Daniel D. McCall

Visual information and object size in the control of reaching

The role of vision in the control of reaching and grasping was investigated by varying the available visual information. Adults (N = 7) reached in conditions that had full visual information, visual information about the target object but not the hand or surrounding environment, and no visual information. Four different object diameters were used. The results indicated that as visual information and object size decreased, subjects used longer movement times, had slower speeds, and more asymmetrical hand-speed profiles. Subjects matched grasp aperture to object diameter, but overcompensated with larger grasp apertures when visual information was reduced. Subjects also qualitatively differed in reach kinematics when challenged with reduced visual information or smaller object size. These results emphasize the importance of vision of the target in reaching and show that subjects do not simply scale a command template with task difficulty.

Proximodistal structure of early reaching in human infants.

Abstract Nine infants were tested, at the age of onset of reaching, seated on their parent’s lap and reaching for a small plastic toy. Kinematic analysis revealed that infants largely used shoulder and torso rotation to move their hands to the toy. Many changes in hand direction were observed during reaching, with later direction changes correcting for earlier directional errors. Approximately half of the infants started many reaches by bringing their hands backward or upward to a starting location that was similar across reaches. Individual infants often achieved highly similar peak speeds across their reaches. These results support the hypothesis that infants reduce the complexity of movement by using a limited number of degrees-of-freedom, which could simplify and accelerate the learning process. The proximodistal direction of maturation of the neural and muscular systems appears to restrict arm and hand movement in a way that simplifies learning to reach.

Does reaching in the dark for unseen objects reflect representation in infants

Five groups of 7-month-old infants had varying amounts of experience with a sounding object in the light before being presented with it in the dark. Neither visual nor auditory experience was necessary for accurate reaching in the dark, suggesting that infants represented the unseen object.

Infants’ means-end search for hidden objects in the absence of visual feedback

Abstract Infants beyond 8 months of age typically succeed in search tasks that require them to sequence multiple-stage, means-end actions. However, it is unclear whether infants depend upon visual feedback of limb movements and their consequences during search. We examined whether means-end search is dependent upon visual feedback by testing 8.5-month-olds’ ability to uncover and retrieve objects in the dark. Sound was used to direct infants’ initial responses toward a covered object, but was terminated as soon as they opened the cover, forcing them to execute the second stage of the search behavior with no further feedback. An additional manipulation involved presenting ‘no-toy’ trials in the dark or ‘surprise’ trials, on which the toy was surreptitiously removed after the lights had been extinguished. Infants successfully retrieved the toy on dark trials, albeit less frequently than in the light. The organization of their means-ends behavior in the dark was highly similar to that in the light, in terms of the number of reaches made on each trial, the accuracy of their reaches, and the latency to respond before and after opening the cover. Infants were just as likely to search on dark trials with and without a toy, suggesting that infants failed to notice or did not consider the implications of no toy put under the cover. The ability of infants to perform the relatively complicated, means-end search in the dark suggests that this sequential behavior can be carried out on the basis of the infant’s memory of the covered object and the actions necessary to achieve the goal.

Sudden changes in spectrum of an echo cause a breakdown of the precedence effect

The effect of changing the frequency components of an echo relative to the sound source was examined in a two-choice discrimination task. Subjects sat in an anechoic chamber and discriminated the direction of the lag noise burst within a lead-lag pair presented over loudspeakers. The leading noise burst was broadband, and the lagging burst was either high- or low-pass filtered. On some conditions, this test burst pair was preceded by a conditioning train of burst pairs, which also had a broadband lead and either a high- or low-frequency lag. When the frequency content of the echo was held constant across the conditioning train and test burst pair, echo suppression that was built up during the repeating train was maintained for the test burst pair, shown by the subjects’ poor performance in detecting the location of the lagging burst. By comparison, subjects had little difficulty in localizing the lagging burst when the frequency content of the echo changed between the conditioning train and the test burst, indicating that any buildup of suppression during the train was broken when the lagging burst’s spectrum shifted. The data are consistent with an interpretation in which echo suppression is temporarily broken when listeners’ built-up expectations about room acoustics are violated.

Sudden changes in simulated room acoustics influence echo suppression

Listeners seated in an anechoic chamber discriminated the direction of the lag noise burst within a lead‐lag pair. The lead loudspeakers were 42° or 48° left of midline, and the lag loudspeakers were 35° or 55° right. A train of noise bursts, during which the lag was always 45° right, preceded the test noise in some conditions. In the first experiment the lead was broadband (‘‘B’’) and lag was either high‐(‘‘H’’) or low‐pass (‘‘L’’) filtered. Discrimination of the test noise location was poorer when the B‐L (lead‐lag) test was preceded by a B‐L train than when the test noise was presented in isolation, suggesting that echo suppression built up during the repetitious train. By comparison, discrimination of the B‐L test was unaffected by a B‐H train, indicating that any buildup of suppression during the train was broken when the lag’s frequency shifted from H to L, which simulated a change in absorption of echoes by room surfaces. A second experiment in which the intensity of the lag varied between train an...

Perception of multiple echoes

Since Ebata et al. [ 537–541 (1968)], little experimental or theoretical work has been done on the perception of multiple echoes. In the current experiment, listeners seated in an anechoic chamber were presented with 4‐ms noise bursts in a lead‐lag‐lag configuration. The lead burst was presented from a loudspeaker at 45° left of midline and one lag burst (lag A) was presented at midline. Listeners discriminated the location of a second lag burst (lag B) presented from a loudspeaker at either 35° or 55° right of midline. Keeping the delay between lead and lag B constant, a range of delays between lead and lag A was presented to determine whether changing the delay of lag A had an effect upon the perception of lag B. Presentation of lag A interfered with the perception of lag B over a wide range of lag A delays, such that subjects’ discrimination of the location of lag B was poorer when compared to the discrimination of lag B when lag A was absent. The data suggest that the introduction of an earlier echo serves to suppress directional information from a subsequent echo. [Work supported by NIH grant DC01625.]

The influence of two‐source noise on echo suppression

When listeners are presented with lead and lag pairs of clicks from two loudspeakers in an anechoic room, they hear a single image near the location of the lead loudspeaker, provided that the delay of the lag is only a few ms. However, Thurlow and Parks [Perceptual and Motor Skills 13, 7–12 (1961)] reported that the addition of a continuous background of noise from a single loudspeaker causes the lag click to become audible as a separate auditory event. The current experiment investigated whether this apparent breakdown of echo suppression would also occur if the background noise was presented from two loudspeakers with one delayed relative to the other. The signal was a pair of brief white‐noise bursts presented from loudspeakers located at 45 deg right and 45 deg left in an anechoic chamber, with delays ranging from 2 to 14 ms. A continuous background white noise presented from the same two loudspeakers disrupted echo suppression only when the direction of its time delay was inconsistent with the signal...