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R A Andersen

Publications and source records attributed to R A Andersen.

At least 37 records · Page 2Linked to original sources
PubMed2000

Eye position signal modulates a human parietal pointing region during memory-guided movements.

Using functional magnetic resonance imaging, we examined the signal in parietal regions that were selectively activated during delayed pointing to flashed visual targets and determined whether this signal was dependent on the fixation position of the eyes. Delayed pointing activated a bilateral parietal area in the intraparietal sulcus (rIPS), rostral/anterior to areas activated by saccades. During right-hand pointing to centrally located targets, the left rIPS region showed a significant increase in activation when the eye position was rightward compared with leftward. As expected, activation in motor cortex showed no modulation when only eye position changed. During pointing to retinotopically identical targets, the left rIPS region again showed a significant increased signal when the eye position was rightward compared with leftward. Conversely, when pointing with the left arm, the right rIPS showed an increase in signal when eye position was leftward compared with rightward. The results suggest that the human parietal hand/arm movement region (rIPS), like monkey parietal areas (Andersen et al., 1985), exhibits an eye position modulation of its activity; modulation that may be used to transform the coordinates of the retinotopically coded target position into a motor error command appropriate for the wrist.

Adult↗
PubMed2000

Intention-related activity in the posterior parietal cortex: a review.

Over the last few years it is becoming increasingly apparent that an important role of the posterior parietal cortex is to process sensory information for the purpose of planning actions. We review studies showing that a large component of neural activity in area LIP is related to planning saccades and activity in a nearby parietal reach region (PRR) to reaches. This intention related activity dominates the delay period in delayed movement tasks, and also comprises a substantial component of the transient response. These findings, along with additional anatomical and physiological evidence, lends support to the idea that different cortical areas within the PPC represent plans for different actions. We also found strong modulation of activity when movement plans were changed without changes in the locus of attention. This result suggests that PPC, which has been postulated to play a role in shifting attention, may also play a role in changing movement intentions. Sensory related activity was also present in these tasks and may be related to the stimulus or to attention. These experiments show that there are intention and sensory related activities in the PPC consistent with its proposed role in sensory-motor transformations. These studies also show that care must be taken to measure intention-related signals and not assume that all task dependent modulation in the PPC reflects attention.

Movement↗
PubMed2000

Correlations between cadmium treatment, oxygen uptake and metallothionein response in liver and kidney from two mice strains.

Oxygen uptake was determined in liver and kidney slices from Balb/C and C3H/HeJ mice at various time points related to a 4-day cadmium (Cd) i.p. injection regime, the latter strain being not only the higher metallothionein (MT) responder, but also more Cd sensitive. In both strains Cd injections caused an increase in O2 uptake in the tissues, being more expressed in the higher MT responsive C3H/HeJ strain. An ability for fast, additional increase of O2 uptake in response to subsequent Cd exposure was also induced during the injection regime. Maximal obtainable MT levels in the two strains were estimated by forced overwhelming pneumococcal infection challenge, giving induction values above those obtained for the Cd injections in the Balb/C strain and equal to those obtained for Cd in the more responsive C3H/HeJ strain. Together with direct defendants against harmful oxygen radicals, MT may act complementary as a radical scavenger and by Cd inactivation, but to a higher degree in the C3H/HeJ compared to the Balb/C strain.

Animals↗
PubMed2000

Reaches to sounds encoded in an eye-centered reference frame.

A recent hypothesis suggests that neurons in the lateral intraparietal area (LIP) and the parietal reach region (PRR) encode movement plans in a common eye-centered reference frame. To test this hypothesis further, we examined how PRR neurons encode reach plans to auditory stimuli. We found that PRR activity was affected by eye and initial hand position. Population analyses, however, indicated that PRR neurons were affected more strongly by eye position than by initial hand position. These eye position effects were appropriate to maintain coding in eye coordinates. Indeed, a significant population of PRR neurons encoded reaches to auditory stimuli in an eye-centered reference frame. These results extend the hypothesis that, regardless of the modality of the sensory input or the eventual action, PRR and LIP neurons represent movement plans in a common, eye-centered representation.

Acoustic Stimulation↗
PubMed2000

Exercise and older patients: guidelines for the clinician.

Sedentary persons who improve their physical fitness are less likely to die of all causes and of cardiovascular disease than are those who remain sedentary. There now exists a wealth of data demonstrating that physical activity and exercise may ameliorate disease and delay decline in function in the geriatric population. We review evidence that exercise can improve body composition, diminish falls, increase strength, reduce depression, reduce arthritis pain, reduce risks for diabetes and coronary artery disease, and improve longevity. However, many healthcare professionals do not feel adequately prepared to design and prescribe exercise programs for their patients. This review provides a basic overview of the benefits of exercise in the geriatric population and guidelines indicating how to prescribe and facilitate adherence to an exercise program. Healthcare providers are strongly encouraged to promote a less sedentary life style for their older patients, which may augment quality of life in these older individuals.

Accidental Falls↗
PubMed2000

Saccade-related activity in the parietal reach region.

In previous experiments, we showed that cells in the parietal reach region (PRR) in monkey posterior parietal cortex code intended reaching movements in an eye-centered frame of reference. These cells are more active when an arm compared with an eye movement is being planned. Despite this clear preference for arm movements, we now report that PRR neurons also fire around the time of a saccade. Of 206 cells tested, 29% had perisaccadic activity in a delayed-saccade task. Two findings indicate that saccade-related activity does not reflect saccade planning or execution. First, activity is often peri- or postsaccadic but seldom presaccadic. Second, cells with saccade-related activity were no more likely to show strong saccadic delay period activity than cells without saccade-related activity. These findings indicate that PRR cells do not take part in saccade planning. Instead, the saccade-related activity in PRR may reflect cross-coupling between reach and saccade pathways that may be used to facilitate eye-hand coordination. Alternatively, saccade-related activity may reflect eye position information that could be used to maintain an eye-centered representation of intended reach targets across eye movements.

Animals↗
PubMed2000

Memory activity of LIP neurons for sequential eye movements simulated with neural networks.

Many neurons in macaque lateral intraparietal cortex (LIP) maintain elevated activity induced by visual or auditory targets during tasks in which monkeys are required to withhold one or more planned eye movements. We studied the mechanisms for such memory activity with neural network modeling. Recurrent connections among simulated LIP neurons were used to model memory responses of LIP neurons. The connection weights were computed using an optimization procedure to produce desired outputs in memory-saccade tasks. One constraint for the training process is the "single-purpose" rule, which mimics the fact that once LIP neurons hold the memory activity of a saccade, they are insensitive to further stimuli until the motor action is completed. After training, excitatory connections were developed between units with similar preferred saccade directions, while inhibitory connections were formed between units with dissimilar directions. This "push-pull" mechanism enables the network to encode the next intended eye movement and is essential for programming sequential saccades. In simulating double saccades, the push-pull connections locked the on-going activity in the network for the first saccade until the saccade was made, then a new population of units became active to prepare for the second saccade. The simulated LIP neurons exhibited sensory responses and memory activities similar to those recorded in LIP neurons. We propose that push-pull recurrent connections might be the basic structure mediating the memory activity of area LIP in planning sequential eye movements.

Animals↗
PubMed2000

Models of the posterior parietal cortex which perform multimodal integration and represent space in several coordinate frames.

Many neurons in the posterior-parietal cortex (PPC) have saccadic responses to visual and auditory targets. The responses are modulated by eye position and head position. These findings suggest that PPC integrates multisensory inputs and may provide information about saccadic targets represented in different coordinate frames. In addition to an eyecentered output representation, PPC may also project to brain areas which contain head-centered and body-centered representations of the space. In this report, possible coordinate transformations in PPC were examined by comparing several sets of models of PPC, each having different representations in the output layer: (i) an eye-centered map only; (ii) a head-centered map only; (iii) an eye-centered map and a head-centered map; and (iv) an eye-centered map, a head-centered map, and a body-centered map. These output maps correctly encoded saccades to visual and auditory targets through training. The units in the hidden layers of the models exhibited the following properties: (1) The units had gain fields (GFs) for eye position, and also for head position if the model had a body-centered output representation; (2) As the result of the GF and the nonlinear activation function of the units, the hidden layers often employed "intermediate" coding, e.g., the hidden units coded targets partially in eye-centered coordinates and, partially, in head-centered coordinates; (3) Different types of coordinate transformations in these models were carried out by different relationships between the receptive fields (RFs) and the GFs of the hidden units; and (4) The properties of PPC neurons are in better accordance with the hidden units of the models that had multiple-output representations than the models that had only one single-output representation. In conclusion, the results show that the GF is an effective mechanism for performing coordinate transformations. The models also suggest that neurons with intermediate coding are to be expected in the process of coordinate transformations.

Algorithms↗
PubMed1999

Reach plans in eye-centered coordinates.

The neural events associated with visually guided reaching begin with an image on the retina and end with impulses to the muscles. In between, a reaching plan is formed. This plan could be in the coordinates of the arm, specifying the direction and amplitude of the movement, or it could be in the coordinates of the eye because visual information is initially gathered in this reference frame. In a reach-planning area of the posterior parietal cortex, neural activity was found to be more consistent with an eye-centered than an arm-centered coding of reach targets. Coding of arm movements in an eye-centered reference frame is advantageous because obstacles that affect planning as well as errors in reaching are registered in this reference frame. Also, eye movements are planned in eye coordinates, and the use of similar coordinates for reaching may facilitate hand-eye coordination.

Animals↗
PubMed1999

The contributions of vestibular signals to the representations of space in the posterior parietal cortex.

Vestibular signals play an important role in spatial orientation, perception of object location, and control of self-motion. Prior physiological research on vestibular information processing has focused on brainstem mechanisms; relatively little is known about the processing of vestibular information at the level of the cerebral cortex. Recent electrophysiological experiments examining the use of vestibular canal signals in two different perceptual tasks are described: computation of self motion and localization of visual stimuli in a world-centered reference frame. These two perceptual functions are mediated by different parts of the posterior parietal cortex, the former in the dorsal aspect of the medial superior temporal area (MSTd) and the latter in area 7a.

Animals↗
PubMed1999

Phylogenetic analysis of the SSU rRNA from members of the Chrysophyceae.

The nucleotide sequence for the nuclear-encoded small subunit ribosomal RNA gene (SSU rRNA) was determined for 24 species of the Chrysophyceae sensu stricto. These sequences were aligned, using primary and secondary structure, with nine previously published sequences for the Chrysophyceae, 14 for the Synurophyceae, and five for the Eustigmatophyceae (outgroup). Data analyses were the substitution rate calibration distance method using neighbor-joining (TREECON), Kimura 2-parameter neighbor-joining method (PAUP) and the maximum parsimony method (PAUP, PHYLIP). Trees from the analyses were largely congruent, but bootstrap support was weak at many nodes. The analyses recovered clades of uniflagellate and biflagellate organisms associated with current higher level taxonomy (e.g., subclass, order). The genus Ochromonas was polyphyletic, and O. tuberculata in particular was distantly related to the other Ochromonas species in the analysis. The family Paraphysomonadaceae occupied a basal position in three of four analyses. The class Synurophyceae appeared to be embedded within the Chrysophyceae, but bootstrap support was weak (< 50%) in all analyses except the PHYLIP parsimony analysis (= 81%). It was considered premature to place the Synurophyceae back into the Chrysophyceae based upon the analysis of one gene, especially given the ultrastructural and pigment differences between the two groups, but the relationship of these two groups deserves further study.

Biological Evolution↗
PubMed1999

Effect of reversible inactivation of macaque lateral intraparietal area on visual and memory saccades.

Previous studies from our laboratory identified a parietal eye field in the primate lateral intraparietal sulcus, the lateral intraparietal area (area LIP). Here we further explore the role of area LIP in processing saccadic eye movements by observing the effects of reversible inactivation of this area. One to 2 microl of muscimol (8 mg/ml) were injected at locations where saccade-related activities were recorded for each lesion experiment. After the muscimol injection we observed in two macaque monkeys consistent effects on both the metrics and dynamics of saccadic eye movements at many injection sites. These effects usually took place within 10-30 min and disappeared after 5-6 h in most cases and certainly when tested the next day. After muscimol injection memory saccades directed toward the contralesional and upper space became hypometric, and in one monkey those to the ipsilesional space were slightly but significantly hypermetric. In some cases, the scatter of the end points of memory saccades was also increased. On the other hand, the metrics of visual saccades remained relatively intact. Latency for both visual and memory saccades toward the contralesional space was increased and in many cases displayed a higher variance after muscimol lesion. At many injection sites we also observed an increase of latency for visual and memory saccades toward the upper space. The peak velocities for memory saccades toward the contralesional space were decreased after muscimol injection. The peak velocities of visual saccades were not significantly different from those of the controls. The duration of saccadic eye movements either to the ipsilesional or contralesional space remained relatively the same for both visual and memory saccades. Overall these results demonstrated that we were able to selectively inactivate area LIP and observe effects on saccadic eye movements. Together with our previous recording studies these results futher support the view that area LIP plays a direct role in processing incoming sensory information to program saccadic eye movements. The results are consistent with our unit recording data and microstimulation studies, which suggest that area LIP represents contralateral space and also has a bias for the upper visual field.

Animals↗
PubMed1999

Influence of gaze rotation on the visual response of primate MSTd neurons.

When we move forward, the visual image on our retina expands. Humans rely on the focus, or center, of this expansion to estimate their direction of heading and, as long as the eyes are still, the retinal focus corresponds to the heading. However, smooth rotation of the eyes adds nearly uniform visual motion to the expanding retinal image and causes a displacement of the retinal focus. In spite of this, humans accurately judge their heading during pursuit eye movements and during active, smooth head rotations even though the retinal focus no longer corresponds to the heading. Recent studies in macaque suggest that correction for pursuit may occur in the dorsal aspect of the medial superior temporal area (MSTd) because these neurons are tuned to the retinal position of the focus and they modify their tuning during pursuit to compensate partially for the focus shift. However, the question remains whether these neurons also shift focus tuning to compensate for smooth head rotations that commonly occur during gaze tracking. To investigate this question, we recorded from 80 MSTd neurons while monkeys tracked a visual target either by pursuing with their eyes or by vestibulo-ocular reflex cancellation (VORC; whole-body rotation with eyes fixed in head and head fixed on body). VORC is a passive, smooth head rotation condition that selectively activates the vestibular canals. We found that neurons shift their focus tuning in a similar way whether focus displacement is caused by pursuit or by VORC. Across the population, compensation averaged 88 and 77% during pursuit and VORC, respectively (tuning shift divided by the retinal focus to true heading difference). Moreover the degree of compensation during pursuit and VORC was correlated in individual cells (P < 0.001). Finally neurons that did not compensate appreciably tended to be gain-modulated during pursuit and VORC and may constitute an intermediate stage in the compensation process. These results indicate that many MSTd cells compensate for general gaze rotation, whether produced by eye-in-head or head-in-world rotation, and further implicate MSTd as a critical stage in the computation of heading. Interestingly vestibular cues present during VORC allow many cells to compensate even though humans do not accurately judge their heading in this condition. This suggests that MSTd may use vestibular information to create a compensated heading representation within at least a subpopulation of cells, which is accessed perceptually only when additional cues related to active head rotations are also present.

Action Potentials↗
PubMed1999

Responses to auditory stimuli in macaque lateral intraparietal area. I. Effects of training.

The lateral intraparietal area (LIP) of macaques has been considered unresponsive to auditory stimulation. Recent reports, however, indicate that neurons in this area respond to auditory stimuli in the context of an auditory-saccade task. Is this difference in auditory responsiveness of LIP due to auditory-saccade training? To address this issue, LIP responses in two monkeys were recorded at two different times: before and after auditory-saccade training. Before auditory-saccade training, the animals had never been trained on any auditory task, but had been trained on visual tasks. In both sets of experiments, activity of LIP neurons was recorded while auditory and visual stimuli were presented and the animals were fixating. Before training, 172 LIP neurons were recorded. Among these, the number of cells responding to auditory stimuli did not reach significance, whereas about one-half of the cells responded to visual stimuli. An information theory analysis confirmed that no information about auditory stimulus location was available in LIP neurons in the experiments before training. After training, activity from 160 cells was recorded. These experiments showed that 12% of cells in area LIP responded to auditory stimuli, whereas the proportion of cells responding to visual stimuli remained about the same as before training. The information theory analysis confirmed that, after training, information about auditory stimulus location was available in LIP neurons. Auditory-saccade training therefore generated responsiveness to auditory stimuli de novo in LIP neurons. Thus some LIP cells become active for auditory stimuli in a passive fixation task, once the animals have learned that these stimuli are important for oculomotor behavior.

Acoustic Stimulation↗
PubMed1999

Responses to auditory stimuli in macaque lateral intraparietal area. II. Behavioral modulation.

The lateral intraparietal area (LIP), a region of posterior parietal cortex, was once thought to be unresponsive to auditory stimulation. However, recent reports have indicated that neurons in area LIP respond to auditory stimuli during an auditory-saccade task. To what extent are auditory responses in area LIP dependent on the performance of an auditory-saccade task? To address this question, recordings were made from 160 LIP neurons in two monkeys while the animals performed auditory and visual memory-saccade and fixation tasks. Responses to auditory stimuli were significantly stronger during the memory-saccade task than during the fixation task, whereas responses to visual stimuli were not. Moreover, neurons responsive to auditory stimuli tended also to be visually responsive and to exhibit delay or saccade activity in the memory-saccade task. These results indicate that, in general, auditory responses in area LIP are modulated by behavioral context, are associated with visual responses, and are predictive of delay or saccade activity. Responses to auditory stimuli in area LIP may therefore be best interpreted as supramodal responses, and similar in nature to the delay activity, rather than as modality-specific sensory responses. The apparent link between auditory activity and oculomotor behavior suggests that the behavioral modulation of responses to auditory stimuli in area LIP reflects the selection of auditory stimuli as targets for eye movements.

Acoustic Stimulation↗
PubMed1998

Center-surround antagonism based on disparity in primate area MT.

Most neurons in primate visual area MT have a large, modulatory region surrounding their classically defined receptive field, or center. The velocity tuning of this "surround" is generally antagonistic to the center, making it potentially useful for detecting image discontinuities on the basis of differential motion. Because classical MT receptive fields are also disparity-selective, one might expect to find disparity-based surround antagonism as well; this would provide additional information about image discontinuities. However, the effects of disparity in the MT surround have not been studied previously. We measured single-neuron responses to variable-disparity moving patterns in the MT surround while holding a central moving pattern at a fixed disparity. Of the 130 neurons tested, 84% exhibited a modulatory surround, and in 52% of these, responses were significantly affected by disparity in the surround. In most cases, disparity effects in the surround were antagonistic to the center; that is, neurons were generally suppressed when center and surround stimuli had the same disparity, with decreasing suppression as the center and surround stimuli became separated in depth. Also, the effects of disparity and direction were mainly additive; i.e., disparity effects were generally independent of direction, and vice versa. These results suggest that the MT center-surround apparatus provides information about image discontinuities, not only on the basis of velocity differences but on the basis of depth differences as well. This supports the hypothesis that MT surrounds have a role in image segmentation.

Analysis of Variance↗