The spectrum of fatty liver is larger than detected by screening for abnormal serum aminotransferases alone. Abnormalities in serum ALT occur exclusively in more severe cases of fatty liver.

174For 7 decades, Kenneth W. Spence's (1937) theory of discrimination learning has provided an elegant and sometimes counterintuitive account of several important behavioral phenomena, including transposition, which was first explored by Köhler (1918Köhler ( /1938. In a typical transposition task, a subject may be given a single pair of simultaneously presented stimuli (e.g., a 3-cm circle and a 4-cm circle) and may be required to select the smaller one (the 3-cm circle). Originally, transposition was said to occur if the subject selected the smaller, relationally correct item in a novel pair of stimuli (e.g., a 2-cm circle and a 3-cm circle), instead of the previously reinforced stimulus (the 3-cm circle). We will term the choice of the relationally correct stimulus a transposition response and the choice of the relationally incorrect stimulus an absolute response.Spence's (1937) stimulus generalization theory explained choice of the relationally correct stimulus with interacting gradients of excitation and inhibition, which arose from absolute stimulus-response tendencies. The theory is based on three simple premises. (1) When a stimulus is associated with reinforcement, a gradient of excitation develops around it; maximal excitation is produced by the training stimulus, with orderly decreases in excitation occurring as stimuli are increasingly removed from it.(2) When a stimulus is associated with nonreinforcement, a less peaked and broader gradient of inhibition develops around it, with orderly decreases in inhibition occurring as stimuli are increasingly removed from it. (3) If the S and the S lie along the same stimulus dimension, then in order to predict responding to any given stimulus, one algebraically sums its excitatory value (on the excitatory gradient) with its inhibitory value (on the inhibitory gradient); doing so along the entire dimension yields a theoretical or derived postdiscrimination stimulus generalization gradient.It is precisely through the algebraic summation of excitation and inhibition that Spence's (1937) theory predicts choice of the relationally correct stimulus. This summation may result in maximal net excitation (and hence, conditioned responding) not at the S , but rather at a point displaced away from the S (e.g., the 2-cm circle, instead of the previously reinforced 3-cm circle). In essence, Spence (1937) proposed that the choice of the previously untrained stimulus over the former S may be no more relational than the tendency to approach S and the tendency to avoid S . Later, peak shift (reviewed by Purtle, 1973) was experimentally documented, providing further support for Spence's (1937) theory.Note that peak shift is not a necessary consequence of discrimination training. For example, when the training stimuli are widely spaced along the sensory dimension (in other words, when the training stimuli are highly dissimilar), a summation of excitatory and inhibitory gradients does not produce a peak shift, a result that has been We studied transposition in pigeons by training t...

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