiments.

In the light-adaptation experiment, Rushton took advantage of an ingenious technique where he stabilized the light stimuli on the retina. The background was a black-red grating of period 0.5º (presumed to be less than the receptive field of the summation pool) and the test flash, a black-green similar size grating (presumed to activate rod receptors). The test flash superimposed on the background was alternately in- and out-phase.