On the timing of reference frames for action control

This study investigated the time course and automaticity of spatial coding of visual targets for pointing movements. To provide an allocentric reference, placeholders appeared on a touch screen either 500 ms before target onset, or simultaneously with target onset, or at movement onset, or not at all (baseline). With both blocked and randomized placeholder timing, movements to the most distant targets were only facilitated when placeholders were visible before movement onset. This result suggests that allocentric target coding is most useful during movement planning and that this visuo-spatial coding mechanism is not sensitive to strategic effects.     

Localization of the plane of regard in space

When we fixate an object in space, the rotation centers of the eyes, together with the object, define a plane of regard. People perceive the elevation of objects relative to this plane accurately, irrespective of eye or head orientation (Poljac et al. (2004) Vision Res, in press). Yet, to create a correct representation of objects in space, the orientation of the plane of regard in space is required. Subjects pointed along an eccentric vertical line on a touch screen to the location where their plane of regard intersected the touch screen positioned on their right. The distance of the vertical line to the subjects eyes varied from 10 to 40 cm. Subjects were sitting upright and fixating one of the nine randomly presented directions ranging from 20° left and down to 20° right and up relative to their straight ahead. The eccentricity of fixations relative to the pointing location varied by up to 40°. Subjects underestimated the elevation of their plane of regard (on average by 3.69 cm, SD=1.44 cm), regardless of the fixation direction or pointing distance. However, when the targets were shown on a display mounted in a table, to provide support of the subjects hand throughout the trial, subjects pointed accurately (average error 0.3 cm, SD=0.8 cm). In addition, head tilt 20° to the left or right did not cause any change in accuracy. The bias observed in the first task could be caused by maintained tonus in arm muscles when the arm is raised, that might interfere with the transformation from visual to motor signals needed to perform the pointing movement. We conclude that the plane of regard is correctly localized in space. This may be a good starting point for representing objects in head-centric coordinates.     

Multiple frames of reference for pointing to a remembered target

Pointing with an unseen hand to a visual target that disappears prior to movement requires maintaining a memory representation about the target location. The target location can be transformed either into a hand-centered frame of reference during target presentation and remembered under that form, or remembered in terms of retinal and extra-retinal cues and transformed into a body-centered frame of reference before movement initiation. The main goal of the present study was to investigate whether the target is stored in memory in an eye-centered frame, a hand-centered frame or in both frames of reference concomitantly. The task was to locate, memorize, and point to a target in a dark environment. Hand movement was not visible. During the recall delay, participants were asked to move their hand or their eyes in order to disrupt the memory representation of the target. Movement of the eyes during the recall delay was expected to disrupt an eye-centered memory representation whereas movement of the hand was expected to disrupt a hand-centered memory representation by increasing movement variability to the target. Variability of movement amplitude and direction was examined. Results showed that participants were more variable on the directional component of the movement when required to move their hand during recall delay. On the contrary, moving the eyes caused an increase in variability only in the amplitude component of the pointing movement. Taken together, these results suggest that the direction of the movement is coded and remembered in a frame of reference linked to the arm, whereas the amplitude of the movement is remembered in an eye-centered frame of reference.     

Interactions of different body parts in peripersonal space: how vision of the foot influences tactile perception at the hand

The body schema, a constantly updated representation of the body and its parts, has been suggested to emerge from body part-specific representations which integrate tactile, visual, and proprioceptive information about the identity and posture of the body. Studies using different approaches have provided evidence for a distinct representation of the visual space ~30 cm around the upper body, and predominantly the hands, termed the peripersonal space. In humans, peripersonal space representations have often been investigated with a visual–tactile crossmodal congruency task. We used this task to test if a representation of peripersonal space exists also around the feet, and to explore possible interactions of peripersonal space representations of different body parts. In Experiment 1, tactile stimuli to the hands and feet were judged according to their elevation while visual distractors presented near the same limbs had to be ignored. Crossmodal congruency effects did not differ between the two types of limbs, suggesting a representation of peripersonal space also around the feet. In Experiment 2, tactile stimuli were presented to the hands, and visual distractors were flashed either near the participant’s foot, near a fake foot, or in distant space. Crossmodal congruency effects were larger in the real foot condition than in the two other conditions, indicating interactions between the peripersonal space representations of foot and hand. Furthermore, results of all three conditions showed that vision of the stimulated body part, compared to only proprioceptive input about its location, strongly influences crossmodal interactions for tactile perception, affirming the central role of vision in the construction of the body schema.

Viewer-centered and body-centered frames of reference in direct visuomotor transformations

It has been hypothesized that the end-point position of reaching may be specified in an egocentric frame of reference. In most previous studies, however, reaching was toward a memorized target, rather than an actual target. Thus, the role played by sensorimotor transformation could not be disassociated from the role played by storage in short-term memory. In the present study the direct process of sensorimotor transformation was investigated in reaching toward continuously visible targets that need not be stored in memory. A virtual reality system was used to present visual targets in different three-dimensional (3D) locations in two different tasks, one with visual feedback of the hand and arm position (Seen Hand) and the other without such feedback (Unseen Hand). In the Seen Hand task, the axes of maximum variability and of maximum contraction converge toward the mid-point between the eyes. In the Unseen Hand task only the maximum contraction correlates with the sight-line and the axes of maximum variability are not viewer-centered but rotate anti-clockwise around the body and the effector arm during the move from the right to the left workspace. The bulk of findings from these and previous experiments support the hypothesis of a two-stage process, with a gradual transformation from viewer-centered to body-centered and arm-centered coordinates. Retinal, extra-retinal and arm-related signals appear to be progressively combined in superior and inferior parietal areas, giving rise to egocentric representations of the end-point position of reaching. Received: 25 November 1998 / Accepted: 8 July 1999     

Goal-directed arm movements in absence of visual guidance: evidence for amplitude rather than position control

Summary 1. The control of pointing arm movements in the absence of visual guidance was investigated in unpracticed human subjects. The right arm grasped a lever which restricted the movement of the right index fingertip to a horizontal arc, centered between the axes of eye rotation. A horizontal panel directly above the arm prevented visual feedback of the movement. Visual stimuli were presented in discrete positions just above panel and fingertip. A flag provided visual feedback on fingertip position before each pointing movement (Exp. A and B), or before a movement sequence (Exp. C). 2. When subjects pointed from straight ahead to eccentric stimulus positions (Exp. A), systematic and variable pointing errors were observed; both kinds of errors increased with stimulus eccentricity. When subjects pointed from 30 deg left to stimuli located further right (Exp. B), errors increased with stimulus position to the right. Taken together, these findings suggest that pointing accuracy depends not primarily on stimulus position, but rather on required movement amplitude. 3. When subjects performed sequences of unidirectional movements (Exp. C), systematic and variable errors increased within the sequence. A quantitative analysis revealed that this increase can be best described as an accumulation of successive pointing errors. 4. We conclude that both findings, error increase with amplitude, and accumulation of successive errors, when considered together strongly support the hypothesis that amplitude, rather than final position, is the controlled variable of the investigated movements.     

Numbers are represented in egocentric space: Effects of numerical cues and spatial reference frames on hand laterality judgements

Convergent findings demonstrate that numbers can be represented according to a spatially oriented mental number line. However, it is not established whether a default organization of the mental number line exists (i.e., a left-to-right orientation) or whether its spatial arrangement is only the epiphenomenon of specific task requirements. To address this issue we performed two experiments in which subjects were required to judge laterality of hand stimuli preceded by small, medium or large numerical cues; hand stimuli were compatible with egocentric or allocentric perspectives. We found evidence of a left-to-right number–hand association in processing stimuli compatible with an egocentric perspective, whereas the reverse mapping was found with hands compatible with an allocentric perspective. These findings demonstrate that the basic left-to-right arrangement of the mental number line is defined with respect to the body-centred egocentric reference frame.     

Systematic errors of planar arm movements provide evidence for space categorization effects and interaction of multiple frames of reference

Healthy humans performed arm movements in a horizontal plane, from an initial position toward remembered targets, while the movement and the targets were projected on a vertical computer monitor. We analyzed the mean error of movement endpoints and we observed two distinct systematic error patterns. The first pattern resulted in the clustering of movement endpoints toward the diagonals of the four quadrants of an imaginary circular area encompassing all target locations (oblique effect). The second pattern resulted in a tendency of movement endpoints to be closer to the body or equivalently lower than the actual target positions on the computer monitor (y-effect). Both these patterns of systematic error increased in magnitude when a time delay was imposed between target presentation and initiation of movement. In addition, the presence of a stable visual cue in the vicinity of some targets imposed a novel pattern of systematic errors, including minimal errors near the cue and a tendency for other movement endpoints within the cue quadrant to err away from the cue location. A pattern of systematic errors similar to the oblique effect has already been reported in the literature and is attributed to the subject's conceptual categorization of space. Given the properties of the errors in the present work, we discuss the possibility that such conceptual effects could be reflected in a broad variety of visuomotor tasks. Our results also provide insight into the problem of reference frames used in the execution of these aiming movements. Thus, the oblique effect could reflect a hand-centered reference frame while the y-effect could reflect a body or eye-centered reference frame. The presence of the stable visual cue may impose an additional cue-centered (allocentric) reference frame. Electronic Publication     

Perceptual distortions in the neural representation of visual space

 The visual mechanism by which human observers determine the separation between objects has long been of interest. This study examines the extent to which separation in visual space can be misperceived in foveal and extrafoveal vision. Foveally, vertical separations were consistently overestimated relative to horizontal separations, a result which is consistent with the well-documented horizontal-vertical illusion (HVI). Extrafoveally, much larger misrepresentations of visual space were perceived. In addition, separations tangential to fixation were consistently perceived as being greater than separations in a radial direction. These marked misperceptions of visual space which occur in extrafoveal vision take the form of a radial/tangential anisotropy combined with an overestimation of vertical distance. The results have important implications for meridional anisotropies which have previously been documented in a number of visual performance tasks. Received: 17 July 1998 / Accepted: 14 October 1998     

Postural costs of performing cognitive tasks in non-coincident reference frames

Dual-task interactions in posture and cognitive tasks have been explained as a competition for spatial processing structures or as interference in the online sensorimotor adjustments required for sensory integration. Going beyond these general terms accounts, we propose that interference between spatial and temporal operations in posture–cognition arises at least partly from the need to share a common behavioral context, such as a spatial frame of reference. Using immersive visualization and motion-tracking techniques, we manipulated the spatial reference frames for a standing task and a conjunction visual search task into or out of coincidence. Aside from performance trade-offs due to task-load manipulations, performing visual search in a non-coincident reference frame led to cognitive task and postural task performance decrements (Experiment 1). Postural dual-task decrements were also observed when visual search was split between coincident and non-coincident frames and both frame conditions rendered identical in visual information relevant to posture control (Experiment 2). We concluded that the postural control costs observed for posture–cognition dual-tasking may in part reflect costs of keeping tasks’ reference frames in register.

Gaze effects in the cerebral cortex: reference frames for space coding and action

Visual information is mapped with respect to the retina within the early stages of the visual cortex. On the other hand, the brain has to achieve a representation of object location in a coordinate system that matches the reference frame used by the motor cortex to code reaching movement in space. The mechanism of the necessary coordinate transformation between the different frames of reference from the visual to the motor system as well as its localization within the cerebral cortex is still unclear. Coordinate transformation is traditionally described as a series of elementary computations along the visuomotor cortical pathways, and the motor system is thought to receive target information in a body-centered reference frame. However, neurons along these pathways have a number of similar properties and receive common input signals, suggesting that a non-retinocentric representation of object location in space might be available for sensory and motor purposes throughout the visuomotor pathway. This paper reviews recent findings showing that elementary input signals, such as retinal and eye position signals, reach the dorsal premotor cortex. We will also compare eye position effects in the premotor cortex with those described in the posterior parietal cortex. Our main thesis is that appropriate sensory input signals are distributed across the visuomotor continuum, and could potentially allow, in parallel, the emergence of multiple and task-dependent reference frames. Received: 21 September 1998 / Accepted: 19 March 1999     

Allocentric and egocentric reference frames in the processing of three-dimensional haptic space

In an earlier experiment we showed that selective attention plays a critical role in rabbit eye blink conditioning (Steele-Russell et al. in Exp Brain Res 173:587–602, 2006). The present experiments are concerned to examine the extent to which visual recognition processes are a separate component from the motor learning that is also involved in conditioning. This was achieved by midline section of the optic chiasma which disconnected the direct retinal projections via the brainstem to the cerebellar oculomotor control system. By comparing both normal and chiasma-sectioned rabbits it was possible to determine the dependence or independence of conditioning on the motor expression of the eye blink response during training. Both normal and chiasma-sectioned animals were tested using a multiple test battery to determine the effect of this redirection of the visual input pathways on conditioning. All animals were first tested for any impairment in visual capability following section of the optic chiasma. Despite the loss of 90% of retinal ganglion cell fibres, no visual impairment for either intensity or pattern vision was seen in the chiasma animals. Also no difference was seen in nictitating membrane (NM) conditioning to an auditory signal between normal and chiasma animals. Testing for motor learning to a visual signal, the chiasma rabbits showed a complete lack of any NM conditioning. However the sensory tests of visual conditioning showed that chiasma-sectioned animals had completely normal sensory recognition learning. These results show that NM Pavlovian conditioning involves anatomically separate and independent sensory recognition and motor output components of the learning. This research was supported by S&W research grants ID# 1810 to ISR and ID# 7985 to JAC.     

Representation of heading direction in far and near head space

Manipulation of objects around the head requires an accurate and stable internal representation of their locations in space, also during movements such as that of the eye or head. For far space, the representation of visual stimuli for goal-directed arm movements relies on retinal updating, if eye movements are involved. Recent neurophysiological studies led us to infer that a transformation of visual space from retinocentric to a head-centric representation may be involved for visual objects in close proximity to the head. The first aim of this study was to investigate if there is indeed such a representation for remembered visual targets of goal-directed arm movements. Participants had to point toward an initially foveated central target after an intervening saccade. Participants made errors that reflect a bias in the visuomotor transformation that depends on eye displacement rather than any head-centred variable. The second issue addressed was if pointing toward the centre of a wide-field expanding motion pattern involves a retinal updating mechanism or a transformation to a head-centric map and if that process is distance dependent. The same pattern of pointing errors in relation to gaze displacement was found independent of depth. We conclude that for goal-directed arm movements, representation of the remembered visual targets is updated in a retinal frame, a mechanism that is actively used regardless of target distance, stimulus characteristics or the requirements of the task.     

Haptic localization and the internal representation of the hand in space

As the hand actively explores the environment, contact with an object leads to neuronal activity in the topographic maps of somatosensory cortex. However, the brain must combine this somatotopically encoded tactile information with an internal representation of the hand's location in space if it is to determine the position of the object in three-dimensional space (3-D haptic localization). To investigate the fidelity of this internal representation in human subjects, a small tactual stimulator, light enough to be worn on the subject's hand, was used to present a brief mechanical pulse (6-ms duration) to the right index finger before, during, or after a fast, visually evoked movement of the right hand. In experiment 1, subjects responded by pointing to the perceived location of the mechanical stimulus in 3-D space. Stimuli presented shortly before or during the visually evoked movement were systematically mislocalized, with the reported location of the stimulus approximately equal to the location occupied by the hand 90 ms after stimulus onset. This pattern of errors indicates a representation of the movement that fails to account for the change in the hand's location during somatosensory delays and, in some subjects, inaccurately depicts the velocity of the actual movement. In experiment 2, subjects were instructed to verbally indicate the perceived temporal relationship of the stimulus and the visually evoked movement (i.e., by reporting whether the stimulus was presented before, during, or after the movement). On average, stimuli presented in the 38-ms period before movement onset were more likely to be perceived as having occurred during rather than before the movement. Similarly, stimuli in the 145-ms period before movement termination were more likely to be perceived as having occurred after rather than during the movement. The analogous findings of experiments 1 and 2 indicate that the same inaccurate representation of dynamic hand position is used to both localize tactual stimuli in 3-D space and construct the perception of arm movement.     

Parietal cortical neurons responding to rotary movement of visual stimulus in space

Summary We found in the posterior parietal association cortex (area 7a) of alert monkeys a group of neurons that were specifically sensitive to the rotation of a visual stimulus (N=21). They responded to rotation of a stimulus in a particular direction much better than to the linear movement in any direction, regardless of shape or orientation of the stimulus. Responses were relatively independent of stimulus position within relatively large receptive fields. The majority of these neurons (N=13) responded to rotation in depth either in the saggital, horizontal or diagonal plane rather than in the frontoparallel plane. These neurons were localized in a small region on the anterior bank of the superior temporal sulcus and may be related directly to the perception of rotation of visual objects in space.     

Frequency and space representation in the inferior colliculus of the FM bat,Eptesicus fuscus

Summary The tonotopic organization and spatial sensitivity of 217 inferior collicular (IC) neurons of Eptesicus fuscus were studied under free field stimulation conditions. Acoustic stimuli were delivered from a loudspeaker placed 21 cm ahead of the bat to determine the best frequency (BF) and minimum threshold (MT) of isolated IC neurons. A BF stimulus was then delivered as the loudspeaker was moved horizontally across the frontal auditory space of the bat to locate the best azimuthal angle (BAZ) at which the neuron had its lowest MT. The stimulus was then raised 3 dB above the lowest MT to determine the horizontal extent of the auditory space within which a sound could elicit responses from the neuron. This was done by moving the loudspeaker laterally at every 5° or 10° until the neuron failed to respond. These measurements also allowed us to redetermine the BAZ at which the neuron fired maximal number of impulses. Electrodes were placed evenly across the whole IC surface and IC neurons were sampled as many as possible within each electrode penetration. Tonotopic organization and spatial sensitivity were examined among all 217 IC neurons as a whole as well as among IC neurons sequentially sampled within individual electrode penetrations. The whole population of 217 IC neurons is organized tonotopically along the dorsoventral axis of the IC. Thus, low frequency neurons are mostly located dorsally and high frequency neurons ventrally with median frequency neurons intervening in between. The BAZ of these 217 IC neurons tend to shift from lateral to medial portions of the contralateral frontal auditory space with increasing BF. Thus, the auditory space appears to have an orderly representation along the tonotopic axis of the IC. The lateral space is represented dorsally and the medial space ventrally. Nevertheless, tonotopic organization and spatial sensitivitty of sequentially isolated IC neurons within each electrode penetration may vary with the point of electrode penetration. This variation may be explained on the basis of the arrangement and thickness of each frequency lamina within the IC.     

Strategy for deletion of complete open reading frames in Saccharomyces cerevisiae

The classical disruption method for yeast genes is by using in vitro deletion of the gene of interest, or of a part of it, with restriction enzymes. We are now routinely using a strategy that takes advantage of polymerase chain reactions (PCRs) which amplify large pieces of DNA. Since this approach results in a complete, precise deletion of the open reading frame, which is replaced by a unique restriction site, the ligated PCR can be used for the insertion of different markers of for two-step gene disruptions without an inserted marker. As we have now used this strategy for the deletion of more than ten genes we have in this report included some hints based on our experience.     

Lymphocyte immunophenotype reference ranges in healthy Indian adults: implications for management of HIV/AIDS in India

With the advent and application of standard and sensitive flow cytometry methods, it became essential to establish reference intervals in healthy individuals to demarcate between health and disease. A reference range of lymphocyte populations for normal individuals is important in the diagnosis and prognosis of immunodeficiency diseases like AIDS. We tried to accomplish this by studying the values for T lymphocyte subsets for 200 healthy North Indian adults between 18 and 55 years. We obtained the following reference ranges for various T lymphocyte subsets: CD4 count (304-1864 cells/microl with the median of 666 cells/microl), CD4% (17.5-50.6% with the median of 35%), CD8% (14-53% with the median of 32.3%), CD3% (43-89% with the median of 70.5%), and CD4/CD8 ratio (0.04-3.5 with the median of 1.04). Significant variations were observed for normal reference intervals for T lymphocyte subsets according to the race, ethnic origin, age group, and gender.     

Selection of reference genes for expression analyses in liver of rats with impaired glucose metabolism

Hepatic gene expression studies are vital for identification of molecular factors involved in insulin resistance. However, the need of normalized gene expression data has led to the search of stable genes which are useful as a reference in specific experimental conditions. The aim of this study was to evaluate expression stability of potential reference genes for real-time PCR gene expression studies, in rats with insulin resistance, early programmed in intrauterine environment of maternal insulin resistance and triggered by exposure to a high sucrose and fat diet in adult life. Male rats coming from insulin resistant (F₁IR) mothers or normal (F₁N) mothers were fed a standard rodent diet from postnatal day 21 to day 56, and then divided in two groups each. One of each subgroups were fed a high sucrose and fat diet (groups F₁IR + HSFD and F₁N + HSFD respectively), the rest were fed a control diet (groups F₁IR + CD and F₁N + CD) for 14 days. Glucose metabolism related tests were later performed. After liver extraction, RNA was isolated and gene expression analyzes of seven potential reference genes (Actb, Gapdh, Gusb, Hprt1, Ldha, Rpl13a and Rplp1) were carried out. LinRegPCR software was used to analyze raw data and determinate baseline corrections, threshold lines, efficiency of PCR reactions and corrected Cq values. Evaluations of gene expression stabilities as well as the number of necessary genes for normalization were assessed with geNorm tool. All samples from all groups showed acceptable PCR amplification efficiencies. The most stable genes were Rplp1, Ldha, Hprt1 and Rpl13a and the less stable was Gapdh. For all groups, just 2 to 3 of the most stable genes were necessary for optimal gene expression data normalization in rat liver. Genes encoding ribosomal proteins are the most appropriated for normalization of expression data in the presented animal model. By contrast, Gapdh, one of the most used genes in normalization, is not recommendable due to its high intergroup variation.