Both Nevin (1969) and Shimp (1966) found on different choice procedures that pigeons equate (match) the proportion of their choices to the proportion of reinforcers each choice delivers. Their results differed in terms of the order of successive choices: Shimp found pigeons ordered successive choices so as to maximize the reinforcement rate, whereas Nevin found no evidence of such an ordering. Experiment 1 replicated both studies and found in both: (a) matching relations and (b) sequential dependencies of choice that corresponded with Shimp's maximizing prediction. The next three experiments studied the order of choices in three other choice procedures: (a) concurrent variable-interval schedules with a changeover delay, (b) concurrent variable-interval schedules without a changeover delay, and (c) concurrent-chains schedules. In all of these procedures, control of choice at the level of the response sequence was evident. The major features of the data from all four experiments were attributed to two molecular processes: response perseveration and reinforcement maximization. This evidence for a microstructure of choice suggests that the molar matching law is not isomorphic with the molecular processes governing concurrent performances.In a study by Herrnstein (1961), pigeons chose between two response keys, each associated with an independent variable-interval (VI) schedule of food reinforcement. A changeover delay (COD) that specified a minimum interval during which reinforcement was unavailable following a switch of choice-between keys was used to minimize interaction between these schedules. Herrnstein found with several pairs of VI schedules that the proportion of responses to a key (responses to a key divided by response total to both keys) equaled or "matched" the proportion of reinforcements that that

Pigeons keypecked on a two-key procedure in which their choice ratios during one time period determined the reinforcement rates assigned to each key during the next period (Vaughan, 1981). During each of four phases, which differed in the reinforcement rates they provided for different choice ratios, the duration of these periods was four minutes, duplicating one condition from Vaughan's study. During the other four phases, these periods lasted six seconds. When these periods were long, the results were similar to Vaughan's and appeared compatible with melioration theory. But when these periods were short, the data were consistent with molecular maximizing (see Silberberg & Ziriax, 1982) and were incompatible with melioration, molar maximizing, and matching. In a simulation, swa birds following a molecular-maximizing algorithm responded on the short-and long-period conditions of this experiment. When the time periods lasted four minutes, the results were similar to Vaughan's and to the results of the four-minute conditions of this study; when the time periods lasted six seconds, the choice data were similar to the data from real subjects for the six-second conditions. Thus, a molecular-maximizing response rule generated choice data comparable to those from the short-and long-period conditions of this experiment. These data show that, among extant accounts, choice on the Vaughan procedure is most compatible with molecular maximizing.Key words: maximizing, matching, melioration, choice, key peck, pigeons When animals are given choices between two concurrently available variable-interval (VI) schedules, the proportion of their responses or time allocations on these schedules typically approximates the proportion of reinforcements these schedules deliver (e.g., see de Villiers, 1977 (1) (2) where P and T refer to responses and time allocated to a schedule, R refers to reinforcement frequency, and the subscripts 1 and 2 distinguish between the two VI schedules.

In Experiment 1, food reinforcement depended on pigeons' discriminations among three bands of key-peck duration: (a) 0-msec peck (no peck); (b) 0-20-msec peck (short pecks) ; and (c) peck durations between 60 and 90, 110, or 180 msec, depending on the subject (long pecks). Trials began with the random selection of a peck-duration band and the presentation of a blue center key. The occurrence of a blue-key response within the criterion-band duration accessed the discrimination phase. This phase consisted of three differently colored keys, each color associated with a response-duration band. Only selection of the discrimination-phase color appropriate to the prior blue-key response-duration band was reinforced. All birds accurately discriminated prior response bands. In Experiment 2, trials began with a color indicating the appropriate response band to be emitted, either a short or long peck. Birds successfully emitted and discriminated both bands, a result inconsistent with Schwartz and Williams's finding that only long pecks can be reinforced. An interpretation of the peck-duration literature is presented, which views duration as a correlate of response strength.Recent work, based in large measure on a the response-independent delivery of grain, study by Brown and Jenkins (1968), sug-Successive key light-food trials were sepagests that key pecking is not invariably sensi-rated by an intertrial interval (ITI) of 1 min. tive to its consequences. In their study, food-After 20 to 200 trials, pigeons began to peck deprived pigeons were exposed to the illumi-the lighted key, even though key pecking was nation of a response key for 8 sec prior to unrelated to food delivery.Brown and Jenkins noted that while these ~~" . ~T key pecks were gratuitous, in that reinforce-This research was supported by National Insti-inHpnwiHwil-nf tVipir nrrnrrmre tute of Mental Health Grant MH22881 to The ment was independent ot their occurrence, American University. This report is based on a they nevertheless might have been shaped by thesis submitted by J. Ziriax to The American Uni-an operant process. One need only posit that versity in partial fulfillment of the requirements the onset O f fa e ^ey Ug^t produced an for a Master's degree. We would like to thank . . , , . c . Scott Parker for helpful comments and suggestions orientation movement toward the key. Since and Daniel Najjar for technical assistance. orientation movements would be closely fol-Requests for reprints should be sent to John l owe d by reinforcement, perhaps key pecking ?^^SS^S^^^. *» automatically shaped, as the name given 20016.this phenomenon, autoshaping, suggests.

Digital anatomical models of man and animals are available for use in numerical calculations to predict electromagnetic field (EMF)-induced specific absorption rate (SAR) values. To use these models, permittivity values are assigned to the various tissues for the EMF frequencies of interest. There is, as yet, no consensus on what are the best permittivity data. This study analyzed the variability in published permittivity data and investigated the effects of permittivity values that are proportional on SAR calculations. Whole-sphere averaged and localized SAR values along the diameter of a 4-cm sphere are calculated for EMF exposures in the radio frequency range of 1 MHz to 1 GHz. When the dimensions of a sphere are small compared to the wavelength (i.e., wavelength inside the material is greater than ten times the dimensions of the object), the whole-sphere averaged SAR is inversely proportional to the permittivity of the material composing the sphere. However, the localized SAR values generally do not have the same relation and, as a matter of fact, vary greatly depending on the location within the sphere. These results indicate that care must be taken in choosing the permittivity values used in calculating SAR values and some estimate of the dependence of the calculated SAR values on variability in permittivity should be determined.

This work describes and presents results from a new three-dimensional whole-body model of human thermoregulation. The model has been implemented using a version of the "Brooks Man" anatomical data set, consisting of 1.3x10(8) cubic volume elements (voxels) measuring 0.2 cm/side. The model simulates thermoregulation through passive mechanisms (metabolism, blood flow, respiration, and transpiration) and active mechanisms (vasodilatation, vasoconstriction, sweating, and shivering). Compared with lumped or compartment models, a voxel model is capable of high spatial resolution and can capture a level of anatomical detail not achievable otherwise. A high spatial resolution model can predict detailed heating patterns from localized or nonuniform heating patterns, such as from some radio frequency sources. Exposures to warm and hot environments (ambient temperatures of 33-48 degrees C) were simulated with the current voxel model and with a recent compartment model. Results from the two models (core temperature, skin temperature, metabolic rate, and evaporative cooling rate) were compared with published experimental results obtained under similar conditions. Under the most severe environmental conditions considered (47.8 degrees C, 27% RH for 2 h), the voxel model predicted a rectal temperature increase of 0.56 degrees C, compared with a core temperature increase of 0.45 degrees C from the compartment model and an experimental mean rectal temperature increase of 0.6 degrees C. Similar, good agreement was noted for other thermal variables and under other environmental conditions. Results suggest that the voxel model is capable of predicting temperature response (core temperature and skin temperature) to certain warm or hot environments, with accuracy comparable to that of a compartment model. In addition, the voxel model is able to predict internal tissue temperatures and surface temperatures, over time, with a level of specificity and spatial resolution not achievable with compartment models. The development of voxel models and related computational tools may be useful for thermal dosimetry applications involving mild temperature hyperthermia and for the assessment of safe exposure to certain nonionizing radiation sources.