By the method of electrophysiology Granit (1945) has obtained evidence in the cat's eye of the presence of activity of specialized mechanisms which exhibit selective sensitivity to wave-length. Using this procedure, a great deal of information on the spectral distribution of sensitivity of receptor organs in different states of adaptation has been accumulated.The spectral sensitivity curve is likely to correspond to a reaction of the retina which manifests itself as a measurable electrical response; however, from a physiological point of view, these responses provide information only about retinal activity. The electrical method cannot tell us what the animal actually sees, because the function of seeing involves more than just the generation of electrical impulses. It is the interpretation of these impulses by the higher centres of the living animal which constitutes 'seeing' in its proper sense. In the present experiment, the 'training method' has been employed to obtain quantitative data on the sensitivity of the dark-adapted cat to lights of different wave-length. It is the aim of this experiment to determine to what extent the action potentials found in the cat's retina for this function do, in fact, correspond to the process of 'seeing' in the lving animal. METHOD Discrimination apparatusIn this apparatus the stimulus lights of different intensities were projected on a milk-glass screen, situated in each of the two doors of a Y-shaped discrimination apparatus. The animal made its choice by pushing open one of the doors and obtaining a food reward behind the one bearing the positive stimulus. A detailed description and a diagram of the apparatus are given in an earlier publication (Gunter, 1951a). ControlsThe precautions taken to ensure equality of olfactory stimulation, and the tests for prevention of light-adaptation and for light.tightness of the apparatus, were the same as those carried out previously
All the available evidence indicates that colour vision is a photopic function and as such dependent on cone activity. Thus, Granit (1942) finds on the basis of his electrophysiological studies that animals with good colour vision always have a broad dominator band of spectral sensitivity.If a retina containing a sufficient number of cones is light-adapted, then the spectral sensitivity of most of the elements is shifted towards the red end of the spectrum. Such work has been reported by Honigmann (1921) on the hen, by Grundfest (1932) on the sun-fish, by Brecher (1936) on the monkey and by Brown (1937) on the rabbit.For the cat, an animal which, according to Walls (1942), has a very respectable number of cones in its retina, Granit (1943) found that only 36 % of the retinal elements investigated showed a Purkinje shift. Physiological studies involving the whole eye have not, so far, shown such a shift of spectral sensitivity on changing from scotopic to photopic levels of adaptation. The present experiment was designed to determine-by a behavioural method-the spectral sensitivity of the cat under conditions of light-adaptation. Furthermore, previous work on the spectral sensitivity of the darkadapted cat (Gunter, 1952) by a method of behaviour has shown the maximum of sensitivity to lie in the blue-green region of the spectrum and to agree well with the electrophysiological data obtained by Granit (1943) on the same species.It would be of interest to determine if such agreement between the two methods extends over into the photopic range. METHODWith minor modifications, the same apparatus as described previously (Gunter, 1951) was used again, and a similar method was employed as in earlier experiments on dark-adapted cats (Gunter, 1952). For these reasons only the briefest description will be given here.
In recent investigations of vision in animals the methods of electrophysiology have proved an invaluable tool for the elucidation of functions of the visual apparatus. Action potentials in optic nerve fibres, as well as electro-retinograms, have been employed to this end; so that we now have a body of accurate and objective information regarding the capacities of the animal's peripheral visual system.Much of this information has been obtained by taling as an index of retinal sensitivity the relative energy in different parts of the spectrum necessary to produce a constant physiological effect, such as the height of the b-wave of the retinogram (Chaffee & Hampson, 1924), or a constant retinal potential (Graham & Riggs, 1935), or a constant number of optic nerve impulses (Graham & Hartline, 1935).By plotting sensitivity as the reciprocal of the amount of light energy necessary to produce a threshold response (or any other constant response) against wave-length, luminosity curves for various animals under different conditions of adaptation have been obtained. These curves, which describe the sensitivity of the eye to lights of various wave-length show, in most cases, fairly good agreement with the spectral distribution of sensitivity of the human eye obtained by subjective methods under similar conditions of adaptation. It is reasonable to suggest that the luminosity curve as a psychophysical function corresponds to a reaction of the retina, which manifests itself as a measurable electrical response. This quantitative electrical response has often been assumed to be the likely physiological basis of the sensation of brightness, so that, on this assumption, a constant retinal potential can be taken as a measure of the sensitivity of the retina to lights of different wave-length. This method has been widely used as a means of investigating colour vision. From the point of view of physiology, however, we are merely dealing with the spectral distribution of sensitivity under various conditions of adaptation, while properties of
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