Substitution and caloric regulation in a closed economy.

Three experiments were conducted to study the effect of an imperfect substitute for food on demand for food in a closed economy. In Experiments 1 and 2, rats pressed a lever for their entire daily food ration, and a fixed ratio of presses was required for each food pellet. In both experiments, the fixed ratio was held constant during a daily session but was increased between sessions. The fixed ratio was increased over a series of daily sessions once in the absence of concurrently available sucrose and again when sucrose pellets were freely available. For both series, increases in the fixed ratio reduced food intake, but body weight was reduced only in the no-sucrose condition. In the sucrose condition, body weight and total caloric intake (sucrose plus food) were relatively unaffected by increases in the fixed ratio. At all fixed ratios, food intake was proportionally reduced by the intake of sucrose. In Experiment 3, monkeys obtained food or saccharin by pressing keys; the fixed ratio of presses per food pellet was increased once when tap water was each monkey's only source of fluid, again when each monkey's water was sweetened with saccharin, and a third time when each monkey had concurrent access to the saccharin solution and plain water. Increases in the fixed ratio, but not the intake of the saccharin solution, reduced each monkey's food intake. Because neither rats' sucrose nor monkeys' saccharin intakes affected the slope of the respective demand curves for food, monkeys and rats increased their daily output of presses and thereby defended their daily intake of those complementary elements of food. However, sucrose reduced rats' food intake. The relative constancy of body weight and total caloric intake in the sucrose condition is consistent with the possibility that rats tended to regulate caloric intake.

Modification of acoustic and tactile startle by single microwave pulses.

Single microwave pulses at 1.25 GHz were delivered to the head and neck of male Long-Evans rats as a prestimulus to acoustic and tactile startle. For acoustic startle, pulses averaging 0.96 microsecond in duration were tested with two specific absorption rate (specific absorption) ranges, 15.0-30.0 kW/kg (16.0-44.2 mJ/kg) and 35.5-86.0 kW/kg (66.6-141.8 mJ/kg), delivered 201, 101, 51, 3, and 1 ms before and 1 ms after onset of a startling noise. The low-intensity pulse did not affect peak amplitude, integral, or latency of the whole-body startle response. The high-intensity pulse at 101 and 51 ms inhibited the startle response by decreasing peak amplitude and integral; at 201 and 51 ms latency was increased. The high-intensity pulse at 1 ms enhanced the startle response by increasing peak amplitude and at 3 ms by increasing integral. For tactile startle, either microwave pulses averaging 7.82 microseconds in duration and 55.9-113.3 kW/kg (525.0-1055.7 mJ/kg) or 94 dB SPL clicks were delivered 157, 107, 57, and 7 ms before and 43 ms after onset of a startling air burst. The microwave pulse at 57 ms inhibited the startle response by decreasing peak amplitude; at 157, 107, 57, and 7 ms it increased latency. The microwave pulse at 43 ms after onset enhanced the startle response by increasing peak amplitude. The acoustic click at 157 and 57 ms inhibited the startle response by decreasing peak amplitude; at 157,2 107, and 57 ms it increased latency.(ABSTRACT TRUNCATED AT 250 WORDS)

A surgical technique for bilateral cochleotomy in the Long-Evans rat.

A bilateral cochleotomized surgical rat model, needed for a study involving microwave effects, was developed, standardized, and assessed for reproducibility. After a review of the literature concerning attempts and approaches with various species, a technique involving an approach through the external auditory canal was chosen and modified. Using a stereomicroscope, a cutaneous incision in the intertragic notch was made and extended medially along the ventral aspect of the external auditory canal to the depth of the external auditory meatus. The tympanic membrane was ruptured and the malleus removed with splinter forceps, allowing visualization of the cochlea. The lateral wall of the cochlea was penetrated with a 0.024-in. wire gauge drill bit and endolymph was suctioned from the cochlea. A 5-mm piece of 3-O silk suture, inserted into the cochlear opening, maintained patency. Appraisal of the reliability and standardization of the procedure was performed utilizing startleometry. Histology assessed completeness of the procedure and any evidence of cochlear infection.

Temporal bisection in rats: the effects of high-peak-power pulsed microwave irradiation.

The effects of high-peak-power, pulsed microwaves on a time perception and discrimination task were studied in rats. Exposures were performed with the TEMPO exposure system, which produces an 80 nanosecond pulse with peak-power levels in excess of 700 megawatts. The ability to expose animals to such fields within a controlled environment is unique. As determined by calorimetry, a maximal, whole-body-averaged, specific-absorption rate of 0.072 W/kg was produced. Thus exposures were well below a recommended SAR limit of 0.4 W/kg. Power levels of transmitted microwaves were varied over a 50 dB range to obtain ascending and descending dose-response functions for each of the behavioral measures. Measures of time perception, response bias, and total trials did not change with power level. Dose-response effects were observed for discriminability (ability to distinguish between durations), session time, and trial completions (null responses, failures to respond on a trial). Covarying sound and X-ray exposures produced by TEMPO did not reliably correlate with the observed microwave effects. The observation of repeatable dose-response effects on discriminability and null responses indicates that the microwave exposures were affecting cognitive function in the rats, particularly the decision-making process.

Loudness bisection and masking in the rat (Rattus norvegicus).

The bisection method of animal psychophysical scaling was examined as a measurement procedure. The critical assumptions of bisection scaling, as described by Pfanzagl (1968), were tested to determine if a valid equal-interval scale could be derived. A valid scale was derived in which loudness for the rat (Rattus norvegicus; n = 13) was a power function of sound pressure for 4-kHz tones. Masking noise reduced the discriminability of tonal stimuli but did not affect the bisection point. This result is consistent with an interval scale representation of loudness and demonstrates scale meaningfulness. Loudness bisection data that have been reported in the literature for 3 species (humans, rats, and pigeons) are in substantial agreement with our results.

Intertrial-interval effects on sensitivity (A') and response bias (B") in a temporal discrimination by rats.

Killeen and Fetterman's (1988) behavioral theory of animal timing predicts that decreases in the rate of reinforcement should produce decreases in the sensitivity (A') of temporal discriminations and a decrease in miss and correct rejection rates (decrease in bias toward "long" responses). Eight rats were trained on a 10- versus 0.1-s temporal discrimination with an intertrial interval of 5 s and were subsequently tested on probe days on the same discrimination with intertrial intervals of 1, 2.5, 5, 10, or 20 s. The rate of reinforcement declined for all animals as intertrial interval increased. Although sensitivity (A') decreased with increasing intertrial interval, all rats showed an increase in bias to make long responses.

Perceptual bisection in rats: the effects of physostigmine, scopolamine and pirenzepine.

The effects of cholinergic drugs on three different perceptual bisection tasks were studied in rats. Physostigmine (0.056-0.56 mg/kg), a reversible anticholinesterase, produced dose-dependent decrements in discriminability (A'), but did not affect the bisection point (BP) in visual duration, auditory duration, and auditory intensity bisection tasks. This finding is consistent with results previously obtained in an auditory duration bisection task with an irreversible anticholinesterase, diisopropyl phosphofluoridate. Scopolamine (0.075-0.422 mg/kg), a muscarinic cholinergic-receptor antagonist, produced dose-dependent decrements in both A' and BP in visual and auditory duration bisection tasks. The behavioral antagonism between physostigmine (0.56 mg/kg) and scopolamine (0.075-0.237 mg/kg) was studied in the visual and auditory duration bisection tasks. The BP was not affected by physostigmine alone or in combination with scopolamine, except at the largest dose of scopolamine, which produced a reliable decrement in the BP. A', however, was equally decreased by physostigmine alone and all combinations of physostigmine and scopolamine. Pirenzepine (1, 3 and 10 mg/kg), a selective high-affinity M1 muscarinic antagonist, had no effect on A' or the BP in the duration bisection tasks, suggesting changes in perception produced by muscarinic antagonists do not involve the M1 receptor subtype. The similar drug effects in different sensory modalities (visual and auditory) and perceptual systems (subjective duration and loudness) suggest that cholinergic drugs may affect perceptual mechanisms responsible for sensory coding, such as the output of a neural generator.

Circadian variations in time perception in rats.

The hypothesis that time perception is determined by an internal clock's rate, which is sensitive to physiological circadian changes was examined. This hypothesis suggests that when body temperature and activity increase, clock rate increases causing overestimations of time and a decrease in time interval production. The following experiments examine this hypothesis with rats. Since rats are nocturnal, and body temperature is highest at night, time estimation should be longer and time production shorter than during the day. All experiments were run 24 hr a day under a 12:12 light:dark cycle. In the first experiment, rats discriminated among stimulus durations. Consistent with the hypothesis, both rats reliably judged standard durations as "longer" during the night than during the day. In Experiment 2, fixed-interval (FI) schedules of 60, 120 and 180 sec were used to examine circadian variations in time production. All seven rats produced longer postreinforcement pause (PRP) durations as FI length increased, and shorter PRP durations at night than during the day. However, absolute differences in PRP durations between day and night did not increase as FI duration increased, suggesting the PRP durations alone do not directly characterize proportional changes in internal clock rate. To more directly examine circadian variation in clock rate and the production of shorter intervals (i.e., between 1 and 16 sec) Experiment 3 used a lever holding procedure. Since this procedure requires the measured lever hold duration to meet or exceed the required hold duration, it more accurately characterizes the timing process.(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminability, loudness, and masking in the rat (Rattus norvegicus): a confirmation and extension.

In Experiment 1, rats discriminated between two sound pressure levels (SPL) of a pure tone: standard (STD) SPLs of 84 and 74 dB and comparison (CO) SPLs 4, 14, and 24 dB below STD were tested in quiet and 60 dB noise at 4 and 12.5 kHz (24 conditions). The decibel difference between STD and CO accounted for only 43.52% of the variance in the signal detection measure of sensitivity, d', across conditions, whereas the loudness difference (LD = STD0.35 - CO0.35) accounted for 89.82% of the variance in d'. These results confirm and extend previous observations that: (a) equal decibel differences are not equally discriminable; (b) loudness for the rat increases as a power function of SPL with an exponent of 0.35: and (c) masked loudness is a linear function of loudness in quiet. In Experiment 2, the assumptions of normal distribution and equal variance implicit in the use of the d' measure were examined. Receiver operating characteristic curves that were well approximated by straight lines of unit slope in normal-normal coordinates were obtained and thereby validated the use of d' in Experiment 1.

A cost-benefit analysis of demand for food.

Laboratory studies of consumer demand theory require assumptions regarding the definition of price in the absence of a medium of exchange (money). In this study we test the proposition that the fundamental dimension of price is a cost-benefit ratio expressed as the effort expended per unit of food value consumed. Using rats as subjects, we tested the generality of this "unit price" concept by varying four dimensions of price: fixed-ratio schedule, number of food pellets per fixed-ratio completion, probability of reinforcement, and response lever weight or effort. Two levels of the last three factors were combined in a 2 x 2 x 2 design giving eight groups. Each group was studied under a series of six FR schedules. Using the nominal values of all factors to determine unit price, we found that grams of food consumed plotted as a function of unit price followed a single demand curve. Similarly, total work output (responses x effort) conformed to a single function when plotted in terms of unit price. These observations provided a template for interpreting the effects of biological factors, such as brain lesions or drugs, that might alter the cost-benefit ratio.

The effects of diisopropyl phosphorofluoridate (DFP) on time perception in rats.

The hypothesis that DFP alters circadian rhythms by altering the output of an "internal clock" which is also used to time events in behavioral tasks was tested. Since any clock has a mean rate (ticks/unit time) and an associated variance (changes in the rate across time), measures of time perception which depend upon both the mean clock rate and its variance (discriminability, A'), or only the mean clock rate (Bisection Point) were examined. In Experiment 1, two groups of rats were trained to discriminate between a standard duration and six comparison durations of a light. Six weeks following three injections of DFP (1.0 mg/kg/week) or vehicle (saline and 5% alcohol), the discriminability (A') between the standard and comparison durations was reliably reduced for the DFP-treated animals. In Experiment 2, rats were trained to perform on a temporal bisection task. Relative to performance during the weeks following vehicle (peanut oil) treatments, discriminability (A') during the weeks following treatment with DFP (1.0 mg/kg/week) was reliably degraded but measures of the Bisection Point were unaffected. Since Experiments 1 and 2 both used a light duration as a discriminative stimulus, Experiment 3 examined the possibility that DFP treatments produced a change in visual function rather than clock function. Two groups of rats were trained to discriminate between light-on and light-off periods in a standard free-operant successive discrimination paradigm. No changes in discriminability or response rates were evident following two injections of DFP (1.0 mg/kg/week) or vehicle (peanut oil).(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term sequelae of soman exposure: hormonal rhythms two weeks postexposure to a single dose.

Two weeks following a single exposure to either soman (100 micrograms/kg, sc) or saline, rats were sacrificed at 2-hr intervals over a 26-hr period. Trunk blood was collected and plasma was stored until assayed for corticosterone, prolactin, growth hormone, adrenocorticotropin, beta-endorphin, and beta-lipotropin. Rats surviving for 2 weeks following soman appeared well groomed and were gaining weight at a rate similar to saline-treated rats at the time of termination. Thus, they appeared to have recovered from the initial physiological effects of soman exposure. However, substantial differences in plasma levels of most hormones were seen in comparing soman- vs saline-treated rats. Levels of prolactin were suppressed at all time points in soman-treated rats. Growth hormone secretion was also suppressed and the normal episodic peaks of growth hormone were missing in soman-treated rats. Both soman- and saline-treated rats displayed circadian rhythms in levels of plasma corticosterone, but the usual late afternoon rise in plasma corticosterone levels was shifted to earlier time points in the soman-treated rats. Levels of beta-endorphin and beta-LPH were slightly but significantly suppressed in soman-treated rats at almost all time points. Levels of adrenocorticotropin were similar in control and soman-treated rats. The results of this experiment demonstrate that a single exposure to soman may have long-lasting effects on neuroendocrine function.

The effects of diisopropyl phosphorofluoridate (DFP) on inter-response time and circadian patterns of lever-pressing in rats.

Rats were injected with saline, 1.0 mg/kg or 2.6 mg/kg of diisopropyl phosphorofluoridate (DFP). Three days later the animals were placed in cages in which they could press a lever to obtain their entire daily ration of food. The time of day at which responses occurred and the time between successive responses were recorded over a six day period to determine the circadian pattern of lever-pressing and the distribution of inter-response times (IRTs). The saline injected rats exhibited a normal nocturnal pattern of feeding, while both DFP treated groups exhibited a significantly greater tendency to eat during the day. Analysis of the IRT distributions of the three groups showed a different pattern of results. The saline and 1.0 mg/kg DFP groups produced nearly identical IRT distributions, while the 2.6 mg/kg group produced an IRT distribution which was marked by significant increases in the interquartile range and median IRT. Since the 1.0 mg/kg dose of DFP produced a circadian disruption but did not affect the IRT distribution, it appears that the disruption of circadian activity by DFP which was reported by Raslear and Kaufman cannot be solely explained by simple changes in the motor response.

Diisopropyl phosphorofluoridate (DFP) disrupts circadian activity patterns.

A single dose (1.75 mg/kg SC) of diisopropyl phosphorofluoridate (DFP) was given to rats living continuously in modified activity cages in which wheel running, lever pressing and feeding were monitored twenty four hours per day. The first twenty four hours following DFP injection was characterized by a marked reduction in all monitored activities. Within a few days, food intake and lever pressing had recovered to pre-injection levels, but running remained suppressed throughout the remainder of the experiment. A comparison of the circadian distribution of activities for individual subjects showed phase shifts and a reduction in amplitude of all three monitored activities, persisting well beyond the initial reductions in the daily totals.

Loudness scaling and masking in rats.

Two groups of rats were trained on operant discriminations in which the discriminanda were two different sound pressure levels of a 4-kHz tone. The discriminanda were chosen so that the loudness difference between stimuli was equivalent for each group when calculated from a power function with an exponent of .35. Half of each group learned the discrimination in quiet, and the other half learned it in a background of white noise. Within the quiet and the noise conditions, the asymptotic discriminability of stimuli separated by equal loudness differences was equivalent, and discriminability was lower in noise. This is consistent with both the human literature on masked loudness and a model of psychophysical scaling (Pierrel-Sorrentino & Raslear, 1980) in which animals judge perceived differences between stimuli.

A test of the Pfanzagl bisection model in rats.

The bisection method of animal psychophysical scaling was examined as a measurement procedure. The critical assumptions of bisection scaling, as described by Pfanzagl (1968), were tested to determine if a valid equal-interval scale could be derived. A valid scale was derived in which subjective time for the rat was a power function of real time. Bisection points were found to be context dependent, because the spacing of stimuli significantly affected the bisection points. Such context effects were directly related to the size of the interval bisected, whereas measurement errors in estimates of the exponent of the power function were an inverse function of interval size.

Differential responding without differential reinforcement: Intensity difference, continuum position, and reinforcement density effects.

The response rates of five groups of rats were observed during exposure to different intensities of a four kilohertz tone within a two-component multiple schedule of nondifferential reinforcement. Response rates were found to be higher during the multiple schedule component which contained the higher intensity tone. Larger differences in response rates between the two multiple schedule components occurred with greater intensity separations (30 versus 20 decibels). At the 30 decibel separation a low absolute magnitude produced larger response rate differences than a high absolute magnitude, while at the 20 decibel separation a high absolute magnitude produced larger response rate differences. Increases in reinforcement density were accompanied by decreases in response rate differences between high and low intensity components only when over-all response rates also increased.

Loudness scaling in rats and chinchillas.

Chinchillas and rats were trained on operant discriminations in which the discriminative stimuli were two different sound pressure levels of a 4-kHz tone. Two or more of these two-intensity discriminations were used at each of three levels of discriminability: high, medium, and low. For any given level, each of the stimulus pairs used differed in decibel separation but were similar in loudness-unit differences calculated from a power function. Different groups of animals trained on stimuli separated by equal numbers of loudness units produced equivalent performances at each of the three levels of discriminability. It is concluded that loudness growth for both of these species, as for man, is well described by a power function (Stevens' law). For the chinchilla the exponent is .25, and for the rat it is .35.

The effects of varying the distribution of generalization stimuli within a constant range upon the bisection of a sound-intensity interval by rats.

Two male, albino rats were trained on a two-valued, self-paced, discrete-trials auditory discrimination. In the presence of a high-intensity stimulus (90 decibels SPL, 4 kiloHertz), response A was reinforced; in the presence of a low-intensity stimulus (50 decibels SPL, 4 kiloHertz), response B was reinforced. When discrimination performance was asymptotic, stimuli intermediate in intensity were presented with the training stimuli in a maintained generalization paradigm. Generalization gradients were derived from the relative frequencies of response A in the presence of each stimulus. A relative frequency of 0.50 was then determined and used as the bisection point of the intensity interval defined by the 90- and 50-decibel stimuli. The bisection point varied with the distribution of the stimuli presented in generalization. This effect was similar to context effects seen in human psychophysics.