Brain Research 881 2000 47–56 www.elsevier.com locate bres
Research report
Organization of efferent neurons in area 19: the projection to extrastriate area 21a
a c
a,b ,
Tara H. Stewart , Jamie D. Boyd , Joanne A. Matsubara
a
Department of Ophthalmology , University of British Columbia, 2550 Willow Street, Vancouver, British Columbia, Canada V5Z 3N9
b
Department of Anatomy , University of British Columbia, 2550 Willow Street, Vancouver, British Columbia, Canada V5Z 3N9
c
Department of Biological Science , Simon Fraser University, 888 University Way, Burnaby, British Columbia, Canada V5A 1S6
Accepted 25 July 2000
Abstract
The organization of efferent neurons in area 19 of the cat was examined by bulk injections of retrograde tracers, WGA–HRP and CTX–Au, into extrastriate area 21a. In one case, the cortex was cut coronally and retrogradely labeled cells in area 19 were present in
columnar register throughout layers 2 to 6, with the majority of labeled cells in layers 2 3. The number of columns per tissue section ranged from 0 to 4 and had a centre-to-centre spacing ranging from 0.6 to 0.9 mm. A few lightly labeled cells were found between the
columns. In six other cases, the visual cortex was flattened, and cut in the tangential plane to reveal a pattern of irregular, widely spaced bands that were elongated in the mediolateral direction with a mean centre-to-centre spacing of 2.6 mm. The density of labeled cells
within these bands fluctuated such that dense aggregates of cells were found, on average, at 0.9 mm intervals along the bands. This tangential heterogeneity in density, along with the patchy columnar staining witnessed in the coronal plane, suggests that the widely
spaced efferent projection bands may have a patchy substructure with a spacing of approximately 1 mm. The pattern of efferent projection bands and its substructure in area 19 is reminiscent of the stripe-like organization of V2 found in primates.
2000 Elsevier Science B.V.
All rights reserved.
Theme : Sensory systems
Topic : Visual cortex: extrastriate
Keywords : Visual cortex; Parallel processing; Cytochrome oxidase; Cortical module
1. Introduction and irregularly spaced stripes running in the mediolateral
direction [3]. Projections from area 19 to extrastriate area Patchy networks are a common feature of cat primary
21a have also been reported to be patchy in the coronal visual cortex. The intrinsic [19,20,28], callosal [3,39,56]
plane [13,43,49]. However, the exact nature of these and corticocortical connections [6,8,32] are all examples of
patches, and their periodicity, has not yet been examined in patchy networks in cat area 17. More recently, a patchy
the tangential plane, which is the preferred plane of section system of cytochrome oxidase CO blobs was identified in
for studying variations in labeling density across an entire cat area 17 [5,15,34]. Cortical areas beyond primary visual
area. cortex also demonstrate patchy architecture, for instance
Of interest in this study is whether the pathway from area 19 of the cat forms reciprocal and often patchy
area 19 to area 21a demonstrates a patterned organization. connections with as many as 13 other visual areas [49].
This type of organization is a common feature of parallel Area 19 also receives patchy thalamocortical innervation
processing streams in the cat and may bear a relationship [25] and the callosal innervation is patchy with elongated
with the functional specialization of area 21a [38]. Segre- gation of inputs to area 21a have already been found in the
primary visual cortex where efferents to area 21a originate
Corresponding author. Eye Care Centre, 2550 Willow Street, Van-
in the patchy CO blob columns of the primary visual
couver, British Columbia, Canada V5Z 3N9. Tel.: 11-604-875-4383; fax:
cortex [8]. Area 21a also demonstrates several physiologi-
11-604-875-4663. E-mail address
: JMSinterchange.ubc.ca J.A. Matsubara.
cal features that suggest it is functionally specialized. For
0006-8993 00 – see front matter
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48 T
instance, most neurons in area 21a exhibit high spatial mg kg, Austin Laboratories were given to the animals to
frequency selectivity and are orientation, but not motion, reduce brain edema.
selective [14,31,53]. This study addresses area 21a inputs arising from area 19, as area 19 is a pivotal visual area
2.2. Histology with connections to over 11 extrastriate visual areas
[4,18,49], suggesting that it plays a role in several process- The animals were given an overdose of barbiturate
ing streams in the cat visual system. anesthetic three days after surgery. They were then per-
In this study we made large injections of a retrograde fused transcardially with 700 ml of phosphate buffer 0.1
tracer to saturate area 21a, as identified stereotaxically. The M, pH 7.2 with 0.5 sodium nitrite. In six cases, a
organization of the efferent projection to area 21a was solution of 2 paraformaldehyde in phosphate buffer was
examined in both the tangential and coronal planes. delivered with a perfusion pump at a rate of 60 ml min for
Retrograde labeling in area 19 revealed a complex pattern 4 min. The visual cortex was then unfolded and flattened
of bands in area 19, elongated in the mediolateral direc- tangentially as described by Olavarria and Van Sluyters
tion. Within these bands, clusters of dense staining was [36]. The visual hemispheres were left between two glass
found, which suggests a substructure to the overall banded slides submersed in 4 paraformaldehyde and 20 suc-
pattern. rose in phosphate buffer. After 5 h, the glass slides were
removed and the sections were allowed to free float for 8 h. The tissue was then cut tangentially at 50 mm on a
2. Materials and methods microtome. In one case with WGA–HRP injected, 4
