Abstract
It is well known that damage to the peripheral auditory system causes deficits in tone detection as well as pitch and loudness perception across a wide range of frequencies. However, the extent to which to which the auditory cortex plays a critical role in these basic aspects of spectral processing, especially with regard to speech, music, and environmental sound perception, remains unclear. Recent experiments indicate that primary auditory cortex is necessary for the normally-high perceptual acuity exhibited by humans in pure-tone frequency discrimination. The present study assessed whether the auditory cortex plays a similar role in the intensity domain and contrasted its contribution to sensory versus discriminative aspects of intensity processing. We measured intensity thresholds for pure-tone detection and pure-tone loudness discrimination in a population of healthy adults and a middle-aged man with complete or near-complete lesions of the auditory cortex bilaterally. Detection thresholds in his left and right ears were 16 and 7 dB HL, respectively, within clinically-defined normal limits. In contrast, the intensity threshold for monaural loudness discrimination at 1 kHz was 6.5±2.1 dB in the left ear and 6.5±1.9 dB in the right ear at 40 dB sensation level, well above the means of the control population (left ear: 1.6±0.22 dB; right ear: 1.7±0.19 dB). The results indicate that auditory cortex lowers just-noticeable differences for loudness discrimination by approximately 5 dB but is not necessary for tone detection in quiet. Previous human and Old-world monkey experiments employing lesion-effect, neurophysiology, and neuroimaging methods to investigate the role of auditory cortex in intensity processing are reviewed.
Highlights
By the close of the 20th century, it seemed reasonably wellestablished on the basis of neuropsychology studies in patients and selective-ablation experiments in animals that auditory cortex was devoted to higher functions such as pattern recognition but played little or no role in elementary functions involving discrimination of a single acoustic feature and its corresponding percept
Experiments with neurological patients employing rigorous psychoacoustic methods and in vivo lesion localization have since demonstrated that lesions of auditory cortex impair pure-tone frequency processing and pitch discrimination, even when puretone audiograms are within normal limits [3,4,5,6]
The present study focuses on pure-tone intensity processing in relation to tone detection and loudness perception
Summary
By the close of the 20th century, it seemed reasonably wellestablished on the basis of neuropsychology studies in patients and selective-ablation experiments in animals that auditory cortex was devoted to higher functions such as pattern recognition but played little or no role in elementary functions involving discrimination of a single acoustic feature and its corresponding percept (e.g., puretone intensity and loudness; for review see [1,2]). Experiments with neurological patients employing rigorous psychoacoustic methods and in vivo lesion localization have since demonstrated that lesions of auditory cortex impair pure-tone frequency processing and pitch discrimination, even when puretone audiograms are within normal limits [3,4,5,6]. These observations suggest a dissociation between the effects of auditory cortex lesions on auditory sensation (i.e., detecting the presence of a sound) and auditory perception (i.e., determining whether and how two sounds differ in one or more attributes). Given recent evidence that auditory cortex supports high perceptual acuity for pure-tone pitch perception [3,6], we hypothesized that it subserves finegrained loudness perception
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