Polytomous IRT models and monotone likelihood ratio of the total score

In a broad class of item response theory (IRT) models for dichotomous items the unweighted total score has monotone likelihood ratio (MLR) in the latent trait. In this study, it is shown that for polytomous items MLR holds for the partial credit model and a trivial generalization of this model. MLR does not necessarily hold if the slopes of the item step response functions vary over items, item steps, or both. MLR holds neither for Samejima's graded response model, nor for nonparametric versions of these three polytomous models. These results are surprising in the context of Grayson's and Huynh's results on MLR for nonparametric dichotomous IRT models, and suggest that establishing stochastic ordering properties for nonparametric polytomous IRT models will be much harder.Hemker's research was supported by the Netherlands Research Council, Grant 575-67-034. Junker's research was supported in part by the National Institutes of Health, Grant CA54852, and by the National Science Foundation, Grant DMS-94.04438.     

脑出血后智力障碍与局部脑血流量相关性研究

作者用长谷川智力测定量表法测定了６１例脑出血患者的智力,用ＳＰＥＣＴ定量测定法测定了双侧大脑半球各１５个脑区的局部血流量,并计算其比值。分析了脑出血、局部脑血流量、局部脑血流量比值与长谷川分数的关系。结果提示脑出血时,除由于病变直接破坏与智力有关的结构外,左半球（特别是左颞叶及左缘上回和角回）局部脑血流量下降,缘上回和角回、颞上回后部、颞中回后部、颞下回局部脑血流量比值失衡亦可能是造成智力下降的原因。     

Pausing under variable-ratio schedules: Interaction of reinforcer magnitude, variable-ratio size, and lowest ratio

Pigeons pecked a key under two-component multiple variable-ratio schedules that offered 8-s or 2-s access to grain. Postreinforcement pausing and the rates of responding following the pause (run rates) in each component were measured as a function of variable-ratio size and the size of the lowest ratio in the configuration of ratios comprising each schedule. In one group of subjects, variable-ratio size was varied while the size of the lowest ratio was held constant. In a second group, the size of the lowest ratio was varied while variable-ratio size was held constant. For all subjects, the mean duration of postreinforcement pausing increased in the 2-s component but not in the 8-s component. Postreinforcement pauses increased with increases in variable-ratio size (Group 1) and with increases in the lowest ratio (Group 2). In both groups, run rates were slightly higher in the 8-s component than in the 2-s component. Run rates decreased slightly as variable-ratio size increased, but were unaffected by increases in the size of the lowest ratio. These results suggest that variable-ratio size, the size of the lowest ratio, and reinforcer magnitude interact to determine the duration of postreinforcement pauses.     

Response suppression by visual stimuli paired with postsession d-amphetamine injections in the pigeon

Responding of pigeons, maintained under a fixed-interval 3-minute schedule of food presentation, was decreased on days that the color of the lights illuminating the food magazine was changed and d-amphetamine (1.0 mg/kg, i.m.) was injected after the session. Responding was not decreased by keylight color changes paired with postsession d-amphetamine or by postsession injections of saline. Administration of pentobarbital (3.0 to 5.6 mg/kg), but not d-amphetamine (.3 to 3.0 mg/kg), before the session increased rates of responding suppressed by drug-paired magazine lights. Responding maintained under a fixed-ratio 30-response schedule was not decreased when differently colored magazine lights were paired with a low (.3 mg/kg) postsession dose of d-amphetamine; with high (3.0 mg/kg) postsession doses, however, responding was completely suppressed after two pairings. The effects of pairing magazine stimuli with an intermediate (1.0 mg/kg) postsession dose of d-amphetamine depended upon the magnitude of prior postsession doses. After being paired with a low dose, stimuli paired with 1.0 mg/kg did not suppress responding. After being paired with a high dose, stimuli paired with 1.0 mg/kg completely suppressed responding. The suppression of food-maintained responding by stimuli paired with postsession drug administration depends upon both behavioral and pharmacological variables.     

Ratio responding as a function of concurrent avoidance schedules, yoked shocks, and ratio value

Fixed-ratio food-reinforced responding in rats was studied alone and with concurrent shock avoidance or with concurrent response-independent shocks matched to those that occurred in the avoidance condition. Under each condition, fixed-ratio size was increased over successive daily sessions. Fixed-ratio response rate generally passed through a maximum as a function of fixed-ratio size. Decreased fixed-ratio responding at values beyond the maximum occurred when (1) the time to complete a fixed ratio approximated the response-shock interval of the avoidance schedule, (2) the shock rate increased, and/or (3) the ratio requirements were so high that ratio strain occurred. Avoidance rates decreased slightly as fixed-ratio size increased.     

RESPONSE REDUCTION THROUGH THE SUPERIMPOSITION OF CONTINUOUS REINFORCEMENT: A SYSTEMATIC REPLICATION

Prior clinical research suggests that superimposition and subsequent removal of a schedule of continuous reinforcement (CRF) may be a viable rate-decreasing procedure in that an extinction-like condition is arranged. The arrangement of similar conditions in the laboratory, however, resulted in the quick recovery of baseline rates. Lever-pressing patterns of eight male rats maintained by different schedules of variable-ratio and variable-interval food reinforcement were examined in an A-B-A experimental design of CRF food superimposition and removal. Responding was substantially reduced during the superimposition of CRF. Upon removal of the superimposed schedule, responding quickly approached presuperimposition baseline rates.     

An inverse relationship between baseline fixed-interval response rate and the effects of a tandem response requirement.

Previous experiments examining the effects of adding a tandem fixed-ratio response requirement on fixed-interval schedule performance have reported inconsistent results. One variable that may account for such inconsistencies is the baseline response rate in the fixed-interval condition. This possibility was investigated in the present study. Rats were given histories with either interresponse times greater than 11 s or fixed-ratio 40 schedules of reinforcement, which engendered either relatively low or high rates of responding, respectively, in the subsequent fixed-interval condition. A tandem ratio response requirement (fixed-ratio 9) was then introduced. The effects of adding this tandem response requirement were inversely related to the baseline fixed-interval response rates; low rates of responding in the fixed-interval condition were markedly increased, whereas high rates of responding were relatively unaffected. This inverse relationship appears to be similar to the rate-dependent relations observed in behavioral pharmacology. These results may provide an explanation for the inconsistent findings reported in previous studies on tandem fixed-interval fixed-ratio schedules and suggest that principles of behavioral pharmacology research may be applicable to the study of the effects of nonpharmacological variables on schedule-controlled behavior.     

The linear system theory's account of behavior maintained by variable-ratio schedules.

The mathematical theory of linear systems, which has been used successfully to describe behavior maintained by variable-interval schedules, is extended to describe behavior maintained by variable-ratio schedules. The result of the analysis is a pair of equations, one of which expresses response rate on a variable-ratio schedule as a function of the mean ratio requirement (n) that the schedule arranges. The other equation expresses response rate on a variable-ratio schedule as a function of reinforcement rate. Both equations accurately describe existing data from variable-ratio schedules. The theory accounts for two additional characteristics of behavior maintained by variable-ratio schedules; namely, the appearance of strained, two-valued (i.e., zero or very rapid) responding at large ns, and the abrupt cessation of responding at a boundary n. The theory also accounts for differences between behavior on variable-interval and variable-ratio schedules, including (a) the occurrence of strained responding on variable-ratio but not on variable-interval schedules, (b) the abrupt cessation of responding on occurrence of higher response rates on variable-ratio than on variable-interval schedules. Furthermore, given data from a series of variable-interval schedules and from a series of concurrent variable-ratio variable-interval schedules, the theory permits quantitative prediction of many properties of behavior on single-alternative variable-ratio schedules. The linear system theory's combined account of behavior on variable-interval and variable-ratio schedules is superior to existing versions of six other mathematical theories of variable-interval and variable-ratio responding.     

Determinants of pausing under variable-ratio schedules: Reinforcer magnitude, ratio size, and schedule configuration

Pigeons pecked a key under two-component multiple variable-ratio schedules that offered 8-s or 2-s access to grain. Phase 1 assessed the effects of differences in reinforcer magnitude on postreinforcement pausing, as a function of ratio size. In Phase 2, postreinforcement pausing and the first five interresponse times in each ratio were measured as a function of differences in reinforcer magnitude under equal variable-ratio schedules consisting of different configurations of individual ratios. Rates were also calculated exclusive of postreinforcement pause times in both phases. The results from Phase 1 showed that as ratio size increased, the differences in pausing educed by unequal reinforcer magnitudes also increased. The results of Phase 2 showed that the effects of reinforcer magnitude on pausing and IRT durations were a function of schedule configuration. Under one configuration, in which the smallest ratio was a fixed-ratio 1, pauses were unaffected by magnitude but the first five interresponse times were affected. Under the other configuration, in which the smallest ratio was a fixed-ratio 7, pauses were affected by reinforcer magnitude but the first five interresponse times were not. The effect of each configuration seemed to be determined by the value of the smallest individual ratio. Rates calculated exclusive of postreinforcement pause times were, in general, directly related to reinforcer magnitude, and the relation was shown to be a function of schedule configuration.     

A comparison of response patterns on fixed-, variable-, and random-ratio schedules

The behavior of individual pigeons on fixed-ratio, variable-ratio, and random-ratio schedules was examined. Within each type of ratio schedule the size of the ratio was varied in an irregular sequence. At various ratio sizes (5, 10, 40, 80) no differences were found among overall response rates (postreinforcement pause plus running response rate) as a function of ratio type. This similarity in overall response rates held despite noticeable differences in the microstructure of performance both within and across subjects; the primary performance difference on the three types of ratio schedules was the relatively longer postreinforcement pause duration on the fixed-ratio schedule. We concluded that the gross temporal characteristics of performance determined by the relative weightings of the postreinforcement pause and running response rate were primarily controlled by the type of ratio schedule (fixed, variable, or random), whereas the overall rate of responding was controlled by the size of the ratio.