Stimulus duration as a measure of stimulus generalization

Four pigeons in the line-positive group were trained with a vertical line on a green background that signalled intermittent reinforcement while a plain green field signalled extinction. Four pigeons in the line-negative group were trained with the opposite discrimination. Response to a control key terminated any trial and initiated the next trial. The birds also used the control key during generalization tests to control the durations of trials in which various line orientations were presented. These durations were summed to provide generalization gradients of stimulus duration that were positive or negative in accordance with the trained discriminations. In Experiment 2, birds from the line-positive group were tested with a procedure in which the control key was not available on some trials. This provided an independent assessment of response rates to the test stimuli. These rates were used to predict the stimulus durations obtained when the control key was available. The findings supported a general model for the prediction of response distributions among concurrent stimuli from rates observed with single stimuli.     

Time-based and count-based measurement of preference

Rats' pressing on two levers was reinforced according to two independent variable-interval schedules that were varied during the experiment. Since the levers were connected directly to the programming equipment, bypassing the standard pulseformers, reinforcement could occur while a lever was held down. Although the time a lever was pressed might, therefore, have varied independently of number of presses, these two measures covaried substantially, because the average duration of the presses remained roughly constant. This rough invariance may have resulted from the rats' tendency to make bursts of brief presses (i.e., to jiggle the levers), even though the contingencies encouraged holding. When duration did vary, presses on the two levers tended to vary together. As a result, relative time spent pressing corresponded closely to relative number of presses. Both of these measures conformed well to the matching law. Absolute behavioral frequency at a lever, measured either way, varied directly with proportion of reinforcement for that lever, in accordance with the generalized version of the matching law. Number of presses seemed, on balance, to be a slightly more reliable measure than pressing time. The substantial interchangeability may prove more significant than the slight disparity, however, because it supports the notion that all behavior can be measured on a common scale of time.     

Matching in a network

Pigeons were given practice choosing between pairs of alternatives yielding different frequencies of reinforcement. Four individual alternatives were set into four pairwise choices. Averaged over subjects, the distribution of responses in each choice approximated matching. The four individual alternatives were then presented, two by two, in two pairwise choices for which there had been no initial practice. No further reinforcement was given during the tests with the new pairs. Transfer to the two test pairs deviated systematically from matching in most cases by exaggerating the preference for the alternative that had had the higher frequency of reinforcement.     

Preference for mixed versus constant delay of reinforcement

Preference for constant and mixed delay of reinforcement was studied using concurrent equal variable-interval schedules. For four pigeons, pecking one key was reinforced following constant delays of 8 sec and mixed delays of 6 or 10 and 2 or 14 sec. Pecking a second key was reinforced following constant delays of 0, 8, 16, and 32 sec. For two additional pigeons, pecking one key was reinforced following delays of 30, 15 or 45, 5 or 55, and 0 or 60 sec. Reinforcements on the other key were delayed 30 sec. It was found that (a) pigeons preferred mixed relative to constant delay of reinforcement, and (b) preference for mixed delay of reinforcement increased as the range of delay interval variability increased.     

Human observing: Maintained by stimuli correlated with reinforcement but not extinction

College students received points exchangeable for money (reinforcement) on a variable-time 60-second schedule that alternated randomly with an extinction component. Subjects were informed that responding would not influence either the rate or distribution of reinforcement. Instead, presses on either of two levers (“observing responses”) produced stimuli. In each of four experiments, stimuli positively correlated with reinforcement and/or stimuli uncorrelated with reinforcement were each chosen over stimuli correlated with extinction. These results are consistent with prior results from pigeons in supporting the conditioned-reinforcement hypothesis of observing and in not supporting the uncertainty-reduction hypothesis.     

Choice as a dependent measure in autoshaping: sensitivity to frequency and duration of food presentation.

Previous investigations have shown that rate, latency, and percentage of trials with at least one response are somewhat insensitive measures of the strength of autoshaped responding. In the present studies, these measures were contrasted with the allocation of responding during simultaneous choice tests, a measure of response strength frequently used in operant paradigms. In two experiments, nine pigeons were exposed to a forward pairing autoshaping procedure. Training sessions consisted of the successive presentation of three stimuli, each followed by food on either 100%, 50%, or 0% of the trials. Choice testing involved the simultaneous presentation of the three stimuli. In Experiment I, all pigeons consistently directed their initial choice responses and the majority of subsequent responses to the stimulus always followed by food, despite the fact that during training sessions the response rates of most birds were highest in the presence of the stimulus followed by food on 50% of the trials. In Experiment II, rate, latency, and percentage of trials with at least one response did not change appreciably as a function of duration of feeder presentations. However, choice responding was lawfully affected by duration of feeder presentations. These data suggest that choice is perhaps a more sensitive measure of the strength of autoshaped responding than other, more commonly employed, indices.     

Optimal choice

We present a classification and theoretical analysis of discrete-trial and free-operant choice procedures in which reinforcement is assigned to one alternative only, or independently to both, is either always available or conditionally available, and is either "held" or not from trial to trial. Momentary-maximizing and (globally) optimal choice sequences are defined in terms of initializing and marker events. Free-operant choice is analyzed in terms of a clock space whose axes are the times since the last A and B choices. The analysis shows that most molar matching data are derivable from momentary maximizing, and that the momentary-maximizing hypothesis has not been adequately tested in either discrete-trial or free-operant situations.     

Discrete-trial choice in pigeons: Effects of reinforcer magnitude

The preference of pigeons for large reinforcers which occasionally followed a response versus small reinforcers which invariably followed a response was studied in a discrete-trial situation. Two differently colored keys were associated with the two reinforcement alternatives, and preference was measured as the proportion of choice trials on which the key associated with uncertain reinforcement was pecked. A combination of choice and guidance trials insured that received distributions of reinforcement equalled the scheduled distributions. For five of six subjects, preference for the uncertain reinforcer appeared to be a linear function of the magnitude of the certain reinforcer. In addition, there was greater preference for the response alternative associated with uncertain reinforcement than would be expected on the basis of net reinforcer value.     

Choice between food and heroin: effects of morphine, naloxone, and secobarbital.

Baboons responded on a choice task on which discrete trials involved choosing between an intravenous injection of heroin (.32 or 1.0 mg/kg) or the availability of food pellets. An intertrial interval of three hours followed the completion of each trial. Under baseline conditions baboons consistently completed the eight available trials each day. Typically, animals chose heroin on three or four trials a day and food on the remaining trials. Animals tended to space the selection of heroin rather than choosing heroin on consecutive trials. A series of single-day experimental manipulations was undertaken to characterize performance further. Manipulation of the heroin dose produced shifts in the relative frequency of choosing the drug option which were inversely related to dose. Manipulation of number of pellets per food trial produced little change in distribution of choices. Noncontingent administration of morphine produced dose-related decreases in relative frequency of heroin choices, and a higher dose decreased the number of trials completed. Noncontingent naloxone produced dose-related increases in the relative frequency of heroin choices. Noncontingent secobarbital had no effect on distribution of choices, and high doses reduced the number of trials completed per day. The results suggest that morphine and naloxone produce shifts in this choice behavior by selectively interacting with the reinforcing properties of the option involving heroin.     

The influence of "preparedness" on autoshaping, schedule performance, and choice.

Two groups of experimentally naive pigeons were exposed to an autoshaping procedure in which the response key was mounted on the wall (the conventional location) or on the floor of the chamber. In two experiments, subjects readily responded to the wall key, but floor-key subjects required shaping. A subsequent experiment compared performance of wall- and floor-key groups on an ascending series of fixed-ratio schedule values, resistance to extinction, differential reinforcement of other behavior, and reversal of key assignment. Each experiment was followed by several sessions of fixed-ratio training; the performance of the wall- and floor-key groups was almost identical throughout. In the final experiment, a fixed-ratio requirement could be completed on either or both keys. Birds initially chose the key on which they had responded during the preceding (reversal of key assignment) experiment. However, within a few sessions both groups showed almost exclusive preference for the floor key. Preference for a key located on the floor may follow from the fact that pigeons are ground feeders and may thus be more "prepared" to peck the floor than to peck a wall. However, autoshaping, under the conditions prevailing here, occurred much more readily to the wall key, suggesting that pecking a vertical surface is more highly prepared. Difficulties in determining relative preparedness seem moot, however, given the lack of between-group differences in the intervening experiments. It is thus unlikely that schedule performances critically depend upon the specific operant response involved.