Differences in reaction time as a function of ear stimulated, processing task, and mode of stimulation

Reaction time measures were utilized to investigate the influences of attentional factors and of variations in processing task and stimulus presentation mode on asymmetries in auditory perception. Individual subject deviations from group performance profiles were described, and reaction time and accuracy measures of inferred laterality were compared.

Measures of right-handed motor reaction times and performance accuracy in response to right and left ear presentations of speech stimuli were obtained from l2 young adult subjects in four processing tasks: s simple reaction time (Experiment I); stimulus discrimination (Experiment II); stimulus recognition (Experiment III); and stimulus identification (Experiment IV). Within tasks, mode of stimulus presentation varied along two dimensions: monotic versus dichotic; and expected versus random ear of presentation. Therefore, four possible modes of presentation (monotic expected, monotic random, dichotic expected, dichotic random) were available when appropriate. Each subject participated in the l3 experimental conditions developed in this manner and also completed a standard dichotic listening pre- and post-test. Major results and conclusions are summarized below.

1. Data were consistent with the hypothesis that at least two mechanisms underlie auditory perceptual asymmetries iii

iv associated with attention. . First, as predicted by the Kinsbourne selective attention model, a tendency toward right ear attending appears to exist under monotic conditions of stimulus uncertainty. In the monotic random mode, right ear reaction times were significantly faster than left ear times, and the size of these right ear advantages appeared relatively unaffected by different tasks. However, dichotic mode expected-random shifts were inconsistent with model predictions. Second, with attention focused during monotic stimulation, asymmetries appear related to the degree of advantage (or disadvantage) associated with access to the hypothesized left hemisphere language processor. Specifically, in the monotic expected mode, the range of reaction time laterality effects was greater than in the monotic random mode. Laterality effects differed significantly between Experiments I and Il. Rank order of task laterality effects closely paralleled dichotic condition rankings, and subjects' reaction time laterality effects in different processing tasks correlated with standard dichotic accuracy measures.

2. Significantly faster right ear reaction times were obtained for dichotic presentations in all processing tasks (except nontarget responses in Experiment IIl). Laterality effects were consistently larger in dichotic (as compared with monotic) stimulus presentation conditions. Faster right ear reaction times were also obtained in monotic modes for the majority of processing tasks. The processing task

rankings from most to least lateralized were consistently Experiments IV, III, and II in all but the monotic random mode. Considerable subject performance variability was noted, however, in Experiments II and III. Data from Experiments II and III are discussed with reference to two optional processing strategies (acoustic template analysis versus linguistic coding) available to subjects. It is hypothesized that linguistic stimuli may or may not be processed in a linguistically relevant manner depending upon: a) the degree to which acoustic information can be or is available to be utilized in task performance; and b) the memory storage requirements of the task.

3. Individual subject performance profiles of reaction time Ear Difference Scores were highly variable within Experiments II and III and within conditions of directed attention, but tended to be more consistent with group performance data and with dichotic listening accuracy measures within Experiments I and IV.

4. Auditory perceptual asymmetries are more evident with reaction time, as compared with accuracy, performance measures. Significant ear differences in reaction time were obtained in all processing tasks (except for nontarget response data in Experiment IIl), and significant differences between reaction time Ear Difference Scores were obtained for two or more processing tasks within all but the monotic random mode.