Jasper Robinson

Lesions in the anterior thalamic nuclei of rats do not disrupt acquisition of stimulus sequence learning

Sequence learning and spatial alternation were examined in rats with anterior thalamic lesions or sham surgeries. There was a lesion-induced deficit in spatial alternation but not in sequence learning. During sequence learning, rats discriminated between six different sequentially presented compounds (e.g., reinforce A before B, but not B before A), composed of audio-visual elements. The solution required rats to learn both specific stimulus sequences and the reward contingencies associated with these specific temporal relationships. The failure of anterior thalamic lesions to affect the acquisition of this sequential configural task complements the recent finding that anterior thalamic lesions also spare the acquisition of a configural task involving specific stimulus pairings and their spatial relationships. These findings suggest that such “structural” learning is more reliant on cortico-hippocampal than thalamo-hippocampal interactions.

Dissociations in the effect of delay on object recognition: evidence for an associative model of recognition memory.

Rats were administered 3 versions of an object recognition task: In the spontaneous object recognition task (SOR) animals discriminated between a familiar object and a novel object; in the temporal order task they discriminated between 2 familiar objects, 1 of which had been presented more recently than the other; and, in the object-in-place task, they discriminated among 4 previously presented objects, 2 of which were presented in the same locations as in preexposure and 2 in different but familiar locations. In each task animals were tested at 2 delays (5 min and 2 hr) between the sample and test phases in the SOR and object-in-place task, and between the 2 sample phases in the temporal order task. Performance in the SOR was poorer with the longer delay, whereas in the temporal order task performance improved with delay. There was no effect of delay on object-in-place performance. In addition the performance of animals with neurotoxic lesions of the dorsal hippocampus was selectively impaired in the object-in-place task at the longer delay. These findings are interpreted within the framework of Wagners (1981) model of memory.

An associative analysis of object memory.

Different aspects of recognition memory in rodents are commonly assessed using variants of the spontaneous object recognition procedure in which animals explore objects that differ in terms of their novelty, recency, or where they have previously been presented. The present article describes three standard variants of this procedure, and outlines a theory of associative learning, SOP which can offer an explanation of performance on all three types of task. The implications of this for theoretical interpretations of recognition memory and the procedures used to explore it are discussed.

Analysis of an ambiguous-feature discrimination.

An autoshaping experiment with pigeons and three appetitive Pavlovian conditioning experiments with rats investigated the course of an ambiguous-feature discrimination in which trials with stimuli A and BC were followed by food, and trials with B and AC were not. The discrimination between A and B was acquired more rapidly than the discrimination between BC and AC in all experiments. Furthermore, the acquisition of conditioned responding with A was faster than that with BC for the three rat experiments. The results of these experiments are discussed in terms of elemental and configural theories of associative learning.

Relative recency influences object-in-context memory

Highlights • Results of object-in-context experiments can be influenced by relative recency.• Data from two experiments presented supporting this suggestion.• This may complicate interpretation of results of object-in-context experiments.• Recommendations are made on how to address this.• Results consistent with an associative account of recognition memory.

Familiarity-Based Stimulus Generalization of Conditioned Suppression

We report that stimulus novelty/familiarity is able to modulate stimulus generalization and discuss the theoretical implications of novelty/familiarity coding. Rats in Skinner boxes received clicker → shock pairings before generalization testing to a tone. Before clicker training, different groups of rats received preexposure treatments designed to systematically modulate the clicker and the tone’s novelty and familiarity. Rats whose preexposure matched novelty/familiarity (i.e., either both or neither clicker and tone were preexposed) showed enhanced suppression to the tone relative to rats whose preexposure mixed novelty/familiarity (i.e., only clicker or tone was preexposed). This was not the result of sensory preconditioning to clicker and tone.

Diminished acquired equivalence yet good discrimination performance in older participants

We asked younger and older human participants to perform computer-based configural discriminations that were designed to detect acquired equivalence. Both groups solved the discriminations but only the younger participants demonstrated acquired equivalence. The discriminations involved learning the preferences [“like” (+) or “dislike” (−)] for sports [e.g., tennis (t) and hockey (h)] of four fictitious people [e.g., Alice (A), Beth (B), Charlotte (C), and Dorothy (D)]. In one experiment, the discrimination had the form: At+, Bt−, Ct+, Dt−, Ah−, Bh+, Ch−, Dh+. Notice that, e.g., Alice and Charlotte are “equivalent” in liking tennis but disliking hockey. Acquired equivalence was assessed in ancillary components of the discrimination (e.g., by looking at the subsequent rate of “whole” versus “partial” reversal learning). Acquired equivalence is anticipated by a network whose hidden units are shared when inputs (e.g., A and C) signal the same outcome (e.g., +) when accompanied by the same input (t). One interpretation of these results is that there are age-related differences in the mechanisms of configural acquired equivalence.

Ratios and effect size.

Responding to a related pair of measurements is often expressed as a single discrimination ratio. Authors have used various discrimination ratios; yet, little information exists to guide their choice. A second use of ratios is to correct for the influence of a nuisance variable on the measurement of interest. I examine 4 discrimination ratios using simulated data sets. Three ratios, of the form a/(a + b), b/(a + b), and (a − b)/(a + b), introduced distortions to their raw data. The fourth ratio, (b − a)/b largely avoided such distortions and was the most sensitive at detecting statistical differences. Effect size statistics were also often improved with a correction ratio. Gustatory sensory preconditioning experiments involved measurement of rats’ sucrose and saline consumption; these flavors served as either a target flavor or a control flavor and were counterbalanced across rats. However, sensory preconditioning was often masked by a bias for sucrose over saline. Sucrose and saline consumption scores were multiplied by the ratio of the overall consumption to the consumption of that flavor alone, which corrected the bias. The general utility of discrimination and correction ratios for data treatment is discussed.