W. David Stahlman

Increased amplitude and duration of acoustic stimuli enhance distraction

Extraneous sounds have a variety of effects on animals; they may interfere with communication, cause physical harm, increase wariness, influence settlement decisions, or they may cause distractions in ways that increase vulnerability to predation. We designed a study to investigate the effects of changing both the amplitude and duration of an acoustic stimulus on distraction in a terrestrial hermit crab (Coenobita clypeatus). In experiment 1, we replicated the key findings from a field result: crabs hid more slowly in response to a silent visual stimulus when we simultaneously broadcast a white noise than they did when in a silent condition. In experiment 2, we altered the noise duration and found that a long noise generated greater latencies to hide than a short noise. In experiment 3, we increased the noise amplitude and found that hide latency increased with higher-intensity auditory stimuli. These experiments demonstrate a variety of stimulus factors that influence distraction. Our results suggest that prey animals could be in greater danger from predators when in an environment with auditory distractions.

Effect of Reward Probability on Spatial and Temporal Variation

Gharib, Derby, and Roberts (2001) proposed that reducing reward expectation increases variation of response form. We tested this rule in a new situation and asked if it also applied to variation of response location and timing. In 2 discrete-trial experiments, pigeons pecked colored circles for food. The circles were of 6 possible colors, each associated with a different probability of reward. Reducing reward expectation did not affect peck duration (a measure of form) but did increase horizontal variation of peck location and interpeck-interval variation. The effect of reward probability on the standard deviation of interpeck intervals was clearer (larger t value) than its effect on mean interpeck interval. Two datasets from rats had similar interresponse-interval effects.

Response variability in pigeons in a pavlovian task

Instrumental response variation is inversely related to reward probability. Gharib, Derby, and Roberts (2001) theorized that individuals behave more variably when their expectation of reward is low. They postulate that this behavioral rule assists the discovery of alternative actions when a target response is unlikely to be reinforced. This suggests that response variability may be unaffected in a situation in which an animal’s behavior is inconsequential to outcome delivery. We trained 6 pigeons in a within-subjects Pavlovian autoshaping procedure. On any given trial, the pigeons were presented with one of six colored discs on a touchscreen; each stimulus was associated with a particular probability of food, ranging from 100% to 0.6%. Pecking was more variable with low probabilities of food delivery, thus extending the rule relating variability and expectation to a Pavlovian situation.

Quantifying personality in the terrestrial hermit crab: Different measures, different inferences

There is much interest in studying animal personalities but considerable debate as to how to define and evaluate them. We assessed the utility of one proposed framework while studying personality in terrestrial hermit crabs (Coenobita clypeatus). We recorded the latency of individuals to emerge from their shells over multiple trials in four unique manipulations. We used the specific testing situations within these manipulations to define two temperament categories (shyness-boldness and exploration-avoidance). Our results identified individual behavioral consistency (i.e., personality) across repeated trials of the same situations, within both categories. Additionally, we found correlations between behaviors across contexts (traits) that suggested that the crabs had behavioral syndromes. While we found some correlations between behaviors that are supposed to measure the same temperament trait, these correlations were not inevitable. Furthermore, a principal component analysis (PCA) of our data revealed new relationships between behaviors and provided the foundation for an alternate interpretation: measured behaviors may be situation-specific, and may not reflect general personality traits at all. These results suggest that more attention must be placed on how we infer personalities from standardized methods, and that we must be careful to not force our data to fit our frameworks.

Blocking of spatial control by landmarks in rats.

We investigated spatial blocking among landmarks in an open-field foraging task in rats. In Phase 1, rats were presented with A+ trials during which landmark (LM) A signaled the location of hidden food. In Phase 2, rats were given AX+ trials in which LM X served as a redundant spatial cue to the location of food. Additionally, BY+ trials were given as a within-subjects overshadowing-control procedure. At test, rats received nonreinforced presentations of LM X and LM Y on separate trials. Rats took longer to find the training goal location in the presence of LM X than of LM Y, thereby demonstrating that spatial control by LM X was blocked by prior learning with LM A. This constitutes the first evidence in rats for spatial blocking of one proximal landmark by another-approximating a conventional blocking design.

Auditory stimulation dishabituates anti-predator escape behavior in hermit crabs (Coenobita clypeatus)

Responses to innocuous stimuli often habituate with repeated stimulation, but the mechanisms involved in dishabituation are less well studied. Chan et al. (2010b) found that hermit crabs were quicker to perform an anti-predator withdrawal response in the presence of a short-duration white noise relative to a longer noise stimulus. In two experiments, we examined whether this effect could be explicable in terms of a non-associative learning process. We delivered repeated presentations of a simulated visual predator to hermit crabs, which initially caused the crabs to withdraw into their shells. After a number of trials, the visual stimulus lost the ability to elicit the withdrawal response. We then presented the crabs with an auditory stimulus prior to an additional presentation of the visual predator. In Experiment 1, the presentation of a 10-s, 89-dB SPL noise produced no significant dishabituation of the response. In Experiment 2 we increased the duration (50s) and intensity (95dB) of the noise, and found that the crabs recovered their withdrawal response to the visual predator. This finding illustrates dishabituation of an antipredator response and suggests two distinct processes-distraction and sensitization-are influenced by the same stimulus parameters, and interact to modulate the strength of the anti-predator response.

A non-selective serotonin antagonist promotes rapid habituation in the terrestrial hermit crab

Research has indicated that serotonin (5-HT) modulates non-associative learning in a variety of invertebrate species. Recent work has demonstrated that the terrestrial hermit crab is a suitable animal model for non-associative learning phenomena, including habituation, sensitization, and dishabituation. We examined the potential role of a non-selective 5-HT antagonist, methysergide, in non-associative learning in the hermit crab. We administered methysergide prior to delivering repeated stimulus presentations of a looming visual predator. We found evidence for more rapid habituation relative to a control condition in which crabs did not receive the drug. These results indicate a role for 5-HT in the defensive behavior of the hermit crab and importantly, suggest a conserved role for 5-HT in modulating basic learning processes in invertebrates.

Overshadowing between visual and tactile stimulus elements in an object recognition task

In recent decades there has been great progress in discovering the conditions under which cue competition occurs during animal learning. In humans, however, the evidence remains equivocal regarding the degree to which stimuli compete with one another for behavioral control. We report here the results of a single experiment wherein thirty-nine college students completed a novel cue competition task with visual and tactile stimuli. Participants visually and/or haptically examined a series of novel objects. They were then asked to select the objects with which they had interacted from a larger pool of both novel and familiar objects. Potentiation (or facilitation) by simultaneous visual and haptic inspection was possible. Alternatively, stimulus elements may have competed with one another (i.e., overshadowing), which would present as poorer recognition at test for objects to which participants had simultaneous, dual-modality training exposure. We report the latter effect. We situate these findings in the broader context of associative learning and suggest that our data is relevant to applied settings.