The functional anatomy of sound intensity discrimination

Abstract

The human neuroanatomical substrate of sound intensity discrimination was investigated by combining psychoacoustics and functional neuroimaging. Seven normal subjects were trained to detect deviant sounds presented with a slightly higher intensity than a standard harmonic sound, using a Go/No Go paradigm. Individual psychometric curves were carefully assessed using a three-step psychoacoustic procedure. Subjects were scanned while passively listening to the standard sound and while discriminating changes in sound intensity at four different performance levels (d' = 1.5, 2.5, 3.5, and 4.5). Analysis of regional cerebral blood flow data outlined activation, during the discrimination conditions, of a right hemispheric frontoparietal network already reported in other studies of selective or sustained attention to sensory input, and in which activity appeared inversely proportional to intensity discriminability. Conversely, a right posterior temporal region included in secondary auditory cortex was activated during discrimination of sound intensity independently of performance level. These findings suggest that discrimination of sound intensity involves two different cortical networks: a supramodal right frontoparietal network responsible for allocation of sensory attentional resources, and a region of secondary auditory cortex specifically involved in sensory computation of sound intensity differences.

Fig. 1.

Psychophysical data for Subject 2. The two estimated thresholds from the three down/one up and eight down/one up adaptive procedures (Phase 1: Adaptive tracking, top panel) are shown with downward arrows. From these two values, the five stimulus levels for the constant stimuli procedure (Phase 2: Constant stimuli, top panel) were derived. A linear psychometric function (solid line) was fitted to the first four of the five data points obtained from this procedure (•). From this function, the five stimulus levels for the Go/No Go procedure (Phase 3: Go/No Go) were determined. A linear psychometric function (solid line) was then fitted to the five data points represented as open circles in the lower panel. Finally, the four stimulus levels used in the imaging study were derived from this latter function as indicated by the downward arrowsin the bottom panel.

Fig. 2.

Surface rendering, on a T1 image of a right hemisphere, of the four cortical regions significantly activated during intensity discrimination (all performance levels pooled) as compared with the passive baseline. Yellow diagrams represent, for each of these regions, the mean (bar) and individual (red dots) rCBF values corresponding to the baseline (B) and each of the four intensity discriminations (d′ = 1.5, 2.5, 3.5, 4.5), in arbitrary units. Decreasing discriminability leads to increased activation in the frontoparietal network, but not in the posterior temporal region.

Fig. 3.

First eigenimage of the dataset (84 scans), explaining 56.4% of the variance. The corresponding activation/deactivation pattern is indicated by the left(negative changes) and middle (positive changes)panels. Composition of the eigenimage in terms of the scans corresponding to each experimental condition (four baseline scans; eight intensity discrimination tasks, two per d′ value) is indicated in the right panel.