PubMed Health⌕ Search

Biomedical subjects

R W Powell

Publications and source records attributed to R W Powell.

87 records · Page 5Linked to original sources
PubMed1970

The effect of punishment shock intensity upon responding under multiple schedules.

In the first of two experiments, responses of two pigeons were maintained by multiple variable-interval, variable-ratio schedules of food reinforcement. Concurrent punishment was introduced, which consisted of a brief electric shock after each tenth response. The initial punishment intensities had no lasting effect upon responding. Then, as shock intensity increased, variable-ratio response rates were suppressed more quickly than variable-interval response rates. When shock intensity decreased, variable-interval responding recovered more quickly, but the rates under both schedules eventually returned to their pre-punishment levels. In the second experiment, the following conditions were studied in three additional pigeons: (1) With each shock intensity in effect for a number of sessions, punishment shock intensity was gradually increased and decreased and responding was maintained by multiple variable-ratio, fixed-ratio schedules of food reinforcement; (2) Changes in punishment shock intensity as described above with responding maintained by either a variable-ratio or a fixed-ratio schedule, which were presented on alternate days; (3) Session-to-session changes in shock intensity with responding maintained by multiple variable-ratio, fixed-ratio schedules. Responding under the two schedules was suppressed to approximately the same extent by a particular shock intensity. Also, post-reinforcement pauses under the fixed-ratio schedule increased as response suppression increased.

Journal Article↗
PubMed1970

The effect of shock intensity upon responding under a multiple-avoidance schedule.

The effect of two shock intensities (1.00 and 2.00 mA) were studied in the acquisition, maintenance, and extinction of unsignalled avoidance by albino rats. Single and multiple avoidance schedules were employed, with shock intensity being the principal condition that differed between schedule components. The higher shock intensity was generally more effective in producing avoidance. Higher response rates and lower shock rates were observed under high-intensity shock when performance stabilized. When the multiple schedule was introduced, the six rats trained under a single shock intensity all showed poorer performance under the new shock intensity, whether it was higher or lower than the training intensity. Performance under the original shock intensity did not change substantially with the introduction of a different shock intensity in the other multiple schedule component. Performance under the new shock intensity showed gradual improvement with continued exposure to it. All of the rats showed persistent "warm-up", receiving approximately 40% of the total session shocks in the first one-sixth of the session. The degree of warm-up was unrelated to avoidance shock intensity.

Journal Article↗
PubMed1969

Persistent shock-elicited responding engendered by a negative-reinforcement procedure.

A procedure in which responses reduced intermittently presented electric shocks to one quarter of their originally scheduled intensity, effectively engendered and maintained lever pressing in hooded rats. This contingency also markedly increased the response rates of rats initially trained under an unsignaled avoidance procedure. The responding of all animals extinguished rapidly when shock was withdrawn. Subsequently, it was discovered that high response rates could be maintained solely through presentation of shocks that were not affected by responses. Variations in the interval between shocks and changes in shock intensity over a wide range did not attenuate responding. Terminal performance was characterized by a consistent pattern of shock-elicited responses. Responses were also elicited by a tone following repeated tone-shock pairings. Finally, responding that was maintained by response-independent shocks was quickly suppressed by response-contingent shocks of the same intensity.

Journal Article↗
PubMed1969

Continuous punishment of free-operant avoidance in the rat.

Three groups of albino rats were trained under a free-operant avoidance (Sidman) procedure with equal shock-shock and response-shock intervals. After stable performance was achieved, the animals were concurrently exposed to a brief electric shock after each response. The procedures were as follows: Punishment Schedule I: punishment shock was introduced at an intensity approximately one quarter that of avoidance shock; increments of nearly this same size were made as stable performance was achieved at succeeding punishment shock intensities. Punishment Schedule II: punishment shock was introduced at approximately one-half the intensity of avoidance shock; after stable performance, punishment shock was increased to the same intensity as avoidance shock. Punishment Schedule III: punishment shock was introduced and maintained at the same intensity as avoidance shock. Punishment was continued for all groups until one of two suppression criteria was attained. All animals made fewer responses and received more avoidance shocks as a function of increasing punishment shock. Half of the animals under Punishment Schedule I required punishment shock higher than avoidance shock to meet their assigned suppression criterion. A comparison of all procedures showed that suppression was greater when punishment shock was initially at high intensity.

Journal Article↗
PubMed1969

The effect of reinforcement magnitude upon responding under fixed-ratio schedules.

Responding under fixed-ratio schedules was studied as a function of two durations of food presentation. Latency of the first response after food presentation (post-reinforcement pause) was consistently shorter when food was presented for the longer duration. Only one of the four pigeons studied showed a consistently higher response rate, exclusive of post-reinforcement pause, as a function of the longer access to food. When ratio size was reduced, pause durations decreased, and the differences related to the two durations of food presentations became progressively smaller.

Journal Article↗
PubMed1969

Running-wheel activity and avoidance in the mongolian gerbil.

In the first of two experiments, running-wheel activity and unsignaled (Sidman) avoidance were studied in gerbils and albino rats. All gerbils ran at higher rates than any of the four rats studied. Under the avoidance procedure, four rats developed effective responding; the other two performed much less successfully. While avoidance developed more slowly in the gerbils, all showed asymptotic performance as effective as the four superior rats. The rats showed a consistent warm-up effect, receiving 60% to 80% of the total shocks in the initial third of the session. The gerbils displayed no warm-up, with shocks being evenly distributed over the session. Warm-up in the rats was not related to either response rate or to how effectively the animal was avoiding. When shock was removed, extinction occurred more rapidly in the rats than the gerbils. In the second experiment, which involved two-way shuttle avoidance, gerbils and albino mice quickly acquired the response. All animals met a criterion of 90% avoidance over 80 trials.

Journal Article↗
PubMed1968

A comparison of escape and avoidance conditioning in wild and domesticated rats.

In the first of two experiments, three cotton rats (Sigmodon hispidus) and three albino rats were exposed to instrumental escape, unsignaled avoidance, and signaled avoidance, in that order. All subjects learned the escape procedure quickly, with the albino rats having generally shorter latencies, higher response rates, and requiring fewer sessions to reach the criterion. When the avoidance contingency was introduced, the cotton rats continued to respond almost entirely in the presence of the shock, whereas the albino rats responded in its absence, thus displaying effective avoidance behavior. Introduction of a pre-aversive stimulus did not improve the performance of the cotton rats. In the second experiment, five cotton rats and four albino rats were exposed to a free-operant (Sidman) avoidance procedure with a shock-shock interval of 3 sec and a response-shock interval of 20 sec. The cotton rats initiated responding at lower shock intensities than the albino rats, but their asymptotic avoidance responding was far less effective.

Animals↗
PubMed1968

The effect of small sequential changes in fixed-ratio size upon the post-reinforcement pause.

Duration of the post-reinforcement pause was measured for three pigeons on fixed-ratio schedules of reinforcement ranging from 10 to 160. Small sequential changes were made in the ratio values without disrupting stable performance. The post-reinforcement pause increased consistently for all birds within three sessions as the ratio requirement increased. A frequency analysis of the individual pauses at selected fixed ratios revealed an increase in dispersion for all animals as the ratio size increased. Response rate tended to decrease for two of the birds and remained relatively stable for the third; but there were many reversals in these data.

Animals↗
PubMedDate not supplied

Determination of ambulatory blood pressure control in treated patients with controlled office blood pressures.

Office blood pressure measurement is the standard for assessing blood pressure control. Many patients, however, take their antihypertensive medication in the morning, so they are likely to have their office blood pressure measured during the maximal antihypertensive effect. It is therefore unknown whether patients deemed by office blood pressure to be controlled do in fact have 24h blood pressure control. The objectives of this study were to determine blood pressure control, including blood pressure control while the patients were awake and during the first 6 hours after awakening, by ambulatory blood pressure monitoring (ABPM) in treated hypertensive patients deemed by office blood pressure measurements to be controlled. A total of 103 patients on a stable antihypertensive regimen and deemed to be controlled in terms of office blood pressure values (mean office blood pressure <140/90mmHg) were enrolled. Patients were stratified by cardiovascular risk status and the number of antihypertensive medications that they were taking. Seventy-eight out of 103 participants successfully completed ABPM. The mean ambulatory blood pressure was greater than 135/85mmHg and 140/90mmHg while awake for 37% (95% confidence interval [CI] 26-48%) and 23% (95% CI 14-32%) of all patients respectively. Forty-eight per cent (95% CI 33-63%) of patients taking monotherapy versus 25% (95% CI 11-39%) of patients on multiple antihypertensive medications were uncontrolled (P=0.039) using 135/85mmHg as the reference value. Thirty-one per cent (95% CI, 17-44%) of patients on monotherapy versus 14% (95% CI 3-25%) of patients on multiple antihypertensive medication were uncontrolled (P=0.064) using 140/90mmHg instead. These results demonstrate that a high number of patients deemed by office blood pressure to be under control do not have adequate blood pressure control based on ABPM.

Adult↗
PubMedDate not supplied

Organ blood flow redistribution in response to hypoxemia in neonatal piglets.

This study was designed to determine the effects of severe hypoxemia on newborn piglet visceral blood flow. While the hemodynamic effects of a severe hypoxemic insult are well characterized in newborn animals, its impact on organ perfusion in premature infants is not well characterized. Cannulas were placed in the femoral vessels and left atrium of term (1-14 days old) and prematurely delivered (cesarean section at 90% of term gestation) piglets. After stabilization, some animals were subjected to 1 h of ventilator-controlled hypoxia (yielding PaO2 approximately = 30-40 torr) followed by 30 min of reoxygenation; the remaining animals served as unchallenged controls. Radiolabeled microspheres were injected in all animals at times 0 min (baseline), 5 and 60 min (hypoxia), and 90 min (reoxygenation). Blood flows (mL/min/g tissue) to organs were determined using reference organ techniques. Control animals displayed no alterations in any of the variables monitored. Throughout the experimental period, organ blood flows were almost uniformly lower (p<.05, ANOVA) in premature versus term animals. The trend toward increased cerebral and cardiac blood flows during hypoxia observed in the premature piglets was similar to that of term animals, but of lower magnitude. In term piglets, hypoxia produced an immediate and significant (*p<.05) decline in small-intestinal blood flow followed by autoregulatory escape (2.02+/-0.17 mL/min/g at time 0, 1.56+/-0.15 mL/min/g at 5 min hypoxia, 1.88+/-0.18 mL/min/g at 60 min hypoxia, 2.26+/-0.19 mL/min/g at 30 min reoxygenation), an effect not readily observed in the premature piglets (0.48+/-0.10 mL/min/g at time 0, 0.44+/-0.07 mL/min/g at 5 min hypoxia, 0.46+/-0.10 mL/min/g at 60 min hypoxia, 0.42+/-0.08 mL/min/g at 30 min reoxygenation). However, mucosal blood flows measured in these younger animals declined throughout the experimental period to almost 50% of baseline, compared to a complete restoration to baseline blood flow observed following reoxygenation of term piglets. Intestinal blood flow in premature infants is small when compared to term animals, and alterations in small intestinal blood mucosal flow induced by hypoxia appear less well tolerated by the premature animals. Taken together, this may in part account for the increased risk of developing intestinal ischemic diseases in premature infants who are even temporarily exposed to a severe hypoxic challenge.

Analysis of Variance↗