Time Course of Functional Connectivity in Primate Dorsolateral Prefrontal and Posterior Parietal Cortex during Working Memory

The dorsolateral prefrontal and posterior parietal cortex play critical roles in mediating attention, working memory, and executive function. Despite proposed dynamic modulation of connectivity strength within each area according to task demands, scant empirical data exist about the time course of the strength of effective connectivity, particularly in tasks requiring information to be sustained in working memory. We investigated this question by performing time-resolved cross-correlation analysis for pairs of neurons recorded simultaneously at distances of 0.2–1.5 mm apart of each other while monkeys were engaged in working memory tasks. The strength of effective connectivity determined in this manner was higher throughout the trial in the posterior parietal cortex than the dorsolateral prefrontal cortex. Significantly higher levels of parietal effective connectivity were observed specifically during the delay period of the task. These differences could not be accounted for by differences in firing rate, or electrode distance in the samples recorded in the posterior parietal and prefrontal cortex. Differences were present when we restricted our analysis to only neurons with significant delay period activity and overlapping receptive fields. Our results indicate that dynamic changes in connectivity strength are present but area-specific intrinsic organization is the predominant factor that determines the strength of connections between neurons in each of the two areas.     

Evidence that Extracellular Concentrations of Dopamine Are Regulated by Noradrenergic Neurons in the Frontal Cortex of Rats

Abstract: Experiments were performed to confirm that noradrenergic terminals regulate extracellular concentrations of dopamine (DA) in the frontal cortex of rats. The effects of 20 mg/kg 1-[2-[bis(4-fluorphenyl)methoxy]-ethyl]-4-(3-phenylpropyl)piperazine (GBR 12909), a selective inhibitor of DA uptake, and 2.5 mg/kg desipramine (DMI) on the extracellular concentrations of DA in the frontal cortex and striatum were studied in rats given 6-hydroxydopamine (6 µg/µl) bilaterally into the locus coeruleus to destroy noradrenergic terminals. GBR 12909 increased dialysate DA similarly in the striatum of vehicle and 6-hydroxydopamine-treated rats, whereas in the frontal cortex it raised DA concentrations only in lesioned animals. DMI raised extracellular DA concentrations in the frontal cortex but not in the striatum of controls. The effect of DMI on cortical DA was abolished by the 6-hydroxydopamine lesion. GBR 12909, at a subcutaneous dose of 20 mg/kg, further increased cortical dialysate DA in rats given DMI intraperitoneally at 20 mg/kg or through the probe at 10⁻⁵ mol/L. The data support the hypothesis of an important regulation of the extracellular concentrations of DA in the frontal cortex by noradrenergic terminals.     

The Role of Right Inferior Parietal Cortex in Auditory Spatial Attention: A Repetitive Transcranial Magnetic Stimulation Study

Behavioral studies support the concept of an auditory spatial attention gradient by demonstrating that attentional benefits progressively diminish as distance increases from an attended location. Damage to the right inferior parietal cortex can induce a rightward attention bias, which implicates this region in the construction of attention gradients. This study used event-related potentials (ERPs) to define attention-related gradients before and after repetitive transcranial magnetic stimulation (rTMS) to the right inferior parietal cortex. Subjects (n = 16) listened to noise bursts at five azimuth locations (left to right: -90°, -45°, 0° midline, +45°, +90°) and responded to stimuli at one target location (-90°, +90°, separate blocks). ERPs as a function of non-target location were examined before (baseline) and after 0.9 Hz rTMS. Results showed that ERP attention gradients were observed in three time windows (frontal 230–340, parietal 400–460, frontal 550–750 ms). Significant transient rTMS effects were seen in the first and third windows. The first window had a voltage decrease at the farthest location when attending to either the left or right side. The third window had on overall increase in positivity, but only when attending to the left side. These findings suggest that rTMS induced a small contraction in spatial attention gradients within the first time window. The asymmetric effect of attended location on gradients in the third time window may relate to neglect of the left hemispace after right parietal injury. Together, these results highlight the role of the right inferior parietal cortex in modulating frontal lobe attention network activity.     

Somatosensory-Motor Adaptation of Orofacial Actions in Posterior Parietal and Ventral Premotor Cortices

Recent studies have provided evidence for sensory-motor adaptive changes and action goal coding of visually guided manual action in premotor and posterior parietal cortices. To extend these results to orofacial actions, devoid of auditory and visual feedback, we used a repetition suppression paradigm while measuring neural activity with functional magnetic resonance imaging during repeated intransitive and silent lip, jaw and tongue movements. In the motor domain, this paradigm refers to decreased activity in specific neural populations due to repeated motor acts and has been proposed to reflect sensory-motor adaptation. Orofacial movements activated a set of largely overlapping, common brain areas forming a core neural network classically involved in orofacial motor control. Crucially, suppressed neural responses during repeated orofacial actions were specifically observed in the left ventral premotor cortex, the intraparietal sulcus, the inferior parietal lobule and the superior parietal lobule. Since no visual and auditory feedback were provided during orofacial actions, these results suggest somatosensory-motor adaptive control of intransitive and silent orofacial actions in these premotor and parietal regions.     

听觉中枢神经元对声信号的识别和处理

人类听觉的基本特性和机制与其他哺乳动物相似,因此,利用动物所作的听觉研究和获得的结果,有助于认识人类自身的听觉.围绕听觉中枢神经元对不同模式的声信号的识别和处理,简要综述了这方面的研究.声信号和声模式识别在听觉中枢对声信号的感受和加工中具有重要意义.听神经元作为声模式识别的结构和功能基础,对不同的声刺激模式产生不同反应,甚至是在同一声刺激模式下,改变其中的某个声参数,神经元的反应也会发生相应改变,而其反应的特性和机制均需要更多研究来解答.另外,声信号作为声信息的载体,不同的声信息寓于不同的声参数和声特征之中,研究发现,听觉中枢神经元存在相应的声信息甄别和选择的神经基础,能对动态变化的声频率、幅度和时程等进行反应和编码,并且,在不同种类动物上获得的研究结果极为相似,表明听觉中枢对不同声信号和声刺激模式的识别、分析和加工,具有共同性和普遍性.     

Different Polypeptides Are Rapidly Transported in Auditory and Optic Neurons

Abstract: Rapidly transported proteins and glycoproteins in the auditory and optic nerves of the guinea pig were analyzed by electrophoresis and two-dimensional electrofocusing/electrophoresis. Proteins transported in the auditory nerve were analyzed in the cochlear nucleus 3 h after cochlear injection of radioactive precursor, and proteins transported in the optic nerve were analyzed in the superior colliculus 6 h after intraocular injection of radioactive precursor. Two-dimensional analysis showed that several rapidly transported polypeptides were present in one system, but not in the other. By use of [³H]fucose as a precursor or by separating [³⁵S]methionine-labeled polypeptides on immobilized concanavalin A or wheat germ agglutinin, it was shown that most of the proteins transported in only one system are glycoproteins. As previously reported a polypeptide of molecular weight 140,000 was a major labeled species in the auditory nerve. This polypeptide was also found in the optic nerve, but only as a minor species. Two other polypeptides with molecular weights and isoelectric points similar to those of the 140,000 molecular weight polypeptide were present in both systems, but were much more abundant in the optic nerve. The major labeled polypeptide in both systems had a molecular weight of 25,000.     

Minimal Conductance-Based Model of Auditory Coincidence Detector Neurons

Sound localization is a fundamental sensory function of a wide variety of animals. The interaural time difference (ITD), an important cue for sound localization, is computed in the auditory brainstem. In our previous modeling study, we introduced a two-compartment Hodgkin-Huxley type model to investigate how cellular and synaptic specializations may contribute to precise ITD computation of the barn owl''s auditory coincidence detector neuron. Although our model successfully reproduced fundamental physiological properties observed in vivo, it was unsuitable for mathematical analyses and large scale simulations because of a number of nonlinear variables. In the present study, we reduce our former model into three types of conductance-based integrate-and-fire (IF) models. We test their electrophysiological properties using data from published in vivo and in vitro studies. Their robustness to parameter changes and computational efficiencies are also examined. Our numerical results suggest that the single-compartment active IF model is superior to other reduced models in terms of physiological reproducibility and computational performance. This model will allow future theoretical studies that use more rigorous mathematical analysis and network simulations.     

Changes in the Number of Primary Sensory Neurons in Normal and Vitamin-E-Deficient Rats during Aging

In the dorsal root ganglia (DRGs) of vitamin-E-deficient rats, we previously found an increase in the number of neurons during the first 5 months of life (Cecchini et al., 1993, 1994). This neurogenetic event seems to bring forward in time the increase in the number of primary sensory neurons that Devor et al. (1985) found in normal rats aged more than 1 year, but that other authors have not confirmed. The present study had two aims: first, to verify whether neurogenesis spontaneously occurs in DRGs of 14-month-old Sprague-Dawley rats; and, second, to determine whether the neurogenesis enhanced by vitamin E deficiency continues further in the long run, or whether it stops or reverses into neuron loss.

A quantitative and morphometric analysis was performed on neurons of L₃-L₆ DRGs in 14-month-old normal and vitamin-E-deficient rats: the results obtained were compared to those previously obtained in 1-month-old and 5-month-old animals of both dietetic treatment groups, in order to observe the effects of aging on these neuronal populations. The total number of DRG neurons in the control group was higher in older than in younger animals, whereas the value in the vitamin-E-deficient group was lower in older than in younger animals. The present data confirm that neurogenesis occurs in DRGs of normal rats during adult life. Moreover, they show that once the premature neurogenesis in the deficient rats is completed, no further increase in the number of neurons takes place.     

儿茶酚胺能神经元与鸣禽鸣唱行为和听觉信息处理

鸣禽脑部的一些鸣唱核团与听觉核团接受来自中脑儿茶酚胺(catecholamine,CA)能神经元发出的纤维投射,并且存在多种儿茶酚胺类受体的表达。研究发现在不同的鸣唱环境下,中脑儿茶酚胺能神经元活性及其支配靶区即早基因的表达水平均存在显著差异。表明中脑儿茶酚胺能神经元在调节鸣唱行为和听觉信息处理等方面发挥重要作用。介绍了近年来有关儿茶酚胺能神经元活动与鸣唱行为和听觉信息处理的研究进展。     

Processing of Auditory Midbrain Interspike Intervals by Model Neurons

A question central to sensory processing is how signal information is encoded and processed by single neurons. Stimulus features can be represented through rate coding (via firing rate), temporal coding (via firing synchronization to temporal periodicities), or temporal encoding (via intricate patterns of spike trains). Of the three, examples of temporal encoding are the least documented. One region in which temporal encoding is currently being explored is the auditory midbrain. Midbrain neurons in the plainfin midshipman generate different interspike interval (ISI) distributions depending on the frequencies of the concurrent vocal signals. However, these distributions differ only along certain lengths of ISIs, so that any neurons trying to distinguish the distributions would have to respond selectively to specific ISI ranges. We used this empirical observation as a realistic challenge with which to explore the plausibility of ISI-tuned neurons that could validate this form of temporal encoding. The resulting modeled cells—point neurons optimized through multidimensional searching—were successfully tuned to discriminate patterns in specific ranges of ISIs. Achieving this task, particularly with simplified neurons, strengthens the credibility of ISI coding in the brain and lends credence to its role in auditory processing.     

Motor Preparatory Activity in Posterior Parietal Cortex is Modulated by Subjective Absolute Value

For optimal response selection, the consequences associated with behavioral success or failure must be appraised. To determine how monetary consequences influence the neural representations of motor preparation, human brain activity was scanned with fMRI while subjects performed a complex spatial visuomotor task. At the beginning of each trial, reward context cues indicated the potential gain and loss imposed for correct or incorrect trial completion. FMRI-activity in canonical reward structures reflected the expected value related to the context. In contrast, motor preparatory activity in posterior parietal and premotor cortex peaked in high “absolute value” (high gain or loss) conditions: being highest for large gains in subjects who believed they performed well while being highest for large losses in those who believed they performed poorly. These results suggest that the neural activity preceding goal-directed actions incorporates the absolute value of that action, predicated upon subjective, rather than objective, estimates of one''s performance.     

Spectrotemporal Response Properties of Core Auditory Cortex Neurons in Awake Monkey

So far, most studies of core auditory cortex (AC) have characterized the spectral and temporal tuning properties of cells in non-awake, anesthetized preparations. As experiments in awake animals are scarce, we here used dynamic spectral-temporal broadband ripples to study the properties of the spectrotemporal receptive fields (STRFs) of AC cells in awake monkeys. We show that AC neurons were typically most sensitive to low ripple densities (spectral) and low velocities (temporal), and that most cells were not selective for a particular spectrotemporal sweep direction. A substantial proportion of neurons preferred amplitude-modulated sounds (at zero ripple density) to dynamic ripples (at non-zero densities). The vast majority (>93%) of modulation transfer functions were separable with respect to spectral and temporal modulations, indicating that time and spectrum are independently processed in AC neurons. We also analyzed the linear predictability of AC responses to natural vocalizations on the basis of the STRF. We discuss our findings in the light of results obtained from the monkey midbrain inferior colliculus by comparing the spectrotemporal tuning properties and linear predictability of these two important auditory stages.     

Functional Interaction between Right Parietal and Bilateral Frontal Cortices during Visual Search Tasks Revealed Using Functional Magnetic Imaging and Transcranial Direct Current Stimulation

The existence of a network of brain regions which are activated when one undertakes a difficult visual search task is well established. Two primary nodes on this network are right posterior parietal cortex (rPPC) and right frontal eye fields. Both have been shown to be involved in the orientation of attention, but the contingency that the activity of one of these areas has on the other is less clear. We sought to investigate this question by using transcranial direct current stimulation (tDCS) to selectively decrease activity in rPPC and then asking participants to perform a visual search task whilst undergoing functional magnetic resonance imaging. Comparison with a condition in which sham tDCS was applied revealed that cathodal tDCS over rPPC causes a selective bilateral decrease in frontal activity when performing a visual search task. This result demonstrates for the first time that premotor regions within the frontal lobe and rPPC are not only necessary to carry out a visual search task, but that they work together to bring about normal function.