Effects of unilateral removal of basal forebrain cholinergic neurons on cued target detection in rats

Speed and Oscillations: Medial Septum Integration of Attention and Navigation

Several cortical and diencephalic limbic brain regions incorporate neurons that fire in correlation with the speed of whole-body motion, also known as linear velocity. Besides the field mapping and head-directional information, the linear velocity is among the major signals that guide animal’s spatial navigation. Large neuronal populations in the same limbic regions oscillate with theta rhythm during spatial navigation or attention episodes; and the frequency of theta also correlates with linear velocity. A functional similarity between these brain areas is that their inactivation impairs the ability to form new spatial memories; whereas an anatomical similarity is that they all receive projections from medial septum-diagonal band of Broca complex. We review recent findings supporting the model that septal theta rhythm integrates different sensorimotor signals necessary for spatial navigation. The medial septal is described here as a circuitry that mediates experience-dependent balance of sustained attention and path integration during navigation. We discuss the hypothesis that theta rhythm serves as a key mechanism for the aligning of intrinsic spatial representation to: (1) rapid change of position in the spatial environment; (2) continuous alteration of sensory signals throughout navigation; and (3) adapting levels of attentional behavior. The synchronization of these spatial, somatosensory and neuromodulatory signals is proposed here to be anatomically and physiologically mediated by the medial septum.

Resting state functional connectivity of the basal nucleus of Meynert in humans: in comparison to the ventral striatum and the effects of age

The basal nucleus of Meynert (BNM) provides the primary cholinergic inputs to the cerebral cortex. Loss of neurons in the BNM is linked to cognitive deficits in Alzheimer's disease and other degenerative conditions. Numerous animal studies described cholinergic and non-cholinergic neuronal responses in the BNM; however, work in humans has been hampered by the difficulty of defining the BNM anatomically. Here, on the basis of a previous study that delineated the BNM of post-mortem human brains in a standard stereotaxic space, we sought to examine functional connectivity of the BNM, as compared to the nucleus accumbens (or ventral striatum, VS), in a large resting state functional magnetic resonance imaging data set. The BNM and VS shared but also showed a distinct pattern of cortical and subcortical connectivity. Compared to the VS, the BNM showed stronger positive connectivity with the putamen, pallidum, thalamus, amygdala and midbrain, as well as the anterior cingulate cortex, supplementary motor area and pre-supplementary motor area, a network of brain regions that respond to salient stimuli and orchestrate motor behavior. In contrast, compared to the BNM, the VS showed stronger positive connectivity with the ventral caudate and medial orbitofrontal cortex, areas implicated in reward processing and motivated behavior. Furthermore, the BNM and VS each showed extensive negative connectivity with visual and lateral prefrontal cortices. Together, the distinct cerebral functional connectivities support the role of the BNM in arousal, saliency responses and cognitive motor control and the VS in reward related behavior. Considering the importance of BNM in age-related cognitive decline, we explored the effects of age on BNM and VS connectivities. BNM connectivity to the visual and somatomotor cortices decreases while connectivity to subcortical structures including the midbrain, thalamus, and pallidum increases with age. These findings of age-related changes of cerebral functional connectivity of the BNM may facilitate research of the neural bases of cognitive decline in health and illness.

Posterior parietal cortex dynamically ranks topographic signals via cholinergic influence

The hypothesis to be discussed in this review is that posterior parietal cortex (PPC) is directly involved in selecting relevant stimuli and filtering irrelevant distractors. The PPC receives input from several sensory modalities and integrates them in part to direct the allocation of resources to optimize gains. In conjunction with prefrontal cortex, nucleus accumbens, and basal forebrain cholinergic nuclei, it comprises a network mediating sustained attentional performance. Numerous anatomical, neurophysiological, and lesion studies have substantiated the notion that the basic functions of the PPC are conserved from rodents to humans. One such function is the detection and selection of relevant stimuli necessary for making optimal choices or responses. The issues to be addressed here are how behaviorally relevant targets recruit oscillatory potentials and spiking activity of posterior parietal neurons compared to similar yet irrelevant stimuli. Further, the influence of cortical cholinergic input to PPC in learning and decision-making is also discussed. I propose that these neurophysiological correlates of attention are transmitted to frontal cortical areas contributing to the top-down selection of stimuli in a timely manner.

Spatial and sustained attention in relation to smoking status: behavioural performance and brain activation patterns

Nicotine enhances attentional functions. Since chronic nicotine exposure through smoking induces neuroadaptive changes in the brain at a structural and molecular level, the present functional MRI (fMRI) study aimed at investigating the neural mechanisms underlying visuospatial and sustained attention in smokers and non-smokers. Visuospatial attention was assessed with a location-cueing paradigm, while sustained attention was measured by changes in response speed over time. During invalid trials, neural activity within the basal forebrain was selectively enhanced in smokers and higher basal forebrain activity was associated with increased parietal cortex activation. Moreover, higher levels of expired carbon monoxide in smokers before scanning were associated with higher parietal cortex activation and faster responses to invalidly cued targets. Smokers showed a slowing of responses and additionally recruited an area within the right supramarginal gyrus with increasing time on task. Activity decreases over time were observed in visual areas in smokers. The data provide evidence for altered attentional functions in smokers as compared with non-smokers, which were partly modulated by residual nicotine levels and were observed at a behavioural level for sustained and at a neural level for spatial and sustained attention.

Cholinergic modulation of cognition: insights from human pharmacological functional neuroimaging

Evidence from lesion and cortical-slice studies implicate the neocortical cholinergic system in the modulation of sensory, attentional and memory processing. In this review we consider findings from sixty-three healthy human cholinergic functional neuroimaging studies that probe interactions of cholinergic drugs with brain activation profiles, and relate these to contemporary neurobiological models. Consistent patterns that emerge are: (1) the direction of cholinergic modulation of sensory cortex activations depends upon top-down influences; (2) cholinergic hyperstimulation reduces top-down selective modulation of sensory cortices; (3) cholinergic hyperstimulation interacts with task-specific frontoparietal activations according to one of several patterns, including: suppression of parietal-mediated reorienting; decreasing ‘effort’-associated activations in prefrontal regions; and deactivation of a ‘resting-state network’ in medial cortex, with reciprocal recruitment of dorsolateral frontoparietal regions during performance-challenging conditions; (4) encoding-related activations in both neocortical and hippocampal regions are disrupted by cholinergic blockade, or enhanced with cholinergic stimulation, while the opposite profile is observed during retrieval; (5) many examples exist of an ‘inverted-U shaped’ pattern of cholinergic influences by which the direction of functional neural activation (and performance) depends upon both task (e.g. relative difficulty) and subject (e.g. age) factors. Overall, human cholinergic functional neuroimaging studies both corroborate and extend physiological accounts of cholinergic function arising from other experimental contexts, while providing mechanistic insights into cholinergic-acting drugs and their potential clinical applications.

Effects of right-hemisphere cortical infarction and muscarinic acetylcholine receptor blockade on spatial visual attention performance in rats

The syndrome of hemispatial neglect is defined as an inability to report, respond or orient to stimuli contralateral to a cerebral lesion despite intact elementary sensory or motor function. This syndrome is typically observed after lesions of the right cerebral cortex, and has been associated with impairment of attention. We studied whether visual attention performance is impaired after right-hemisphere infarction in rats. Using a behavioural paradigm measuring spatial visual attention, we tested the effects of photothrombotic infarction to either the frontal cortex or the parietal cortex on attention performance. Since the cholinergic system is known to modulate attention performance, we additionally evaluated the role of cholinergic receptor blockade with scopolamine in our task paradigm. Our results show a transient response bias immediately after cortical infarction, with a decrease in contralesional responses and an increase in contralesional omissions after frontal infarction. Parietal infarction and systemic administration of scopolamine also resulted in a decrease in correct responses and an increase in omissions, but without a difference in side responding. In conclusion, right frontal infarction induces a transient impairment in contralesional spatial visual attention that we explain as left-sided neglect. Right parietal infarction and cholinergic blockade shows non-lateralized deficits in spatial visual attention, suggestive of global attentional impairment. We postulate that both effects of cortical infarction on attention performance may be related to cholinergic dysfunction. Our study confirms the role of frontal and parietal cortices in attention performance in rats, and corroborates the theory that attention performance is impaired in hemispatial neglect in human stroke patients.

Performance of galanin transgenic mice in the 5-choice serial reaction time attentional task

The neuropeptide galanin impairs learning and memory in rodents. The mechanism underlying the cognitive effects of galanin may be related to inhibitory effects of galanin on cholinergic transmission. As cholinergic function is thought to modulate sustained attention, the present study examined whether galanin-overexpressing transgenic mice have impairments in sustained attention. Galanin transgenic (GAL-tg) mice and wild-type (WT) littermate controls were trained in a 5-choice serial reaction time task, modified to assess sustained attention. GAL-tg and WT mice performed similarly during acquisition with respect to accuracy, total omissions, and response speed. Attentional mechanisms were challenged by parametric changes including increased event rate, event asynchrony, or decreased stimulus duration. Singly, these challenges did not differentially affect performance between genotypes. Concurrent administration of these challenges, which represents an optimal test of sustained attention, also had similar effects on GAL-tg and WT mice. When stimulus discriminability was reduced by constant illumination of the house light, GAL-tg mice omitted more trials than WT mice, but other measures of performance did not differ by genotype. Moreover, intraventricular injection of galanin in WT mice did not affect sustained attention. These data indicate that previously reported learning and memory effects of galanin are not secondary to attentional dysfunction.

Microsphere embolism-induced cortical cholinergic deafferentation and impairments in attentional performance

Ischemic events have been hypothesized to play a critical role on the pathogenesis of dementia and the acceleration of cognitive impairments. This experiment was designed to determine the consequences of microvascular ischemia on the cortical cholinergic input system and associated attention capacities. Injections of microspheres ( approximately 50 microm diameter; approximately 5000 microspheres/100 microL) into the right common carotid artery of rats served as a model of microvascular ischemia and resulted in decreases in the density of cholinergic fibers in the ipsilateral medial prefrontal cortex and frontoparietal areas. Furthermore, dense astrogliosis, indicated by glial fibrillary acidic protein (GFAP) immunohistochemistry, was observed in the globus pallidus, including the areas of origin of cholinergic projections to the cortex. Fluoro-Jade B staining indicated that loss of neurons in the cortex was restricted to areas of microsphere-induced infarcts. Attentional performance was assessed using an operant sustained attention task; performance in this task was previously demonstrated to reflect the integrity and activity of the cortical cholinergic input system. Embolized animals' performance was characterized by a decrease in the animals' ability to detect signals. Their performance in non-signal trials remained unaffected. The residual density of cholinergic axons in prefrontal and frontoparietal areas correlated with the animals' performance. The present data support the hypothesis that microvascular ischemia results in loss of cortical cholinergic inputs and impairs associated attentional performance. Microsphere embolism represents a useful animal model for studying the role of interactions between microvascular disorder and impaired forebrain cholinergic neurotransmission in the manifestation of cognitive impairments.

Unraveling the attentional functions of cortical cholinergic inputs: interactions between signal-driven and cognitive modulation of signal detection

Neurophysiological studies demonstrated that increases in cholinergic transmission in sensory areas enhance the cortical processing of thalamic inputs. Cholinergic activity also suppresses the retrieval of internal associations, thereby further promoting sensory input processing. Behavioral studies documented the role of cortical cholinergic inputs in attentional functions and capacities by demonstrating, for example, that the integrity of the cortical cholinergic input system is necessary for attentional performance, and that the activity of cortical cholinergic inputs is selectively enhanced during attentional performance. This review aims at integrating the neurophysiological and behavioral evidence on the functions of cortical cholinergic inputs and hypothesizes that the cortical cholinergic input system generally acts to optimize the processing of signals in attention-demanding contexts. Such signals 'recruit', via activation of basal forebrain corticopetal cholinergic projections, the cortical attention systems and thereby amplify the processing of attention-demanding signals (termed 'signal-driven cholinergic modulation of detection'). The activity of corticopetal cholinergic projections is also modulated by direct prefrontal projections to the basal forebrain and, indirectly, to cholinergic terminals elsewhere in the cortex; thus, cortical cholinergic inputs are also involved in the mediation of top-down effects, such as the knowledge-based augmentation of detection (see Footnote 1) of signals and the filtering of irrelevant information (termed 'cognitive cholinergic modulation of detection'). Thus, depending on the quality of signals and task characteristics, cortical cholinergic activity reflects the combined effects of signal-driven and cognitive modulation of detection. This hypothesis begins to explain signal intensity or duration-dependent performance in attention tasks, the distinct effects of cortex-wide versus prefrontal cholinergic deafferentation on attention performance, and it generates specific predictions concerning cortical acetylcholine (ACh) release in attention task-performing animals. Finally, the consequences of abnormalities in the regulation of cortical cholinergic inputs for the manifestation of the symptoms of major neuropsychiatric disorders are conceptualized in terms of dysregulation in the signal-driven and cognitive cholinergic modulation of detection processes.

Norepinephrine and acetylcholine mediation of the components of reflexive attention: implications for attention deficit disorders

Attention deficit hyperactivity disorders (ADHD) create major learning barriers for children and significant social and legal handicaps for adults worldwide. Important advances in the genetic basis of the disease have been made, but reliable, biological, diagnostic markers remain elusive. This review takes the position that future progress in treating the core symptom of attention deficits requires a clearer understanding of the neuroscience of attention in normal individuals. Two important achievements in this direction have been the development of tasks that identify activity in the orienting, alerting and conflict networks, and the identification of neurotransmitters that mediate these components. The proven ability of these tasks to identify and characterize response components of "normal" attention argues that they could be used advantageously with patient populations. The categorization of neurotransmitter abnormalities in those with ADHD could clarify whether attention deficits occur within or across attention networks. To realize these goals, we evaluate laboratory studies of attention in humans and animals that address the underlying neurotransmitter systems, primarily norepinephrine and acetylcholine. We propose that key facts about deficits in reflexive and voluntary attention may be understood by a model that includes deficits in brain norepinephrine release and its effects on cholinergic activity in the parietal cortex.

Lateralized Attentional Functions of Cortical Cholinergic Inputs

The integrity of the cortical cholinergic input system is necessary for attention performance. This experiment tested hypotheses concerning the lateralized contributions of cortical cholinergic inputs to attention performance by assessing the effects of unilateral lesions of basal forebrain cholinergic neurons on sustained attention performance. Loss of right-hemispheric cortical cholinergic inputs impaired the rats' ability to detect signals but did not affect nonsignal trial performance. Conversely, loss of left-hemispheric cortical cholinergic inputs increased the number of false alarms in nonsignal trials. These data correspond with hypotheses about the mediation of detection processes primarily by right-hemispheric circuits and executive aspects of attention performance by left-hemispheric systems. Cortical cholinergic inputs represent a major component of the brain's lateralized attention systems.

Parafascicular electrical stimulation attenuates nucleus basalis magnocellularis lesion-induced active avoidance retention deficit

Previous experiments from our laboratory showed that retention of two-way active avoidance learning is improved by post-training intracranial electrical stimulation (ICS) of the parafascicular nucleus (PF) and impaired by pre-training electrolytic lesions of the nucleus basalis magnocellularis (NBM). The question investigated here was whether post-training PF ICS is able to attenuate the active avoidance retention deficit observed in rats lesioned pre-training in the NBM. To this goal, the following experimental design was used: rats bilaterally lesioned in the NBM and stimulated in the PF, rats lesioned in the NBM, rats stimulated in the PF, control rats implanted in the PF, and sham-operated rats were first trained in a shuttle-box for a single 30-trial session and tested again following two successive retention intervals (24 h and 11 days). The results showed that: (1) NBM lesions impaired the 11-day performance without affecting either the acquisition or the 24-h retention of the avoidance learning; (2) PF ICS treatment in unlesioned rats improved performance in both retention sessions only when the stimulation was applied in the posterior region of the nucleus; and (3) stimulation of the posterior PF compensated the 11-day retention impairment induced by NBM lesions. These results are discussed in relation to the interaction of arousal systems in the modulation of cognitive processes.

192 IgG-saporin lesions to the nucleus basalis magnocellularis (nBM) disrupt acquisition of learning set formation

Rats with bilateral 192 IgG-saporin lesions to the nucleus basalis magnocellularis (nBM) were tested on olfactory discrimination learning set (ODLS), olfactory discrimination reversal learning set (DRLS), and open field activity. Control animals demonstrated learning set in both the ODLS and DRLS tasks. The nBM-lesioned animals showed initial acquisition impairment in learning set in the ODLS task but eventually demonstrated learning set in both ODLS and DRLS tasks. There were no group differences in open-field activity. Results suggest that removal of the nBM cholinergic system through 192 IgG-saporin lesions impairs early acquisition of learning set compared to control animals, but does not prevent later use of learning set formation. Implications for the non-cholinergic basal forebrain cells in learning set are discussed.

Two forms of spatial memory: a double dissociation between the parietal cortex and the hippocampus in the rat

Two variants of a continuous recognition training procedure were designed in order to query 2 forms of spatial memory. A continuous reinforcement condition (reflecting perceptual memory) and a differential reinforcement condition (reflecting episodic-like memory) were used to test rats on a 12-arm radial maze. After total hippocampal lesions, rats demonstrated intact performance on the continuous reinforcement condition, but impaired performance on the differential reinforcement condition. After parietal lesions, rats demonstrated the reverse pattern of performance: impaired performance on the continuous reinforcement condition and intact performance on the differential reinforcement condition. Thus, a double dissociation appears to exist between parietal cortex and hippocampus for the continuous reinforcement condition (reflecting perceptual memory) versus the differential reinforcement condition (reflecting episodic memory) for spatial location information.

The cognitive neuroscience of sustained attention: where top-down meets bottom-up

The psychological construct 'sustained attention' describes a fundamental component of attention characterized by the subject's readiness to detect rarely and unpredictably occurring signals over prolonged periods of time. Human imaging studies have demonstrated that activation of frontal and parietal cortical areas, mostly in the right hemisphere, are associated with sustained attention performance. Animal neuroscientific research has focused on cortical afferent systems, particularly on the cholinergic inputs originating in the basal forebrain, as crucial components of the neuronal network mediating sustained attentional performance. Sustained attention performance-associated activation of the basal forebrain corticopetal cholinergic system is conceptualized as a component of the 'top-down' processes initiated by activation of the 'anterior attention system' and designed to mediate knowledge-driven detection and selection of target stimuli. Activated cortical cholinergic inputs facilitate these processes, particularly under taxing attentional conditions, by enhancing cortical sensory and sensory-associational information processing, including the filtering of noise and distractors. Collectively, the findings from human and animal studies provide the basis for a relatively precise description of the neuronal circuits mediating sustained attention, and the dissociation between these circuits and those mediating the 'arousal' components of attention.

Behavioural, histological and immunocytochemical consequences following 192 IgG-saporin immunolesions of the basal forebrain cholinergic system

Use of the selective immunotoxin; 192 IgG-saporin, is helping to elucidate the role of the cholinergic system in cognition by overcoming the problems of interpretation associated with the use of non-specific lesioning agents. In separate studies, we have compared the long- and short-term effects of single site and combined saporin lesions of the nucleus basalis magnocellularis and medial septal area, on spatial learning and memory in radial arm and water maze tasks. At 11 months, only rats with combined lesions showed deficits in both radial and water maze tasks, although terminal cholinergic deafferentation was substantial and extensive tissue loss was seen at the injection sites in both single and combined lesions. However, the extensive tissue loss with long-term lesions suggested that behavioural deficits were not solely attributable to cholinergic deafferentation. In contrast, when rats with combined lesions were tested 5 months after lesioning, no deficits were apparent, although there was almost complete loss of choline acetyltransferase- and nerve growth factor receptor-immunoreactivity in the basal forebrain with no tissue damage at the injection sites. This study supports existing literature that selective loss of cholinergic neurons in the basal forebrain does not produce behavioural impairments in standard tasks of learning and memory, but deficits are apparent when damage is non-selective as occurs late after lesioning, confounding interpretation of behavioural data. It further highlights potential problems with this immunotoxin in long-term studies.

Attention as a target of intoxication: insights and methods from studies of drug abuse

A symposium was convened to discuss recent developments in the assessment of attention and the effects of drugs and toxic chemicals on attention at the 17th annual meeting of the Behavioral Toxicology Society on May 1, 1999, in Research Triangle Park, NC. Speakers addressed issues including the methodology of assessing cognitive function, the neurobiology of specific aspects of attention, the dual roles of attention as a target of intoxication and as a mediating variable in the development of addiction to psychoactive drugs, the changes in attention that accompany neuropsychological disorders of schizophrenia, senile dementia of the Alzheimer type and attention deficit hyperactivity disorder, and potential therapies for these disorders. This article provides an overview of the objectives of the symposium, followed by summaries of each of the talks given.

Behavioral and electrophysiological estimates of visual thresholds in awake rats treated with 3,3',4,4',5-pentachlorobiphenyl (PCB 126)

Visual thresholds for luminance increments were obtained behaviorally and electrophysiologically from rats exposed to a polychlorinated biphenyl (PCB) during development. Male Long-Evans rats exposed to 0, 0.25, or 1.0 microg/kg/day of 3,3',4,4', 5-pentachlorobiphenyl (PCB 126) through gestation and weaning were trained as adults to perform a signal detection task. Estimates of threshold were derived from psychometric functions for each animal relating the proportion of hits to signal intensity. Thresholds derived under three luminance conditions did not differ significantly among the PCB-treated groups. After behavioral testing was completed, flash-evoked potentials were recorded from dark-adapted awake animals. Peak amplitudes increased linearly over approximately 3 log units of intensity. Extrapolations to 0 amplitude along the linear portion of the amplitude-log intensity functions produced estimates of absolute threshold of -5.44 to -5.53 log cd/m(2)-s. Waveforms recorded from awake animals had a large late negative component that was absent in previously reported anesthetized preparations. Developmental exposure to PCB 126 had no significant effect on absolute threshold or peak amplitudes and latencies.

Threshold relationship between lesion extent of the cholinergic basal forebrain in the rat and working memory impairment in the radial maze

The cholinergic basal forebrain (CBF) degenerates in Alzheimer's Disease (AD), and the degree of this degeneration correlates with the degree of dementia. In the present study we have modeled this degeneration in the rat by injecting various doses of the highly selective immunotoxin 192 IgG-saporin (192-sap) into the ventricular system. The ability of 192-sap-treated rats to perform in a previously learned radial maze working memory task was then tested. We report here that 192-sap created lesions of the CBF and, to a lesser extent, cerebellar Purkinje cells in a dose-dependent fashion. Furthermore, we found that rats harboring lesions of the entire CBF greater than 75% had impaired spatial working memory in the radial maze. Correlational analysis of working memory impairment and lesion extent of the component parts of the CBF revealed that high-grade lesions of the hippocampal-projecting neurons of the CBF were not sufficient to impair working memory. Only rats with high-grade lesions of the hippocampal and cortical projecting neurons of the CBF had impaired working memory. These data are consistent with other 192-sap reports that found behavioral deficits only with high-grade CBF lesions and indicate that the relationship between CBF lesion extent and working memory impairment is a threshold relationship in which a high degree of neuronal loss can be tolerated without detectable consequences. Additionally, the data suggest that the CBF modulates spatial working memory via its connections to both the hippocampus and cortex.

Behavioral assessments of learning and attention in rats exposed perinatally to 3,3',4,4',5-pentachlorobiphenyl (PCB 126)

Evidence from humans suggests that cognitive dysfunction may result from perinatal exposure to polychlorinated biphenyls (PCBs), and the results of some animal research with PCBs have been interpreted in terms of possible impairment of attention. Long-Evans rats were fed 3,3',4,4',5-pentachlorobiphenyl (PCB 126), a coplanar congener, at doses of 0.25 or 1 microgram/kg/day [corrected] throughout gestation and nursing. Male offspring of these rats were trained as adults to perform 2 tests of attention for food reward. First, a cued target-detection task, modeled after Posner's covert orienting method for humans, was used to assess visuospatial attention. In this task, a visual target stimulus was presented in 1 visual hemifield on each trial, preceded either by a valid cue, an invalid cue, or no cue. A valid cue appeared in the same hemifield as the target, and an invalid cue appeared in the opposite hemifield. As expected, valid cues increased accuracy and speed of target detection and invalid cues decreased accuracy and speed; moreover, these effects were systematically related to changes in cue intensity and target duration. However, perinatal exposure to PCB 126 did not affect acquisition or performance of this task. The second task assessed sustained attention by means of a signal detection method in which a brief, spatially-constant but temporally unpredictable, visual signal indicated which of 2 responses would yield food. Varying the intensity of the signal greatly affected the probability of correctly reporting the signal. Perinatal exposure to PCB 126 did not affect acquisition of the response rule or performance of the task. Finally, all rats were challenged with chlordiazepoxide (CDP) at doses of 0, 3, 5, 8, or 12 mg/kg SC, 20 min before testing in the sustained attention task. In control rats, low doses (3, 5, and 8 mg/kg) of CDP reduced accuracy at low signal intensities only, suggesting an increase in visual threshold. The high dose of CDP reduced accuracy at all signal intensities and increased the false-alarm rate as well, suggesting an impairment of attention. The rats exposed perinatally to PCB 126 at 0.25 micrograms/kg [corrected] were unaffected by CDP, and those exposed to PCB 126 at 1 microgram/kg [corrected] showed a smaller decrement in performance after CDP than did the controls. Taken together, these data provide little support for the possibility that perinatal exposure to PCB 126 causes deficits in attention, but suggest that PCB 126 may alter GABA-mediated pathways in the CNS during development.

The behavioral functions of the cholinergic basal forebrain: lessons from 192 IgG-saporin

Until recently our understanding of the functional neuroanatomy of the cholinergic basal forebrain (CBF) has been hindered by the lack of a lesioning technique that is truly selective. The development of the immunotoxin 192 IgG-saporin (192-sap) has greatly improved our ability to create specific lesions of the CBF. Rats with such lesions have been studied in a wide variety of behavioral paradigms of learning, memory, and attention. Complete or near-complete destruction of the CBF results in deficits in a variety of behavior paradigms including passive avoidance, spatial tasks (water and radial mazes), delayed matching to position/sample, and attentional tasks. However, interpretation of many experiments is hampered by incomplete lesions and/or concomitant damage to cerebellar Purkinje neurons. Future studies will need to address these issues. Recent development of a similar immunotoxin that is effective in primates should permit more sophisticated behavioral analysis of CBF function. Additionally, immunotoxins selective for other types of neurons, such as the noradrenergic selective anti-DBH-saporin, will permit analysis of the behavioral functions of other diffusely projecting systems and how these other systems may interact with the CBF.