Donald M. Wilkie

Backward Conditioning: A Reevaluation of the Empirical Evidence

There is an apparent discrepancy between the widespread view that backward conditioning does not occur and the experimental evidence which suggests that it does. Backward pairing of conditioned and unconditioned stimuli frequently has resulted in effects similar to those produced by forward pairing, and the results of several recent experiments have established that such effects cannot be attributed to factors other than stimulus pairing per se. Surprisingly, even some of the earlier experiments that provided the basis for the current skepticism concerning backward conditioning provide evidence of its existence. The failure to recognize backward conditioning as a legitimate phenomenon seems to reflect theoretical biases rather than a paucity of empirical evidence. Thus backward conditioning and its properties merit renewed interest and rcexamination.

Spatial Working Memory and Hippocampal Size across Pregnancy in Rats

The present experiments investigated the effects of pregnancy on performance in the Morris water maze and on hippocampal volume. In the first study, pregnant rats (in between the first and second trimester) outperformed nonpregnant rats on the Morris water maze on 1 day of testing. In the second study, rats were tested in a working memory variation of the maze in which the spatial location of the platform varied. Pregnant females traveled shorter distances than nonpregnant females during the first two trimesters, but performed worse than nonpregnant females during the third trimester. Latency measures showed a similar profile. Group differences in performance were not related to changes in swim speed. However, changes in performance in pregnant females may be related to estrogen, progesterone, and/or corticosterone levels during pregnancy, with low levels of estradiol and high levels of progesterone being associated with better performance. There were no significant differences between pregnant and nonpregnant animals on any of the brain measures, although pregnant animals tended to have a smaller hippocampus than nonpregnant animals. These results indicate that pregnancy can affect performance, possibly related to the hormonal changes that accompany pregnancy.

Time-of-day discrimination by pigeons, Columba livia

Pigeons were trained to discriminate between four keys. One provided food in the mornings, another provided food in the afternoons, and two never provided food. Three experiments were performed to determine whether pigeons could track food availability over a 24-h period. All the subjects appeared to demonstrate time-place associative learning. A fourth experiment was designed to investigate the mechanisms underlying the timing behavior. Lights-on time was shifted back by 6 h, and no decrease in performance was found during the first session following the phase shift. This suggests that a circadian type of timing mechanism with a self-sustaining oscillator mediates time-place learning over a period of 24 h. Further support for this notion was found in a fifth experiment, in which the subjects were tested in constant dim light. In that experiment, the subjects’ continued correct responding provides additional support for a self-sustaining circadian timer.

Discriminal distance analysis supports the hypothesis that pigeons retrospectively encode event duration

A discriminal distance analysis procedure similar to that used by Roitblat (1980) was employed to test the hypotheses that animals either retrospectively (Spetch & Wilkie, 1983) or prospectively (Kraemer, Mazmanian, & Roberts, 1985) encode durations of events. Pigeons were required to discriminate 2-, 8-, and 10-sec presentations of light. Choices of red, orange, and green keys were correct after 2, 8, and 10 sec, respectively. The key elements in this design were (1) that some samples (8 and 10 sec) and some choice stimuli (red and orange) were more difficult to discriminate than were others, and (2) that an easy sample discrimination (2 vs. 8 sec) was mapped onto a difficult choice discrimination (red vs. orange), and vice versa. An examination of raw error scores and calculated confusion indexes in three experiments supported the hypothesis that subjects retrospectively, rather than prospectively, encode event duration.

Rats use an ordinal timer in a daily time-place learning task.

Rats received 2 daily sessions in a large clear chamber. A lever was mounted on each of the 4 chamber walls. For each rat, a different lever provided food during 0930 and 1530 sessions. The rats learned which lever would provide food at 0930 and 1530. Probe tests suggested that the rats learned to press 1 lever during their 1st session of each day and to then press a 2nd lever during their 2nd session of each day. We propose that this knowledge of the order of a set of events within a period of time constitutes ordinal timing. We contrast the temporal information provided by ordinal, phase, and interval timing and consider why multiple timing systems have evolved in animals.

Disruption of autoshaped responding to a signal of brain-stimulation reward by neuroleptic drugs

Repeated pairing of the onset of a stationary light (CS) that signalled electrical stimulation of brain-stimulation reward sites in the mesencephalon (US) resulted in autoshaped approach behavior to the CS. After acquisition of approach to the CS two groups of rats were injected with either pimozide (0.15, 0.50, or 1.0 mg/kg) or haloperidol (0.05, 0.10, or 0.15 mg/kg) prior to test sessions consisting of 30 CS-US pairings. Both neuroleptic drugs caused a significant dose-related attenuation of the autoshaped CS-approach. A within-session analysis of responding after treatment with the high dose of each drug indicated that most responses occurred in the first 10 trials, a result that appears to rule out a direct effect of the drugs on sensory processes and orientation. The effect of repeated testing with pimozide (1.0 mg/kg) or haloperidol (0.15 mg/kg) was compared to three sessions with CS alone (extinction). Autoshaped CS-approaches declined gradually over the three extinction sessions, in contrast to the immediate and sustained disruption of approaching during the three drug sessions. These data suggest that neuroleptic-induced suppression of autoshaped CS-approach with brain-stimulation reward cannot be attributed solely to a block of reward processes. It is suggested that neuroleptic drugs disrupt neural mechanisms by which signals of impending reward release pre-organized response patterns.

Stimulus intensity affects pigeons’ timing behavior: Implications for an internal clock model

Five pigeons were trained to discriminate between 2- and 10-sec illuminations of a white light; choice of a red pecking key was correct and rewarded after presentation of the short stimulus, whereas choice of a green key was correct and rewarded after presentation of the long stimulus. On half the trials, the light was bright; on the others, it was dim. Durations of 4, 6, and 8 sec of both dim and bright light were also presented; choices on these trials were not rewarded. The probability of the pigeons’ choosing the short alternative decreased in a graded manner as duration of both bright and dim light increased from 2, to 4, to 6, to 8, and to 10 sec. However, the pigeons were more likely to choose the short alternative with longer durations of the dim light than the bright light, a result that implies that the perceived duration of a dim light was shorter than that of a bright light of equal length. One interpretation of this effect is that stimulus intensity affects the rate of the pacemaker in an internal clock mechanism subserving timing of event duration.

Effects of haloperidol and d-amphetamine on perceived quantity of foods and tones

The hypothesis that dopamine (DA) receptor agonists and antagonists affect “hedonia” associated with natural rewards was tested, using a psychophysical procedure previously shown to be sensitive to both the sweetness of food and the motivational state of rats. Rats were first trained to discriminate between two different quantities of a rewarding stimulus by pressing one of two levers. Perceived quantity was subsequently derived from generalization trials of intermediate quantities. Haloperidol (0.03–0.083 mg/kg), a DA receptor antagonist, did not influence perceived food quantity, an indirect marker of hedonic value. On the other hand, d-amphetamine (0.25–1.0 mg/kg) affected perceived food quantity in a dose-dependent fashion, and in the same direction as occurs after increasing hunger or food sweetness. Both haloperidol and amphetamine influenced the perceived quantity of a stimulus without natural reinforcing properties (a tone), but the effect of amphetamine on the perceived quantity of this initially neutral stimulus was opposite in direction to that observed with food. These results suggest that whereas amphetamine affects hedonic processes, haloperidol does not. In addition, it seems that haloperidol probably produces its actions through effects on motor mechanisms or by interfering with the response-facilitating properties of rewards.

Unconfounding time and number discrimination in a Mechner counting schedule

Food-deprived pigeons were trained on a modified Mechner counting schedule under which a peck at the center key produced grain reinforcement provided that this response was preceded by n or more consecutive pecks at the left key. Each peck at the left key produced either a fixed- or a variable-duration blackout. In the former, but not the latter, procedure, confounding of a fixed time period between the first and last responses on the left key and number of responses on the left key was possible. The results showed that the count of responses, independent of the duration of responding, may act as a controlling stimulus.

Stimulus control of defensive burying in the rat

Abstract Rats shocked once through a wire-wrapped stationary prod mounted on the wall of a test chamber incorporated the bedding material covering the chamber floor into a defensive reaction. When tested 1 min later, they approached the prod and buried it. Evidence was provided by three separate studies of this burying response that rats had learned about the association of both the position and brightness of the prod with shock after this single conditioning trial. In Experiment 1, the amount of burying decreased if either the position or brightness of the prod had been changed prior to the test. In Experiments 2 and 3, rats were shocked through one of two prods (white or black) mounted on opposite walls of the test chamber. When the positions of the prods were unchanged for the test, almost every subject buried the prod through which it had been shocked, even when the conditioning-test interval was 24 hr; whereas, each rat directed substantial amounts of bedding material at both prods when the positions of the two prods were reversed. Thus, discriminated “avoidance” learning can be rapid, reliable, and enduring when shock is administered “by” a clearly defined stimulus object, i.e., when cue and consequence are spatially contiguous.