Wellcome Department of Cognitive Neurology, University College London, London WC1E 6BT, United Kingdom
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ABSTRACT |
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Adams, Daniel L. and Semir Zeki. Functional Organization of Macaque V3 for Stereoscopic Depth. J. Neurophysiol. 86: 2195-2203, 2001. Recordings were made from single and small groups of cells in prestriate area V3 of the visual cortex of the Cynomolgus macaque (Macaca fascicularis). The majority of cells in V3 were selective for orientation and stereoscopic depth, these cells being segregated into two sets of functionally distinct columns. Orientation columns in V3 have been previously demonstrated; here we show that V3 also contains columns of segregated disparity-selective cells. On the basis of its cellular properties, functional organization, and intra-cortical connections, we propose that V3 contributes to the processing of stereoscopic depth information and that the parietal areas to which it projects use this information for the analysis of object depth and three-dimensional form.
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INTRODUCTION |
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Among the least studied of
the cortical visual areas is V3, which lies deep within the lunate and
inferior-occipital sulci in the macaque. The paucity of studies on V3
is perhaps surprising considering that it appears to occupy an
"early" stage of visual processing and was among the first visual
areas to be identified. V3 was discovered by virtue of its direct
topographic projection from V1 (Cragg 1969; Zeki
1969
) which originates in layer 4b (Burkhalter et al.
1986
; Lund et al. 1975
).
The posterior border of V3 lies at the representation of the horizontal
meridian of V2 and V3. The anterior extent of dorsal V3 borders area
V3A at the vertical meridian representation of these areas. Because
parts of visual areas that represent the vertical meridian in each
hemisphere are connected via the corpus callosum (Whitteridge
1965), V3's border with V3A can be visualized by staining for
degenerated fibers after section of the corpus callosum (Van
Essen and Zeki 1978
; Zeki 1978a
;
Zeki and Sandeman 1976
). Both V3 and V3A are organized
retinotopically (Cragg 1969
; Perkel et al.
1986
; Zeki 1969
, 1980
). V3 consists of two
portions: the dorsal portion of V3 (V3d) contains a representation of
the lower contralateral quadrant of the visual field and ventral V3 (V3v) contains the upper contralateral quadrant (Gattass et al. 1988
; Lyon and Kaas 2001
).
Both V4 and V5 receive a forward projection from V3 (Zeki and
Shipp 1988). Projections have been found directly from V3 and indirectly via V3A to the posterior parietal lobule (PPL) (Adams 1997
). The stronger of the projections to the PPL is to cortex within the confines of the dorsal prelunate area (DP) (Andersen et al. 1990
) and the caudal intraparietal area (CIP)
(Endo et al. 2000
; Kusunoki et al. 1993
;
Sakata et al. 1997
). In addition, the lateral
intraparietal area (LIP) has asymmetric connections with V3 and V3A
(Blatt et al. 1990
). The pattern of connections to
parietal (DP, CIP and LIP) and temporal (V4) areas places V3 in an
hierarchically intermediate position between the V4 and V5 complexes.
V3, in common with V3A, contains a predominance of
orientation-selective cells (Baizer 1982;
Burkhalter et al. 1986
; Felleman and Van Essen
1987
; Zeki 1978a
,b
). V3 cells with similar
orientation preference are grouped into columns (Zeki
1978a
). A similar functional architecture has been demonstrated
in V5, where the property of direction selectivity is organized in a
columnar fashion (Albright et al. 1984
; Zeki
1974a
). The grouping of cells into columns occurs in many areas
of the neocortex. A clue to the functional tasks executed by any given
area is afforded by the types of columnar systems it contains
(Mountcastle 1997
). The prevalence of
orientation-selective cells in V3, their preference for moving stimuli,
and their grouping into columns have led some to identify it as an area
specialized for the processing of dynamic form (Gegenfurtner et
al. 1997
; Zeki and Shipp 1988
). Given the close
connection between orientation selectivity and stereoscopic disparity
tuning (Hubel and Wiesel 1970
), we wondered whether
there was any systematic relationship in the representation of the two
within V3.
The horizontal separation of the eyes causes a slightly different image
to fall onto each retina. The brain gains knowledge about depth by
analyzing the disparity between the two retinal images. The brain's
capacity to perceive depth from retinal disparity is called stereopsis.
Cells sensitive to disparity have been found in most visual areas and
have been classified according to the shape of their disparity tuning
curve. Disparity-selective cells fall into two groups: tuned and
asymmetric (Poggio and Fischer 1977; Poggio et
al. 1988
). Tuned cells respond with a narrow profile at a
particular disparity (usually within ±1° of the fixation plane) and
asymmetric cells respond over a broad range of disparities either in
front of or behind the fixation plane.
Many V3 cells have strong binocular interactions, i.e., their binocular
response are greatly facilitated or attenuated compared with the sum of
their monocular responses (Zeki 1978a,b
). Many V3 cells
are also disparity-selective (Felleman and Van Essen 1987
; Poggio et al. 1988
). We report that the
disparity-selective cells of V3 are organized into columns of similar
cell type. The organization of orientation- and disparity-selective
cells in V3 leads us to propose that this area participates in the
analysis of three-dimensional form.
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METHODS |
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Thirteen Cynomolgus macaques (Macaca fascicularis)
weighing between 1.5 and 2.0 kg were used. Monkeys were initially
anesthetized with ketamine (Vetalar, Parke-Davis; 25 mg/kg im),
intubated, and anesthetized by continuous intravenous infusion of
propofol (Diprivan, Zeneca; 4-12 mg · kg1 · h
1 iv) and
sufentanil citrate (Janssen; 3-8 µg · kg
1 · h
1 iv). A
5-mm craniotomy was made over the lunate sulcus and the dura reflected.
A cylindrical plastic recording chamber was then bonded to the skull
with dental cement and filled with 2% agar-in-saline. After surgery, a
neuromuscular blocking agent, pancuronium bromide (Pavulon, Organon
Teknika; 0.4 mg · kg
1 · h
1 iv), was infused to prevent eye movements.
Streak retinoscopy was used to select appropriate lenses to focus the
eyes onto a tangent screen 114 cm from the animal. Plano contact lenses
were inserted to prevent corneal drying. The foveae and some
distinctive retinal vascular landmarks were plotted onto the screen
with a reversible ophthalmoscope and monitored regularly to track any residual eye drift. Recordings were made with glass-coated
platinum-plated tungsten microelectrodes (Merrill and Ainsworth
1972
) with an exposed tip length of 10-12 µm. Spikes were
sorted by amplitude using a window discriminator (Neurolog), digitized,
and recorded by a computer at 20-ms binwidth. Signals from the stimulus
computer were integrated with the spike data and receptive field
location to produce on-line poststimulus time histograms and to
calculate average spike rates for each trial. During recording, animals were maintained under anesthetic and ventilated while their
electrocardiogram (ECG), temperature, and end-tidal
CO2 were monitored. All procedures were done in
accordance with British Home Office regulations under the Animals
(Scientific Procedures) Act 1986.
Multiple electrode penetrations were made over a period of 4-5 days.
The reconstruction of electrode tracks from horizontal sections was
aided by placing small electrolytic lesions at recording sites by
passing current through the electrode (7 µA for 7 s, tip
positive). Immediately after the experiment, the monkey was killed with
an overdose of pentobarbitone sodium and perfused transcardially with
warm saline, followed by 3 l of 0.1 M PO4 buffered 4% paraformaldehyde. Buffered sucrose solution was then run
through sequentially at 10, 20, and 30% concentrations. The brain was
removed and placed in 0.1 M PO4 buffered 30%
sucrose solution and kept at 4°C until no longer buoyant (2-4 days).
Sections were cut horizontally at 60 µm on a freezing microtome.
Every section was saved and stored at 4°C in 0.1 M phosphate buffer prior to histological reaction. Adjacent sections were stained for
cytochrome oxidase (Wong-Riley 1979), myelin
(Gallyas 1979
), and Nissl.
Stimuli
Stimuli consisted of moving or stationary luminance contrast bars presented on an 18-in RGB computer monitor (Grundig), refresh rate 60 Hz, positioned 114 cm from the animal. The orientation, size, shape, and color of the computer-generated stimuli could be changed to best suit the cell being studied. Receptive fields were plotted manually by moving a bar stimulus on the screen with a track ball. Both receptive fields of a binocular cell were plotted while occluding each eye in turn. Four boundaries were plotted for each receptive field to form two rectangular minimum response fields. Once receptive fields were digitized, computer-controlled trials were presented to discover the cell's most effective stimulus orientation, speed, color, size, and aspect ratio. Generally, 4-10 presentations of each stimulus were necessary to produce a clean spike histogram, depending on the firing characteristics of the cell.
An adaption of the method used first by Henry et al.
(1969) was employed to stimulate cells at different
disparities. During paralysis, receptive fields became arbitrarily
separated on the tangent screen because fusion was disrupted. A pair of
optimal stimulus bars with a constant separation was swept over the
screen so that each bar passed through one of the monocular receptive fields. Changing the stimulus bars' horizontal separation relative to
the receptive field separation changed the disparity of the stimulus.
Typically, cells were stimulated over a broad range of disparity
(~5°) in steps as small as 3 min of arc (1 mm on the tangent
screen). This method of presenting simultaneously two monocular stimuli
removed the need to superimpose the receptive field pairs with prisms.
A limitation to studying disparity tuning in the anesthetized paralyzed
preparation is that the eyes assume a divergent, drifting gaze, and a
correction has to be used to stimulate corresponding points on each
retina. We used the angular separation of the fovea's on the tangent
screen as a measure of the angle between corresponding points in the
two retinas. To correct our disparities for divergence and drift and
calculate the position of the fixation plane, eye position was
monitored between each recorded cell by projecting retinal vascular
landmarks onto the tangent screen using a reversible ophthalmoscope. We found the reversible ophthalmoscope to be accurate to ±1.0° when plotting the positions of clearly visible vascular features of the
retina, and this inaccuracy was reflected in our calculations of
fixation plane position.
Quantitative assessment of responses
As well as qualitative assessment at the time of recording, we applied a quantitative method to judge cell selectivities. An index was calculated for each cell's responses to orientation, direction and color stimulus properties. Cells with index values >0.5 were considered biased and those with index values >0.7 were considered selective for the property in question.
Orientation index (OI) was calculated by the following formula
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RESULTS |
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The areal assignment of cells was done on the basis of sulcal
topography and the locations of borders in brain sections from previous
anatomical mapping studies. To visualize the V3/V3A border, sections
from brains where the splenium of the corpus callosum had been cut and
the brain stained for degeneration were consulted (Van Essen and
Zeki 1978; Zeki 1978a
). Maps derived from V1
lesion studies (Cragg 1969
; Zeki 1969
)
were used to locate the V2/V3 border. Receptive field sizes were also
used to distinguish between V2 and V3, although they could not be used
to differentiate V3 and V3A, whose cells have similarly sized RFs
(Zeki 1978a
). We could therefore be confident that cells
were in V3 rather than V2 but not always as certain that they were in
V3 rather than V3A.
A total of 225 cells was recorded from V3, 100 of which were tested for
their selectivity for horizontal disparity. All of the units had their
receptive fields within 20° of the fovea and the majority were within
10°. Since we restricted our recording sites to the region of V3 at
the fundus and deep anterior bank of the lunate sulcus (V3d), most
receptive fields were restricted to the horizontal meridian and lower
visual field (see Fig. 1A). However, a few V3 cells' field centers represent the ipsilateral visual field, a finding also demonstrated with fMRI in human V3 complex
(Tootell et al. 1998). Figure 1B shows how
receptive field size varied as a function of eccentricity.
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Orientation/direction selectivity
We confirmed that orientation selectivity was a characteristic property of V3 cells; the majority (85%) was orientation selective as qualitatively judged by the experimenter. We calculated orientation indices (OI) for our population of V3 cells to quantify these judgments (see METHODS). In our study, 82% of cells had an OI >0.7, signifying strong selectivity, a proportion remarkably similar to the fraction qualitatively judged by ear to be orientation selective at the time of recording. Values >1 were common and occurred when there was inhibition of the spike rate to a level below baseline by the orthogonally oriented stimulus.
Qualitatively, 25% of our V3 cell population were judged to be
direction selective. Previous reports have found proportions ranging
from 15% (qualitatively judged) (Baizer 1982) to 57%
(quantitatively judged) (Gegenfurtner et al. 1997
) of
direction-selective or biased cells in V3. When we calculated direction
indices (DI) for these cells, 25% were classified as direction
selective and 19% direction biased. OI versus DI was plotted for the
V3 cell population in Fig. 2.
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Color properties
We routinely tested cells in V3 for color properties by presenting a series of roughly isoluminant bars of various colors on a black background at the cell's preferred orientation, size and speed. Very few cells (5%) appeared to modulate their responses with wavelength as judged qualitatively at the time of recording. The few that did have wavelength preferences were often characterized by a lack of response to, rather than selectivity for, a particular wavelength. Cells of this nature were nevertheless classified as color-selective. When color indices were calculated for our population of V3 cells the proportion of selective cells was 9% and that of biased cells was 6%.
Disparity selectivity
The primary focus of this study was the organization of
disparity-selective cells in V3. To investigate their arrangement, long
electrode penetrations were made at differing angles through V3d.
Disparity-selective cells were classified into the groups first defined
by Poggio and Fischer (1977). The following criteria were used for classification of disparity-selective cells:
ASYMMETRIC CELLS. Excitatory responses greater than the sum of the monocular responses to a broad range of disparities either in front of the fixation plane (for near cells) or behind it (for far cells), commonly accompanied by inhibition, or a response smaller in magnitude than either monocular response, to the inverse range of disparities.
TUNED CELLS. A narrow (<1° half-width at half height) peak in response occurring either at zero disparity (for T0 cells) or at near/far disparities for TN and TF cells. TI cells were characterized by excitation to all disparities except a narrow range (<1° half-width at half height) of inhibition/response smaller than either of the monocular responses. Figure 3 shows the spike histograms and tuning curves for two typical disparity-selective cells in V3. The foveal separation during the recording of these cells was 1°, so we estimate the fixation plane to lie at 1° on the x axis. The upper cell was classified as a near cell, and the lower one, a T0 cell. Of our sample of 100 V3d cells tested for disparity, 61 were judged as disparity-selective using the preceding criterion.
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Representative penetrations
Three representative electrode penetrations will be presented. The first (Fig. 4A) passed through the cortical sheet at an angle of 44° from normal; the second (Fig. 4B) traversed the cortical sheet at 73°. The third penetration (Fig. 5) passed through the cortex at the fundus of the lunate sulcus and therefore started normally, became tangential, and then exited the cortex normally again. Because this penetration changed angle as it passed through the cortex, it was divided into three equal segments, a-c. The average angle of each segment from normal was measured as 53, 90, and 36°.
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In the first penetration (Fig. 4A), orientation preference changed by 36° in a single step between two consecutive recording sites 100 µm apart. All cells recorded in this penetration were near cells except the last one, which was unselective for disparity. In the second penetration (Fig. 4B) the orientation preference changed between consecutive cells in a progressive manner and covered 90°. Five separate types of disparity-selective cells were present along the length of the penetration, some in clusters.
In the third penetration (Fig. 5), orientation preference changed
little at the beginning and the end of the passage through V3 where the
electrode ran nearly perpendicular to the cortical layers. In the
middle of the electrode track where the electrode was passing more
tangentially, orientation preference changed most rapidly, covering a
90° range over 1 mm. The spans of disparity-selective cells changed
in a similar way: long clusters were observed at the beginning and end
of the penetration, whereas in the middle, the cell properties changed
frequently. The clustering pattern of orientation selectivity
properties in these electrode tracks confirms previous reports of a
columnar organization of orientation-selective cells in V3 (Zeki
1978a). Additionally, disparity-selective cells also appear to
be organized into columns that stretch from the cortical surface to
white matter boundary.
The progression of receptive field (RF) centers for each of these penetrations is shown in Fig. 6, A-C. The first penetration's RF centers were more clustered than those of the second and third penetration illustrated. They also showed less net displacement. The RF centers in penetrations (B) and (C) progressed in an average downward direction because the electrode was advancing from posterior V3d, where the horizontal meridian is represented, toward the V3/V3A border where the lower vertical meridian lies. The large amount of scatter in these progressions compared with that observed in similar penetrations through V1, reflects the coarse point-to-point retinotopy in V3.
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Columnar organization
To distinguish between the clustering of cells with similar properties and the organization of cells into columns that stretch from the cortical surface to white matter boundary, we made penetrations through V3 over a range of angles to the cortical layers. If cells were organized into spherical clusters, the penetration angle would not affect the frequency with which different cell types were encountered. However, if cells were organized in a columnar fashion, one would encounter fewer shifts in disparity cell type for normal rather than tangential penetrations. To quantify this parameter, one first has to calculate the penetration angle. The electrode was always advanced close to the plane of section (horizontal), but the cortical sheet was rarely perfectly vertical. Thus measuring the horizontal angle of the electrode from the cortical layers in horizontal sections was not sufficient to describe the penetration angle in three dimensions. To overcome this geometrical problem, we used the following method to calculate the penetration angle. We first measured the length of the electrode track within V3. If one assumes an average cortical thickness of 1.8 mm these two lengths form two sides of a right-angle triangle. With this information, we calculated the penetration angle and the distance traversed parallel to the surface of the cortex for each penetration. We then counted the number of switches from one disparity tuned cell type to another that occurred over this tangential distance. The result was an index expressed as the number of cell type changes per mm measured parallel to the cortical surface. This parameter is plotted against penetration angle in Fig. 7. The positive correlation between these measures suggests an organization of disparity tuned cells into cortical columns rather than the simple clustering of cell properties.
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DISCUSSION |
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Previous physiological investigations of V3 have shown it to be an
area containing a predominance of orientation-selective cells, a subset
of which are additionally direction-selective (Baizer
1982; Felleman and Van Essen 1987
;
Gegenfurtner et al. 1997
; Zeki 1978a
).
Our present results are in agreement. On the basis of V3's complement
of orientation-selective cells, its functional organization and
preference for stimuli in motion, it has been proposed that V3 is an
area specialized for the processing of dynamic form
(Gegenfurtner et al. 1997
; Zeki and Shipp
1988
). The issue of whether color is processed within V3 is
more contentious. Felleman and Van Essen (1987)
found
that 20% of V3 cells were color selective when judged by their
quantitative color index criterion. Our results are not in agreement
with those of Gegenfurtner et al. (1997)
, who found that
50% of V3 cells exhibited color selectivity as judged by an
index-based method. Using the same quantitative method, about which we
have reservations, we found only 15% of cells to be color selective,
and when qualitatively judged at the time of recording, we categorized
only 5% as color selective or biased. We conclude, as previously, that
V3 is not involved in color processing.
Our finding that 61% of V3 cells were disparity selective strongly suggests that V3 is an area involved in the processing of disparity information. Moreover, our discovery of columns of cells with similar disparity-tuning properties demonstrates a functional specialization within V3 for stereoscopic depth. While the exact function of disparity columns has yet to be elucidated, it is likely that this functional architecture is important for simplifying both inter- and intra-areal connections between disparity-selective cells. Connections between disparity-selective cells are presumably important for the generation of cells with more sophisticated response properties, for example, selectivity for absolute distance using additional information about eye position or selectivity for disparity gradients using the outputs from many disparity-selective cells.
DeAngelis et al. (DeAngelis and Newsome 1999) have
recently reported finding a similar organization of disparity columns
in visual area V5 (MT) of the macaque. The columnar organization we
describe bears a resemblance to their V5 disparity columns except that
the cells described here are orientation/disparity selective while
those in V5 are direction/disparity selective. They also demonstrated
that microstimulation of these columns influenced the depth judgements
of macaques (DeAngelis et al. 1998
). They concluded that
V5 plays an important role in stereoscopic depth perception in addition
to its well known role in motion perception. It is very likely that
cells in V5 tuned for direction of motion and disparity are signaling
the depth of moving objects. V5 cells capable of signaling motion in
depth have also been found (Zeki 1974b
; but see
Maunsell and Van Essen 1983
). Since V3 appears to be an
area specialized for form vision, its high proportion and functional
organization of disparity tuned cells would suggest that cells there
are signaling three-dimensional form. It would therefore be interesting
to discover if the processing of 3D information in V3 could be
correlated with perception of depth in the same way that has been
demonstrated in V5 or whether the disparity-selective cells of V3 are
serving a "nonconscious" function, such as the guidance of vergence
eye movements (Cumming and Parker 1997
; Masson et
al. 1997
).
We found a higher proportion of near cells than any other type of
disparity-selective cell in V3. Previous studies of disparity tuning in
prestriate cortex have found that there is an increase in the ratio of
asymmetric to tuned cells as one advances from V1-V5 to V5A (MST) and
LIP (Gnadt and Mays 1991; Maunsell and Van Essen
1983
; Poggio et al. 1988
; Roy et al.
1992
; Trotter 1995
; Trotter et
al. 1992
). The functional significance of this proportional change is not yet fully understood because the functions of each type
of disparity tuned cell is not known. It was originally suggested that
tuned cells (T0, TN, and TF) might have a role in fine stereoscopic discrimination around the fixation plane, whereas asymmetric cells might participate in coarse stereoscopic discrimination necessary to
drive vergence eye movements (Poggio and Fischer
1977
). However, it has since been demonstrated that cells with
asymmetric disparity tuning can be better suited to providing fine
stereoscopic information than those with symmetric tuning (Lehky
et al. 1990
). Other recent studies have questioned the
classification scheme of Poggio and Fischer altogether, suggesting that
there is in fact a smooth distribution of disparity tuning curve shape
and preference (DeAngelis and Newsome 1999
; LeVay
and Voigt 1988
). Most of the disparity tuned cells recorded in
this study were readily classifiable into the groups defined by
Poggio et al. (Poggio and Fischer 1977
; Poggio et al. 1988
) but it must be noted that this
classification was not always straightforward, and no bimodal
distribution of width of tuning was seen. Instead we chose a somewhat
arbitrary threshold of 1° half-width at half-height to distinguish
symmetric and asymmetric cells, and our data are not incompatible with
the idea that disparity tuning forms a continuum of shapes and preferences.
Recently V3's connections with posterior parietal areas involved in
spatial vision have been demonstrated. V3 sends ascending connections
to V6 (PO) (Shipp et al. 1998), an area in the anterior bank of the parieto-occipital sulcus heavily involved in sensory motor
integration of reaching arm movements (Galletti et al.
1997
). V6 also contains a population of "real position"
cells, thought to be engaged in the spatial encoding of extrapersonal
visual space (Galletti et al. 1995
). These cellular
properties would require, or at least benefit from, stereoscopic depth
information. In a concurrent study (Adams 1997
), we have
found connections from V3 (via V3A) to an area at the caudal extent of
the intraparietal sulcus (CIP) (Kusunoki et al. 1993
).
CIP contains cells that use disparity information to code the axis- and
surface-orientation and shape of objects in three dimensions
(Endo et al. 2000
; Sakata et al. 1997
).
We suggest that the disparity-selective cells of V3 provide at least
some of the information required for the generation of more complex
responses to three-dimensional forms found in CIP cells.
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ACKNOWLEDGMENTS |
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We thank Drs. Stuart Shipp and Konstantinos Moutoussis for participating in the experiments, J. Romaya for stimulus and data collection programming, G. Wray for technical assistance, and Drs. Jonathan Horton and Lawrence Sincich and an anonymous referee for helpful comments on the manuscript.
This work was supported by the Wellcome Trust.
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FOOTNOTES |
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Present address and address for reprint requests: D. L. Adams, Dept. of Ophthalmology, University of California, 10 Koret Way, San Francisco, CA 94143-0730 (E-mail: dadams{at}itsa.ucsf.edu).
Received 30 October 2000; accepted in final form 18 July 2001.
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REFERENCES |
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