Contingency and stimulus change in chained schedules of reinforcement.

Higher rates of pecking were maintained by pigeons in the middle component of three-component chained fixed-interval schedules than in that component of corresponding multiple schedules (two extinction components followed by a fixed-interval component). This rate difference did not occur in equivalent tandem and mixed schedules, in which a single stimulus was correlated with the three components. The higher rates in components of chained schedules demonstrate a reinforcing effect of the stimulus correlated with the next component; the acquired functions of this stimulus make the vocabulary of conditioned reinforcement appropriate. Problems in defining conditioned reinforcement arise not from difficulties in demonstrating reinforcing effects but from disagreements about which experimental operations allow such reinforcing effects to be called conditioned.

Yoked variable-ratio and variable-interval responding in pigeons.

Pigeons' key pecks were maintained by variable-ratio or variable-interval schedules of food reinforcement. For pairs of pigeons in one group, variable-ratio reinforcement was arranged for one pigeon's pecks; for the second pigeon, reinforcement was arranged according to a variable-interval schedule yoked to the interreinforcement times produced by the first pigeon. For pairs of pigeons in another group, variable-interval reinforcement was arranged for one pigeon's pecks; for the second pigeon, reinforcement was arranged according to a variable-ratio schedule yoked to the interreinforcement responses produced by the first pigeon. For each pair, the yoking procedure was maintained for four or five consecutive sessions of 50 reinforcements each. In more than three-quarters of the pairs, variable-ratio response rates were higher than variable-interval rates within two sessions; in all cases, the rate difference developed within four sessions.

Angiographic features of peripancreatic malignant lymphoma.

Angiographic features are described in 5 patients with peripancreatic lymphomas who initially presented with clinical features of primary pancreatic disease. Each patient demonstrated a large hypovascular mass with considerable displacement of pancreatic and peripancreatic arteries and associated compression of adjacent veins. Smooth arterial encasement was present in 2 cases. Signs of mass out of proportion to signs of invasion mitigated against the diagnosis of carcinoma in each case. Other differential diagnostic considerations are pancreatic pseudocyst and cystadenoma.

Concurrent performances: stimulus-control gradients during schedules of signalled and unsignalled concurrent reinforcement.

On one key, pigeons' pecks were reinforced according to a variable-interval schedule in the presence of vertical lines, and were not reinforced in the presence of oblique lines. On a second key, pecks were reinforced according to a variable-interval schedule in the presence of blue, according to a signalled variable-interval schedule in the presence of red, and were not reinforced in the presence of white. Subsequently, during extinction, stimulus-control gradients were obtained by presenting eight different line orientations on the first key concurrent with each of the three colors on the second key. On the first key, line-orientation gradients tended to be lower, narrower, and less shifted in peak or area when the second-key stimulus was blue or red, the stimuli respectively correlated with unsignalled and signalled reinforcement, than when it was white, the stimulus correlated with extinction. Thus, the effect on first-key line-orientation gradients depended on second-key stimuli correlated with concurrent reinforcement, whether or not these stimuli were also correlated with concurrent responding. As a function of first-key line orientation, an inverted gradient was obtained on the second key during blue; during both red and white, rates of pecking on the second key were near zero.

Second-order schedules: brief shock at the completion of each component.

Pigeons worked on second-order schedules in which completion of fixed-interval component schedules was reinforced with food according to a variable-interval schedule of reinforcement. The completion of each fixed-interval component resulted in the presentation of a brief electric shock. In one condition (shock-paired), the completion of every fixed-interval component, including those that ended in food, resulted in the shock. In another condition (shock-nonpaired), completion resulted in shock except for those components that ended in food. Shock presentations resulted in a positively accelerated rate within fixed-interval components. This patterning within components was similar whether the shock was intermittently paired with food or not. Response rates tended to decrease as shock intensity increased. The characteristic fixed-interval response pattern within components did not occur when shock presentations were omitted at the end of each component (tandem schedule). When shocks were scheduled but food was no longer presented (extinction) response rates declined to a near-zero level. The performance under shock conditions is similar to that in other studies in which visual and auditory stimuli are presented at the completion of component schedules.

Chained and tandem fixed-interval schedules of punishment.

Pigeons' key pecks were both punished with electric shock on four-component chained and tandem fixed-interval schedules and reinforced on a variable-interval schedule of food presentation. Pecking was suppressed less in the early components of the chained schedule than in the early components of the tandem schedule. Related multiple and mixed schedules of punishment were also presented; these schedules were identical to the chained and tandem schedules, respectively, except that components changed independently of responding. Similar effects were obtained, in that responding was suppressed in all components of the mixed schedule and only in the fourth component of the multiple schedule of punishment. The performances maintained on the chained and tandem schedules of punishment were generally symmetrical to those found in analogous chained and tandem schedules of food reinforcement.