Stabilization of the total force in multi-finger pressing tasks studied with the 'inverse piano' technique

When one finger changes its force, other fingers of the hand can show unintended force changes in the same direction (enslaving) and in the opposite direction (error compensation). We tested a hypothesis that externally imposed changes in finger force predominantly lead to error compensation effects in other fingers thus stabilizing the total force. A novel device, the “inverse piano”, was used to impose controlled displacements to one of the fingers over different magnitudes and at different rates. Subjects (n = 10) pressed with four fingers at a constant force level and then one of the fingers was unexpectedly raised. The subjects were instructed not to interfere with possible changes in the finger forces. Raising a finger caused an increase in its force and a drop in the force of the other three fingers. Overall, total force showed a small increase. Larger force drops were seen in neighbors of the raised finger (proximity effect). The results showed that multi-finger force stabilizing synergies dominate during involuntary reactions to externally imposed finger force changes. Within the referent configuration hypothesis, the data suggest that the instruction “not to interfere” leads to adjustments of the referent coordinates of all the individual fingers.     

Integration of hand and finger location in external spatial coordinates for tactile localization

Tactile stimulus location is automatically transformed from somatotopic into external spatial coordinates, rendering information about the location of touch in three-dimensional space. This process is referred to as tactile remapping. Whereas remapping seems to occur automatically for the hands and feet, the fingers may constitute an exception in that some studies have implied purely somatotopic coding of touch to the fingers. When participants judge the order of two tactile stimuli, they often err when the stimulated body parts (usually the two hands) are crossed, presumably because somatotopic and external coordinates are in conflict in crossed postures. Using this task, we investigated, first, whether the fingers are unlike other limbs with regard to spatial coding, by testing whether crossing effects, indicative of external coding, were observable when stimulating two fingers, either on the same or on different hands. Second, we investigated the interaction of hand and finger posture in tactile localization of finger stimuli. Crossing effects emerged when fingers and hands were crossed, suggesting external coding for all body parts. Crossing effects were larger when both hand and finger were located in the hemifield opposite to their body side, and smaller when only hand or finger lay in the opposite hemifield. We suggest that tactile location is estimated by integrating the external location of all relevant body parts, here of a finger and its belonging hand, and that such integrative coding may represent a general principle for body part processing as well as for tool use.     

Finger interdependence and unintentional force drifts: Lessons from manipulations of visual feedback

We explored the phenomenon of unintentional finger force drift by using visual feedback on the force produced either by explicitly instructed (master) finger pairs or by non-instructed (enslaved) finger pairs. In particular, we drew contrasting predictions from two hypotheses: that force drifts represented consequences of drifts in effector referent coordinates at the level of individual fingers vs. at the level of finger modes (hypothetical variables accounting for the finger force interdependence). Subjects performed accurate force production with two fingers of a hand, index-ring or middle-little. They received visual feedback on the force produced either by the master fingers or by the other two, enslaved, fingers. The feedback scale was adjusted to ensure that the subjects did not know the difference between these two, randomly presented, conditions. Under feedback on the master finger force, enslaved force showed a consistent drift upward. Under feedback on the enslaved finger force, master force showed a consistent drift downward. The subjects were unaware of the force drifts, which could reach over 35% of the initial force magnitude. The data support the hypothesis on drifts in the referent coordinate at the level of individual digits, not finger modes, as the origin of unintentional force drifts. The consistent increase in the relative amount of force produced by the enslaved fingers suggests that the commonly used methods to quantify enslaving should include relatively brief force production tasks.     

Manipulating response set in a choice reaction task: evidence for anatomical coding in response selection

Accurate motor performance requires a process of response selection that chooses the correct response out of a set of possible ones. Most theories of response selection assume that this selection process operates on spatial codes, which define the location of stimuli and responses in environmental coordinates, with little or no role for the anatomical codes of the effectors involved. In this study, I systematically manipulated the anatomical identities of the response fingers in a four-choice reaction task, while keeping the stimulus and response locations constant. Results showed that homologous fingers on different hands modulated the reaction time profiles and error patterns in a mirror-symmetrical way. This finding indicates that anatomical factors may play a more substantial role in response selection than often assumed.     

An extension of the Riggs projection colorimeter

A method for the construction and calculation of CIE chromaticity coordinates from a simple and inexpensive projection colorimeter is presented. The device allows variation along the dimensions of hue, saturation, and brightness and specification of these variations in terms of chromaticity coordinates, dominant wavelength, relative luminance, and excitation purity.     

Instrumentation for an electrocutaneous communication device

The device described is a prototype that uses spatiotemporal resolution for electrocutaneous stimulation. Signals initiated through a keyboard activate a matrix of four electrodes that may be applied to the fingers, forearm, abdomen, back, or other parts of the integument. The instrumentation described includes a pulse generator, the active time and delay time controls, the control of the total length of a pulse train, and the constant-current generators whose output is fed into the electrodes. This device may be used as an interface with an alphanumeric device (e.g., a calculator or computer) and as a basic instrument in research investigating the spatial and temporal resolving capacities of the skin senses.     

Perceived roughness as a function of body locus

Twenty subjects made magnitude estimates of the roughness of grooved metallic surfaces applied to 10 body loci. To a first approximation, perceived roughness grew as a power function of groove width, in accordance with earlier studies. The exponents and intercepts (up-down position in log-log coordinates) of the power function turn out to depend strongly on body locus. The straight lines in log-log coordinates tend to diverge with groove width so that differences among body loci are especially pronounced at large groove widths. Sensitivity to roughness was greatest for the lips, fingers, and forearm, and least for the heel, back, and thigh. The rank order of the body loci in terms of roughness sensitivity closely parallels the rank order for punctate pressure sensitivity, as reported by von Frey in 1894, but apparently not for other measures of tactile sensitivity, such as vibration thresholds to various frequencies, two-point thresholds, and error of point localization.     

Response-repetition effects depend on motor set: Evidence for anatomical coding in response selection

Successful motor performance requires a process of response selection that chooses the correct response out of a set of possible ones. Most theories of response selection assume that this selection process operates on spatial codes, which define the location of stimuli and responses in environmental coordinates, with little or no role for the anatomical codes of the effectors involved. In this study, we tested this assumption by investigating response-repetition effects in a response-cuing paradigm using two motor sets (fingers on one hand vs. fingers on two hands). Reaction time results demonstrated a robust response-repetition benefit that was greater for the one-hand set than for the two-hands set. Furthermore, with the one-hand set the repetition benefit was independent of cue type and cue-stimulus interval on the previous trial, whereas with the two-hands set it was strongly modulated by these two factors. These differential response-repetition effects for one- and two-hands motor sets demonstrate the important role of the neuro-anatomical hand distinction in response selection, thereby supporting multiple coding notions.     

A stereotaxic positioning platform for double-angle placements

A simple and inexpensive device for double-angled positioning of electrode carriers during stereotaxic surgery is described. The device consists of a base and adjustable vertical point that is used to locate the position of final stereotaxic coordinates in space. This allows reproducible placements for deep midline brain structures that require double-angle approaches. Such placements are easily obtained with this instrument on standard and even single-carrier stereotaxic instruments.     

The television camera used to measure movement

A method is described in which the signals from a vidicon television camera are converted into vertical and horizontal voltage coordinates to reference the position of a moving target. Although limited in frequency response, the device is capable of measuring various types of movement activity.     

Performance of able-bodied subjects on a text-typing task using a head-operated device and expanded membrane cursor keys

Children and adults with disabilities frequently rely on computers to complete written tasks. Those with significant motor limitations typically use alternative computer-input devices since the traditional keyboard and mouse are insufficient to accommodate their abilities. For persons unable to isolate their fingers, input devices controlled by movements of the head or whole hand or arm movements may be among the options considered. This study investigated the performance of a head-operated device and expanded membrane cursor keys for text entry. Data from 24 young adults indicated the head-operated device performed significantly faster given reduced cognitive demands for device operation, increased stimulus-response compatibility, and simplicity of movement. Use of the expanded membrane cursor keys resulted in significantly lower error rates. No significant differences in comfort or ease of use were reported for the two devices. The relative performance of device options for users sharing similar motor challenges provides rehabilitation specialists with important clinical information.     

A rotator for X/Y oscilloscope displays

A simple and relatively inexpensive electronic circuit is described that will generate the Cartesian coordinates of a rotated display for both the X- and Y-axes of a conventional oscilloscope type of display monitor. The device will rotate the display through fixed angles and possesses the merits of very low intrinsic noise and extremely high speeds of response. It is particularly suited for computer control of stimulus orientation in psychophysical investigations of spatial contrast vision.     

A simple method for measuring brain asymmetry in children: Application to autism

A device for measuring signal transfer within and between hemispheres has been developed at the Center for Neuropsychological Research at the University of Trier, Germany. It contains two identical panels allowing both tactile stimulation and motor response with buttons for the fingers of each hand. The buttons have two functions. They can exert a slight tactile stimulation to a finger, and they can be pressed down by the finger to provide a motor response to the tactile stimulation allowing measuring the response time. The device was used for measuring brain asymmetry in tactile processing autistic children. The participants were given a finger tapping test followed by the procedures with unilateral and bilateral processing of tactile stimulation. All participants responded positively to the test procedure and accepted it as a kind of game. The results indicated that brains were more asymmetrical in autistic children than in controls: The right hemisphere functioned quicker than the left hemisphere.     

Perceptual learning following a long-lasting tactile reversal.

Crossing 2 adjacent fingers produces a distorted perception when 2 tactile stimuli are touched to the crossed fingertips. This is because a given pair of fingers has a functional range of action within which spatial perception is correct (uncrossed fingers) and beyond which perceived location of tactile stimuli is wrong (crossed fingers). The present study tested the possibility that this range of action could be modified following a long-lasting crossing. Therefore, the functional range of action of the 2nd and 3rd fingers was tested every month in 6 volunteers who maintained the finger crossing for as long as 6 months. The results show that after a variable period of time the range of action was enlarged such that spatial perception was correct with crossed fingers as well. The perceptual organization of the human hand therefore depends on experience and is not rigidly determined on a genetic basis.     

The virtual haptic display: A device for exploring 2-D virtual shapes in the tactile modality

In order to understand better the processes involved in the perception of shape through touch, someelement of control is required over the nature of the shape presented to the hand and the presentation timing. To that end, we have developed a cost-effective, computer-controlled apparatus for presenting haptic stimuli using active touch, known as a virtual haptic display (VHD). The operational principle behind this device is that it translates black and white visual images into topographic, 2-D taxel (tactile pixel) arrays, along the same principle using in Braille letters. These taxels are either elevated or depressed at any one time representing white and black pixel colors of the visual image, respectively. To feel the taxels, the participant places their fingers onto a carriage which can be moved over the surface of the device to reveal a virtual shape. We conducted two experiments and the results show that untrained participants are able to recognize different, simple and complex, shapes using this apparatus. The VHD apparatus is therefore ideal at presenting 2-Dshapes through touch alone. Moreover,this device and its supporting software can also be used for presenting computer-controlled stimuli in cross-modal experiments.     

Reorganization of tactile perception following the simulated amputation of one finger.

The tactile counterpart of diplopia has recently been described: tactile diplopia or diplesthesia. The human fingers show a rather precise organization of the cutaneous areas eliciting diplesthesia. In particular, these areas are very extensive between nonadjacent fingers (second and fourth, second and fifth, and third and fifth), suggesting that the illusory doubling depends on the lack of reciprocal interaction between cutaneous zones during manipulation. In the present study, four subjects agreed to explore and manipulate objects only through nonadjacent fingers for a period of up to six months. The amputation of the third finger was simulated by excluding it from active exploration by means of a bandage. After six months, the diplesthetic areas of the nonadjacent fingers decreased in size or completely disappeared. It is suggested that this reorganization of tactile perception is due to the increase in temporal correlation of the sensory inputs between nonadjacent fingers.     

Dependence of finger flexion force on the posture of the nonperforming fingers during key pressing tasks

The influence of different positions of the nonperforming (idle) fingers on the maximal force contraction of flexion (master) fingers during key pressing tasks was investigated. Ten participants performed maximal voluntary flexion contractions with various combinations of the index, middle, ring, and little fingers while the idle fingers rested on or were lifted away from the supporting surface. The effect of idle finger posture on total finger force production of master fingers was dependent on finger combination. In general, force production by master fingers was higher when the idle fingers were lifted away from the supporting surface than when they rested on it. The average increase in total force production by master fingers caused by the lifting of idle fingers was +12.4% (from -8.3% to +30.2%). Force-production capability of individual master fingers can be facilitated (as high as 34.1%), unchanged, or depressed (as high as -29.0%) by lifting the idle fingers. The effect of idle finger posture on finger force production of master fingers led to changes in force deficit. Neural, anatomical, and mechanical factors might account for the dependence of finger flexion force of master fingers on the posture of the idle fingers.     

Assessment of handedness using finger spreading

In 50 right handed and 50 left handed subjects we researched the phenomenon of spreading fingers. In the right hand of right-handers we found the smallest angels between fingers, in the right hand of left-handers we found the biggest angels between the spreaded fingers. The higher the dominance of the hand, the lower is the phenomenon of spreading fingers. We assumed smaller motor units at the side of dominance.     

Haptic subitizing across the fingers

Numerosity judgments of small sets of items (≤ 3) are generally fast and error free, while response times and error rates increase rapidly for larger numbers of items. We investigated an efficient process used for judging small numbers of items (known as subitizing) in active touch. We hypothesized that this efficient process for numerosity judgment might be related to stimulus properties that allow for efficient (parallel) search. Our results showed that subitizing was not possible for raised lines among flat surfaces, whereas this type of stimulus could be detected in parallel over the fingers. However, subitizing was possible when the number of fingers touching a surface had to be judged while the other fingers were lowered in mid-air. In the latter case, the lack of tactile input is essential, since subitizing was not enabled by differences in proprioceptive information from the fingers. Our results show that subitizing using haptic information from the fingers is possible only when some fingers receive tactile information while other fingers do not.