Visual adaptation and spectral sensitivity in rainbow trout

Abstract

The first part of this study is an investigation of the photomechanical movements in the retina of rainbow trout. These movements were followed during natural twilight periods and their rates of light and dark adaptation determined in the laboratory following a sudden change in the adapting illumination. The results are discussed in relation to previous data and form an introduction to the remainder of the study. The possible existence of an endogenous rhythm of photomechanical movements was subsequently examined in both laboratory and naturally entrained fish. A pattern of retinomotor movements during extended periods of darkness was found that is unique among species so far examined, with peaks of light adaptation coincident with dawn and dusk. It is suggested that such an apparently non-adaptive physiological rhythm is related to trout behavioural patterns and reveals a basic crepuscular controlling mechanism. No endogenous rhythm was observed in constant light. The function of photomechanical movements was examined in two ways. Firstly, the level of extractable visual pigment was found to remain constant during both dawn and dusk periods, suggesting that part of the function of photomechanical movements is to protect the rod visual pigment. The intensity of bleaching light needed to bleach a criterion amount of visual pigment in the light adapted condition was further found to be thirty-six times as great as in the dark adapted condition, suggesting the pigment epithelium is very effective in its shielding function. Secondly, the close temporal relation observed between sensitivity changes, measured using the ERG b-wave, during both light and dark adaptation and the position of the retinal elements further suggests that photomechanical movements determine sensitivity. Form changes of the ERG during light and dark adaptation are also briefly discussed. The second part of this study concerns the spectral sensitivity and response to flicker of the rainbow trout. The scotopic action spectrum, determined using the ERG criterion b-wave technique, agreed well with the absorption spectrum of the extractable visual pigment. The photopic action spectrum, on the other hand, obtained using both the ERG and a behavioural appetitive training technique, was observed to be more complex, showing three distinct maxima. The position of these maxima and general shape of the photopic spectral sensitivity differed depending on the technique used. It is suggested that ERG photopic action spectra are largely determined by inhibitory interactions between cones, while the behavioural spectral sensitivity curve is best explained by independent receptor interaction at most wavelengths, with some inhibition between the red and green receptors.