In Experiments 1 and 2, the time to locate and identify a visual target (visual search performance in a two-alternative forced-choice paradigm) was measured as a function of the location of the target relative to the subject's initial line of gaze. In Experiment 1, tests were conducted within a 260°region on the horizontal plane at a fixed elevation (eye level). In Experiment 2, the position of the target was varied in both the horizontal (260°) and the vertical (±46°from the initial line of gaze) planes. In both experiments, and for all locations tested, the time required to conduct a visual search was reduced substantially (175-1,200 msec) when a 10-Hz click train was presented from the same location as that occupied by the visual target. Significant differences in latencies were still evident when the visual target was located within 10°of the initial line of gaze (central visual field). In Experiment 3, we examined head and eye movements that occur as subjects attempt to locate a sound source. Concurrent movements of the head and eyes are commonly encountered during auditorily directed search behavior. In over half of the trials, eyelid closures were apparent as the subjects attempted to orient themselves toward the sound source. The results from these experiments support the hypothesis that the auditory spatial channel has a significant role in regulating visual gaze. Statement of the ProblemThe auditory system in human beings has only limited spatial resolving power; the ability to discriminate the location of a sound source, for example, is seldom better than 1°-2 0.1 Although an extensive literature exists on the topic of auditory spatial processes, little attention has been paid to evaluating the function of this system. In our search for a role for the auditory spatial system, we assumed that the function it serves must require no more than the limited resolution normally observed. We wish to suggest the following hypothesis: The primary function of the auditory spatial system may be to provide information that allows the individual to redirect the eyes in order to bring the fovea into line with an acoustically active object. Since the fovea, which is the most powerful information processing segment of the retina, extends over several degrees of visual angle, additional auditory spatial capacity may not have had any adaptive value. In the following section, we will attempt to present the arguments that led us to this conclusion. OverviewIn human beings, the eyes are located relatively close together at the front of the head. One cost of this arrangement is that people have available only a limited sample of the immediate environment. As noted by Gibson

Particle health effects studies that include both ambient PM2.5 and gaseous concentrations as independent variables must be analyzed carefully and interpreted cautiously, since both parameters may be serving as surrogates for PM2.5 exposures.

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