Neural processing underlying tactile microspatial discrimination in the blind: a functional magnetic resonance imaging study

Abstract

Although blindness alters neocortical processing of non-visual tasks, previous studies do not allow clear conclusions about purely perceptual tasks. We used functional magnetic resonance imaging (fMRI) to examine the neural processing underlying tactile microspatial discrimination in the blind. Activity during the tactile microspatial task was contrasted against that during a tactile temporal discrimination task. The spatially selective network included frontoparietal and visual cortical regions. Activation magnitudes in left primary somatosensory cortex and in visual cortical foci predicted acuity thresholds. Effective connectivity was investigated using multivariate Granger causality analyses. Bilateral primary somatosensory cortical foci and a left inferior temporal focus were important sources of connections. Visual cortical regions interacted mainly with one another and with somatosensory cortical regions. Among a set of distributed cortical regions exhibiting greater spatial selectivity in early blind compared to late blind individuals, the age of complete blindness was predicted by activity in a subset of frontoparietal regions and by the weight of a path from the right lateral occipital complex to right occipitopolar cortex. Thus, many aspects of neural processing during tactile microspatial discrimination differ between the blind and sighted, with some of the key differences reflecting visual cortical engagement in the blind.

Figure 1

A. MRI-compatible pneumatic stimulator. Stimuli were mounted face-down on the square base-plate at the bottom of the drive shaft. The finger mold used to immobilize the finger was mounted on the base of the device. Arrows indicate direction of airflow. B: Stimulus configurations in spatial task; central dot in array was offset either to the right or left. C: Stimulus array for the temporal task used an array without spatial offset. (Reproduced, with permission, from Stilla et al., 2007).

Figure 2

Common spatially-selective activations across EB, LB groups, displayed on inflated hemispheric representations. Top panels: lateral views; bottom panels: ventral views. RH: right hemisphere; LH: left hemisphere. Other abbreviations as in text. Color t scale on right.

Figure 3

Spatially-selective activations EB > LB, displayed on inflated hemispheric representations. Top panels: lateral views; middle panels: ventral views; bottom panels: medial views. RH: right hemisphere; LH: left hemisphere. Other abbreviations as in text. Color t scale on right.

Figure 4

Visual cortical regions showing spatially-selective activations EB > LB, displayed on flat maps of right and left hemisphere (RH, LH). A: anterior; P: posterior; other abbreviations as in text. Color t scale on right.

Figure 5

Regions where ANCOVA showed negative correlations between degree of spatial selectivity and age of total blindness, displayed on representative axial slices. Talairach z plane is indicated below each slice. R: right; other abbreviations as in text. Color r scale on right.

Figure 6

BOLD signal time-courses in regions where ANCOVA showed negative correlations between degree of spatial selectivity and age of total blindness. Error bars: SEM; abbreviations as in text.

Figure 7

Regions where ANCOVA showed negative correlations between activation magnitude and acuity threshold, displayed on representative coronal/axial slices. Talairach y/z plane is indicated below each slice. R: right; other abbreviations as in text. Color r scale on right.

Figure 8

Visual cortical regions where ANCOVA showed negative correlations between activation magnitude and acuity threshold, displayed on flat maps of right and left hemisphere (RH, LH). A: anterior; P: posterior; other abbreviations as in text. Color r scale on right.

Figure 9

BOLD signal time-courses in regions where ANCOVA showed negative correlations between activation magnitude and acuity threshold. Error bars: SEM; abbreviations as in text.

Figure 10

Effective connectivity as revealed by Granger causality analysis. The left aIPS focus also extended into the SMG. Abbreviations as in text. Color scale at right indicates path weights (arbitrary units).