PubMed Health⌕ Search
Home/Archive/PubMed

PubMed · 15296007

External and internal limitations in amplitude-modulation processing.

Stephan D Ewert·Torsten Dau
View original source ↗Export metadata ↓

Abstract

Three experiments are presented to explore the relative role of "external" signal variability and "internal" resolution limitations of the auditory system in the detection and discrimination of amplitude modulations (AM). In the first experiment, AM-depth discrimination performance was determined using sinusoidally modulated broadband-noise and pure-tone carriers. The AM index, m, of the standard ranged from -28 to -3 dB (expressed as 20 log m). AM-depth discrimination thresholds were found to be a fraction of the AM depth of the standard for standards down to -18 dB, in the case of the pure-tone carrier, and down to -8 dB, in the case of the broadband-noise carrier. For smaller standards, AM-depth discrimination required a fixed increase in AM depth, independent of the AM depth of the standard. In the second experiment, AM-detection thresholds were obtained for signal-modulation frequencies of 4, 16, 64, and 256 Hz, applied to either a band-limited random-noise carrier or a deterministic ("frozen") noise carrier, as a function of carrier bandwidth (8 to 2048 Hz). In general, detection thresholds were higher for the random- than for the frozen-noise carriers. For both carrier types, thresholds followed the pattern expected from frequency-selective processing of the stimulus envelope. The third experiment investigated AM masking at 4, 16, and 64 Hz in the presence of a narrow-band masker modulation. The variability of the masker was changed from entirely frozen to entirely random, while the long-term average envelope power spectrum was held constant. The experiment examined the validity of a long-term average quantity as the decision variable, and the role of memory in experiments with frozen-noise maskers. The empirical results were compared to predictions obtained with two modulation-filterbank models. The predictions revealed that AM-depth discrimination and AM detection are limited by a combination of the external signal variability and an internal "Weber-fraction" noise process.

Explore related subjects

Acoustic StimulationAdultAudiometry, Pure-ToneAuditory PerceptionAuditory ThresholdHumansMalePerceptual MaskingPsychoacousticsJournal ArticleResearch Support, Non-U.S. Gov't

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

Stephan D Ewert, Torsten Dau. 2004. External and internal limitations in amplitude-modulation processing.. https://doi.org/10.1121/1.1737399

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related citations

PubMed2006

Subcortical reorganization in amyotrophic lateral sclerosis.

The cerebral cortex reorganizes in response to central or peripheral lesions. Although basal ganglia and cerebellum are key components of the network dedicated to movement control, their role in motor reorganization remains elusive. We therefore tested if slowly progressive neurodegenerative motor disease alters the subcortical functional anatomy of the basal ganglia-thalamo-cerebellar circuitry. Ten patients with amyotrophic lateral sclerosis (ALS) and ten healthy controls underwent functional magnetic resonance imaging (fMRI), while executing a simple finger flexion task. Cued by an acoustic trigger, they squeezed a handgrip force transducer with their right hand at 10% of their maximum voluntary contraction force. Movement frequency, amplitude, and force were controlled. Statistical parametric mapping of task-related BOLD-response revealed increased activation in ALS patients as compared to healthy controls. The main activation increases were found in the supplementary motor area, basal ganglia, brainstem, and cerebellum. These findings suggest that degeneration of cortical and spinal motor neurons in ALS leads to a recruitment of subcortical motor structures. These subcortical activation patterns strongly resemble functional activation in motor learning and might therefore represent adaptations of cortico-subcortical motor loops as a - albeit finally ineffective - mechanism to compensate for the ongoing loss of motor neurons in ALS.

Acoustic Stimulation↗
PubMed2006

Interaural timing difference circuits in the auditory brainstem of the emu (Dromaius novaehollandiae).

In the auditory system, precise encoding of temporal information is critical for sound localization, a task with direct behavioral relevance. Interaural timing differences (ITDs) are computed using axonal delay lines and cellular coincidence detectors in nucleus laminaris (NL). We present morphological and physiological data on the timing circuits in the emu, Dromaius novaehollandiae, and compare these results with those from the barn owl (Tyto alba) and the domestic chick (Gallus gallus). Emu NL was composed of a compact monolayer of bitufted neurons whose two thick primary dendrites were oriented dorsoventrally. They showed a gradient in dendritic length along the presumed tonotopic axis. The NL and nucleus magnocellularis (NM) neurons were strongly immunoreactive for parvalbumin, a calcium-binding protein. Antibodies against synaptic vesicle protein 2 and glutamic acid decarboxlyase revealed that excitatory synapses terminated heavily on the dendritic tufts, while inhibitory terminals were distributed more uniformly. Physiological recordings from brainstem slices demonstrated contralateral delay lines from NM to NL. During whole-cell patch-clamp recordings, NM and NL neurons fired single spikes and were doubly rectifying. NL and NM neurons had input resistances of 30.0 +/- 19.9 Momega and 49.0 +/- 25.6 Momega, respectively, and membrane time constants of 12.8 +/- 3.8 ms and 3.9 +/- 0.2 ms. These results provide further support for the Jeffress model for sound localization in birds. The emu timing circuits showed the ancestral (plesiomorphic) pattern in their anatomy and physiology, while differences in dendritic structure compared to chick and owl may indicate specialization for encoding ITDs at low best frequencies.

Acoustic Stimulation↗
PubMed2006

Selection of currently relevant words: an auditory verbal memory study using positron emission tomography.

Clinical and imaging studies have shown that the posterior and anterior parts of the limbic system make distinct contributions to memory processing. Whereas the medial temporal lobes and connected structures are important for learning and recognition of new information, the posterior orbitofrontal cortex appears to be crucial for the selection of currently relevant memories, that is, the ability to distinguish between memories that pertain to ongoing reality and memories that do not. Previous studies demonstrating this dissociation used repeated continuous recognition tasks with visual stimuli. In this H2O positron emission tomography study, we demonstrate that this same organizing principle also applies to auditory verbal information, that is, to the processing of auditorily presented words.

Acoustic Stimulation↗