Biomedical subjects

D H Ashmead

Publications and source records attributed to D H Ashmead.

At least 19 recordsLinked to original sources

PubMed1999

Visual control of reaching and grasping in infants.

The role of visual input during reaching and grasping was evaluated. Groups of infants (5, 7, and 9 months old) and adults reached for an illuminated object that sometimes darkened during the reach. Behavioral and kinematic measures were assessed during transport and grasp. Both infants and adults could complete a reach and grasp to a darkened object. However, vision was used during the reach when the object remained visible. Infants contacted the object more often when it remained visible, though they had longer durations and more movement units. In contrast, adults reached faster and more precisely during transport and grasp when the object remained visible. Thus, continuous sight of the object was not necessary, but when it was available, infants used it for contacting the object whereas adults used it to reach and grasp more efficiently.

Adult↗

PubMed1998

Spatial hearing in children with visual disabilities.

A study is reported of the effect of early visual experience on the development of auditory space perception. The spatial hearing of thirty-five children with visual disabilities (twenty-two with congenital total blindness) was compared with that of eighteen sighted children and seventeen sighted adults. The tests provided a comprehensive assessment of spatial-hearing ability, including psychophysical estimates of spatial resolution in the horizontal, vertical, and distance dimensions, as well as measures of reaching and walking to the locations of sound sources. The spatial hearing of the children with visual disabilities was comparable to or somewhat better than that of the sighted children and adults. This pattern held even when the group with visual disabilities was restricted to those children with congenital total blindness; in fact, some of those children had exceptionally good spatial hearing. These findings imply that the developmental calibration of human spatial hearing is not dependent on a history of visual experience. It seems likely that this calibration arises from the experience of changes in sound-localization cues arising from self-motion, such as turning the head or walking. As a practical matter, orientation and mobility instructors may reasonably assume that individuals with visual disabilities can use their hearing effectively in day-to-day travel situations.

Adolescent↗

PubMed1998

Temporal processing in the aging auditory system.

Measures of monaural temporal processing and binaural sensitivity were obtained from 12 young (mean age = 26.1 years) and 12 elderly (mean age = 70.9 years) adults with clinically normal hearing (pure-tone thresholds < or = 20 dB HL from 250 to 6000 Hz). Monaural temporal processing was measured by gap detection thresholds. Binaural sensitivity was measured by interaural time difference (ITD) thresholds. Gap and ITD thresholds were obtained at three sound levels (4, 8, or 16 dB above individual threshold). Subjects were also tested on two measures of speech perception, a masking level difference (MLD) task, and a syllable identification/discrimination task that included phonemes varying in voice onset time (VOT). Elderly listeners displayed poorer monaural temporal analysis (higher gap detection thresholds) and poorer binaural processing (higher ITD thresholds) at all sound levels. There were significant interactions between age and sound level, indicating that the age difference was larger at lower stimulus levels. Gap detection performance was found to correlate significantly with performance on the ITD task for young, but not elderly adult listeners. Elderly listeners also performed more poorly than younger listeners on both speech measures; however, there was no significant correlation between psychoacoustic and speech measures of temporal processing. Findings suggest that age-related factors other than peripheral hearing loss contribute to temporal processing deficits of elderly listeners.

Adult↗

PubMed1996

Measuring habituation in infants: an approach using regression analysis.

The effectiveness of different habituation criteria was examined by means of computer simulations. A criterion based on fitting a second-order polynomial regression function to the looking time data was described. This criterion produced more accurate estimation of looking times as well as higher experimental power for detecting novelty effects, compared to the traditional windowed running average criterion or to a criterion based on linear regression. The polynomial regression approach probably has this advantage because it utilizes all of the available looking time data, rather than just the data in the current windowed average, and because it is sensitive to nonlinear trends in looking time. This new habituation criterion is easy to implement on a laboratory computer, and it should increase session lengths by no more than one trial or so, compared to windowed average criteria. With regard to test-retest reliability, all of the habituation criteria that were evaluated appear to have low reliability on average, with high sample-to-sample variability. These undesirable reliability characteristics are attributable to the high variability of infants' attentional behavior.

Attention↗

PubMed1995

Contribution of listeners' approaching motion to auditory distance perception.

Of the several sources of acoustic information for distance perception, those arising from motion of the listener or sound source have received little attention. This motion-related information (recently called acoustic tau) is described, and experiments evaluating its utilization are presented. Accuracy and consistency at walking to the locations of briefly presented sounds were better when people listened while walking than while standing still. Manipulations of the sound to simulate shorter or longer target distances produced appropriate undershooting but not overshooting. The results indicate that people use motion-related acoustic information about distance to guide their locomotor actions, although they do not take full advantage of this information.

Adult↗

PubMed1995

Calibration of human locomotion and models of perceptual-motor organization.

People coordinate the force and direction of skilled actions with target locations and adjust the calibrations to compensate for changing circumstances. Are the adjustments globally organized (adjusting a particular action to fit a particular circumstance would generalize to all actions in the same circumstance); anatomically specific (every effector is adjusted independently of others); of functional (adjustments would generalize to all actions serving the same goal and generating the same perceptible consequences)? Across 10 experiments, changes in the calibration of walking, throwing, and turning-in-place were induced, and generalization of changes in calibration to functionally related and unrelated actions were tested. The experiments demonstrate that humans rapidly adjust the calibration of their walking, turning, and throwing to changing circumstances, and a functional model of perceptual-motor organization is suggested.

Adult↗

PubMed1994

MRI asymmetries and language dominance.

OBJECTIVE: To examine the relationship between language dominance, as measured by Wada testing, and hemispheric asymmetries on MR brain images. BACKGROUND: A previous report that did not include verification of language dominance compared the length of the planum temporale with hemispheric asymmetries seen on CT and inferred that occipital lobe asymmetry is related to language dominance. METHODS: Language dominance was identified by the Wada test in 57 patients evaluated for surgical treatment of epilepsy. Fifty-five had an MRI scan that allowed accurate measurement. In a blinded fashion, two examiners independently measured bilateral frontal, parietal, and occipital lobe lengths on MR scan for each patient. Measurements of asymmetries were compared with language dominance established by the Wada test. RESULTS: Reliability of measurement between the examiners was 97%. Asymmetry of the occipital lobe length on MR scan 10 mm above the tentorium was the only measurement significantly related to language dominance (p < 0.01). Occipital lobe length was longer on the left in 19 (40%) and on the right in 10 (21%) patients with left dominance. The right lobe was longer in six of seven (86%) patients with bilateral dominance. One patient with right hemisphere dominance had a longer left lobe. None of the measurements significantly related to handedness. CONCLUSION: Asymmetries of occipital lobe length relate to language dominance, but such dominance cannot be reliably identified by MR in an individual patient.

Adolescent↗

PubMed1993

Is visually guided reaching in early infancy a myth?

The issue examined was whether infants require sight of their hand when first beginning to reach for, contact, and grasp objects. 7 infants were repeatedly tested between 6 and 25 weeks of age. Each session consisted of 8 trials of objects presented in the light and 8 trials of glowing or sounding objects in complete darkness. Infants first contacted the object in both conditions at comparable ages (mean age for light, 12.3 weeks, and for dark, 11.9 weeks). Infants first grasped the object in the light at 16.0 weeks and in the dark at 14.7 weeks, a nonsignificant difference. Once contact was observed, infants continued to touch and grasp the objects in both light and dark throughout all sessions. Because infants could not see their hand or arm in the dark, their early success in contacting the glowing and sounding objects indicates that proprioceptive cues, not sight of the limb, guided their early reaching. Reaching in the light developed in parallel with reaching in the dark, suggesting that visual guidance of the hand is not necessary to achieve object contact either at the onset of successful reaching or in the succeeding weeks.

Attention↗

PubMed1993

Visual guidance in infants' reaching toward suddenly displaced targets.

This experiment evaluated the role of visual input about the location of a target object and the location of the hand in reaching by infants and adults. 5- and 9-month-old infants were presented with illuminated toys to reach for in a dark room. On no-switch trials, the toy remained illuminated throughout the infant's reach, whereas on switch trials the first-lit toy was replaced during the reach by a second-lit toy at a different position. On approximately half of the trials of each type a luminescent marker was attached to the reaching hand. Adult subjects (tested without the hand marker) fully compensated to the second-lit toy on switch trials, during a second reaching segment. On switch trials, 9-month-olds partially adjusted to the second-lit toy when wearing the hand marker and did not adjust without it. On no-switch trials, 9-month-olds reached just as accurately with or without the hand marker. 5-month-olds were generally inaccurate in their reaching and were unaffected by the presence or absence of the hand marker. The findings suggest that during the development of reaching there is an increase in visual guidance during the approach phase of reaches.

Adult↗

PubMed1993

Computer simulation technique for assessing pediatric auditory test protocols.

Adaptive testing procedures are widely used for evaluating the hearing of young children. A standard protocol for such testing, however, has not been recommended and, as a result, many variations of the procedure are used clinically. This study, by utilizing computer simulations, varied several test parameters and examined the resulting test outcomes. We evaluated the effects of starting level, stimulus step size, and the use of conditioning trials on test outcome, while also varying the hearing levels and false positive and and negative rates of our simulated subjects. Results indicated that a low starting level, with a 20-dB down 10-dB up step size, and no conditioning trials produced the most accurate estimates of thresholds under most conditions.

Acoustic Stimulation↗

PubMed1991

Sound localization and sensitivity to interaural time differences in human infants.

The role of interaural time differences in infants' sound localization was investigated. One experiment on free-field sound localization corroborated previous findings that the minimum audible angle changes substantially toward the end of the first half year after birth, and 3 experiments explored interaural time discrimination in that age range. The first of these 3 experiments used an adaptive psychophysical procedure, showing that infants responded appropriately in a sound lateralization task that provides a direct measure of interaural time discrimination. The other 2 experiments improved on the psychophysical procedure by taking into account the ceiling level on performance in the task. Infants aged 16, 20, and 28 weeks had thresholds in the range of 50 to 75 microsec, with no apparent age difference. These thresholds were much lower than would be predicted from studies of free-field sound localization, indicating that sensitivity to interaural time differences is not a limiting factor for the precision of sound localization in this age range. Instead, age-related changes in free-field sound localization may reflect the need to integrate across different localization cues and to calibrate the changing values of cues due to head growth.

Acoustic Stimulation↗

PubMed1991

Postural sway of human infants while standing in light and dark.

Postural sway was measured in 12-14-month-old human infants and in adults while they were standing in the light and dark. Spectral density analyses conducted on all frequencies, at specific frequencies, and for individual subjects showed that infants generally did not sway significantly more in the dark than in the light, whereas adults did. For example, infants' dark/light sway proportions were 1.12 and 1.21 for the anterior-posterior and lateral dimensions, respectively, compared to adult values of 2.23 and 3.43 for one-footed stance, and 1.43 and 2.13 for two-footed stance. A statistical power analysis indicated that if the dark/light proportions for infants had been comparable to those for adults, significant differences could have been detected. These findings indicate that the early regulation of standing posture does not depend on the continuous availability of visual information.

Dark Adaptation↗

PubMed1990

Visual perception and the guidance of locomotion without vision to previously seen targets.

Two experiments were performed to assess the accuracy and precision with which adults perceive absolute egocentric distances to visible targets and coordinate their actions with them when walking without vision. In experiment 1 subjects stood in a large open field and attempted to judge the midpoint of self-to-target distances of between 4 and 24 m. In experiment 2 both highly practiced and unpracticed subjects stood in the same open field, viewed the same targets, and attempted to walk to them without vision or other environmental feedback under three conditions designed to assess the effects on accuracy of time-based memory decay and of walking at an unusually rapid pace. In experiment 1 the visual judgments were quite accurate and showed no systematic constant error. The small variable errors were linearly related to target distance. In experiment 2 the briskly paced walks were accurate, showing no systematic constant error, and the small, variable errors were a linear function of target distance and averaged about 8% of the target distance. Unlike Thomson's (1983) findings, there was not an abrupt increase in variable error at around 9 m, and no significant time-based effects were observed. The results demonstrate the accuracy of people's visual perception of absolute egocentric distances out to 24 m under open field conditions. The accuracy of people's walking without vision to previously seen targets shows that efferent and proprioceptive information about locomotion is closely calibrated to visually perceived distance. Sensitivity to the correlation of optical flow with efferent/proprioceptive information while walking with vision may provide the basis for this calibration when walking without vision.

Adult↗

PubMed1990

Perception of the relative distances of nearby sound sources.

The pressure of a sound varies systematically with a listener's distance from a sound source, providing a useful cue for perceiving changes in the distance between a listener and a sound-producing object. The pressure-discrimination hypothesis predicts that thresholds for discriminating changes in distance are constrained by the underlying ability to discriminate the resulting changes in sound pressure--specifically, that the smallest discriminable change in distance should be about 5% of the reference distance. Previous studies reported thresholds of about 5% for reference distances greater than a few meters but surprisingly worse thresholds for closer reference distances. In the present study, thresholds at two close distances, 1 and 2 m, were within the 5% range predicted from the pressure-discrimination hypothesis. Moreover, thresholds were significantly worse in a control condition in which the pressure cue was removed. Results of previous studies were adjusted to take into account the possibility of conservative response tendencies by the subjects. These adjusted findings agree well with the results of the present study and the pressure-discrimination hypothesis. It is concluded that variations in sound pressure are very useful for perceiving changes in listener-source distances, even at close distances.

Adult↗

PubMed1989

Obstacle perception by congenitally blind children.

The ability to perceive objects from a distance and navigate without vision depends principally on auditory information. Two experiments were conducted in order to assess this ability in congenitally blind children aged 4 to 12 years who had negligible amounts of visual experience or formal mobility training. In Experiment 1, children walked along a sidewalk toward a target location to get some candy. A box was placed along the path on some trials, and the children were instructed to avoid the box if it was present. The children spent more time in the region just in front of the box than in the region just behind it, indicating that they perceived the box and acted so as to navigate around it. In Experiment 2, children attempted to discriminate whether a nearby disk was on their left or on their right. The children performed at above-chance levels, again indicating distal perception of objects. The results of both experiments suggest that blind children with little or no visual experience or formal training utilize nonvisual information, presumably auditory, to perceive objects. The specific nature of this auditory information requires further investigation, but these findings imply that the underlying perceptual ability does not require experience in spatial vision or deliberate training and intervention.

Auditory Perception↗

PubMed1986

Development of auditory localization in dogs: single source and precedence effect sounds.

The development of auditory localization in dogs was investigated in a litter of 12 pups. Behavioral auditory localization consisted of orienting responses to dog vocalizations presented from loudspeakers 90 degrees to each side. Sounds were presented in two configurations, single source (only one loudspeaker) and precedence effect (both loudspeakers, with one slightly leading the other). Localization began around 16 days after birth, for single-source sounds. This is consistent with previous observations and with findings on dogs' auditory neural development. Single-source sounds were localized earlier during development than precedence-effect sounds. This ordering resembles findings on human infants and can be related to neuroanatomical investigations of mammalian brain structures mediating single source versus precedence effect localization.

Age Factors↗

PubMed1984

The development of anticipatory hand orientation during infancy.

The development of infants' prehensile adjustments regarding the orientation of objects was investigated. Five- and nine-month-olds (N = 16 per group) were presented with horizontally and vertically oriented dowels to determine at what point during the reach, hand orientation approximated that of the dowel. Nine-month-olds rotated their hands appropriately, early in the course of the reach, i.e., before tactual contact of the dowel, whereas five-month-olds did so mostly after tactual contact. Analyses of the effects of trials within the experimental session indicated that this age difference was not associated with practice or fatigue effects. The results are discussed in relation to the development of visual control of prehensile behavior.

Attention↗

PubMed1983

The coordination of eye and head movements during early infancy: a selective review.

A review of work dealing with the human infant's ability to use coordinated eye and head movements is presented. Primary emphasis is given to performance under two conditions: (1) the shifting of fixation to a visual stimulus in the periphery; and (2) the use of eye movements to maintain fixation by compensating for the effects of externally imposed head movements.

Adult↗