Informational Masking in Aging and Brain-lesioned Individuals

Abstract

Auditory stream segregation and informational masking were investigated in brain-lesioned individuals, age-matched controls with no neurological disease, and young college-age students. A psychophysical paradigm known as rhythmic masking release (RMR) was used to examine the ability of participants to identify a change in the rhythmic sequence of 20-ms Gaussian noise bursts presented through headphones and filtered through generalized head-related transfer functions to produce the percept of an externalized auditory image (i.e., a 3D virtual reality sound). The target rhythm was temporally interleaved with a masker sequence comprising similar noise bursts in a manner that resulted in a uniform sequence with no information remaining about the target rhythm when the target and masker were presented from the same location (an impossible task). Spatially separating the target and masker sequences allowed participants to determine if there was a change in the target rhythm midway during its presentation. RMR thresholds were defined as the minimum spatial separation between target and masker sequences that resulted in 70.7% correct-performance level in a single-interval 2-alternative forced-choice adaptive tracking procedure. The main findings were (1) significantly higher RMR thresholds for individuals with brain lesions (especially those with damage to parietal areas) and (2) a left-right spatial asymmetry in performance for lesion (but not control) participants. These findings contribute to a better understanding of spatiotemporal relations in informational masking and the neural bases of auditory scene analysis.

Keywords: Aphasia; Brain lesion; Masking; Parietal; RMR; Stream segregation.

Fig. 1

Stimuli used in the current study. On each trial either a sequence with a change or without a change was presented (A). The listener had to determine if the target rhythm changed midway through the sequence. The target was always presented in the presence of a masker sequence that precisely “filled in” the gaps in the target sequence. When there was spatial separation between masker and target sequences (B) the task could be performed. When there was no spatial separation (C), the task was impossible. RMR threshold was defined as the angular spatial separation between masker and target sequences that resulted in 70.7% correct performance

Fig. 2

A RMR thresholds for the three participant groups (see C for legend). Thresholds are shown for the left and right hemifields (negative and positive numbers, respectively). The zero point on the ordinate designates a spatial position directly ahead of the participant. Smaller RMR thresholds near zero represent better performance. B Averaged RMR thresholds for the three groups (absolute values). C RMR thresholds averaged across the left and right hemifields and plotted in ascending order. D RMR thresholds for lesion participants and older controls as a function of age. No significant correlation as a function of age was observed. N = 55 (lesion) N = 15 (older adults) N = 13 (younger adults)

Fig. 3

RMR thresholds for 55 brain-lesioned participants. Labels show lesion locations at the level of lobe or major structure, and aphasia description. Abbreviations: L, left hemisphere, R, right hemisphere

Fig. 4

Orthogonal views of an overlap map of the areas of damage in the stroke group participants with cerebral damage, N = 52. Montreal Neurological Institute coordinates are provided for each slice. The area of max overlap, N = 25, is in the inferior frontal lobe, coordinates − 50 − 4 11. Not pictured are the single participants with left cerebellar, right cerebellar, and brain stem damage, respectively

Fig. 5

A RMR thresholds for those with damage to parietal regions compared to other lesion participants (N = 55). B No correlation was observed between RMR threshold and lesion volume (N = 54). C RMR thresholds for three subgroups of brain lesioned individuals (Broca’s N = 19, anomic N = 12, conduction N = 8. D Higher thresholds for Broca’s aphasics may be explained by damage to parietal regions. N = 19

Fig. 6

A RMR thresholds as a function of basic localization thresholds (interaural delays) measured from the same brain-lesioned individuals. B Histogram of interaural delay thresholds shows a bimodal pattern. C Histogram of RMR thresholds. N = 55