Dissociation of Detection and Discrimination of Pure Tones following Bilateral Lesions of Auditory Cortex

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.

Auditory intensity processing: Categorization versus comparison

“…Similarly, categorization of frequency modulated (FM) tones based on their intensity seems to mainly engage the left auditory cortex (AC) shown by an fMRI experiment with contralateral noise presentation (Angenstein and Brechmann, 2013a). Yet another psychoacoustic study did not find any difference between left and right ear presentation during loudness discrimination on pure tones with 20 dB softer contralateral noise (Dykstra et al, 2012).…”

“…We derived this hypothesis from the previous study in which FM tones had to be categorized based on their intensity (Angenstein and Brechmann, 2013a). With respect to the comparison of tones based on their intensity no specific hypothesis can be formulated due to the equivocal results of the previous studies employing comparison of intensity within pairs of sounds (Dykstra et al, 2012;Milner, 1962;Reiterer et al, 2008). However, our own results on sequential comparisons of other features of sounds would suggest a bias towards the left AC (Angenstein and Brechmann, 2013b;Brechmann et al, 2007).…”

“…In addition, Angenstein and Brechmann (2013a) found a stronger left-hemispheric involvement in intensity processing with FM sweeps. Moreover, studies with tones found diverse results regarding hemispheric involvement (Belin et al, 1998;Dykstra et al, 2012;Milner, 1962;Reiterer et al, 2008). Thus, the kind of stimuli does not seem to be an important factor for biasing lateralization of intensity processing.…”

“…In most of the studies mentioned above, the intensity had to be compared within pairs of sounds (Brancucci et al, 2005;Dykstra et al, 2012;Milner, 1962;Reiterer et al, 2008) and thus required sequential comparison between the actual and the previous tone. This means with each presented pair the reference for comparison had to be updated.…”

“…Such sequential comparison can change the lateralization of activity towards the left AC even though the processing of the feature itself is lateralized to the right AC (Angenstein and Brechmann, 2013b;Brechmann et al, 2007). Whatsoever, the majority of studies on intensity discrimination employing such comparisons did not find a bias towards the left hemisphere (Brancucci et al, 2005;Dykstra et al, 2012;Milner, 1962; but see Reiterer et al, 2008).…”