Figure 4.

Possible neural mechanisms underlying increased functional selectivity with development. In this schematic, we consider how exposure to faces might change neural selectivity for faces and how it might manifest with fMRI. This figure illustrates responses in a putative voxel containing two neural populations – the red population, which prefers faces, and the blue population, which prefers objects. In all panels, the left column depicts neuronal firing rates; the inset illustrates the distribution of face (red) and object (blue) preferring neurons (represented by circles) in a voxel; and the right column illustrates the BOLD responses to faces and objects. Neural firing rates were modeled with a Gaussian with σ = 1.5 and were centered at two stimulus locations 1.5σ apart. The red curve reflects the firing rate for face-selective neurons, and the blue reflects the firing of object-selective neurons. In all panels (except d) half of the neurons in the voxel prefer faces and half prefer objects. BOLD responses (right column) were estimated for faces and objects as a sum of neural-population responses (i.e. firing rates) to each of these stimuli weighted by the proportion of each neural population in a voxel. (a) Immature region. The immature voxel contains an equal number of face and object neurons. Neural tuning to faces and objects is broad and overlapping. Because the sizes of the neural populations preferring faces and objects in the voxel are equal, the BOLD response to objects and faces is the same. (b) Sharpening: After exposure to faces, the tuning of face-selective neurons (red-left) becomes narrower around the preferred stimulus. This is modeled by a smaller σ (narrower tuning width) of the neural tuning curve for faces (in this example σ decreased by a factor of two). There is no change in the tuning to objects. As a consequence, the BOLD response to objects will decrease. If the initial tuning to faces and objects in (a) is not overlapping, sharpening at the neural level might not produce detectable changes with BOLD. (c) Increased firing rate: After exposure to faces, the magnitude of responses to faces increases (in this example the magnitude doubled), but there is no change in the tuning width. This will produce increased BOLD responses to both faces and objects and a larger increase for faces. (d) Increase in the number of selective neurons: after exposure to faces, more neurons in the voxel will respond preferentially to faces than to objects (more red neurons than blue in inset; in this example there are twice as many face-selective versus object-selective neurons in a voxel). As a consequence, BOLD responses will increase for faces and decrease for objects.