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PubMed1990

Detection of an orientation singularity in Gabor textures: effect of signal density and spatial-frequency.

This work presents evidence for a second stage of spatial filtering in early vision. This second stage operates on the output of the well known linear spatial filters and integrates their thresholded responses with a center-surround weighting function. The evidence for the existence of this second stage comes from experiments where observers have to detect a Gabor signal with known parameters (target) among a varied number of other Gabor signals having orthogonal orientation (distractors). Detection performance on this task depends on the number of distractors and the distance between them: when the number of distractors is small performance deteriorates with an increasing number of distractors; however, when the number of distractors becomes larger performance improves with an increasing number of distractors. This improvement depends on the spatial-frequency of the signals and their spatial separation. Best performance is achieved when the spatial separation between signals is larger than three times their center wavelength but smaller than nine times their wavelength, implying a second stage filtering with a center size of six wavelengths and a total size of 18 wavelengths. This second stage of filtering may underlie our ability to detect certain texture boundaries preatentively.

Discrimination, Psychological↗
PubMed1990

Consequences of spatial sampling for human motion perception.

This paper describes evidence for spatial aliasing in human motion perception. For a certain range of spatial frequencies, interference fringes drifting across the extrafoveal retina resemble two-dimensional spatial noise drifting in the opposite direction. For retinal locations within 10 deg of the fovea, the perceived direction of motion is veridical up to spatial frequencies near the cone Nyquist frequency, reverses between one and two times the cone Nyquist frequency, and sometimes reverses back to the correct direction at still higher frequencies. Thus two "motion nulls", or spatial frequencies at which the direction of motion is ambiguous, are typically observed at each retinal eccentricity. A computational model is described in which sinusoidal gratings are sampled by a cone mosaic and the direction of motion of the filtered output is computed. The model predicts that the second motion null, but not the first, should be relatively immune to postreceptoral processing and should roughly equal twice the cone Nyquist frequency. This prediction is confirmed by psychophysical experiments, providing a new technique to estimate cone spacing in the living human eye.

Discrimination, Psychological↗
PubMed1991

Spatial contrast sensitivity of goldfish: mean luminance, temporal frequency and a new psychophysical technique.

Behavioral contrast sensitivity in goldfish was examined at various mean luminances and stimulus drift rates. Goldfish were classically conditioned to suppress respiration upon presentation of a drifting sinusoidal grating. Contrast threshold at each spatial frequency was determined by means of a new two-alternative forced-choice procedure in which the observer's decision about the presence of the stimulus was based on the animal's respiration pattern. The results show that: (1) as mean luminance decreases, contrast sensitivity to high spatial frequencies decreases and peak sensitivity shifts to lower spatial frequencies; (2) as drift rate increases, contrast sensitivity to low spatial frequencies increases, but sensitivity to high spatial frequencies is relatively unaffected by stimulus drift rate. Both the mean luminance and temporal frequency of the stimulus clearly influence the behavioral contrast sensitivity of the goldfish in ways that would be predicted from behavioral results from other species. We conclude that the mechanisms that mediate contrast sensitivity in goldfish are similar to those that mediate contrast sensitivity in other vertebrates.

Animals↗
PubMed1992

Moving two-dimensional patterns can capture the perceived directions of lower or higher spatial frequency gratings.

Coherent plaid motion is produced by superimposing two one-dimensional gratings of the same spatial frequency moving +/- 60 degrees from the intersection-of-constraints (IOC) resultant direction. These moving plaids were found to change the perceived direction of a third one-dimensional grating, either 6-fold lower or higher in spatial frequency, from traveling in one of the plaid's component direction to the IOC resultant direction. We describe this phenomenon as coherence capture. Coherence capture was found to be effective between plaids with 0.5, 1.0, and 1.5 c/deg components and gratings of 3.0, 6.0 and 9.0 c/deg respectively. It was also found to be effective between plaids with 3.0 c/deg components and gratings of 0.5 c/deg. However, coherence capture between higher spatial frequency plaids and lower spatial frequency gratings became less effective when the component spatial frequencies of the plaid increased.

Humans↗
PubMed1992

Spatial frequency mechanisms with short-wavelength-sensitive cone inputs.

We have estimated the minimum number and frequency tuning of spatial mechanisms with Short-Wavelength-Sensitive (SWS)-cone inputs. This was accomplished by isolating SWS cones with intense, long-wavelength (yellow) adaptation, and measuring threshold elevation functions for short-wavelength, spatially localized test stimuli masked by obliquely oriented, short-wavelength, cosine gratings. Peak spatial frequency of the cosine gratings and test stimuli varied from 0.25 to 2.83 cpd in 0.5 octave steps. Results derived in this manner demonstrate that SWS cones input to at least two orientation selective mechanisms with peak spatial frequencies of approx. 0.7 and 1.4 cpd, respectively. The frequency tuning of the isolated-SWS-cone, spatial frequency mechanisms resemble closely the lowest two mechanisms measured with luminance modulation (i.e. normal viewing conditions where all cones contribute to the response).

Contrast Sensitivity↗
PubMed1993

The effects of grating area and spatial frequency on contrast sensitivity as a function of light level.

Contrast sensitivity was measured as a function of retinal illuminance (I) for vertical cosine gratings of various circular areas (A) and spatial frequencies (f < or = 4 c/deg). Spatial frequency and grating diameter varied in inverse proportion to each other in order to keep the relative grating area (A x f2) constant at either 3.14, 12.6, 50.3, or 201 square cycles. At all grating areas and spatial frequencies contrast sensitivity in dim light first increased in proportion to the square root of retinal illuminance. Then the increase saturated and contrast sensitivity became independent of luminance level in bright light. For gratings with constant relative area contrast sensitivity functions were similar in shape and had the same maximum sensitivity but were shifted horizontally towards lower illuminances with decreasing spatial frequency. However, when replotted as a function of retinal illuminance divided by spatial frequency squared, contrast sensitivity functions fell on a common curve at all levels of relative retinal illuminance (I/f2).

Adult↗
PubMed1993

Spatial and chromatic interactions in the human pattern electroretinogram.

The spectral sensitivity and the spatial selectivity was studied both psychophysically and electroretinographically using the pattern onset-offset paradigm. All measurements were made under intensive yellow adaptation. The spectral sensitivity functions of both measures were in close agreement. They showed a peak at 460 nm (blue-sensitive mechanism) and a shoulder around 550 nm (red-green-sensitive mechanism). The luminance curves of the pattern onset ERG obtained with long wavelengths had a steeper slope and reached larger amplitudes than those obtained with short wavelengths. In addition the response-peak times were longer with short wavelengths. When the spatial frequency of the pattern was varied the 460 nm-onset responses showed very little or no spatial tuning and long peak times (around 60 msec). This was ascribed to the contribution from only one type of ganglion cell, namely the blue-yellow opponent receptive fields lacking a center-surround organization. The 550 nm-onset responses showed a clear spatial tuning (4 c/deg) and an increase in peak time (40-50 msec) with increasing spatial frequency (0.26-9.2 c/deg). This was ascribed to different types of receptive fields having a center-surround structure.

Adaptation, Ocular↗
PubMed1993

Effects of luminance and exposure time on contrast sensitivity in spatial noise.

Using gratings with and without spatial noise we measured r.m.s. contrast sensitivity as a function of (i) retinal illuminance, (ii) amount of light added onto the screen, and (iii) exposure time. Our experiments showed that contrast sensitivity in external spatial noise was independent of decreasing retinal illuminance and exposure time as long as contrast sensitivity was lower with noise than without. Thereafter the decrease of contrast sensitivity was identical with and without noise. Although contrast sensitivity without external spatial noise was independent of the amount of added light, contrast sensitivity in external spatial noise was found to increase with the amount of added light until it reached the sensitivity measured without noise. Thereafter contrast sensitivities with and without noise were identical. Our results (i) provide experimental evidence for the validity of the generally accepted hypothesis that in the human contrast detection mechanism signal-to-noise ratio is constant at threshold and (ii) indicate that external spatial noise is the principal source of noise when it reduces contrast sensitivity.

Adult↗
PubMed1994

The spatial properties of binocular suppression zone.

Spatial properties of binocular suppression zones were studied with one-dimensional difference of Gaussian (DOG) patterns. Two parallel DOG patterns were presented in one eye and a single DOG, perpendicular to the double DOG, was presented to the other eye. When viewed together, the probability of seeing the portion of the single DOG in between of the double DOG depended on the separation of the double DOG stimulus. Threshold separations for detecting an intermediate segment of the single DOG pattern (binocular suppression zones) were measured as a function of spatial frequency, contrast and orientation of the stimuli. The diameter of suppression zone decreased proportionally with increasing peak spatial frequency of the DOG patterns. The vertical dimension of the suppression zone was larger than the horizontal dimension at low spatial frequencies but the inverse tended to occur at high spatial frequencies. Diameter of the suppression zone increased rapidly with increasing contrast of the stimuli when the contrast was lower than 20-30% but asymptoted at higher contrasts.

Contrast Sensitivity↗
PubMed1994

Spatial summation and its interaction with the temporal integration mechanism in human motion perception.

The combination of visual motion information over visual space (spatial summation) and stimulus duration (temporal integration) was investigated using a random-pixel array (spatiotemporally broad-band) apparent motion stimulus designed to isolate specific populations of visual motion detectors. The results indicate that, in agreement with results from spatiotemporally narrow-band stimuli, spatial summation follows the form of linear probabilistic summation rather than non-linear probabilistic summation. Linear probabilistic summation holds for a wide range of stimulus parameters and when changing either motion stimulus height or width. Linear probabilistic summation breaks down when the motion display region approaches a height and/or width that is related to the spatial displacement size, not the speed, of the random-pixel array. This height and width (termed the critical height and width, or critical dimension), increases with spatial displacement size and can be interpreted as a measure of the basic dimensions of the selected motion detector population's receptive field. The critical height is smaller than the critical width, a result that is consistent with a motion detector receptive field that is elongated in the direction of motion. Perhaps most importantly, the mechanisms of temporal integration and spatial summation can work independently under a wide range of conditions. Finally, the results provide evidence for a short-term inhibitory phenomenon from the edges of the useful display area that affects the visibility of the motion.

Contrast Sensitivity↗
PubMed1996

Positional loss in strabismic amblyopia: inter-relationship of alignment threshold, bias, spatial scale and eccentricity.

In order to understand the spatial loss in strabismic amblyopia and its relationship to the contrast sensitivity deficit, we measured alignment performance for a three element vertical alignment task in which the elements were equi-visible, spatial Gabors. We derived the threshold and bias and compared these for stimuli of different spatial scale and eccentricity. Our results suggest that: (1) the deficits for alignment thresholds and bias are uncorrelated; (2) in the majority of strabismic amblyopes, both deficits are scale invariant; (3) the form of the regional distribution depends on the spatial measure used and the scale at which it is measured; and (4) there is a poor correlation between the deficit for either spatial measure and the contrast sensitivity loss.

Amblyopia↗
PubMed1992

Diazepam impairs retention of spatial information without affecting retrieval or cue learning.

We further investigated the effect of diazepam on the processing of spatial information in a water maze task. Diazepam significantly impaired the retention of spatial information in a group of rats trained to locate a hidden platform. In a free swim trial carried out after training, diazepam-treated rats showed no bias to the target quadrant. There was no effect of diazepam on retrieval of spatial information in well-trained rats, and diazepam was devoid of any effect on cue learning in the water maze. However, diazepam blocked latent place learning during cue training in the water maze. Our results indicate that the GABA-BZD receptor modulates spatial information processing and that diazepam specifically impairs the retention of spatial information without affecting retrieval or cue learning.

Animals↗
PubMed1992

Effects of vinconate, a novel vinca alkaloid, on spatial learning deficits induced by the basal forebrain lesion in rats.

We investigated the effects of vinconate, a novel vinca alkaloid, on spatial learning deficits induced by the basal forebrain (BF) lesion in rats. Bilateral BF lesions were produced by injecting ibotenic acid (6 micrograms/0.5 microliter/side). In BF-lesioned rats, impairment of spatial learning in escaping onto the platform during training and decrease in spatial bias during the spatial probe trial in Morris's water maze task were both observed. Vinconate (5 and 10 mg/kg) treatment shortened the increase of escape latency to the platform in BF-lesioned rats and significantly reversed the decrease in spatial bias induced by the BF lesion. Vinconate (10 mg/kg) attenuated the decrease in choline acetyltransferase activity in the frontoparietal cortex caused by the BF lesion. The present study suggests that vinconate has an antiamnesic effect on the BF-lesion-induced amnesia by ameliorating the dysfunction in cholinergic neurons.

Animals↗
PubMed1992

Relation of rotation to egocentric and allocentric spatial learning in the rat.

In this experiment, we asked whether the relation between amphetamine-induced rotation and the learning and retention of left-right discrimination extends to allocentric spatial learning or is limited to egocentric spatial tasks. Rotation was established following injections of d-amphetamine sulfate, and rats were classified as nonrotators, midrotators, or strong rotators. Animals were successively trained on navigation in the Morris water maze (allocentric) and delayed spatial alternation in a water T-maze (egocentric). There were no rotation effects in water maze learning but rotators and nonrotators differed significantly in delayed spatial alternation learning but not relearning. Strong rotators learned more slowly than midrotators, clearly implying that rotational bias and directional learning are not linearly related. We show that it is egocentric spatial learning that is facilitated by a nigrostriatal dopamine asymmetry and extend the generality of the left-right discrimination findings.

Amphetamine↗
PubMed1996

Consequences of selective blockade of septal noradrenergic afferents on anxiety and spatial working memory performance in mice.

This experiment was designed to investigate the role of septal noradrenergic (NA) afferents in the control of anxiety and spatial working memory. To this end, C57Bl/6 mice were infused bilaterally into the lateral septal nuclei with 500 ng/0.2 microliter of BE 2254, a selective alpha 1 postsynaptic adrenoceptor antagonist. The consequences of this reversible treatment were evaluated 20 min later on the anxiety level measured in an elevated plus-maze and on spatial working memory, evaluated under four different conditions via the learning of a delayed nonmatching to place (DNMTP) rule achieved in an eight-arm radial maze. In these conditions, the BE 2254, as well as the saline-injected control group, showed an elevation of the anxiety level that may be the indirect expression of a nonspecific septal dysfunction induced by the vehicle injection rather than the normal behavioral response produced by the decrease of septal NA activity. This septal dysfunction also impaired spatial working memory but only when mnesic difficulty of the task is increased, suggesting that this impairment expresses a general memory deficit rather than a working memory deficit per se. A lack of spatial working memory deficits in BE 2254 or saline-injected animals was also observed in two other conditions of the behavioral protocol. However, when treatments were applied before the first exposure of animals to the radial maze (exploration session), only the group which received BE 2254 was impaired during the acquisition session for the rule performed 24 h later. This delayed perturbation seems to be linked, at this stage of the learning procedure, to the lack of NA-dependent processes taking place during the exploration session. Taken together, these data suggest that septal NA mechanisms are more essential at initial stage of this learning, when animals process new features of the situation, than during the expression of spatial working memory per se.

Adrenergic Fibers↗
PubMed1991

The effect of flumazenil on acquisition, retention, and retrieval of spatial information.

The effect of flumazenil, a benzodiazepine-receptor antagonist, was evaluated in a spatial-reference memory procedure in a water maze. Flumazenil (1.0, 3.0, and 10.0 mg/kg, ip) did not modify acquisition of spatial information. Retention was similar between control and experimental rats 24 h after the training phase, as all groups showed bias to the target quadrant in a free swim trial. However, 10 days later, only flumazenil-injected rats (3.0 mg/kg) showed bias to the target quadrant. Flumazenil did not affect retrieval of spatial information in a group of well-trained rats. These results suggest that a benzodiazepine-receptor mediated endogenous mechanism is activated during learning of spatial tasks and that its blockade facilitates retention of spatial information.

Animals↗
PubMed1992

Strategy selection in a task with spatial and nonspatial components: effects of fimbria-fornix lesions in rats.

The main purpose of the present research was to investigate the ability of rats to learn a 12-arm radial maze task that requires the concurrent utilization of both spatial and intramaze cue information. The task involves in a single trial both place and cue learning as well as reference memory (RM) and working memory (WM). Since the animal can choose place and cue arms in any order, the strategies employed to learn the task can be studied as well as the kinds of memory errors that are made. The results of Experiment 1 showed that the number of errors made on the place and cue components of the task did not differ, and that more RM than WM errors were made early during learning. As the task was learned, the animals tended to choose the place arms before choosing the intramaze cue arms, thus suggesting that a spatial strategy was employed first followed by a cue strategy. In Experiment 2 lesions of the fimbria-fornix resulted in temporary impairments in both RM and WM that were especially apparent on the spatial component of the task. The lesioned rats also switched from choosing mostly place arms early during the trial to choosing more cue arms. While fimbria-fornix lesioned rats recovered from the memory impairments with training, the change in response strategy persisted throughout postoperative testing. The procedure of combining both spatial and non-spatial components concurrently in the same task should prove of value in studying response strategies in animals.

Animals↗
PubMed1982

Dissociation of the contributions of the prefrontal cortex and dorsomedial thalamic nucleus to spatially guided behavior in the rat.

Rats with lesions of the medial frontal cortex or dorsomedial nucleus of the thalamus (MD) were studied in two spatial localization tasks: the Morris water task and the 8-arm radial arm maze. Rats with medial frontal lesions failed to learn to swim from different locations to a hidden platform located at a specific place in a large tank (Morris task) and were impaired at learning the location of reward in the radial arm maze. Rats with MD lesions were not impaired at spatial orientation in either task. The results provide the first unequivocal evidence of a spatial orientation deficit following frontal lesions and lend support to the notion that the frontal cortex forms part of a 'spatial mapping system'. The results suggest that although MD is the major afferent to the frontal cortex, it does not provide necessary spatially-relevant input.

Afferent Pathways↗