Orthogonal representation of sound dimensions in the primate midbrain

Abstract

Natural sounds are characterized by their spectral content and the modulation of energy over time. Using functional magnetic resonance imaging in awake macaques, we observed topographical representations of these spectral and temporal dimensions in a single structure, the inferior colliculus, the principal auditory nucleus in the midbrain. These representations are organized as a map with two approximately perpendicular axes: one representing increasing temporal rate and the other increasing spectral frequency.

Figure 1. BOLD response maps in the inferior colliculus (IC)

A. Location of the analysed MRI slices (in red) on a sagittal structural MRI from one animal. V and D indicate the ventral and the dorsal edges of the slice. B. t-value map of all sound stimuli versus no-sound for one of the slices in A (t-value shown by scale on right). Robust responses from the inferior colliculi are clearly visible. C. Subtraction maps for the white framed area in B. These show the BOLD responses to high spectral sound frequencies (hf, left column) or temporal rates (hr, right column) vs. the low spectral frequencies (lf) or temporal rates (lr) respectively for three animals. Change towards blue indicates increasing dominance of high frequencies or rates. Change towards red indicates increasing dominance of low frequencies or rates. Minimal and maximal response estimate coefficients (beta-values) are displayed above and below respectively the colour scale. Letters in upper left corner of left column indicate animal ID.

Figure 2. Estimation of gradient directions by two dimensional regression analysis

These are shown for the spectral experiment (left) and the temporal experiment (right) in the left IC of animal Ws. Above: subtraction maps for the response estimates. Middle: The fitting of a plane to the values of the response-estimate coefficients (black dots). Below: gradient direction is displayed by the contours of the subtraction maps. Gradient directions are indicated relative to the dorsal-ventral axis. The angle between the axes of the tonotopic and periodotopic gradients is shown below the figure.