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Review
. 1998 Feb 3;95(3):811-7.
doi: 10.1073/pnas.95.3.811.

Functional analysis of primary visual cortex (V1) in humans

R B Tootell  1 , N K Hadjikhani, W Vanduffel, A K Liu, J D Mendola, M I Sereno, A M Dale
Affiliations
Review

Functional analysis of primary visual cortex (V1) in humans

R B Tootell et al. Proc Natl Acad Sci U S A. .

Abstract

Human area V1 offers an excellent opportunity to study, using functional MRI, a range of properties in a specific cortical visual area, whose borders are defined objectively and convergently by retinotopic criteria. The retinotopy in V1 (also known as primary visual cortex, striate cortex, or Brodmann's area 17) was defined in each subject by using both stationary and phase-encoded polar coordinate stimuli. Data from V1 and neighboring retinotopic areas were displayed on flattened cortical maps. In additional tests we revealed the paired cortical representations of the monocular "blind spot." We also activated area V1 preferentially (relative to other extrastriate areas) by presenting radial gratings alternating between 6% and 100% contrast. Finally, we showed evidence for orientation selectivity in V1 by measuring transient functional MRI increases produced at the change in response to gratings of differing orientations. By systematically varying the orientations presented, we were able to measure the bandwidth of the orientation "transients" (45 degrees).

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Figures

Figure 1
Figure 1
Topography of primary visual cortex and surrounding areas. (A and B) Field sign analysis (12, 13) of retinotopic cortical visual areas from right and left hemispheres (respectively) in a single subject. Both hemispheres are views of the medial bank, in its normal, folded configuration. Thus in A, anterior is to the left, and posterior to the right. In B, this is reversed. The field sign maps are based on two scans measuring polar angle (rotating thin ray stimulus), and two scans measuring eccentricity (expanding thin ring stimulus), acquired from echo-planar images in a 3-T scanner (General Electric/ANMR), using a bilateral, send/receive quadrature coil. Both stimuli extended 18–25° in eccentricity (36–50° extent). (C and D) Same data, in a cortically “inflated” format, now viewed from a more posterior–inferior vantage point. Again the left panel shows the right hemisphere, and the right panel shows the left hemisphere from the same subject. Human retinotopic areas revealed by the field sign analysis have been labeled (V1, V2, V3, VP, V3A, V4v). Cortical areas with a visual field sign (polarity) similar to that in the actual visual field are coded blue, and those areas showing a mirror-reversed field polarity are coded yellow. Also labeled is the foveal representation in V1 (black asterisks). Gyri and sulci in the folded state (e.g., A and B) are coded in lighter and darker shades of gray (respectively) in the inflated format (C and D). In this subject, area V1 is somewhat larger than normal, extending well past the lips of the calcarine fissure. However, as in most subjects, the V1 representation of the extrafoveal horizontal meridian lies near the fundus of the calcarine fissure.
Figure 2
Figure 2
Representation of stationary polar coordinate stimuli (rays and rings, respectively) in human V1. This experiment was designed to produce a “bulls-eye” or “spider web” pattern in area V1 and examine the response by using fMRI, analogous to the pattern produced in macaque V1 previously by using different functional imaging techniques (34, 35). Here, the ray and ring stimuli were presented separately, during different scan acquisitions. During the first scan, the subjects viewed alternating 16-sec epochs of ray stimuli (“S1” and “S2”) composed of flickering black and white checks. This viewing produced the activity patterns shown in A and B. Preferential activation by the first stimulus is coded in red, and preferential activation by the second stimulus is coded green. A is a flattened section of the cortical surface from the right hemisphere, and B is from the left hemisphere. Both sections are taken from the same hemisphere shown in Fig. 1. Area V1 is the large middle region enclosed in dotted lines (i.e., the representation of the vertical meridian, based on the field sign map). V1 is flanked on both sides by V2, then V3/VP. The foveal representation is represented by a white asterisk. As one would expect from previous retinotopic maps in macaques and humans, rays of equal polar angle produce bands of approximately equal width in flattened cortex. During the second scan, the stimuli were composed of interleaved rings, again composed of flickering black and white checks (“S3” and “S4”). The rings were of equal polar width, thus quite unequal in width in the visual display. This stimulus produced activity bands of approximately equal width in cortex (C and D, same red–green pseudocolor conventions), oriented roughly at right angles to the bands of equal-polar-angle in A and B. During the third scan, stimuli were circular in shape (S5 and S6). The diameters of the stimulus circles were equal in polar coordinates. Thus the circles were much larger in angular subtense at greater eccentricities (large blue arrows), compared with the circles at more central eccentricities (smaller blue arrows). Nevertheless, the roughly circular activity representation of the two sets of circles in V1 was approximately equal in cortical extent (E and F). The stimulus circles are rerepresented, but progressively more faintly, in V2 and V3/VP. [The scale bar represents 1 cm (on average) across the cortical surface.]
Figure 3
Figure 3
Ophthalmological view of the normal retina, including the fovea (asterisk) and the optic nerve head (“blind spot,” delineated by the dashed white line). In each retina, the blind spot is located nasally, and slightly superior to, the fovea. The blind spot comprises a significant region in the retina. However, its relative contribution to the cortical retinotopic map is minimized by its relatively peripheral location, convolved by the cortical magnification factor (see Fig. 4).
Figure 4
Figure 4
Functional labeling of the representation of the retinal “blind spot,” as a monocular region in V1. B and C show flattened cortical surfaces including the central two-thirds of V1, V2, and V3/VP from both left and right hemispheres (B and C, respectively) from the same subject shown in Figs. 1 and 2. During the associated experiment, the subject viewed a large stimulus composed of flickering checks, using alternating monocular stimulation in alternating 16-sec epochs. Relative to nonstimulated baseline, the stimulus produced robust activation across all of these cortical surfaces (not shown). Preferential activation by right vs. left eye is coded in red–orange vs. blue–cyan, respectively. The data are accumulated from one 4 min and 16 sec scan with a 3-T scanner. The high field strength is partly why the significance levels (f test; see statistical logo at lower right) are relatively high. The foveal representation is marked by an asterisk, and the area borders revealed by the field sign maps (Fig. 1) are transposed onto the flattened maps, as in Fig. 2. The time course of the activity in these two “blind spot” representations is shown in A (orange for right eye, cyan for left eye stimulation); the mutual alternation of magnetic resonance (MR) signals is quite clear. Differential activation of similar-sized stimulus “circles” in the visual field produces resolvable activation in both V1 and V2 (Fig. 2 E and F). Thus it is interesting that the representation of the blind spot does not show up in V2 (B and C), in the same subject. However, no attempt was made to equate activity thresholds across these two experiments.
Figure 5
Figure 5
Preferential activation of human V1 produced by alternating stimulus contrast. A shows the contrast gain function based on averaged MR time courses from three visual cortical areas: V1, V3, and MT (5). The data predict that stimulus alternation between 6% and 100% contrast should produce robust MR modulation (80% maximum) of V1, but essentially no modulation in MT and V3. C shows the result of this experiment. Significant activation (red through yellow) is largely confined to area V1. Area borders in C have been transposed from field sign tests from the same subject, shown in B. To minimize cortical distortion, area V1 has been artificially bisected (as in refs. and 12).
Figure 6
Figure 6
Transient MR signal changes reveal orientation selectivity in human V1. Subjects fixated the center of the stimulus screen during MR scans, in a 1.5-T scanner. In A, scans were 4 min and 40 sec long. During the first 40 sec, the stimulus was a uniform gray field (dark gray time bin in A). During the remaining 4 min, the grating was presented continuously, with stripe width (0.1–2°) and position varied randomly every 0.4 sec. The flickering gratings were presented at a single orientation for 40 sec at a time. Every 40 sec, the grating orientation changed from one oblique orientation to the other, each 90° different from each other (white and light gray time bins in A). Signals were acquired continuously (1 image per slice per 2 sec) from voxels in area V1, selected on the basis of field sign map boundaries. A shows the averaged time course from 20 scans in one subject, in one ≈2-hr scan session. B shows the averaged response to each change in orientation; thus it represents the average of all epochs of a single orientation except the first grating presentation in each scan. Approximately 7 sec (the expected hemodynamic delay) following each change in orientation, a positive inflection (the “orientation transient”) is produced in the averaged MR signal (A and B). The second set of experiments (C and D) shows that smaller angular changes in orientation produce correspondingly smaller fMRI “transients.” C shows the average increase in signal between the actual time of orientation change (time = 0 sec) and the MR signal 6–8 sec later, when the transient MR response occurred. These changes were measured during two sets of scans, corresponding to the two curves shown in C. In one set of scans, the change in orientation between epochs began at 90°, becoming progressively smaller (67.5°, 45°, 22.5°, then 0°) at each subsequent epoch. During the second set of scans (presented in interleaved fashion with the first set of scans), the order of the size of orientation change was reversed (0°, 22.5°, 45°, 67.5°, then 90°). The values obtained after a specific size of orientation change were similar irrespective of presentation order, and they were averaged together to produce the average bandwidth shown in D. Averaged values for each orientation change in C are displayed twice in D (excepting the 90° change), for illustrative purposes.
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