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Cats were trained to respond on a multiple discriminative schedule, with milk as reinforcement. Two subjects did not immediately consume the reinforcer when they were injected with 6 mg of methylphenidate before the experiment. This observation could be repeated in one of the subjects under various conditions of reinforcement and various doses of the drug. Control experiments showed that under normal conditions the same cats never ignored the reinforcer. The modification induced by the drug in the relationship between behavior and the reinforcement is discussed in its bearing on the notion of reinforcer.

The presence of cones in potto's retina has been proved beyond doubt although they are very restricted in number (1 cone for 300 rods). Morphologically, speaking there is no point in calling these cones "rudimentary" except for their slender outer segment. There are red sensitive elements in that retina at wavelengths beyond the spectral sensitivity of visual purple and it is tempting to assume that these elements are cones. The ERG evoked from these elements by red light differs from that in response to white and blue light. They dark-adapt faster than the receptors sensitive to blue and white flashes. However in some of their properties, for example fusion frequency, these cones behave like rods in other species. As these few cones seem to activate the bipolar cells nearly as effectively as the numerous rods, it is suggested that these cones may be responsible for day vision in the potto.