Visual topography of human intraparietal sulcus

Abstract

Human parietal cortex is implicated in a wide variety of sensory and cognitive functions, yet its precise organization remains unclear. Visual field maps provide a potential structural basis for descriptions of functional organization. Here, we detail the topography of a series of five maps of the contralateral visual hemifield within human posterior parietal cortex. These maps are located along the medial bank of the intraparietal sulcus (IPS) and are revealed by direct visual stimulation during functional magnetic resonance imaging, allowing these parietal regions to be routinely and reliably identified simultaneously with occipital visual areas. Two of these maps (IPS3 and IPS4) are novel, whereas two others (IPS1 and IPS2) have previously been revealed only by higher-order cognitive tasks. Area V7, a previously identified visual map, is observed to lie within posterior IPS and to share a foveal representation with IPS1. These parietal maps are reliably observed across scan sessions; however, their precise topography varies between individuals. The multimodal organization of posterior IPS mirrors this variability in visual topography, with complementary tactile activations found immediately adjacent to the visual maps both medially and laterally. These visual maps may provide a practical framework in which to characterize the functional organization of human IPS.

Figures

Figure 1.

The angular mapping wedge stimulus reveals visuotopically specific activation throughout a substantial portion of the posterior cerebral cortex. The angular position of the wedge resulting in the greatest significant (p < 0.05) response is indicated by the colored overlay (inset), with red representing the upper visual meridian, blue representing the contralateral horizontal meridian, and green representing the lower meridian. Potential ipsilateral responses are indicated in yellow. The right hemisphere (RH) of a single subject (Subj 1) is shown here. A, The reconstructed pial surface is shown from posterior lateral (top left), posterior medial (bottom left), and posterior (right) views. B, Computational inflation of the folded cortical surface reveals visuotopically specific activation within the sulci. Reversals in the represented angular position correspond to areal boundaries in the early visual areas and are shown as overlaid lines. Dashed lines indicate reversals at representations of the vertical meridian, whereas solid lines indicate reversals at horizontal meridian representations. We find five hemifield maps along the medial bank of the IPS, including area V7 and IPS1–IPS4. C, A flattened representation of the occipital cortex (extending into parietal and temporal regions) showing 14 simultaneously mapped visual regions. The disorganized responses in the peripheral regions of V1–V3 likely represent negative blood oxygenation level-dependent signal, leading to lagged or antiphase responses to the visual stimulus (Shmuel et al., 2002).

Figure 2.

Coronal slices. The location of V3A, V3B, V7, and IPS1–4 is indicated in coronal slices through the brain of subject 1 (Subj 1). Area V7 occupies a substantial portion of posterior/inferior IPS. RH, Right hemisphere; ANT, anterior; POS, posterior.

Figure 3.

Radial histograms of preferred angular response phase. These rose diagrams show the distribution of response phase for surface vertices responding significantly to the angular mapping stimulus, combined across all five intensively mapped subjects. The histograms have been rotated so that the angular position on the plot corresponds to the angular position within the visual field. A, B, Histograms for left and right combined V1, V2, and V3. C, D, Histograms for left and right combined V7 and IPS1–4. All respond predominately to contralateral stimuli. Both early visual and parietal cortex overrepresent the horizontal meridian, although the overrepresentation is somewhat more pronounced in IPS. The color maps used to display the response to the angular mapping stimulus (as in Fig. 1) are defined to similarly overrepresent the contralateral hemisphere. LH, Left hemisphere; RH, Right hemisphere.

Figure 4.

Angular and eccentricity patches for the left (LH) and right (RH) hemispheres of a second subject (Subj 2). The location of the patches is indicated by shading on the inflated hemisphere (far left). The patches are aligned so that posterior IPS runs vertically, from V3A/B at the bottom (inferiorly) to IPS4 at the top (superiorly). Angular (left) and eccentricity (Eccen; right) data are displayed on the same anatomical patches, with identical borders overlaid. The eccentricity mapping color key goes from red at central eccentricities (here ∼0–3.5° radius) through mid-eccentricities in blue and the periphery in green. Foveal responses are indicated by circles.

Figure 5.

Average amplitude and reproducibility of visuotopic activity across areas. In A, gray bars show the average signal change (mean to peak) within the retinotopically defined visual maps at the mapping stimulus frequency across the full population of 20 subjects (subjs) scanned. Black bars show the signal change estimate at a frequency five cycles per run greater than the mapping stimulus, at which no significant periodic component is expected. In B, bars show the circular correlation between the phases of points significantly active in both mapping sessions over the set of five repeatedly scanned subjects. Error bars indicate SEM. *p < 0.05; **p < 0.01.

Figure 6.

Test–retest reliability of visual field maps. Maps acquired during separate sessions of stimulus-driven mapping (at least 2 months apart) are shown projected onto parietal patches for both hemispheres of a single representative subject. The maps show qualitatively good reproducibility across sessions. In addition to the reproducibility of the maps themselves, large dropouts in the visuotopic response [as seen for instance at the level of IPS1 in the left hemisphere of subject 1 (subj 1)] are also reliably observed in the same positions across sessions. A, Left hemisphere (LH). B, Right hemisphere (RH).

Figure 7.

Comparison of stimulus-driven and saccade maps. Both stimulus-driven and saccade maps acquired in separate sessions for a hemisphere with relatively extensive saccade responses are shown. Each stimulus type produces similar angular phase maps, although we typically find a larger region of significant response during stimulus-driven mapping. Subj 2, Subject 2; LH, left hemisphere.

Figure 8.

Visuotopic and tactile activations form complementary patterns in posterior parietal cortex. Four hemispheres from three subjects are shown, each with visuotopic maps, tactile activations, and the conjunction (Conjunct) of visuotopic and tactile activations overlaid. Tactile patches show activation in the contrast of roughness judgment versus a motor control. The discrete heat scale for the tactile map indicates significances at levels of p < 0.1, 0.01, 10−5, and 10−20 in progressively warmer colors. In the conjunction maps, significant tactile activity is indicated in warm colors along the vertical axis using the same scale as in the tactile maps, whereas significant visuotopic activity is shown in cool colors along the horizontal axis at levels of p < 0.1, 0.05, 0.01, and 10−5. The conjunction of significant activation in both tasks (logical “and”) is shown in shades of green. The minimum threshold for both tasks is set at a lenient 0.1 level, because the logical and conjunction is intrinsically a conservative statistic (Nichols et al., 2005). The activation patterns overlap at the edges; however, in no case have we observed tactile activation fully overlapping a connected region of visuotopic activation (or vice versa). Notably, in hemispheres where the visuotopic activation pattern is separated into two noncontiguous clusters of significant activity, the tactile activation “fills in” this gap (Subj 1 LH, Subj 3 RH). Conversely, in hemispheres with a continuous band of significant visuotopic activity, the tactile activation pattern is divided by this band into distinct, noncontiguous medial and lateral foci (Subj 1 RH, Subj 4 RH). Subj, Subject; LH, left hemisphere; RH, Right hemisphere.