Muscle-based perception: theory, research and implications for rehabilitation: [review] / Percepção baseada nos músculos: teoria, pesquisa e implicações na reabilitação: [revisão]

BACKGROUND: Muscle-based perception of the spatial properties of limbs constrains the patterning, timing and magnitude of muscle forces while performing motor activities. The centrality of muscle-based perception to both ordinary and skilled actions warrants attention from the rehabilitation community, since deficits in its functioning would be related to important functional limitations. In this overview, we summarize a body of research that may be used to guide the development of effective assessment tools and rehabilitation programs that are specifically directed towards such deficits. OBJECTIVES: There were four specific aims: first, to present an information-based approach to muscle-based perception that is grounded in physical laws; second to identify central principles underlying muscle-based perception that have been revealed and supported by empirical work; third, to summarize reports that have investigated whether the principles identified can be generalized to muscle-based perception in individuals with sensory-motor impairments; and fourth to provide a preliminary discussion of the potential implications of the research presented here for issues relating to rehabilitation.

INTRODUÇÃO: A percepção muscular das propriedades espaciais dos membros restringe o padrão, período e magnitude das forças exercidas durante a execução de atividades motoras. A importância central da percepção muscular, tanto para ações rotineiras quanto para ações especializadas, merece atenção da comunidade envolvida na área de reabilitação, uma vez que alterações em suas funções podem estar relacionadas a importantes limitações funcionais. Nesta revisão, os autores apresentam um resumo da pesquisa que pode ser utilizada para guiar o desenvolvimento de ferramentas de avaliação eficazes bem como programas de reabilitação que sejam especificamente direcionados para estas disfunções. OBJETIVOS: Quatro pontos específicos foram incluídos: primeiro, a apresentação da abordagem com base em informações relativas à percepção muscular de acordo com as leis da física; segundo, a identificação dos princípios centrais determinantes da percepção muscular que vem sendo revelada e apoiada por trabalhos empíricos; terceiro, um resumo dos relatos que investigaram e se os princípios identificados poderiam ser generalizados para a percepção muscular dos indivíduos com alterações motoras e sensitivas; e quarto, uma discussão preliminar sobre as implicações potenciais da pesquisa aqui apresentada, no tocante aos assuntos relacionados à reabilitação.

Somatosensory attunement to the rigid body laws.

In the most general case, haptic perception of an object's heaviness is most likely the perception of the object's resistance to movement, determined jointly by the object's mass and mass distribution. In two experiments with occluded objects wielded freely in three dimensions, we showed additive effects on perceived heaviness of mass and the inertia tensor. Our manipulations of the inertia tensor were directed specifically at the volume and symmetry of the inertia ellipsoid, quantities that can be understood as important to controlling the level and patterning of muscular forces, respectively. Ellipsoid volume and symmetry were found to have separate effects on perceptual reports of heaviness that were invariant over different tensors. Independent sensitivities to translational inertia and particular characterizations of rotational inertia suggest specialized somatosensory attunement to the rigid body laws.

Inertia tensor and weight-percept models of length perception by static holding.

S. J. Lederman, S. R. Ganeshan, and R. E. Ellis (1996) reported an experiment demonstrating that for occluded rods of equal mass and length but different diameters length perception by static holding was larger for rods of smaller diameter. They concluded that participants inferred length from illusory weight percepts. However, rods of equal mass and length that differ in diameter also differ in the eigenvalues of their respective inertia tensors. In the present experiments, the authors manipulated the diameters (Experiment 1) and the inertial eigenvalues (Experiments 4 and 5) of statically held objects. As has been shown with wielded objects, perceived length was a function of the eigenvalues. Additional experiments failed to confirm the expectation from the weight-percept model that perceived length maps to the estimated weight (Experiments 2 and 3). Physical quantities, not psychological quantities, seem to explain length perception by static holding.

Perceptual independence of whole length, partial length, and hand position in wielding a rod.

Previous investigations of dynamic touch have shown that, in wielding an occluded rod, the nonvisible perceptions of total rod length, hand position, and fractional rod length above or below the hand are different functions of the eigenvalues and eigenvectors of the inertia tensor. The implication that the 3 perceptions are independent covariants of the inertia tensor of a wielded object was tested with the complete identification experimental procedure using the statistical prescriptions of F. G. Ashby and J. T. Townsend (1986). The confirmed independence is discussed in the context of the generalized psychological or perception-information hypothesis.

Affordance, proper function, and the physical basis of perceived heaviness.

The physical basis of perceived heaviness requires consideration of the haptic perceptual system's role in controlling actions (the system's proper function) and the relation of an object's inertial properties to properties of the human movement system (the object's affordance). We show that the mass of a wielded object and particular scalar variables calculated from the object's inertia tensor combine linearly in determining perceived heaviness. The tensor-derived scalars reflect the symmetry and volume of the corresponding inertia ellipsoid. These measures bear directly on the object's wieldability, that is, on the patterning and level of muscular forces required to move the object in a controlled fashion.

Perceiving the sweet spot.

Many sports involve aligning a hitting implement with a ball trajectory such that contact is made at the implement's center of percussion or 'sweet spot'. This spot is not visibly distinct; its perception must be haptic. Although it is functionally defined with respect to contact--it is the point of impact that produces the least vibration in the hand holding the implement--hitting success requires appreciating the location of the sweet spot prior to contact. Two experiments verified that perceivers (novices as well as expert tennis players) distinguished perception of length from perception of the position of the sweet spot simply on the basis of wielding, both for tennis rackets and for bats contrived from wooden rods with attached masses. Results conformed to previous research on dynamic touch in showing that perceiving the lengths of wielded objects, including selectively perceiving partial lengths, is constrained by inertial properties of the object.

When nonwords activate semantics better than words.

We conducted a strong test of the idea that visual word processing and the activation of a printed word's meaning proceeds at a rate scaled by the temporal evolution of a unique and stable phonological code. Using the lexical decision task, and readers fluent in the two alphabets of Serbo-Croatian, we compared the priming of a target word such as automat by the semantically related word ROBOT and by the nonword ROBOT. Whereas the Serbo-Croatian word ROBOT can support two phonological codes, /robot/ and /rovot/, the nonword ROBOT composed by illegally mixing Roman and Cyrillic letters can support only the phonological code /robot/, that corresponding to the word whose meaning is related to automat's. At a prime duration of 35 ms, the lexical decision on the target automat was facilitated by ROBOT but not by ROBOT. At a prime duration of 125 ms, the word ROBOT was the more effective prime. One consequence of phonology's leading role in visual word recognition is that a nonword can sometimes activate a given word's meaning better than the word itself.

Perceiving the width and height of a hand-held object by dynamic touch.

The haptic perceptual sybsystem of dynamic touch is prominent in manipulating and transporting objects, providing a nonvisible awareness of their linear dimensions. The hypothesis that perceptions of object width and height by dynamic touch are different functions of the inertia tensor is addressed. In two experiments heights and widths of nonvisible wielded objects were judged separately. Experiment 1 used solid rectangular parallelepipeds of different sizes; Experiment 2 used objects of identical mass and linear dimensions but nonidentical inertia ellipsoids. Width and height perceptions of comparable reliability and accuracy were found to vary as distinct functions of the objects' inertial eigenvalues. Discussion focused on the notion of tangible shape and on the selectivity of attention within dynamic touch.

Inertial eigenvalues, rod density, and rod diameter in length perception by dynamic touch.

Four experiments addressed the relevance of the eigenvalues Ik of the inertia tensor for perceiving length by dynamic touch. Experiments 1-2 focused on the consequences of limiting variation in the minimum eigenvalue I3. Both revealed that perceived length is a function of Ik. Whether the contribution of I3 is detected, however, depends on the range of values that characterize a particular object set. Experiments 3-4 considered the relationship between an independent index of a rod's diameter, which does not affect Ik, and actual manipulation of a rod's diameter, which does affect Ik. Whereas the former appeared as satisfaction of implicit instructions to alter reports of perceived length, the latter entailed actual differences in perceived length in accordance with Ik. Results are discussed with respect to the links among actual length, perceived length, and Ik, as well as, in particular, how these links guarantee that perceived length is in the range of actual lengths.

Nonvisible perception of segments of a hand-held object and the attitude spinor.

When a rod is grasped at a place intermediate between its ends, the nonvisible perception of length by wielding can be directed to either side of the hand. The selective perception of the lengths of rod segments relative to the hand is hypothesized to depend jointly on the rod's inertia tensor about a fixed rotation point and on the 2-valued attitude spinor connecting the rod's reference frame to the hand's. That hypothesis was tested in 3 experiments m which 8-10 subjects participated; asymmetry in the rod's mass distribution relative to the hand was induced either by the addition of a metal ring to one end or by grasping the rod at a place other than its midpoint. Planes of wielding, style of wielding, and object size were varied across the experiments. The results conformed to expectation: For a given asymmetric rod configuration, perceived length for attending to one direction from the hand (e g., above or left) differed from perceived length for attending to the other direction (below or right); for a given segment of an asymmetric rod, perception of its length did not differ as a function of us direction from the hand. In each experiment, variance in perceived partial length was accommodated by the rod's major eigenvalue and the spinor rotation angle, with rotation sense dictated by the direction of attention.

Selective perception by dynamic touch.

Perceiving the length of a rod by dynamic touch is tied to the inertia tensor Iij, a quantification of its resistance to rotational acceleration. Perception of the portion extending in front of the grasp has previously been ascribed to decomposing one component of Iij by attention. The tensorial nature of dynamic touch suggests that this ability must be anchored wholly in the tensor. Three experiments show that perceived partial length is a function of two components of the tensor, one tied primarily to magnitude and the other tied primarily to direction, whereas perceived whole length is a function of a magnitude component alone. Dynamic touch is characterized in terms of a haptic perceptual instrument that softly assembles to exploit Iij differently depending on the intention, producing 1:1 maps that are appropriately scaled for each intention.

Exteroception and exproprioception by dynamic touch are different functions of the inertia tensor.

When an object is held and wielded, a time-invariant quantity of the wielding dynamics is the inertia tensor Iij. The 3 x 3 quantity Iij is composed of moments of inertia (on the diagonal) and products of inertia (off the diagonal). Examination of Iij as a function of different locations at which a cylindrical object is grasped revealed that the products related systematically to grip position (a direction), and both the products and moments taken together related systematically to the extent of the rod to one side of the hand (a magnitude in a direction). In two experiments, observers wielded an occluded rod that was held at an intermediate point along its length and reproduced both the felt grip position and partial rod length. In both experiments, perceived grip position was a function of the rod's products of inertia and perceived partial rod length was a function of the moments and products. Discussion focuses on the specificity of exteroception and exproprioception to Iij.

Effects of association, frequency, and stimulus quality on naming words in the presence or absence of pseudowords.

According to classical dual-route theory, effects of associative priming and frequency on the naming of printed words arise from lexical access and should be weak or absent when word names are assembled prelexically. Assembled naming would be more likely in a shallow orthography, especially in the presence of nonwords. This hypothesis was examined with the shallow Serbo-Croatian orthography. Interactions between association, frequency, and stimulus quality were also examined in both Serbo-Croatian and English. Contrary to classical dual-route theory, both lexical effects were found for naming words in Serbo-Croatian, with or without nonwords. Neither interaction was significant in Serbo-Croatian and only association x quality was significant in English. Discussion focused on (a) the claim that lexical effects on naming in a shallow orthography constitute prima facie evidence against either prelexical phonology or the orthographic depth hypothesis, and (b) the possible factorization of frequency and active associative knowledge in naming words.

Eigenvalues of the inertia tensor and exteroception by the "muscular sense".

Muscle spindles and Golgi tendon organs constitute the receptor foundation to the "muscle sense." Muscle sensitivity has long been assumed relevant to the non-visual perception of the positions and motions of the body's segments and of the properties of hand-held objects. Dynamic touch is the label given to the particular kind of tactile exteroperception that involves a non-spatial input from muscles and tendons. When a hand-held object is wielded, hefted, carried and so on, the hand movements, together with the physical properties of the object, produce torques and angular motions that change in time with the movement. There is, however, an unchanging quantity that relates the variable torques and angular motions, namely, the object's inertia for rotation about a fixed point in the wrist. Our research revealed that the non-visual perception of the length of a wielded object by dynamic touch is a function of muscular sensitivity to the principal moments or eigenvalues of the inertia tensor. Across four experiments, variations in object length were accompanied by variations in width, spatial and material heterogeneity, the relation of the tensorial components to mass, and geometric shape. Subjects had no foreknowledge of the variations in object dimensions. Perceived lengths of occluded objects were reported by adjusting a visible marker so that its position corresponded to the position of the felt end of the object. In each experiment, perceived length was closely related to actual length and uniquely constrained by the major and minor eigenvalues of the inertia tensor. The present results, in conjunction with previous research, suggest that the inertia tensor provides the domains for two sets of functions realized by the "muscular sense," one consisting of the principal moments of inertia or eigenvalues, which map on to perceived object magnitudes (e.g. length, weight), and one consisting of the principal directions or eigenvectors, which map on to perceived relations between hand and object (e.g. position of grasp). The significance of information-perception specificity over cognitive mechanisms is underlined and perspectives on dynamic touch and its underlying muscular sensitivity, including a general tensorial analysis, are discussed.

Lexical involvement in naming does not contravene prelexical phonology: comment on Sebastián-Gallés (1991).

N. Sebastián-Gallés (1991) showed lexical involvement in naming Spanish. Her results were purported to be a failure to substantiate claims for prelexical phonology that characterize Serbo-Croatian. Summaries of several experimental demonstrations of lexical involvement in naming Serbo-Croatian are used to show that such results are only interpretable consistently in a model that assumes prelexical phonology. The lexicon is accessed through assembled phonology, but assembled and lexical phonology interact in resolving a unique pronunciation when several pronunciations are assembled and in assigning stress, which is not assembled. The authors argue that lexical access need not be different in different orthographies but that the weights on connections between orthographic and phonological substructures established through covariant learning will distinguish orthographies in the rate and degree of phonological involvement in word recognition.

Reduction in alphabet priming with delay and degradation.

Phonologically ambiguous Serbo-Croatian words are named more slowly than their phonologically unique partners. This difference is reduced by nonword primes containing consonants unique to one or the other alphabet. In 2 experiments we investigated the hypothesis that alphabet priming is the inhibition of unique and ambiguous letter units of one alphabet by the unique letter units of the other alphabet. In Experiment 1, ambiguous and unique words followed alphabet-specific nonwords at lags between 100 ms and 1,550 ms. The ambiguous-unique difference increased from 1 ms to 45 ms, consistent with a relaxing inhibitory process. In Experiment 2 we compared priming of ambiguous words with and without visual noise. Priming was less for noisy than for intact stimuli, as would be expected if noise slows processing and if the inhibition responsible for priming weakens further during the additional processing time.

Dynamical constraints on pictorial action lines.

Pictorial action lines are an effective way of portraying movement in a static drawing. When such lines emanate from the backs of characters, they can give a sense of the path or style of movement. Seven experiments assessed whether photographic streak lines-lines that depict actual movement paths-can, in and of themselves, be informative about the act that produced them. Lines were produced as a darkly clad actor with point-lights attached to his major joints performed a number of actions in front of an open-lens camera. Completely naive Ss had little success identifying events in these photographs. Once Ss were told of the photographic technique, however, striking proficiency was achieved. Subtle distinctions (e.g., whether the movement was forward or backward or performed while wearing weights) were made for some of the events. Results are discussed in terms of various treatments of pictorial information.

Optical information about the severity of upcoming contacts.

The time derivative (tau) of the inverse of the relative rate of optical expansion (tau) may have critical values with potential implications for controlling activity. The present research addresses the particular hypothesis that tau < -0.5 specifies "unsafe" collision courses and tau > or = -0.5 specifies "safe" collision courses. Optical expansion patterns were simulated on a computer with -1.0 < or = tau < 0 and judged as suggesting a "hard" or "soft" collision. tau < -0.5 led to significantly different decisions from tau > or = -0.5, but the critical value of -0.5 was not perceived reliably as soft, a deviation possibly due to discretely approximating continuous functions. Additional experiments evaluated terminal rates of change and display duration and examined the effects of biasing the presented displays toward the soft or the hard end of the tau continuum. The results were consistent with the tau hypothesis.

Haptic probing: perceiving the length of a probe and the distance of a surface probed.

Knowing about the properties of objects by wielding them and knowing about the distances of surfaces by striking them with objects as probes are examples of dynamic or effortful touch. Six experiments focused on the invariant mechanical parameters that couple the time-varying states (displacements, velocities) of hand-held rods to the time-varying torques and forces imposed upon them by wielding and probing. There were three major conclusions. First, when a probe is wielded without contact, perceived probe length is a function of the probe's rotational inertia; however, with contact, perceived probe length is affected by the rotational inertia and the distance of the point of contact from the probe's center of percussion. Second, when a surface is struck with a probe, perceived surface distance is affected by the probe's rotational inertia and the angle of inclination of the probe at contact. Third, under seemingly identical conditions of probing, either probe length or surface distance can be perceived selectively without confusion. Results were discussed in terms of haptic information, haptic attention, and the dynamics of probing.

Phase transitions and critical fluctuations in the visual coordination of rhythmic movements between people.

By watching each other's lower oscillating leg, 2 seated Ss kept a common tempo and a particular phase relation of either 0 degrees (symmetric mode) or 180 degrees (alternate mode). This study investigated the differential stability of the 2 phase modes. In Experiment 1, in which Ss were instructed to remain in the initial phase mode, the alternate phase mode was found to be less stable as the frequency of oscillation increased. In addition, analysis of the nonsteady state cycles revealed evidence of a switching to the symmetric phase mode for the initial alternate phase mode trials. In Experiments 2 and 3, Ss were instructed to remain at a noninitial phase angle if it was found to be more comfortable. The transition observed between the 2 phase modes satisfies the criteria of a physical bifurcation--hysteresis, critical fluctuations, and divergence--and is consonant with previous findings on transitions in limb coordination within a person.