Color and retinotopy in human cortical visual area V8

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nature neuroscience • volume 1 no 3 • july 1998 235
In Old World primates such as macaque monkeys and humans,
visual information about color is processed in anatomically
segregated columns, layers, channels or areas. It is important
to know to what extent color is processed in separate versus
convergent visual information pathways, because the added
dimension of color is so rich in visual information. For exam-
ple, we can discriminate about fifteen hundred different levels
of luminance1, whereas we can make several million discrimi-
nations if we also consider variations in color2. It is likely that
this glut of color information is incorporated into the labeled
lines of the neural architecture in some organized way.
In macaque monkeys, an anatomical segregation between chro-
matic-opponent versus achromatic-opponent cells has been report-
ed as early as the lateral geniculate nucleus. Color-specific anatomical
segregation has also been described in primary (V1) and secondary
(V2) visual cortex. In V1, prominent populations of color-selective
cells have been reported in specific layers3–5 and in the cytochrome-
oxidase blobs4–6, though the latter claim has been disputed7,8. Sim-
ilar (and equally controversial) claims have been made about the
prominence of color-opponent cells in the ‘thin’ stripes in area V2,
to which the V1 blobs project (ref. 9,10, but see 11).
However, the most prominent controversy about the
anatomical segregation of color-selective neurons occurs at a
higher level, in cortical area V4. According to different reports,
a high percentage of color-selective cells is either present12–15 or
absent16 in the largest and best-studied portion of that area,
dorsal V4 (V4d). A high percentage of color-selective cells has
not been reported in the smaller, ventral subdivision of V4
(V4v). More recent evidence suggests that brain mechanisms
critical for color selectivity are located not in macaque V4, but
rather in areas anterior to it (ref. 17–19, Vanduffel et al. Soc.
Neurosci. Abstr. 23, 845, 1997).
This controversy about color selectivity in V4 has now been
extended to human visual cortex. Based on human neuroimag-
ing studies, a small patch of color-selective activity near the mid-
dle of the collateral sulcus has been named ‘V4’ (ref. 20–22).
This choice of name presupposes that (1) an area homologous to
macaque V4 exists in humans, (2) V4 is color-selective, and (3)
this region in or near the collateral sulcus is the macaque V4
homolog. However, in humans, the location of this color-selec-
tive region has not yet been compared with the map of retino-
topic areas, to see whether color selectivity is really is in a
retinotopically defined human area V4.Furthermore, the degree
of color selectivity in macaque V4 is itself controversial17–19.
This issue is not just of academic interest. In an intriguing
clinical syndrome (‘achromatopsia’), human patients report
that the visual world becomes colorless following damage to a
cortical region that apparently includes this color-selective
area in the collateral sulcus23–25. This suggests that the con-
scious percept of ‘color’ involves that area, although it is
known that physical wavelength-dependent differences are
coded throughout prior levels of the visual system as well. If
we can define better which area this is in humans, we can learn
something about where conscious perceptions of color arise.
Accurate localization in humans should also make it possible
to study the homologous area in macaques using more inci-
sive (but invasive) classical neurobiological techniques.
We have attempted to clarify these issues in humans using
functional magnetic resonance imaging (fMRI). Technical details
were similar to those described elsewhere26, except that here we
manipulated the color content of the visual stimuli. We also used
a high-field MRI scanner and other improvements to substan-
tially increase the sensitivity of the retinotopic maps (Methods).
Results
COLOR- VERSUS LUMINANCE-VARYING STIMULI
First, we compared the activity produced by color variations to
that produced by variations in luminance, in the same sub-
articles
Retinotopy and color sensitivity in
human visual cortical area V8
Nouchine Hadjikhani, Arthur K. Liu, Anders M. Dale, Patrick Cavanagh
and Roger B. H. Tootell
Nuclear Magnetic Resonance Center, Massachusetts General Hospital, 149 13th Street, Charlestown, Massachusetts 02129, USA
Correspondence should be addressed to N.H. (nouchine@nmr.mgh.harvard.edu)
Prior studies suggest the presence of a color-selective area in the inferior occipital-temporal region
of human visual cortex. It has been proposed that this human area is homologous to macaque area
V4, which is arguably color selective, but this has never been tested directly. To test this model, we
compared the location of the human color-selective region to the retinotopic area boundaries in the
same subjects, using functional magnetic resonance imaging (fMRI), cortical flattening and
retinotopic mapping techniques. The human color-selective region did not match the location of
area V4 (neither its dorsal nor ventral subdivisions), as extrapolated from macaque maps. Instead
this region coincides with a new retinotopic area that we call ‘V8’, which includes a distinct
representation of the fovea and both upper and lower visual fields. We also tested the response to
stimuli that produce color afterimages and found that these stimuli, like real colors, caused preferen-
tial activation of V8 but not V4.
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236 nature neuroscience • volume 1 no 3 • july 1998
jects (Fig. 1). Our stimuli consisted of slowly moving sinu-
soidal radial gratings (‘pinwheels’) of low spatial frequency,
defined by either color or luminance contrast (Methods). As
shown earlier in V1 and V2 (ref. 27), we found that both
color- and luminance-varying stimuli produced robust acti-
vation in many areas of visual cortex, when compared with a
uniform gray field (data not shown).
Here we focused on those locations where the color stimuli
produced more activation than the luminance stimuli. In the
classically retinotopic visual areas (V1, V2, V3/VP, V3A and
V4v), we found prominent color-selective activation in the rep-
resentations of the fovea (center of gaze) but not in peripheral
representations (Fig. 1). A foveal color bias has not been report-
ed in previous imaging studies, perhaps because it is more obvi-
ous in our flattened maps. However, such a foveal color bias is
consistent with the well known predominance of cone pho-
toreceptors, and the corresponding absence of rods, in the fovea
of the retina. A similar foveal color bias is found in routine clin-
ical perimetry and in numerous psychophysical studies.
In 25 of 26 hemispheres (13 subjects) tested, we found an
additional region that responded preferentially to color, locat-
ed midway along the length of the collateral sulcus. Based on
the anatomical location and the nature of the functional com-
parison used here, this collateral color-selective patch appears
equivalent to the previously reported area involved in achro-
matopsia20,22, which has been proposed as the human homo-
logue of macaque area V4 (refs 20–22, 28–30).
However, when we compared the location of that collateral
color-selective patch to the retinotopic borders in the same sub-
jects, we found that the color-selective patch was consistently
located just beyond the most anterior retinotopic area defined
previously, area V4v. Earlier reports26,31–33 suggested that
human V4v is a quarter-field representation of the contralater-
al upper visual field. The more sensitive retinotopic mapping
articles
Fig. 1. Topography of
color-selective activity in
human visual cortex.
(a, b) The inferior,
‘inflated’ cortex, with
posterior to the left and
anterior to the right, in
two subjects. (c, d) The
posterior portion of
cortex in fully flattened
format, another view of
the same data shown in
(a) and (b), respectively.
In all panels, gyri from
the original brain are
shown as light gray and
sulci as dark gray. The
fundus of the collateral
sulcus (cs) is indicated
by the dashed black line.
The borders of previ-
ously described retinotopic areas (V1, V2, V3, VP, V3A, and V4v) are indicated in white (horizontal meridians, solid lines, upper vertical
meridians, dotted lines, lower vertical meridians, dashed lines). Typically, color-varying stimuli produced relatively higher activation in the
foveal representation of V1 and often V2 and V3/VP and a distinctive patch of color-selective activation approximately midway in the collat-
eral sulcus. When present, the latter patch was always located just anterior to the horizontal meridian representation marking the anterior
border of area V4v, rather than within V4v.
Fig. 2. Retinotopic features of area V8 by fMRI
mapping. (a–c) Retinotopy of polar angle in the infe-
rior row of cortical areas, from three flattened
hemispheres. From left to right, each panel shows
the representations of the contralateral upper quar-
ter field (red through blue or vice versa; see
pseudocolor logo) in inferior V1, then inferior V2,
then VP, then V4v. To the right of (anterior to) V4v
is the distinctive half-field representation comprising
V8 (green through blue through red, from upper to
lower in this figure). (d) Retinotopic representation
of eccentricity (the other dimension in polar space),
from the same hemisphere shown in (c). The repre-
sentations of central-through-more-peripheral
eccentricities are coded in red-through-blue-
through-green, respectively (see pseudocolor logo,
bottom right). The representations of the center of gaze are indicated with an asterisk. Area V8 has its own representation of the fovea, quite
distinct (and 3.5 cm) from the foveal representation in adjacent area V4v.
a b
cd
ab
c d
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nature neuroscience • volume 1 no 3 • july 1998 237
methods used here confirmed that V4v represents just that
quarter field, with its foveal representation located superiorly
alongside that of adjacent areas V3/VP (Figs 2 and 3).
The improved retinotopic methods also revealed additional
retinotopic features anterior to V4v. Taken together, these fea-
tures indicate the presence of an additional retinotopic map,
comprising a previously undifferentiated cor-
tical area that we call ‘V8’. This continues the
naming scheme begun by Zeki and colleagues,
who identified areas V1 through V612–15,21,22,34.
(We also identified an area ‘V7’, which is a rep-
resentation of the contralateral lower visual
field anterior to human V3A.)
Area V8 has a unique polar angle retinotopy
and a distinctive foveal representation. This
contrasts significantly with the three extrastri-
ate representations posterior to V8 (V4v, VP,
and the inferior wing of V2), all of which are
quarter-field representations of the contralat-
eral upper visual field. Although the polar-
angle retinotopy in V8 includes an additional
representation of this quarter field, it also
extends further to include a lower-field repre-
sentation as well (Figs 2 and 3). These three
extrastriate visual areas (V4v, VP and inferior
V2) also share a contiguous representation of
the fovea, at the top (superior) end of this row
of areas (Fig. 2d). However, the foveal repre-
sentation in V8 is not part of this contiguous
foveal band; instead it is located about 3.5 cen-
timeters away along the cortical surface, at the
anterior border of V8 (Fig. 2d).
In conventional Talairach coordi-
nates, foveal V4v (as defined retino-
topically in this study) was centered at
± 32, -87, -16, whereas foveal V8 was
centered at ± 33, -65, -14. When we
averaged the Talairach coordinates of
the color-selective area ‘V4’ described
in previous studies ( ± 26, -67, -9), we
found that it was about twice as close
to the location of our retinotopically
defined V8, compared to our retino-
topically defined V4v. This supports
all the other evidence that the color-
selective activity is located in area V8,
rather than in ‘V4’.
In each hemisphere, V8 comprises
a continuous representation of the
entire contralateral half of the visual
field. Although the authors did not
attempt polar-coordinate retinotopy or
flattened mapping, a prior study also
concluded that this human color-selec-
tive region included a representation
of upper and lower visual fields22.
Together with V8 in the opposing
hemisphere, the entire visual field is
thus represented. Such a complete rep-
resentation of the visual field would be
appropriate in an area processing high-
er-order, large-field color information.
HUMAN VERSUS MACAQUE MAPS
Because so much of the historical controversy about corti-
cal color processing arose in studies of macaque monkey, it is
natural to wonder which area in macaque corresponds to
area V8 in humans. To clarify the topographic relationship
of the human and macaque maps, we first averaged together
articles
Fig. 4. Comparison of the polar angle retinotopy in human visual cortex, relative to
that reported in macaque monkeys. In both species, visual cortex is shown in flat-
tened format, with visual area boundaries and polar angle continua as in Fig. 2a–c.
Area MT is shown in gray. In macaque, dorsal area V4 is also indicated (V4d). The
retinotopy of V8 is similar to that reported in area TEO, in that both areas are located
immediately adjacent to area V4v. However, the two areas differ in overall shape, and
the retinotopy of V8 is rotated approximately 90orelative to that reported in TEO.
Fig. 3. Detailed retinotopy of the polar angle representation, from the same hemisphere shown
in Fig. 2a. This figure shows the peak fMRI response (noisy white lines) corresponding to polar
angle gradients of approximately 20o, superimposed on the standard pseudocolor rendering of
areas V1, V2, VP and V4v (drawn in a). To the right of each panel is a logo indicating the specific
polar angle (white line) stimulated. The complete contralateral visual field is represented in V8,
from the lower visual field (aand b), across the horizontal meridian (c–e) to the upper visual
field (f–h). Note in (e–h) that the upper visual field representation in V8 can clearly be distin-
guished from, and is mirror symmetric to, that in adjacent V4v.
a
b
c
d
e
f
g
h
ab
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238 nature neuroscience • volume 1 no 3 • july 1998
six of the most robust human retinotopic maps, using digital
morphing techniques (Fig. 4b) as described26. These aver-
aged maps could then be compared to the map of macaque
retinotopy, as estimated from single-unit mapping35
(Fig. 4a). This comparison suggests that human area V8
shows some retinotopic and topographic similarity to
macaque area TEO. Furthermore, TEO and/or more anteri-
or areas have appeared strongly color selective in recent stud-
ies of macaque visual cortex (refs 17–19, 28, Vanduffel et al.
Soc Neurosci. Abstr. 23, 845, 1997).
However, the retinotopic similarity between V8 and TEO is
far from exact (Fig. 4). Furthermore, other investigators have
proposed different area boundaries in this region of macaque
cortex36–38. Unfortunately, those alternative models of the
macaque maps are even less similar to the empirical human
maps in this region of cortex. Thus it is not clear which macaque
area is homologous to human area V8. However, the flat maps
in Fig. 4 do make it clear that this human collateral color area is
not topographically similar to macaque V4 (neither dorsal nor
ventral subdivisions). Human V8 is also topographically incon-
sistent with the location of subdivisions proposed in macaque
dorsal V4, such as V4t (ref. 39) and V4A (e.g. ref. 14).
COLOR AFTERIMAGES
Another way to assess functional selectivity is by measuring the
fMRI responses during visual aftereffects, rather than during
the fMRI effects produced by the visual stimuli themselves. In
other dimensions such as motion40 and orientation41, such indi-
rect aftereffects have ironically proven to be functionally more
selective than the effects themselves. Here we tested whether
illusory color would also activate V8, as did real color stimuli.
If one stares for a time at a saturated color, then looks away
at a uniform gray field, one sees an illusory percept of the com-
plementary color. Unlike motion or orientation aftereffects,
these negative color afterimages are thought to arise primarily
in the retina42,43. However, they also presumably trigger activ-
ity at higher levels, as would a real stimulus that was similarly
stabilized on the retina29,44. To test for the presence of fMRI
responses to these illusory colors, we produced such color after-
images in the MR scanner, along with control stimuli that were
very similar but did not produce color afterimages.
Figure 5 shows the time course of fMRI activity produced
by these stimuli in area V8. As expected, the colored stimuli
produced robust fMRI activity in V8, whether alternating or
constant. However, only the constant-colored stimuli pro-
duced a perceptual color afterimage and a corresponding fMRI
aftereffect in V8 during subsequent viewing of the uniform
gray field (Fig. 5a). The alternating-colored stimuli produced
neither a perceptual color afterimage nor a prolongation of
the normal fMRI return to baseline during the subsequent
viewing period (Fig. 5a). The duration of the isolated fMRI
color aftereffect was prolonged, consistent with the prolonged
duration of the illusory color percept (Fig. 5b). Overall, this
strongly suggests that these fMRI responses were related to
the processing of the illusory colors.
One unexpected finding was that the stimuli with alter-
nating colors produced slightly more activity than the stim-
uli in which color remained constant (Fig. 5a). This may
reflect the fact that the hues in the constant-colored stimuli
become progressively less saturated (less densely colored) with
time because of chromatic adaptation45. Essentially one begins
to see the mixture of the color afterimage and the actual color,
Because these two colors are complementary, they produce a
less saturated, more ‘washed-out’ color. This decreased fMRI
response to the decreasingly saturated colors supports the
other evidence that V8 is involved in color perception.
Findings similar to our fMRI afterimages in V8 were
reported from scattered voxels in single-slice imaging through
nearby posterior fusiform gyrus29, but the location of those
voxels was not localized to any specific cortical area. However,
articles
Fig. 5. The time course of V8 activity is related to the perception of color afterimages. (a) Time course from all voxels in retinotopically
defined V8 that responded (p<0.00001) to the colored stimuli, relative to the initial presentation of the uniform gray stimulus, averaged
across 16 MR scans, showing the response to both the actual and the illusory color stimuli. Epochs in which subjects viewed a uniform grey
field, or an illusory afterimage on a gray background, are indicated with a white background; epochs when the subjects viewed colored stim-
uli (alternating- or constant-colored) are indicated with a gray background. After the color stimulus, subjects viewed a uniform gray field, or
an illusory afterimage on a gray background. During a color afterimage, the fMRI response was quite prolonged, consistent with the time
course of the percept of the illusory colors. (b) The MR afterimage is shown more directly by subtracting signal during constant color and
subsequent gray period from alternating color and subsequent grey period.
a b
Color
Fixation
Time (s)
Time (s)
∆ % MIRI signal
% MIRI isgnal
Fixation
Constant
colors
FixationFixation
Alternating
colors
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nature neuroscience • volume 1 no 3 • july 1998 239
none of these data address the possibility that similar fMRI
afterimages occur nonspecifically throughout much wider
areas of visual cortex. This could arise from retinal color pro-
cessing that is transmitted passively to cortex, or from glob-
ally increased attention when viewing the color afterimages.
To test this, we reanalyzed our data to find the areas in an
activity map that respond differentially during the presence
of the visual afterimage (Fig. 6). At lower levels of significance
than shown here, a number of additional visual areas (e.g. V1,
V2) do respond more to color afterimages. However, at the
more strict significance threshold used in Fig. 6,the activity
in V8 was more prominent than in any other area. In partic-
ular, the wider areas of foveal color selective activities includ-
ing areas such as V1, V2, etc., were relatively less prominent
in the afterimage test, compared to the direct comparisons of
color versus luminance (see Fig. 1c and Fig. 6, showing the
same hemisphere).
Discussion
The retinotopic maps make it clear that an additional area
(V8) exists beyond those areas described previously in human
visual cortex. Area V8 is retinotopically distinct from the pre-
viously described area V4v, based on at least four different cri-
teria. First, V4v and V8 have separate foveal representations,
approximately 3.5 cm apart along the cortical surface. Second,
V4v and V8 each include separate representations of the upper
visual field, separated from each other by a representation of
the horizontal meridian. Third, V8 differs from V4v in its
global functional properties, including but not limited to color
sensitivity. Fourth, the nature of the retinotopy in V8 is dif-
ferent from that in V4v.
The direct comparisons between color- and luminance-vary-
ing stimuli (Fig. 1) indicate that area V8 responds slightly better
than neighboring cortical regions to colored stimuli. However,
we also found that the color-varying stimuli produce preferen-
tial activation in the foveal representation of all retinotopic areas.
Thus, V8 may seem especially biased for color stimuli merely
because its foveal representation sets it topographically apart
from the conjoined foveal color responses of its neighbors (Figs
2and 3). This is a relatively trivial explanation for the color selec-
tivity reported earlier, but we cannot rule it out completely. This
idea is further supported by the fact that area V8 responds at
reasonable levels to a wider variety of visual stimuli.
However, other evidence argues that V8 is involved in
wavelength-dependent processing and perhaps in the con-
scious perception of color itself. The robust and selective
response to illusory colors (Figs 5 and 6) strongly supports
this idea.Also, the anatomical colocalization of V8 compared
with the previous clinical data makes it likely that area V8 is
damaged in achromatopsic patients23–25.
What do these human data tell us about macaque visual
cortex? This question is constrained by several factors. The
human data are based on clinical and neuroimaging data,
whereas the macaque data are derived from different tech-
niques (e.g., single units, lesions and DG imaging), which
could conceivably produce different results. Also, there may
be significant biological differences between the cortical orga-
nization of color sensitivity in humans compared with
macaques. As we learn more about human and macaque visu-
al cortex, the number of differences between these species are
increasing correspondingly26,46–48.
Despite these caveats, the data suggest that the area of
macaque cortex that is homologous to the human ‘achro-
matopsia’ area should be located in or anterior to TEO, rather
than in V4. This is supported by data from macaque17–19 as
well as the present data from human V8.
Methods
GENERAL PROCEDURES. Except for modifications described below, the
methods in this study are similar to those described26. Informed writ-
ten consent was obtained for each subject prior to the scanning ses-
sion, and all procedures were approved by Massachusetts General
Hospital Human Studies Protocol numbers 90–7227 and 96–7464.
Normal human subjects, with (or corrected to) emmetropic vision,
were scanned in General Electric magnetic resonance (MR) scanners
retrofitted with ANMR echo-planar imaging. Most scans were
acquired in a high-field (3 T) scanner, but some early scans were
acquired in a scanner of conventional (1.5 T) field strength. Based on
signal-to-noise ratios obtained during otherwise comparable condi-
tions, four functional scans at 1.5 T were found to be approximately
equal to one functional scan at 3 T, so this was the ratio used to equate
data acquired from the two scanners. Head motion was minimized
by using bite bars with deep, individually molded dental impressions.
The subject’s task in all experiments was to fixate the center of each
type of visual stimulus throughout the period of scan acquisition.
MR images were acquired using a custom-built quadrature surface
coil, shaped to fit the posterior portion of the head. MR slices were
3-4 mm thick, with an in-plane resolution of 3.1 x 3.1 mm, oriented
approximately perpendicular to the calcarine fissure. Each scan took
either 4 min 16 s (color-versus-luminance and color afterimage scans)
or 8 min 32 s (retinotopy), using a TR of either two or four seconds,
respectively. Each scan included 2,048 images, comprised of 128
images per slice in 16 contiguous slices.
Improved retinotopic maps were obtained from 32 subjects (79
scans polar angle, 79 scans eccentricity, 323,584 images total). Among
them, 13 subjects were also tested for color-versus-luminance (112
scans, 229,376 images total). Of these, five subjects were tested exten-
sively for color afterimages (100 scans; 204,800 images total). In most
subjects, additional scans were done to clarify the location of area
MT and other visual areas.
articles
Fig. 6. The perception of color afterimages produces relatively
higher activation in cortical area V8, compared with other cortical
areas. The activation shown here represents all regions that
responded significantly more (p≤0.00001) during viewing of the uni-
form gray stimulus following the constant color stimulus, compared
with viewing of the same gray stimulus following the alternating
color stimulus.
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240 nature neuroscience • volume 1 no 3 • july 1998
VISUAL STIMULI. The goal of the first color-related experiment (Fig. 1)
was to map the MR activity produced by color- versus luminance-
varying stimuli throughout visual cortex, using conventional psy-
chophysical stimuli. Prior to scanning, the equiluminance values for
different color combinations (red-cyan, CIE x and y coordinates 0.645,
0.345 and 0.185, 0.248 respectively, or green-purple, CIE x and y coor-
dinates 0.277, 0.684 and 0.351, 0.220, respectively) were measured for
each subject, outside the scanner. Equiluminance was measured using
a motion-null test, with the same stimulus projector (NEC model MT
800), lens and color software used subsequently in the MR experi-
ments. In the first experiment, both color- and luminance-varying
stimuli were produced using slowly moving (0.5 Hz) sinusoidal radi-
al gratings (‘pinwheels’) of low spatial frequency (3 cycles per revo-
lution). The gratings varied either in achromatic luminance
(maximum, 95% luminance contrast) or in equiluminant color (at
maximum available saturations of the display device within con-
straints of approximately 140 cd per m2mean luminance and approx-
imately white mean chromaticity), in direct alternation, in 16-second
epochs, using 16 epochs per scan.
In a second experiment (Figs 5 and 6), color afterimages were pro-
duced by showing subjects colored adaptation patterns. Subjects
adapted to two general types of stimuli; one produced a pronounced
color afterimage when subjects subsequently viewed a uniform gray
field, but a very similar control stimulus did not produce such a color
afterimage. There were five epochs in each scan, presented in the fol-
lowing order: (1) uniform gray, (2) alternating-color (control adap-
tation), (3) uniform gray, (4) constant-color (experimental
adaptation) and (5) uniform gray. All epochs were 48-s long, except
that the final fixation period was prolonged 16 s to reveal the final
traces of the MR afterimage. In the analysis for Fig. 5b, the last 16 s
in that final epoch was truncated to match the duration of all other
epochs. Both the constant- and the alternating-colored patterns were
comprised of complementary colors (red-cyan or green-purple), spa-
tially arranged in opposed quarter-fields (i.e. two-cycle-per-revolu-
tion polar square waves), akin to interleaved bow ties. All hues were
presented at equal luminance, based on motion-null tests in each sub-
ject. Following adaptation to the constant colors, subjects initially
experienced a prominent color afterimage against the uniform gray
background, which faded over tens of seconds. The afterimage was
retinotopically similar to the adaptation stimulus, but of comple-
mentary color. As controls, we presented stimuli equivalent to the
constant colors, except that the colors alternated between color and
complementary color, reversing every 2 s. The latter condition pro-
duced no perceptual color afterimages during the subsequent epoch of
uniform gray stimulation.
Stimuli for retinotopic mapping were slowly moving, phase-encod-
ed thin rays or rings comprised of counterphasing black and white
checks, scaled according to polar coordinates, similar to those
described26,31,32,50. However, to produce the most informative retino-
topic maps possible, several stimulus modifications and new proce-
dures were implemented. First, all retinotopic measurements were
made in the 3 T scanner. This increased the MR amplitudes by a factor
near four, and the physiological signal-to-noise ratio by a factor near
two. Second, we signal-averaged the information from 4–12 scans
(8,192–24,576 MR images) of polar angle or eccentricity. Data were
also combined from different slice prescriptions on the same cortical
surface, to reduce intervoxel aliasing. Third, the retinotopic stimuli
were increased in extent both foveally and peripherally, to extend from
0.2° through 18–30°. This activated correspondingly more surface in
each cortical area. Fourth, the visual stimuli were presented using a
new LCD projector of higher spatial resolution (800 x 600), using bet-
ter optics than previously (aperture lens, bypassing shielding screen,
etc.). Fifth, the retinotopic stimuli varied in color as well as luminance,
to better activate any color-selective cells in the region. The sum of all
these manipulations produced very robust retinotopic maps.
DATA ANALYSIS. Data from two-condition experiments (e.g., color-versus-
luminance comparisons) and phase-encoded retinotopic experiments were
initially analyzed by doing a fast Fourier transform on the MR time course
from each voxel. Statistical significance was calculated by converting the
Fourier magnitude of the response to an f-statistic. The phase of the sig-
nal at the stimulus frequency was used to track stimulus location in the
case of retinotopic stimuli, and to distinguish between positive- or negative-
going MR fluctuations in the case of two-condition stimulus comparisons.
Scans comparing more than two stimuli (e.g., the color afterimage
data) were analyzed by selective averaging of two conditions. This was
followed by statistical comparison using a t-test of the difference of the
first seconds following onset of the next epoch (here stimulus offset).
For topographic clarity, all data were analyzed and displayed in cor-
tical surface format, as described36,43,44. This made it possible to
extract the MR time courses from voxels in specific cortical areas,
which were defined in the same subjects. The specific areas sampled
were V1, V2, V3/VP, V3A, V4v, MT+, and V8. Area ‘MT+’ was defined
on the basis of additional scans comparing moving and stationary
stimuli26,40. All other areas were based on retinotopic criteria.
For ease of comparison, all hemispheres are shown in right hemi-
sphere format. Above a minimum threshold, the statistical significance
of the displayed pseudocolor range has been normalized according to
the overall sensitivity of each subject, as described elsewhere.
Acknowledgments
This work was supported by grants from the Human Frontiers Science
Foundation and NEI EY07980 to R.B.H.T., NEI EY09258 to P.C. and Swiss
Fonds National de la Recherche Scientifique to N.H. We thank Terry
Campbell and Mary Foley for scanning and participation in these
experiments, Robert Savoy, Ken Kwong, Bruce Fischl and Kevin Hall for
advice, Tommy Vaughan for coil design and manufacture and Martin Sereno
for modifying pilot stimuli. Wim Vanduffel, Ekkehardt Kustermann and
Irene Tracy also helped in preliminary versions of this experiment.
RECEIVED 16 APRIL: ACCEPTED 21 MAY 1998
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