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PubMed1992

GABA-induced inactivation of functionally characterized sites in cat visual cortex (area 18): effects on orientation tuning.

Microiontophoresis of the inhibitory transmitter GABA was used to reversibly inactivate small sites of defined orientation specificity at a horizontal distance of some 600 microns from single cells recorded in area 18 of cat visual cortex, and the effects on orientation tuning were studied. The receptive fields of cells at the recording and inactivation sites overlapped extensively. During the inactivation of sites where the orientation preference differed by 45 degrees or more from that of a recorded cell ("cross-orientation" sites), 65% of 54 cells tested showed significant broadening of orientation tuning, with a mean increase in tuning width (measured at half the maximum response) of 93%, and an almost fourfold increase in the relative response to the orientation orthogonal to the optimum, compared with the response to the optimum; four cells essentially lost their orientation tuning. Broadening of tuning reflected an increase in response to nonoptimal orientations and was reversible upon termination of GABA application. The effects on orientation tuning typically peaked within 10-15 min of the onset of GABA iontophoresis with 50-100 nA ejecting currents, and could not be replicated by inactivating sites where the orientation preference was similar to that of a recorded cell; when the orientation preference at the inactivation sites was within 22.5 degrees of that of a recorded cell ("iso-orientation" sites), only 3 of 22 cells showed significant broadening of tuning, and in these cases, the effects were relatively weak (mean increase in tuning width of 39% and a negligible change in the relative response to the orientation orthogonal to the optimum). The effect of inactivating "iso-orientation" sites consisted primarily in an increase in response magnitude. The difference in the magnitude of the effects on orientation tuning elicited by inactivating "cross-orientation" and "iso-orientation" sites was highly statistically significant. Additionally, inactivation of "cross-orientation" or "iso-orientation" sites elicited differential effects on orientation tuning in 10 of the 13 cells in which direct comparisons were made. It is argued that the observed broadening of tuning was due to the loss of a "cross-orientation" inhibitory input, which normally sharpens orientation tuning by suppressing responses to nonoptimal orientations.

Animals↗
PubMed1994

Orientation-sensitive amacrine and ganglion cells in the rabbit retina.

1. Intracellular recordings were obtained from amacrine and ganglion cells in the isolated, superfused retina-eyecup preparation of the rabbit to test the orientation sensitivity of their responses. Cell identification was based on morphological criteria following injection of horseradish peroxidase (HRP) or N-(2-aminoethyl)-biotinamide hydrochloride (Neurobiotin) to visualize soma-dendritic architectures. 2. In terms of the physiological mechanisms generating their sensitivity, two types of orientation-sensitive amacrine cell and a single type of orientation-sensitive ganglion cell were found. These cell types were termed orientation selective and orientation biased. Cells were subtypes further into on- or off-center receptive-field categories. 3. The receptive fields of orientation-selective amacrine and ganglion cells were composed of two inhibitory fields that flanked the excitatory center receptive field along the preferred orientation. These inhibitory flanks produced a center receptive-field anisotropy with its major axis corresponding to the preferred orientation: either parallel or orthogonal to the visual streak. When a stimulus was oriented orthogonal to the preferred orientation (i.e., at the null orientation), the inhibitory fields were stimulated, resulting in a null inhibition that blocked the center-mediated excitation. Stimulation of these inhibitory flanks was absolutely essential to evoke the orientation selectivity of these cells. The null response reflected inhibition associated with a conductance increase and not disfacilitation. 4. Orientation-biased amacrine cells displayed a center receptive-field anisotropy with its major axis oriented either parallel or orthogonal to the visual streak. These cells preferred light stimuli oriented along the major axis of the center receptive field. However, whereas the excitatory response of these cells was reduced when a stimulus was rotated from the preferred orientation, there was no corresponding hyperpolarization. No null inhibition was detected even after modulation of the membrane potential with extrinsic current. 5. Although orientation-biased amacrine cells were morphologically heterogeneous, they all displayed dendritic arbors that were markedly elongated along an axis corresponding to their physiological preferred orientation. Thus it appears that the elongated dendritic fields of these cells may provide for the anisotropy of their center receptive fields and, in turn, their orientation sensitivity. 6. Orientation-selective amacrine cells formed a rather homogeneous morphological group of cells. These neurons displayed large, radially symmetric dendritic arbors with diameters averaging 1,100 microns. There were no asymmetries in their dendritic fields and thus no clear structural basis for their orientation selectivity. 7. In contrast, orientation-selective ganglion cells displayed diverse soma-dendritic architecture and thus could not be placed into a single morphological class.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1979

Inhibitory mechanisms influencing complex cell orientation selectivity and their modification at high resting discharge levels.

1. These experiments have investigated the contribution made by GABA-mediated inhibitory processes to the orientation tuning of complex cells in the cat's striate cortex. The GABA antagonist bicuculline has been ionophoretically applied to individual complex cells and the modifications produced in their orientation tuning documented. 2. In terms of the type of change produced in orientation tuning by the application of bicuculline, it seems that there are two categories of complex cells. 3. In one of these categories the orientation selectivity was eliminated during bicuculline application. The excitatory input to these cells would therefore appear to be non-orientation specific. Their orientation selectivity is presumably generated by a GABA-mediated inhibitory input. 4. In the other category of complex cells, although the orientation selectivity was decreased during bicuculline application, the cells retained a preference for a range of orientations that was generally centred around the original optimal orientation. It is suggested that for these cells the inhibitory input enhances the orientation tuning of an excitatory input that is already broadly orientation tuned. 5. Comparison of normal orientation tuning curves with those observed during the application of bicuculline provides a basis for estimating the orientation tuning of the GABA-mediated inhibitory input. In all cases, it is clear that at normal resting discharge levels, orientations either side of the optimal, and not those centred on the optimal, generate the most powerful inhibitory input. 6. These results would seem to be best explained by inhibitory interconnexions between cortical columns sensitive to different orientations. This type of lateral interaction between columns may serve to enhance the contrast in the orientation domain for the cortical representation of a specific stimulus orientation. 7. Increasing the resting discharge level of a complex cell, without blocking the action of GABA appeared to increase the gain of the inhibitory mechanisms acting on the cell. The normal excitatory responses to optimal or near optimal orientations were greatly reduced, or replaced by inhibitory responses, and non-optimal orientations produced only inhibitory responses. These inhibitory effects were blocked on the context of other observations in the literature. It is tentatively suggested that the interneurones providing the inhibitory drive to complex cells receive an input from recurrent collaterals of the recipient complex cells. Their other inputs would derive from neighbouring colums and from the afferent input to the parent column. The inputs from neighbouring columns would mediate the lateral inhibitory interactions in the orientation domain, and the recurrent collateral feed-back the decreased responsiveness at high resting discharge levels.

Animals↗
PubMed1986

Retinal constraints on orientation specificity in cat visual cortex.

Most retinal ganglion cells (Levick and Thibos, 1982) and cortical cells (Leventhal, 1983; Leventhal et al., 1984) subserving peripheral vision respond best to stimuli that are oriented radially, i.e., like the spokes of a wheel with the area centralis at the hub. We have extended this work by comparing directly the distributions of orientations represented in topographically corresponding regions of retina and visual cortex. Both central and peripheral regions were studied. The relations between the orientations of neighboring ganglion cells and the manner in which the overrepresentation of radial orientations is accommodated in the functional architecture of visual cortex were also studied. Our results are based on an analysis of the orientations of the dendritic fields of 1296 ganglion cells throughout the retina and the preferred orientations of 1389 cells located in retinotopically corresponding regions of cortical areas 17, 18, and 19 in the cat. We find that horizontal and vertical orientations are overrepresented in regions of both retina and visual cortex subserving the central 5 degrees of vision. The distributions of the orientations of retinal ganglion cells and cortical cells subserving the horizontal, vertical, and diagonal meridians outside the area centralis differ significantly. The distribution of the preferred orientations of the S (simple) cells in areas 17, 18 and 19 subserving a given part of the retina corresponds to the distribution of the dendritic field orientations of the ganglion cells in that part of retina. The distribution of the preferred orientations of C (complex) cells with narrow receptive fields in area 17 but not C cells with wide receptive fields in areas 17, 18, or 19 subserving a given part of the retina matches the distribution of the orientations of the ganglion cells in that part of retina. The orientations of all of the alpha-cells in 5-9 mm2 patches of retina along the horizontal, vertical, and oblique meridians were determined. A comparison of the orientations of neighboring cells indicates that other than a mutual tendency to be oriented radially, ganglion cells with similar orientations are not clustered in the retina. Reconstructions of electrode penetrations into regions of visual cortex representing peripheral retina indicate that columns subserving radial orientations are wider than those subserving nonradial orientations. Our results provide evidence that the distribution of the preferred orientations of simple cells in visual cortex subserving any region of the visual field matches the distribution of the orientations of the ganglion cells subserving the same region of the visual field.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1980

Topographic organization of the orientation column system in the striate cortex of the tree shrew (Tupaia glis). I. Microelectrode recording.

Microelectrode recordings were made in the binocular portion of the tree shrew striate cortex to determine how orientation selective cells are distributed topographically in area 17 of this species. Seventy-five percent of the cells sampled were activated well by elongated visual stimuli and were quite selective for stimulus orientation. Ninety-five percent of the orientation-selective cells had orientation tuning ranges (Wilson and Sherman, '76) between +/- 5 degrees and +/- 40 degrees from their optimal orientation. Orientation-selective cells with the same or similar optimal orientations were distributed in cortex in a columnar manner (Hubel and Wiesel, '62), as determined from electrode penetrations nearly normal to the cortical surface. Penetrations parallel to the cortical surface revealed a highly ordered representation of optimal stimulus orientation, generally characterized by sequential changes in optimal orientation with electrode movement across the striate cortex. In addition, relatively consistent differences were observed in the rates and patterns of orientation shift on these penetrations depending on the direction of electrode movement across the cortex. Penetrations parallel to the 17--18 border yielded moderate-to-high rates of orientation change (mean slope = 434 degrees/mm), with the changes generally progressing through a complete clockwise or counterclockwise cycle of 180 degrees or more before a major reversal in the direction of orientation shift was encountered. In contrast, penetrations perpendicular to the border yielded low-to-moderate slopes (mean slope = 239 degrees/mm). On these penetrations a more limited range of optimal orientations (< 180 degrees) was usually encountered, due to frequent reversals in the direction of orientation shift. Also, extended regions (100--200) micrometers long) of constant optimal orientation were observed in these penetrations. The different patterns of orientation change found on these orthogonal penetrations across the striate cortex indicate that the orientation column system in this species is anisotropically organized with respect to the 17--18 border. Further, the regions of constant optimal orientation frequently encountered on penetrations perpendicular to the 17--18 border suggest that the anisotropy is subserved by a system of elongated zones of iso-orientation arranged approximately perpendicular to the 17--18 border.

Animals↗
PubMed1985

Mutual spatial orientation of hexons in the adenovirus capsid by electron microscopy and modelling.

On the basis of electron microscopic analysis of the virion and model experiments there are four hexon species in the adenovirus capsid which have different positional status and spatial orientation as compared to the six nearest neighbouring capsomers. The binding pattern of the polypeptide subunits according to their orientation is also different. The four hexon species are situated beside one another in the capsid and they form the capsid of 240 hexons besides the penton by repeating symmetrically 60 times, as a group of four hexons (GOF). Electron microscopic analysis and model experiments lead to the conclusion that the mutual spatial orientation of GOF hexons can be parallel (their longitudinal axes are parallel), or the direction of their longitudinal axes is different in one or two planes, thus they form different angles with the longitudinal axes of the neighbouring hexons. In the latter cases therefore, a bend and torsion of the bindings (connective elements) can be supposed as compared to the tendency-plane of parallel bindings. Based on the different combinations of the three kinds of rotational orientation of polypeptide subunits and of the different spatial orientation of hexons, six kinds of interhexonal bindings can be found in the adenovirus capsid. The distribution and characteristics of the 690 interhexonal bindings are the following; 240 bindings of "one-to-two" polypeptide orientation with parallel spatial orientation; 180 bindings of "one-to-two" polypeptide with spatial orientation divergent in two planes; 30 bindings of "one-to-one" polypeptide orientation with parallel spatial orientation; 60 bindings of "one-to-one" polypeptide orientation with spatial orientations divergent in one plane; 60 bindings of "one-to-one" polypeptide orientation with spatial orientation divergent in two planes; 120 bindings of "two-to-two" polypeptide orientation with spatial orientation divergent in two planes. The dissociational sequence of the capsid suggests that the firmest bindings are the ones having "one-to-two" polypeptide orientation and parallel spatial orientation. It may be supposed that any two hexons are able to connect to each other according to all three polypeptide orientations and the connections can tolerate the bends and torsions arising from the icosahedral structure. This suggests that only one kind of hexon is existing and the adenovirus capsid is formed by a homogeneous hexon population.

Adenoviridae↗
PubMed1988

Maturation of rat visual cortex: IV. The generation, migration, morphogenesis, and connectivity of atypically oriented pyramidal neurons.

The generation, migration, and morphogenesis of atypically oriented pyramidal neurons in the rat visual cortex were examined. In the mature cortex, these neurons were distributed through layers II-VI. Moreover, the atypically oriented pyramidal neurons in a particular layer tended to be oriented in a specific way; atypically oriented pyramidal neurons in layer II, layers III-VIa, and layer VIb were obliquely, radially, and obliquely oriented, respectively. Ultrastructurally, the somata of atypically oriented pyramidal neurons contained large euchromatic ovoid nuclei and cytoplasm that was replete with rough endoplasmic reticulum and Golgi apparatus. These somata formed only symmetric axosomatic synapses. Many atypically oriented pyramidal neurons projected axons into the white matter as demonstrated by a Golgi method and by a retrograde tract-tracing technique; however, some of these pyramidal neurons in layers III-V had axons that ascended to layer I. By using a technique which combined retrograde tract tracing with [3H]thymidine autoradiography, it was determined that most atypically oriented pyramidal neurons in layers V and VIa, layer IV, and layer II were generated on gestational days (GD) 15-17, GD 17-19, and GD 20-21, respectively. Atypically oreinted pyramidal neurons were identified during the period from postnatal day 0 (day of birth) to day 30. On day 0, obliquely oriented pyramidal neurons were distributed in the deep cortical plate, i.e., immature layer VI. On day 3, the youngest atypically oriented pyramidal neurons were radially oriented and were located in layer IV. Some obliquely oriented pyramidal neurons were present in layer II on day 6, but the greatest number and the most severely canted pyramidal neurons in layer II were evident on day 9. The orientations of the cell body and the apical dendrite did not change appreciably after migration was complete, except for those in layers V and VI with obliquely oriented cell bodies and radially oriented apical dendrites. The second and third postnatal weeks were marked by substantial morphological differentiation of all pyramidal neurons as noted by the lengthening and branching of dendrites and by the appearance of dendritic spines. By the fourth postnatal week, atypically oriented pyramidal neurons achieved their mature morphology. The generation, migration, and morphogenesis of atypically oriented pyramidal neurons proceed by an inside-to-outside sequence. This development is similar and concurrent with that of typically oriented pyramidal neurons.

Aging↗
PubMed1996

Mechanisms underlying orientation selectivity of neurons in the primary visual cortex of the macaque.

1. Effects of blocking intracortical inhibition by microiontophoretic administration of bicuculline methiodide (BMI), a selective antagonist for GABAA receptors, on orientation selectivity of 109 neurones were studied in the primary visual cortex (V1) of anaesthetized and paralysed monkeys. 2. The averaged orientation tuning of visual responses of cells was poor in cytochrome oxidaserich blobs of layer II/III and in layer IVc beta, moderate in layers IVb, IVc alpha and V, and sharp in the interblob region of layer II/III and in layers IVa and VI. 3. Iontophoretic administration of BMI reduced the sharpness of orientation tuning of cells to a varying extent in each layer. In most cells, furthermore, the originally ineffective stimuli induced visual responses during the BMI administration, suggesting that excitatory inputs evoked by the non-optimally oriented stimuli were masked by GABAergic inhibition. Nevertheless, the maximal facilitation was observed in the response to the optimally or near-optimally oriented stimuli. 4. There was a difference in such an effect of BMI among layers. Orientation selectivity of cells in interblobs in layer II/III and in layer IVb was sensitive to BMI whereas that of cells in layer VI was relatively insensitive to BMI, suggesting a larger contribution of excitatory mechanisms to the orientation selectivity in this layer. 5. In the orientation-selective cells, an analysis of the magnitude of excitation and inhibition evoked by stimuli at various orientations suggests that both inputs tune around the optimal orientation and their magnitudes are almost proportional to each other except at the optimal orientation. This analysis also indicates that the orientation tuning of inhibition had a less prominent peak around the optimal orientation than that of excitation. This dominance of excitation over inhibition around the optimal orientation may function to accentuate the response to the optimally oriented stimulus. 6. These results suggest that, in the monkey V1, the orientation selectivity of cells is largely dependent on the orientation-biased excitatory and inhibitory inputs which have a broader tuning profile, covering from the optimal to null-orientation, than that observed in extracellularly recorded responses at the control level.

Animals↗
PubMed2005

Orientation discrimination in human vision: psychophysics and modeling.

We evaluated orientation discrimination thresholds using an external noise paradigm. Stimuli were spatiotemporal Gaussian patches of 2D orientation noise band-pass filtered in Fourier domain. Orientation acuity was measured for various combinations of stimulus spatial bandwidth, spatial frequency, and size as a function of orientation bandwidths of the stimuli. Stimulus contrast was matched in multiples of detection threshold. Consistent with the idea that stimulus orientation bandwidth acts as a source of external noise, orientation discrimination thresholds increased monotonically in all conditions with stimulus bandwidth. To interpret these results quantitatively, we first fitted a variance summation model to the data and derived the internal orientation noise, relative sampling efficiency, and orientation tuning of the mechanism underlying orientation discrimination. Due to the equivocal biological nature of these parameters for orientation discrimination, we investigated, with a modeling approach, how neural detectors characterized by a broad orientation tuning may support orientation discrimination. We demonstrated using the ideal-observer theory that while linear models, based on either unimodal filtering or center-surround opponency, predict the monotonic relationship between orientation discrimination threshold and orientation noise, a nonlinear model incorporating a broadband divisive inhibition in the orientation domain is a better candidate due to its contrast invariance. This model, using broad and similar orientation tuning for its excitatory and inhibitory inputs, accounts for the acute orientation acuity of human vision and proves to be robust despite the variance found in natural stimuli.

Discrimination, Psychological↗
PubMedDate not supplied

Orientation-specific relationship between populations of excitatory and inhibitory lateral connections in the visual cortex of the cat.

The topography of lateral excitatory and lateral inhibitory connections was studied in relation to orientation maps obtained in areas 17 and 18. Small iontophoretic injections of biocytin were delivered to the superficial layers in regions where orientation selectivity had been mapped using electrode recordings of single- and multi-unit activity from various cortical depths. Biocytin revealed extensive patchy axonal projections of up to 3.5 mm in both areas while labelled somata occurred chiefly at the injection site, indicating that the labelling was primarily anterograde. Two types of boutons could be clearly distinguished: (i) putative excitatory boutons either en passant or having a short stalk and (ii) inhibitory boutons which were invariably of the basket-type. Three-dimensional reconstructions of all labelled boutons showed that the excitatory and the inhibitory networks had a distinctively different relationship to orientation maps. The overall distribution of connections showed that 53-59% of excitatory and 46-48% of inhibitory connections were at iso-orientation, +/-30 degrees; oblique-orientation, +/-(30-60) degrees, was shown by 30% of excitatory and 28-39% of inhibitory connections; cross-orientation was shown by 11-17% of excitatory and 15-24% of inhibitory connections. Although excitatory patches occupied mainly iso-orientation locations, interpatch regions representing chiefly non-iso-orientations (oblique + cross orientation) were also innervated. There was considerable overlap between the excitatory and inhibitory network. Nonetheless, inhibitory connections were more common than excitatory connections with non-iso-orientation locations. There was no significant difference between the orientation topography of area 17 and area 18 projections. The results suggest that in general the lateral connectivity system is not orientation specific, but shows a moderate iso-orientation preference for excitation and an even weaker iso-orientation preference for inhibition. The broad orientation spectrum of lateral connections could provide the basis for mechanisms that requiring different orientations, as for example in detecting orientation discontinuities.

Animals↗
PubMed1991

Influence of GABA-induced remote inactivation on the orientation tuning of cells in area 18 of feline visual cortex: a comparison with area 17.

We have investigated the effect of iontophoretically applying the inhibitory transmitter gamma-aminobutyric acid (GABA) through four pipettes, each located at a horizontal distance of some 500-600 microns from the recording site, on the orientation tuning of cells in areas 17 and 18 of the cat visual cortex for moving the stationary flash-presented bar stimuli. Forty-five of 74 cells tested in area 18 (61%) showed a significant (greater than 25%) increase in orientation tuning width (at half the maximum response) during GABA application, which reflected an increase in response to non-optimal orientations. The mean orientation tuning width of these cells increased by 79%, and the ratio of responses to the orientation orthogonal to the optimum and to the optimum increased from 0.16 to 0.46. The results were similar to those from area 17, in which 36 of 54 cells (66%) showed significant broadening of orientation tuning during GABA application, with a 90% increase in mean tuning width and an increase in the relative response to the orientation orthogonal to the optimum from 0.17 to 0.42. The distributions of cells in areas 17 and 18 with respect to the magnitude of GABA-induced effects on orientation tuning width were not significantly different (mean increase in tuning width: area 17, 102%; area 18, 87%). Although most cells were tested only with moving bars, comparable effects of remote GABA application on orientation tuning were observed when stationary flash-presented bars were used. Of 11 cells thus tested in area 18, seven showed significantly broader tuning during GABA application, with a 132% increase in mean tuning width. In some 25% of cells in each area which showed a significant effect of GABA application on orientation tuning the response to at least one non-optimal orientation exceeded, during GABA application, the response to the previous optimum. There was essentially no correlation between the changes in orientation tuning and changes in the level of spontaneous activity or in the response to the optimum orientation during GABA application. Thus, an increase in the general excitability of recorded cells or the loss of an unspecific inhibitory input cannot account for the effects of GABA application on orientation tuning. Remote GABA application presumably inactivated cells with different preferred orientations from that of the recorded cell. It is thus argued that the observed broadening of orientation tuning during GABA application reflected the loss of an inhibitory input tuned to non-optimal orientations.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2000

A maximum entropy analysis of protein orientations using fluorescence polarization data from multiple probes.

Techniques have recently become available to label protein subunits with fluorescent probes at predetermined orientation relative to the protein coordinates. The known local orientation enables quantitative interpretation of fluorescence polarization experiments in terms of orientation and motions of the protein within a larger macromolecular assembly. Combining data obtained from probes placed at several distinct orientations relative to the protein structure reveals functionally relevant information about the axial and azimuthal orientation of the labeled protein segment relative to its surroundings. Here we present an analytical method to determine the protein orientational distribution from such data. The method produces the broadest distribution compatible with the data by maximizing its informational entropy. The key advantages of this approach are that no a priori assumptions are required about the shape of the distribution and that a unique, exact fit to the data is obtained. The relative orientations of the probes used for the experiments have great influence on information content of the maximum entropy distribution. Therefore, the choice of probe orientations is crucial. In particular, the probes must access independent aspects of the protein orientation, and two-fold rotational symmetries must be avoided. For a set of probes, a "figure of merit" is proposed, based on the independence among the probe orientations. With simulated fluorescence polarization data, we tested the capacity of maximum entropy analysis to recover specific protein orientational distributions and found that it is capable of recovering orientational distributions with one and two peaks. The similarity between the maximum entropy distribution and the test distribution improves gradually as the number of independent probe orientations increases. As a practical example, ME distributions were determined with experimental data from muscle fibers labeled with bifunctional rhodamine at known orientations with respect to the myosin regulatory light chain (RLC). These distributions show a complex relationship between the axial orientation of the RLC relative to the fiber axis and the azimuthal orientation of the RLC about its own axis. Maximum entropy analysis reveals limitations in available experimental data and supports the design of further probe angles to resolve details of the orientational distribution.

Biophysical Phenomena↗
PubMed2005

Control of hand orientation and arm movement during reach and grasp.

We studied the coordination of arm and wrist motion in a task requiring fine control of hand orientation. Subjects were instructed to reach and grasp one of two targets positioned in the frontal plane at various orientations. The task was performed under three target conditions: fixed orientation, predictably perturbed orientation, and randomly perturbed orientation. For fixed target orientations, the hand began to rotate to the required orientation from the beginning of the reach. Hand peak supination angles scaled linearly with target orientations. The trajectories of hand/arm joint angles also had a one-to-one relationship with different target orientations. These demonstrate that target orientation is a constraint on the hand/arm final orientation, a control variable to be specified in advance by the central nervous system (CNS). Under perturbation conditions, subjects were still able to complete the task smoothly. In the early trials of the predictable perturbation, the hand rotated first to the original target orientation and then corrected for the final target orientation. Initial corrections occurred about 200 ms after the onset of perturbation. This latency decreased as the subjects adapted to the perturbation, and the hand orientation trajectory shifted to match the unperturbed trajectory for the final orientation. By contrast, we observed no clear changes in orientation trajectory under the randomly perturbed conditions. These suggest that feedback control is important to the execution of the movement, but that the CNS tends to optimize feedforward planning rather than feedback correction when the disturbance information is predictable.

Adult↗
PubMed2001

Responses of macaque V1 neurons to binocular orientation differences.

Interocular differences in orientation occur during binocular viewing of a surface slanted in depth. These orientation disparities could be exploited by the visual system to provide information about surface slant, but gradients of positional disparity provide an equally effective means to the same end. We examined the encoding of orientation disparities in V1 neurons that were recorded from two awake fixating monkeys. Monocular orientation selectivity was measured separately in each eye. Although the preferred monocular orientation in the left and right eyes was highly correlated (r = 0.98), 19 of 61 cells showed a significant interocular difference in preferred orientation (IDPO). By itself, an IDPO does not imply a specific binocular selectivity for orientation differences. We therefore examined the response to 25 binocular combinations of orientations by pairing each of five orientations in one eye with five in the other. Forty-four of 64 neurons showed responses that reflected the monocular orientation tuning selectivity; the preferred orientation disparity changed when the monocular orientation was changed in either eye. The remaining third (20 of 64) responded to a consistent orientation disparity in a way that was not simply predictable from monocular orientation selectivity. However, nearly all of these neurons were selective for positional disparity, and several characteristics of the responses suggest that the apparent selectivity for orientation disparities was just a consequence of the positional disparity sensitivity. Neither the data presented here nor previous data from the cat (Blakemore et al., 1972; Nelson et al., 1977) support the idea that a population of neurons early in the visual system has a separate encoding scheme for orientation disparities.

Action Potentials↗
PubMed1997

The computation of orientation statistics from visual texture.

This paper examines how observers estimate the overall orientation of spatially disorganised textures containing variable orientation. Experiments used asymmetrical distributions of orientations to separate the predictions from different models of average orientation estimation. Stimuli were composed of two spatially intermingled sets of oriented patches, each set having Gaussian distributed element orientation. The threshold separation of the means of the two sets was determined for a variety of tasks. Discrimination of these textures from a reference composed of two sets with the same mean orientation was well predicted by discrimination of orientation variability. A single interval judgement of which set contained more elements required a greater separation of the set orientations and suggested that the sets must be resolved in the orientation domain for independent representation of their properties. That resolution is required to perform this task further suggests that orientational skew is not coded. Threshold offsets for judgement of average orientation were re-expressed as shifts of four candidate features for coding the central tendency of texel orientations. Comparison with similar thresholds for single distributions of orientations indicated that average orientation is assigned to the centroid of a set of orientation measures.

Discrimination, Psychological↗
PubMed2004

Cross-orientation summation in texture segregation.

Human texture vision has been modeled as a filter-rectify-filter (FRF) process, in which '2nd-order' filters detect changes in the rectified outputs of luminance-based '1st-order' filters. This study tested the validity of the two basic assumptions of the standard FRF model, namely (a) that the 2nd-order filters are sensitive to spatial modulations in both contrast and orientation, and (b) that the 2nd-order filters are tuned to different 1st-order orientations. In the first experiment, we tested subthreshold summation between two orthogonal carrier orientations in detection of a texture region, which was defined by contrast modulations across regions in the two carrier orientations, while systematically varying the relative change magnitudes between the two orientations. The results showed that the detection thresholds were determined by spatial difference in the contrast integrated over the two orientations. Orientation difference did act as a segregation cue, but only when there was no differences in carrier contrast. This suggests that two mechanisms are involved in texture segregation; one that detects changes in luminance contrast and another that detects changes in orientation. To further analyze the latter mechanism, a second experiment measured cross-orientation summation in the detection of purely orientation-defined textures, using stimuli that were density modulations of two orientations presented among randomly-orientated distractors. Again, the relative modulation magnitudes between the two orientations was systematically varied. The results are consistent with the notions that (a) the dominant orientation is extracted from the 1st-order outputs before the 2nd-order process, and that (b) the 2nd-order, spatial comparison process integrates those dominant signals over different orientations.

Field Dependence-Independence↗
PubMed2001

Temporal resolution of orientation-based texture segregation.

We analysed the temporal-frequency characteristics of two functional processes involved in orientation-based texture segregation: local orientation coding and subsequent orientation-contrast coding. Two texture images, in which each micropattern was rotated by 90 degrees, were alternated at various temporal frequencies. A micropattern was a second-derivative (D2) of a Gaussian that loses orientation information when temporally fused with the orthogonal D2 pattern. We measured the upper temporal-frequency limits for localising the target region whose mean orientation differed from the background by 90 degrees or by 45 degrees. If the temporal limit of the texture perception is determined by the most sluggish processing stage, the temporal limit for the 90 degrees texture should be determined by local orientation coding or by orientation-contrast coding, depending on which stage has the lower temporal precision. On the other hand, the 45 degrees texture should always be segregated below the temporal limit of local orientation coding regardless of the temporal limit of orientation-contrast coding. We found that the temporal limit for the 90 degrees texture was slightly higher than that for the 45 degrees texture under spatial conditions appropriate for texture segregation. Moreover, an orientation-noise analysis of segregation performance for a wide range of temporal frequencies revealed that the temporal-frequency sensitivities for the two textures were nearly identical. These results imply that the temporal limit for orientation-based texture segregation depends only on that of local orientation coding. This conclusion further suggests that the potential temporal resolution of orientation-contrast coding is not lower than that of local orientation coding, which would imply that the orientation-contrast coding is unlikely to be mediated by sluggish neural processes.

Contrast Sensitivity↗
PubMed2001

Neural model for processing the influence of visual orientation on visually perceived eye level (VPEL).

An individual line or a combination of lines viewed in darkness has a large influence on the elevation to which an observer sets a target so that it is perceived to lie at eye level (VPEL). These influences are systematically related to the orientation of pitched-from-vertical lines on pitched plane(s) and to the lengths of the lines, as well as to the orientations of lines of 'equivalent pitch' that lie on frontoparallel planes. A three-stage model processes the visual influence: The first stage parallel processes the orientations of the lines utilizing 2 classes of orientation-sensitive neural units in each hemisphere, with the two classes sensitive to opposing ranges of orientations; the signal delivered by each class is of opposite sign in the two hemispheres. The second stage generates the total visual influence from the parallel combination of inputs delivered by the 4 groups of the first stage, and a third stage combines the total visual influence from the second stage with signals from the body-referenced mechanism that contains information about the position and orientation of the eyes, head, and body. The circuit equation describing the combined influence of n separate inputs from stage 1 on the output of the stage 2 integrating neuron is derived for n stimulus lines which possess any combination of orientations and lengths; Each of the n lines is assumed to stimulate one of the groups of orientation-sensitive units in visual cortex (stage 1) whose signals converge on to a dendrite of the integrating neuron (stage 2), and to produce changes in postsynaptic membrane conductance (g(i)) and potential (V(i)) there. The net current from the n dendrites results in a voltage change (V(A)) at the initial segment of the axon of the integrating neuron. Nerve impulse frequency proportional to this voltage change signals the total visual influence on perceived elevation of the visual field. The circuit equation corresponding to the total visual influence for n equal length inducing lines is V(A)= sum V(i)/[n+(g(A)/g(S))], where the potential change due to line i, V(i), is proportional to line orientation, g(A) is the conductance at the axon's summing point, and g(S)=g(i) for each i for the equal length case; the net conductance change due to a line is proportional to the line's length. The circuit equation is interpreted as a basis for quantitative predictions from the model that can be compared to psychophysical measurements of the elevation of VPEL. The interpretation provides the predicted relation for the visual influence on VPEL, V, by n inducing lines each with length l: thus, V=a+[k(i) sum theta(i)/n+(k(2)/l)], where theta(i) is the orientation of line i, a is the effect of the body-referenced mechanism, and k(1) and k(2) are constants. The model's output is fitted to the results of five sets of experiments in which the elevation of VPEL measured with a small target in the median plane is systematically influenced by distantly located 1-line or 2-line inducing stimuli varying in orientation and length and viewed in otherwise total darkness with gaze restricted to the median plane; each line is located at either 25 degrees eccentricity to the left or right of the median plane. The model predicts the negatively accelerated growth of VPEL with line length for each orientation and the change of slope constant of the linear combination rule among lines from 1.00 (linear summation; short lines) to 0.61 (near-averaging; long lines). Fits to the data are obtained over a range of orientations from -30 degrees to +30 degrees of pitch for 1-line visual fields from lengths of 3 degrees to 64 degrees, for parallel 2-line visual fields over the same range of lengths and orientations, for short and long 2-line combinations in which each of the two members may have any orientation (parallel or nonparallel pairs), and for the well-illuminated and fully structured pitchroom. In addition, similar experiments with 2-line stimuli of equivalent pitch in the frontoparallel plane were also fitted to the model. The model accounts for more than 98% of the variance of the results in each case.

Humans↗