In two awake, fixating monkeys (‘CH’, ‘TO’), we performed a phase-encoded retinotopic mapping experiment at 7 Tesla (Sereno et al., 1995
) using stimuli with eccentricities between 1-12 degrees and a full variation of polar-angle. As predicted from single unit and anatomical tract tracing studies (Komatsu and Wurtz, 1988
; Tanaka et al., 1993
; Maunsell and Van Essen, 1987
; Desimone and Ungerleider, 1986
; Mikami et al., 1986
), we identified a foveal representation near the fundus of the posterior portion of the STS ( [monkey CH], and figs. S7A and S7B
[monkey TO]) that is anatomically distinct from the foveal representation in early visual areas (V1, V2, V3, V3A and V4). In addition, the fMRI data revealed (i) a continuous eccentricity map spanning a three quarter circle surrounding this singular foveal representation (, and figs. S7A and S7B
), (ii) a polar angle map showing eight alternating representations of horizontal and vertical meridians (, and figs. S7C and S7D
), and (iii) a field sign map indicating the existence of four individual areas joined at this foveal representation (figs. S3E
). Thus, one contralateral quarter-field plus three complete contralateral hemifield representations surrounding a common foveal representation and sharing a continuous circular eccentricity map in this portion of the STS are found consistently in all four hemispheres.
shows the polar angle phase variation as a function of distance along a curved line surrounding the foveal MT/V5 representation based on a detailed analysis of the polar angle data. We sub-sampled the polar angle data, interpolated it onto a Cartesian grid of 0.5 mm, and smoothed it with a Gaussian kernel of 0.5 mm to restore the initial resolution of 0.75 mm. We selected grid points that coincided with points along a pre-selected circular path on the grid for display. Both test and re-test data were projected onto the same surfaces and we used the same circular path on the Cartesian grid for both data sets.
Figure 3 Polar angle phase variation within the MT/V5 cluster; subject CH. (A) Polar angle phase as a function of distance along a semi-circle within the MT/V5 cluster (CH); top: from a to b for the left hemisphere (B); bottom: from c to d for the right hemisphere (more ...)
The data confirms multiple phase reversals, thereby corroborating the existence of one contralateral quarter-field and three contralateral hemifield representations. The individual field maps are separated by representations of the upper and lower vertical meridians, as indicated by the color-coded wedges in . show enlarged versions of the polar angle maps from the posterior STS of the left and right hemisphere (subject CH, same data as in ). Two consecutive cycles of lower and upper-field representations can be observed if one moves along the curved lines (). Further, illustrates test-retest reproducibility and across-hemisphere symmetry. The data revealed excellent agreement for polar angle phase reversals (i) between two independently acquired data sets from the same monkey in experiments separated by ~5.5 months (open vs. closed symbols) and (ii) between two hemispheres in the same animal (upper vs. lower traces). This topographic organization of MT/V5 and its satellites was consistently found in all four hemispheres imaged as shown by a test-retest analysis (figs. S3
). In general, the observed functional layout resembles a pinwheel structure, exactly the hallmark of a field map cluster (Wandell et al., 2005
In , we show the detailed spatial relationship of the polar angle and eccentricity representations between a flattened reconstruction of the STS and the corresponding coronal sections through area MT/V5 and its satellites. The eccentricity and polar angle lines in the schematics of were constructed based on a combination of polar angle, eccentricity, and field sign maps as described in figs. S3
and methods. In we indicated on three coronal T1-weighted sections the position of several arbitrarily chosen anatomical landmarks at the boundary between grey and white matter -such as the ventral lip of the STS (grey diamonds), the boundaries of the floor of the STS (purple diamonds), and points in between (white diamonds). These landmarks were projected onto a 2D-reconstructed portion of the STS as shown on the flat maps in . The same procedure was performed for all coronal sections covering this portion of the STS, hence, these flat maps show the exact location of the field maps with respect to the posterior lip of the STS (grey dotted line), and the dorsal and ventral limits of the floor of the fundus (purple dashed lines).
Figure 4 Anatomical location of polar angle and eccentricity representations in the MT/V5 cluster and surrounding areas; subject CH. (A-C) Three coronal sections (T1-weighted images) at −4.90 mm, −3.15 and −1.40 mm relative to the inter-aural (more ...)
Based on the detailed anatomical location of the meridian representations (), we attribute the quarter and the three hemifield representations to areas V4t, MT/V5, MSTv, and FST (Komatsu and Wurtz, 1988
; Desimone and Ungerleider, 1986
; Tanaka et al., 1993
; Andersen et al., 1990
). We found weak evidence for an additional quarterfield representation of the upper visual field ventrally to V4t, which, based on polar angle and field sign map, could be a complementary part of a hemifield at this location. At present, however, we lack further evidence from functional data for this part of the cortex and are not able to attribute this quadrant to a specific area. Therefore, we have assigned areas based on what is supported by literature, i.e. V4t is a quarterfield representation. To illustrate the quality of the fMRI signal changes within the MT/V5 cluster, we show session-averaged time courses of five selected voxels, which are located in the lower bank of the STS and which are confined to V4t and MT/V5 (). It is apparent from the time courses that the receptive field size of the neurons within these voxels is sufficiently small to allow reliable phase analyses since the signal returns to baseline between two periods of activations.
Figure 5 Amplitude of BOLD signal changes in single voxels from lateral to medial in the STS; subject TO, left hemisphere. (A) Coronal section (T1-weigthed image) approximately 10 mm posterior to the inter-aural plane. The position of five voxels are indicated (more ...) Figures S3
show evidence of two additional central or near-central representations with associated field maps. One is located ventral  and another dorsal  relative to the MT/V5 cluster (see also ). The dorsal near-central representation  is most obvious in subject TO, right hemisphere (fig. S5
) with minimum eccentricity values near 3 degrees. It is associated with a hemifield map with perpendicular polar angle and eccentricity representations and can be seen in all tested hemispheres (figs. S3
), except for the left hemisphere of subject TO (fig. S6
). The ventral central representation can be seen in all tested hemispheres with eccentricity values as low as 1 degree, which is the lowest eccentricity tested by the stimulus (figs. S3
). It is associated with a hemifield in all hemispheres (figs. S3
), except for the left hemisphere in subject TO (fig. S6
). This organization supports a dorsal and ventral field map with a full hemifield representation, distinct from the MT/V5 cluster.
The more dorsally located near-central representation  lacks the blue color-code in the eccentricity map, as one would expect for a true central visual field. However, the general circular structure of the eccentricity map and the associated hemifield representation suggest it is a near-central, and not a mid-peripheral, representation. We attribute this effect to a combination of larger receptive field sizes in  compared to the foveal representation in the MT/V5 cluster and averaging effects due to a finite imaging resolution.
The apparent activation seen in the foveal representation of areas V1, V2, and V3 can be caused by surround inhibition beyond the actual stimulus leading to a negative BOLD signal (Brewer et al., 2002
; Sereno and Tootell, 2005
; Saygin and Sereno, 2008
). In eccentricity measurements this can lead to wrap around of the phase angle at the stimulus edge at low and high eccentricities resulting in a continuation of the eccentricity map beyond this edge. This effect is more pronounced in areas with a large cortical magnification factor and relatively small receptive field sizes such as V1 and V2, and vanishes for small areas with larger receptive field sizes such as in the MT/V5 cluster. We have observed this effect in all four hemispheres near the fovea of the primary visual areas and indicated the approximate edge of the stimulus by a dashed-dotted grey line in and fig. S7