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PubMed1997

Predictions of a recurrent model of orientation selectivity.

Recurrent models of orientation selectivity in the visual cortex postulate that an initially broad tuning given by the pattern of geniculate afferents is substantially sharpened by intracortical feedback. We show that these models can be tested on the basis of their predicted responses to certain visual stimuli, without the need for pharmacological or physiological manipulations. First, we consider a detailed recurrent model proposed by Somers, Nelson and Sur [(1995) Journal of Neuroscience, 15, 5448-5465] and show that it can be simplified to a single equation: a center-surround feedback filter in the orientation domain. Then, we explore the responses of the simplified model to stimuli containing two or more orientations. We find that the model exhibits peculiar responses to stimuli containing two orientations, such as plaids or crosses: if the component orientations differ by less than 45 deg the model cannot distinguish between them; if the orientations differ by more than 45 deg the model overestimates their angle by as much as 30 deg. Moreover, the model cannot signal the presence of three orientations separated by 60 deg (it responds as if there were only two orientations), and the addition of two-dimensional visual noise to an oriented stimulus results in strong spurious responses at the orthogonal orientation. We argue that the effects of attraction and repulsion between orientations and the emergence of responses at off-optimal orientations are common to a wide class of feedback models of orientation selectivity. These models could thus be tested by measuring the visual responses of cortical neurons to stimuli containing multiple orientations.

Feedback↗
PubMed1990

Orientation bias of neurons in the lateral geniculate nucleus of macaque monkeys.

The purpose of this investigation was to analyze the influence of stimulus orientation on the responses of individual neurons in the monkey's lateral geniculate nucleus (LGN). Our specific goals were to assess the prevalence and the degree of orientation tuning in the monkey LGN and to determine if the preferred stimulus orientations of LGN neurons varied as a function of receptive-field position. The primary motivation for this research was to gain insight into the receptive-field configuration of LGN neurons and consequently into the neural mechanisms which determine the spatial organization of LGN receptive fields in primates. In both the parvocellular and magnocellular layers, the responses of the majority of individual neurons to sine-wave gratings varied as a function of stimulus orientation. The influence of stimulus orientation was, however, highly dependent on the spatial characteristics of the stimulus; the greatest degree of orientation bias was observed for spatial frequencies higher than the cell's optimal spatial frequency. On a population basis, the degree of orientation bias was similar for all major classes of LGN neurons (e.g. ON vs. OFF center; parvocellular vs. magnocellular) and did not vary systematically with receptive-field eccentricity. At a given receptive-field location, LGN neurons, particularly cells in the parvocellular laminae, tended to prefer either radially oriented stimuli or stimuli oriented more horizontally than their polar axis. Our analyses of the orientation-dependent changes in spatial-frequency response functions, which was based on the Soodak et al., (1987; Soodak, 1986) two-dimensional, difference-of-Gaussian receptive-field model, suggested that the orientation bias in LGN neurons was due to an elongation of the receptive-field center mechanism which in some cases appeared to consist of multiple subunits. Direct comparisons of the orientation-tuning characteristics of LGN cells and their retinal inputs (S potentials) indicated that the orientation bias in the monkey LGN reflects primarily the functional properties of individual retinal ganglion cells. We conclude that orientation sensitivity is a significant property of subcortical neurons in the primate's geniculo-cortical pathway.

Action Potentials↗
PubMed2001

Factors determining the orientation of axially coordinated imidazoles in heme proteins.

Factors determining conformations of imidazole axially coordinated to heme in heme proteins were investigated by analyzing 693 hemes in 432 different crystal structures of heme proteins from the Protein Data Bank (PDB), where at least one histidine is ligated to heme. The results from a search of the PDB for protein structures were interpreted with molecular force field computations. Analysis of data from these crystal structures indicated that there are two main factors that determine the orientations of imidazole ligated to heme. These are the interactions of imidazole with the propionic acid side chains of heme and with the histidine backbone. From the analysis of the crystal structures of heme proteins, it turned out that the hydrogen bonding pattern is often not decisive, though it is probably used by nature to fine-tune the orientation of imidazole axially ligated to heme. We found that in many heme proteins the NdeltaH group of imidazole ligated to heme can assume a number of different hydrogen bonds and that in mutant structures the orientation of the ligated imidazole often does not change significantly, although the mutant altered the hydrogen bonding scheme involving the imidazole. Data from crystal structures of heme proteins show that there are preferred orientations of imidazoles with respect to heme. Generally, the NdeltaH group of imidazole is oriented toward the propionic acid groups of the heme. In some cases, the NdeltaH group of imidazole is close to only one of the propionic acid groups, but it is practically never oriented in the opposite direction. The imidazole also adopts a preferred orientation with respect to its histidine backbone such that the plane of the imidazole ring is practically never parallel to the Calpha-Cbeta bond of its histidine backbone. For a given conformation of histidine backbone with respect to heme, as well as imidazole with respect to histidine backbone, the orientation of the imidazole with respect to heme is uniquely determined, since the three orientations depend on each other. Hence, the interaction of the imidazole with the backbone also influences the orientation of the imidazole with respect to the heme. Force field computations are in agreement with experimental data. With this method, we showed that there is an energy minimum when the NdeltaH group of the imidazole is oriented toward the propionic acid groups and that there are minima of energy for orientations where the imidazole ring is orthogonal to the plane defined by the Calpha-Cbeta and Cbeta-Cgamma bonds of the histidine. The computations also demonstrated that these interactions are mainly of electrostatic origin. By taking into account these two major factors, we were able to understand the orientations of axially coordinated imidazoles for all groups of heme proteins, except for the group of cytochrome c peroxidase. In this group, the orientation of the imidazole is determined by a strong hydrogen bond of the NdeltaH group with Asp235.

Crystallography, X-Ray↗
PubMed1996

Determination of the orientation of a band 3 affinity spin-label relative to the membrane normal axis of the human erythrocyte.

The orientation of the nitroxide moiety of an isotopically substituted spin-labeled derivative of dihydrostilbenedisulfonate ([15N,2H13]-SL-H2DADS-maleimide) covalently coupled at the extracellular stilbenedisulfonate binding site of the human erythrocyte anion exchange protein, band 3, has been determined relative to the membrane normal axis of intact cells. The X-band linear electron paramagnetic resonance (EPR) spectra of [15N,2H13]-SL-H2DADS-maleimide-labeled band 3 in intact erythrocytes oriented by flow through an EPR flat cell have been obtained for two orthogonal orientations of the sample in the DC magnetic field. Two different methods of analysis have provided very similar values for the angles alpha 1 and beta 1 which uniquely define the orientation of the nitroxide axis frame relative to the membrane normal axis. In the first approach, a variable fraction of the cells, f, were taken to be biconcave disks perfectly oriented relative to the flat cell surface with the remainder, 1-f, isotropically oriented. Simultaneous nonlinear least squares analysis of the spectra obtained at the two sample orientations yielded best fit values of f = 0.60, alpha 1 = 58 degrees, and beta 1 = 36 degrees. In the second approach, the EPR spectra of flow-oriented intact erythrocytes labeled with the fatty acid spin-label, [15N,2H12]-5-nitroxyl stearate, have been obtained at the two sample orientations. These two spectra have been used to determine a model-independent distribution of membrane normal orientations in the sample. Using this experimentally determined membrane normal orientation distribution, the EPR spectra of [15N,2H13]-SL-H2DADS-maleimide-labeled erythrocytes were then reanalyzed to obtain a second determination of the nitroxide orientation, alpha 1 = 61 degrees and beta 1 = 37 degrees. The orientation of the nitroxide with respect to the membrane normal axis determined in the present study is nearly identical to the orientation of the nitroxide with respect to the uniaxial rotational diffusion axis, alpha = 66 degrees and beta = 34 degrees, as determined from saturation transfer EPR (ST-EPR) studies [Hustedt, E. H., & Beth, A. H. (1995) Biophys. J. 69, 1409-1423]. This result supports the conclusion that the motion observed using ST-EPR spectroscopy is, in fact, the uniaxial rotational diffusion of band 3 about the membrane normal.

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗
PubMed2006

Enzymatic, alkaline, and autocatalytic degradation of poly(L-lactic acid): effects of biaxial orientation.

The hydrolytic degradation of biaxially oriented and de-oriented (melt-crystallized) poly(l-lactic acid) (PLLA) films was investigated in Tris-HCl-buffered solution (pH 8.6) with proteinase K, alkaline solution, and phosphate-buffered solution (pH 7.4) by the use of gravimetry, gel permeation chromatography, differential scanning calorimetry, and scanning electron microscopy. Biaxial orientation disturbed the proteinase K-catalyzed enzymatic degradation of PLLA films and the effects of biaxial orientation overcame those of crystallinity. The former may be due to the fact the enzyme cannot attach to the extended (strained) chains in the amorphous regions of the biaxially oriented PLLA film or cannot catalyze the cleavage of the strained chains. Another probable cause is that the enzyme can act only at the film surface of the biaxially oriented PLLA film, in marked contrast with the case of the de-oriented PLLA films where enzymatic degradation can proceed beneath the spherulitic crystalline residues. The effects of biaxial orientation on the alkaline and autocatalytic degradation of the PLLA films were insignificant for the periods studied here. The crystallinity rather than the biaxial orientation seems to determine the alkaline and autocatalytic degradation rates of the PLLA films. The accumulation of crystalline residues formed as a result of selective cleavage and removal of the amorphous chains was observed for the de-oriented PLLA films, but not for the biaxially oriented PLLA film, when degraded in the presence of proteinase K. This means the facile release of formed crystalline residues from the surface of the biaxially oriented PLLA film during enzymatic degradation, due to the fact that the crystalline regions of the biaxially oriented PLLA film were oriented with their c axis parallel to the film surface.

Alkalies↗
PubMed2006

Beam orientation optimization for intensity-modulated radiation therapy using mixed integer programming.

The purpose of this study is to extend an algorithm proposed for beam orientation optimization in classical conformal radiotherapy to intensity-modulated radiation therapy (IMRT) and to evaluate the algorithm's performance in IMRT scenarios. In addition, the effect of the candidate pool of beam orientations, in terms of beam orientation resolution and starting orientation, on the optimized beam configuration, plan quality and optimization time is also explored. The algorithm is based on the technique of mixed integer linear programming in which binary and positive float variables are employed to represent candidates for beam orientation and beamlet weights in beam intensity maps. Both beam orientations and beam intensity maps are simultaneously optimized in the algorithm with a deterministic method. Several different clinical cases were used to test the algorithm and the results show that both target coverage and critical structures sparing were significantly improved for the plans with optimized beam orientations compared to those with equi-spaced beam orientations. The calculation time was less than an hour for the cases with 36 binary variables on a PC with a Pentium IV 2.66 GHz processor. It is also found that decreasing beam orientation resolution to 10 degrees greatly reduced the size of the candidate pool of beam orientations without significant influence on the optimized beam configuration and plan quality, while selecting different starting orientations had large influence. Our study demonstrates that the algorithm can be applied to IMRT scenarios, and better beam orientation configurations can be obtained using this algorithm. Furthermore, the optimization efficiency can be greatly increased through proper selection of beam orientation resolution and starting beam orientation while guaranteeing the optimized beam configurations and plan quality.

Algorithms↗
PubMed1991

A detailed model of the primary visual pathway in the cat: comparison of afferent excitatory and intracortical inhibitory connection schemes for orientation selectivity.

In order to arrive at a quantitative understanding of the dynamics of cortical neuronal networks, we simulated a detailed model of the primary visual pathway of the adult cat. This computer model comprises a 5 degrees x 5 degrees patch of the visual field at a retinal eccentricity of 4.5 degrees and includes 2048 ON- and OFF-center retinal beta-ganglion cells, 8192 geniculate X-cells, and 4096 simple cells in layer IV in area 17. The neurons are implemented as improved integrate-and-fire units. Cortical receptive fields are determined by the pattern of afferent convergence and by inhibitory intracortical connections. Orientation columns are implemented continuously with a realistic receptive field scatter and jitter in the preferred orientations. We first show that realistic ON-OFF-responses, orientation selectivity, velocity low-pass behaviour, null response, and responses to spot stimuli can be obtained with an appropriate alignment of geniculate neurons converging onto the cortical simple cell (Hubel and Wiesel, 1962) and in the absence of intracortical connections. However, the average receptive field elongation (length to width) required to obtain realistic orientation tuning is 4.0, much higher than the average observed elongation. This strongly argues for additional intracortical mechanisms sharpening orientation selectivity. In the second stage, we simulated five different inhibitory intracortical connection patterns (random, local, sparse-local, circular, and cross-orientation) in order to investigate the connection specificity necessary to achieve orientation tuning. Inhibitory connection schemes were superimposed onto Hubel and Wiesel-type receptive fields with an elongation of 1.78. Cross-orientation inhibition gave rise to different horizontal and vertical orientation tuning curves, something not observed experimentally. A combination of two inhibitory schemes, local and circular inhibition (a weak form of cross-orientation inhibition), is in good agreement with observed receptive field properties. The specificity required to establish these connections during development is low. We propose that orientation selectivity is caused by at least three different mechanisms ("eclectic" model): a weak afferent geniculate bias, broadly tuned cross-orientation inhibition, and some iso-orientation inhibition. The most surprising finding is that an isotropic connection scheme, circular inhibition, in which a cell inhibits all of its postsynaptic target cells at a distance of approximately 500 microns, enhances orientation tuning and leads to a significant directional bias. This is caused by the embedding of cortical cells within a columnar structure and does not depend on our specific assumptions.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1984

Relationship between preferred orientation and receptive field position of neurons in extrastriate cortex (area 19) in the cat.

Orientation sensitivity is a characteristic of most retinal ganglion cells (Levick and Thibos, '82), most relay cells in the dorsal lateral geniculate nucleus (Vidyasagar and Urbas, '82), and most neurons in the visual cortex (Hubel and Wiesel, '62) in the cat. In the retina there is a systematic relationship between receptive field position (polar angle) and preferred orientation. Outside of the area centralis most retinal ganglion cells respond best to stimuli oriented radially, i.e., oriented parallel to the line connecting their receptive fields to the area centralis (Levick and Thibos, '82). This relationship is strongest along the horizontal meridian (the visual streak) and appears to reflect the innate, radial orientation of retinal ganglion cell dendritic fields (Leventhal and Schall, '83). A relationship between preferred orientation and polar angle also exists in cat striate cortex; outside of the area centralis representation most cells respond best to lines oriented radially. This relationship is strongest for S-type cells, the most orientation-selective cells, and cells in regions representing the horizontal meridian (Leventhal, '83). To determine if similar relationships exist in cat extrastriate cortex, the preferred orientations and receptive field positions of 226 neurons in area 19 were studied. We find that, as in area 17, most area 19 cells outside of the representation of the area centralis respond best to lines oriented radially; this relationship is strongest for the cells having the narrowest receptive fields and in regions subserving the horizontal meridian. Unlike in striate cortex, in area 19 the relationship between preferred orientation and polar angle is not dependent upon cell type (S or C) or to the degree of orientation sensitivity exhibited. Also, in area 19, but not in area 17, the relationship between preferred orientation and polar angle fails for the cells having the widest receptive fields.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Inactivation of the infragranular striate cortex broadens orientation tuning of supragranular visual neurons in the cat.

Intracortical inhibition is believed to enhance the orientation tuning of striate cortical neurons, but the origin of this inhibition is unclear. To examine the possible influence of ascending inhibitory projections from the infragranular layers of striate cortex on the orientation selectivity of neurons in the supragranular layers, we measured the spatiotemporal response properties of 32 supragranular neurons in the cat before, during, and after neural activity in the infragranular layers beneath the recorded cells was inactivated by iontophoretic administration of GABA. During GABA iontophoresis, the orientation tuning bandwidth of 15 (46.9%) supragranular neurons broadened as a result of increases in response amplitude to stimuli oriented about +/- 20 degrees away from the preferred stimulus angle. The mean (+/- SD) baseline orientation tuning bandwidth (half width at half height) of these neurons was 13.08 +/- 2.3 degrees. Their mean tuning bandwidth during inactivation of the infragranular layers increased to 19.59 +/- 2.54 degrees, an increase of 49.7%. The mean percentage increase in orientation tuning bandwidth of the individual neurons was 47.4%. Four neurons exhibited symmetrical changes in their orientation tuning functions, while 11 neurons displayed asymmetrical changes. The change in form of the orientation tuning functions appeared to depend on the relative vertical alignment of the recorded neuron and the infragranular region of inactivation. Neurons located in close vertical register with the inactivated infragranular tissue exhibited symmetric changes in their orientation tuning functions. The neurons exhibiting asymmetric changes in their orientation tuning functions were located just outside the vertical register. Eight of these 11 neurons also demonstrated a mean shift of 6.67 +/- 5.77 degrees in their preferred stimulus orientation. The magnitude of change in the orientation tuning functions increased as the delivery of GABA was prolonged. Responses returned to normal approximately 30 min after the delivery of GABA was discontinued. We conclude that inhibitory projections from neurons within the infragranular layers of striate cortex in cats can enhance the orientation selectivity of supragranular striate cortical neurons.

Animals↗
PubMed1991

Quantitative study of striate single unit responses in monkeys performing an orientation discrimination task.

Contour orientation discrimination accuracy is determined by the orientation bandwidth, response variance and response strength of single units that code for orientation. We measured the latter three properties for V1 cells of monkeys which were performing an orientation discrimination of the grating stimulating the cell under study. We recorded from 285 cells, of which 76% responded to the grating. The orientation bandwidth, measured as full width at half height of the tuning curve, varied over a wide range amongst cells. The median bandwidth was 41 degrees. The response variance of the cells also varied considerably between cells; on average it was about two times the response strength. We also studied the temporal properties of the responses. Most of our cells had a latency between 40 and 100 ms. The response variance was found to be smaller in the initial phases of the response than at the later response stages. In some cells the orientation tuning varied in successive stages of the response, while in others the orientation bandwidth and preferred orientation remained stable throughout the response. However, all orientation sensitive cells were orientation tuned from the start of the response, a property which contribute to the fast and reliable coding of contour orientation. These results provide for the first time an estimation of the orientation tuning properties of V1 cells during visual orientation discrimination. They will be very useful to compare single cell properties of other areas to as well as in simulation studies of models of primate visual discriminations.

Animals↗
PubMed2005

The oblique effect depends on perceived, rather than physical, orientation and direction.

Observers can better discriminate orientation or direction near the cardinal axes than near an oblique axis. We investigated whether this well-known oblique effect is determined by the physical or the perceived axis of the stimuli. Using the simultaneous tilt illusion, we generated perceptually different orientations for the same inner (target) grating by contrasting it with differently oriented outer gratings. Subjects compared the target orientation with a set of reference orientations. If orientation discriminability was determined by the physical orientations, the psychometric curves for the same target grating would be identical. Instead, all subjects produced steeper curves when perceiving target gratings near vertically as opposed to more obliquely. This result of orientation discrimination was confirmed by using adaptation-generated tilt aftereffect to manipulate the perceived orientation of a given physical orientation. Moreover, we obtained the same result in direction discrimination by using motion repulsion to alter the perceived direction of a given physical direction. We conclude that when the perceived orientation or direction differs from the physical orientation or direction, the oblique effect depends on perceived, rather than physical, orientation or direction. Finally, as a by-product of the study, we found that, around the vertical direction, motion repulsion is much stronger when the inducing direction is more clockwise to the test direction than when it is more counterclockwise.

Discrimination, Psychological↗
PubMed2000

Neuronal representation of object orientation.

The dissociation between object identity and object orientation observed in six patients with brain damage, has been taken as evidence for a view-invariant model of object recognition. However, there was also some indication that these patients were not generally agnosic for object orientation but were able to gain access to at least some information about objects' canonical upright. We studied a new case (KB) with spared knowledge of object identity and impaired perception of object orientation using a forced choice paradigm to contrast directly the patient's ability to perceive objects' canonical upright vs non-upright orientations. We presented 2D-pictures of objects with unambiguous canonical upright orientations in four different orientations (0 degrees, -90 degrees, +90 degrees, 180 degrees ). KB showed no impairment in identifying letters, objects, animals, or faces irrespective of their given orientation. Also, her knowledge of upright orientation of stimuli was perfectly preserved. In sharp contrast, KB was not able to judge the orientation when the stimuli were presented in a non-upright orientation. The findings give further support for a distributed view-based representation of objects in which neurons become tuned to the features present in certain views of an object. Since we see more upright than inverted animals and familiar objects, the statistics of these images leads to a larger number of neurons tuned for objects in an upright orientation. We suppose that probably for this reason KB's knowledge of upright orientation was found to be more robust against neuronal damage than knowledge of other orientations.

Acute Disease↗
PubMed2000

Are there separate first-order and second-order mechanisms for orientation discrimination?

In a series of experiments we compared orientation discrimination performance for Gabor stimuli in which the stimulus profile was either matched to the receptive field profile of single V1 simple cells ('simple'), or in which the carrier and envelope orientations were different ('tigertails'). In the first Experiment, using small, high spatial frequency, peripheral stimuli to minimise the number of detectors involved, we found that simple stimuli were more detectable than tigertails of the same contrast energy, and that orientation discrimination thresholds for simple stimuli were lower than for tigertails of equal detectability. In later experiments with larger stimuli we measured thresholds for detecting tilts of the envelope with the carrier fixed in orientation. Envelope thresholds were similar for different carrier orientations, but carrier orientation had a strong biasing effect upon perceived envelope orientation. When the orientation difference between envelope and carrier was small, the carrier orientation was attracted to that of the envelope; when the difference was large (>10 degrees ) repulsion was found. The biases were reduced by half-wave rectifying the stimuli, putatively making the envelope visible to a first-order filter (Experiment 2). Discrimination thresholds for envelope orientation were higher than those for carrier orientation, and this difference was greater for briefly-presented parafoveal stimuli than for long duration foveal stimuli (Experiments 3 and 4). We conclude from these results that there are separate mechanisms for envelope and carrier orientation discriminations for large stimuli, but that first- and second-order mechanisms are not independent in the discrimination of orientation.

Analysis of Variance↗
PubMedDate not supplied

Linear and nonlinear contributions to orientation tuning of simple cells in the cat's striate cortex.

Orientation selectivity is one of the most conspicuous receptive-field (RF) properties that distinguishes neurons in the striate cortex from those in the lateral geniculate nucleus (LGN). It has been suggested that orientation selectivity arises from an elongated array of feedforward LGN inputs (Hubel & Wiesel, 1962). Others have argued that cortical mechanisms underlie orientation selectivity (e.g. Sillito, 1975; Somers et al., 1995). However, isolation of each mechanism is experimentally difficult and no single study has analyzed both processes simultaneously to address their relative roles. An alternative approach, which we have employed in this study, is to examine the relative contributions of linear and nonlinear mechanisms in sharpening orientation tuning. Since the input stage of simple cells is remarkably linear, the nonlinear contribution can be attributed solely to cortical factors. Therefore, if the nonlinear component is substantial compared to the linear contribution, it can be concluded that cortical factors play a prominent role in sharpening orientation tuning. To obtain the linear contribution, we first measure RF profiles of simple cells in the cat's striate cortex using a binary m-sequence noise stimulus. Then, based on linear spatial summation of the RF profile, we obtain a predicted orientation-tuning curve, which represents the linear contribution. The nonlinear contribution is estimated as the difference between the predicted tuning curve and that measured with drifting sinusoidal gratings. We find that measured tuning curves are generally more sharply tuned for orientation than predicted curves, which indicates that the linear mechanism is not enough to account for the sharpness of orientation-tuning. Therefore, cortical factors must play an important role in sharpening orientation tuning of simple cells. We also examine the relationship of RF shape (subregion aspect ratio) and size (subregion length and width) to orientation-tuning halfwidth. As expected, predicted tuning halfwidths are found to depend strongly on both subregion length and subregion aspect ratio. However, we find that measured tuning halfwidths show only a weak correlation with subregion aspect ratio, and no significant correlation with RF length and width. These results suggest that cortical mechanisms not only serve to sharpen orientation tuning, but also serve to make orientation tuning less dependent on the size and shape of the RF. This ensures that orientation is represented equally well regardless of RF size and shape.

Animals↗
PubMed2005

Influence of path integration versus environmental orientation on place cell remapping between visually identical environments.

To assess the effects of interactions between angular path integration and visual landmarks on the firing of hippocampal neurons, we recorded from CA1 pyramidal cells as rats foraged in two identical boxes with polarizing internal cues. In the same-orientation condition, following an earlier experiment by Skaggs and McNaughton, the boxes were oriented identically and connected by a corridor. In the opposite-orientation condition, the boxes were abutted by rotating them 90 degrees in opposite directions, so that their orientations differed by 180 degrees . After 16-23 days of pretraining on the same-orientation condition, three rats experienced both conditions in counterbalanced order on each of two consecutive days. On the third day they ran two opposite-orientation trials. Although Skaggs and McNaughton observed stable partial "remapping" of place fields, none of the fields in this experiment remapped in the same-orientation condition. In the opposite-orientation condition, place fields in the first box were isomorphic with those in the same-orientation condition, whereas in the second box the rats eventually exhibited completely different fields. The rats differed as to the trial in which this first occurred. Once the second box exhibited different fields, it continued to do so in all subsequent opposite-orientation trials, yet fields remained the same in subsequent same-orientation trials. The results demonstrate that when animals move actively between environments, and are thus potentially able to maintain their inertial angular orientation, discordance between environmental orientation and the rat's idiothetic direction sense can profoundly affect the hippocampal map-either immediately, or as a result of cumulative experience.

Analysis of Variance↗
PubMed1977

Discrimination of orientation and position disparities by binocularly activated neurons in cat straite cortex.

1. We have examined and compared the ability of binocularly activated striate neurons to make both position disparity and orientation disparity discrimination in the anesthetized (N2O/O2) and paralyzed cat preparation. 2. Accurate knowledge of eye position is essential for disparity studies. Using a retinal projection technique able to detect eye drifts of less than 3' arc per retinal landmark and less than 18' arc cyclorotation disparity, we determined eye drift during the course of 2- to 4-day experiments. After the initial eye rotation due to the anesthesia and the onset of paralysis (see below), rotational drift thereafter was mainly excyclorotatory and, from all causes, rarely totaled more than 4 degrees disparity. All our data have been corrected for this residual cyclorotatory drift. 3. Optimal stimulus orientation disparities were determined from quantitative monocular orientation tuning curves for 74 binocularly activated striate cells (37 simple, 3 hypercomplex I, 31 complex, 3 hypercomplex II) from nine cats. Without exception, the mean optimal stimulus orientation disparity in each of our animals showed a departure from zero disparity equivalent to an incyclorotation of the eyes (mean, 9.2 degrees; range, 2.7 degrees-15.9 degrees). 4. We attribute this mean optimal stimulus orientation disparity shift to ocular cyclorotation as a result of the initial anesthesia and paralysis. Assuming equal intortion, incyclorotation for each eye averages 4.6 degrees. On the assumption that the mean optimal stimulus orientation disparity is zero in normal life, we pooled results from the nine animals about their individual means. For the 74 cells the resulting distribution of the optimal stimulus orientation disparities had a range of about +/-15 degrees (simple cells: SD 4.9 degrees; complex cells: SD 7.4 degrees). 5. We examined the relationship of the sharpness of the orientation tuning curves to ocular dominance, to absolute orientation preference, and to other unit properties. The striking observation was the high correlation between the sharpness of orientation tuning curves for the two eyes of a binocular neuron. For simple cells the mean difference for the half-widths of half-height was only 2.54 degrees, with sharpness showing a high correlation between the two eyes (r=0.915) over half-width at half-heights ranging from 8.5 degrees to 41.8 degrees. Complex cells showed a similar, albeit weaker, correlation. 6. Having shown that, assessed monocularly binocular units show different orientation tunings in the two eyes, we undertook binocular experiments to ascertain if these differences were the optimal disparities of sharply tuned stimulus orientation disparity channels. Using a matrix stimulation paradigm to minimize the effects of spontaneous changes in responsiveness, we have simultaneously extracted bionocular stimulus orientation disparity and position disparity tuning curves from single striate neurons...

Animals↗
PubMed1987

Surface organization of orientation and direction selectivity in cat area 18.

Two-dimensional maps of orientation and direction preference were made in area 18 of the cat's visual cortex using multiple electrode penetrations 150-300 micron apart. The maps were then analyzed by autocorrelation and Fourier transformation. The power spectrum of the orientation map was sharply peaked below the theoretical cutoff frequency for the sampling function (the pattern of electrode penetrations) used to obtain the map. This suggested that it would be possible to interpolate orientation values between the sample points in the cortex. This was done and the resulting fine-grain maps of orientation preference were studied. Empirical testing showed that the interpolated orientation values were accurate to within +/- 30 degrees. Study of the fine-grain maps showed that iso-orientation domains for broad orientation ranges (0 degrees-90 degrees) were branching bands running from an anterolateral to a posteromedial direction across the surface of the cortex with a periodicity of 1.25 +/- 0.13 mm. Domains for smaller orientation ranges (0 degrees-30 degrees) were periodically spaced, but somewhat irregularly shaped patches. The orientation maps contained numerous point singularities where orientation changed discontinuously. These were spaced about 750 micron apart and most were surrounded by a single 180 degrees cycle of orientations. Autocorrelation analysis of the maps of preferred direction revealed local clustering that extended over a distance of 250-300 micron, but spectral analysis failed to reveal any evidence of periodicity. The absence of periodicity was probably due to a relatively large number of 180 degrees reversals in the map of direction preference, which do not affect the continuity of the orientation map and are not sufficiently numerous to destroy local continuity in the direction map. It is shown that the direction map, if it is to be as continuous as possible, must nevertheless contain lines across which direction preference reverses by 180 degrees. These lines run from one orientation singularity to another. Evidence that the direction map approaches this degree of continuity is presented.

Action Potentials↗
PubMedDate not supplied

Orientation of DNA and the agarose gel matrix in pulsed electric fields.

Transient electric birefringence has been used as an analytical tool to study the orientation of DNA in agarose gels, and to study the orientation of the matrix alone. The sign of the birefringence of DNA oriented in an agarose gel is negative, as observed in free solution, indicating that the DNA molecules orient parallel to the direction of the electric field. If the median pore diameter of the gel is larger than the contour length of the DNA molecule, the DNA effectively does not see the matrix and the birefringence relaxation time is the same as observed in free solution. However, if the median pore diameter of the gel is smaller than the contour length of the DNA, the DNA molecule becomes stretched as well as oriented. For DNA molecules of moderate size (less than or equal to 4 kb), stretching in the gel causes the birefringence relaxation times to increase to the values expected for fully stretched molecules. Complete stretching is not observed for larger DNA molecules. The orientation and stretching of DNA molecules in the gel matrix indicates that end-on migration, or reptation, is a likely mechanism for DNA electrophoresis in agarose gels. When the electric field is rapidly reversed in polarity, very little change in the orientation of the DNA is observed if the DNA molecules were completely stretched and had reached their equilibrium orientation before the field was reversed in direction. Hence completely stretched, oriented DNA molecules are able to reverse their direction of migration in the electric field with little or no loss of orientation. However, if the DNA molecules were not completely stretched or if the equilibrium orientation had not been reached, substantial disorientation of the DNA molecules is observed at field reversal. The forced rate of disorientation in the reversing field is faster than the field-free rate of disorientation. Complicated patterns of reorientation can be observed after field reversal, depending on the degree of orientation in the original field direction. The effect of pulsed electric fields on the orientation of the agarose gel matrix itself was also investigated.(ABSTRACT TRUNCATED AT 400 WORDS)

Bacteriophage lambda↗