Behavioral Memory Performance Research Articles

Individual Differences in Older Adult Frontal Lobe Function Relate to Memory and Neural Activity for Self-Relevant and Emotional Content.

Older adults show memory benefits for self-relevant and emotional content, but there are individual differences in this effect. It has been debated whether processing of self-relevant and emotional information relies on similar processes to one another. We examined whether variation in frontal lobe (FL) function among older adults related similarly to the processing of self-relevant information as it did to emotional information, or whether these relations diverged. While undergoing fMRI, participants (ages 60-88) viewed positive, negative, and neutral objects, and imagined placing those objects in either their home or a stranger's home. Participants completed a surprise memory test outside of the MRI. In a separate session, a cognitive battery was collected and composite scores measuring FL and medial temporal lobe function were computed and related to the behavioral memory performance and the neural engagement during fMRI. Behaviorally, FL function related to memory for self-relevant, but not emotional content. Older adults with higher FL function demonstrated reduced self-bias in memory performance. During the processing of self-relevant stimuli, independent of emotion, levels of activity in the middle frontal gyrus showed positive associations with FL function. This relationship was not driven by compensatory activity or disruptions to nonself-relevant neutral content. These findings point to divergence in the cognitive functions relating to memory enhancements for self- and emotional-relevance. The results further suggest self-relevance as a mnemonic device for older adults, especially in those with lower FL function.

Structural and Functional MRI Data Differentially Predict Chronological Age and Behavioral Memory Performance.

Human cognitive abilities decline with increasing chronological age, with decreased explicit memory performance being most strongly affected. However, some older adults show "successful aging," that is, relatively preserved cognitive ability in old age. One explanation for this could be higher brain-structural integrity in these individuals. Alternatively, the brain might recruit existing resources more efficiently or employ compensatory cognitive strategies. Here, we approached this question by testing multiple candidate variables from structural and functional neuroimaging for their ability to predict chronological age and memory performance, respectively. Prediction was performed using support vector machine (SVM) classification and regression across and within two samples of young (N = 106) and older (N = 153) adults. The candidate variables were (1) behavioral response frequencies in an episodic memory test; (2) recently described functional magnetic resonance imaging (fMRI) scores reflecting preservation of functional memory networks; (3) whole-brain fMRI contrasts for novelty processing and subsequent memory; (4) resting-state fMRI maps quantifying voxel-wise signal fluctuation; and (5) gray matter volume estimated from structural MRIs. While age group could be reliably decoded from all variables, chronological age within young and older subjects was best predicted from gray matter volume. In contrast, memory performance was best predicted from task-based fMRI contrasts and particularly single-value fMRI scores, whereas gray matter volume has no predictive power with respect to memory performance in healthy adults. Our results suggest that superior memory performance in healthy older adults is better explained by efficient recruitment of memory networks rather than by preserved brain structure.

Seeing What I Did (Not): Cerebral and Behavioral Effects of Agency and Perspective on Episodic Memory Re-activation

Intuitively, we assume that we remember episodes better when we actively participated in them and were not mere observers. Independently of this, we can recall episodes from either the first-person perspective (1pp) or the third-person perspective (3pp). In this functional magnetic resonance imaging (fMRI) study, we tested whether agency and perspective modulate neural activity during memory retrieval and subsequently enhance memory performance. Subjects encoded a set of different episodes by either imitating or only observing videos that showed short toy stories. A week later, we conducted fMRI and cued episodic retrieval by presenting the original videos, or slightly modified versions thereof, from 1pp or from 3pp. The hippocampal formation was sensitive to self-performed vs. only observed actions only when there was an episodic mismatch. In a post-fMRI memory test a history of self-performance did not improve behavioral memory performance. However, modified videos were often (falsely) accepted as showing truly experienced episodes when: (i) they were already presented in this modified version during fMRI or (ii) they were presented in their original form during fMRI but from 3pp. While the overall effect of modification was strong, the effects of perspective and agency were more subtle. Together, our findings demonstrate that self-performance and self-perspective modulate the strength of a memory trace in different ways. Even when memory performance remains the same for different agentive states, the brain is capable of detecting mismatching information. Re-experiencing the latter impairs memory performance as well as retrieving encoded episodes from 3pp.

Neural correlates of episodic memory modulated by temporally delayed rewards.

When a stimulus is associated with an external reward, its chance of being consolidated into long-term memory is boosted via dopaminergic facilitation of long-term potentiation in the hippocampus. Given that higher temporal distance (TD) has been found to discount the subjective value of a reward, we hypothesized that memory performance associated with a more immediate reward will result in better memory performance. We tested this hypothesis by measuring both behavioral memory performance and brain activation using functional magnetic resonance imaging (fMRI) during memory encoding and retrieval tasks. Contrary to our hypothesis, both behavioral and fMRI results suggest that the TD of rewards might enhance the chance of the associated stimulus being remembered. The fMRI data demonstrate that the lateral prefrontal cortex, which shows encoding-related activation proportional to the TD, is reactivated when searching for regions that show activation proportional to the TD during retrieval. This is not surprising given that this region is not only activated to discriminate between future vs. immediate rewards, it is also a part of the retrieval-success network. These results provide support for the conclusion that the encoding-retrieval overlap provoked as the rewards are more delayed may lead to better memory performance of the items associated with the rewards.

Polyhedral Compilation for Racetrack Memories

Traditional memory hierarchy designs, primarily based on SRAM and DRAM, become increasingly unsuitable to meet the performance, energy, bandwidth, and area requirements of modern embedded and high-performance computer systems. Racetrack memory (RTM), an emerging nonvolatile memory technology, promises to meet these conflicting demands by offering simultaneously high speed, higher density, and nonvolatility. RTM provides these efficiency gains by not providing immediate access to all storage locations, but by instead storing data sequentially in the equivalent to nanoscale tapes called tracks . Before any data can be accessed, explicit shift operations must be issued that cost energy and increase access latency. The result is a fundamental change in memory performance behavior: the address distance between subsequent memory accesses now has a linear effect on memory performance. While there are first techniques to optimize programs for linear-latency memories, such as RTM, existing automatic solutions treat only scalar memory accesses. This work presents the first automatic compilation framework that optimizes static loop programs over arrays for linear-latency memories. We extend the polyhedral compilation framework Polly to generate code that maximizes accesses to the same or consecutive locations, thereby minimizing the number of shifts. Our experimental results show that the optimized code incurs up to 85% fewer shifts (average 41%), improving both performance and energy consumption by an average of 17.9% and 39.8%, respectively. Our results show that automatic techniques make it possible to effectively program linear-latency memory architectures such as RTM.

GJ-4 ameliorates memory impairment in focal cerebral ischemia/reperfusion of rats via inhibiting JAK2/STAT1-mediated neuroinflammation

Ethnopharmacological relevanceGardenia jasminoides J. Ellis (Fructus Gardenia) is a traditional Chinese medicine with diverse pharmacological functions, such as anti-inflammation, anti-depression, as well as improvement of cognition and ischemia brain injury. GJ-4 is a natural extract from Gardenia jasminoides J. Ellis (Fructus Gardenia) and has been proved to improve memory impairment in Alzheimer's disease (AD) mouse model in our previous studies. Aim of the studyThis study aimed to evaluate the therapeutic effects of GJ-4 on vascular dementia (VD) and explore the potential mechanisms. Material and methodsIn our experiment, a focal cerebral ischemia and reperfusion rat model was successfully developed by the middle cerebral artery occlusion and reperfusion (MCAO/R). GJ-4 (10 mg/kg, 25 mg/kg, 50 mg/kg) and nimodipine (10 mg/kg) were orally administered to rats once a day for consecutive 12 days. Learning and memory behavioral performance was assayed by step-down test and Morris water maze test. The neurological scoring test was performed to evaluate the neurological function of rats. 2,3,5-Triphenyltetrazolium chloride (TTC) staining and Nissl staining were respectively employed to determine the infarct condition and neuronal injury of the brain. Iba1 immunohistochemistry was used to show the activation of microglia. Moreover, the synaptic damage and inflammatory level were detected by Western blot. ResultsGJ-4 could significantly improve memory impairment, cerebral infraction, as well as neurological deficits of VD rats induced by MCAO/R. Further research indicated VD-induced neuronal injury was alleviated by GJ-4. In addition, GJ-4 could protect synapse of VD rats by upregulating synaptophysin (SYP) expression, post synaptic density 95 protein (PSD95) expression, and downregulating N-Methyl-D-Aspartate receptor 1 (NMDAR1) expression. Subsequent investigation of the underlying mechanisms identified that GJ-4 could suppress neuroinflammatory responses, supported by inhibited activation of microglia and reduced expression of inflammatory proteins, which ultimately exerted neuroprotective effects on VD. Further mechanistic study indicated that janus kinase 2 (JAK2)/signal transducer and activator of transcription 1 (STAT1) pathway was inhibited by GJ-4 treatment. ConclusionThese results suggested that GJ-4 might serve as a potential drug to improve VD. In addition, our study indicated that inhibition of neuroinflammation might be a promising target to treat VD.

Habituation during encoding: A new approach to the evaluation of memory deficits in schizophrenia

BackgroundMemory is significantly impaired in schizophrenia. However, memory measures are often complex and confounded by additional impairments such as motivation and task comprehension, which can affect behavioral performance and obscure neural function during memory tasks. Neural signatures of memory encoding that are robust to potential confounds may shed additional light on neural deficits contributing to memory impairment in schizophrenia. MethodsHere, we investigate a potential neural signature of memory—habituation—and its relationship with healthy and impaired memory function. To limit potential confounds, we used a passive depth of encoding memory task designed to elicit neural responses associated with memory encoding while limiting other cognitive demands. To determine whether habituation during encoding was predictive of intact memory processing, we first compared neural habituation over repeated encoding exposures with subsequent explicit memory in healthy individuals. We then tested whether a similar relationship existed in patients with schizophrenia. ResultsExplicit memory performance was impaired in patients with schizophrenia relative to healthy control subjects. In healthy participants, more habituation over repeated exposures during encoding was associated with greater repetition-related increases in accuracy during testing. However, in patients with schizophrenia, better performance was associated with less habituation, or a more sustained neural response during encoding. ConclusionsThese results suggest that sustained neural activity is required for normal repetition-related improvements in memory performance in schizophrenia, in line with a neural inefficiency model. Habituation may serve as a valuable index of neural processes that underlie behavioral memory performance.

Working memory performance impaired after exposure to acute social stress: The evidence comes from ERPs.

Using event-related potentials (ERPs), we examined the time course of 39 healthy novice drivers during a blocked working memory task (numerical N-back) under acute social stress or control conditions, which were induced by the Trier Social Stress Task (TSST) or control procedure. Subjective measures were used to assess stress manipulation throughout the experiment. An elevated negative effect in response to a stress condition indicated a successful stress induction. The behavioral results showed that the stress group had a longer response time and larger differences in accuracy than the control group. On a neural level, the control group had larger P3 amplitude in the 1-back condition than in the 2-back condition; this load effect, however, disappeared in the stress group. These results revealed that acute social stress had a disruptive effect on both working memory behavioral performance and cognitive neural process. These findings provide us with a basis to understand the correlation between acute stress and cognitive processes of working memory at a cognitive neural level.

Hippocampal maturity promotes memory distinctiveness in childhood and adolescence

Adaptive learning systems need to meet two complementary and partially conflicting goals: detecting regularities in the world versus remembering specific events. The hippocampus (HC) keeps a fine balance between computations that extract commonalities of incoming information (i.e., pattern completion) and computations that enable encoding of highly similar events into unique representations (i.e., pattern separation). Histological evidence from young rhesus monkeys suggests that HC development is characterized by the differential development of intrahippocampal subfields and associated networks. However, due to challenges in the in vivo investigation of such developmental organization, the ontogenetic timing of HC subfield maturation remains controversial. Delineating its course is important, as it directly influences the fine balance between pattern separation and pattern completion operations and, thus, developmental changes in learning and memory. Here, we relate in vivo, high-resolution structural magnetic resonance imaging data of HC subfields to behavioral memory performance in children aged 6-14 y and in young adults. We identify a multivariate profile of age-related differences in intrahippocampal structures and show that HC maturity as captured by this pattern is associated with age differences in the differential encoding of unique memory representations.

Study of absence of theta oscillation of working memory dysfunction in temporal lobe epilepsy

Objective Temporal lobe epilepsy (TLE), frequently accompanied by working memory dysfunction, is an important form of adult epilepsy syndrome. Study on neural mechanisms of TLE with working memory dysfunction has important scientific value and clinical significance. Theta oscillations are synchronous activity of the brain in the 4-8 Hz frequency range. It is well recognized that there is a close relationship between the theta oscillations and working memory behavior. TLE patients with working memory dysfunction were taken as the research subjects, and the theta oscillations absence in TLE with working memory behavioral disorder was carried out, which provide the reference for further research into the neural mechanisms of TLE with working memory dysfunction. Methods The 34-channel electroencephalographs (EEGs) were recorded from TLE group (18 TLE patients) and control groups (18 healthy subjects) while subjects were performing visual working memory (delayed matching-to-sample) tasks. The EEGs during the 3 s delay period was analyzed as experimental data. Fourier transform was used to assess the EEGs spectrum. The channel with the strongest spectrum was selected as the feature channel. Short-time Fourier transform algosiths was employed to calculate the time-frequency representation of the feature channel for the TLE and control groups. Frequency band with the strongest spectrum was selected for control group as the feature frequency band in working memory. Then, the topographical maps of the feature frequency band spectrum were calculated for the TLE and control groups. Results Compared with the control group, the working memory behavioral performance of the TLE group was lower: accuracy of the TLE group was significantly lower than that of the control group (P<0.05), and reaction time of the TLE group was significantly longer than that of the control group (P<0.001). The feature channel in working memory was frontal midline (Fz) and the feature frequency band was theta band. The Fz spectrum of the feature frequency for TLE was lower than that for control (P<0.05). The frontal spectrum (seven channels) of the feature frequency for TLE was lower than that of control (P<0.01). Conclusions Theta oscillations for TLE with working memory behavioral disorder is absent, which maybe one of the possible neural mechanism of TLE with working memory dysfunction. Key words: Temporal lobe epilepsy; Working memory dysfunction; Multi-channel electroencephalographs; Theta oscillations

Commentary: Episodic Memory Retrieval Functionally Relies on Very Rapid Reactivation of Sensory Information.

Commentary: Episodic Memory Retrieval Functionally Relies on Very Rapid Reactivation of Sensory Information.

Neuroprotection, learning and memory improvement of a standardized extract from Renshen Shouwu against neuronal injury and vascular dementia in rats with brain ischemia

Ethnopharmacological relevanceThe Renshen Shouwu capsule (RSSW) is a patented Traditional Chinese Medicine (TCM), that has been proven to improve memory and is widely used in China to apoplexy syndrome and memory deficits. To investigate the neuroprotective and therapeutic effect of the Renshen Shouwu standardized extract (RSSW) on ischemic brain neuronal injury and impairment of learning and memory related to Vascular Dementia (VD) induced by a focal and global cerebral ischemia-reperfusion injury in rats. Material and methodsUsing in vivo rat models of both focal ischemia/reperfusion (I/R) injuries induced by a middle cerebral artery occlusion (MCAO), and VD with transient global brain I/R neuronal injuries induced by a four-vessel occlusion (4-VO) in Sprague–Dawley (SD) rats, RSSW (50,100, and 200mgkg−1 body weights) and Egb761® (80mgkg−1) were administered orally for 20 days (preventively 6 days+therapeutically 14 days) in 4-VO rats, and for 7 days (3 days preventively+4 days therapeutically) in MCAO rats. Learning and memory behavioral performance was assayed using a Morris water maze test including a place navigation trial and a spatial probe trial. Brain histochemical morphology and hippocampal neuron survival was quantified using microscope assay of a puffin brain/hippocampus slice with cresyl violet staining. ResultsMCAO ischemia/reperfusion caused infarct damage in rat brain tissue. 4-VO ischemia/reperfusion caused a hippocampal neuronal lesion and learning and memory deficits in rats. Administration of RSSW (50, 100, and 200mg/kg) or EGb761 significantly reduced the size of the insulted brain hemisphere lesion and improved the neurological behavior of MCAO rats. In addition, RSSW markedly reduced an increase in the brain infarct volume from an I/R-induced MCAO and reduced the cerebral water content in a dose-dependent way. Administration of RSSW also increased the pyramidal neuronal density in the hippocampus of surviving rats after transient global brain ischemia and improved the learning and memory ability of rats with 4-VO induced vascular dementia in a dose-dependent manner. ConclusionsThe in vivo results suggested that RSSW has significant neuroprotective effects against MCAO and 4-VO I/R injury and a therapeutic effect on cognitive disorders in VD rats. RSSW also improved the learning and memory ability of VD rats. These results convincingly demonstrated that RSSW may be useful to prevent and treat ischemia/reperfusion injury and vascular dementia disease.

Real-time fMRI training-induced changes in regional connectivity mediating verbal working memory behavioral performance

Working memory refers to the ability to temporarily store and manipulate information that is necessary for complex cognition activities. Previous studies have demonstrated that working memory capacity can be improved by behavioral training, and brain activities in the frontal and parietal cortices and the connections between these regions are also altered by training. Our recent neurofeedback training has proven that the regulation of the left dorsal lateral prefrontal cortex (DLPFC) activity using real-time functional magnetic resonance imaging (rtfMRI) can improve working memory performance. However, how working memory training promotes interaction between brain regions and whether this promotion correlates with performance improvement remain unclear. In this study, we employed structural equation modeling (SEM) to calculate the interactions between the regions within the working memory network during neurofeedback training. The results revealed that the direct effect of the frontoparietal connection in the left hemisphere was enhanced by the rtfMRI training. Specifically, the increase in the path from the left DLPFC to the left inferior parietal lobule (IPL) was positively correlated with improved performance in verbal working memory. These findings demonstrate the important role of the frontoparietal connection in working memory training and suggest that increases in frontoparietal connectivity might be a key factor associated with behavioral improvement.

Immediate Memory and Electro Physiologic Effects of Prefrontal Cortex Transcranial Direct Current Stimulation on a Chronic Traumatic Brain Injury Survivor: A Case Report

Ongoing memory impairment is a common long-term consequence of traumatic brain injury (TBI) that negatively impacts everyday function. Traditional rehabilitation to improve memory function focuses on use of external and/or internal behavioral memory strategies, the benefits of which are supported by varying levels of evidence. This case report examined immediate behavioral and electrophysiological effects of three conditions of left dorsolateral prefrontal cortex Transcranial direct current stimulation (tDCS) on auditory working memory in a chronic TBI survivor with persisting post-injury memory problems. Pre- and post-tDCS behavioral memory performance was measured, as were auditory event-related potentials (P300 activity) and power of alpha and theta EEG bands. Behavioral and electrophysiological results were specific to tDCS condition, with anodal tDCS, versus cathodal and sham, significantly enhancing memory function and related cortical activity. Rehabilitation implications of these findings are discussed.

Development in the neurophysiology of emotion processing and memory in school-age children

In the adult literature, emotional arousal is regarded as a source of the enhancing effect of emotion on subsequent memory. Here, we used behavioral, electrophysiological, and psychophysiological methods to examine the role of emotional arousal on subsequent memory in school-age children. Five- to 8-year-olds, divided into younger and older groups, viewed emotional scenes as EEG, heart rate, and respiration was recorded, and participated in a memory task 24 hours later where EEG and behavioral responses were recorded; participants provided subjective ratings of the scenes after the memory task. All measures indicated emotion responses in both groups, and in ERP measures the effects were stronger for older children. Emotion responses were more consistent across measures for negative than positive stimuli. Behavioral memory performance was strong but did not differ by emotion condition. Emotion influenced the ERP index of recognition memory in the older group only (enhanced recognition of negative scenes). The findings an increasing interaction of emotion and memory during the school years. Further, the findings impress the value of combining multiple methods to assess emotion and memory in development. Development in the neurophysiology of emotion processing and memory in school-age children.

A pilot study examining functional brain activity 6months after memory retraining in MS: the MEMREHAB trial.

Cognitive impairment in individuals with multiple sclerosis (MS) is now well recognized. One of the most common cognitive deficits is found in memory functioning, largely due to impaired acquisition. We examined functional brain activity 6months after memory retraining in individuals with MS. The current report presents long term follow-up results from a randomized clinical trial on a memory rehabilitation protocol known as the modified Story Memory Technique. Behavioral memory performance and brain activity of all participants were evaluated at baseline, immediately after treatment, and 6months after treatment. Results revealed that previously observed increases in patterns of cerebral activation during learning immediately after memory training were maintained 6months post training.

Menstrual-cycle dependent fluctuations in ovarian hormones affect emotional memory

The hormones progesterone and estradiol modulate neural plasticity in the hippocampus, the amygdala and the prefrontal cortex. These structures are involved in the superior memory for emotionally arousing information (EEM effects). Therefore, fluctuations in hormonal levels across the menstrual cycle are expected to influence activity in these areas as well as behavioral memory performance for emotionally arousing events. To test this hypothesis, naturally cycling women underwent functional magnetic resonance imaging during the encoding of emotional and neutral stimuli in the low-hormone early follicular and the high-hormone luteal phase. Their memory was tested after an interval of 48h, because emotional arousal primarily enhances the consolidation of new memories. Whereas overall recognition accuracy remained stable across cycle phases, recognition quality varied with menstrual cycle phases. Particularly recollection-based recognition memory for negative items tended to decrease from early follicular to luteal phase. EEM effects for both valences were associated with higher activity in the right anterior hippocampus during early follicular compared to luteal phase. Valence-specific modulations were found in the anterior cingulate, the amygdala and the posterior hippocampus. Current findings connect to anxiolytic actions of estradiol and progesterone as well as to studies on fear conditioning. Moreover, they are in line with differential networks involved in EEM effects for positive and negative items.

Nonlinear correlations impair quantification of episodic memory by mesial temporal BOLD activity.

Episodic memory processes can be investigated using different functional MRI (fMRI) paradigms. The purpose of the present study was to examine correlations between neuropsychological memory test scores and BOLD signal changes during fMRI scanning using three different memory tasks. Twenty-eight right-handed healthy subjects underwent three paradigms, (a) a word pair, (b) a space-labyrinth, and (c) a face-name association paradigm. These paradigms were compared for their value in memory quantification and lateralization by calculating correlations between the BOLD signals in the mesial temporal lobe and behavioral data derived from a neuropsychological test battery. As expected, group analysis showed left-sided activation for the verbal, a tendency to right-sided activation for the spatial, and bilateral activation for the face-name paradigm. No linear correlations were observed between neuropsychological data and activation in the temporo-mesial region. However, we found significant u-shaped correlations between behavioral memory performance and activation in both the verbal and the face-name paradigms, that is, BOLD signal changes were greater not only among participants who performed best on the neuropsychological tests, but also among the poorest performers. The figural learning task did not correlate with the activations in the space-labyrinth paradigm at all. We interpreted the u-shaped correlations to be due to compensatory hippocampal activations associated with low performance when people try unsuccessfully to remember presented items. Because activation levels did not linearly increase with memory performance, the latter cannot be quantified by fMRI alone, but only be used in conjunction with neuropsychological testing.

Developmental fMRI study of episodic verbal memory encoding in children

Understanding the maturation and organization of cognitive function in the brain is a central objective of both child neurology and developmental cognitive neuroscience. This study focuses on episodic memory encoding of verbal information by children, a cognitive domain not previously studied using fMRI. Children from 7 to 19 years of age were scanned at 1.5-T field strength using event-related fMRI while performing a novel verbal memory encoding paradigm in which words were incidentally encoded. A subsequent memory analysis was performed. SPM2 was utilized for whole brain and region-of-interest analyses of data. Both whole-sample intragroup analyses and intergroup analyses of the sample divided into 2 subgroups by age were conducted. Importantly, behavioral memory performance was equal across the age range of children studied. Encoding-related activation in the left hippocampus and bilateral basal ganglia declined as age increased. In addition, while robust blood oxygen level-dependent signal was found in left prefrontal cortex with task performance, no encoding-related age-modulated prefrontal activation was observed in either hemisphere. These data are consistent with a developmental pattern of verbal memory encoding function in which left hippocampal and bilateral basal ganglionic activations are more robust earlier in childhood but then decline with age. No encoding-related activation was found in prefrontal cortex which may relate to this region's recognized delay in biologic maturation in humans. These data represent the first fMRI demonstration of verbal encoding function in children and are relevant developmentally and clinically.

Prefrontocortical serotonin depletion results in plastic changes of prefrontocortical pyramidal neurons, underlying a greater efficiency of short-term memory

The prefrontal cortex activity is involved in organizing the short-term memory. Although the involvement of serotonin for an appropriate performance in learning and memory tests is well known, its role is still unclear; as is the cellular basis of short-term memory behavioral performance. Sprague–Dawley rats were stereotactically injected with 1 μg/μl of 5,7-dihydroxitryptamine to cause a lesion to the dorsal raphe nucleus. Sham-operated or intact rats were also studied as control groups. Before surgery and 20 days post-operatively, each animal was placed in the Biel maze for five consecutive trials. In the pre-treatment test, all three groups decreased significantly the number of errors beginning with the fourth trial. The same occurred in the post-treatment test, except for the experimental group, whose animals committed less errors beginning with the second trial. After behavioral testing, the dorsomedial prefrontal cerebral cortex was dissected out, and the Golgi study of the third-layer pyramidal neurons revealed that the length of both the apical and the basilar dendrites was smaller than that of controls, and that the apical and oblique dendrites had a greater spine density. A major proportion of thin spines was also seen on the basilar and oblique dendrites, and more stubby spines were seen on the apical dendrite. Serotonin depletion in the prefrontal cerebral cortex resulted in cytoarchitectural alterations of the prefrontocortical pyramidal neurons, which may be underlying partially the greater efficiency observed in the short-term memory behavioral performance.