Different patterns of human discrimination learning for two interaural cues to sound-source location

Abstract

Two of the primary cues used to localize the sources of sounds are interaural level differences (ILDs) and interaural time differences (ITDs). We conducted two experiments to explore how practice affects the human discrimination of values of ILDs and ongoing ITDs presented over headphones. We measured discrimination thresholds of 13 to 32 naive listeners in a variety of conditions during a pretest and again, 2 weeks later, during a posttest. Between those two tests, we trained a subset of listeners 1 h per day for 9 days on a single ILD or ITD condition. Listeners improved on both ILD and ITD discrimination. Improvement was initially rapid for both cue types and appeared to generalize broadly across conditions, indicating conceptual or procedural learning. A subsequent slower-improvement stage, which occurred solely for the ILD cue, only affected conditions with the trained stimulus frequency, suggesting that stimulus processing had fundamentally changed. These different learning patterns indicate that practice affects the attention to, or low-level encoding of, ILDs and ITDs at sites at which the two cue types are processed separately. Thus, these data reveal differences in the effect of practice on ILD and ITD discrimination, and provide insight into the encoding of these two cues to sound-source location in humans.

Figure 1

Learning in the trained conditions. Pretest, posttest, and training-session thresholds for the discrimination of ILDs (Left) and ITDs (Right) in the trained conditions for trained (□) and control (▴) listeners. Shown are the group mean (Top) and individual (Bottom) thresholds. The error bars represent ± one SEM across listeners (Top) and within listeners (Bottom). Two 300-ms tones at the indicated frequencies (4 kHz or 0.5 kHz) were presented either with the indicated standard value of interaural difference (0 dB or 0 μs) or with a larger value that favored the right ear. Threshold is expressed as the difference in the values necessary to discriminate the larger from the standard values on 79% of trials.

Figure 2

Generalization to untrained conditions. Mean pretest (open symbols) and posttest (filled symbols) z scores for the five conditions tested in the ILD training (Top) and ITD training (Bottom) experiments. Results are plotted for the trained (squares) and control (triangles; same data plotted in both panels for conditions common to both experiments) listeners. Error bars represent ± 1 SEM across listeners. Asterisks indicate significant differences between the trained and control listeners on the pre- or posttest z scores. The number of listeners tested (n) is listed for each listener group and condition.

Figure 3

Relationship between pretest thresholds and the amount of learning by individual listeners on the ILD conditions. Pretest (abscissa) and posttest (ordinate) thresholds for individual listeners (open symbols), and their group means (filled symbols), in the four ILD conditions. Results are shown for the ILD-trained (Left) and control (Right) listeners. We determined the linear regression of y on x for each data set (dashed lines), excluding values (hourglasses) for which the pretest threshold was either more than two standard deviations above or below the mean of all listeners, or was flagged by a Leverage test (30) during the fitting process. The regression-line slope and intercept, as well as the sample r² and standard error of the estimate for the fit (in parentheses), are listed in each panel. Asterisks indicate slopes that were significantly different from zero.

Figure 4

Relationship between pretest thresholds and the amount of learning by individual listeners on the ITD conditions. Same as Fig. 3, but for the three ITD conditions.