Journal of Neurophysiology

Information processing in the cerebral cortex involves interactions among distributed areas. Anatomical connectivity suggests that certain areas form local hierarchical relations such as within the visual system. Other connectivity patterns, particularly among association areas, suggest the presence of large-scale circuits without clear hierarchical relations. In this study the organization of networks in the human cerebrum was explored using resting-state functional connectivity MRI. Data from 1,000 subjects were registered using surface-based alignment. A clustering approach was employed to identify and replicate networks of functionally coupled regions across the cerebral cortex. The results revealed local networks confined to sensory and motor cortices as well as distributed networks of association regions. Within the sensory and motor cortices, functional connectivity followed topographic representations across adjacent areas. In association cortex, the connectivity patterns often showed abrupt transitions between network boundaries. Focused analyses were performed to better understand properties of network connectivity. A canonical sensory-motor pathway involving primary visual area, putative middle temporal area complex (MT+), lateral intraparietal area, and frontal eye field was analyzed to explore how interactions might arise within and between networks. Results showed that adjacent regions of the MT+ complex demonstrate differential connectivity consistent with a hierarchical pathway that spans networks. The functional connectivity of parietal and prefrontal association cortices was next explored. Distinct connectivity profiles of neighboring regions suggest they participate in distributed networks that, while showing evidence for interactions, are embedded within largely parallel, interdigitated circuits. We conclude by discussing the organization of these large-scale cerebral networks in relation to monkey anatomy and their potential evolutionary expansion in humans to support cognition.

Schultz, Wolfram. Tín hiệu phần thưởng dự đoán của các nơron dopamine. J. Neurophysiol. 80: 1–27, 1998. Các tác động của tổn thương, chặn thụ thể, tự kích thích điện, và các loại thuốc gây nghiện cho thấy rằng các hệ thống dopamine ở giữa não có liên quan đến việc xử lý thông tin phần thưởng và học hỏi hành vi tiếp cận. Hầu hết các nơron dopamine thể hiện sự kích hoạt pha sau các phần thưởng chất lỏng và thực phẩm cơ bản và các kích thích thị giác và thính giác đã được điều kiện hóa, dự đoán phần thưởng. Chúng thể hiện các phản ứng kích hoạt-ức chế hai pha sau các kích thích giống các kích thích dự đoán phần thưởng hoặc là mới lạ hoặc đặc biệt nổi bật. Tuy nhiên, chỉ có một vài sự kích hoạt pha theo sau các kích thích khó chịu. Do đó, các nơron dopamine gán nhãn các kích thích môi trường với giá trị hấp dẫn, dự đoán và phát hiện các phần thưởng và đưa ra tín hiệu cảnh báo và động lực cho các sự kiện. Bằng cách không phân biệt giữa các phần thưởng khác nhau, các nơron dopamine dường như phát đi một thông điệp cảnh báo về sự hiện diện hoặc vắng mặt đầy bất ngờ của các phần thưởng. Tất cả các phản ứng đối với phần thưởng và kích thích dự đoán phần thưởng phụ thuộc vào độ dự đoán của sự kiện. Các nơron dopamine được kích hoạt bởi các sự kiện thưởng tốt hơn dự đoán, giữ nguyên không bị ảnh hưởng bởi các sự kiện tốt như dự đoán, và bị ức chế bởi các sự kiện tồi hơn dự đoán. Bằng cách tín hiệu phần thưởng theo lỗi dự đoán, các phản ứng dopamine có các đặc điểm hình thức của một tín hiệu giảng dạy mà các lý thuyết học củng cố đã giả định. Các phản ứng dopamine chuyển giao trong suốt quá trình học từ các phần thưởng cơ bản sang các kích thích dự đoán phần thưởng. Điều này có thể góp phần vào các cơ chế neuron dưới đây tác động ngược của các phần thưởng, một trong những câu đố chính trong việc học củng cố. Phản ứng xung phát ra một xung dopamine ngắn lên nhiều nhánh, do đó phát sóng một tín hiệu củng cố khá toàn cầu đến các nơron hậu synap. Tín hiệu này có thể cải thiện hành vi tiếp cận bằng cách cung cấp thông tin phần thưởng trước khi hành vi xảy ra và có thể góp phần vào việc học bằng cách thay đổi truyền dẫn synap. Tín hiệu phần thưởng dopamine được bổ sung bởi hoạt động trong các nơron ở striatum, vỏ não trán, và amygdala, những vùng xử lý thông tin phần thưởng cụ thể nhưng không đưa ra tín hiệu lỗi dự đoán phần thưởng toàn cầu. Sự hợp tác giữa các tín hiệu phần thưởng khác nhau có thể đảm bảo việc sử dụng các phần thưởng cụ thể để củng cố một cách chọn lọc các hành vi. Giữa các hệ thống chiếu sáng khác, các nơron noradrenaline chủ yếu phục vụ cho các cơ chế chú ý và các nơron hạt nhân basalis mã hóa phần thưởng một cách dị biệt. Các sợi leo trong tiểu não tín hiệu lỗi trong hiệu suất motor hoặc lỗi trong việc dự đoán các sự kiện khó chịu đến các tế bào Purkinje của tiểu não. Hầu hết các khiếm khuyết sau khi tổn thương làm giảm dopamine khó có thể giải thích dễ dàng bằng một tín hiệu phần thưởng thiếu sót nhưng có thể phản ánh sự thiếu vắng của một chức năng cho phép chung của mức dopamine ngoại bào tĩnh. Do đó, các hệ thống dopamine có thể có hai chức năng, truyền tải pha thông tin phần thưởng và cho phép tĩnh cho các nơron hậu synap.

1. An oculomotor delayed-response task was used to examine the spatial memory functions of neurons in primate prefrontal cortex. Monkeys were trained to fixate a central spot during a brief presentation (0.5 s) of a peripheral cue and throughout a subsequent delay period (1-6 s), and then, upon the extinction of the fixation target, to make a saccadic eye movement to where the cue had been presented. Cues were usually presented in one of eight different locations separated by 45 degrees. This task thus requires monkeys to direct their gaze to the location of a remembered visual cue, controls the retinal coordinates of the visual cues, controls the monkey's oculomotor behavior during the delay period, and also allows precise measurement of the timing and direction of the relevant behavioral responses. 2. Recordings were obtained from 288 neurons in the prefrontal cortex within and surrounding the principal sulcus (PS) while monkeys performed this task. An additional 31 neurons in the frontal eye fields (FEF) region within and near the anterior bank of the arcuate sulcus were also studied. 3. Of the 288 PS neurons, 170 exhibited task-related activity during at least one phase of this task and, of these, 87 showed significant excitation or inhibition of activity during the delay period relative to activity during the intertrial interval. 4. Delay period activity was classified as directional for 79% of these 87 neurons in that significant responses only occurred following cues located over a certain range of visual field directions and were weak or absent for other cue directions. The remaining 21% were omnidirectional, i.e., showed comparable delay period activity for all visual field locations tested. Directional preferences, or lack thereof, were maintained across different delay intervals (1-6 s). 5. For 50 of the 87 PS neurons, activity during the delay period was significantly elevated above the neuron's spontaneous rate for at least one cue location; for the remaining 37 neurons only inhibitory delay period activity was seen. Nearly all (92%) neurons with excitatory delay period activity were directional and few (8%) were omnidirectional. Most (62%) neurons with purely inhibitory delay period activity were directional, but a substantial minority (38%) was omnidirectional. 6. Fifteen of the neurons with excitatory directional delay period activity also had significant inhibitory delay period activity for other cue directions. These inhibitory responses were usually strongest for, or centered about, cue directions roughly opposite those optimal for excitatory responses.(ABSTRACT TRUNCATED AT 400 WORDS)

We studied the extent to which automatic postural actions in standing human subjects are organized by a limited repertoire of central motor programs. Subjects stood on support surfaces of various lengths, which forced them to adopt different postural movement strategies to compensate for the same external perturbations. We assessed whether a continuum or a limited set of muscle activation patterns was used to produce different movement patterns and the extent to which movement patterns were influenced by prior experience. Exposing subjects standing on a normal support surface to brief forward and backward horizontal surface perturbations elicited relatively stereotyped patterns of leg and trunk muscle activation with 73- to 110-ms latencies. Activity began in the ankle joint muscles and then radiated in sequence to thigh and then trunk muscles on the same dorsal or ventral aspect of the body. This activation pattern exerted compensatory torques about the ankle joints, which restored equilibrium by moving the body center of mass forward or backward. This pattern has been termed the ankle strategy because it restores equilibrium by moving the body primarily around the ankle joints. To successfully maintain balance while standing on a support surface short in relation to foot length, subjects activated leg and trunk muscles at similar latencies but organized the activity differently. The trunk and thigh muscles antagonistic to those used in the ankle strategy were activated in the opposite proximal-to-distal sequence, whereas the ankle muscles were generally unresponsive. This activation pattern produced a compensatory horizontal shear force against the support surface but little, if any, ankle torque. This pattern has been termed the hip strategy, because the resulting motion is focused primarily about the hip joints. Exposing subjects to horizontal surface perturbations while standing on support surfaces intermediate in length between the shortest and longest elicited more complex postural movements and associated muscle activation patterns that resembled ankle and hip strategies combined in different temporal relations. These complex postural movements were executed with combinations of torque and horizontal shear forces and motions of ankle and hip joints. During the first 5-20 practice trials immediately following changes from one support surface length to another, response latencies were unchanged. The activation patterns, however, were complex and resembled the patterns observed during well-practiced stance on surfaces of intermediate lengths.(ABSTRACT TRUNCATED AT 400 WORDS)

1. We stimulated the motor cortex of normal subjects (transcranial magnetic stimulation) while they 1) observed an experimenter grasping 3D-objects, 2) looked at the same 3D-objects, 3) observed an experimenter tracing geometrical figures in the air with his arm, and 4) detected the dimming of a light. Motor evoked potentials (MEPs) were recorded from hand muscles. 2. We found that MEPs significantly increased during the conditions in which subjects observed movements. The MEP pattern reflected the pattern of muscle activity recorded when the subjects executed the observed actions. 3. We conclude that in humans there is a system matching action observation and execution. This system resembles the one recently described in the monkey.

Experiments were made on the posterior parietal association cortical areas 5 and in 17 hemispheres of 11 monkeys, 6 M. mulatta and 5 M. arctoides. The electrical signs of the activity of single cortical cells were recorded with microelectrodes in waking animals as they carried out certain behavioral acts in response to a series of sensory cues. The behavioral paradigms were one for detection alone, and a second for detection plus projection of the arm to contact a stationary or moving target placed at arm's length. Of the 125 microelectrode penetrations made, 1,451 neurons were identified in terms of the correlation of their activity with the behavioral acts and their sensitivity or lack of it to sensory stimuli delivered passively; 180 were studied quantitatively. The locations of cortical neurons were identified in serial sections; 94 penetrations and 1,058 neurons were located with certainty. About two-thirds of the neurons of area 5 were activated by passive rotation of the limbs at their joints; of these, 82% were related to single, contralateral joints, 10% to two or more contralateral joints, 6% to ipsilateral, and 2% to joints on both sides of the body. A few of the latter were active during complex bodily postures. A large proportion of area 5 neurons were relatively insensitive to passive joint rotations, as compared with similar neurons of the postcentral gyrus, but were driven to high rates of discharge when the same joint was rotated during an active movement of the animal...

Slices of sensorimotor and anterior cingulate cortex from guinea pigs were maintained in vitro and bathed in a normal physiological medium. Electrophysiological properties of neurons were assessed with intracellular recording techniques. Some neurons were identified morphologically by intracellular injection of the fluorescent dye Lucifer yellow CH. Three distinct neuronal classes of electrophysiological behavior were observed; these were termed regular spiking, bursting, and fast spiking. The physiological properties of neurons from sensorimotor and anterior cingulate areas did not differ significantly. Regular-spiking cells were characterized by action potentials with a mean duration of 0.80 ms at one-half amplitude, a ratio of maximum rate of spike rise to maximum rate of fall of 4.12, and a prominent afterhyperpolarization following a train of spikes. The primary slope of initial spike frequency versus injected current intensity was 241 Hz/nA. During prolonged suprathreshold current pulses the frequency of firing adapted strongly. When local synaptic pathways were activated, all cells were transiently excited and then strongly inhibited. Bursting cells were distinguished by their ability to generate endogenous, all-or-none bursts of three to five action potentials. Their properties were otherwise very similar to regular-spiking cells. The ability to generate a burst was eliminated when the membrane was depolarized to near the firing threshold with tonic current. By contrast, hyperpolarization of regular-spiking (i.e., nonbursting) cells did not uncover latent bursting tendencies. The action potentials of fast-spiking cells were much briefer (mean of 0.32 ms) than those of the other cell types.(ABSTRACT TRUNCATED AT 250 WORDS)