Ben A. Williams

Active Spatial Perception in the Vibrissa Scanning Sensorimotor System

Haptic perception is an active process that provides an awareness of objects that are encountered as an organism scans its environment. In contrast to the sensation of touch produced by contact with an object, the perception of object location arises from the interpretation of tactile signals in the context of the changing configuration of the body. A discrete sensory representation and a low number of degrees of freedom in the motor plant make the ethologically prominent rat vibrissa system an ideal model for the study of the neuronal computations that underlie this perception. We found that rats with only a single vibrissa can combine touch and movement to distinguish the location of objects that vary in angle along the sweep of vibrissa motion. The patterns of this motion and of the corresponding behavioral responses show that rats can scan potential locations and decide which location contains a stimulus within 150 ms. This interval is consistent with just one to two whisk cycles and provides constraints on the underlying perceptual computation. Our data argue against strategies that do not require the integration of sensory and motor modalities. The ability to judge angular position with a single vibrissa thus connects previously described, motion-sensitive neurophysiological signals to perception in the behaving animal.

Conditioned Reinforcement: Experimental and Theoretical Issues

The concept of conditioned reinforcement has received decreased attention in learning textbooks over the past decade, in part because of criticisms of its validity by major behavior theorists and in part because its explanatory function in a variety of different conditioning procedures has become uncertain. Critical data from the major procedures that have been used to investigate the concept (second-order schedules, chain schedules, concurrent chains, observing responses, delay-of-reinforcement procedures) are reviewed, along with the major issues of interpretation. Although the role played by conditioned reinforcement in some procedures remains unresolved, the results taken together leave little doubt that the underlying idea of conditioned value is a critical component of behavior theory that is necessary to explain many different types of data. Other processes (marking, bridging) may also operate to produce effects similar to those of conditioned reinforcement, but these clearly cannot explain the full domain of experimental effects ascribed to conditioned reinforcement and should be regarded as complements to the concept rather than theoretical competitors. Examples of practical and theoretical applications of the concept of conditioned reinforcement are also considered.

Conditioned reinforcement: Neglected or outmoded explanatory construct?

The construct of conditioned reinforcement has been challenged over the past two decades, both as an adequate explanation of behavior in various situations (e.g., behavioral chains) in which it was previously regarded as crucial and in terms of the validity of the concept of conditioned value on which it is based. Recent research is reviewed that answers these criticisms and vindicates the importance of the construct. Also discussed are unanswered empirical issues regarding the concept and its implications for behavioral phenomena of major importance for general-process learning theory, such as autoshaping and imprinting.

Behavioral contrast redux.

Behavioral contrast is defined as a change in response rate during a stimulus associated with a constant reinforcement schedule, in inverse relation to the rates of reinforcement in the surrounding stimulus conditions. Contrast has at least two functionally separable components: local contrast, which occurs after component transition, and molar contrast. Local contrast contributes to molar contrast under some conditions, but not generally. Molar contrast is due primarily to anticipatory contrast. However, anticipatory contrast with respect to response rate has been shown to be inversely related to stimulus preference, which challenges the widely held view that contrast effects reflect changes in stimulus value owing to the reinforcement context. More recent data demonstrate that the inverse relation between response rate and preference with respect to anticipatory contrast is due to Pavlovian contingencies embedded in anticipatory contrast procedures. When those contingencies are weakened, anticipatory contrast and stimulus preference are positively related, thus reaffirming the view that the reinforcing effectiveness of a constant schedule is inversely related to the value of the context of reinforcement in which it occurs. The underlying basis of how the context of reinforcement controls reinforcement value remains uncertain, although clear parallels exist between contrast and the effects of contingency in both Pavlovian and operant conditioning.

Information effects on the response-reinforcer association

Pigeons were trained on a discrete-trial delayed reinforcement procedure with respect to one response key that was periodically illuminated. In some conditions, a second response key, or a tone, both previously paired with reinforcement, was interpolated in the delay-of-reinforcement interval. In comparison to a control condition with neither stimulus in the delay interval, the interpolated stimulus attenuated (blocked) the amount of responding that was maintained by the delayed reinforcement contingency. The degree of blocking was unaffected by whether the interpolated stimulus was the tone or keylight, in spite of the fact that the keylight evoked responding and the tone did not. A second study showed that the blocking effects involved the response-reinforcer association in that blocking occurred when the delayed reinforcement was response-dependent but did not occur when reinforcement was response-independent. The results thus show that response-reinforcer associations are affected by informational variables in the same manner as has been shown for stimulus-reinforcer associations. They also demonstrate that preexisting stimulus-reinforcer associations can block response-reinforcer associations, thus suggesting that both types of association depend upon the same associative process.

Perils of Evidence-Based Medicine

Evidenced-based medicine views random-assignment clinical trials as the gold standard of evidence. Because patient populations are heterogeneous, large numbers of patients must be studied in order to achieve statistically significant results, but the means or medians of these large samples have weak predictive validity for individual patients. Further, the logic of random-assignment clinical trials allows only the inference that some subset of patients benefits from the treatment. Post-hoc analysis is therefore necessary to identify those patients. Otherwise, many patients may receive treatments that are useless and potentially harmful.

Multiple determinants of "blocking" effects on operant behavior

Blocking was investigated in a free-operant procedure by presenting a response-contingent signal prior to reinforcer delivery. At issue was the way in which blocking effects previously reported with this procedure are related to conditioned reinforcement effects, also previously found with similar procedures. Signal presentation decreased response rate when delay of reinforcement was 0 or .5 sec, but the signal increased response rate when the delay of reinforcement was increased to 3 sec. Thus, which effect (blocking or conditioned reinforcement) occurred depended critically on the response-reinforcer interval.

Associative competition in operant conditioning: blocking the response-reinforcer association.

Naive rats were trained to leverpress with a 30-sec delay-of-reinforcement contingency from the start of training. In Experiment 1, the delay interval for different groups of subjects included a signal in the first 5 sec, a signal in the last 5 sec, or no signal at any time. Rats with the signal at the start of the delay interval learned most rapidly. Rats with the signal at the end of the delay failed to learn. In Experiment 2, a choice procedure was used, in which each of two levers was associated with its own 30-sec delay of reinforcement. The delay for one lever included a 5-sec signal at the end of the delay. The delay for the second lever had no signal in its 30-sec delay. Preference was in favor of the lever without the signal in the delay interval. The results demonstrate that the acquisition of new response can be blocked in a manner analogous to the blocking of Pavlovian conditioning.

Behavioral contrast and reinforcement value

Behavioral contrast was produced in two target components of a four-component multiple schedule by having two target stimuli followed either by a higher rate of reinforcement or by extinction. Response rate was higher in the target followed by extinction. Periodic probe trials were then presented in which the two target stimuli were presented together. Choice on these probe trials was in favor of the stimulus followed by the higher rate of reinforcement during regular training. Experiment 2 replicated this finding but with probe trials presented throughout training. Here, preference for the stimulus followed by the higher rate of reinforcement was evident early in training, substantially before the contrast effects developed. The results challenge interpretations of contrast based on the concept of relative value.

Marking and bridging versus conditioned reinforcement

Rats were trained on a series of reversals of a two-choice conditional discrimination. Choice responses were followed by different delays of reinforcement, which were either unsignaled or filled with either a brief or a long tone. In some conditions, the tone occurred following both correct and incorrect choices; in other conditions, the tone occurred only after correct choices. Presentation of the tone following only correct choices greatly facilitated the acquisition of the discrimination, and there was little effect of the tone’s duration. Presentation of the tone following all choices did not improve discrimination acquisition relative to the no-signal condition. The results demonstrate facilitatory effects of a signal during a delay-of-reinforcement interval that are caused by the conditioned-reinforcement properties of the signal and cannot he explained by the alternative mechanisms of marking or bridging.