Area postrema lesions impair flavor-toxin aversion learning but not flavor-nutrient preference learning

Rats with lesions of the area postrema (APX) or sham lesions were trained to associate flavored solutions with positive or negative postingestive consequences. The APX rats were similar to controls in learning preferences for flavors paired with concurrent intragastric infusions of maltodextrin or corn oil and for a flavor paired with delayed maltodextrin infusions. In contrast, the APX rats displayed impaired aversion learning for flavors paired with toxic drug treatments (lithium chloride infusion or methylscopolamine injection). The aversion learning deficit ranged from mild to total, depending on training procedures. These findings confirm the important role of the area postrema in flavor-toxin learning but provide no evidence for its involvement in flavor-nutrient conditioning.     

Zona incerta lesions impair osmotic but not hypovolemic thirst

Male albino rats with bilateral lesions of the anterior zona incerta are hypodipsic when dry food is available and show little or no regulatory drinking when food deprived. They do, however, drink normal quantities of water in response to subcutaneous injections of polyethylene glycol that produce extracellular hypovolemia. In contrast, they show a markedly reduced response to intracellular dehydration produced by intraperitoneal injections of hypertonic saline. The osmotic thirst deficit is not due to an increased latency of response to the injections.     

Perirhinal cortex and anterior thalamic lesions: comparative effects on learning and memory

Three learning and memory tasks were used to compare the effects of neurotoxic anterior thalamic nuclei (ATN) and perirhinal cortex (PRC) lesions in rats. Rats with ATN lesions showed impaired spatial memory in a 12-arm radial maze, whereas rats with PRC lesions showed intact spatial memory, despite the use of minimal pretraining and extensive within-session delays (to 40 rain). PRC, but not ATN, lesions produced impairments on a configural learning task using complex visual-tactile cues in the radial maze. Neither ATN nor PRC lesions consistently affected spontaneous object recognition across extended sample-test delays (to 40 min). These findings confirm the differential involvement of the ATN and PRC in learning and memory.     

Perirhinal cortex and place-object conditional learning in the rat

The present study examined whether excitotoxic lesions of the perirhinal cortex can affect acquisition of a place-object conditional task in which object and spatial information must be integrated. Testing was carried out in a double Y-maze apparatus, in which rats learned a conditional rule of the type, "In Place X, choose Object A, not Object B (A+ vs. B-); in Place Y, choose Object B, not Object A (A- vs. B+)." Perirhinal cortex lesions significantly impaired acquisition of this task while sparing performance of an allocentric spatial memory task performed in a radial arm maze. Perirhinal cortex lesions also had no apparent effect on a 1-pair object discrimination task performed in the double Y maze or on retention and acquisition of 4-pair concurrent discrimination problems performed in a computer-automated touch screen testing apparatus. The results suggest that, although the perirhinal cortex and hippocampus can be functionally dissociated, their normal mode of operation includes the integration of object and spatial information.     

Dorsal striatum and stimulus-response learning: lesions of the dorsolateral, but not dorsomedial, striatum impair acquisition of a stimulus-response-based instrumental discrimination task, while sparing conditioned place preference learning

While some evidence suggests that the dorsal striatum is important for stimulus-response learning, disagreement exists about the relative contribution of the dorsolateral and dorsomedial striatum to this form of learning. In the present experiment, the effects of lesions of the dorsolateral and dorsomedial striatum were investigated on two tasks that differentially require the development of stimulus-response learning. The first task utilized an operant conditional discrimination task, which is likely to rely heavily upon stimulus-response learning. The second task looked conditioned place preference learning, a task that is unlikely to require the development of stimulus-response associations. Animals with lesions of the dorsolateral striatum were impaired on the operant conditional discrimination task, but retained the ability to learn the conditioned place preference task. In contrast, animals with lesions of the dorsomedial striatum were not found to be impaired on either task used in the present experiment. These results suggest that the dorsolateral striatum is necessary for the successful acquisition of tasks that place a demand upon stimulus-response learning, while the dorsomedial striatum is not involved in this type of learning.     

Excitotoxic lesions to the prefrontal cortex of Sprague-Dawley rats do not impair response matching

In previous studies, chronic low-dose methylphenidate (MPH) administration during early development has been shown to increase emotional responding in adulthood. However, most studies employed male subjects, which generally show enhanced fear relative to females in laboratory tests of anxious behaviors. The present study examined the sex-dependent effects of MPH treatment on innate and learned fear behaviors. Rats were treated for 4 weeks from periadolescence through early adulthood with oral MPH. In open field testing, females showed greater levels of activity than males, and MPH (5mg/kg) decreased locomotion relative to control and 2mg/kg treatment in both sexes. In contextual fear conditioning, females exhibited less freezing than males at all retention intervals. Both sexes treated with 5mg/kg MPH showed increased fear to the shock context, although MPH treatment did not interfere with contextual discrimination in either sex. Upon reexposure to the shock context at 24h, only females treated with 5mg/kg MPH exhibited increases in freezing. MPH treatment did not disrupt extinction of contextual fear (48 h post-conditioning) in either sex. These findings illustrate subtle sex differences in the effects of prolonged MPH exposure on fear behaviors, and highlight the need to examine further the underlying mechanisms in both sexes.     

Basal forebrain lesions impair tactile discrimination and working memory

Rats received bilateral injections of the excitotoxin, N-methyl-D,L aspartate, which resulted in degeneration of basal forebrain cholinergic (BFC) neurons in the nucleus basalis magnocellularis. Most tests of general neurological function revealed no differences between control rats and those with BFC lesions and where differences were found they appeared to be due to hyperemotionality. Rats with BFC lesions demonstrated significant deficits in working memory, as evaluated in an 8-arm radial maze. In addition, these rats showed a severe impairment in tactile discrimination learning, an effect of BFC lesions not previously demonstrated. We propose that cholinergic deafferentation of the somatosensory cortex with consequent disruption in somatosensory information processing might account at least in part for this effect.     

Lateral,but not medial,frontal lesions impair fixed ratio performance in rats

Preoperatively, male hooded rats were reduced to 85% of their free-feeding weight, and trained to receive food reinforcement on a fixed ratio 32 schedule in standard operant chambers. When response rates stabilized, the rats were randomly assigned to one of the following surgical treatments: lateral frontal ablation, medial frontal ablation, or sham operate control (N=6/group). On the day following surgery, and continuing for a total of 15 postoperative days, the rats were retested on FR32. Lateral frontal lesions produced a dramatic reduction in response rate, while medial frontal lesions produced only a mild and transient effect. The sham operative procedure did not influence performance. These results suggest a dissociation of motor and associative functions in the frontal cortex of the rat and support anatomical evidence demonstrating this division.     

Cytotoxic lesions of the hippocampus increase social investigation but do not impair social-recognition memory

A number of studies have implicated the hippocampal formation in social-recognition memory in the rat. The present study addressed this issue directly by assessing the effects of cytotoxic lesions confined to the hippocampus proper, encompassing the four CA subfields and the dentate gyrus, on this behavioural task. Ibotenate-induced hippocampal lesions led to locomotor hyperactivity and a marked spatial working-memory impairment on the elevated T-maze. In addition, they also led to increased social investigation. However, despite these clear effects, there was no effect of the lesions on social-recognition memory. These results suggest that the hippocampus proper does not subserve social-recognition memory; but does not, however, preclude the possibility that other areas of the hippocampal formation (e.g. entorhinal cortex or subiculum) may support this memory process.     

Fornix lesions impair context-related cingulothalamic neuronal patterns and concurrent discrimination learning in rabbits (Oryctolagus cuniculus)

Cingulothalamic neurons develop topographic patterns of cue-elicited neuronal activity during discrimination learning. These patterns are context-related and are degraded by hippocampal lesions, suggesting that hippocampal modulation of cingulothalamic activity results in the expression of the patterns, which could promote the retrieval of context-appropriate responses and memories. This hypothesis was tested by training rabbits (Oryctolagus cuniculus) with fornix lesions concurrently on two discrimination tasks (approach and avoidance) in different contexts. Because the same conditioned stimuli were used for both tasks, contextual information was critical for overcoming intertask interference during concurrent task acquisition. The lesions degraded the topographic patterns and significantly impaired concurrent learning, suggesting that hippocampal-cingulothalamic interactions and the resulting topographic patterns are critical for processing contextual information needed to defeat interference.     

Do pretectal lesions impair visual discrimination acquisition in rats?

The effects of lesions in the anterior and posterior pretectum on acquisition of an intensity and a line orientation discrimination were examined. The lesions impaired line orientation, but not intensity discrimination acquisition. Anterior lesions had more effect than posterior, possibly due to more extensive involvement of the medical posterior thalamus in the former group.     

Lesions of visual cortex impair discrimination of hidden figures by cats

Cats lacking cortical visual areas 17, 18 and 19 can discriminate between patterns equal in luminous flux. The present experiment investigated the extent to which this ability varies with differences in the amount of contour and complexity of the patterns. To test this, cats were required to discriminate between patterns that were made progressively more alike in contour and complexity by increasing the number of masking stripes superimposed on each of a pair of stimuli. A mild deficit in shape discrimination among cats with lesions of areas 17, 18 and 19 was greatly exacerbated by masking the stimuli with superimposed stripes, and the severity of the deficit for each operated cat was correlated with the size of the contour and complexity difference between the stimuli. Finally, the severity of the deficit was related to the amount of geniculocortical damage. The results suggest that lesions largely confined to areas 17, 18 and 19 cause profound deficits on masked pattern discrimination even though performance with unmasked stimuli is relatively unimpaired. Indirect evidence suggests that this effect is not due solely to a reduction in visual acuity.     

Familiar contextual odors promote discrimination learning in preweanling but not in older rats

For rats 16- or 28-days-old postnatal, we tested the source of the facilitation in instrumental learning provided by odors from home nest materials. Three experiments confirmed that acquisition of a spatial discrimination (to escape footshock) was facilitated not only by the presence of typical nest odors in the training context, but also by the presence of a non-rat odor (banana) to which the animals had been familiarized for 4 hr/day for 7 days. There was a borderline tendency for a similar facilitation after familiarization to the latter odor for only 10 min. These effects occurred for a similar facilition after familiarization to the latter odor for only 10 min. These effects occurred for rats 16 days postnatal but not for those 28 days old. The experiments also confirmed that the prior exposure to non-rat odors did not in itself, in the absence of that odor during learning, affect discriminated escape learning, and that enhanced affinity for a contextual odor is not a sufficent condition for enhancement of learning in its presence. There was some indication that the 16-day-old rat was more likely to select a discriminative odor to guide their choice of spatial locations if a familiar contextual odor was present.     

Perirhinal cortex ablation in rats selectively impairs object identification in a simultaneous visual comparison task

In Experiment 1, rats discriminated among computer-generated visual displays (scenes) comprising 3 different shapes (objects). One constant scene (unrewarded) appeared on every trial together with a trial-unique variable scene (rewarded). Four types of variable scene were intermingled: (a) unfamiliar objects in different positions from the constant; (b) unfamiliar objects in same positions as the constant; (c) same objects as the constant in different positions; (d) same objects and positions, recombined. Aspiration lesions of perirhinal cortex impaired performance with type (b) only. Experiment 2 tested spatial delayed nonmatching-to-sample. The perirhinal group were impaired nonsignificantly, and less than fornix-transected rats in an earlier study. Rats' perirhinal cortex, like monkeys', subserves object identification in the absence of memory requirement but does not contribute substantially to hippocampal system spatial memory function.     

Effects of serial lesions of somatosensory cortex and further neodecortication on retention of a rough-smooth discrimination in rats

Summary Five groups of rats with bilateral lesions of the somatosensory cortex and one of animals sustaining only sham operations were tested for retention of a rough-smooth discrimination. Two of the lesion groups had sequential unilateral ablations, in one case with interoperative testing, and three groups had one-stage bilateral lesions. The two groups of animals with serial lesions did not differ from each other or from sham operates in relearning the task. Rats with one-stage lesions and preoperative overtraining also performed well, but the other one-stage groups showed deficits relative to control and serial lesion groups. In the second experiment the sham operated rats from Experiment 1 experienced lesions anterior and posterior to the somatosensory zones. These lesions did not affect retention. Somatosensory cortex then was ablated in one operation and severe performance decrements were seen. Removal of additional neocortex in a sample of animals that had relearned the discrimination after one-stage somatosensory cortex lesions (Exp. 1) also affected retention. In contrast, retention was not impaired on some of the measures in those animals that originally had two-stage ablations. The findings from these two experiments show that some ablation effects can be circumvented with overtraining or serial lesion techniques. The data also indicate that non-somatosensory cortex may play a role in recovery after somatic cortex lesions, but that the substrates underlying recovery might, not be the same after one-stage and two-stage ablations.     

Amygdala lesions do not impair shock-probe avoidance retention performance

The present experiment used the shock-probe paradigm, a procedure usually used to assess anxiolytic processes, to assess memory in amygdala-lesioned rats. Rats were placed in a chamber that contained a probe protruding from 1 of 4 walls and were kept there for 15 min after they contacted the probe. For half the rats, the probe was electrified (2 mA). Four days later, sham or neurotoxic amygdala lesions were induced. Retention performance was assessed 8 days later by measuring the latency to contact the probe and the number of contact-induced shocks. The results indicated that, although shock-naive amygdala-lesioned rats were impaired on the 2nd shock-probe test, shock-experienced amygdala-lesioned rats were not. These data indicate that the memory of a shock experience, as indexed with a shock-probe avoidance response, is spared in rats with large amygdala lesions.     

Cerebellar cortical degeneration disrupts discrimination learning but not delay or trace classical eyeblink conditioning

The authors investigated classical eyeblink conditioning in a relatively rare patient, B.R., with extensive cerebellar cortical atrophy and marked sparing of the dentate nucleus. Patient B.R.'s ability to acquire and extinguish simple associations (delay and trace conditioning tasks) as well as her ability to acquire more complex associations (temporal and simple discrimination tasks) were examined. There are 2 primary findings from this study. First, B.R. showed normal acquisition and extinction in delay and trace conditioning. Second, she demonstrated a complete inability to learn associative discriminations, even in the case of a simple 2-tone discrimination within the context of a delay paradigm. The latter finding was unexpected because of the sparing of her deep cerebellar nuclei. These data suggest that the cerebellar cortex, or pathways traversing cerebellar cortex, play an important role in classical eyeblink discrimination learning.     

Impairments in negative patterning, but not simple discrimination learning, in rats with 192 IgG-saporin lesions of the nucleus basalis magnocellularis

Rats with 192 IgG-saporin lesions of the nucleus basalis magnocellularis (NBM) and sham-operated rats were trained in either a simple discrimination paradigm assessing simple association learning or a negative patterning paradigm assessing configural association learning. In the simple discrimination task, rats were reinforced for responding to a light but were not reinforced for responding to a tone. In the negative patterning discrimination task, rats were reinforced for responding to either a light or a tone presented alone but were not reinforced for responding to both stimuli presented simultaneously. Simple discrimination learning was not affected, whereas acquisition of negative patterning was impaired by NBM lesions. Impaired configural association learning may reflect a loss in the ability of rats with NBM lesions to attend to multiple sensory stimuli or to cope with conflicting response strategies.     

Hippocampal lesions impair navigational learning in experienced homing pigeons.

Adult experienced homing pigeons from Maryland were subjected to hippocampal lesion and then transferred to a new loft in Ohio to examine what effect such treatment may have on learning to navigate to a new home loft. When subsequently released from an unfamiliar site, the hippocampal lesioned birds were impaired in taking up a vanishing bearing toward their new Ohio loft. This deficit is interpreted as an impairment in hippocampal-lesioned birds learning a new navigational map. Together with a previous study, the results suggest that an intact hippocampus is necessary if young naive or adult experienced homing pigeons are to learn a navigational map.     

Role of cortex in Pavlovian discrimination learning.

Two totally neodecorticate rabbits and two groups of normal rabbits were trained on a light-tone differentiation using a Pavlovian nictitating membrane response. The significance of the two stimuli involved in the differentiation was then reversed. Excellent initial differentiations were produced by both decorticates and normals, although there was some evidence of a small retardation of conditional response acquisition in the decorticates in this first stage of the experiment. Under reversal conditions the decorticates extinguished responding to the formerly positive conditional stimulus in fewer trials and produced more complete differentiation performances than normal animals. The decorticates, unlike the normal animals, failed to show a reduction in conditional response onset latencies during either differentiation or reversal training.