Showing papers in "Psychobiology in 1998"

Toward direct visualization of the internal shape representation space by fMRI

Abstract: Reports of columnar organization of the macaque inferotemporal cortex (Tanaka, 1992, 1993a) indicate that ensembles of cells responding to particular objects may be both sufficiently extensive and properly localized to allow their detection and discrimination by means of functional magnetic resonance imaging (fMRI). A recently developed theory of object representation by ensembles of coarsely tuned units (Edelman, 1998; Edelman & Duvdevani-Bar, 1997b) and its implementation as a computer model of recognition and categorization (Cutzu & Edelman, 1998; Edelman & Duvdevani-Bar, 1997a) provide a computational framework in which such findings can be interpreted in a straightforward fashion. Taken together, these developments in the study of object representation and recognition suggest that direct visualization of the internal representations may be easier than was previously thought. In this paper, we show how fMRI techniques can be used to investigate the internal representation of objects in the human visual cortex. Our initial results reveal that the activation of most voxels in object-related areas remains unaffected by a coarse scrambling of the natural images used as stimuli and that a map of the representation space of object categories in individual subjects can be derived from the distributed pattern of voxel activation in those areas.

Neural systems that encode categorical versus coordinate spatial relations: PET investigations

Abstract: Participants received three sets of trials while regional cerebral blood flow was assessed using positron emission tomography (PET). In one set, the baseline, they responded when they detected a horizontal bar with an × either above or below it. In another set, the categorical spatial relations judgment condition, they decided whether the × was above or below the bar. In the third set, the coordinate spatial relations judgment condition, they decided whether the × was within 0.5 in. of the bar. In Experiment 1, the precise locations of the bars and × marks were varied, which required participants to focus attention selectively. Consistent with previous behavioral and neuropsychological findings, the left hemisphere was generally more activated during the categorical judgment task than during the coordinate judgment task, whereas the right hemisphere was generally more activated during the coordinate judgment task. In addition, these apparently simple tasks drew on large and different networks of areas. Experiment 2 was the same as Experiment 1 except that the stimuli were always in the same position and were presented very briefly. Three areas in the right parietal lobe were activated more by the coordinate task than by the categorical task in both experiments. In contrast, in neither experiment were any common areas activated more by the categorical task than by the coordinate task.

Illumination effects in face recognition

Abstract: How do observers recognize faces despite dramatic image variations that arise from changes in illumination? This paper examines (1) whether face recognition is sensitive to illumination direction and (2) whether cast shadows improve performance by providing information about illumination or hinder performance by introducing spurious edges. In Experiment 1, observers judged whether two sequentially presented faces, illuminated from the same direction or different directions, were the same individual or not. Cast shadows were present for half of the observers. Performance was impaired by a change in the illumination direction and by the presence of shadows. In Experiment 2, observers learned to name eight faces under one illumination direction (left/right) and one cast-shadow condition (present/absent); they were later tested under novel illumination and shadow conditions. Performance declined for unfamiliar illumination directions, but not for unfamiliar shadow conditions. The finding that face recognition is illumination dependent is consistent with the use of image-based representations. The results indicate that face recognition processes are sensitive to either the direction of lighting or the resultant pattern of shading, and that cast shadows can hinder recognition, possibly by masking informative features or leading to spurious contours.

Human brain potentials related to the emotional expression, repetition, and gender of faces

Abstract: Event-related potentials were recorded from 20 healthy male subjects in response to a large number of color slides of unfamiliar faces with happy, sad, or no emotional expression. In an initial task, the subjects rated the emotional valence of the faces with a joystick. In comparison with neutral faces, both happy and sad faces evoked a larger lateral occipito-temporal negativity from 200 to 400 msec post-stimulus onset. Modulation of late positive complex (LPC: 450–600 msec) by emotional expressions was observed at the frontal sites only in this task, when attention to the emotional valence was required. In a second task, the subjects detected repeating faces among nonrepeating, novel faces. Emotionally expressive faces evoked more negative potential than neutral faces occipito-temporally between 270 and 540 msec latency. Although repetition had a large effect in decreasing the N4 and increasing the LPC, it did not interact with emotional expression, supporting previously proposed independence between processing of a face identity and emotional expression. These findings imply that emotional expression affects early perceptual stages as well as later cognitive stages of face processing. Nonrepeated male faces in both tasks evoked a larger late negativity than female faces.

Rodent posterior parietal cortex as a component of a cortical network mediating directed spatial attention

Abstract: The rat posterior parietal cortex has been found to be a multimodal convergence area identifiable by its pattern of neuroanatomical connections with the cortex and thalamus. The rat posterior parietal cortex is part of a cortical network that also includes the medial agranular and ventrolateral orbital areas. Each of these three cortical areas is a region of multimodal convergence. Bilateral destruction of any of these areas produces deficits in spatial learning, whereas unilateral lesions induce deficits in directed attention. These findings suggest that together these cortical regions form an integrated network for spatial processing and orientation. In a recent test of this hypothesis, the posterior parietal area was selectively disconnected from the medial agranular cortex without direct damage to either area. This resulted in severe neglect which was qualitatively and quantitatively similar to that produced by unilateral destruction of wither cortical area, thereby supporting the network hypothesis. Some earlier conflicting reports bearing on the functional role of rat posterior parietal cortex may be explained in light of these disconnection data. The anatomical and behavioral findings suggest that in rats, as in primates, a parieto-frontal network mediates the integrated, dynamic spatial representations essential for normal directed attention and spatial orientation.

Locating an invisible goal in a water maze requires at least two landmarks

Abstract: In three experiments, rats were trained in a spatial task in a swimming pool similar to that used by Morris (1981), but surrounded by black curtains that provided a great level of control over the landmarks that defined the location of the goal. Experiment 1 showed that rats can learn to find an invisible platform in a fixed location relative to four landmarks in a very homogeneous environment. Experiments 2 and 3 demonstrated that the animals need two landmarks to recover the information about the environment stored in their memory as a cognitive map.

Object-centered attentional biases and object recognition contributions to scene segmentation in left- and right-hemisphere-damaged patients

Abstract: Participants viewed elongated rectangular displays in which two regions shared a central contour. In experimental stimuli, the central contour portrayed a known object on one, high-denotative, side. In control stimuli, no known objects were portrayed on either side of the central contour, but one side of each control stimulus was a scrambled version of one of the high-denotative regions, matching it on all factors known to influence scene segmentation other than object recognition. For each display, participants decided whether the left or the right region was more likely to be an object. Paradoxically, both right-hemisphere- (RH) and left-hemisphere- (LH) damaged individuals were more likely to see objects lying on the contralesional rather than the ipsilesional side of the central contour. This tendency is attributed to an object-centered attentional bias toward the central contour when objects lie on its contralesional side and away from the central contour when objects lie on its ipsilesional side. Object-centered attentional biases were stronger following RH than LH damage. Elderly control participants showed a slight bias in the same direction as RH-damaged individuals. More high-denotative regions than scrambled regions were seen as objects, even when object-centered attention was biased away from the central contour carrying the object recognition information. The latter result suggests that the object recognition processes contributing to scene segmentation are preattentive.

Head-centered representation and spatial memory in rat posterior parietal cortex

Abstract: Rodent posterior parietal cortex, similar to its homologue in primates, has been shown to be critically involved in spatial navigation. We summarize here selected neurophysiological data and their relevance to recent lesion-behavioral studies. The findings indicate that the rat posterior parietal cortex contains a significant proportion of neurons that code for head-centered (or body-centered) space by making use of the vestibular and proprioceptive inputs. Some posterior parietal neurons may store a working representation of space, which appears to be allocentric.

The dopamine D2 agonist quinpirole disrupts attention to temporal signals without selectively altering the speed of the internal clock

Abstract: Three groups of rats were trained to discriminate between 2 sec and 8 sec of darkness by responding to either the left or the right lever. Following acquisition of this temporal discrimination, psychophysical functions were obtained by presenting unreinforced signals of intermediate duration. Two groups of rats were trained with saline and subsequently tested with the specific D2 dopamine agonist quinpirole (0.08 mg/kg). One of these groups was naive to the drug prior to testing (DN), whereas the other had exposure to the drug but not during training sessions (DE). A third group (DT) was trained under quinpirole and tested with saline. The temporal discrimination was acquired rapidly and equiv-alently in Groups DN and DE. However, rats in Group DT were severely impaired in acquiring the discrimination. During psychophysical testing, quinpirole disrupted the accuracy of temporal discrimination equivalently in Groups DN and DE. Both the Weber fraction (WF) and the difference limen (DL) increased significantly in Groups DN and DE; however, the point of subjective equality (PSE) was not affected. In Group DT, the shift to saline during psychophysical testing did not result in any changes to the PSE, DL, or WF. These findings are not consistent with the hypothesis that the speed of the internal clock is selectively affected by D2 dopaminergic manipulations. Prior exposure to the drug does not appear to be a critical variable in the failure to observe a selective adjustment of the internal clock. The D2 agonist quinpirole appears to affect the accuracy of temporal discriminations generally, without altering the speed of the internal clock.

Further implications of a computational model of septohippocampal cholinergic modulation in eyeblink conditioning

Abstract: Previously we have shown that Gluck and Myers’s (1993) corticohippocampal model could be extended to incorporate Hasselmo and Schnell’s (1994) hypothesis that septohippocampal cholinergic processes regulate the amount of information storage in hippocampus. The generalized model could account for the effect of the anticholinergic drug scopolamine in delaying onset of eyeblink conditioning (Myers et al., 1996). Here, we show that the model also accounts for additional eyeblink results, including quick recovery after scopolamine is removed, preserved latent inhibition, learned irrelevance and extinction under scopolamine, and no effect of systemic scopolamine after hippocampal lesion. Additionally, the model is consistent with data concerning localized scopolamine injections to the medial septum, the lateral septum, and the hippocampus and their effect on eyeblink conditioning.

Configural learning in humans : The transverse patterning problem

Abstract: Most learning theorists concur that some sort of configural representation system is required for animals to solve certain compound discriminations. Transverse patterning (A+ vs. B−, B+ vs. C−, C+ vs. A−) is one example of a problem that requires a configurai solution. It has been reported in the past that adult humans are unable to solve this problem (Berch & Israel, 1971; Franks, 1976). We investigated the generality of these reports. Moreover, we examined whether a stepwise approach to training this problem would facilitate learning and at what stage in training subjects adopt a configurai strategy. We found that college-aged adults had little difficulty solving the transverse patterning problem, and that their learning was greatly facilitated by using the stepwise approach described by Alvarado and Rudy (1989). Moreover, we found that subjects seem to adopt a configural strategy when faced with ambiguous stimulus pairs even when an elemental strategy would suffice. These results provide insight into how humans solve configurai problems and also suggest some direct tests of the role of the hippocampus in configural associations.

A connectionist model of path integration with and without a representation of distance to the starting point

Abstract: Path integration (dead reckoning) is the process by which animals use self-generated movement information to continuously update a representation of their position relative to some starting point. This allows them to return to this point, even in the absence of exteroceptive positional information. Path integration is a bicoordinate process: Both distance and direction to (or from) the starting point have to be maintained in the representation. Recent theoretical work based on neurobiological observations seems to lead to the conclusion that, in rodents at least, path integration is map based, implying a geocentered (earthbound) coordinate system. The present paper discusses, as a word of caution, the opposite notion that path integration in an impoverished, single-coordinate egocentered (animal-bound) polar coordinate system could account for some rather puzzling behavioral data previously recorded on hamsters. This admittedly strange position hinges on observations made on connectionist models of path integration that were developed to simulate the homing behavior of hamsters.

The contribution of the associative parietal cortex and hippocampus to spatial processing in rodents

Abstract: A growing volume of data supports the notion that the associative parietal cortex (APC) in rodents plays an important role in the processing of spatial information. The present paper reviews the literature and available data, emphasizing the respective contribution of the APC and the hippocampus to spatial processing. It is proposed that the APC has a function of interfacing between the egocentric and allocentric coding of space. According to this view, the APC would associate visuospatial and internal movement-related information and thus would mediate the integration of multiple egocentric frames of reference into allocentric spatial representations. The allocentric coding of space would be completed and implemented in the hippocampal formation.

The effects of parietal cortex lesions on an object/spatial location paired-associate task in rats

Abstract: The present experiment was conducted in order to test the hypotheses (1) that the posterior parietal cortex (PPC) serves as a neural system that is critical for binding spatial location and object information in long-term memory and (2) that even restricted lesions of the PPC would result in similar deficits. Long-Evans rats were given either a large or a small PPC lesion or a control surgery under Nembutal anesthesia. After a 1-week recovery period, the rats were tested on either an object or a spatial location go/no-go successive discrimination task. After reaching criterion (a minimum of a 5 sec difference between reward and nonreward trials), they were trained on the other discrimination. After reaching criterion on the second discrimination, all of the rats were trained on a successive discrimination go/no-go task in which they had to remember which object/spatial location pairs had been associated with reward. As compared with controls, neither the small nor the large PPC lesion impaired object or spatial location discrimination. In the paired-associate object/spatial location task, both large and small PPC lesioned rats were impaired, relative to controls. These data suggest that the rodent PPC is not involved in object or spatial location discrimination but rather is involved in discrimination and long-term memory for the combination of object and spatial location information.

Human visual object recognition: What have we learned from neuroimaging?

Abstract: We review the neuroimaging literature for studies whose designs permit the identification of regions specialized for visual recognition. The results of these 17 studies are combined and analyzed with the goal of answering the following questions: (1) How well localized are areas involved in visual recognition across subjects? (2) Are there cortical areas that are specialized for the perception of different categories of stimuli (e.g., faces, words, and general objects)? The concept of specialization is defined and examined, as are the inferential limitations of neuroimaging methodology. Local maxima across studies were poorly colocalized within posterior inferior cortex, and there was no consistent segregation of activation sites dependent on the category of stimulus used. We discuss several possible reasons why the results of this review do not agree with the predictions of lesion and neurophysiology studies.

Effects of genetic selection for fearfulness or social reinstatement behavior on adult social and sexual behavior in domestic quail (Coturnix japonica)

Abstract: Male quail (Coturnix japonica) were genetically selected for high or low levels of social reinstatement behavior (weighted for independence from inherent fearfulness) and high or low fearfulness (weighted for independence from social reinstatement behavior). Genetic selection was based on tests conducted during the first 10 days after hatching. Inherent fearfulness was inferred from a test of the tonic immobility response, and social reinstatement behavior was measured in a treadmill test. When tested in adulthood, males selected for high levels of social reinstatement behavior showed more social proximity behavior, shorter latencies to copulate with a female, larger numbers of copulatory responses, and greater copulatory efficiency than did males selected for low inherent fearfulness. The other genetic lines performed at intermediate levels. Thus, the effects of genetic selection extended into adulthood and influenced responses that were not in the behavioral repertoire of the birds at the time of selection. Interestingly, selection for inherent fearfulness did not necessarily disrupt sexual performance. In fact, fearfulness appeared, at least to some extent, to be conducive to the expression of male sexual behavior.

Acquisition and retention of visual discrimination learning after ablation of perirhinal cortex in the rat

Abstract: Eight Dark Agouti rats were trained in a version of concurrent discrimination learning in a computer-controlled automated apparatus that allowed the use of complex abstract patterns as stimuli. Pre-operatively, they learned a five-pair concurrent discrimination and were tested for their postoperative retention of this discrimination. Rats with ablation of the perirhinal cortex were impaired in their retention of the concurrent discriminations. However, they were totally unimpaired in postoperative acquisition of two new concurrent discrimination sets. Moreover, they were also completely unimpaired in postoperative retention of these postoperatively learned discrimination sets. The deficit therefore appears to be a specific retrograde amnesia, with no evidence for anterograde effects.

Updating the path integrator through a visual fix

Abstract: Mammals can navigate through path integration (dead reckoning) by updating their position on the basis of internal signals generated during locomotion, without using any external references. However, being open to cumulative errors, path integration remains functional over short excursions only, unless corrected by familiar landmarks. That such a corrective process may occur was examined in golden hamsters during hoarding excursions occurring in darkness, within a large open arena. The subjects proceeded from their peripheral nest to a feeding site on a platform. If the animals were rotated during food pouching, their subsequent homing behavior was disoriented, self-generated positional information having been disrupted. By contrast, when the subjects were rotated at the food source and then briefly presented with the familiar visual environment, they returned homeward, albeit not very precisely. Thus, the animals may have taken an approximate positional fix, or reset their internal compass only.

Absence of recovery or dendritic reorganization after neonatal posterior parietal lesions

Abstract: Rats with bilateral removal of the posterior parietal cortex on Postnatal Day 5 or 10 were compared behaviorally and neuroanatomically to littermate control rats. The lesioned rats were impaired at the Morris water task but performed the Whishaw reaching task as well as control rats. Although the lesioned rats learned to find the hidden platform nearly as quickly as the control animals, their swim paths were less accurate, reflecting a chronic deficit in orienting the body through space. In contrast to the effects of lesions in the frontal or occipital cortex, posterior parietal lesions were not associated with compensatory dendritic growth in pyramidal cells in adjacent parietal cortex, a result that is consistent with an absence of functional recovery. Also in contrast to the effects of frontal lesions, there was no sex difference in the effects of posterior parietal lesions on either behavior or dendritic arborization in nearby parietal cortex. These results imply that the posterior parietal cortex is fundamentally different in its response to neonatal injury from other neocortical areas such as frontal, motor, or occipital cortex.

Rats’ object-in-place encoding and the effect of fornix transection

Abstract: We studied the perception of simple computer-generated scenes by normal and fornix-transected Dark Agouti rats. In Experiment 1, the rats were rewarded for approaching trial-unique variable scenes differing from a constant scene that was the same across trials (constant-negative paradigm). The groups performed equivalently when scenes differed only in their objects or only in the occupied positions; however, when two scenes shared an object—place combination, the normal rats were more likely to see them as similar than were the fornix-transected rats. In Experiment 2, the rats learned to discriminate pairs of scenes. Again, there was no lesion effect when scenes differed by a single cue, object or position, but when the two scenes comprised the same objects interchanged in position, fornix-transected rats learned relatively easily. Fornix transection reduces rats’ sensitivity to object—place combinations within scenes, consistent with D. Gaffan’s account of scene memory as an animal analogue of episodic memory deficits in amnesia.

Contributions of posterior parietal cortex to cognitive functions in primates

Abstract: The posterior parietal cortex plays an important role in a variety of cognitive functions in the primate brain. Parietal lesions result in profound defects in the perception and memory of spatial relationships, in the guidance of reaching motions and of the grasping of nearby objects, and in the control of eye movements to visual targets. The ability to direct attention to visual stimuli is also severely impaired after parietal damage. Anatomical and physiological studies in the monkey cortex have identified subdivisions with specialized properties and neural circuits that are capable of Unking visual information to areas involved in motor control. Neurophysiological experiments with animal behavior are now uncovering the neuronal mechanisms that transform information from visual to motor coordinates, and providing important insight into the mechanisms that direct attention to salient stimuli.

Parietal cortex and a spatial cognitive map

Abstract: A framework is proposed, based on the assumption that the posterior parietal cortex stores long-term memory information in the form of a spatial cognitive map, reflecting the operation of perceptual representations of relations among spatial features and landmarks as well as the operation of attentional processes to bind spatial features and landmarks and to selectively attend to the critical features that form spatial cognitive maps. Evidence in support of this assumption, based on an analysis of specific properties of neural systems that store long-term memory information, is presented.

Visual object representation: An introduction

Abstract: What are the computational, behavioral, and neural mechanisms that give rise to object perception? In this review, I present a cognitive neuroscience overview of the literature on object representation. Marr’s (1982) framework for studying complex tasks is used as a guide for the review. This framework involves analyzing a problem on three levels: (1) the computational theory, which asks what is computed and how; (2) the representation and algorithm, which focus on the representations and processes that underlie a computation; and (3) the hardware implementation, which deals with the implementation of the representations and processes. Computational considerations of object recognition raise the importance of the object invariances, which allow viewers to perceive an object as remaining stable despite changes in the retinal image. I then use the invariances to guide my review of the representations and processes involved in human object recognition, Marr’s second level, and of the hardware implementation, Marr’s third level. Throughout the review, my focus is on integrating across disciplines and across the levels of Marr’s framework.

Effects of time of night and sleep stage on perception of sleep in subjects with sleep state misperception

Abstract: Although previous studies have documented that some persons who complain of poor sleep report having been awake when questioned following an awakening from Stage 2 sleep induced by an auditory stimulus, such observations have been confined to the first NREM-REM cycle. We have examined the possibility that the perception of having been awake or asleep may differ across the night. Eight subjects with sleep state misperception were awakened by a 500-Hz tone of progressive amplitude at five time points and were then asked to report whether they believed themselves to have been awake or asleep. This procedure was carried out on an early night during the first sleep cycle, and on a late night during the third sleep cycle, in random sequence. During the early Stage 2 trial, subjects were more likely to report having been awake, whereas in the late Stage 2 trial, they were more likely to report that they had been asleep. Conversely, in the REM sleep trial on the early night, subjects were more likely to report that they had been asleep. On the late night they were more likely to report that they had been awake, a change which was paralleled by a significant drop in scores on the Foulkes Dream Complexity Scale. Calculation of the duration of uninterrupted sleep prior to the trials suggests that this may have been a factor in the Stage 2, but not the REM, findings. There were no significant differences between the early and late REM trials in the frequency of dream reports, quality of dreams, or fragmentation of REM. These data indicate that subjects with sleep state misperception may experience REM sleep very differently in the first and third NREM-REM cycles.

Turn-signal utilization by rats with either unilateral or bilateral posterior parietal cortex injuries

Abstract: Rats with sham, unilateral, or bilateral aspiration lesions of the posterior parietal cortex (PPC) were trained in a water T-maze to use flashing lights located along the starting alley (two each) and inner walls of the goal alleys (two each) to find a hidden escape platform. Thereafter, the performance of the spatial response was tested under several conditions. Rats with bilateral PPC lesions were significantly inferior to unilateral- and sham-injured rats in learning the “turn-signal” cued spatial task. Also, left-PPC-injured rats committed significantly more errors than did the control animals. After mastering the task, cue saliency was reduced and the amount of spatial discontiguity between the stimuli and escape site was increased in two stages. That is, the flashing light closest to the escape platform was turned off for one testing session. On the following day, animals were required to rely on information located within the starting alley to make the correct spatial response at the choice point. Animals with bilateral or left PPC lesions were significantly impaired on the task with cues located only in the starting alley. The animals were then tested with competing constant illumination of the lights on the side of the apparatus opposite the flashing-light cues. The performance of all animals dropped to chance and failed to improve with training. Finally, three of the sham-operated controls were retrained to criterion on the original discrimination and prepared with bilateral PPC injuries. Substantial savings was observed. The results reveal a greater role of the left PPC than the right in the use of local sensory cues for spatial navigation, and they show that the PPC is not the repository of the “engram” for this learned visuospatial behavior.

Effects of delay and duration of light deprivation on recovery of function from neglect induced by unilateral medial agranular prefrontal cortex lesions in rats

Abstract: Forty-eight hours of postoperation light deprivation (LD) has been found to produce complete and permanent behavioral recovery of function from neglect induced by unilateral lesions of medial agranular cortex (AGm) in rats. The two experiments in the present study parametrically examined the postoperation delay prior to LD and the duration of LD necessary to produce recovery from severe neglect. Subjects in both experiments received unilateral AGm lesions and were tested for the degree of neglect of visual, tactile, and auditory stimuli prior to and after experiencing LD. The results of the study of delay of LD indicated that LD administered 4 h postsurgery produced dramatic immediate recovery from severe neglect, and the recovery lasted for the duration of behavioral testing (3 weeks) following LD. The longer delay groups (52 and 100 h) and the no-manipulation controls did not demonstrate behavioral recovery. The 28-h delay group demonstrated an intermediate effect following LD. The results of the study of duration of LD indicated that the therapeutic effect of LD was duration dependent. Forty-eight hours of LD produced a significant reduction in the severity of neglect, but shorter durations (4 and 24 h) did not. The results of the present study indicate that a critical postoperative period exists in which LD must be initiated in order to produce recovery from severe neglect and that LD has to be administered for a period of at least 24 h to produce any evidence of recovery of function. These findings may have clinical implications for the treatment of neglect in humans.

Auditory conditional discrimination deficits without delays in rats with lesions of either frontal cortex or medial thalamus

Abstract: An auditory match-to-position (AMTP) task was developed to compare the effects of medial thalamic (L-IML) and prefrontal cortical (MW) lesions on the ability of rats to perform conditional discriminations with and without memory delays. Both lesions affected AMTP when there was no delay, and these impairments were not exacerbated when memory delays were imposed. The MW group performed normally, whereas the L-IML group was impaired in learning a go/no-go discrimination based on the same auditory stimuli as those used for AMTP. These findings are consistent with other evidence that the medial prefrontal cortex is critical for conditional associative learning when the same response alternatives are presented repeatedly across trials. The impairments observed for AMTP when there was no memory delay suggest that the effects of these lesions on the rate of forgetting may have been masked by the effects of these lesions on nonmnemonic aspects of the AMTP task.

Environmental control of morphine withdrawal : Context specificity or stimulus novelty ?

Abstract: Past investigations of context-specific morphine withdrawal have demonstrated that when rats are tested while undrugged, withdrawal behaviors are more pronounced in an environment previously paired with drug administration than in an environment previously paired with saline administration (even in rats that have had exposure to only low doses of morphine prior to testing). In these studies, rearing is a commonly used index of morphine withdrawal. Rearing is also an exploratory behavior. The higher levels of rearing displayed in a morphine-paired, compared with a saline-paired, environment may be a manifestation of a tendency to explore this environment, rather than a tendency to display withdrawal symptoms in this environment. We evaluated the extent to which rearing and other measures of morphine withdrawal are displayed in drug-paired, saline-paired, and novel environments. The results suggest that, in some cases, apparent context specificity of morphine withdrawal actually results from novelty-elicited exploration.

Effects of controllability of stress on hippocampal pharmacology

Abstract: To examine the effect of the controllability of stress on aspects of hippocampal noradrenergic, opiate, and cholinergic pharmacology, we trained rats in a controllability paradigm and assessed [3H]desmethylimipramine, [3H]quinuclidinyl benzilate, or [3H]naloxone binding in hippocampal areas CA1, CA3, and the dentate gyrus with quantitative autoradiography. Rats that could control shock termination were yoked to rats that could not control termination of equivalent shock; a third group of rats received no shock. When the rats that could terminate shock responded at an 85% rate, their brains were removed and sectioned, incubated with tritiated ligands, and exposed to film for an appropriate period. Quantitative densitometry revealed a 10%–22% decrease in naloxone binding in CA3 in rats receiving uncontrollable shock; no significant changes were observed in rats that could control shock. Thus, the ability to control shock prevented stress-induced changes in µ opiate receptor binding in area CA3 of the hippocampus.

Strain differences in latent inhibition

Abstract: Latent inhibition (LI)—that is, retarded conditioning to a stimulus following its nonreinforced pre-exposure—was tested in inbred Buffalo Dark-Agouti (DA) rats. Unlike albino outbred Wistar rats, which showed LI to both tone and flashing light in a conditioned emotional response (CER) procedure, DA rats showed LI to light but not to tone. The same pattern was obtained in DA rats in a two-way active-avoidance procedure. In addition, DA rats failed to show LI to white noise. LI to tone in CER was not restored by increasing the number of preexposures or by the administration of haloperidol. This is a first demonstration of strain differences in LI acquisition. Since LI disruption characterizes specific neuropsychiatric disorders, the elucidation of the mechanisms underlying the failure to develop LI, when this failure is not caused by external manipulations (e.g., by a drug or a lesion), may be important for understanding the neural substrates of this deficit.