Selectivity to translational egomotion in human brain motion areas

Abstract

The optic flow generated when a person moves through the environment can be locally decomposed into several basic components, including radial, circular, translational and spiral motion. Since their analysis plays an important part in the visual perception and control of locomotion and posture it is likely that some brain regions in the primate dorsal visual pathway are specialized to distinguish among them. The aim of this study is to explore the sensitivity to different types of egomotion-compatible visual stimulations in the human motion-sensitive regions of the brain. Event-related fMRI experiments, 3D motion and wide-field stimulation, functional localizers and brain mapping methods were used to study the sensitivity of six distinct motion areas (V6, MT, MST+, V3A, CSv and an Intra-Parietal Sulcus motion [IPSmot] region) to different types of optic flow stimuli. Results show that only areas V6, MST+ and IPSmot are specialized in distinguishing among the various types of flow patterns, with a high response for the translational flow which was maximum in V6 and IPSmot and less marked in MST+. Given that during egomotion the translational optic flow conveys differential information about the near and far external objects, areas V6 and IPSmot likely process visual egomotion signals to extract information about the relative distance of objects with respect to the observer. Since area V6 is also involved in distinguishing object-motion from self-motion, it could provide information about location in space of moving and static objects during self-motion, particularly in a dynamically unstable environment.

Figure 1. Schematic representation of the stimuli we used in the main fMRI experiment (ff-3D).

Figure 2. Psycophysical results.

Histograms show the Intensity of self motion sensation revealed by VAS scale across subjects. Bars represent the mean VAS scores ± standard error of the mean across runs and participants.

Figure 3. Motion areas.

In color are the regions more activated in at least one of the motion (M) conditions relative to the static (S) condition (contrast M-S): V6; MT+, middle temporal complex; V3A; CSv, visual cingulate sulcus area; IPSmot, Intra Parietal Sulcus motion area. The plots represent the averaged BOLD percent signal change ± standard error of the mean across subjects and hemispheres for each experimental condition labelled as follows: TRA, translational; CIR, circular; RAD, radial; SPI, spiral; RAND, random; STA, static.

Figure 4. Motion areas mapped by localizers.

(A) Area V6 mapped with the functional localizer, i.e., coherent flow versus randomly moving dots. Results are displayed on the medial folded representation of the right hemisphere of the template brain. The anatomical location of V6 can be better appreciated in the close-up of the flattened surface (white box) taken from the same average brain. Dashed lines, fundus of the main sulci; POs, parieto-occipital sulcus; pIPs, posterior end of the intraparietal sulcus. (B) Plots represent the averaged BOLD percent signal changes ± standard error of the mean in the localizer-defined area V6. (C) Imaging results from the functional localizer used to map areas MT and MST+ (i.e., ipsilateral vs contralateral radial motion). Results are displayed on the lateral folded representation of the left hemisphere of the template brain. (D) Plots represent the averaged BOLD percent signal changes ± standard error of the mean in the localizer-defined areas MT and MST+.

Figure 5. Motion and other visual areas of the brain.

An inflated (medial and dorso-lateral views) and flattened representation of the left hemisphere of a representative participant marked with the locations of the six average regions of interest (ROIs) that were studied: V6, MT, MST+, V3A, CSv and IPSmot. The six ROIs are displayed together with the borders of the visual areas identified in this subject by retinotopic mapping. Area V6 as defined by the retinotopic mapping is indicated by white outline and label on the POs, which overlaps with the V6 ROI in green. The dashed lines reported on both the flat map and the inflated representations indicate the fundus of the major sulci. Dotted and continuos lines indicate the vertical and horizontal meridian representations in visual areas, respectively. Major sulci (dark gray) are labeled as follows: ITs, Inferior Temporal sulcus; MTs, Middle Temporal sulcus; STs, Superior Temporal sulcus; hIPs, horizontal segment of the intraparietal sulcus; PCs, post-central sulcus; Cs, Central sulcus; LOR, Lateral Occipital Region; COs, Collateral sulcus. Other labels are as in Figure 4.

Figure 6. Cortical responses to motion coherency.

Average motion coherence coefficients extracted from the functionally defined ROIs (see Materials and Methods). (A) MC/MI coefficient in areas CSv, V3A, and IPSmot, as defined by the group statistical contrast (M-S) and in areas V6, MT and MST+, as defined by functional localizer. (B) MC/MI coefficient in area V6, as defined by the group statistical contrast (M-S) and by functional localizer. (C) MC/MI coefficient in area MT+, as defined by the statistical contrast (M-S) and in its functional subdivisions MT and MST+, as defined by functional localizer. The MC/MI coefficient of 1 is marked by a thicker orange horizontal line to indicate identical response to both kinds of motion. Bars represent the mean coefficients ± standard error of the mean across runs and participants (n indicates number of hemispheres).