1Laboratorios de Neurociencia y Computacion Neuronal (asociados al Instituto Cajal-CSIC), Facultad de Medicina y Servicio Neurofisiologia Clinica-Complejo Hospitalario Universidad de Santiago de Compostela, Santiago de Compostela E-15705, Spain; 2Departamento de Ciencias de la Salud II, Universidad de A Coruña, A Coruña E-15006 Spain; and 3Department of Optometry and Vision Sciences, University of Manchester Institute of Science and Technology, Manchester M60 1QD, United Kingdom
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ABSTRACT |
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Martinez-Conde, Susana, Javier Cudeiro, Kenneth L. Grieve, Rosa Rodriguez, Casto Rivadulla, and Carlos Acuña. Effects of Feedback Projections From Area 18 Layers 2/3 to Area 17 Layers 2/3 in the Cat Visual Cortex. J. Neurophysiol. 82: 2667-2675, 1999. In the absence of a direct geniculate input, area 17 cells in the cat are nevertheless able to respond to visual stimuli because of feedback connections from area 18. Anatomic studies have shown that, in the cat visual cortex, layer 5 of area 18 projects to layer 5 of area 17, and layers 2/3 of area 18 project to layers 2/3 of area 17. What is the specific role of these connections? Previous studies have examined the effect of area 18 layer 5 blockade on cells in area 17 layer 5. Here we examine whether the feedback connections from layers 2/3 of area 18 influence the orientation tuning and velocity tuning of cells in layers 2/3 of area 17. Experiments were carried out in anesthetized and paralyzed cats. We blocked reversibly a small region (300 µm radius) in layers 2/3 of area 18 by iontophoretic application of GABA and recorded simultaneously from cells in layers 2/3 of area 17 while stimulating with oriented sweeping bars. Area 17 cells showed either enhanced or suppressed visual responses to sweeping bars of various orientations and velocities during area 18 blockade. For most area 17 cells, orientation bandwidths remained unaltered, and we never observed visual responses during blockade that were absent completely in the preblockade condition. This suggests that area 18 layers 2/3 modulate visual responses in area 17 layers 2/3 without fundamentally altering their specificity.
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INTRODUCTION |
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To date, very little is known about the role of
feedback connections in the visual system, either at the
corticocortical or corticothalamic levels. In the cat visual cortex,
area 17 and adjacent area 18 are reciprocally connected (Bullier
et al. 1984; Ferrer et al. 1988
,
1992
; Salin et al. 1992
,
1995
; Squatrito et al. 1981
;
Symonds and Rosenquist 1984a
,b
). Input from area 18 can
drive visual responses from area 17 cells in the absence of a direct
geniculate input (Mignard and Malpeli 1991
). This
connection (often called a "feedback" connection) arises from two
levels: one from layer 5 of area 18 and projecting to layer 5 of area 17, and the other from layers 2/3 of area 18 projecting to layers 2/3
of area 17 (Henry et al. 1991
; Salin and Bullier
1995
). Focal lidocaine blockade in layer 5 of area 18 can
affect both the orientation and velocity selectivity of cells within
layer 5 of area 17 (Alonso et al. 1993a
,b
). Here we
investigate the effect of focal GABA blockade of layers 2/3 of area 18 on the visual responses of cells in layers 2/3 of area 17 to sweeping
bars of various orientations and velocities. Preliminary data have been
presented in abstract form (Acuña et al. 1995
).
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METHODS |
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Animal preparation
Experiments were carried out on 32 adult cats (1.8-3.0 kg). The
animals were anesthetized with halothane (5% for induction; 1.5-2%
for surgery and 0.1-0.5% for maintenance) in
NO2 (70%) and O2 (30%)
and paralyzed with gallamine triethiodide (40 mg iv for induction, 10 mg · kg1 · h
1 iv
for maintenance). Lidocaine hydrochloride with adrenaline was
administered subcutaneously to all wound margins, and the ear bars of
the stereotaxic frame were covered with lidocaine gel (1%). Solutions
of atropine methonitrate and phenylephrine hydrochloride were applied
to each eye to dilate the pupils, paralyze accommodation, and retract
the nictitating membranes. Zero power contact lenses, 3-mm artificial
pupils, and supplementary lenses were used to bring the eyes to focus
on a tangent screen at a distance of 57 cm. End-tidal
CO2 levels (maintained between 3.8 and 4.2%),
electroencephalogram (EEG), electrocardiogram (ECG), and temperature of
the animal (maintained between 37.5 and 38.5°C) were monitored
continuously throughout the experiment. All of the experimental
procedures were approved by the National Committee (Spain) from the
International Council for Laboratory Animal Science, protocol
86/809/EEC.
Simultaneous recording and iontophoretic administration of drugs
Single-unit extracellular activity was recorded through a
tungsten microelectrode placed in layers 2/3 of area 17 of visual cortex. In area 18, single-unit activity from layers 2/3 was recorded simultaneously through the central barrel of a seven-barreled micropipette, with a tip broken back to 3-10 µm diam. The recording barrel contained 3 M NaCl. Five of the six other barrels contained -aminobutyric acid (GABA, 0.5 M, pH 3.5), and the sixth barrel was
filled with Pontamine sky blue (PSB, 2% wt/vol in 0.5 M sodium acetate
solution) for histological reconstruction of the micropipette position.
Focal reversible blockade was achieved by the iontophoretic application
of GABA through one or more barrels of the micropipette (range: 30-100
nA). The extent of the blockade was <300 µm in radius, as shown by
control experiments in which a second recording microelectrode was
placed at various distances from the micropipette. To avoid leakage
from the micropipette, we applied a small retention current of 5-25 nA
in the polarity opposite to the ions being injected. In the recovery
phase, visual responses were tested again in both cells, 5-8 min after
cessation of GABA iontophoresis.
Although single-cell recording in area 18 was necessary to differentiate between cells in layers 2/3 and layer 4 (or deeper), GABA blockade certainly inhibited cells with receptive fields beyond the receptive field of the cell we were recording from. Thus orientation selectivity, direction preference, receptive field position, and velocity tuning of individual cells in area 18 may not be exactly the same for the local population of inactivated cells.
Visual stimulation system
Cells of both areas were first studied qualitatively by
hand-mapping. Each cell was classified as simple or complex
(Hubel and Wiesel 1962), and ocular dominance was
measured on a five-point scale (Wilson and Sherman
1976
), simplified from the original seven-point scale of
Hubel and Wiesel (1962)
. Each pair of cells was
quantitatively studied using computer-controlled stimuli (Visual Stimulation System, Cambridge Electronic Design, Cambridge, UK). Stimulus contrast [(Lmax
Lmin)/(Lmax + Lmin)] was held to within a
nonsaturating range (0.36-0.7), with a mean luminance of 14 cd/m2. The receptive fields of all cells were
within 12° of the area centralis.
Experimental protocol
Because of time limitations, each pair of cells was tested either for velocity or orientation, but not both. We studied three successive conditions for each pair of cells: control (preblockade), GABA (in which we infused GABA into area 18), and recovery. During the velocity experiments, visual stimuli consisted of bars of light with optimal width and orientation for each cell, sweeping at six different velocities (2, 4, 8, 15, 25, and 32°/s). Bar length (10°) always extended beyond the receptive fields in the two areas. When the preferred orientation for the cell in area 17 differed from the preferred orientation for the cell in area 18, we ran the control and recovery conditions twice (once for each orientation). During the blockade situation, the cell in area 18 was inactive, so we optimized the stimulus orientation for the cell in area 17 only (we ran the test only once, even if cells in areas 17 and 18 had different orientations). Each velocity was presented 15 times, in a randomly interleaved fashion.
During the orientation experiments, visual stimuli consisted of 10° long bars of light with optimal width and velocity for each cell, sweeping at six different orientations at intervals of 30° (each orientation having 2 directions of movement). When the preferred velocity for the cell in area 17 differed from the preferred velocity for the cell in area 18, we ran the control and recovery conditions twice (once for each velocity). In the blockade situation, the stimulus velocity was optimized for the cell in area 17 only. Each orientation was presented 10 times in a randomly interleaved fashion.
Statistical analysis
Visual responses were measured in spikes per second for each
velocity or orientation tested. We collected peristimulus time histograms (PSTHs) with binwidths of 100 ms. In the velocity
experiments we compared the peak bin in each histogram for every
condition and across the various velocities. It was necessary to
compare only peak rates of firing when comparing responses in the
velocity domain, because trials with slow velocities are longer in
duration and therefore collect larger numbers of spikes than trials
with fast velocities, even if the instantaneous firing rate was higher for the fast velocities. In the orientation experiments the trials were
of constant duration, so we compared both the peak firing rate (as in
the velocity tests), as well as the average firing rate for the entire
sweep of the bar. Both methods generated precisely the same results. We
considered there to have been a change in the response of each area 17 cell, during area 18 blockade, when the change in response was
statistically significant (P 0.05 using both 2-way
ANOVA Friedman test and Wilcoxon rank sum test). If changes were less
than significant (P > 0.05), we categorized these
cells as "unchanged." All cells used in the analysis were moreover
required to recover completely from the blockade, so that the control
and recovery conditions were not significantly different
(P > 0.05) in both area 18 and 17. This was necessary to be sure that any changes in firing rate in area 17 were due to the
administration of GABA in area 18, rather than to changes in EEG
activity during our experiment, or other factors. Orientation tuning
curves were smoothed and normalized to calculate their half bandwidths
at half height (Orban 1984
).
Histological reconstruction
Recording sites were marked (area 17 microelectrode: 30 µA
20 s; area 18 multipipette: PSB application, 5 mA 30 min). At the end of the experiment animals were overdosed with pentobarbital and
their brains removed and fixed by immersion in 10% formaldehyde in
normal saline. After cryoprotection in 30% sucrose solution, 60-µm
frozen sections were Nissl stained. Areas and laminae marked were then
identified (Garey 1971).
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RESULTS |
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Velocity experiments
We tested 31 pairs of cells for bars sweeping at various velocities. During area 18 blockade, 17/31 (55%) area 17 cells showed increased responses, 5/31 (16%) cells showed decreased responses, and 9/31 (29%) cells showed no response changes.
An example of an increased response is illustrated in Fig. 1A. This area 17 complex cell had an optimum response to bars sweeping at a rate of 4°/s. The other cell of the pair (Fig. 1B), an area 18 complex cell, had an optimum response to an appropriately oriented light bar drifting at ~15°/s. The two cells had the same orientation and direction preferences and had the same ocular dominance. The area 17 cell receptive field was partially overlapped (24%) by the receptive field of the cell in area 18. During area 18 blockade the responses of the area 17 cell increased for all velocities to which the cell responded without blockade, and no responses were seen to the highest velocities tested in any condition. Figure 2 shows the PSTHs from which Fig. 1 was derived. The lack of activity of the area 17 cell to stimuli of velocities of 25 and 32°/s is striking (Fig. 2, A-C), given the relatively robust responses seen in the cell in area 18 (Fig. 2, D and E).
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An example of a decreased response in an area 17 complex cell is illustrated in Fig. 3A. The cell in area 18 (Fig. 3B) was simple, and its receptive field almost completely overlapped (87%) that of the area 17 complex cell. The two cells had the same orientation preference (although with opposite direction preference), and ocular dominance differed by one point. During GABA blockade of area 18 the response of this area 17 cell was reduced, except for the response to the lowest velocity tested, which was unaffected by the blockade. Figure 4 shows the PSTHs corresponding to these two cells.
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In 9/31 cell pairs, no effect of blockade in area 18 was seen in area 17. An example of this is illustrated in Fig. 5. The receptive fields of these two cells were completely nonoverlapping, which may explain the lack of changes in responsiveness in the area 17 cell. The optimum velocity response for this area 17 complex cell was 15°/s (Fig. 5A). The area 18 cell (Fig. 5B) was a simple cell, with an optimal response at 32°/s or perhaps higher. Both cells had the same orientation selectivity but opposite direction preference and differed in ocular dominance by one point.
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Orientation experiments
We recorded from 66 cell pairs to test area 18 blockade effects on area 17 in the orientation domain. Most area 17 cells [49/66 (74%)] showed clear changes in their responses during area 18 blockade. Decreased responses were found in 27 (41%) cells, increased responses in 22 (33%) cells, and 17 (26%) cells showed no changes.
Figure 6 is an example of an area 17 cell with an increased response to area 18 blockade: before the blockade (A), during the blockade (B), and after the blockade (C). The optimum orientation was between 270 and 300°. Responses were increased especially at the optimum orientations. Figure 6D shows the responses of a complex cell recorded simultaneously in area 18 before (solid line) and after the blockade (dotted line). The cell had an optimum orientation of 120°. Most (87%) of the area 17 cell's receptive field was overlapped by the receptive field of the cell in area 18.
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Figure 7 illustrates an example of a narrowly tuned area 17 cell with a decreased response to area 18 blockade. The optimum orientations of both cells were very similar (although preferred directions of motion were opposite) and their receptive fields almost fully overlapped (89%). During GABA application in area 18 (Fig. 7B), the area 17 cell showed a decrease in response of 50% at the preferred orientation (90°). After the recovery from the blockade, the responses of both cells returned to their preblockade levels.
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Some area 17 cells (17/66) showed no changes in their rate of discharge during GABA application in area 18 at any of the orientations tested. Figure 8 illustrates an example on an area 17 complex cell. Unlike the cells illustrated in Figs. 6 and 7 that changed in responsiveness, the receptive fields from these two cells were overlapping only marginally (16%), suggesting that projections from area 18 to area 17 tend to match retinotopically.
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Those area 17 cells unaffected by area 18 blockade (Figs. 8 and 5) furthermore illustrate the good stability (statistical stationarity) of responses throughout the different conditions.
Strength of the effects
We have separated cells with significant changes in response into two groups: one for response increments and one for response decrements. The reason for separating the data in this way is that cells can decrease their rate of discharge up to 100%, but increases in firing rate are not limited to 100%, so comparing the magnitude of these effects in terms of percent change would be inappropriate between groups.
The average peak increase in area 17 responses during area 18 blockade was 127 ± 18% (mean ± SE) in the velocity experiments and 113 ± 23% in the orientation experiments. The average peak decrease in area 17 responses was 63 ± 9% in the velocity experiments and 43 ± 3% in the orientation experiments.
The average P-value (2-way ANOVA Friedman test) for the significant changes was 0.0233 ± 3.887E-03 in the orientation experiments and 0.0238 ± 2.366E-03 in the velocity experiments.
Figure 9 plots the magnitude of these effects for each individual cell in area 17 against their significance.
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Width of tuning curves
Velocity tuning curves bandwidths could not be properly measured for most of the cells recorded, because we only tested a small range of velocities to which cells were sensitive. We observed, however, that either most of the velocities to which cells in area 17 responded to (before area 18 blockade) were affected, or none were. Cells did not become sensitive to velocities to which they were insensitive before the blockade. Furthermore, none of the area 17 cells responded during area 18 blockade with an increase in sensitivity to some velocities and a decrease to other velocities.
Similarly, in the orientation domain, we observed no changes in the
optimum orientation, and we rarely saw changes in sharpness of tuning.
Figure 10 compares the orientation
half-bandwidths for the area 17 cells tested with different
orientations (n = 66). Optimum orientations of most
cells were unaffected by area 18 blockade, and only 8 of 66 cells
showed a change in half-bandwidth of 10° during area 18 blockade.
Broader tuning curves were found in six of these cells, and sharper
tuning curves were found in two cells. One cell showed an increase in
half-bandwidth of 41°, but this was a rare exception. As with
velocity tuning, none of the cells increased their responses to some
orientations and decreased their responses to others.
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Retinotopic location between recording sites in areas 18 and 17
As illustrated in Figs. 1, 3, 6, and 7, area 17 cells that changed their responsiveness appeared to be retinotopically related to the cells recorded simultaneously in area 18 (although one should keep in mind that the recordings in area 18 were from single units within a population of blockaded cells). Figure 11A shows the amount of overlap between the receptive fields of cells simultaneously recorded in areas 17 and 18. Not surprisingly, the percentage of overlap was larger in the groups that showed increases (40.69 ± 6.13%; n = 39 cells) and decreases (50.71 ± 6.76%; n = 32 cells) than in the group of cells that showed no change (26.38 ± 7.03%; n = 26 cells). This difference was statistically significant for the decrease versus the nonchange group of cells (P = 0.01; t-test). We also calculated the distance (in degrees of visual angle) between the receptive field centers of cells in both areas (Fig. 11B). When there was no effect of area 18 blockade, receptive field center distances were relatively larger (mean, 1.63 ± 0.19 SE) than when there was either a decrease in firing (mean, 1.11 ± 0.13 SE; P = 0.0283) or an increase in firing (mean, 1.14 ± 0.13 SE; P = 0.0380) in area 17. These results are only an approximate indicator, because in each case the receptive fields of the population of cells inactivated in area 18 must have scattered to cover a wider area than that covered by the single receptive field we mapped.
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DISCUSSION |
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General comments
Our results indicate that GABA-induced focal blockade of layers
2/3 of area 18 can produce changes in the responses of cells in layers
2/3 of area 17 to oriented bars, revealing functional connections
between the upper layers of both areas, in agreement with previous
studies (Mignard and Malpeli 1991). The contribution of
these connections to visual perception nevertheless remains unclear.
The finding that area 18 blockade produced two types of effects on the
response of area 17 cells, increases and decreases, may seem puzzling.
It suggests a more complex role of area 18 feedback connections than
would have been suggested had we found only increases or decreases in
responsiveness in area 17 cells or if we had found changes in only one
of the domains, orientation or velocity. As it stands, the only
perceptual role we might suspect for the connections between the upper
layers from area 18 to area 17 is the modulation of sensitivity in area
17 cells, perhaps for the purposes of adaptation or attention.
Consideration of the circuits involved in these connections may provide
us with clues concerning the functional anatomy. Although our results
do not rule out indirect connections through the lateral geniculate
nucleus (LGN) or other cortical areas receiving input from area 18, the
simplest explanation is probably the interruption of a direct
corticocortical projection from area 18 to area 17, such as those in
the upper layers (Henry et al. 1991; Mignard and
Malpeli 1991
). Decreased responses in area 17 cells are easy to
imagine as the loss of a direct excitatory projection from area 18 to
area 17, and increased responses could be the result of the loss of an
inhibitory connection. Because long corticocortical connections are
exclusively excitatory in the visual cortex of the cat (Bullier
et al. 1988
; Pérez-Cerdá et al.
1996
), the inhibition would presumably be exerted through a
GABAergic inhibitory interneuron within area 17. If inhibitory
interneurons within area 17 can account for some of our results, they
could be located either in layers 2/3 or in layer 5 (thereby acting
through rising collaterals to layers 2/3). Some projections from layers
2/3 of area 18 collateralize in layer 5 of area 17 (Henry et al.
1991
) and inhibitory projections from layer 5 to layers 2/3 in
area 17 have been described in anatomic (Kisvárday
1992
) and physiological studies (Allison and Bonds
1994
). GABAergic interneurons in area 17, moreover, have been
found in all layers (Gabbott and Somogyi 1986
).
These findings would not necessarily apply to primate cortex. In the
cat, areas 17 and 18 both receive strong geniculate input (Stone
and Dreher 1973), and both areas may be thus considered, in
some sense, as primary visual cortex (Tretter et al.
1975
). Primate area 18 receives LGN input only from the
interlaminar zones of the geniculate (Bullier and Kennedy
1983
), so that feedback connections from area 18 to area 17 in
the monkey may play a different role than in the cat. In the squirrel
monkey, Sandell and Schiller (1982)
found that most area
17 cells showed decreased visual responses when area 18 was reversibly
cooled, although a few cells became more active. Bullier et al.
(1996)
similarly reported in the cynomolgus monkey that, when
area 18 had been inactivated by GABA, area 17 cells showed decreased or
unchanged visual responses in the center of the classical receptive
field, but increased responses in the region surrounding it. These
results have been strengthened by recent findings in areas V1, V2, and
V3 following area MT inactivation (Hupé et al.
1998
).
Retinotopic relationship between area 18 and area 17
As expected from the retinotopic specificity of anatomic feedback
connections (Salin et al. 1992), previous physiological studies have reported a functional projection from area 18 cells to
area 17 cells with a similar retinotopy (Alonso et al.
1993b
; Bullier et al. 1988
; Salin et al.
1992
, 1995
). Our results show that the area 17 cells affected by the blockade tended to have receptive fields
overlapped to a greater extent by the receptive fields in area 18 than
those area 17 cells unaffected by the blockade (Fig. 11A).
We also found that the distance between the receptive field centers of
area 17 and area 18 tended to be smaller in cells affected by the
blockade (Fig. 11B). Although the cell we recorded from in
area 18 was probably in the center of the blocked region, many other
cells were presumably inactivated, and the receptive fields from the
entire population of blockaded cells most likely scattered to some
extent with respect to the one we mapped, so we must consider these
measures approximate.
Role of feedback connections from area 18 to area 17
In the light of anatomic evidence suggesting separate feedback
pathways from area 18 to area 17 (Henry et al. 1991),
the complex relationship between the two areas might be better
understood by focal blockade experiments restricted to superficial or
deep layers, rather than by global inactivation studies. We have found some properties of these two pathways, the superficial and the deep, we
consider similar: blockade of area 18 layers 2/3 resulted in increased
responses in some area 17 layers 2/3 cells and decreased responses in
others, in agreement with the results reported in layer 5 (Alonso et al. 1993b
).
Other properties were found to differ between superficial and deep
pathways. In the original observations in cat area 17, Hubel and
Wiesel (1962) suggested that the orientation selectivity of
simple cells was the result of the alignment of receptive fields from
afferent inputs from lateral geniculate cells. Although some studies
have pointed toward local inhibition within cortex as important to
generating or sharpening orientation selectivity (Bishop et al.
1971
; Blakemore and Tobin 1972
; Bonds
1989
; Eysel et al. 1990
; Crook et al.
1991
; Sillito 1975
; Volgushev et al. 1996
), other evidence seems to run counter to this suggestion (Ferster et al. 1996
; Reid and Alonso
1995
). Previous work on the projections from layer 5 to layer 5 suggests that area 18 can significantly alter the bandwidth of
orientation tuning curves in area 17 cells. Alonso et al.
(1993b)
reported that 46% of area 17 layer 5 cells changed
their half-bandwidths during area 18 blockade (22% cells broadening
and 24% cells sharpening their tuning curves). We have examined this
possibility for the layers 2/3 projections and found no changes in
orientation preference and relative lack of change in orientation
bandwidth of area 17 cells.
It would not be surprising if area 18 blockade were to affect the
velocity preferences of cells in area 17. In the cat, as already
mentioned, cortical areas 17 and 18 receive parallel geniculate inputs.
Area 17 has a major input from LGN X cells, and in a lesser extent from
Y cells, whereas area 18 receives exclusively Y cell input
(Ferster 1990a,b
; Stone and Dreher 1973
).
Several characteristics of cortical cells may thus be determined by
these afferents (Ferster and Jagadeesh 1991
;
Stone et al. 1979
): Y cells respond better to higher
velocity stimuli than X cells (Cleland et al. 1971
), and
area 18 cells similarly have an average preference for higher velocity
stimuli than cells in area 17 (Tretter et al. 1975
). Alonso et al. (1993a)
showed that area 18 layer 5 blockade could reveal responses to high velocity stimuli in area 17 layer 5 cells, to which they were normally unresponsive. In the present
study we never observed that effect: our results instead show changes in response magnitude in most or all the velocities that area 17 cells
were responsive to before area 18 blockade. Although it is possible
that these differences between layer 5 and layers 2/3 results are due
to the different methods involved in both studies (i.e., lidocaine vs.
GABA blockade), they could reflect a different functional role for the
projections from area 18 at these two different levels. Thus, although
both layer 5 and layer 3 pathways from area 18 seem to be involved in
the modification of area 17 cells responses, the mechanisms involved
and their perceptual implications could be different.
Our results show that area 18 layers 2/3 affect the responses of cells in area 17 layers 2/3 to stimuli of different orientations and velocities without fundamentally altering their specificity. These enhancements and decreases in the cells' responsiveness may suggest some kind of gain modulation from area 18 to area 17 superficial layers. Although the role of such modulation in our visual perception remains unclear, such effects could be part of a mechanism of attention or adaptation.
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ACKNOWLEDGMENTS |
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We thank S. L. Macknik and D. H. Hubel for comments on the manuscript, J.-M. Alonso for suggestions during the development of these experiments, and J.-L. Otero and P. Vazquez for advice on statistics and data analysis.
This project was funded by Grant PB93-0347 from Direción General de Investigación Científicay Técnica to C. Acuña, Grants FIS-97/0402 and PGID799PXI-113401B to J. Cudeiro, and training grants from the Spanish Ministry of Education and Science to S. Martinez-Conde and C. Rivadulla and from the Universidad de Santiago de Compostela to R. Rodriguez.
Present addresses: S. Martinez-Conde, Dept. of Neurobiology, Harvard Medical School, Boston, MA 02115; R. Rodriguez, Dept. of Neurophysiology, Max-Planck-Institute for Brain Research, Frankfurt D-60528, Germany; C. Rivadulla, Dept. of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139.
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FOOTNOTES |
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Address for reprint requests: C. Acuña, Departamento de Fisiologia, Facultad de Medicina San Francisco, 1, Santiago de Compostela 15705, Spain.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 1 December 1998; accepted in final form 1 July 1999.
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REFERENCES |
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