Unconscious updating of grasp motor program

The Role of Haptic Expectations in Reaching to Grasp: From Pantomime to Natural Grasps and Back Again

When we reach to pick up an object, our actions are effortlessly informed by the object's spatial information, the position of our limbs, stored knowledge of the object's material properties, and what we want to do with the object. A substantial body of evidence suggests that grasps are under the control of "automatic, unconscious" sensorimotor modules housed in the "dorsal stream" of the posterior parietal cortex. Visual online feedback has a strong effect on the hand's in-flight grasp aperture. Previous work of ours exploited this effect to show that grasps are refractory to cued expectations for visual feedback. Nonetheless, when we reach out to pretend to grasp an object (pantomime grasp), our actions are performed with greater cognitive effort and they engage structures outside of the dorsal stream, including the ventral stream. Here we ask whether our previous finding would extend to cued expectations for haptic feedback. Our method involved a mirror apparatus that allowed participants to see a "virtual" target cylinder as a reflection in the mirror at the start of all trials. On "haptic feedback" trials, participants reached behind the mirror to grasp a size-matched cylinder, spatially coincident with the virtual one. On "no-haptic feedback" trials, participants reached behind the mirror and grasped into "thin air" because no cylinder was present. To manipulate haptic expectation, we organized the haptic conditions into blocked, alternating, and randomized schedules with and without verbal cues about the availability of haptic feedback. Replicating earlier work, we found the strongest haptic effects with the blocked schedules and the weakest effects in the randomized uncued schedule. Crucially, the haptic effects in the cued randomized schedule was intermediate. An analysis of the influence of the upcoming and immediately preceding haptic feedback condition in the cued and uncued random schedules showed that cuing the upcoming haptic condition shifted the haptic influence on grip aperture from the immediately preceding trial to the upcoming trial. These findings indicate that, unlike cues to the availability of visual feedback, participants take advantage of cues to the availability of haptic feedback, flexibly engaging pantomime, and natural modes of grasping to optimize the movement.

How does ageing affect grasp adaptation to a visual-haptic size conflict?

Previous research suggests that the ability to adapt motor behaviour to sudden environmental changes may be impaired in older adults. Here, we investigated whether the adaptation of grasping behaviour in response to a visual-haptic size conflict is also affected by increasing age. 30 older and 18 young adults were instructed to grasp a hidden block whilst viewing a second block in a congruent position. Initially block sizes were equal, but after a set number of trials a sensory conflict was introduced by covertly changing the hidden block for a smaller or larger block. The scale and speed of maximum grasp aperture adaptation to the increase or decrease in the size of the hidden block was measured. Older adults successfully adapted to the visual-haptic size conflict in a similar manner to young adults, despite a tendency to adapt less when the hidden block increased in size. This finding is attributed to the physical capabilities of the grasping hand of older adults, rather than an effect of age-related sensory or cognitive decline. The speed of grasp adaptation did not differ between age groups; however, awareness of the visual-haptic conflict lead to faster adaptation. These findings suggest that sensorimotor adaptation for grasping is intact for cognitively healthy older adults.

Unconscious Imagination and the Mental Imagery Debate

Traditionally, philosophers have appealed to the phenomenological similarity between visual experience and visual imagery to support the hypothesis that there is significant overlap between the perceptual and imaginative domains. The current evidence, however, is inconclusive: while evidence from transcranial brain stimulation seems to support this conclusion, neurophysiological evidence from brain lesion studies (e.g., from patients with brain lesions resulting in a loss of mental imagery but not a corresponding loss of perception and vice versa) indicates that there are functional and anatomical dissociations between mental imagery and perception. Assuming that the mental imagery and perception do not overlap, at least, to the extent traditionally assumed, then the question arises as to what exactly mental imagery is and whether it parallels perception by proceeding via several functionally distinct mechanisms. In this review, we argue that even though there may not be a shared mechanism underlying vision for perception and conscious imagery, there is an overlap between the mechanisms underlying vision for action and unconscious visual imagery. On the basis of these findings, we propose a modification of Kosslyn's model of imagery that accommodates unconscious imagination and explore possible explanations of the quasi-pictorial phenomenology of conscious visual imagery in light of the fact that its underlying neural substrates and mechanisms typically are distinct from those of visual experience.

Through the forest of motor representations

Following neuroscience, and using different labels, several philosophers have addressed the idea of the presence of a single representational mechanism lying in between (visual) perceptual processes and motor processes involved in different functions and useful for shaping suitable action performances: a motor representation (MR). MRs are the naturalized mental antecedents of action. This paper presents a new, non-monolithic view of MRs, according to which, contrarily to the received view, when looking at in between (visual) perceptual processes and motor processes, we find not only a single representational mechanism with different functions, but an ensemble of different sub-representational phenomena, each of which with a different function. This new view is able to avoid several issues emerging from the literature and to address something the literature is silent about, which however turns out to be crucial for a theory of MRs.

Real-time vision, tactile cues, and visual form agnosia: removing haptic feedback from a "natural" grasping task induces pantomime-like grasps

Investigators study the kinematics of grasping movements (prehension) under a variety of conditions to probe visuomotor function in normal and brain-damaged individuals. “Natural” prehensile acts are directed at the goal object and are executed using real-time vision. Typically, they also entail the use of tactile, proprioceptive, and kinesthetic sources of haptic feedback about the object (“haptics-based object information”) once contact with the object has been made. Natural and simulated (pantomimed) forms of prehension are thought to recruit different cortical structures: patient DF, who has visual form agnosia following bilateral damage to her temporal-occipital cortex, loses her ability to scale her grasp aperture to the size of targets (“grip scaling”) when her prehensile movements are based on a memory of a target previewed 2 s before the cue to respond or when her grasps are directed towards a visible virtual target but she is denied haptics-based information about the target. In the first of two experiments, we show that when DF performs real-time pantomimed grasps towards a 7.5 cm displaced imagined copy of a visible object such that her fingers make contact with the surface of the table, her grip scaling is in fact quite normal. This finding suggests that real-time vision and terminal tactile feedback are sufficient to preserve DF’s grip scaling slopes. In the second experiment, we examined an “unnatural” grasping task variant in which a tangible target (along with any proxy such as the surface of the table) is denied (i.e., no terminal tactile feedback). To do this, we used a mirror-apparatus to present virtual targets with and without a spatially coincident copy for the participants to grasp. We compared the grasp kinematics from trials with and without terminal tactile feedback to a real-time-pantomimed grasping task (one without tactile feedback) in which participants visualized a copy of the visible target as instructed in our laboratory in the past. Compared to natural grasps, removing tactile feedback increased RT, slowed the velocity of the reach, reduced in-flight grip aperture, increased the slopes relating grip aperture to target width, and reduced the final grip aperture (FGA). All of these effects were also observed in the real time-pantomime grasping task. These effects seem to be independent of those that arise from using the mirror in general as we also compared grasps directed towards virtual targets to those directed at real ones viewed directly through a pane of glass. These comparisons showed that the grasps directed at virtual targets increased grip aperture, slowed the velocity of the reach, and reduced the slopes relating grip aperture to the widths of the target. Thus, using the mirror has real consequences on grasp kinematics, reflecting the importance of task-relevant sources of online visual information for the programming and updating of natural prehensile movements. Taken together, these results provide compelling support for the view that removing terminal tactile feedback, even when the grasps are target-directed, induces a switch from real-time visual control towards one that depends more on visual perception and cognitive supervision. Providing terminal tactile feedback and real-time visual information can evidently keep the dorsal visuomotor system operating normally for prehensile acts.

Effect of visual and haptic feedback on grasping movements

Perceptual estimates of three-dimensional (3D) properties, such as the distance and depth of an object, are often inaccurate. Given the accuracy and ease with which we pick up objects, it may be expected that perceptual distortions do not affect how the brain processes 3D information for reach-to-grasp movements. Nonetheless, empirical results show that grasping accuracy is reduced when visual feedback of the hand is removed. Here we studied whether specific types of training could correct grasping behavior to perform adequately even when any form of feedback is absent. Using a block design paradigm, we recorded the movement kinematics of subjects grasping virtual objects located at different distances in the absence of visual feedback of the hand and haptic feedback of the object, before and after different training blocks with different feedback combinations (vision of the thumb and vision of thumb and index finger, with and without tactile feedback of the object). In the Pretraining block, we found systematic biases of the terminal hand position, the final grip aperture, and the maximum grip aperture like those reported in perceptual tasks. Importantly, the distance at which the object was presented modulated all these biases. In the Posttraining blocks only the hand position was partially adjusted, but final and maximum grip apertures remained unchanged. These findings show that when visual and haptic feedback are absent systematic distortions of 3D estimates affect reach-to-grasp movements in the same way as they affect perceptual estimates. Most importantly, accuracy cannot be learned, even after extensive training with feedback.

Mirror reversal and visual rotation are learned and consolidated via separate mechanisms: recalibrating or learning de novo?

Motor learning tasks are often classified into adaptation tasks, which involve the recalibration of an existing control policy (the mapping that determines both feedforward and feedback commands), and skill-learning tasks, requiring the acquisition of new control policies. We show here that this distinction also applies to two different visuomotor transformations during reaching in humans: Mirror-reversal (left-right reversal over a mid-sagittal axis) of visual feedback versus rotation of visual feedback around the movement origin. During mirror-reversal learning, correct movement initiation (feedforward commands) and online corrections (feedback responses) were only generated at longer latencies. The earliest responses were directed into a nonmirrored direction, even after two training sessions. In contrast, for visual rotation learning, no dependency of directional error on reaction time emerged, and fast feedback responses to visual displacements of the cursor were immediately adapted. These results suggest that the motor system acquires a new control policy for mirror reversal, which initially requires extra processing time, while it recalibrates an existing control policy for visual rotations, exploiting established fast computational processes. Importantly, memory for visual rotation decayed between sessions, whereas memory for mirror reversals showed offline gains, leading to better performance at the beginning of the second session than in the end of the first. With shifts in time-accuracy tradeoff and offline gains, mirror-reversal learning shares common features with other skill-learning tasks. We suggest that different neuronal mechanisms underlie the recalibration of an existing versus acquisition of a new control policy and that offline gains between sessions are a characteristic of latter.

Basic principles of sensorimotor adaptation to different distortions with different effectors and movement types: a review and synthesis of behavioral findings

This article reviews seemingly conflicting behavioral data about sensorimotor adaptation. Some earlier studies assert that one common mechanism exists for multiple distortions, and others that multiple mechanisms exist for one given distortion. Some but not others report that adaptation is direction-selective. Some submit that adaptation transfers across effectors, and others that a single effector can adapt to multiple distortions. A model is proposed to account for all these findings. It stipulates that adaptive mechanisms respond to multiple distortions, consist of directionally tuned special-purpose modules, can be switched in dependence on contextual cues, and are connected to practiced movement types with a higher weight than to unpracticed ones.

Direct and indirect haptic calibration of visual size judgments

It has long been suspected that touch plays a fundamental role in the calibration of visual perception, and much recent evidence supports this idea. However, as the haptic exploration workspace is limited by the kinematics of the body, the contribution of haptic information to the calibration process should occur only within the region of the haptic workspace reachable by a limb (peripersonal space). To test this hypothesis we evaluated visual size perception and showed that it is indeed more accurate inside the peripersonal space. We then show that allowing subjects to touch the (unseen) stimulus after observation restores accurate size perception; the accuracy persists for some time, implying that calibration has occurred. Finally, we show that observing an actor grasp the object also produces accurate (and lasting) size perception, suggesting that the calibration can also occur indirectly by observing goal-directed actions, implicating the involvement of the “mirror system”.

From Intention to Input: Motor Cognition, Motor Performance, and the Control of Technology

Fundamental knowledge of motor cognition is an important component in a human factors repertoire, and this chapter serves as a guide to the history, theory, and application of motor cognition research.“From intention to input” captures the scope of this chapter in that cognitive theories of motor control, neural control of movement, and the effects of feedback on movement are all discussed. The chapter progresses from an overview and history of motor cognition theories down to the neural basis for movement, then to an application of these theories via the study of specific actions. From there, rooted in the scientist-practitioner paradigm of human factors, the chapter covers applied considerations for designing control tasks and their associated inputs, taking into account individual differences in motor cognition and control and identifying critical issues in designing for input. General, applied guidelines are provided for use with current and future systems that have a motor cognition component.

Collision error avoidance: influence of proportion congruency and sensorimotor memory on open-loop grasp control

Grasping behaviour involves the integration of current and historical knowledge about an object, a process that can be influenced by sensory uncertainty. In the present study, participants simultaneously interacted with a visual cue and a haptic cue before reaching to grasp a target object. The visual cue was either congruent (equal in size to haptic cue and target) or incongruent (larger than haptic cue and target). To enhance sensory uncertainty, we manipulated the proportion of congruent trials to be either 80 or 20%. We compared grasp kinematics and forces between congruent and incongruent trials and between the 20 and 80% proportion congruency groups. We also studied the effects of trial history by comparing the performance of congruent and incongruent trials preceded by either the same or opposite trial type. Proportion congruency did not affect temporal kinematics but did affect maximum grip aperture (MGA) as the 80% proportion congruency group used a greater MGA, regardless of trial type. For grasping forces, an interaction effect showed that the 20% proportion congruency group used a greater peak load force on congruent trials. Incongruent trials that followed congruent trials had decreased movement time, increased MGA and increased grasping forces, relative to those that followed incongruent trials. We interpret the data to suggest that the grasp control system integrates multisensory information using flexible, yet specific criteria regarding task constraints. The prevention of collision error (i.e., an inadequate MGA when contacting the target) may be one guiding principle in the control process.

Prediction of object contact during grasping

The maximum grip aperture (MGA) during prehension is linearly related to the size of objects to be grasped and is adapted to the haptically sensed object size when there is a discrepancy between visual and haptic information. We have investigated what information is used to drive this adaptation process and how the onset of fingertip forces on the object is triggered. Subjects performed a reach-to-grasp task, where the object seen and the object grasped physically never were the same. We measured the movements of the index finger and the thumb and the contact forces between each fingertip and the object. The subjects' adaptation of the MGA was unrelated both to different fingertip velocities at the moment of object contact, or the fingertip forces. Instead, the 'timing' of contact between the fingers and the object was most consistently influenced by introducing a size discrepancy. Specifically, if the object was larger than expected, the moment of contact occurred earlier, and if the object was decreased in size, then the contact occurred later. During adaptation, these timing differences were markedly reduced. Also, the motor command for applying forces on the object seemed to be released in anticipation of the predicted moment of contact. We therefore conclude that the CNS dynamically predicts when contact between the fingertips and objects occur and that aperture adaptation is primarily driven by timing prediction errors.

Calibrating grasp size and reach distance: interactions reveal integral organization of reaching-to-grasp movements

Feedback is a central feature of neural systems and of crucial importance to human behaviour as shown in goal directed actions such as reaching-to-grasp. One important source of feedback in reach-to-grasp behaviour arises from the haptic information obtained after grasping an object. We manipulated the felt distance and/or size of a visually constant object to explore the role of haptic information in the calibration of reaching and grasping. Crucially, our design explored post-adaptation effects rather than the previously documented role of haptic information in movement organisation. A post-adaptation reach-to-grasp task showed: (1) distorted haptic feedback caused recalibration; (2) reach distance and grasp size could be calibrated separately but, if calibrated simultaneously, then (3) recalibration was greater when distance and size changed in a consistent (e.g. reaching for a larger object at a greater distance) rather than an inconsistent (e.g. a smaller object at a greater distance) fashion. These interactions reveal the integral nature of reach-to-grasp organization, that is, that reaching and grasping are integrated components of a single action system.

Cortical Connections of the Macaque Anterior Intraparietal (AIP) Area

We traced the cortical connections of the anterior intraparietal (AIP) area, which is known to play a crucial role in visuomotor transformations for grasping. AIP displayed major connections with 1) areas of the inferior parietal lobule convexity, the rostral part of the lateral intraparietal area and the SII region; 2) ventral visual stream areas of the lower bank of the superior temporal sulcus and the middle temporal gyrus; and 3) the premotor area F5 and prefrontal areas 46 and 12. Additional connections were observed with the caudal intraparietal area and the ventral part of the frontal eye field. This study suggests that visuomotor transformations for object-oriented actions, processed in AIP, rely not only on dorsal visual stream information related to the object's physical properties but also on ventral visual stream information related to object identity. The identification of direct anatomical connections with the inferotemporal cortex suggests that AIP also has a unique role in linking the parietofrontal network of areas involved in sensorimotor transformations for grasping with areas involved in object recognition. Thus, AIP could represent a crucial node in a cortical circuit in which hand-related sensory and motor signals gain access to representations of object identity for tactile object recognition.

A new perspective in the understanding of hand dysfunction following neurological injury

The human hand is inherently complex and versatile. Its use in everyday activities requires its careful positioning relative to the arm and fine adjustments of the fingers to secure the object in the hand to perform a desired task. Understanding the mechanics of prehension requires an appreciation of the anatomy, biomechanics, kinematics, and control of the hand. This article summarizes these complex mechanisms as well as the central nervous system control of hand movement. We propose a measure to characterize the biomechanics of palmar arch modulation during grasping. We also highlight questions to be investigated in future studies to stimulate further understanding of the motor control of hand function and of the recovery of hand functioning after neurological injury.

Adaptation of grasping responses to distorted object size and orientation

The present study investigated the adaptive plasticity of the grasp component of prehensile movements. Subjects saw visual objects (V) of various sizes and orientations and were instructed to seize them with the thumb and index finger without manipulating them. The subjects' hand disappeared from view during the response and made contact with a haptic object (H), which could differ with respect to size or orientation from V. In Experiment A, we found that the grasp aperture adaptively increased when H > V and decreased when H < V. This confirms earlier observations and expands them by documenting that grasp size adaptation occurs even when grasping is not integrated into an intentional behavioral context (i.e., object manipulation). However, the magnitude of the observed adaptive size change was not monotonously related to the visual-haptic size difference, as one would expect for an adequate adaptive response. In Experiment B, we observed for the first time that the orientation of the grasp aperture adaptively changed when H was differently oriented than V. This change generalized to an unpracticed size and orientation of V, even in the absence of confirmative haptic feedback about dowel orientation. This outcome indicates that grasp orientation adaptation was at least partly based on the recalibration of sensory-to-motor transformation rules, rather than being completely due to strategic adjustments.

Acquiring and adapting a novel audiomotor map in human grasping

For sensorimotor transformations to be executed accurately, there must be mechanisms that can both establish and modify mappings between sensory and motor coordinates. Such mechanisms were investigated in normal subjects using a reach-to-grasp task. First, we replaced the normal input of visual information about object size with auditory information, i.e., we attempted to establish an 'audiomotor map'. The size of the object was log linearly related to the frequency of the sound, and we measured the maximum grip aperture (MGA) during the reaching phase to determine if the subjects had learned the relationship. Second, we changed the frequency-object size relationship to study adaptation in the newly acquired map. Our results demonstrate that learning of an audiomotor map consisted of three distinct phases: during the first stage (approximately 10-15 trials) subjects simply used MGAs large enough to grasp any reasonably sized object and there were no overt signs of learning. During the second stage, there was a period of fast learning where the slope of the relationship between MGA and object size became steeper until the third stage where the slope was constant. In contrast, when sensorimotor adaptation was studied in the established audiomotor map, there was rapid learning from the start of a size perturbation. We conclude that different learning strategies are employed when sensorimotor transformations are established compared to when existing transformations are modified.

Effects of an orientation illusion on motor performance and motor imagery

Although the effect of visual illusions on overt actions has been an area of keen interest in motor performance, no study has yet examined whether illusions have similar or different effects on overt and imagined movements. Two experiments were conducted that compared the effects of an orientation illusion on an overt posture selection task and an imagined posture selection task. In Experiment 1 subjects were given a choice of grasping a bar with the thumb on the left side or right side of the bar. In Experiment 2 subjects were instructed to only imagine grasping the bar while remaining motionless. Subjects then reported which side of the bar their thumb had been placed in imagined grasping. Both the overt selection and imagined selection tasks were found to be sensitive to the orientation illusion, suggesting that similar visual information is used for overt and imagined movements, with both being sensitive to an orientation illusion. The results are discussed in terms of the visual processing and representation of real and imagined actions.

Short-term plasticity of the visuomotor map during grasping movements in humans

During visually guided grasping movements, visual information is transformed into motor commands. This transformation is known as the "visuomotor map." To investigate limitations in the short-term plasticity of the visuomotor map in normal humans, we studied the maximum grip aperture (MGA) during the reaching phase while subjects grasped objects of various sizes. The objects seen and the objects grasped were physically never the same. When a discrepancy had been introduced between the size of the visual and the grasped objects, and the subjects were fully adapted to it, they all readily interpolated and extrapolated the MGA to objects not included in training trials. In contrast, when the subjects were exposed to discrepancies that required a slope change in the visuomotor map, they were unable to adapt adequately. They instead retained a subject-specific slope of the relationship between the visual size and MGA. We conclude from these results that during reaching for grasping, normal subjects are unable to abandon a straight linear function determining the relationship between visual object size and MGA. Moreover, the plasticity of the visuomotor map is, at least in short term, constrained to allow only offset changes, that is, only "rigid shifts" are possible between the visual and motor coordinate systems.

Task requirements influence sensory integration during grasping in humans

The sensorimotor transformations necessary for generating appropriate motor commands depend on both current and previously acquired sensory information. To investigate the relative impact (or weighting) of visual and haptic information about object size during grasping movements, we let normal subjects perform a task in which, unbeknownst to the subjects, the object seen (visual object) and the object grasped (haptic object) were never the same physically. When the haptic object abruptly became larger or smaller than the visual object, subjects in the following trials automatically adapted their maximum grip aperture when reaching for the object. This adaptation was not dependent on conscious processes. We analyzed how visual and haptic information were weighted during the course of sensorimotor adaptation. The adaptation process was quicker and relied more on haptic information when the haptic objects increased in size than when they decreased in size. As such, sensory weighting seemed to be molded to avoid prehension error. We conclude from these results that the impact of a specific source of sensory information on the sensorimotor transformation is regulated to satisfy task requirements.

Optic ataxia as a deficit specific to the on-line control of actions

Optic ataxia is characterized by inaccuracies in body movements under visual control, and is a common consequence of damage to the posterior parietal lobes in humans. It is argued here that optic ataxia can be characterized as a deficit in the visual on-line guidance of actions, with action planning remaining relatively intact. This contrasts with the common view of optic ataxia as representing a deficit in the transformations that take place between visual inputs and motor outputs. Evidence in support of the planning-control view comes from the pattern of spared and disrupted behaviors in patients with optic ataxia. It is shown that spared behaviors are those that emphasize planning, whereas disrupted behaviors are those that emphasize control. In particular, recent studies have highlighted the inability of a patient with optic ataxia to make on-line adjustments to targets that change position during the movement. Taken in sum, the data from patients with optic ataxia is more consistent with the planning-control interpretation of optic ataxia than with the visuomotor transformation interpretation.

Grasping after a delay shifts size-scaling from absolute to relative metrics

We carried out three experiments designed to compare the effects of relative and absolute size on manual prehension and manual estimates of perceived size. In each experiment, right-handed subjects were presented with two different-sized 3-D objects in a virtual display and were instructed to pick up or estimate the size of one of them. In Experiment 1, subjects were requested to pick up the smaller one of two virtual objects under one condition and the larger one under the other condition. In fact, the target object was identical on all trials; it was simply paired with a smaller object on some trials and a larger object on others. To provide veridical haptic feedback, a real object was positioned beneath a mirror at the same location as the virtual target object. In Experiment 2, one of the virtual objects was marked with a red dot on its top surface. From trial to trial, the marked object was paired with a larger, smaller, or same-sized object. Subjects were instructed to always pick up the marked object on each trial. In both Experiment 1 and 2, half the subjects were tested in delayed grasping with a 5-sec delay between viewing the objects and initiating the grasp, and half in real-time grasping without a delay. Using the same display of virtual objects as in Experiment 2, subjects in Experiment 3 were requested to estimate the size of the marked object using their index finger and thumb (i.e., they showed us how big the object looked to them). After estimating the target object's size, they picked it up. All subjects gave their estimates either immediately or after a delay. Recording of hand movements revealed that when subjects in Experiments 1 and 2 picked up the target object in real time, their grip aperture in flight was not significantly affected whether the object was accompanied by a larger object or a smaller one. When subjects picked up the target object after a delay, however, their grip aperture in flight was larger when the target object was accompanied by a smaller object than when it was accompanied by a larger object. A similar size-contrast effect was also observed in Experiment 3 in which subjects gave manual estimates of the perceived size of the target object. This perceptual effect was observed both when the estimates were given immediately and when they were given after a 5-sec delay. These results suggest that normal (real-time) visuomotor control relies on absolute metrics, whereas delayed grasping utilizes the same relative metrics used by conscious perception.

Intersensory conflict between vision and touch: the response modality dominates when precise, attention-riveting judgments are required

In four experiments, reducing lenses were used to minify vision and generate intersensory size conflicts between vision and touch. Subjects made size judgments, using either visual matching or haptic matching. In visual matching, the subjects chose from a set of visible squares that progressively increased in size. In haptic matching, the subjects selected matches from an array of tangible wooden squares. In Experiment 1, it was found that neither sense dominated when subjects exposed to an intersensory discrepancy made their size estimates by using either visual matching or haptic matching. Size judgments were nearly indentical for conflict subjects making visual or haptic matches. Thus, matching modality did not matter in Experiment 1. In Experiment 2, it was found that subjects were influenced by the sight of their hands, which led to increases in the magnitude of their size judgments. Sight of the hands produced more accurate judgments, with subjects being better able to compensate for the illusory effects of the reducing lens. In two additional experiments, it was found that when more precise judgments were required and subjects had to generate their own size estimates, the response modality dominated. Thus, vision dominated in Experiment 3, where size judgments derived from viewing a metric ruler, whereas touch dominated in Experiment 4, where subjects made size estimates with a pincers posture of their hands. It is suggested that matching procedures are inadequate for assessing intersensory dominance relations. These results qualify the position (Hershberger & Misceo, 1996) that the modality of size estimates influences the resolution of intersensory conflicts. Only when required to self-generate more precise judgments did subjects rely on one sense, either vision or touch. Thus, task and attentional requirements influence dominance relations, and vision does not invariably prevail over touch.

Modeling parietal-premotor interactions in primate control of grasping

Visual information is processed in the posterior parietal cortex for the hypothesized purpose of extracting a variety of affordances for the generation of motor behavior. The term affordance is used to mean that visual cues are mapped directly to parameters that are relevant for motor interaction. In this paper, we present the FARS model of the cortical involvement in grasping, a model which focuses on the interaction between anterior intra-parietal area (AIP) and premotor area F5. The model represents the role of other intra-parietal areas, working in concert with inferotemporal cortex and F5, to provide AIP with a full range of information from which affordances may be derived. The model also suggests how task information and other constraints may resolve the action opportunities provided by multiple affordances. Our model demonstrates not only that posterior parietal cortex is a network of interacting subsystems, but also that it functions through a pattern of "cooperative computation" with a multiplicity of other brain regions. Finally, through the use of several novel tasks, the model allows us to make specific predictions regarding neural firing patterns at both the single unit and population levels, which aids in our further understanding of information encoding in these brain regions.

Implicit short-lived motor representations of space in brain damaged and healthy subjects

This article reviews experimental evidence for a specific sensorimotor function which can be dissociated from higher level representations of space. It attempts to delineate this function on the basis of results obtained by psychophysical experiments performed with brain damaged and healthy subjects. Eye and hand movement control exhibit automatic features, such that they are incompatible with conscious control. In addition, they rely on a reference frame different from the one used by conscious perception. Neuropsychological cases provide a strong support for this specific motor representation of space, which can be spared in patients with lesions of primary sensory systems who have lost conscious perception of visual, tactile or proprioceptive stimuli. Observation of these patients also showed that their motor behavior can be "attracted" by a goal only under specific conditions, that is, when the response is immediate and when no cognitive representation of this goal is elaborated at the same time. Beyond the issue of the dissociation between an implicit motor representation and more cognitive processing of spatial information, the issue of the interaction between these two systems is thus a matter of interest. It is suggested that the conscious, cognitive representation of a stimulus can contaminate or override the short-lived motor representation, but no reciprocal influence seem to occur. The interaction observed in patients can also be investigated in normals. The literature provides examples of interaction between sensorimotor and cognitive framing of space, which confirm that immediate action is not mediated by the same system as delayed action, and that elaborating a categorial representation of the action goal prevents the expression of the short-lived sensorimotor representation. It is concluded that action can be controlled by a sensory system which is specialized for on-line processing of relevant goal characteristics. The temporal constraints of this system are such that it can affect the action before a full sensory analysis of this goal has been completed. The performance obtained on the basis of this spatial sensory processing suggests that short-lived motor representations may rather be considered as real "presentation" of the action world, which share its metric properties.

Implicit visual analysis in handedness recognition

In the present study, we addressed the problem of whether hand representations, derived from the control of hand gesture, are used in handedness recognition. Pictures of hands and fingers, assuming either common or uncommon postures, were presented to right-handed subjects, who were required to judge their handedness. In agreement with previous results (Parsons, 1987, 1994; Gentilucci, Daprati, & Gangitano, 1998), subjects recognized handedness through mental movement of their own hand in order to match the posture of the presented hand. This was proved by a control experiment of physical matching. The new finding was that presentation of common finger postures affected responses differently from presentation of less common finger postures. These effects could be not attributed to mental matching movements nor related to richness in hand-finger cues useful for handedness recognition. The results of the present study are discussed in the context of the notion that implicit visual analysis of the presented hands is performed before mental movement of one's hand takes place (Parsons, 1987; Gentilucci et al., 1998). In this process, hand representation acquired by experience in the control and observation of one's and other people's hand gestures is used. We propose that such an immediate recognition mechanism belongs to the class of mental processes which are grouped under the name of intuition, that is, the processes by which situations or people's intentions are immediately understood, without conscious reasoning.

The development of speech adaptation to an artificial palate

An investigation of adaptation to palatal modification in [s] production was conducted using acoustic and perceptual analyses. The experiment assessed whether adaptation would occur subsequent to a brief period of intensive, target-specific practice. Productions of [sa] were elicited at five time intervals, 15 min apart, with an artificial palate in place. Between measurement intervals, subjects read [s]-laden passages to promote adaptation. Results revealed improvement in both acoustic and perceptual measures at the final time interval relative to the initial measurement period. Interestingly, the data also suggested changes to normal (unperturbed) articulation patterns during the same interval. Results are discussed in relation to the development of speech adaptation to a structural modification of the oral cavity.