Acquisition of conditional discriminations in hippocampal lesioned and decorticated rats: evidence for learning that is separate from both simple classical conditioning and configural learning

A Receptor Tyrosine Kinase Plays Separate Roles in Sensory Integration and Associative Learning in C. elegans

Associative learning and sensory integration are two behavioral processes that involve the sensation and processing of stimuli followed by an altered behavioral response to these stimuli, with learning requiring memory formation and retrieval. We found that the cellular and molecular actions of scd-2 dissociate sensory integration and associative learning. This was discovered through investigation of a Caenorhabditis elegans mutation (lrn-2 (mm99)) affecting both processes. After mapping and sequencing, lrn-2 was found to be allelic to the gene, scd-2. scd-2-mediated associative learning and sensory integration operate in separate neurons as separate processes. We also find that memories can form from associations that are processed and stored independently from the integration of stimuli preceding an immediate behavioral decision.

Occasion setting

Occasion setting refers to the ability of 1 stimulus, an occasion setter, to modulate the efficacy of the association between another, conditioned stimulus (CS) and an unconditioned stimulus (US) or reinforcer. Occasion setters and simple CSs are readily distinguished. For example, occasion setters are relatively immune to extinction and counterconditioning, and their combination and transfer functions differ substantially from those of simple CSs. Similarly, the acquisition of occasion setting is favored when stimuli are separated by longer intervals, by empty trace intervals, and are of different modalities, whereas the opposite conditions typically favor the acquisition of simple associations. Furthermore, the simple conditioning and occasion setting properties of a single stimulus can be independent, for example, that stimulus may simultaneously predict the occurrence of a reinforcer and indicate that another stimulus will not be reinforced. Many behavioral phenomena that are intractable to simple associative analysis are better understood within an occasion setting framework. Besides capturing the distinction between direct and modulatory control common to many arenas in neuroscience, occasion setting provides a model for the hierarchical organization of memory for events and event relations, and for contextual control more broadly. Although early lesion studies further differentiated between occasion setting and simple conditioning functions, little is known about the neurobiology of occasion setting. Modern techniques for precise manipulation and monitoring of neuronal activity in multiple brain regions are ideally suited for disentangling contributions of simple conditioning and occasion setting in associative learning. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Conditioned Taste Avoidance Drug Discrimination Procedure: Assessments and Applications

In the present chapter, we summarize much of the work on the taste avoidance drug discrimination procedure, presenting the logic for its initial introduction and the extension of the procedure in the investigation of the discriminative properties of various drugs. Results from these assessments parallel those from more traditional operant and maze designs in classifying and characterizing the discriminative properties of drug. At the same time, this design reveals a procedure that is sensitive in such assessments by indexing these stimulus properties more rapidly and at lower doses than in the more traditional procedures (in some cases for drugs heretofore resistant in their detection). Importantly, much remains to be learned about the taste avoidance procedure in that the nature of such learning remains unknown and the specific parameters under which it can be established and generalized and its neurochemical and neuroanatomical bases are largely unexplored. The application of drug discrimination learning to human drug abuse continues to be an important consideration for this specific design (as well as that of drug discrimination procedures in general), and recent parallels between drug use and food intake in terms of its regulation by interoceptive stimuli suggests a possible role of the loss of stimulus control in drug escalation and addiction (with possible therapeutic implications via the modulation of these interoceptive cues).

Environmental complexity affects contextual fear conditioning following hippocampal lesions in rats

Contextual fear conditioning has become a benchmark measure for hippocampal function, even though several studies report successful acquisition in hippocampal-damaged rodents. The current study examined whether environmental complexity may account for these discrepancies. We directly compared single-session contextual fear conditioning in rats in a simple vs. complex environment. Hippocampal lesions led to reduced fear conditioning in both contexts, as measured by freezing, but the effect was significantly greater in the complex context. As well, lesions led to generalized fear when the complex context was paired with shock, but not when the simple context was paired. We suggest that the representation of the simple context formed by rats with hippocampal lesions was adequate to support associative learning, but the representation of the complex context, which depended to a greater extent on relational learning, was not. The results were interpreted as consistent with theories of hippocampal function that emphasize its role in integrating multiple stimulus elements in a memory trace.

Roles of the auditory cortex in discrimination learning by rats

We investigated the roles of the auditory cortex in sound discrimination learning in Wistar rats. Absolute pitch or relative pitch can be used as discrimination cues in sound frequency discrimination. To clarify this, rats were trained to discriminate between rewarded (S+) and unrewarded (S-) test stimuli (S+ frequency>S- frequency). After learning was acquired by rats, performance was tested in a new test in which S+ frequency was constant but S+ frequency<S- frequency, or S+ frequency>S- frequency but both frequencies were increased. If the discrimination cue of the first test was preserved in the new test, performance following change of testing procedures was expected to remain high. The measured performance suggested that rats used relative pitch in half octave discrimination (difference between S+ and S- frequencies, 0.5 octave), and absolute pitch in octave discrimination (difference between S+ and S- frequencies, 1.0 octave). Bilateral lesions in the auditory cortex had almost no effect on performance before procedure change. Furthermore, performance following procedure change was not affected by lesions in the auditory cortex when the discrimination cue was preserved. However, performance was impaired by lesions in the auditory cortex when a new discrimination cue was used following procedure change. Lesions in the auditory cortex also impaired multimodal discrimination between sound and sound plus light. The present findings suggest that the auditory cortex plays a role as a sensory interface of the higher cortices required for flexible learning and multimodal discrimination.

Perirhinal cortex lesions impair context aversion learning

Rats with perirhinal cortex lesions were compared with sham controls on a conditional discrimination in which saccharin was paired with LiCl in context 1, but paired with saline in context 2. Perirhinal-lesioned rats were slightly slower to acquire the discrimination but reached control levels by the end of acquisition. Both groups showed transfer to familiar tap water, consuming less in context 1 than in context 2. Unlike sham rats, perirhinal rats failed to show an aversion to context 1 on a place choice test. These data provide neuroanatomical support for the postulated difference between Pavlovian conditioning and conditional learning.

Conditional discrimination learning and negative patterning in rats with neonatal hippocampal lesion induced by ionizing radiation

This study was undertaken to investigate the associative process underlying serial feature positive conditional discrimination learning (X-->A+/A-) and the role of the hippocampus in the solution of tasks demanding a configural association strategy such as the negative patterning discrimination (XA-/X+/A+). It has been suggested that the hippocampus is essential for the learning of complex tasks, so, it is expected that hippocampal lesions would prove equally detrimental to performance in both tasks, but would not interfere with simple discrimination learning. Hippocampal lesions were made with X-radiation exposure to neonate rats after completion of a parametric study 'J. Neurosci. Methods 75 (1997) 41' that established the best radiation parameters to selectively lesion the hippocampal dentate gyrus. When adults, rats were submitted to a serial feature positive conditional discrimination task with the trials 'House light/Tone: water (H-->T+)', 'Tone: no water (T-)', and two simple discrimination with the trials 'Clicker: water (C+)' and 'Noise: no water (N-)' in Experiment I. In Experiment II, adult rats, irradiated and control, were submitted to the negative patterning task with the trials 'House light/Tone: no water (HT-)', 'House light: water (H+)', 'Tone: water (T+)', and to the non-conditional discrimination with the trial Noise: no water (N-)'. In contrast to the expectation of impaired performance in these tasks by lesioned rats, animals with damage to the hippocampal dentate gyrus learned the complex and the simple tasks as well as control subjects. These results suggest that the dentate gyrus does not participate directly in the modulation of acquisition of tasks demanding a complex strategy of occasion setting in procedures of serial conditional discrimination or a configural strategy, important for the negative patterning discrimination solution.

Applications of the Morris water maze in the study of learning and memory

The Morris water maze (MWM) was described 20 years ago as a device to investigate spatial learning and memory in laboratory rats. In the meanwhile, it has become one of the most frequently used laboratory tools in behavioral neuroscience. Many methodological variations of the MWM task have been and are being used by research groups in many different applications. However, researchers have become increasingly aware that MWM performance is influenced by factors such as apparatus or training procedure as well as by the characteristics of the experimental animals (sex, species/strain, age, nutritional state, exposure to stress or infection). Lesions in distinct brain regions like hippocampus, striatum, basal forebrain, cerebellum and cerebral cortex were shown to impair MWM performance, but disconnecting rather than destroying brain regions relevant for spatial learning may impair MWM performance as well. Spatial learning in general and MWM performance in particular appear to depend upon the coordinated action of different brain regions and neurotransmitter systems constituting a functionally integrated neural network. Finally, the MWM task has often been used in the validation of rodent models for neurocognitive disorders and the evaluation of possible neurocognitive treatments. Through its many applications, MWM testing gained a position at the very core of contemporary neuroscience research.

Conditional discriminations based on external and internal cues in rats with cytotoxic hippocampal lesions

Septal-hippocampal system lesions, mostly using aspiration techniques, have been reported to impair performance of conditional tasks. Rats with axon-sparing cytotoxic hippocampal lesions were therefore tested in a range of instrumental conditional paradigms. They did not differ from controls in their ability to choose the appropriate object in a conditional object discrimination cued by internal state (hunger or thirst) or on performance of conditional visuospatial object discriminations. Acquisition of a conditional visuospatial discrimination with black and white boxes as stimuli was also unimpaired. In contrast, lesioned rats were profoundly impaired on an open T-maze task when cued by either their internal state (reference memory task) or their previous response (working memory task). The results indicate that perception or use of spatial cues, rather than conditional responding per se, is impaired by cytotoxic hippocampal lesions.

Memory for duration: role of hippocampus and medial prefrontal cortex

In this task rats had to learn that a three-dimensional object stimulus (a rectangle) that was visible for 2 s would result in a positive (go) reinforcement for one object (a ball) and no reinforcement (no go) for a different object (a bottle). However, if the rectangle stimulus was visible for 8 s then there would be no reinforcement for the ball (no go), but a reinforcement for the bottle (go). After rats learned this conditional discrimination by responding differentially in terms of latency to approach the object, they received large (dorsal and ventral) lesions of the hippocampus, lesions of the medial prefrontal cortex (anterior cingulate and precentral cortex), lesions of the cortex dorsal to the dorsal hippocampus, or served as sham-operated controls. Following recovery from surgery they were retested. The results indicate that there were major impairments following hippocampal lesions, in contrast to cortical control and medial prefrontal cortex lesions, as indicated by smaller latency differences between positive and negative trials on postsurgery tests. In order to ensure that the deficits observed with hippocampal lesions were not due to a discrimination problem, new rats were trained in an object (gray cylinder) duration discrimination task. In this go/no go procedure, the rats were reinforced for a 2-s exposure (duration) of the gray cylinder, but not a 10-s duration, or vice versa. The results indicate that after hippocampal lesions, there was an initial deficit followed by complete recovery. There were no significant changes for the medial prefrontal, cortical control, or sham-operated animals. It appears that the hippocampus, but not the medial prefrontal cortex, is actively involved in representing in short-term memory temporal attribute information based on the use of markers for the beginning and end of the presence (duration) of a stimulus (object).

Hippocampal lesions alter conditioning to conditional and contextual stimuli

The control of conditioned fear behaviour by a conditional stimulus (CS) and contextual stimuli (CXT) was compared in rats with lesions to the hippocampus (HPC) or neocortex (CO), and operated controls (OC). After classical fear conditioning in a distinctive context, rats were subsequently tested in the presence of the CS and CXT (CS + CXT), the CS alone (CS-only), or context alone (CXT-only). Two experiments were conducted in which conditioned fear was measured by an active avoidance response (experiment 1) or by response suppression (experiment 2). Groups did not differ in acquiring the conditioned fear response, as measured in the CS + CON test but, in both experiments, hippocampal (HPC) groups exhibited more conditioned fear behaviour than controls in the CXT-Only and CS-Only conditions. It was suggested that control rats conditioned the fear response to a stimulus complex that incorporated the CS and CTX. Rats with HPC lesions did not form this association between the stimulus elements; instead they segregated the CS and CXT and formed independent associations between the conditioned response (CR) and each component. In showing that HPC damage disrupts the process of forming associations between environmental stimuli and that the effect is not restricted to contextual cues, the results help to resolve apparently contradictory findings regarding the role of HPC in contextual information processing.

Systematic comparison of the effects of hippocampal and fornix-fimbria lesions on acquisition of three configural discriminations

The effects of lesions to the hippocampal system on acquisition of three different configural tasks by rats were tested. Lesions of either the hippocampus (kainic acid/colchicine) or fornix-fimbria (radiofrequency current) were made before training. After recovery from surgery, rats were trained to discriminate between simple and compound-configural cues that signaled the availability or nonavailability of food when a bar was pressed. When positive cues were present, one food pellet could be earned by pressing a lever after a variable time had elapsed. The trial terminated on food delivery (variable interval 15 s). This procedure eliminates some possible alternative explanations of the results of previous experiments on configural learning. Hippocampal lesions increased rates of responding and retarded acquisition of a negative patterning task (A+, B+, AB-); using a ratio measure of discrimination performance these lesions had a milder retarding effect on a biconditional discrimination (AX+, AY-, BY+, BX-), and they had no effect on a conditional context discrimination (X: A+, B-; Y: A-, B+). Fornix-fimbria lesions did not affect acquisition of any of these tasks but increased rates of responding. The results suggest that several task parameters determine the involvement of the hippocampus in configural learning; however, all tasks tested can also be learned to some extent in the absence of an intact hippocampal system, presumably by other learning/memory systems that remain intact following surgery. The lack of effect of fornix-fimbria lesions on any of these tasks suggests that retrohippocampal connections with other brain areas may mediate hippocampal contributions to the learning of some configural tasks. An analysis of these results and of experiments on spatial learning situations suggests that involvement of the hippocampus is a function of the degree to which correct performance depends on a knowledge of relationships among cues in a situation.