The use of ischemic nerve block procedures in the investigation of the sensory control of movements.

In this study the ischemic nerve block (INB) technique is investigated as a procedure for blocking afferent information in the study of the sensory control of movements. A movement replication task is used. The results suggest that the elbow, wrist and metacarpal-phalangeal joints are at three different stages of sensory information loss or reduction. With the index finger, replications of joint position are not made in the correct direction significantly greater than chance. Error is also largest for the finger. The elbow joint appears to be least affected by the block. Motor impairment under the block is also investigated and the results show that there is a marked decrement in the force exerted in flexion and extension of the index finger, wrist and elbow joints after sensory drop-out. This finding indicates that there is substantial motor impairment of movements distal to the block. The limitations of the INB technique as a procedure for investigating the control of movement under conditions of sensory loss are discussed.

Acuity of sound localisation: a topography of auditory space. III. Monaural hearing conditions.

A study is reported in which the acuity of azimuth and elevation discrimination under monaural listening conditions was measured. Six subjects localised a sound source (white noise through a speaker) which varied in position over a range of elevations (-40 degrees to +40 degrees) and azimuths (0 degrees to 180 degrees), at 10 degrees intervals, on the left side of the head. Monaural listening conditions were established by the fitting of an ear defender and one earmuff to the right ear. The absolute and algebraic, azimuth and elevation errors were measured for all subjects at each position of the source. The results indicate that all subjects suffered a marked reduction of azimuth acuity under monaural conditions, although a coarse capacity to discriminate azimuth still remained. Considerable between-subject variability was observed. Front/back discrimination was retained, although it was slightly impaired compared to that observed under normal listening conditions. Elevation discrimination was, on the whole, quite good under monaural conditions. However, a comparison of the performance of these subjects under monaural conditions with that observed under normal listening conditions indicated that some reduction in elevation localisation acuity occurred in the frontal quadrants in the median plane and in the upper quadrants of more lateral source positions. The reduction in acuity seen in these regions is attributed to the loss of information from the pinna of the occluded ear rather than to the observed reduction in azimuth error. The results provide partial support for the binaural pinna disparity model.

The spatial characteristics of tactile form perception.

The perceived orientation of a raised letterform indenting the skin of the finger reverses (from normal to its mirror-image) when the letter is held in contact with the finger and both are rotated through 180 degrees about the axis of the finger. Thus, though the pattern of stimulated skin receptors remains constant, the perceived orientation of the letter reverses. On the basis of this observation it is proposed that tactual perception of object form involves assignment of a spatial coordinate system within which the patterns of skin stimulation are interpreted. In experiments in which the orientations of letters and subjects are systematically varied within the environment, the occurrence or nonoccurrence of reversal in perceived orientation of letters has been used to investigate the origin of the proposed spatial coordinate system; that is, whether it is assigned with respect to the observer ( egocentrically ) or with respect to the environment ( geocentrically ). The results indicate that the assignment of coordinates is determined by both egocentric and geocentric factors. It is proposed that the reversal phenomena observed in these experiments, and in experiments of others which involve drawing characters on the skin, are consistent with Gibson's proposal that it is object form which is directly perceived, but that this involves interpretation of the patterns of skin stimulation within a framework of spatial coordinates.

Acuity of sound localisation: a topography of auditory space. I. Normal hearing conditions.

Eight subjects were required to localise a sound source (white noise through a speaker) which varied in position on both sides of the head over a range of elevations (-40 degrees to +40 degrees) and azimuths (0 degree to 180 degrees) at 10 degrees intervals. The perceived position of the source was indicated by pointing a special gun. Depression of the trigger activated a photographic system which recorded two views of the subject, the sound source, and the gun. The absolute and algebraic, azimuth and elevation errors were measured for all subjects at each position of the source. The variability of azimuth and elevation error was also computed. In a second experiment, four of the same subjects performed the same task but in this case visually located the sources. This experiment provided an estimate of inherent motor error in the pointing task. No differences in localisation acuity between sides were found, but there were significant differences between front and back regions. Azimuth and elevation error were well matched and low in the front. However, azimuth error increased in the regions behind the head, particularly for azimuth positions 120 degrees to 160 degrees. Larger increases were found for positions in the upper elevations of this region. Elevation error also increased in the upper elevations behind the head. A comparison of the auditory and visual data indicates that this pattern of error is not due to motor factors. The results are discussed in relation to the structural characteristics of the pinnae and modifications that they impose on incoming sound energy.

Acuity of sound localisation: a topography of auditory space. II. Pinna cues absent.

The acuity of azimuth and elevation discrimination was measured under conditions in which the cues to localisation provided by the pinnae were removed. Four subjects localised a sound source (white noise through a speaker) which varied in position over a range of elevations (-40 degrees to +40 degrees) and azimuths (0 degree to 180 degrees), at 10 degrees intervals, on the left side of the head. Pinna cues were removed by the insertion of individually cast moulds in both pinnae. Each mould had an access hole to the auditory canal. The absolute and algebraic, azimuth and elevation errors were measured for all subjects at each position of the source. The variability of azimuth and elevation error was also computed. The performance of the subjects was compared to their performance under normal hearing conditions. Insertion of the pinnae moulds was found to increase substantially elevation error and the number of front/back reversals. The importance of the cues provided by the pinnae in these discriminations was thus confirmed. However, the increase in elevation error did not result in a corresponding increase in azimuth error. These findings provide support for the proposition that azimuth and elevation discrimination are coded independently.

Detection and discrimination of spectral peaks and notches at 1 and 8 kHz.

The ability of subjects to detect and discriminate spectral peaks and notches in noise stimuli was determined for center frequencies fc of 1 and 8 kHz. The signals were delivered using an insert earphone designed to produce a flat frequency response at the eardrum for frequencies up to 14 kHz. In experiment I, subjects were required to distinguish a broadband reference noise with a flat spectrum from a noise with either a peak or a notch at fc. The threshold peak height or notch depth was determined as a function of bandwidth of the peak or notch (0.125, 0.25, or 0.5 times fc). Thresholds increased with decreasing bandwidth, particularly for the notches. In experiment II, subjects were required to detect an increase in the height of a spectral peak or a decrease in the depth of a notch as a function of bandwidth. Performance was worse for notches than for peaks, particularly at narrow bandwidths. For both experiments I and II, randomizing (roving) the overall level of the stimuli had little effect at 1 kHz, but tended to impair performance at 8 kHz, particularly for notches. Experiments III-VI measured thresholds for detecting changes in center frequency of sinusoids, bands of noise, and spectral peaks or notches in a broadband background. Thresholds were lowest for the sinusoids and highest for the peaks and notches. The width of the bands, peaks, or notches had only a small effect on thresholds. For the notches at 8 kHz, thresholds for detecting glides in center frequency were lower than thresholds for detecting a difference in center frequency between two steady sounds. Randomizing the overall level of the stimuli made frequency discrimination of the sinusoids worse, but had little or no effect for the noise stimuli. In all six experiments, performance was generally worse at 8 kHz than at 1 kHz. The results are discussed in terms of their implications for the detectability of spectral cues introduced by the pinnae.

Aurally and visually guided visual search in a virtual environment.

We investigated the time participants took to perform a visual search task for targets outside the visual field of view using a helmet-mounted display. We also measured the effectiveness of visual and auditory cues to target location. The auditory stimuli used to cue location were noise bursts previously recorded from the ear canals of the participants and were either presented briefly at the beginning of a trial or continually updated to compensate for head movements. The visual cue was a dynamic arrow that indicated the direction and angular distance from the instantaneous head position to the target. Both visual and auditory spatial cues reduced search time dramatically, compared with unaided search. The updating audio cue was more effective than the transient audio cue and was as effective as the visual cue in reducing search time. These data show that both spatial auditory and visual cues can markedly improve visual search performance. Potential applications for this research include highly visual environments, such as aviation, where there is risk of overloading the visual modality with information.