Figure-background perception in right and left hemispheres of human commissurotomy subjects

The right and left hemispheres of four complete commissurotomy subjects were tested for the ability to recognize and integrate figure and background elements of composite visual stimuli. In the first experiment the subjects were required to identify from a four-choice array in free vision the stimulus card that matched the briefly lateralized sample stimulus. For all subjects the left hemispheres was proficient at identifying the figure, but performed at near-chance level in recognizing the textured background. In contrast, the right hemisphere was equally adapt at identifying figures and backgrounds. Both hemispheres could easily identify the isolated figure or background from a choice array, demonstrating that the observed hemisphere effects were due to figure-background interactions rather than the result of any difficulty in processing specific elements of the composite stimulus. The second experiment involved the determination of the size and position of a dot that appeared against various plain and textured backgrounds. The right hemisphere of two subjects, but not the left, performed with greater accuracy when the background consisted of a 'natural' texture gradient rather than a plain white backing. Similar though less consistent results were obtained when an inverted gradient or an evenly spaced grid was used as the background. For each condition, right-hemisphere performance resembled that of normal control subjects. In contrast, the left hemisphere provided a pattern of results dissimilar to that of control subjects for the various figure-background tasks described. It appeared to be generally insensitive to background effects, except when the information provided by the background was highly unusual, as from an inverted texture gradient. The results suggest a preeminent role for the right hemisphere in the recognition of background components of a whole-field stimulus, sensitivity to the influence of the background on the perception of an object, and the ability to use natural perspective cues to assist in the accurate perception of an object.     

Mental transformations in the identification of left and right hands.

Subjects determined as rapidly as possible whether each line drawing portrayed a left or a right hand when the drawings were presented in any of four versions (palm or back of either hand) and in any of six orientations in the picture plane. Reaction time varied systematically with orientation and, in the absence of advance information, was over 400 msec longer for the fingers-down orientation. However, when subjects were instructed to imagine a specified (palm or back) view of a specified (left or right) hand in a specified orientation, reaction times to test hands that were consistent with these instructions were short (about 500 msec), independent of orientation, and unacompanied by errors. It is proposed that subjects determine whether a visually presented hand is left or right by moving a mental "phantom" of one of their own hands into the portrayed position and by then comparing its imagined appearance against the appearance of the externally presented hand.     

Pointing with the left and right hands in congenitally blind children

Congenitally blind and blindfolded sighted children at ages of 6, 8, 10 and 12 years performed a pointing task with their left and right index fingers at an array of three targets on a touch screen to immediate (0 s) and delayed (4 s) instructions. Accuracy was greater for immediate than delayed pointing and there was an effect of delay for the orientation of the main axis of the pointing distribution in both groups, indicating distinct spatial representations with development such as ego- and allocentric frames of reference, respectively. The pointing responses of the blind covered less surface area indicating better overall accuracy as compared to the sighted blindfolded. The hands differed for four of the six precision and accuracy parameters. The right hand performed better and seemed relatively contextually oriented, whereas the responses of the left hand were closer to the body and egocentrically oriented. The elongation of the scatter of the pointing responses was greater for the boys and more allocentrically oriented, indicating gender differences in spatial representation. The study provides a first evidence of ego- and allocentric spatial frames of reference in congenitally blind children and an ability to point at targets with the left and right hands in the total absence of vision.     

Production of successive force impulses by the left and right hands

Right-handed participants made ballistic force impulses with the index finger of their left or right hand to match the amplitude of 2 successive visual targets whose onset was separated by 600-850 ms. The targets specified either low- (10% of maximum voluntary contraction; MVC) or high- (25% MVC) force level and were presented in all combinations (low-low, low-high, high-low, high-high). The amplitude of the 1st force impulse affected that of the 2nd, forming an effect that was greatest at the shortest interresponse intervals and that decayed with increasing intervals. This effect was smaller with a shorter time course in the dominant hand than the nondominant hand, suggesting a greater ability of the dominant hand to make successive independent responses.     

Accommodation to increased accuracy demands by the right and left hands

This study was designed to identify the phase of rapid aimed movements responsible for hand differences in motor skill, and to evaluate potential differences between the hands in accommodating to greater accuracy demands. In both experiments, an accelerometer mounted on a stylus allowed key changes in acceleration to be used to partition the movement into phases. In Experiment 1, slower left hand movement times were attributable primarily to a terminal homing-in phase, especially as target size decreased. Since error rates varied as a function of hand and target size, speed-accuracy trade-offs may have occurred. Experiment 2 rigidly controlled error rate and confirmed the major hand difference to occur in the latter phase of the movement where error correction is presumed. Although less pronounced, adjustments were made in the earlier movement phases as well. Accommodation to greater accuracy demands involved moving the stylus closer to the target before decelerating to engage in error correction. This adjustment to gain enhanced precision was more pronounced in the left hand.     

Kinesthetic aspects of mental representations in the identification of left and right hands

Kinesthetic aspects of mental representations of one’s own hands were investigated. Line drawings showed a human hand in one of five versions, in which finger position and wrist rotation varied; each version occurred as a left and as a right hand, and could appear in any one of eight directions in the picture plane. The subject was required to make quick judgments of whether a left or a right hand was represented, under three conditions of head tilt (left, upright, right). Reaction time varied systematically, reflecting the time required to move one’s own hand into congruence with the stimulus. Head tilt influenced the subjective reference frame of mental rotation when the degree of head tilt was 60 deg.     

Prehension and perception of size in left visual neglect

Right hemisphere damaged patients with and without left visual neglect, and age-matched controls had objects of various sizes presented within left or right body hemispace. Subjects were asked to estimate the objects' sizes or to reach out and grasp them, in order to assess visual size processing in perceptual-experiential and action-based contexts respectively. No impairments of size processing were detected in the prehension performance of the neglect patients but a generalised slowing of movement was observed, associated with an extended deceleration phase. Additionally both patient groups reached maximum grip aperture relatively later in the movement than did controls. For the estimation task it was predicted that the left visual neglect group would systematically underestimate the sizes of objects presented within left hemispace but no such abnormalities were observed. Possible reasons for this unexpected null finding are discussed.     

Perception of numerical stimuli felt by fingers of the left and right hands

Three experiments are reported in which blindfolded right-handed adults felt numerical stimuli with the middle fingers of their left or right hands. These stimuli consisted of collections of raised dots in random arrangement to be enumerated (Experiment I), collections of evenly spaced raised dots in a straight line to be enumerated (Experiment II), and raised digits to be identified (Experiment III). Differences between hands were only found in Experiment I. The left hand was faster, apparently reflecting specialisation of the right cerebral hemisphere for the analysis of complex spatial stimuli. A fourth experiment, in which collections of raised dots in random arrangement to be enumerated were felt through a piece of cloth by subjects who were not blindfolded, confirmed the left hand superiority and demonstrated that it had not arisen from loss of sight of the movements of the dominant hand.     

Pushing the limits of task difficulty for the right and left hands in manual aiming

The effects of task difficulty on the performance of the two hands in right-handers were examined in a manual-aiming paradigm. In order to examine both simple and complex tasks, movement amplitude, cursor size, and target size were manipulated, resulting in eight different indices of difficulty (as defined by Fitts' Law). Kinematic parameters examined included reaction time, movement time, time to and time after peak velocity, peak velocity, and resultant accuracy. Analysis revealed no differential effects of task difficulty on the overall movement times of the two hands. These results are discussed in light of current theories of manual asymmetries.     

Interference within hands: Retrieval-induced forgetting of left and right hand movements

We examined retrieval-induced forgetting of motor sequences that were categorized by the effectors (left or right hand) involved in their execution. This left–right categorization was independent from input locations or input devices. In addition, the acquired motor sequences were arbitrarily assigned to left and right. Participants learned twelve sequential joystick movements as responses to letter stimuli. Half of the sequences pertained to the left, half to the right hand. Subsequent retrieval-practice of half the items of one hand induced forgetting for the non-retrieved rest of the items of that hand in a final recall test. This finding demonstrates that the hands were used to organize the memory storage of motor sequences in a way that gave rise to later interference between commonly stored items, that is, linked to the same hand.     

Bilateral tachistoscopic word perception of stutterers and normal subjects

Bilateral tachistoscopic procedures were utilized to investigate the visual half-field preferences of 15 stutterers and a group of 15 normal controls. Statistical analyses indicated a right visual half-field preference for the control group. In contrast, a significant visual half-field preference was not revealed for the stuttering group. However, further analysis revealed that a significantly larger proportion of stutterers, compared to controls, demonstrated a left visual half-field preference. Results indicated reversed cerebral processing for the stuttering group as compared with the control group.     

Temporal regularity of tapping by the left and right hands in timed and untimed finger tapping

The temporal characteristics of repetitive finger tapping by the left and right hands were examined in two experiments. In the first experiment, interresponse intervals (IRIs) were recorded while right-handed male subjects tapped in synchrony with an auditory timing pulse (the synchronization phase) and then attempted to maintain the same tapping rate without the timing pulses (the continuation phase). The left and right hands performed separately, at four different rates (interpulse intervals of 250, 500, 750, and 1500 ms). There was no asymmetry of the asynchronies of the timing pulses and the associated responses in the synchronization phase or of the IRIs in either phase, but there was an asymmetry of chronization phase or of the IRIs in either phase, but there was an asymmetry in the temporal dispersion of the responses in both phases. in the second experiment, right-handed males tapped separately with each hand at three different speeds: as quickly as possible, at a fast but steady rate, and at a slow rhythmical rate. The speed asymmetry present when tapping as quickly as possible (with the preferred hand tapping more quickly ) was reduced when tapping at the fast steady rate and was absent when tapping at the slow rhythmical rate. The temporal dispersion of the IRIs produced by the nonpreferred hand was greater than the temporal dispersion of those produced by the preferred hand in all speed conditions. These results show smaller temporal dispersion of tapping by the preferred hand in right-handed males under different conditions, including submaximal speeds at which both hands respond at the same rate. This suggests that the motor system controlling the preferred hand in right-handers had more precise timing of response output than that controlling the nonpreferred hand.     

Roughness perception across the hands

It has previously been shown that the perceived roughness of a surface touched by one digit is influenced by the roughness of a different surface touched simultaneously by another digit on the same hand. The present study was designed to examine whether this is the case when surfaces of varying roughness are touched using digits on separate hands. Participants touched pairs of sandpaper surfaces, in sequence, using the same digit, and identified which of the two was rougher. Roughness discrimination was measured in the presence of distractor surfaces touched simultaneously with the target surface, but using a different digit either on the same or on the other hand. The overall perception of roughness of the attended surfaces was better on the left than on the right hand. Perceived roughness also varied systematically with the roughness of the distractor surfaces. Attended surfaces were more likely to be perceived as smoother when they were paired with smooth rather than rough distractors. Likewise, attended surfaces tended to be perceived as rougher with rough distractors. This pattern of results occurred whether the attended and distractor digits were on the same hand or different hands. These data confirm that it is difficult to restrict tactile attention for roughness to a single digit and show that this difficulty extends to restricting attention to a single hand. Furthermore, the effect of a stimulus at an unattended body location was not simply to impair perception in general, but to bias it in the roughness direction of the distractor surface.     

Velocities of motor and sensory nerve conduction are the same for right and left arms in right- and left-handed normal subjects

The velocities of motor and sensory conduction of median and ulnar nerves were measured on the left and right arms of 33 right-handed and 12 left-handed normal subjects. Contrary to current knowledge there was no statistically significant difference in the velocities of nerve conduction on the left and right sides of these subjects. It was suggested that the differences in the velocities of nerve conduction cannot contribute to the mechanisms of handedness.     

Differences between right versus left lateral body tilt in its effect on the visual and tactual perception of verticality

Summary Ninety six Ss were subjected to lateral body tilt, ranging from 10° to 90° to the right and to the left, and indicated the visual vertical (by means of a luminescent rod) and the tactual vertical (by means of a metal bar) in a darkroom. There were significant differences for deviations of apparent from true vertical in both series between body tilt to the right versus to the left. With tilts to the right, apparent vertical deviated more opposite the direction of tilt, the E-phenomenon, or less in the direction of tilt, the A-phenomenon, as compared with tilts to the left.This study was supported, in part, by a Public Health Service Research Grant, MH 00348, from the National Institute of Mental Health, while the author was a NATO research fellow at Clark University, Worcester, MA; and in part by a Federal Grant, MR HD 06276, to the Children's Hospital Medical Center, while the author was a research fellow in child psychiatry at Harvard University, School of Medicine, Boston, MA.     

The perception of emotionally toned sentences by right hemisphere-damaged and aphasic subjects

The perception of 60 emotionally toned sentences, divided into meaningful and meaningless utterances, by high- and low-verbal aphasics and right hemisphere-damaged subjects who were not aphasic was investigated. The intended moods were happiness, sadness, and anger. Results indicated (a) significant differences between the aphasic groups in identifying the intended emotion but none between groups in identifying the intended emotion but none between either of these groups and the right hemispheric dysfunction subjects; (b) sentence meaningfulness only affected the responses of high-verbal aphasics; and (c) while the happy mood seemed to be the least frequently selected among the three moods, closer inspection indicated that the subjects in absence of response bias were distinguishing among the three moods with equal success.     

Word length and orthographic neighborhood size effects in the left and right cerebral hemispheres

Previous studies have reported an interaction between visual field (VF) and word length such that word recognition is affected more by length in the left VF (LVF) than in the right VF (RVF). A reanalysis showed that the previously reported effects of length were confounded with orthographic neighborhood size (N). In three experiments we manipulated length and N in lateralized lexical decision tasks. Results showed that length and VF interacted even with N controlled (Experiment 1); that N affected responses to words in the LVF but not the RVF (Experiment 2); and that when length and N were combined, length only affected performance in the LVF for words with few neighbors.