Claudia Freigang

Evaluation of central auditory discrimination abilities in older adults

The present study focuses on auditory discrimination abilities in older adults aged 65–89 years. We applied the “Leipzig inventory for patient psychoacoustic” (LIPP), a psychoacoustic test battery specifically designed to identify deficits in central auditory processing. These tests quantify the just noticeable differences (JND) for the three basic acoustic parameters (i.e., frequency, intensity, and signal duration). Three different test modes [monaural, dichotic signal/noise (s/n) and interaural] were used, stimulus level was 35 dB sensation level. The tests are designed as three-alternative forced-choice procedure with a maximum-likelihood procedure estimating p = 0.5 correct response value. These procedures have proven to be highly efficient and provide a reliable outcome. The measurements yielded significant age-dependent deteriorations in the ability to discriminate single acoustic features pointing to progressive impairments in central auditory processing. The degree of deterioration was correlated to the different acoustic features and to the test modes. Most prominent, interaural frequency and signal duration discrimination at low test frequencies was elevated which indicates a deterioration of time- and phase-dependent processing at brain stem and cortical levels. LIPP proves to be an effective tool to identify basic pathophysiological mechanisms and the source of a specific impairment in auditory processing of the elderly.

Age-related changes in sound localisation ability

Auditory spatial processing is an important ability in everyday life and allows the processing of omnidirectional information. In this review, we report and compare data from psychoacoustic and electrophysiological experiments on sound localisation accuracy and auditory spatial discrimination in infants, children, and young and older adults. The ability to process auditory spatial information changes over lifetime: the perception of the acoustic space develops from an initially imprecise representation in infants and young children to a concise representation of spatial positions in young adults and the respective performance declines again in older adults. Localisation accuracy shows a strong deterioration in older adults, presumably due to declined processing of binaural temporal and monaural spectro-temporal cues. When compared to young adults, the thresholds for spatial discrimination were strongly elevated both in young children and older adults. Despite the consistency of the measured values the underlying causes for the impaired performance might be different: (1) the effect is due to reduced cognitive processing ability and is thus task-related; (2) the effect is due to reduced information about the auditory space and caused by declined processing in auditory brain stem circuits; and (3) the auditory space processing regime in young children is still undergoing developmental changes and the interrelation with spatial visual processing is not yet established. In conclusion, we argue that for studying auditory space processing over the life course, it is beneficial to investigate spatial discrimination ability instead of localisation accuracy because it more reliably indicates changes in the processing ability.

A comparison of visual and auditory representational momentum in spatial tasks

Similarities have been observed in the localization of the final position of moving visual and moving auditory stimuli: Perceived endpoints that are judged to be farther in the direction of motion in both modalities likely reflect extrapolation of the trajectory, mediated by predictive mechanisms at higher cognitive levels. However, actual comparisons of the magnitudes of displacement between visual tasks and auditory tasks using the same experimental setup are rare. As such, the purpose of the present free-field study was to investigate the influences of the spatial location of motion offset, stimulus velocity, and motion direction on the localization of the final positions of moving auditory stimuli (Experiment 1 and 2) and moving visual stimuli (Experiment 3). To assess whether auditory performance is affected by dynamically changing binaural cues that are used for the localization of moving auditory stimuli (interaural time differences for low-frequency sounds and interaural intensity differences for high-frequency sounds), two distinct noise bands were employed in Experiments 1 and 2. In all three experiments, less precise encoding of spatial coordinates in paralateral space resulted in larger forward displacements, but this effect was drowned out by the underestimation of target eccentricity in the extreme periphery. Furthermore, our results revealed clear differences between visual and auditory tasks. Displacements in the visual task were dependent on velocity and the spatial location of the final position, but an additional influence of motion direction was observed in the auditory tasks. Together, these findings indicate that the modality-specific processing of motion parameters affects the extrapolation of the trajectory.

Resolution of lateral acoustic space assessed by electroencephalography and psychoacoustics

The encoding of auditory spatial acuity (measured as the precision to distinguish between two spatially distinct stimuli) by neural circuits in both auditory cortices is a matter of ongoing research. Here, the event-related potential (ERP) mismatch negativity (MMN), a sensitive indicator of preattentive auditory change detection, was used to tap into the underlying mechanism of cortical representation of auditory spatial information. We characterized the MMN response affected by the degree of spatial deviance in lateral acoustic space using a passive oddball paradigm. Two stimulation conditions (SCs)—specifically focusing on the investigation of the mid- and far-lateral acoustic space—were considered: (1) 65° left standard position with deviant positions at 70, 75, and 80°; and (2) 95° left standard position with deviant positions at 90, 85, and 80°. Additionally, behavioral data on the minimum audible angle (MAA) were acquired for the respective standard positions (65, 95° left) to quantify spatial discrimination in separating distinct sound sources. The two measurements disclosed the linkage between the (preattentive) MMN response and the (attentive) behavioral threshold. At 65° spatial deviations as small as 5° reliably elicited MMNs. Thereby, the MMN amplitudes monotonously increased as a function of spatial deviation. At 95°, spatial deviations of 15° were necessary to elicit a valid MMN. The behavioral data, however, yielded no difference in mean MAA thresholds for position 65 and 95°. The different effects of laterality on MMN responses and MAA thresholds suggest a role of spatial selective attention mechanisms particularly relevant in active discrimination of neighboring sound sources, especially in the lateral acoustic space.

Pre-attentive cortical processing of behaviorally perceptible spatial changes in older adults—a mismatch negativity study

From behavioral studies it is known that auditory spatial resolution of azimuthal space declines over age. To date, it is not clear how age affects the respective sensory auditory processing at the pre-attentive level. Here we tested the hypothesis that pre-attentive processing of behaviorally perceptible spatial changes is preserved in older adults. An EEG-study was performed in older adults (65–82 years of age) and a mismatch negativity (MMN) paradigm employed. Sequences of frequent standard stimuli of defined azimuthal positions were presented together with rarely occurring deviants shifted by 10° or 20° to the left or to the right of the standard. Standard positions were at +5° (central condition) from the midsagittal plane and at 65° in both lateral hemifields (±65°; lateral condition). The results suggest an effect of laterality on the pre-attentive change processing of spatial deviations in older adults: While for the central conditions deviants close to MAA threshold (i.e., 10°) yielded discernable MMNs, for lateral positions the respective MMN responses were only elicited by spatial deviations of 20° toward the midline (i.e., ±45°). Furthermore, MMN amplitudes were found to be insensitive to the magnitude of deviation (10°, 20°), which is contrary to recent studies with young adults (Bennemann et al., 2013) and hints to a deteriorated pre-attentive encoding of sound sources in older adults. The discrepancy between behavioral MAA data and present results are discussed with respect to the possibility that under the condition of active stimulus processing older adults might benefit from recruiting additional attentional top-down processes to detect small magnitudes of spatial deviations even within the lateral acoustic field.

Position of acoustic stimulus modulates visual α activity.

It has been repeatedly shown that a unimodal stimulus can modulate oscillatory activity of multiple cortical areas already at early stages of sensory processing. In this way, an influence can be exerted on the response to a subsequent sensory input. Even though this fact is now well established, it is still not clear whether cortical sensory areas are informed about spatial positions of objects of modality other than their preferred one. Here, we test the hypothesis of whether oscillatory activity of the human visual cortex depends on the position of a unimodal auditory object. We recorded electroencephalogram while presenting sounds in an acoustic free-field either at the center of the visual field or at lateral positions. Using independent component analysis, we identified three cortical sources located in the visual cortex, showing stimulus position-specific oscillatory responses. The most pronounced effect was an immediate &agr; (8–12 Hz) power decrease over the entire occipital lobe when the stimulus originated from the center of the binocular visual field. Following a lateral stimulation, the amplitude of &agr; activity decreased slightly over contralateral visual areas, while at the same time a weak &agr; synchronization was observed in corresponding ipsilateral areas. Thus, even in the absence of visual stimuli, the visual cortex is differentially activated depending on the position of an acoustic sound source. Our results show that the visual cortex receives information about the position of auditory stimuli within the visual field.

Perceptual ambiguity — perception and processing of spatially discordant/concordant audiovisual stimuli

It is well known that under certain conditions crossmodal interactions alter our perception of multisensory events, especially in conflicting/ambiguous situations, e.g., presenting physically separated (incongruent) audiovisual cues. In case of the ventriloquist illusion, subjects shift the location of an acoustic signal to a spatially discordant visual cue (perceptual fusion), i.e., subjects perceptually unify visual and acoustic stimuli as coming from the same source although they are spatially separated. In a behavioral experiment applying a simple unification task we presented audiovisual stimuli in the freefield either spatially discordant or spatially concordant. Participants were required to judge spatial relation of co-occurring events (separation/unity). The focus of the present study was to quantify participants’ response behavior using a recently developed nonparametric adaptive sequential sampling procedure (Poppe et al., 2012). We tested four acoustic reference positions in central and para-central space (±8° and ±25° corresponding to left (−) and right (+) hemispace). Deviating visual signals were presented between 30° left and right of acoustic signals. Acquired psychometric functions allowed a detailed analysis of individual response behavior across all sampled conditions. Additionally, estimations of 50% correct response thresholds were used as guideline for the parameter setting of audiovisual stimuli in subsequent EEG experiments. We tested the hypothesis according to which the prestimulus cortical activity modulates the perceptual organization of a given stimulus and therefore determines the perceptual outcome of ambiguous audiovisual stimuli. For that EEG was recorded while presenting physically identical stimuli which, however, generate a perception that varies between two possible alternatives.

Interactions of multisensory integration and spatial attention in auditory space perception

In reverberant environments sound reflections (echoes) interfere with spatial information of the sound source. The precedence effect is a mechanism that suppresses spatial information of echoes arriving with a short latency and fuses them with the sound source, helping listeners to localize sound sources. Two recent papers showed that the strength of the precedence effect can be modulated by spatially and temporally congruent visual information (Bishop et al., 2011) and exogenous spatial attention (London et al., 2012). If a concurrent visual stimulus or a visual cue (300 ms before auditory stimulus) occurred at the location of the sound source, echo suppression was enhanced. If the respective visual stimuli occurred at the location of the echo, the suppression mechanism was inhibited. While multisensory integration was earlier thought to be an automatic process not influenced by attention, in recent years several studies were able to show that attention can influence multisensory integration under specific conditions. In this study we combine visual (multisensory context) and exogenous attentional modulation of echo suppression to examine how interactions of multisensory integration and bottom-up driven attentional modulation shape auditory space perception. From the comparison of listener’s performance in trials with either (i) visual or (ii) attentional modulation of echo suppression alone or with (iii) simultaneous visual and attentional modulation to either the same or different locations the interrelation of the two processes can be disentangled.

Influence of visual cues on localization of acoustic sound sources in old adults

Localization accuracy of stationary acoustic objects is reduced as people grow older. While it is known that this reduction can be caused by many age-related declines in the peripheral sensory system, at central cortical levels as well as in cognitive processes, it is not known how much localization performance is influenced by concurrent congruent and/or incongruent spatial information from a different sensory system, e.g., vision. In the present study we examined localization accuracy of young and old adults to acoustic stimuli that were presented simultaneously to a visual stimulus that was either spatially congruent or spatially disparate (by ±5°/±10°/±15°) in acoustic free field. The acoustic reference position was presented at frontal (9°), para-frontal (30°), and lateral (64°) positions. To infer how strongly the visual cue interacted with the auditory stimulus a unification task was examined. Here, acoustic and visual stimulus combinations were the same as in the localization task. Participants were instructed to indicate whenever both visual and acoustic information matched in terms of their spatial position. Localization accuracy was not influenced by the visual cue in young adults, but the influence of the visual distractor was strong in old adults, i.e., visual bias was strong. These observations were supported by the unification task where old adults had increased perception of congruent audio–visual directions at all reference positions and even at very large disparities (e.g., ±15°). Conclusively, concurrent information from different sensory systems highly influences auditory localization accuracy in older adults, supporting the notion that multisensory integration is enhanced in older adults.