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PubMed1993

"Flip-flop" orientation of agarose gel fibers in pulsed alternating electric fields.

The orientation of the agarose gel matrix in pulsed electric fields has been studied by transient electric birefringence. Two types of agarose with different degrees of charge were studied, in addition to agarose solutions and gels containing beta-carrageenan, a stereoisomer of agarose, and polyacrylamide. Agarose gels exhibit normal orientation behavior when short, high voltage pulses are applied to the gel. The sign of the birefringence is positive and the relaxation times are consistent with the orientation of dangling fiber ends parallel to the electric field. When long, low voltage pulses, of the amplitude and duration used for pulsed field gel electrophoresis, are applied to the gel, completely different orientation effects are observed. The amplitude of the birefringence (i.e., extent of orientation) is much larger than expected from the high field results, and the birefringence decay curves contain multiple components of opposite sign. The relaxation times are consistent with the orientation of long agarose chain bundles or fibers, as well as large three-dimensional domains. Chain bundles or fibers of the lengths observed in the agarose gels are also observed in agarose solutions, suggesting that the fibers that are free to orient in the gels had previously formed in the sol phase and are only weakly integrated into the matrix structure. In rapidly reversing low voltage electric fields, the sign of the birefringence of the agarose gels reverses from positive to negative in phase with the reversing electric field. This alternating change in the sign of the birefringence suggests that the agarose fibers "flip-flop" in orientation from parallel to perpendicular every time the electric field reverses its direction. Similar effects are observed for agarose gels with different charge densities. The flip-flop orientation and reorientation of agarose fibers within the matrix in reversing electric fields may decrease the microscopic viscosity of the gel, increasing the mobility of large DNA molecules migrating through the gel during electrophoresis. Polyacrylamide gels do not exhibit an anomalous reversal of the sign of the birefringence in reversing electric fields. Hence, the orienting fibers in these gels do not change their direction of orientation in reversing electric fields. Extensive orientation is observed in beta-carrageenan gels, similar to that observed in agarose gels. However, little orientation occurs in polyacrylamide gels, which are chemically crosslinked.(ABSTRACT TRUNCATED AT 400 WORDS)

Acrylic Resins↗
PubMed2001

Long-range electrostatic interactions influence the orientation of Fos-Jun binding at AP-1 sites.

Heterodimeric transcription regulatory proteins that bind palindromic DNA sequences can potentially bind their recognition sites in two opposite orientations. The orientation of transcription factor binding can control transcriptional activity by altering interactions with proteins that bind to adjacent regulatory elements. Fos-Jun heterodimers bind to AP-1 sites with different flanking sequences in opposite orientations. A gel-based fluorescence resonance energy transfer assay, gelFRET, was used to define the mechanism whereby amino acid residues and nucleotide base-pairs outside the Fos-Jun-AP-1 contact interface determine the orientation of heterodimer binding. Exchange of three amino acid residues adjacent to the basic DNA contact regions between Fos and Jun reversed the binding orientation. The effects of these amino acid residues on the orientation of heterodimer binding depended on base-pairs flanking the core AP-1 recognition sequence. Single amino acid and base-pair substitutions had parallel effects on DNA bending by Fos-Jun-AP-1 complexes and on heterodimer orientation. The binding orientation exhibited a close correspondence with both the difference in bending propensities of opposite sides of the AP-1 site as well as the difference in bending potentials of the Fos and Jun subunits of the heterodimer. The influence of flanking DNA sequences on heterodimer orientation was attenuated in the presence of high concentrations of multivalent cations. Base substitutions up to one helical turn from the center of the AP-1 site affected the binding orientation. Modification of flanking base-pairs with positively or negatively charged functional groups had opposite effects on the orientation of heterodimer binding. These changes in DNA charge had converse effects on the orientation preferences of heterodimers in which charged amino acid residues adjacent to the basic regions were exchanged between Fos and Jun. These results indicate that the orientation of heterodimer binding is determined primarily by minimization of the electrostatic free energy of the Fos-Jun-AP-1 complex. Consequently, long-range electrostatic interactions influence the architecture of nucleoprotein complexes.

Amino Acid Sequence↗
PubMed2004

Categorical perception of orientation in monkeys.

Structural and functional substrates of orientation processing in monkeys have been clarified. However, orientation perception in monkeys has not been fully studied. In this study, the cognitive mechanism that controls monkeys' perception of orientation was evaluated. After the monkeys were trained to discriminate between a cardinal and an oblique orientation (e.g., 0 degrees and 30 degrees), their perceptual mechanisms underlying orientation discrimination were tested by using six orientations, ranging from 0 degrees to 150 degrees, including ones used in the discrimination training. Generalization tests showed that the monkeys who were trained with cardinal orientations (e.g., 0 degrees) as positive stimuli generalized their responses to the other cardinal orientation (e.g., 90 degrees). Similarly, the monkeys who were trained with oblique orientations (e.g., 30 degrees) as positive stimuli generalized their responses to all other oblique orientations (e.g., 60 degrees, 120 degrees, and 150 degrees). These findings indicated that the monkeys abstracted the quality of the cardinal/oblique category from the physical features of orientation stimuli although they were not trained to do so. Such an abstraction also suggested a discrepancy between a continuously and orderly arranged cortical map and a discontinuously categorized perception of orientation. The present findings provide insight into the learning-correlated plasticity of cortical orientation preference.

Animals↗
PubMed1999

The interaction of first- and second-order cues to orientation.

The visual system is sensitive to orientation information defined both by first-order (luminance) and by second-order (texture) cues. We consider how these orientation cues are computed and how they affect one another. We measured the perceived orientation of the first and second-order components of Gabor patches (the carrier and envelope, respectively) and report a dependence of the perceived orientation of each on the orientation of the other, and on the spatial frequency of the carrier. Fixing the carrier orientation near that of the envelope interferes with envelope orientation judgements. This interference is reduced by adding a small (subthreshold) rotation to the carrier indicating that the site of interference is early. When the gross relative orientation of carrier and envelope is varied, the carrier appears systematically tilted towards the envelope. However, provided envelope and carrier are separated by more than approximately 10 degrees, the perceived envelope orientation appears tilted away from the carrier. The size of these effects increases with decreasing carrier spatial frequency, and with increasing exposure duration. When the envelope and carrier are both non parallel and non-perpendicular Fourier energy is distributed asymmetrically across orientation. We demonstrate that, for a channel-based orientation code, this asymmetry induces a shift in mean orientation that is sufficient to explain illusory tilting of carriers. The illusory tilting of the envelope, as a function of carrier orientation and spatial frequency, demonstrates that human ability to demodulate contrast information is far from ideal and cannot be explained by existing two-stage filter-rectify-filter models. We propose that illusory tilting of the envelope is due to selective connectivity between first- and second-stage filters whose purpose is to dissociate the type of image structure producing each class of cue.

Cues↗
PubMedDate not supplied

Visual experience promotes the isotropic representation of orientation preference.

Within the visual cortex of several mammalian species, more circuitry is devoted to the representation of vertical and horizontal orientations than oblique orientations. The sensitivity of this representation of orientation preference to visual experience during cortical maturation and the overabundance of cardinal contours in the environment suggest that vision promotes the development of this cortical anisotropy. We tested this idea by measuring the distribution of cortical orientation preference and the degree of orientation selectivity in developing normal and dark-reared ferrets using intrinsic signal optical imaging. The area of the angle map of orientation preference representing cardinal and oblique orientations was determined; in addition, orientation selectivity indices were computed separately for cardinal and oblique difference images. In normal juvenile animals, we confirm a small, but statistically significant overrepresentation of near horizontal orientations in the cortical angle map. However, the degree of anisotropy did not increase in the weeks that followed eye opening when orientation selectivity matured; rather, it decreased. In dark-reared ferrets, an even greater cortical anisotropy emerged, but angle maps in these animals developed an apparently anomalous overrepresentation of near vertical orientations. Thus, the overrepresentation of cardinal orientations in the visual cortex does not require experience with an anisotropic visual environment; indeed, cortical anisotropy can develop in the complete absence of vision. These observations suggest that the role of visual experience in cortical maturation is to promote the isotropic representation of orientation preference.

Aging↗
PubMed1998

Limiting factors for the detection of orientation.

First steps of visual-information processing in primates are characterised by a highly ordered representation of the outside world on the cortex. Two prominent features of cortical organisation are the retinotopic mapping of position in the visual field on the first stages of the visual stream, and the systematic variation of orientation preference in the same areas. In an attempt to understand the relation of position and orientation representation, we need to know the minimum spatial requirements for orientation detection. In the present paper, the spatial limits for detecting orientation are analysed by simulating simple orientation filters and testing the ability of human observers to detect the orientation of small lines at various positions in the visual field. At sufficiently high contrast levels, the minimum physical length of a line to discriminate orientation differences of 45 degrees-90 degrees is not constant when presented at various eccentricities, but covaries inversely with the cortical magnification factor. In consequence, a line needs to correspond to about 0.2 mm of cortical surface, independently of the actual eccentricity at which the stimulus is presented, in order to allow observers to recognise its orientation. This has consequences for our understanding of orientation detection. (i) In combination with simulation experiments, it becomes clear that the elementary process underlying orientation detection is a local operation, which seems to focus on small regions compared with cortical receptive fields. (ii) With respect to the number of inputs to the visual cortex, the performance of this local operation approaches the physical limits, requiring hardly more than three-four input LGN axons to be activated for detecting the orientation of a highly visible line segment. Comparing these spatial characteristics with the receptive fields of orientation-sensitive neurons in the primate visual system could suggest new insights into the neuronal circuits underlying orientation mapping in the human cortex.

Computer Simulation↗
PubMed1983

Effects of exposure to lines of one or two orientations on different cell types in striate cortex of cat.

We raised cats using goggles to control early visual exposure (stripe-rearing). Four conditions were used: (a), both eyes exposed to 0 degrees lines, (b), both eyes exposed to 90 degree lines, (c), one eye exposed to 45 degree lines, the other to 135 degree lines, (d), one eye exposed to 0 degree lines, the other to 90 degree lines. At the completion of the rearing, we recorded extracellularly from single cells in striate cortex (area 17) of these animals; circular statistics were used to analyse the distribution of the orientation preferences of neurones recorded. Exposure to either one or two stimulus orientations produced a significant bias in the distribution of the orientation preferences of cells recorded. We found no more non-selective cells in cats exposed to one orientation (15%) than in cats exposed to two orientations (14%). We found about the same mean proportion of binocular cells in cats exposed to one orientation (27%) as in cats exposed to two orientations (24%). Cells were comparable in orientation selectivity in cats exposed to one orientation (mean half-width at half-height = 37 degrees) and in cats exposed to two orientations (mean half-width at half-height = 34 degrees). The effects of the rearing depended upon the receptive field properties of the cells. For cells with narrow receptive fields and low cut-off velocities, the rearing produced no bias in the distribution of the orientation preferences; for all other groups of cells the rearing produced a bias toward the exposed orientation. In addition, the cells with narrow receptive fields and low cut-off velocities were more finely tuned for orientation than the remaining cells. We conclude that there are cell types in the striate cortex of the cat which differ in their susceptibility to the effects of stripe-rearing; these cell types can be discriminated on the basis of their physiological characteristics. We can compare these cell types with morphologically identified cell types that also differ in their susceptibility to the effects of stripe-rearing.

Animals↗
PubMed1992

Differential imaging of ocular dominance and orientation selectivity in monkey striate cortex.

Differential images of ocular dominance, acquired by comparing responses to the two eyes, reveal dark and light bands where cortical cells are dominated by the right and left eyes. These include most (but not all) histochemically stained cytochrome oxidase blobs in their centers. Differential images of orientation, acquired by comparing responses to orthogonal orientations, reveal dark and light bands that are reminiscent of the "orientation columns" reported earlier, on the basis of 2-deoxyglucose (2DG) autoradiograms (Hubel et al., 1978). However, they are shorter and more fragmented because they do not include regions lacking selectivity for orientation. Even though these "bands" derive from orientation-selective areas, comparisons with differential images of other orientations reveal that regions along their centers prefer different orientations. Hence, the orientation preferences inferred from "bands" in single differential images, or single 2DG autoradiograms, are not necessarily incorrect. Interactions between ocular dominance and orientation were investigated by comparing differential images of orientation obtained with binocular and monocular stimulation, as well as by comparing differential images of ocular dominance obtained with different orientations. In both cases, the elicited interactions were minimal, indicating a remarkable and unexpected independence that subsequent experiments revealed arises, at least in part, from a lateral segregation of regions most selective for one eye and regions most selective for one orientation, in the centers and edges of ocular dominance columns. Since this can also be viewed as a lateral correlation between binocularity and orientation selectivity, it fits with the simultaneous emergence of these properties in layers receiving input from layer 4c, and suggests that each of these properties requires the other.

Animals↗
PubMed1997

Optically imaged maps of orientation preference in primary visual cortex of cats and ferrets.

Feature maps in the cerebral cortex constitute orderly representations of response features created within the cortex; an example is the mapping of orientation-selective neurons in visual cortex. We have compared the properties of orientation maps in area 17 of cats and ferrets, obtained by optical imaging of intrinsic signals. Orientation maps in both species contain a quasi-periodic distribution of iso-orientation domains that are organized into a lattice of pinwheels. However, the spatial density of orientation domains and of pinwheels in ferret area 17 is nearly twice that in cat area 17. The ferret map also contains more discontinuities, or fractures, where orientation changes abruptly. The size of orientation domains scales with interdomain spacing, so that the ratio of the two is approximately the same in both species. Consistent with this finding, the orientation tuning width of individual pixels is similar in the two. The magnitude of orientation preference, however, is much lower in ferret compared to cat. The greater incidence of fractures in ferret appears to be due to proportionately greater overlap between domains of different orientations, particularly along fracture lines that link pinwheel centers. We hypothesize that a key determinant of orientation maps, the relationship between orientation domain size and spacing, expresses an anatomical link between sizes of thalamocortical arbors and horizontal intracortical connections in area 17.

Animals↗
PubMed1992

A unique morphological subtype of horizontal cell in the rabbit retina with orientation-sensitive response properties.

Intracellular recordings were obtained from horizontal cells in the rabbit retina to assess the orientation sensitivity of their visual responses to moving and stationary rectangular slits of light. Cells were subsequently labeled with horseradish peroxidase (HRP) for morphological identification. The responses of A-type horizontal cells and those of the somatic and axon terminal endings of B-type horizontal cells (with the exception of one cell) were found to be insensitive to the orientation of light stimuli. However, 20 horizontal cells encountered within or just superior to the visual streak displayed clear orientation-sensitive response properties. These cells were divided into two groups: the majority (70%) showed preference for light stimuli oriented parallel to the visual streak, whereas the remainder preferred stimuli oriented orthogonal to the visual streak. Analysis of the shape of the receptive fields of these cells by means of a narrow, displaced slit of light revealed an anisotropy with the major or elongated axis of the receptive field of each cell aligned along the same angle as its physiological preferred orientation. Morphologically, the orientation-sensitive horizontal cells formed a homogeneous group with an architecture corresponding to that of elongated A-type or Ae-type horizontal cells reported previously in the rabbit retina. These cells showed a marked elongation of their dendritic arbors with the major axes oriented either parallel or orthogonal to the visual streak. Furthermore, the orientation of the dendritic arbor of each cell matched that of its physiological preferred orientation. The present results, then, suggest strongly that the orientation sensitivity of Ae-type horizontal cells results directly from the asymmetry in their dendritic arbors. The spatial location and specialized physiology of Ae-type horizontal cells suggest that they play a role in the formation of orientation-sensitive properties exhibited by more proximal neurons in the rabbit retina.

Animals↗
PubMed1983

Orientation selectivity and the spatial distribution of enhancement and suppression in receptive fields of cat striate cortex cells.

The relationship between orientation selectivity and spatial receptive field organization was analyzed. Receptive field maps were made with a dual stimulus technique where an optimally oriented activation slit was presented in the most responsive region to produce activity against which the effect of a test spot in various positions was determined. Both simple and complex cells had receptive fields which were subdivided into adjacent elongated and antagonistic subregions. When the two stimuli were presented in phase (both ON or OFF simultaneously) the fields had a central enhancement region with a strong suppression flank on one or both sides. Optimal slit orientation was related to the location of the suppression flank relative to the location of the central enhancement region, and the degree of orientation selectivity to the shape of the subregions and the distance between them. Estimated orientation tuning curves calculated from the receptive field maps gave satisfactory first approximations to experimental curves. The relative contribution of enhancement and suppression to orientation selectivity was studied by presenting a test slit in different orientations in phase with an optimally oriented activation slit. The orientation selectivity was produced almost exclusively by the flank suppression indicating that orientation selectivity is produced by inhibitory input. The flank suppression lacked any specific orientation selectivity, and it occurred only when both the central region and the flanks were activated in phase. Orientation selectivity in both simple and complex cells is explained by a receptive field organization where the cells have input from partially overlapping excitatory and inhibitory fields which have their centers slightly displaced from each other.

Animals↗
PubMed1982

Orientation sensitivity of cat LGN neurones with and without inputs from visual cortical areas 17 and 18.

Orientation sensitivity was tested, using moving bars as stimuli, in 136 LGN cells in normal cats and 82 LGN cells in cats with areas 17 and 18 lesioned. The responses of most neurones showed some dependence on the orientation of the line stimulus. The orientation bias was more pronounced for long, narrow bars moving at rather slow velocities. Length-response curves revealed less end-inhibition along the optimum orientation than along the non-optimum orientation. Thiry-two percent of the cells in the normal cats and 50% in the lesioned animals responded best to orientations within 10 degrees of the vertical or horizontal. The oblique orientations were represented poorly in the lesioned group. Thus the corticogeniculate feedback may serve to confer a more uniform distribution of orientation preferences on the LGN. It is suggested that the orientation biases of LGN neurones may play a role in building orientation-selective cells in the visual cortex. Further, the preferences for horizontal and vertical orientations in the LGN may explain the preferences for these orientations reported for visual cortical cells.

Animals↗
PubMed1988

A theory for the use of visual orientation information which exploits the columnar structure of striate cortex.

A neural model is constructed based on the structure of a visual orientation hypercolumn in mammalian striate cortex. It is then assumed that the perceived orientation of visual contours is determined by the pattern of neuronal activity across orientation columns. Using statistical estimation theory, limits on the precision of orientation estimation and discrimination are calculated. These limits are functions of single unit response properties such as orientation tuning width, response amplitude and response variability, as well as the degree of organization in the neural network. It is shown that a network of modest size, consisting of broadly orientation selective units, can reliably discriminate orientation with a precision equivalent to human performance. Of the various network parameters, the discrimination threshold depends most critically on the number of cells in the hypercolumn. The form of the dependence on cell number correctly predicts the results of psychophysical studies of orientation discrimination. The model system's performance is also consistent with psychophysical data in two situations in which human performance is not optimal. First, interference with orientation discrimination occurs when multiple stimuli activate cells in the same hypercolumn. Second, systematic errors in the estimation of orientation can occur when a stimulus is composed of intersecting lines. The results demonstrate that it is possible to relate neural activity to visual performance by an examination of the pattern of activity across orientation columns. This provides support for the hypothesis that perceived orientation is determined by the distributed pattern of neural activity. The results also encourage the view that limits on visual discrimination are determined by the responses of many neurons rather than the sensitivity of individual cells.

Animals↗
PubMed2000

A mean-field model for orientation tuning, contrast saturation, and contextual effects in the primary visual cortex.

Orientation-selective cells in the primary visual cortex of monkeys and cats are often characterized by an orientation-tuning width that is invariant under stimulus contrast. At the same time their contrast response function saturates or even super-saturates for high values of contrast. When two bar stimuli are presented within their classical receptive field, the neuronal response decreases with the intersection angle. When two stimuli are presented inside and outside the classical receptive field, the response of the cell increases with the intersection angle. Both cats and monkeys show iso-orientation suppression, which has sometimes been reported to be combined with cross-orientation facilitation. This property has previously been described as sensitivity to orientation contrast. We address the emergence of these effects with a model that describes the processing of geniculocortical signals through cortical circuitry. We hypothesize that short intracortical fibers mediate the classical receptive field effects, whereas long-range collaterals evoke contextual effects such as sensitivity to orientation contrast. We model this situation by setting up a mean-field description of two neighboring cortical hypercolumns, which can process a nonoverlapping center and a (nonclassical) surround stimulus. Both hypercolumns interact via idealized long-range connections. For an isolated model hypercolumn, we find that either contrast saturation or contrast-invariant orientation tuning emerges, depending on the strength of the lateral excitation. There is no parameter regime, however, where both phenomena emerge simultaneously. In the regime where contrast saturation is found, the model also correctly reproduces suppression due to a second, cross-oriented grid within the classical receptive field. If two model hypercolumns are mutually coupled by long-range connections that are iso-orientation specific, nonclassical surround stimuli show either suppression or facilitation for all surround orientations. Sensitivity to orientation contrast is not observed. This property requires excitatory-to-excitatory long-range couplings that are less orientation specific than those targeting inhibitory neurons.

Animals↗
PubMed1986

Effects of monocular exposure to oriented lines on monkey striate cortex.

This study examines the extent to which the restriction of visual experience to lines of a single orientation influences the organization of the striate cortex in infant monkeys (Macaca mulatta). Previous studies of kittens raised with monocular exposure to a single line orientation have consistently shown the response preference of cells driven by that eye to be biased towards the experienced orientation. Studies of binocular exposure to restricted orientations have been equivocal. In the infant monkey cortex responses to oriented lines have virtually all the specificity of responses seen in the adult animal. In an effort to clarify the phenomenon and the mechanism by which orientation bias might be obtained, we examined the effects of monocular exposure to a restricted orientation in infant macaques. Three monkeys were used. Each monkey was raised with one open eye exposed to lines of a single orientation and one eye occluded by lid suture. As in other cases of monocular deprivation in either cat or monkey, few binocularly driven cells were recorded and the majority of cells were dominated by the open eye. Cells driven by the open eye had normal representation of all orientation preferences and there was no overall increase in the number of cells preferring the orientation to which the eye had been exposed. The cells dominated by the occluded eye, however, showed a lack of cells responding to orientations to which the open eye had been exposed. These findings suggest that a competitive mechanism operates between the two eyes to provide an orientation selective advantage to the open eye.

Animals↗
PubMed2001

Attention-dependent brief adaptation to contour orientation: a high-level aftereffect for convexity?

In contrast to the abundant literature investigating how orientation coding depends on edges defined by various image features, relatively little is known about how coding of orientation might also depend on the two distinct functional roles that oriented edges commonly play. Oriented lines can delineate outline contours of a figure or they can form texture. The results of five experiments using orientation aftereffects measured with brief tests (27 ms, backward masked; adapt-to-test interval=201 ms) provided evidence that brief stimuli (<135 ms) selectively adapt coding of contour-line orientation rather than coding of line-texture orientation. Furthermore, parametric results revealed that the rapidly adapting aftereffects for contour orientation are characterized by (1) broad orientation tuning (peaking at +/-30 degrees to +/-50 degrees from test orientation), (2) indifference as to how the contours are defined (e.g. bright lines, high-pass-filtered lines, faint lines generated by the spatial inhomogeneity of visual sensitivity), (3) rapid saturation at low contrast energy, (4) strong modulation by selective attention, and (5) relative size tolerance. These characteristics appear to parallel those of cells in the high end of the visual form processing pathway (such as inferotemporal cortex). It is thus suggested that the rapidly adapting contour orientation aftereffects reported here may be mediated by high-level neural units that encode global configurations of orientation (e.g. convexity and concavity).

Adaptation, Physiological↗
PubMedDate not supplied

Dynamics of orientation coding in area V1 of the awake primate.

To investigate the importance of feedback loops in visual information processing, we have analyzed the dynamic aspects of neuronal responses to oriented gratings in cortical area V1 of the awake primate. If recurrent feedback is important in generating orientation selectivity, the initial part of the neuronal response should be relatively poorly selective, and full orientation selectivity should only appear after a delay. Thus, by examining the dynamics of the neuronal responses it should be possible to assess the importance of feedback processes in the development of orientation selectivity. The results were base on a sample of 259 cells recorded in two monkeys, of which 89% were visually responsive. Of these, approximately two-thirds were orientation selective. Response latency varied considerably between neurons, ranging from a minimum of 41 ms to over 150 ms, although most had latencies of 50-70 ms. Orientation tuning (defined as the bandwidth at half-height) ranged from 16 deg to over 90 deg, with a mean value of around 55 deg. By examining the selectivity of these different neurons by 10-ms time slices, starting at the onset of the neuronal response, we found that the orientation selectivity of virtually every neuron was fully developed at the very start of the neuronal response. Indeed, many neurons showed a marked tendency to respond at somewhat longer latencies to stimuli that were nonoptimally oriented, with the result that orientation selectivity was highest at the very start of the neuronal response. Furthermore, there was no evidence that the neurons with the shortest onset latencies were less selective. Such evidence is inconsistent with the hypothesis that recurrent intracortical feedback plays an important role in the generation of orientation selectivity. Instead, we suggest that orientation selectivity is primarily generated using feedforward mechanisms, including feedforward inhibition. Such a strategy has the advantage of allowing orientation to be computed rapidly, and avoids the initially poorly selective neuronal responses that characterize processing involving recurrent loops.

Animals↗
PubMedDate not supplied

The influence of input from the lower cortical layers on the orientation tuning of upper layer V1 cells in a primate.

The receptive fields of cells in the primary visual cortex (area 17 or V1) show clear orientation selectivity, unlike those of the lateral geniculate nucleus (LGN) cells which provide their visual input. The intrinsic circuitry of V1 cells is believed to be partly responsible for this selectivity. We investigated the influence of ascending projections from neurons in the lower layers (5 and 6) of V1 on the orientation selectivity of single neurons in the upper layers (2,3, and 4) by reversibly inactivating ("blocking") lower layer neural activity with iontophoretic application of gamma-aminobutyric acid (GABA) while recording from upper layer cells in the prosimian primate, Galago crassicaudatus. During lower layer blocking, the majority (20/28 = 71.4%) of upper layer neurons exhibited a change in the orientation of their preferred stimulus, a reduction in their orientation tuning, and/or an increase in their response amplitude. Twelve (42.9%) neurons exhibited shifts in their preferred orientation averaging 11 (+/- 4) deg. These neurons were located on average, 272 (+/- 120) microns tangential from the vertical axis of the pipette center. Eleven neurons (39.2%) exhibited an average reduced orientation tuning of 52.5%. Their average location was 230 +/- (115) microns away from the vertical axis of the pipette. Five (17.9%) neurons with average location 145 (+/- 75) microns from the vertical axis exhibited both effects. Two (7.1%) neurons that exhibited significant increases in response amplitude to stimulus angles within 10 deg of the peak excitatory stimulus without changes in orientation selectivity or tuning were located less than 100 microns from the vertical axis. The effects on the orientation tuning of cells were restricted in all cases to within +/- 30 deg of the preferred stimulus orientation. This suggests that layer blocking affects cells with preferred stimulus orientations similar to those of the recorded neurons. Only cells located within 500 microns tangential to the vertical axis of the injection site exhibited these effects. These results suggest that cells within layers 5 and 6 provide organized, orientation-tuned inhibition that sharpens the orientation tuning of cells in the upper cortical layers within the same, or closely neighboring, cell columns.

Animals↗