Response properties in the dorsal lateral geniculate nucleus of the adult cat after interruption of prenatal binocular interactions

1989 ◽  
Vol 62 (5) ◽  
pp. 1039-1051 ◽  
Author(s):  
C. A. White ◽  
L. M. Chalupa ◽  
L. Maffei ◽  
M. A. Kirby ◽  
B. Lia
Keyword(s):  
Spatial Resolution ◽  
Receptive Fields ◽  
Spatial Summation ◽  
Field Size ◽  
Dorsal Lateral Geniculate Nucleus ◽  
Lower Percentage ◽  
Binocular Interactions ◽  
Adult Cats ◽  
X Cells ◽  
Y Cells

1. Single-cell recordings were made in the magnocellular layer of the dorsal lateral genicule nucleus (dLGN) of five adult cats in which prenatal binocular interactions were interrupted by monocular enucleation at known gestational ages. Three cats (early enucleates) had one eye removed on either embryonic day 44.48, or 49, before retinogeniculate inputs are segregated into uniocular layers. Two other (late enucleates) underwent this procedure on embryonic days 55 and 58, when segregation is well advanced. Responses were compared with those obtained from recordings in the A and A1 layers of the dLGN of seven normal adult cats. 2. Cells were classified as ON or OFF by the use of spots of light and as X or Y based on a test of linearity of spatial summation with the use of counterphased sinusoidal gratings. Receptive-field size and spatial resolution were also obtained. 3. The dLGN of prenatally enucleated cats contains a dorsal magnocellular layer and a ventral parvocellular layer. In early enucleates, only an occasional hint of a cell-sparse interlaminar zone was apparent, located between the magnocellular and parvocellular layers. In late enucleates, a prominent cell-sparse band was observed contralateral to the remaining eye, in a region that would most likely correspond to layer A1 in the normal dLGN. No such cell-sparse band was seen ipsilateral to the remaining eye in late enucleates. 4. Eighty-six X cells and 22 Y cells were studied in the enucleates. Both cell types were found at all depths of the magnocellular layer. All but a few neurons had concentric ON-center or OFF-center receptive fields that were normal in size. The topography of receptive fields also appeared normal. In addition, spatial resolution of X and Y cells was similar in experimental and control animals. 5. In early enucleates there was a higher percentage of X cells and a lower percentage of Y cells than normal. The change in X-to-Y ratio was shown to be because of both a gain in cells with X properties and a loss of cells with Y properties. The distribution of dLGN somal sizes in the early enucleates was comparable with controls, so the change in X-to-Y ratio most likely did not result from an electrode sampling bias. It was suggested that the X-to-Y ratio difference could stem from the abnormalities in retinogeniculate terminal arbors that have been shown to follow early eye removal.(ABSTRACT TRUNCATED AT 400 WORDS)

Two Different Types of Y Cells in the Cat Lateral Geniculate Nucleus

2003 ◽  
Vol 90 (3) ◽  
pp. 1852-1864 ◽  
Author(s):  
Chun-I Yeh ◽  
Carl R. Stoelzel ◽  
Jose-Manuel Alonso
Keyword(s):  
Lateral Geniculate Nucleus ◽  
Single Channel ◽  
Receptive Fields ◽  
Spatial Summation ◽  
Field Size ◽  
Lateral Geniculate ◽  
Spatial Parameters ◽  
Latency Response ◽  
X Cells ◽  
Y Cells

The Y pathway in the cat visual system is traditionally viewed as a single channel that originates in the retina. However, most Y cells from the contralateral retina diverge to innervate two different layers of the lateral geniculate nucleus, suggesting a possible channel split: YC (Y geniculate cell in layer C) and YA (Y geniculate cell in layer A). We tested the functional significance of this anatomical divergence by comparing the response properties of simultaneously recorded YC and YA geniculate cells with overlapping receptive fields. Our results demonstrate that YC and YA cells significantly differ in a large number of temporal and spatial parameters including response latency, response transiency, receptive-field size, and linearity of spatial summation. Furthermore, for some of these parameters, the differences between YC and YA cells are as pronounced as the differences between Y and X cells in layer A. These results along with results from previous studies strongly suggest that Y retinal afferents diverge into two separate channels at the level of the thalamus.

Effects of strabismus on responsivity, spatial resolution, and contrast sensitivity of cat lateral geniculate neurons

1984 ◽  
Vol 52 (3) ◽  
pp. 538-552 ◽  
Author(s):  
K. R. Jones ◽  
R. E. Kalil ◽  
P. D. Spear
Keyword(s):  
Contrast Sensitivity ◽  
Spatial Resolution ◽  
Striate Cortex ◽  
Receptive Fields ◽  
Orienting Response ◽  
Lateral Geniculate ◽  
Spatial Frequencies ◽  
Area Centralis ◽  
X Cells ◽  
Y Cells

Rearing cats with esotropia is known to cause a number of deficits in visual behavior tested through the deviated eye. These include a loss of orienting response to stimuli presented in the nasal visual field of the deviated eye, a reduction in visual acuity, and a general reduction in contrast sensitivity at all spatial frequencies. To assess the involvement of the lateral geniculate nucleus (LGN) in these deficits, we measured the following: 1) the visual responsiveness of lamina A1 cells with peripheral (more than 10 degrees from area centralis) receptive fields in three esotropic and three normal cats and 2) the spatial resolution and contrast sensitivity of lamina A X-cells with central (within 5 degrees of the area centralis) receptive fields in six esotropic and six normal cats. For comparison, we also measured LGN X-cell spatial resolutions in four exotropic cats and in two cats raised with an esotropia in one eye and the lids of the other eye sutured shut (MD-estropes). Recordings from the lateral portion of lamina A1 in esotropic cats yielded similar numbers of visually responsive cells with far nasal receptive fields as were seen in normal animals. Peak and mean response rates to a flashing spot also were normal. In addition, no differences were found between esotropes and normals in the percentages of X- and Y-cells encountered. These results suggest that the loss of orienting response to stimuli presented in the nasal field (12, 20) is not due to a loss of neural responses in the LGN of esotropic cats. In addition, they suggest that decreases in cell size in lamina A1 of esotropic cats (13, 36; R. E. Kalil, unpublished observations) are not accompanied by marked functional abnormalities of the cells and that cortical abnormalities ipsilateral to the deviated eye (22) are likely to have their origin within striate cortex itself. Recordings from lamina A cells with receptive fields near area centralis revealed that the average X-cell spatial resolution in esotropes (2.1 cycles/deg) was significantly lower than that in normal cats (3.1 cycles/deg). This reduction was seen in all esotropic cats tested and was due both to an increase in the proportion of X-cells with very low spatial resolution and to a loss of X-cells responding to high spatial frequencies (greater than 3.25 cycles/deg). The average spatial resolution of X-cells driven by the deviated eye in MD-esotropes fell midway between those of esotropes and normals. In exotropes, mean X-cell spatial resolution was normal.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of neuronal response properties in the cat dorsal lateral geniculate nucleus during monocular deprivation

1983 ◽  
Vol 50 (1) ◽  
pp. 240-264 ◽  
Author(s):  
S. C. Mangel ◽  
J. R. Wilson ◽  
S. M. Sherman
Keyword(s):  
Lateral Geniculate Nucleus ◽  
Spatial Resolution ◽  
Optic Chiasm ◽  
Dorsal Lateral Geniculate Nucleus ◽  
Response Properties ◽  
Lateral Geniculate ◽  
X And Y Cells ◽  
X Cells ◽  
Y Cells ◽  
Dorsal Lateral Geniculate

We measured response properties of X- and Y-cells from laminae A and A1 of the dorsal lateral geniculate nucleus of monocularly lid-sutured cats at 8, 12, 16, 24, and 52-60 wk of age. Visual stimuli consisted of small spots of light and vertically oriented sine-wave gratings counterphased at a rate of 2 cycles/s. In cats as young as 8 wk of age, nondeprived and deprived neurons could be clearly identified as X-cells or Y-cells with criteria previously established for adult animals. Nonlinear responses of Y-cells from 8- and 12-wk-old cats were often temporally labile; that is, the amplitude of the nonlinear response of nondeprived and deprived cells increased or decreased suddenly. A similar lability was not noted for the linear response component. This phenomenon rarely occurred in older cats. At 8 wk of age, Y-cell proportions (number of Y-cells/total number of cells) in nondeprived and deprived A-laminae were approximately equal. By 12 wk of age and thereafter, the proportion of Y-cells in deprived laminae was significantly lower than that in nondeprived laminae. At no age was there a systematic difference in response properties (spatial resolution, latency to optic chiasm stimulation, etc.) for Y-cells between deprived and nondeprived laminae. Spatial resolution, defined as the highest spatial frequency to which a cell would respond at a contrast of 0.6, was similar for nondeprived and deprived X-cells until 24 wk of age. In these and older cats, the mean spatial resolution of deprived X-cells was lower than that of nondeprived X-cells. This difference was noted first for lamina A1 at 24 wk of age and later for lamina A at 52-60 wk of age. The average latency of X-cells to optic chiasm stimulation was slightly greater in deprived laminae than in nondeprived laminae. No such difference was seen for Y-cells. Cells with poor and inconsistent responses were encountered infrequently but were observed far more often in deprived laminae than in nondeprived laminae. Lid suture appears to affect the development of geniculate X- and Y-cells in very different ways. Not only is the final pattern of abnormalities quite different between these cell groups, but the developmental dynamics of these abnormalities also differ.

Response properties of ganglion cells in the isolated mouse retina

1993 ◽  
Vol 10 (1) ◽  
pp. 31-39 ◽  
Author(s):  
Charlene Stone ◽  
Lawrence H. Pinto
Keyword(s):  
Ganglion Cells ◽  
Cutoff Frequency ◽  
Receptive Fields ◽  
Spatial Summation ◽  
Mouse Retina ◽  
Sinusoidal Grating ◽  
Central Retina ◽  
X Cells ◽  
Y Cells ◽  
Receptive Field Center

AbstractWe have studied the organization of receptive fields of ganglion cells in the isolated mouse retina and have shown that the organization is similar to that of the cat. Based upon responses to circular and annular stimuli, most ganglion cells (90%; N = 83) had receptive fields with concentric center-surround organization, either ON or OFF center. The plot of response amplitude vs. stimulus area for these cells increased to a maximum (corresponding to a diameter of 10.0 ± 2.8 deg S.E.M.; N = 13) and then decreased for larger stimuli, demonstrating the presence of an antagonistic surround. The dark-adapted sensitivity (205 ± 43.8 impulses quantum−1 rod−1; mean ± S.E.M.) did not differ from that measured for the intact preparation. We found a subset of OFF-center cells for which the dark discharge was very regular (mean coefficient of variation = 0.30). Using sinusoidal grating stimuli, we classified ganglion cells as X-like (87%) and Y-like (13%) based on spatial summation properties and the presence of subunit activity in the receptive-field center. We found no difference in the spatial-frequency preference between X-like and Y-like cells in the central retina (high cutoff frequency, 0.20 ± 0.014 cycle/deg, mean ± S.E.M.), in contrast to the marked difference between X cells and Y cells in the cat. Thus, ganglion cell receptive fields in the mouse retina resemble those of the cat, although the spatial characteristics of the receptive fields in the central retina are more homogeneous. This homogeneity would simplify the comparison of retinas from normal and mutant strains of the mouse.

Spatial receptive-field structure of cat retinal W cells

1993 ◽  
Vol 10 (4) ◽  
pp. 765-779 ◽  
Author(s):  
Michael H. Rowe ◽  
James F. Cox
Keyword(s):  
Receptive Field ◽  
Spatial Resolution ◽  
Ganglion Cells ◽  
Second Harmonic ◽  
Receptive Fields ◽  
Spatial Summation ◽  
Field Structure ◽  
Contrast Gain ◽  
Frequency Domain Methods ◽  
Y Cells

AbstractWe have used frequency-domain methods to characterize the spatial receptive-field structure of cat retinal W cells. For most ON- and OFF-center tonic and phasic W cells, measurements of responsivity to drifting gratings at various spatial frequencies could be adequately described by a difference-of-Gaussians (DOG) function, consistent with the presence of center and surround mechanisms that are approximately Gaussian in shape and whose signals are combined additively. Estimates of the responsivity of the center mechanisms of tonic and phasic W cells were similar, but both were significantly lower than the corresponding values for X or Y cells. The width of the center mechanisms of tonic W cells, phasic W cells, and Y cells did not differ significantly from each other, but all were significantly larger than the width of X-cell centers. Surround parameters did not vary significantly among the four groups of ganglion cells. Measurements of contrast gain in both tonic and phasic W cells gave values that were significantly lower than in X or Y cells.Virtually all of the phasic W cells in our sample displayed evidence of spatial non-linearities in their receptive fields, in the form of either d.c. responses to drifting sine-wave gratings or second harmonic responses to counterphased gratings. The spatial resolution of the mechanism underlying these nonlinearities was typically higher than that of the center mechanism of these cells. Most tonic W cells exhibited linear spatial summation, although a subset gave strong second harmonic responses to counterphased gratings.Spatial-responsivity measurements for most ON-OFF and directionally selective W cells were not adequately described by DOG functions. These cells did, however, show evidence of spatial nonlinearities similar to those seen in phasic W cells. Suppressed-by-contrast cells gave both modulated and unmodulated responses to drifting gratings which both appeared to involved rectification, but which differed from each other in both spatial resolution and contrast gain.These data confirm earlier reports that the receptive fields of tonic and most ON- or OFF-center phasic W cells appear to include classical center and surround mechanisms. However, the receptive fields of some phasic cells, as well as ON-OFF and directionally selective W cells may have quite different structures. Our results also suggest that phasic, ON-OFF, directionally selective, suppressed-by-contrast, and a subset of tonic W cells may all receive nonlinear inputs with characteristics similar to those described in the receptive fields of retinal Y cells. If so, this has important implications for identifying and understanding the presynaptic circuitry of W cells, as well as the nature of their output to both telencephalic and midbrain visual targets.

scholarly journals Dynamics of Spatial Resolution of Single Units in the Lateral Geniculate Nucleus of Cat During Brief Visual Stimulation

2007 ◽  
Vol 97 (2) ◽  
pp. 1445-1456 ◽  
Author(s):  
O. Ruksenas ◽  
A. Bulatov ◽  
P. Heggelund
Keyword(s):  
Lateral Geniculate Nucleus ◽  
Spatial Resolution ◽  
Visual Stimulation ◽  
Receptive Fields ◽  
Spatial Summation ◽  
Stimulus Onset ◽  
Dorsal Lateral Geniculate Nucleus ◽  
Lateral Geniculate ◽  
Peak Resolution ◽  
Coarse To Fine

Sharpness of vision depends on the resolution of details conveyed by individual neurons in the visual pathway. In the dorsal lateral geniculate nucleus (LGN), the neurons have receptive fields with center-surround organization, and spatial resolution may be measured as the inverse of center size. We studied dynamics of receptive field center size of single LGN neurons during the response to briefly (400–500 ms) presented static light or dark spots. Center size was estimated from a series of spatial summation curves made for successive 5-ms intervals during the stimulation period. The center was wide at the start of the response, but shrank rapidly over 50–100 ms after stimulus onset, whereupon it widened slightly. Thereby, the spatial resolution changed from coarse-to-fine with average peak resolution occurring ∼70 ms after stimulus onset. The changes in spatial resolution did not follow changes of firing rate; peak firing appeared earlier than the maximal spatial resolution. We suggest that the response initially conveys a strong but spatially coarse message that might have a detection and tune-in function, followed by transient transmission of spatially precise information about the stimulus. Experiments with spots presented inside the maximum but outside the minimum center width suggested a dynamic reduction in number of responding neurons during the stimulation; from many responding neurons initially when the field centers are large to fewer responding neurons as the centers shrink. Thereby, there is a change from coarse-to-fine also in the recruitment of responding neurons during brief static stimulation.

scholarly journals Functional Consequences of Neuronal Divergence Within the Retinogeniculate Pathway

2009 ◽  
Vol 101 (4) ◽  
pp. 2166-2185 ◽  
Author(s):  
Chun-I Yeh ◽  
Carl R. Stoelzel ◽  
Chong Weng ◽  
Jose-Manuel Alonso
Keyword(s):  
Receptive Field ◽  
Functional Anatomy ◽  
Receptive Fields ◽  
Spatial Summation ◽  
High Specificity ◽  
Field Size ◽  
Dorsal Lateral Geniculate Nucleus ◽  
Retinal Ganglion ◽  
Cell Pair ◽  
Functional Consequences

The neuronal connections from the retina to the dorsal lateral geniculate nucleus (dLGN) are characterized by a high specificity. Each retinal ganglion cell diverges to connect to a small group of geniculate cells and each geniculate cell receives input from a small number of retinal ganglion cells. Consistent with the high specificity of the connections, geniculate cells sharing input from the same retinal afferent are thought to have very similar receptive fields. However, the magnitude of the receptive-field mismatches, which has not been systematically measured across the different cell types in dLGN, seems to be in contradiction with the functional anatomy of the Y visual pathway: Y retinal afferents in the cat diverge into two geniculate layers (A and C) that have Y geniculate cells (YA and YC) with different receptive-field sizes, response latencies, nonlinearity of spatial summation, and contrast sensitivity. To better understand the functional consequences of retinogeniculate divergence, we recorded from pairs of geniculate cells that shared input from a common retinal afferent across layers and within the same layer in dLGN. We found that nearly all cell pairs that shared retinal input across layers had Y-type receptive fields of the same sign (i.e., both on-center) that overlapped by >70%, but frequently differed in size and response latency. The receptive-field mismatches were relatively small in value (receptive-field size ratio <5; difference in peak response <5 ms), but were robustly correlated with the strength of the synchronous firing generated by the shared retinal connections ( R2 = 0.75). On average, the percentage of geniculate spikes that could be attributed to shared retinal inputs was about 10% for all cell-pair combinations studied. These results are used to provide new estimates of retinogeniculate divergence for different cell classes.

Morphology of retinogeniculate X and Y axon arbors in cats raised with binocular lid suture

1988 ◽  
Vol 60 (6) ◽  
pp. 2152-2167 ◽  
Author(s):  
D. Raczkowski ◽  
D. J. Uhlrich ◽  
S. M. Sherman
Keyword(s):  
Optic Tract ◽  
Selective Reduction ◽  
Complete Darkness ◽  
Response Properties ◽  
Qualitative And Quantitative ◽  
Essentially Normal ◽  
Adult Cats ◽  
The Individual ◽  
X Cells ◽  
Y Cells

1. We examined the terminal arbors of single, physiologically identified retinogeniculate X and Y axons in 13 adult cats raised from birth with binocular lid suture. We recorded in the optic tract from 146 retinogeniculate axons. We studied the response properties of each axon encountered and attempted to penetrate it for labeling with horseradish peroxidase. 2. We attempted to classify each retinogeniculate axon as X or Y on a standard battery of tests. We thus identified 46 X and 91 Y axons; 5 axons had unusual response properties, and 4 axons were lost before they could be adequately identified. The X and Y axons had response properties that were completely normal by our criteria. The 5 unusual axons exhibited linear spatial and temporal summation, which is a property of X cells, despite all of their other tested response properties being consistent with those of Y cells. 3. We achieved complete, dark labeling of 13 X and 13 Y axons that form the data base for all of our qualitative and quantitative morphological observations. All of these labeled axons had response properties entirely normal for their X or Y class. Nine of the labeled X axons arise from the contralateral retina and 4 from the ipsilateral retina, whereas the respective numbers for the Y axons are 8 and 5. 4. Each of the individual retinogeniculate X axons form terminal arbors that appeared essentially normal in terms of location within geniculate lamina A or A1, shape, volume, and number of terminal boutons. 5. In contrast, the retinogeniculate Y axons form clearly abnormal arbors with diminished projections, both in terms of bouton numbers and arbor volumes. For Y axons from the contralateral retina, a roughly normal arbor is formed in the C-laminae, despite greatly diminished or absent projections formed in lamina A, something never seen in normal cats. For Y axons from the ipsilateral retina, the projections to lamina A1 are also diminished, and the arbors there are all limited to the ventral half of the lamina, a pattern rarely seen for normal Y axons. 6. The selective reduction in retinogeniculate Y axon arbors in these binocularly lid-sutured cats is consistent with similar observations reported for monocularly lid-sutured and strabismic cats but is quite different from the apparently normal development of retinogeniculate axon arbors in cats raised in complete darkness.(ABSTRACT TRUNCATED AT 400 WORDS)

Postnatal development of receptive field surround inhibition in kitten dorsal lateral geniculate nucleus

1986 ◽  
Vol 56 (2) ◽  
pp. 523-541 ◽  
Author(s):  
J. S. Tootle ◽  
M. J. Friedlander
Keyword(s):  
Receptive Field ◽  
Lateral Geniculate Nucleus ◽  
Spatial Summation ◽  
Optic Chiasm ◽  
Dorsal Lateral Geniculate Nucleus ◽  
Surround Inhibition ◽  
Physiological Properties ◽  
Y Cells ◽  
Dorsal Lateral Geniculate ◽  
Stimulation Of

We recorded the responses to visual stimulation of single neurons in the A-layers of the dorsal lateral geniculate nucleus (LGNd) of 4- to 5-wk-old kittens and adult cats. Visual stimuli were generated on a cathode-ray tube (CRT) display and consisted of circular spots and annuli whose contrast was twice the threshold for each neuron and was modulated about a background luminance of 28 cd/m2 at 0.5 Hz. Neural responses were collected as interspike intervals and displayed as instantaneous firing rates for individual trials. From the responses to a series of sizes of spot stimuli, area-response functions were constructed and used to derive a quantitative measure of the strength of the receptive field (RF) surround inhibition of each neuron, the spatial density minimum ([SDmin[). To separate neural from optical factors that affect measurements of surround inhibition, published values for the posterior nodal distances of the kitten and adult eye were used to scale stimuli in terms of the retinal area subtended. Of 153 kitten and 95 adult LGNd neurons studied, the responses to a complete series of spot stimuli of different sizes (areas) were obtained for 52 kitten neurons [44 with linear spatial summation (L) and 8 with nonlinear spatial summation (NL)] and 45 adult (24 X-and 21 Y-) neurons. In addition, intracellular recordings were made from 30 of the kitten neurons that were filled iontophoretically with horseradish peroxidase (HRP) and were evaluated structurally. In the adult, neurons were classified as X-or Y-cells on the basis of a battery of physiological properties, including linearity of spatial summation, latency to electrical stimulation of the optic chiasm, and ability to respond reliably to rapidly moving stimuli. Kitten neuronal responses allowed them to be clearly identified as exhibiting linear or nonlinear spatial summation, but application of additional criteria produced ambiguous results for classification into X-or Y-categories. Kitten L or NL neurons showed differences typical of adult X-and Y-cells on some [e.g., RF center size (P less than 0.01)] but not other [e.g., latency to stimulation of optic chiasm (P greater than 0.40)] properties. In addition, by direct comparison of morphological features with these physiological responses, some kitten cells with adult X-cell physiological properties on these tests were found to have typical adult Y-cell somadendritic structure.(ABSTRACT TRUNCATED AT 400 WORDS)

Directional filter characteristics of optic nerve fibers in California ground squirrel (Spermophilus beecheyi)

1984 ◽  
Vol 52 (6) ◽  
pp. 1200-1212 ◽  
Author(s):  
M. E. McCourt ◽  
G. H. Jacobs
Keyword(s):  
Optic Nerve ◽  
Ground Squirrel ◽  
Receptive Fields ◽  
Spatial Summation ◽  
Nerve Fibers ◽  
Field Size ◽  
Image Motion ◽  
Spermophilus Beecheyi ◽  
California Ground Squirrel ◽  
Preferred Direction

Directional units in the optic nerve of the California ground squirrel (Spermophilus beecheyi) were studied with respect to their response to diffuse light, preferred directions of motion, tuning for preferred direction, the relationship between spatial and directional tuning characteristics, and receptive-field size and areal summating properties. Directional units in the ground squirrel optic nerve are of the “on-off” type. No purely on or off units were encountered in a sample of 356 directionally selective fibers. The distribution of preferred directions of image motion for 356 units was significantly anisotropic; greater than 50% of the directional units prefer motion in the direction of the superior-nasal visual quadrant. Mean directional bandwidth, measured at half-amplitude response, for 39 units was 88.5 degrees. The distribution of directional bandwidths suggests that two subpopulations of directional units may exist a broadly tuned (106.4 degrees bandwidth) group preferring image motion in the superior-nasal direction, and a narrowly tuned group (59.9 degrees bandwidth) with a uniform distribution of preferred direction. Tuning for direction of motion and for spatial frequency were significantly positively correlated in a sample of 35 directional units. Area-vs.-response measures for directional units show that they possess excitatory discharge centers with a concentric antagonistic surround, plus a larger suppressive surround activated specifically by moving luminance contours, which may be asymmetric. Critical activation areas for directional units, as measured along orthogonal orientations, were highly positively correlated. This suggests that these receptive fields possess the property of linear spatial summation, not of luminance flux, but of areas of moving luminance contours.

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