A network analysis of developing brain cultures

We recorded electrical activity from four developing embryonic brain cultures (4-40 days in vitro) using multielectrode arrays (MEAs) with 60 embedded electrodes. Data were filtered for local field potentials (LFPs) and downsampled to 1 ms to yield a matrix of time series consisting of 60 electrode × 60 000 time samples per electrode per day per MEA. Each electrode time series was rendered stationary and nonautocorrelated by applying an ARIMA (25, 1, 1) model and taking the residuals (i.e. innovations). Two kinds of analyses were then performed. First, a pairwise crosscorrelation (CC) analysis (±25 1 ms lags) revealed systematic changes in CC with lag, day in vitro (DIV), and inter-electrode distance. Specifically, (i) positive CCs were 1.76× more prevalent and 1.44× stronger (absolute value) than negative ones, and (ii) the strength of CC increased with DIV and decreased with lag and inter-electrode distance. Second, a network equilibrium analysis was based on the instantaneous (1 ms resolution) logratio of the number of electrodes that were above or below their mean, called simultaneous departure from equilibrium, SDE. This measure possesses a major computational advantage over the pairwise crosscorrelation approach because it is very simple and fast to calculate, an important factor for the analysis of large networks. The results obtained with SDE covaried highly with CC over DIV, which further validates the usefulness of this measure as a computationally effective tool for large scale network analysis.

True associations between resting fMRI time series based on innovations

We calculated voxel-by-voxel pairwise crosscorrelations between prewhitened resting-state BOLD fMRI time series recorded from 60 cortical areas (30 per hemisphere) in 18 human subjects (nine women and nine men). Altogether, more than a billion-and-a-quarter pairs of BOLD time series were analyzed. For each pair, a crosscorrelogram was computed by calculating 21 crosscorrelations, namely at zero lag ± 10 lags of 2 s duration each. For each crosscorrelogram, in turn, the crosscorrelation with the highest absolute value was found and its sign, value, and lag were retained for further analysis. In addition, the crosscorrelations at zero lag (irrespective of the location of the peak) were also analyzed as a special case. Based on known varying density of anatomical connectivity, we distinguished four general brain groups for which we derived summary statistics of crosscorrelations between voxels within an area (group I), between voxels of paired homotopic areas across the two hemispheres (group II), between voxels of an area and all other voxels in the same (ipsilateral) hemisphere (group III), and voxels of an area and all voxels in the opposite (contralateral) hemisphere (except those in the homotopic area) (group IV). We found the following. (a) Most of the crosscorrelogram peaks occurred at zero lag, followed by ± 1 lag; (b) over all groups, positive crosscorrelations were much more frequent than negative ones; (c) average crosscorrelation was highest for group I, and decreased progressively for groups II-IV; (d) the ratio of positive over negative crosscorrelations was highest for group I and progressively smaller for groups II-IV; (e) the highest proportion of positive crosscorrelations (with respect to all positive ones) was observed at zero lag; and (f) the highest proportion of negative crosscorrelations (with respect to all negative ones) was observed at lag = 2. These findings reveal a systematic pattern of crosscorrelations with respect to their sign, magnitude, lag and brain group, as defined above. Given that these groups were defined along a qualitative gradient of known overall anatomical connectivity, our results suggest that functional interactions between two voxels may simply reflect the density of such anatomical connectivity between the areas to which the voxels belong.

Post-traumatic stress disorder: a right temporal lobe syndrome?

In a recent paper (Georgopoulos et al 2010 J. Neural Eng. 7 016011) we reported on the power of the magnetoencephalography (MEG)-based synchronous neural interactions (SNI) test to differentiate post-traumatic stress disorder (PTSD) subjects from healthy control subjects and to classify them with a high degree of accuracy. Here we show that the main differences in cortical communication circuitry between these two groups lie in the miscommunication of temporal and parietal and/or parieto-occipital right hemispheric areas with other brain areas. This lateralized temporal-posterior pattern of miscommunication was very similar but was attenuated in patients with PTSD in remission. These findings are consistent with observations (Penfield 1958 Proc. Natl Acad. Sci. USA 44 51-66, Penfield and Perot 1963 Brain 86 595-696, Gloor 1990 Brain 113 1673-94, Banceaud et al 1994 Brain 117 71-90, Fried 1997 J. Neuropsychiatry Clin. Neurosci. 9 420-8) that electrical stimulation of the temporal cortex in awake human subjects, mostly in the right hemisphere, can elicit the re-enactment and re-living of past experiences. Based on these facts, we attribute our findings to the re-experiencing component of PTSD and hypothesize that it reflects an involuntarily persistent activation of interacting neural networks involved in experiential consolidation.

The synchronous neural interactions test as a functional neuromarker for post-traumatic stress disorder (PTSD): a robust classification method based on the bootstrap

Traumatic experiences can produce post-traumatic stress disorder (PTSD) which is a debilitating condition and for which no biomarker currently exists (Institute of Medicine (US) 2006 Posttraumatic Stress Disorder: Diagnosis and Assessment (Washington, DC: National Academies)). Here we show that the synchronous neural interactions (SNI) test which assesses the functional interactions among neural populations derived from magnetoencephalographic (MEG) recordings (Georgopoulos A P et al 2007 J. Neural Eng. 4 349-55) can successfully differentiate PTSD patients from healthy control subjects. Externally cross-validated, bootstrap-based analyses yielded >90% overall accuracy of classification. In addition, all but one of 18 patients who were not receiving medications for their disease were correctly classified. Altogether, these findings document robust differences in brain function between the PTSD and control groups that can be used for differential diagnosis and which possess the potential for assessing and monitoring disease progression and effects of therapy.

Functional organization of the basal ganglia: contributions of single-cell recording studies

Studies of single-cell discharge in the basal ganglia of behaving primates have revealed: characteristic patterns of spontaneous discharge in the striatum, external (GPe) and internal (GPi) globus pallidus, pars reticulata and pars compacta of the substantia nigra, and the subthalamic nucleus (STN); phasic changes in neural discharge in relation to movements of specific body parts (e.g. leg, arm, neck, face); short-latency (sensory) neural responses to passive joint rotation; a somatotopic organization of movement-related neurons in GPe, GPi, and STN; a clustering of functionally similar neurons in the putamen and globus pallidus; greater representation of the proximal than of the distal portion of the limb; changes in neural activity in reaction-time tasks, suggesting a greater role of the basal ganglia in the execution than in the initiation of movement in this paradigm; a clear relation of neuronal activity to direction, amplitude (?velocity) of movement, and force; a preferential relation of neural activity to the direction of movement, rather than to the pattern of muscular activity. Some of these findings suggest that the basal ganglia may play a role in the control of movement parameters rather than (or independent of) the pattern of muscular activity. Loss of basal ganglia output related to amplitude may account for the bradykinesia in Parkinson's disease. The presence of somatotopic organization in the putamen and globus pallidus, together with known topographic striopallidal connections, suggests that segregated, parallel cortico-subcortical loops subserve 'motor' and 'complex' functions.

Cortical mechanisms subserving reaching

The generation and control of reaching in space is a function of several structures, cortical and subcortical. This paper summarizes some principles of the cortical mechanisms subserving this function, as revealed by recording the impulse activity of neurons in motor cortex and area 5 of the posterior parietal cortex in behaving monkeys. Large populations of neurons in these cortical areas are engaged in reaching. This engagement is early in time; for example, cell activity in the motor cortex begins to change 60-80 ms after target onset, and slightly later in area 5. The time course of cell recruitment in the active population is very similar for reaching movements of equal amplitude directed to different targets. In contrast, the intensity of cell discharge in both motor and parietal cortex is clearly modulated with respect to the direction of reaching. Typically, the firing rate is a cosine function of the direction of the movement in space. An unambiguous distributed code for the direction of reaching exists in neuronal populations in the cortical areas studied. The outcome of this population code can be visualized as a vector in neural space that points in the direction of the upcoming movement.

Neurophysiology of the parieto-frontal system during target interception

We studied the functional properties of neurons of two elements of the parieto-frontal system: area 7a of the PPC and the motor cortex (M1), during an interception task of stimuli moving in real (RM) and apparent motion (AM). The stimulus moved along a circular path with one of 5 speeds, and was intercepted at 6 o'clock by exerting a force pulse on a joystick. A smooth stimulus motion was produced in RM, whereas in AM 5 stimuli were flashed successively at the vertices of a pentagon. The results showed, that a group of neurons in both areas above responded not only during the interception but also during a NOGO task in which the same stimuli were presented in the absence of a motor response. Most of these neurons were tuned to the stimulus angular position. In addition, we found that the time-varying neuronal activity in both areas was related to various aspects of stimulus motion and hand force, with stimulus-related activity prevailing in area 7a and hand-related activity prevailing in M1. Interestingly, the neural activity was selectively associated with the stimulus angle during RM, whereas it was tightly correlated to the time-to-contact during AM. Thus, the results suggest that area 7a was processing high level features of the circularly moving stimuli and was involved in the production of an early command signal for stimulus interception, whereas M1 was still processing some aspect of the visual stimulus that were used to trigger the interception movement using a predictive mechanism.

Impact of path parameters on maze solution time

In order to compare spatial attention and visual processing capabilities of humans and rhesus macaques, we developed a visual maze task both could perform. Maze stimuli were constructed of orthogonal line segments displayed on a monitor. Each was octagonal in outline and contained a central square (the 'start box'). A single ('main') path, containing a random number of turns, extended outward from the start box, and either reached an exit in the maze's perimeter, or a blind ending within the maze. Subjects maintained ocular fixation within the start box, and indicated their judgment whether the path reached an exit or not by depressing one of two keys (humans) or foot pedals (monkeys). Successful maze solution by human subjects required a minimum viewing time. Replacing the maze with a masking stimulus after a variable interval revealed that the percent correct performance increased systematically with greater viewing time, reaching a plateau of approximately 85% correct if mazes were visible for 500 ms or more. A multiple linear regression analysis determined that the response time of both species depended upon several parameters of the main path, including the number of turns, total length, and exist status. Human and nonhuman primates required comparable time to process each turn in the path, whereas monkeys were faster than humans in processing each unit of path length. The data suggest that a covert analysis of the maze proceeds from the center outward along the main path in the absence of saccadic eye movements, and that both monkeys and humans undertake such an analysis during the solution of visual mazes.

Effects of optic flow in motor cortex and area 7a

Moving visual stimuli were presented to behaving monkeys who fixated their eyes and did not move their arm. The stimuli consisted of random dots moving coherently in eight different kinds of motion (right, left, up, downward, expansion, contraction, clockwise, and counterclockwise) and were presented in 25 square patches on a liquid crystal display projection screen. Neuronal activity in the arm area of the motor cortex and area 7a was significantly influenced by the visual stimulation, as assessed using an ANOVA. The percentage of cells with a statistically significant effect of visual stimulation was 3 times greater in area 7a (370/587, 63%) than in motor cortex (148/693, 21.4%). With respect to stimulus properties, its location and kind of motion had differential effects on cell activity in the two areas. Specifically, the percentage of cells with a significant stimulus location effect was approximately 2.5 times higher in area 7a (311/370, 84%) than in motor cortex (48/148, 32.4%), whereas the percentage of cells with a significant stimulus motion effect was approximately 2 times higher in the motor cortex (79/148, 53.4%) than in area 7a (102/370, 27.6%). We also assessed the selectivity of responses to particular stimulus motions using a Poisson train analysis and determined the percentage of cells that showed activation in only one stimulus condition. This percentage was 2 times higher in the motor cortex (73.7%) than in area 7a (37.7%). Of all kinds of stimulus motion tested, responses to expanding optic flow were the strongest in both cortical areas. Finally, we compared the activation of motor cortical cells during visual stimulation to that observed during force exertion in a center --> out task. Of 514 cells analyzed for both the motor and visual tasks, 388 (75.5%) showed a significant relation to either or both tasks, as follows: 284/388 (73.2%) cells showed a significant relation only to the motor task, 27/388 (7%) cells showed a significant relation only to the visual task, whereas the remaining 77/388 (19.8%) cells showed significant relations to both tasks. Therefore a total of 361/514 (70.2%) cells were related to the motor task and 104/514 (20.2%) were related to the visual task. Finally, with respect to receptive fields (RFs), there was no clear visual receptive field structure in the motor cortical neuronal responses, in contrast to area 7a where RFs were present and could be modulated by the type of optic flow stimulus.

Guiding contact by coupling the taus of gaps

Animals control contact with surfaces when locomoting, catching prey, etc. This requires sensorily guiding the rate of closure of gaps between effectors such as the hands, feet or jaws and destinations such as a ball, the ground and a prey. Control is generally rapid, reliable and robust, even with small nervous systems: the sensorimotor processes are therefore probably rather simple. We tested a hypothesis, based on general tau theory, that closing two gaps simultaneously, as required in many actions, might be achieved simply by keeping the taus of the gaps coupled in constant ratio. tau of a changing gap is defined as the time-to-closure of the gap at the current closure-rate. General tau theory shows that tau of a gap could, in principle, be directly sensed without needing to sense either the gap size or its rate of closure. In our experiment, subjects moved an effector (computer cursor) to a destination zone indicated on the computer monitor, to stop in the zone just as a moving target cursor reached it. The results indicated the subjects achieved the task by keeping tau of the gap between effector and target coupled to tau of the gap between the effector and the destination zone. Evidence of tau-coupling has also been found, for example, in bats guiding landing using echolocation. Thus, it appears that a sensorimotor process used by different species for coordinating the closure of two or more gaps between effectors and destinations entails constantly sensing the taus of the gaps and moving so as to keep the taus coupled in constant ratio.

Neuronal Clusters in the Primate Motor Cortex during Interceptin of Moving Targets

Two rhesus monkeys were trained to intercept a moving target at a fixed location with a feedback cursor controlled bya 2-D manipulandum. The direction from which the target appeared, the time from the target onset to its arrival at the interception point, and the target acceleration were randomized for each trial, thus requiring the animal to adjust its movement according to the visual input on a trail-by-trail basis. The two animals adopted different strategies, similar to those identified previously in human subjects. Single-cell activity was recorded from the arm area of the primary motor cortex in these two animals, and the neurons were classified based on the temporal patterns in their activity, using a nonhierarchical cluster analysis. Results of this analysis revealed differences in the complexity and diversity of motor cortical activity between the two animals that paralleled those of behavioral strategies. Most clusters displayed activity closedly related to the kinematics of hand movements. In addition, some clusters displayed patterns of activation that conveyed additional information necessary for successful performance of the task, such as the initial target velocity and the interval between successive submovements, suggesting that such information is represented in selective subpopulations of neurons in the primary motor cortex. These results also suggest that conversion of information about target motion into movement-related signals takes place in a broad network of cortical areas including the primary motor cortex.

Motor Cortical Activity during Interception of Moving Targets

The single-unit activity of 831 cells was recorded in the arm area of the motor cortex of tow monkeys while the monkeys intercepted a moving visual stimulus (interception task) or remained immobile during presentation of the same moving stimulus (no-go task). The moving target traveled on an oblique path from either lower corner of a screen toward the vertical meridian, and its movement time (0.5,1.0, or 1.5 sec) and velocity profile (accelerating, decelerating, or constant velocity) were pseudorandomly varied. The moving target had to be intercepted within 130 msec of target arrival at an interception point. By comparing motor cortical activity at the single-neuron tasks, we tested whether information about parameters of moving target is represented in the primary motor cortex to generate appropriate motor responses. A substantial number of neurons displayed modulation of their activity during the no-go task, and this activity was often affected by the stimulus parameters. These results suggest a role of motor cortex in specifying the timing of movement initiation based on information about target motion. In addition, there was a lack of systematic relation between the onset times of neural activity in the interception and no-go task, suggesting that processing of information concerning target motion and generation of hand movement occurs in parallel. Finally, the activity in the most motor cortical neurons was modulated according to an estimate of the time-to-target interception, raising the possibility that time-to-interception may be coded in the motor cortical activity.

Functional magnetic resonance imaging of visual object construction and shape discrimination : relations among task, hemispheric lateralization, and gender

We studied the brain activation patterns in two visual image processing tasks requiring judgements on object construction (FIT task) or object sameness (SAME task). Eight right-handed healthy human subjects (four women and four men) performed the two tasks in a randomized block design while 5-mm, multislice functional images of the whole brain were acquired using a 4-tesla system using blood oxygenation dependent (BOLD) activation. Pairs of objects were picked randomly from a set of 25 oriented fragments of a square and presented to the subjects approximately every 5 sec. In the FIT task, subjects had to indicate, by pushing one of two buttons, whether the two fragments could match to form a perfect square, whereas in the SAME task they had to decide whether they were the same or not. In a control task, preceding and following each of the two tasks above, a single square was presented at the same rate and subjects pushed any of the two keys at random. Functional activation maps were constructed based on a combination of conservative criteria. The areas with activated pixels were identified using Talairach coordinates and anatomical landmarks, and the number of activated pixels was determined for each area. Altogether, 379 pixels were activated. The counts of activated pixels did not differ significantly between the two tasks or between the two genders. However, there were significantly more activated pixels in the left (n = 218) than the right side of the brain (n = 161). Of the 379 activated pixels, 371 were located in the cerebral cortex. The Talairach coordinates of these pixels were analyzed with respect to their overall distribution in the two tasks. These distributions differed significantly between the two tasks. With respect to individual dimensions, the two tasks differed significantly in the anterior--posterior and superior--inferior distributions but not in the left--right (including mediolateral, within the left or right side) distribution. Specifically, the FIT distribution was, overall, more anterior and inferior than that of the SAME task. A detailed analysis of the counts and spatial distributions of activated pixels was carried out for 15 brain areas (all in the cerebral cortex) in which a consistent activation (in > or = 3 subjects) was observed (n = 323 activated pixels). We found the following. Except for the inferior temporal gyrus, which was activated exclusively in the FIT task, all other areas showed activation in both tasks but to different extents. Based on the extent of activation, areas fell within two distinct groups (FIT or SAME) depending on which pixel count (i.e., FIT or SAME) was greater. The FIT group consisted of the following areas, in decreasing FIT/SAME order (brackets indicate ties): GTi, GTs, GC, GFi, GFd, [GTm, GF], GO. The SAME group consisted of the following areas, in decreasing SAME/FIT order : GOi, LPs, Sca, GPrC, GPoC, [GFs, GFm]. These results indicate that there are distributed, graded, and partially overlapping patterns of activation during performance of the two tasks. We attribute these overlapping patterns of activation to the engagement of partially shared processes. Activated pixels clustered to three types of clusters : FIT-only (111 pixels), SAME-only (97 pixels), and FIT + SAME (115 pixels). Pixels contained in FIT-only and SAME-only clusters were distributed approximately equally between the left and right hemispheres, whereas pixels in the SAME + FIT clusters were located mostly in the left hemisphere. With respect to gender, the left-right distribution of activated pixels was very similar in women and men for the SAME-only and FIT + SAME clusters but differed for the FIT-only case in which there was a prominent left side preponderance for women, in contrast to a right side preponderance for men. We conclude that (a) cortical mechanisms common for processing visual object construction and discrimination involve mostly the left hemisphere, (b) cortical mechanisms specific for these tasks engage both hemispheres, and (c) in object construction only, men engage predominantly the right hemisphere whereas women show a left-hemisphere preponderance.

Mental maze solving

We sought to determine how a visual maze is mentally solved. Human subjects (N = 13) viewed mazes with orthogonal, unbranched paths; each subject solved 200-600 mazes in any specific experiment below. There were four to six openings at the perimeter of the maze, of which four were labeled: one was the entry point and the remainder were potential exits marked by Arabic numerals. Starting at the entry point, in some mazes the path exited, whereas in others it terminated within the maze. Subjects were required to type the number corresponding to the true exit (if the path exited) or type zero (if the path did not exit). In all cases, the only required hand movement was a key press, and thus the hand never physically traveled through the maze. Response times (RT) were recorded and analyzed using a multiple linear regression model. RT increased as a function of key parameters of the maze, namely the length of the main path, the number of turns in the path, the direct distance from entry to termination, and the presence of an exit. The dependence of RT on the number of turns was present even when the path length was fixed in a separate experiment (N = 10 subjects). In a different experiment, subjects solved large and small mazes (N = 3 subjects). The former was the same as the latter but was scaled up by 1.77 times. Thus both kinds of mazes contained the same number of squares but each square subtended 1.77 degrees of visual angle (DVA) in the large maze, as compared to 1 DVA in the small one. We found that the average RT was practically the same in both cases. A multiple regression analysis revealed that the processing coefficients related to maze distance (i.e., path length and direct distance) were reduced by approximately one-half when solving large mazes, as compared to solving small mazes. This means that the efficiency in processing distance-related information almost doubled for scaled-up mazes. In contrast, the processing coefficients for number of turns and exit status were practically the same in the two cases. Finally, the eye movements of three subjects were recorded during maze solution. They consisted of sequences of saccades and fixations. The number of fixations in a trial increased as a linear function of the path length and number of turns. With respect to the fixations themselves, eyes tended to fixate on the main path and to follow it along its course, such that fixations occurring later in time were positioned at progressively longer distances from the entry point. Furthermore, the time the eyes spent at each fixation point increased as a linear function of the length and number of turns in the path segment between the current and the upcoming fixation points. These findings suggest that the maze segment from the current fixation spot to the next is being processed during the fixation time (FT), and that a significant aspect of this processing relates to the length and turns in that segment. We interpreted these relations to mean that the maze was mentally traversed. We then estimated the distance and endpoint of the path mentally traversed within a specific FT; we also hypothesized that the next portion of the main path would be traversed during the ensuing FT, and so on for the whole path. A prediction of this hypothesis is that the upcoming saccade would land the eyes at or near the locus on the path where the mental traversing ended, so that "the eyes would pick up where the mental traversal left off." In this way, a portion of the path would be traversed during a fixation and successive such portions would be strung together closely along the main path to complete the processing of the whole path. We tested this prediction by analyzing the relations between the path distance of mental traverse and the distance along the path between the current and the next fixation spot. (ABSTRACT TRUNCATED)

Neural aspects of cognitive motor control

Traditionally, motor and cognitive functions were studied separately; however, the investigation of processes at the interface between cognition and action has become more and more popular recently. Typical research goals include the identification of the processes involved using experimental psychological methods, and understanding the neural mechanisms underlying these processes using neurophysiological and functional neuroimaging methods. Specifically, there has been a special emphasis during the past few years on timing mechanisms, practice effects, and the application of rules in guiding action. New information concerning the neural mechanisms involved is being acquired at a rapid pace, albeit mostly within a descriptive framework. With respect to specific brain areas, a key finding has been the clear involvement of the primary motor cortex in complex tasks engaging diverse motor and cognitive dimensions.

Motor area activity during mental rotation studied by time-resolved single-trial fMRI

The functional equivalence of overt movements and dynamic imagery is of fundamental importance in neuroscience. Here, we investigated the participation of the neocortical motor areas in a classic task of dynamic imagery, Shepard and Metzler's mental rotation task, by time-resolved single-trial functional Magnetic Resonance Imaging (fMRI). The subjects performed the mental-rotation task 16 times, each time with different object pairs. Functional images were acquired for each pair separately, and the onset times and widths of the activation peaks in each area of interest were compared to the response times. We found a bilateral involvement of the superior parietal lobule, lateral premotor area, and supplementary motor area in all subjects; we found, furthermore, that those areas likely participate in the very act of mental rotation. We also found an activation in the left primary motor cortex, which seemed to be associated with the right-hand button press at the end of the task period.

Varied duration of congenital hypothyroidism potentiates perseveration in a response alternation discrimination task

The behavior of five groups of rats (seven rats per group) made hypothyroid for varying lengths of time and one group of seven normal control rats was assessed under forced alternation fixed-ratio (FR1, FR3, FR5 and FR10), alternating lever cyclic-ratio (ALCR) and progressive-ratio (PR3) schedules of reinforcement. Hypothyroidism was produced by adding methimazole (MMI) to the drinking water of pregnant dams from embryonic day E16 to postnatal day P25. Four groups were given replacement thyroxine (T4) injections beginning at specific time points (P1, P7, P13, and P19). There were no differences in behavioral performance between control and experimental groups under the FR schedule, which indicates that the animals' sensorimotor abilities were intact. Under the forced ALCR schedule, all groups reached criteria similarly. However, under the choice lever ALCR schedule, control animals and those which received T4 replacement from early on (P1, P7, P13 groups) performed well and all had reached criteria by 11 sessions. In contrast, animals which did not receive any T4 replacement or received it late (P19 group) took longer to reach criteria and 5/14 animals had not reached criteria at all by 20 sessions. This deterioration in performance was paralleled by an increase in perseverative behavior as evidenced by an increased frequency of pressing the wrong lever when alternation of lever was required. This suggests that congenital hypothyroidism results in increased perseveration leading to a decrease in learning when a discrimination between correct and incorrect operanda is made available.

Directional tuning profiles of motor cortical cells

The directional tuning profiles of motor cortical cells are commonly described by a cosine tuning function with three adjustable parameters (Georgopoulos, A.P., Kalaska. J.F., Crutcher, M.D., Caminiti, R., Massey, J.T., 1982. On the relations between the direction of two-dimensional (2D) arm movements and cell discharge in primate motor cortex. J. Neurosci. 2, 1527-1537). In this study the variation in the shape of the directional tuning profiles among a population of cells recorded from the arm area of the motor cortex of monkeys using movements in 20 directions, every 18 degrees, was examined systematically. This allowed the investigation of tuning functions with extra parameters to capture additional features of the tuning curve (i.e. tuning breadth, symmetry, and modality) and determine an 'optimal' tuning function. These functions provided better fit than the standard cosine one. The optimal function for the large majority of tuned cells was unimodal (84%), and only for a few of them (16%) it was bimodal. Of the unimodal cells, 73% exhibited symmetric and 27% asymmetric shape. The half-width, sigma, at the midpoint of optimal tuning curves differed among cells from 30 to 90 degrees, with a median at 56 degrees. This is much narrower than in the standard cosine tuning function with a fixed width of sigma = 90 degrees. It was concluded that motor cortical cells are more sharply tuned than previously thought.

Congenital hypothyroidism impairs response alternation discrimination behavior

The behavior of six congenitally hypothyroid and six normal control rats was assessed under forced alternation fixed-ratio, alternating lever cyclic-ratio (ALCR) and progressive-ratio schedules of reinforcement. Hypothyroidism was produced by adding methimazole (MMI) to the drinking water of pregnant dams from embryonic day 16 to postnatal day 25. There were no differences in behavioral performance between MMI-treated and control animals under the fixed-ratio and progressive ratio schedules. There were also no differences in circulating triiodothyronine levels between groups at the end of the study. Under the ALCR schedule, when alternation of responding was forced during the first three cycles but both levers (choice) were presented during the last three cycles (correct lever active), the entire control group reached a competency criteria in nine sessions. In contrast, only two MMI-treated animals reached criteria after 17 sessions, and the remaining four MMI-treated animals did not reach criteria by 30 sessions of training. These results suggest that congenital hypothyroidism impairs learning when a discrimination between correct and incorrect operanda is made available.

Neural coding of finger and wrist movements

Previous work (Schieber and Hibbard, 1993) has shown that single motor cortical neurons do not discharge specifically for a particular flexion-extension finger movement but instead are active with movements of different fingers. In addition, neuronal populations active with movements of different fingers overlap extensively in their spatial locations in the motor cortex. These data suggested that control of any finger movement utilizes a distributed population of neurons. In this study we applied the neuronal population vector analysis (Georgopoulos et al., 1983) to these same data to determine (1) whether single cells are tuned in an abstract, three-dimensional (3D) instructed finger and wrist movement space with hand-like geometry and (2) whether the neuronal population encodes specific finger movements. We found that the activity of 132/176 (75%) motor cortical neurons related to finger movements was indeed tuned in this space. Moreover, the population vector computed in this space predicted well the instructed finger movement. Thus, although single neurons may be related to several disparate finger movements, and neurons related to different finger movements are intermingled throughout the hand area of the motor cortex, the neuronal population activity does specify particular finger movements.

Multidimensional scaling analyses of two construction-related tasks

Representations involved in two construction-related tasks were analyzed by multidimensional scaling (MDS), a statistical technique that allows the dimensions of internal representations to be derived from empirically obtained judgment data. The tasks involved judgments of how similar two objects were and how well they fitted together; these judgments are related to copying and assembly abilities that are impaired in constructional apraxia. Analyses of numerical subjective ratings and response times for these judgments showed that within the same set of geometric objects, different shape-related properties were emphasized under different task conditions. The similarity judgment depended most on a representational dimension related to enclosure of space, while the fit judgment depended to a greater extent on a dimension related to the objects' symmetry properties. This pattern of results was found in both subjective ratings and response times, as analyzed by MDS and by confirmatory classical statistics. The findings suggest that construction-related tasks depend on representations that are context-dependent, and that MDS may be useful in a variety of settings as an intermediate-level tool for analyzing representations related to context-specific abilities.

Motor cortical encoding of serial order in a context-recall task

The neural encoding of serial order was studied in the motor cortex of monkeys performing a context-recall memory scanning task. Up to five visual stimuli were presented successively on a circle (list presentation phase), and then one of them (test stimulus) changed color; the monkeys had to make a single motor response toward the stimulus that immediately followed the test stimulus in the list. Correct performance in this task depends on memorization of the serial order of the stimuli during their presentation. It was found that changes in neural activity during the list presentation phase reflected the serial order of the stimuli; the effect on cell activity of the serial order of stimuli during their presentation was at least as strong as the effect of motor direction on cell activity during the execution of the motor response. This establishes the serial order of stimuli in a motor task as an important determinant of motor cortical activity during stimulus presentation and in the absence of changes in peripheral motor events, in contrast to the commonly held view of the motor cortex as just an "upper motor neuron."

Cortical populations and behaviour: Hebb's thread

This paper discusses work on the function of the motor cortex as revealed by single cell recordings in monkeys and artificial neural network modelling. Our key conceptual approach both in behavioural neuroscience and neural network modeling of motor cortical function relies on reconstructing, visualizing, and modelling the activity in neuronal populations, indeed a key concept advanced by Hebb (1949). The behaviour investigated ranges from exertion of isometric force to pointing movements to complex cognitive processing. The functional properties of single cells with respect to the direction of movement in space are described as well as a population code which provides a unique measure for this direction. Finally, the results of modeling studies are discussed in which directional population activity is used as an input to an artificial neural network to drive a simulated arm.

Drawing under visuomotor incongruence

Six human subjects were asked to draw ellipses presented on a screen by moving a manipulandum that controlled the position of a cursor. Six visual templates were used, which comprised three different ellipses displayed either horizontally or vertically; the ratio between the major and minor axes was 2, 4, or 5. For each visual template, the gains were set such that the movement trajectories required to trace the template with the cursor corresponded to one of six ellipses. Thus these movement ellipses were horizontal or vertical with a ratio between major and minor axes of 2, 4, or 5. All 36 combinations of six visual ellipses and six required movement ellipses were used. Therefore, in some conditions the required movement ellipse had a different orientation (with respect to the major axis) than the visual template. These conditions were called orientation incongruent, whereas, when the orientation of the required movement ellipse matched the orientation of the visual template, the conditions were called orientation congruent. Similarly, eccentricity incongruent referred to conditions where the eccentricities of the visual ellipse and the required movement ellipse were different, as opposed to eccentricity congruent. The main results were as follows: (a) The perimeter of the traced ellipse always tended to be larger than that of the visual template. In addition, it was significantly larger in the orientation incongruent conditions than in the orientation congruent conditions. Nevertheless, the perimeter of the traced figure increased with the template in both orientation congruent and incongruent conditions. (b) The shape of the traced figure varied appropriately with the visual template, but differed significantly between the orientation congruent and incongruent conditions. It was closer to the one of the template in the orientation congruent than in the incongruent conditions. Finally, (c) the instantaneous speed was significantly correlated with curvature but more tightly so in the orientation congruent than in the orientation incongruent conditions. The parameters defining the relation between speed and curvature were affected by the required movement ellipse, but not by the particular visuomotor condition. These results showed that although spatial motor performance was affected by changes in the correspondence between visual and movement coordinates, the relation between the speed and curvature of the movement trajectory was stable despite drastic changes in this correspondence.

Online visual control of the arm

The psychophysical and cerebrocortical mechanisms in visually guided reaching movements and isometric force pulses are discussed. The results of psychophysical studies of pointing movements have demonstrated a tight coupling between the visual information and the direction of the movement, and those of studies of directed isometric force pulses have documented the sensitive dependence of the motor system on the continuous availability of visual information for the ongoing correction of directional deviations from the instructed direction. Recordings of the activity of single cells in the motor cortex and parietal areas 2 and 5 have revealed the same tight, online coupling between visual information and cell discharge, and have partially elucidated the neural mechanisms underlying this function at the cortical level.

Interview with Apostolos P. Georgopoulos

Apostolos P. Georgopoulos studied Medicine and Physiology at the University of Athens in Greece where he obtained his M.D. and Ph.D. degrees. He was trained in neurophysiology by Vernon B. Mountcastle at Johns Hopkins and, after a brief return to Athens, he came back to Johns Hopkins. He ascended the faculty ranks and promoted to Professor of Neuroscience in 1986. He was a member of the Philip Bard Laboratories of Neurophysiology at the Department of Neuroscience until 1991 when he moved to Minnesota as the American Legion Brain Sciences Chair at the Minneapolis Veterans Affairs Medical Center and the University of Minnesota.

Variability and correlated noise in the discharge of neurons in motor and parietal areas of the primate cortex

We analyzed the magnitude and interneuronal correlation of the variability in the activity of single neurons that were recorded simultaneously using a multielectrode array in the primary motor cortex and parietal areas 2/5 in rhesus monkeys. The animals were trained to move their arms in one of eight directions as instructed by a visual target. The relationship between variability (SD) and mean of the discharge rate was described by a power function with a similar exponent ( approximately 0.57), regardless of the cortical area or the behavioral condition. We examined whether the deviation from mean activity between target onset and the end of the movement was correlated on a trial-by-trial basis with variability in activity during the hold period before target onset. In both cortical areas, for about a quarter of the neurons, the neuronal noise of these two periods was positively correlated, whereas significant negative correlations were seldom observed. Overall, neurons with higher signal correlation (i.e., similar directional pattern) showed higher noise correlation in both cortical areas. On the other hand, when the data were divided according to the distance between the electrode tips from which the neurons were recorded, a consistent relationship between the signal and noise correlations was found only for pairs of neurons recorded through the same electrode. These results suggest that nearby neurons with similar directional tuning carry primarily redundant messages, whereas neurons in separate cortical columns perform more independent processing.

Manual interception of moving targets. II. On-line control of overlapping submovements

We studied the kinematic characteristics of arm movements and their relation to a stimulus moving with a wide range of velocity and acceleration. The target traveled at constant acceleration, constant deceleration, or constant velocity for 0.5-2.0 s, until it arrived at a location where it was required to be intercepted. For fast moving targets, subjects produced single movements with symmetrical, bell-shaped velocity profiles. In contrast, for slowly moving targets, hand velocity profiles displayed multiple peaks, which suggests a control mechanism that produces a series of discrete submovements according to characteristics of target motion. To analyze how temporal and spatial aspects of these submovements are influenced by target motion, we decomposed the vertical hand velocity profiles into bell-shaped velocity pulses according to the minimum-jerk model. The number of submovements was roughly proportional to the movement time, resulting in a relatively constant submovement frequency (approximately 2.5 Hz). On the other hand, the submovement onset asynchrony showed significantly more variability than the intersubmovement interval, indicating that the submovement onset was delayed more following a submovement with a longer duration. Examination of submovement amplitude and its relation to target motion revealed that the subjects achieved interception mainly by producing a series of submovements that would keep the displacement of the hand proportional to the first-order estimate of target position at the end of each submovement along the axis of hand movement. Finally, we did not find any evidence that information regarding target acceleration is properly utilized in the production of submovements.

Manual interception of moving targets. I. Performance and movement initiation

We investigated the capacities of human subjects to intercept moving targets in a two-dimensional (2D) space. Subjects were instructed to intercept moving targets on a computer screen using a cursor controlled by an articulated 2D manipulandum. A target was presented in 1 of 18 combinations of three acceleration types (constant acceleration, constant deceleration, and constant velocity) and six target motion times, from 0.5 to 2.0 s. First, subjects held the cursor in a start zone located at the bottom of the screen along the vertical meridian. After a pseudorandom hold period, the target appeared in the lower left or right corner of the screen and traveled at 45 degrees toward an interception zone located on the vertical meridian 12.5 cm above the start zone. For a trial to be considered successful, the subject's cursor had to enter the interception zone within 100 ms of the target's arrival at the center of the interception zone and stay inside a slightly larger hold zone. Trials in which the cursor arrived more than 100 ms before the target were classified as "early errors," whereas trials in which the cursor arrived more than 100 ms after the target were classified as "late errors." Given the criteria above, the task proved to be difficult for the subjects. Only 41.3% (1080 out of 2614) of the movements were successful, whereas the remaining 58.7% were temporal (i.e., early or late) errors. A large majority of the early errors occurred in trials with decelerating targets, and their percentage tended to increase with longer target motion times. In contrast, late errors occurred in relation to all three target acceleration types, and their percentage tended to decrease with longer target motion times. Three models of movement initiation were investigated. First, the threshold-distance model, originally proposed for optokinetic eye movements to constant-velocity visual stimuli, maintains that response time is composed of two parts, a constant processing time and the time required for the stimulus to travel a threshold distance. This model only partially fit our data. Second, the threshold-tau model, originally proposed as a strategy for movement initiation, assumes that the subject uses the first-order estimate of time-to-contact (tau) to determine when to initiate the interception movement. Similar to the threshold distance model, the threshold-tau model only partially fit the data. Finally, a dual-strategy model was developed which allowed for the adoption of either of the two strategies for movement initiation; namely, a strategy based on the threshold-distance model ("reactive" strategy) and another based on the threshold-tau model ("predictive" strategy). This model provided a good fit to the data. In fact, individual subjects preferred to use one or the other strategy. This preference was allowed to be manifested at long target motion times, whereas shorter target motion times (i.e., 0.5 s and 0.8 s) forced the subjects to use only the reactive strategy.

Sequential activity in human motor areas during a delayed cued finger movement task studied by time-resolved fMRI

Activity in the human primary motor cortex, the premotor cortex and the supplementary motor area during a delayed cued finger movement task was measured by time-resolved functional magnetic resonance imaging. Activity during movement preparation can be resolved from activity during movement execution in a single trial. All three areas were active during both movement preparation and movement execution. Activity in the primary motor cortex was considerably weaker during movement preparation than during movement execution; in the premotor cortex and the supplementary motor area, activity was of similar intensity during both periods. These observations are consistent with results from single neuronal recording studies in primates.

Box-Jenkins intervention analysis of functional magnetic resonance imaging data

Data obtained in functional magnetic resonance imaging (fMRI) typically form a time series of MRI signal collected over a period of time at constant intervals. These data are potentially autocorrelated and may contain time trends. Therefore, any assessment of significant changes in the MRI signal over a certain period of time requires the use of specific statistical techniques. For that purpose we used the Box-Jenkins intervention time series analysis to determine brain activation during task performance. We found that for a substantial number of pixels there was significant autocorrelation and, occasionally, time trends. In these cases, use of the classical t-test would not be appropriate. In contrast, Box-Jenkins intervention analysis, by detrending the series and by explicitly taking into account the correlation structure, provides a more appropriate method to determine the presence of significant activation during the task period in fMRI data.

A simulated actuator driven by motor cortical signals

One problem in motor control concerns the mechanism whereby the central nervous system translates the motor cortical command encoded in cell activity into a coordinated contraction of limb muscles to generate a desired motor output. This problem is closely related to the design of adaptive systems that transform neuronal signals chronically recorded from the motor cortex into the physiologically appropriate motor output of multijoint prosthetic limbs. In this study we demonstrated how this transformation can be carried out by an artificial neural network using as command signals the actual impulse activity obtained from recordings in the motor cortex of monkeys during the performance of a task that required the exertion of force in different directions. The network receives experimentally measured brain signals and recodes them into motor actions of a simulated actuator that mimics the primate arm. The actuator responds to the motor cortical commands with surprising fidelity, generating forces in close quantitative agreement with those exerted by trained monkeys, in both the temporal and spatial domains. Moreover, we show that the time-varying motor output may be controlled by the impulse activity of as few as 15 motor cortical cells. These results outline a potentially implementable computation scheme that utilizes raw neuronal signals to drive artificial mechanical systems.

Arm movements in monkeys: behavior and neurophysiology

Reaching to objects of interest is very common in the behavioral repertoire of primates. Monkeys possess keen binocular vision and make graceful and accurate arm movements. This review focuses on behavioral and neurophysiological aspects of eye-hand coordination in behaving monkeys, including neural coding mechanisms at the single cell level and in neuronal populations. The results of these studies have converged to a common behavioral-neurophysiological ground and provided a springboard for studies of brain mechanisms underlying motor cognitive function.

Intercepting real and path-guided apparent motion targets

Human subjects were instructed to intercept with a cursor real and apparent motion targets presented on a computer screen. Targets traveled counterclockwise (CCW) in a circle at one of five angular velocities (180, 300, 420, 480 and 540 deg/s), either smoothly (real motion) or in path-guided apparent motion. Subjects operated a computer mouse and were instructed to intercept targets at the 12 o'clock position; there were no constraints on when to initiate the response, which was a movement from the center of the screen towards and past 12 o'clock. We found the following: (a) for both motion conditions and all target velocities, subjects were late in intercepting the target, especially at higher target velocities; (b) for both motion conditions, the directional variability of the response increased as a linear function of the target velocity; (c) the directional variability of the response was systematically higher for the apparent than the real motion condition; there was no significant interaction between target velocity and target motion type; (d) the response time did not vary significantly with velocity, but was consistently longer for apparent than real motion targets; (e) the movement time was very similar for different target velocities; and (f) the moment of initiation of the interception movement was delayed appreciably at higher target velocities, relative to that dictated for perfect interception at a given target velocity. This delay was greater for the apparent motion target. These results demonstrated the following: (a) for both target motion conditions, interception was not fully predictive but lagged the target in spite of the constant target velocity and the unconstrained time allowed for initiating the interception movement; (b) subjects can intercept an apparent motion target but, compared with real motion, the performance is somewhat degraded overall; (c) the similarities in performance between the two target motion conditions, and the fact that target velocity influenced performance in a similar fashion, suggest that the motor system can access the visual information provided by the moving target; and (d) since movement time was similar for different target velocities, the strategy for interception relied on controlling the moment of initiation of the interception movement. This was successful for low target velocities but became unsuccessful at higher target velocities.

On the relations between single cell activity in the motor cortex and the direction and magnitude of three-dimensional static isometric force

We examined the relations between the steady-state frequency of discharge of cells in the arm area of the motor cortex of the monkey and the direction and magnitude of the three-dimensional static force exerted by the arm on an isometric manipulandum. Data were analyzed from two monkeys (n = 188 cells) using stepwise multiple linear regression. In 154 of 188 (81.9%) cells the regression model was statistically significant (P < 0.05). In 121 of 154 (78.6%) cells the direction but not the magnitude of force had a statistically significant effect on cell activity; in 11 of 154 (7.1%) cells only the magnitude effect was significant; and in 22 of 154 (14.3%) cells both the direction and magnitude effects were significant. The same analysis was used to assess the effect of the direction and magnitude of force on the electromyographic activity of 9 muscles of the arm and shoulder girdle. The regression model was statistically significant. For all the muscles studied in 4 of 9 (44.4%) muscles only the direction effect was significant whereas in the remaining 5 of 9 (55.6%) muscles both the direction and the magnitude were significant. No muscle studied showed a significant effect of force magnitude alone. These differences in the frequency of occurrence of directional and magnitude effects between cells and muscles were statistically significant (P < 0.005, chi 2 test). These findings underscore the fundamental importance of the direction of force in space for both motor cortical cells and proximal muscles and underline the differential relations of the cells and muscles to the direction and magnitude of force. These results indicate that the specification of the magnitude of three-dimensional force is embedded within the directional signal; this combined direction+magnitude effect was 3.9 times more prevalent in the muscles than in the cells studied. In contrast, the pure directional effect was 1.8 times more prevalent in the cells than in the muscles studied. This suggests that the direction of force can be controlled independently of its magnitude and that this direction signal is especially prominent in the motor cortex.

Neural computations underlying the exertion of force: a model

We have developed a model that simulates possible mechanisms by which supraspinal neuronal signals coding forces could converge in the spinal cord and provide an ongoing integrated signal to the motoneuronal pools whose activation results in the exertion of force. The model consists of a three-layered neural network connected to a two-joint-six-muscle model of the arm. The network layers represent supraspinal populations, spinal cord interneurons, and motoneuronal pools. We propose an approach to train the network so that, after the synaptic connections between the layers are adjusted, the performance of the model is consistent with experimental data obtained on different organisms using different experimental paradigms: the stiffness characteristics of human arm; the structure of force fields generated by the stimulation of the frog's spinal cord; and a correlation between motor cortical activity and force exerted by monkey against an immovable object. The model predicts a specific pattern of connections between supraspinal populations coding forces and spinal cord interneurons: the weight of connection should be correlated with directional preference of interconnected units. Finally, our simulations demonstrate that the force generated by the sum of neural signals can be nearly equal to the vector sum of forces generated by each signal independently, in spite of the complex nonlinearities intervening between supraspinal commands and forces exerted by the arm in response to these commands.

Modeling motor cortical operations by an attractor network of stochastic neurons

Understanding the neural computations performed by the motor cortex requires biologically plausible models that account for cell discharge patterns revealed by neurophysiological recordings. In the present study the motor cortical activity underlying movement generation is modeled as the dynamic evolution of a large fully recurrent network of stochastic spiking neurons with noise superimposed on the synaptic transmission. We show that neural representations of the learned movement trajectories can be stored in the connectivity matrix in such a way that, when activated, a particular trajectory evolves in time as a dynamic attractor of the system while individual neurons fire irregularly with large variability in their interspike intervals. Moreover, the encoding of trajectories as attractors ensures high stability of the ensemble dynamics in the presence of synaptic noise. In agreement with neurophysiological findings, the suggested model can provide a wide repertoire of specific motor behaviors, whereas the number of specialized cells and specific connections may be negligibly small if compared with the whole population engaged in trajectory retrieving. To examine the applicability of the model we study quantitatively the relationship between local geometrical and kinematic characteristics of the trajectories generated by the network. The relationship obtained as a result of simulations is close to the '2/3 power law' established by psychophysical and neurophysiological studies.

On the translation of directional motor cortical commands to activation of muscles via spinal interneuronal systems

I discuss in this paper some of the neural mechanisms by which directional motor cortical commands could be potentially translated into multi-muscle activations to generate a directed force (and initial movement) in space. Specifically, I review the results of recent studies in the motor cortex of monkeys and the spinal cord of the frog, and propose a possible mechanism by which these results could be formally connected. It is suggested that spinal mechanisms of the kind described in the spinal frog could serve as substrates for the operation of directionally tuned motor cortical activity to produce an appropriately directed motor output by the limb.

Quantitative relations between parietal activation and performance in mental rotation

The quantitative relationships between functional activation of the superior parietal lobule (SPL) and performance in the Shepard-Metzler mental rotation task were investigated in 16 human subjects using magnetic resonance (MR) imaging at high field (4 Tesla). Subjects were shown pairs of perspective drawings of three-dimensional objects and asked to judge whether they were the same or mirror images. Increased SPL activation was associated with a higher proportion of errors in performance. The increase in errors, and the concomitant increase in SPL activation, could be due to an increased difficulty in, and therefore increased demands for, information processing at several stages involved in making a decision, including encoding of the visual images shown, mentally rotating them, and judging whether they are the same or mirror images.

The mental and the neural: psychological and neural studies of mental rotation and memory scanning

In this article we review studies pertaining to psychophysical measurements and neural correlates of tasks requiring the processing of directional information in spatial motor tasks. The results of psychological studies in human subjects indicate that time-consuming processes underlie mental rotation and memory scanning. Other studies have suggested that these processes may rely on different basic mechanisms. A direct insight into their neural mechanisms was obtained analyzing the activity of single cells and neuronal populations in the brain of behaving monkeys performing the same tasks. These studies revealed the nature of the neural processes underlying mental rotation and memory scanning and confirmed their different nature.

Current issues in directional motor control

Many studies during the past 15 years have shown that the direction of motor output (movement or isometric force) is an important factor for neuronal activity in the motor cortex, both at the level of single cells and at the level of neuronal populations. Recent studies have investigated several new aspects of this problem including the effect of posture, the relations to time-varying movement parameters (for example, position, velocity and acceleration) and the cortical representation of memorized simple movements and complex-movement trajectories. Furthermore, the neural correlates of directional operations, such as mental rotation and memory-scanning of visuomotor directions, have also been investigated. In addition, neural networks have been used to model dynamic, time-varying, spatial motor trajectories.

System for projection of a three-dimensional, moving virtual target for studies of eye-hand coordination

Eye-hand tracking of moving visual objects in three-dimensional (3D) space is common in the behavioral repertoire of primates. However, behavioral and/or neurophysiological studies of this function are lacking mainly because devices do not exist that allow its investigation. We describe a device by which a spot of light can be presented in the immediate extrapersonal space of a subject and can be moved in various trajectories in 3D space. The target is a real image of a circular aperture produced by a system consisting of a light source, aperture, filters, several lenses and fold mirrors, and a large concave mirror to focus the final real image. Rapid, computer-controlled movement of the image is obtained by tilting a gimbal-mounted guide mirror (for x and y motion) and by translating a lens (for motion in the z direction). A second configuration of the system allows movement of a 3D image in the 3D space. Hand motion is monitored by means of a sonic, 3D, position-measurement system.

Motor cortical activity in a context-recall task

A monkey was trained to respond on the basis of the serial position of a test stimulus in a sequence. First, three stimuli were presented successively on a circle. Then one of them (except the last) changed color (test stimulus) and served as the go signal: The monkey was required to produce a motor response in the direction of the stimulus that followed the test stimulus. When the test stimulus was the second in the sequence, there was a change in motor cortical activity from a pattern reflecting the direction of this stimulus to the pattern associated with the direction of the motor response. This change was abrupt, occurred 100 to 150 milliseconds after the go signal, and was evident both in the activity of single cells and in the time-varying neuronal population vector. These findings identify the neural correlates of a switching process that is different from a mental rotation described previously.

Behavioral neurophysiology of the motor cortex

The study of the motor cortex in behaving monkeys during the past 20 years has provided important information on the brain mechanisms underlying motor control. With respect to reaching movements in space, several aspects of motor cortical function concerning the specification of the direction of movement have now been elucidated and are reviewed in this article. The activity of single cells in the motor cortex is broadly tuned with respect to the direction of reaching, so that the discharge rate is highest with movements in a preferred direction and decreases progressively with movements made in directions more and more away from the preferred one. Thus the neural command for the direction of reaching can be regarded as an ensemble of cell vectors, with each vector pointing in the cell's preferred direction and having a length proportional to the change in cell activity. The outcome of this population code can be visualized as a vector that points in the direction of the upcoming movement during the reaction time, during an instructed delay period, and during a memorized delay period. Moreover, when a mental transformation is required for the generation of a reaching movement in a different direction from a reference direction, the population vector provides a direct insight into the nature of the cognitive process by which the required transformation is achieved.

Movement parameters and neural activity in motor cortex and area 5

The relations of ongoing single-cell activity in the arm area of the motor cortex and area 5 to parameters of evolving arm movements in two-dimensional (2D) space were investigated. A multiple linear regression model was used in which the ongoing impulse activity of cells at time t + tau was expressed as a function of the (X, Y) components of the target direction and of position, velocity, and acceleration of the hand at time t, where tau was a time shift (-200 to +200 msec). Analysis was done on 290 cells in the motor cortex and 207 cells in area 5. The time shift at which the highest coefficient of determination (R2) was observed was determined and the statistical significance of the model tested. The median R2 was 0.581 and 0.530 for motor cortex and area 5, respectively. The median shift at which the highest R2 was observed was -90 and +30 msec for motor cortex and area 5, respectively. For most cells statistically significant relations were observed to all four parameters tested; most prominent were the relations to target direction and least prominent those to acceleration.