Stimulus Spatial Frequency Research Articles

The two-dimensional spectral structure of simple receptive fields in cat striate cortex.

1. A quantitative, general purpose method was developed for measuring the responses of visual neurons to stimuli distributed with high resolution over the two-dimensional (2D) spatial frequency domain. The stimuli consisted of drifting sinusoidal gratings of nonsaturating contrasts whose spatial frequency and orientation were drawn in random order from a 16 X 16 array of coordinates covering each neuron's responsive area. This method was applied to a population of 36 simple cells in area 17 of cat. 2. The response of each simple cell to drifting sinusoidal gratings appeared as a rectified sinusoidal modulation of the spike frequency. The degree of rectification varied from cell to cell, but for each cell, the form of the response was constant irrespective of stimulus spatial frequency, orientation, or contrast. The amplitude of the average response at the stimulus temporal frequency was used as the response metric at all spectral coordinates. Variations in this amplitude over two spectral dimensions forms a surface that we call the 2D spectral response profile. 3. For each cell, the 2D spectral response profile was localized to a limited region of the complete 2D spatial frequency domain. In bidirectionally responsive cells, there were two lobes in the surface disposed with mirror symmetry about the origin. In all cells, each lobe exhibited a single maximum and the response decayed smoothly in every direction away from the maximum. Isoresponse amplitude contours were elliptical and often, but not always, elongated about an axis of symmetry passing through the origin. 4. We tested the hypothesis that orientation and spatial frequency tuning are independent by forming scaled radial and angular sections through 2D spectral response profiles. In virtually every case polar separability did not obtain, that is, orientation selectivity depended on spatial frequency and vice versa. 5. In contrast, more than half the cells had 2D spectral response profiles that were Cartesian separable. The 2D spectral response profiles of most of the remaining cells were neither polar nor Cartesian separable, because the response profiles were elongated about an axis of symmetry that did not pass through the origin. 6. These results are discussed in terms of the constraints they place on models of the contributions simple cells make toward the neural representation of images.

Spatial scaling of central and peripheral contrast-sensitivity functions.

Contrast sensitivity was measured as a function of spatial frequency at various eccentricities in the nasal visual field. Eccentricity influenced resolution more for vertical gratings than for horizontal ones, demonstrating a nasal field anisotropy. When grating apertures and spatial frequencies were varied together, by changing viewing distance, peripheral and central contrast-sensitivity functions could not be entirely superimposed by scaling the spatial-frequency stimulus parameter. However, when gratings apertures were designed to project a standard-sized patch on a hypothetical vertical surface at an angle of 9 deg to the line of sight, the superimposition of central and peripheral contrast-sensitivity functions improved. Central and peripheral contrast-sensitivity functions can be superimposed by describing the grating stimulus with reference to this putative surface in terms of cycles per surface distance. The surface-scaling approach leaves a minor effect of eccentricity on contrast sensitivity for vertical gratings but readily accommodates the vertical-horizontal anisotropy in grating resolution. These findings suggest a novel functional role for gradients in the spatial sampling of the visual field, in relation to the internal representation of the structure and geometry of visible surfaces.

Computation of the luminance and pattern components of the bar pattern electroretinogram.

Pattern onset electroretinograms (PERGs) were recorded from four normal subjects. Square-wave gratings of 75% contrast were presented in three approximately contiguous, concentric zones of outer angular radius, 5.1 degrees, 12.6 degrees, and 23.6 degrees. The zones were calculated to give equal numbers of ganglion cell receptive fields. The recorded PERGs were considered to include luminance and pattern components which have low and bandpass spatial tuning functions respectively. These components combine in the PERG to produce a broad spatial tuning characteristic. The amplitude of PERGs in response to low spatial frequency stimuli is widely reported to be linearly related to contrast. The retinal illuminance response at every spatial frequency was computed from the eye's modulation transfer function. This function characterizes the reduction in contrast that occurs because of optical degradation. The computed retinal illuminance response was subtracted from the PERG waveform and a pattern-specific response was revealed. The latter had a highly tuned bandpass function which peaked at higher spatial frequencies than the PERG at corresponding peripheral angles.

Influence of saccades on manual reactions—A reaction time and VEP study

Experiments were performed to investigate the retardation of motor reactions, triggered immediately after saccadic eye movements, by comparing VEP latencies and manual reaction times. As visual stimuli grating patterns of different spatial frequencies were used. They were presented during continuous fixation (“resting eye” condition) as well as shortly after a saccade (“saccadic” condition) with different onset delays (25, 50, 100, 150 msec), in order to determine the influence of saccades on VEP latencies and on manual reaction times. Compared with the reaction times in the “resting eye” condition, the postsaccadic reaction times were much prolonged whereas the respective VEP latencies are almost unchanged. Thus an inhibitory interaction of different motor responses (i.e. saccadic eye movements and manual reactions in this study) at higher levels of the afferent system or at the motor control site is postulated. This result has been confirmed for stimuli of different spatial frequencies.

Visual evoked potentials in parkinsonism and dopamine blockade reveal a stimulus-dependent dopamine function in humans.

VEPs were recorded with three different spatial frequencies of stimulation in patients affected by idiopathic Parkinsonism and by Parkinsonian syndromes. The detection of VEP abnormalities in Parkinson's disease was dependent on the spatial frequency of the visual stimulus (a vertical square wave grating). The VEP latency was normal in Parkinsonian syndrome patients (except in one patient affected by familial Parkinsonism). Dopamine precursor therapy differently reduced the VEP latency, depending on the spatial frequency of the visual stimulus. These findings suggest that the dopaminergic mechanism involved in the generation of VEP delays is sensitive to stimulus spatial frequency. The study of VEPs before and after the administration of haloperidol confirmed this hypothesis. VEP latency did not correlate with the major clinical symptoms of Parkinson's disease and could not predict the results of chronic dopaminergic therapy.

The influence of age on the spatial and temporal contrast sensitivity function.

A clinical technique for sampling the spatial and temporal CSF at low, intermediate and high frequencies is described. Reductions in contrast sensitivity with age were shown with the highest spatial and temporal frequency stimuli. The reductions can be almost completely accounted for by the reduction of pupil area with age, causing a reduction in retinal illuminance. The results show a significant deterioration in vision with age that is not revealed by standard visual acuity testing.

Scalp potential fields evoked by grating stimuli: Effects of spatial frequency and orientation

Visually evoked potentials were recorded from an array of 16 electrodes over the occipital cortex in 12 subjects. Grating stimuli of different spatial frequencies were presented on a circular test field, and potential fields evoked by vertical, horizontal and oblique stimuli were compared. Component latencies were determined by computing a reference-independent measure of field power. Scalp field maps were constructed at component latencies and compared over all conditions. Significant influences of both spatial frequency and orientation on component latencies were found. Component latencies were shortest for 2.3 c/deg gratings, and for vertically oriented stimuli. Latencies increased for horizontal, and further for oblique, gratings. The effect of spatial frequency or orientation on the location of the major components on the scalp was less pronounced. On the other hand, there were significant differences both in component latency and scalp distribution when upper and lower hemiretinal stimuli were compared. Upper hemiretinal stimulation yielded shorter latencies and a more anteriorly located potential peak on the scalp.

The position sensitivity of retinal X- and Y-cells in cats.

We have devised a measure of a retinal ganglion cell's sensitivity to changes in the spatial position of a grating stimulus. At maximum, this relative position sensitivity is a scaled product of the stimulus spatial frequency and the cell's fundamental component of response to that spatial frequency. We obtained the relative position sensitivity as a function of spatial frequency for 13 X-cells and 14 Y-cells. X-cell functions peak at significantly higher spatial frequencies than do those of Y-cells. At their peaks, X-cells display significantly higher values of relative position sensitivity than do Y-cells. However, Y-cells have higher position sensitivity at lower spatial frequencies, but exhibit less of a range of variation from maximum to minimum than do X-cells. These results are consistent with a hypothesis that Y-cells provide the crucial substrate for form vision at lower spatial frequencies, while X-cells are important for details carried by the higher spatial frequencies.

Grating visual evoked potentials in the evaluation of laser bioeffects: twenty nanosecond foveal ruby exposures.

A series of experiments was performed to determine the parameters necessary to produce immediate measurable changes in the visual system of nonhuman primates after exposure of the fovea to laser radiation. The parameters investigated were stimulus spatial frequency (1.6 and 2.8 c/deg), size of the visual field stimulated (30 and 3.6 degrees), retinal diameter of the laser exposure (50 and 500 micron), and total intraocular energy. Steady-state visual evoked potentials (VEP's) were recorded from cynomolgus monkeys in response to an oscillating grating. Single 20 ns Q-switched ruby laser pulses (694.3 nm) were directed into the fovea of the experimental eye during the pattern stimulation. No immediate effects on the VEP were noted. Strong delayed effects occurred 45 to 120 s postexposure and were characterized by large phase shifts in the response signal as referenced to the stimulus, magnitude decreases, variance increases, and a loss of waveform correlation with pre-exposure baseline signals. These effects persisted for approximately 30 s before the VEP again became normally re-entrained.

Spatial frequency and the pattern onset-offset response

Visually evoked responses (VERs) were recorded with vertical sinusoidal gratings presented in the on-off mode at rates of 0.5 and 1 Hz. Stimulus spatial frequency ranged from 0.5 to 16 c/deg and its contrast varied from near-threshold to the value of 0.3. At low spatial frequencies. 0.5 and 1 c/deg, the onset and offset VERs were of similar shape, magnitude and contrast dependence. At spatial frequencies higher than 2 c/deg the onset VERs were usually larger and persisted at lower contrast levels than the offset VERs. The results are consistent with the hypothesis that a gradual transition from the operation of transient channels to the operation of sustained channels takes place on increasing stimulus spatial frequency and that within a wide spatial frequency range (at least from 2 to 8 c/deg) suprathreshold gratings effectively stimulate both types of channels. It is confirmed that both onset and offset VERs are delayed at high spatial frequencies. In addition, a longer delay of the late onset waves was found in comparison with the delay of the early onset waves.

Disparity range for local stereopsis as a function of luminance spatial frequency

The disparity range for stereo sensitivity was investigated with spatially filtered bars, tuned narrowly over a broad range of spatial frequencies. When measured with narrow (high spatial frequency) bars the disparity range for stereopsis exceeded two orders of magnitude. The range was reduced with broad (low spatial frequency) bars by an elevation of stereothreshold that increased according to a constant 6deg phase disparity. The upper disparity limit also increased at broad spatial periods but at a lower rate. These size disparity correlations illustrate quantitative stereopsis along a continuum from fine to coarse disparities ( ± 2deg). A disparity matching task revealed that greater amounts of uncrossed than crossed disparity were required to match suprathreshold disparities. The ratio of standard/matched disparity was lower in both directions for small suprathreshold disparities subtended by broad than by narrow spatial periods. This selective reduction of stereo-efficiency ratio illustrates tuning to coarse disparities subtended by broad (low spatial frequency) stimuli and accounts for the marked reduction of stereoacuity caused by spectacle blur.

The variability of discharge of simple cells in the cat striate cortex.

The relationship between the variance and mean rate of discharges of simple cells in the cat striate cortex has been examined when mean rate was varied by changing either stimulus spatial frequency or contrast. In both cases, the variance was related to the mean discharge rate by an exponent of about 1.15; the relation was thus roughly linear. The discharge variance was on average 1.7 times the mean rate for data obtained from measurements of the neurones' spatial frequency tuning curves, and 1.48 times the mean for data from the response-contrast determination. However, this difference was not statistically significant.

Apparent contrast of suprathreshold gratings varies with stimulus orientation.

Human contrast sensitivity is known to vary with both stimulus spatial frequency and orientation. Previous research has indicated, however, that the apparent contrast of a sinusoidal grating is independent of spatial frequency at suprathreshold levels. We have found that the apparent contrast of suprathreshold gratings is not independent of its orientation. Observers adjusted the apparent contrasts of vertical and oblique gratings to match a 40% standard. Contrast match deviations were found at a number of spatial frequencies, indicating a lack of contrast constancy with stimulus orientation. The apparent contrast of gratings in different orientations could not be predicted in any simple fashion from either contrast threshold differences or ratios.

Retina X- and Y-cells in monocularly lid-sutured cats: normality of spatial and temporal properties

The electrophysiological effects of prolonged neonate monocular lid suture upon the cat's dorsal lateral geniculate nucleus are at least two-fold. First, there is a reduction in the proportion of Y-cells encountered with microelectrodes in the laminae receiving input from the deprived eye12,14. This reduction occurs central to the optic tract. That is, the proportion of retinal Y-cells encountered in the retina and optic tract is not altered by monocular lid suturelL Second, there is a decrease in the spatial resolution of X-cells within these same deprived laminae 5,9,1°. Spatial resolution is defined here as the highest spatial frequency stimulus to which the cell will respond. In the present report we provide evidence that this decrease in spatial resolution of geniculate X-cells is not matched by similar changes in retinal X-cells. Therefore, this deprivation,ind'uced deficit also primarily occurs central to the optic tract. We used standard, extracellular recording techniques to measure spatial and temporal contrast sensitivity functions of single retinal cells in normal and monocularly lid-sutured cats. These functions not only provide accurate estimates of spatial and temporal resolution, but also elucidate the cell's sensitivity to stimulus changes produced throughout the range of spatial and temporal frequencies to which the neurons can respond (see below). Deprived cats had the lids of one eye sutured closed under general anesthesia at 5-8 days of age, and they were used for recording after they had reached adulthood (7-8 months of age). Our recording procedures have been described in previous publications and will only be briefly summarized here 6,s,12. The animals were prepared for recording under Halothane anesthesia mixed in 50 ~ N2050 ~ 02 and were maintained for recording with a 70 ~ N20-30 ~ 02 mixture. The cats were paralyzed with Flaxedil (5 mg/kg/h) and curare (0.5 mg/kg/h) mixed in lactated Ringers with 5 ~ dextrose, and were artificially respired with end-tidal CO2 maintained at 4.0 ~. Sutured eyes were opened during the initial surgical preparation.

Orientation differences for perception of sinusoidal line stimuli

The effect of orientation on the sensitivity of sinusoidal lines of different spatial frequencies, was measured on four observers. As with conventional vernier stimuli the sensitivity for obliquely oriented stimuli was poorer than that for stimuli that were either vertical or horizontal. This reduction in sensitivity was approximately constant at about 30% for stimuli of spatial frequencies between 0.15 and 10c/deg. This pattern of change of sensitivity with spatial frequency could not be mimicked by either spherical or cylindrical optical defocus, by reduction in the mean luminance, or by peripheral viewing. The reduction in periodic vernier acuity for oblique sinusoidal lines is therefore a unique neural loss.

Perception time and spatial frequency

The time of detection of a feature of square-wave gratings (orientation or a small difference in spatial frequency between two adjacent gratings) was studied as a function of stimulus spatial frequency. The relative perception time was measured by the reaction time method or by the method of masking. Perception time was found to increase at grating frequencies above 4–6 c/deg, the effect being partly due to the lower contrast sensitivity at these frequencies. However, a delay was also found when increased contrast at high frequencies compensated for the lower sensitivity. The result suggests the existence of a mechanism for processing a high spatial frequency stimulus which demands more time than that for processing a low spatial frequency.

Reaction time to square-wave gratings as a function of spatial frequency, complexity and contrast

Visuomotor reaction time (RT) of normal human subjects to briefly presented square-wave bar patterns were measured. The pattern varied with respect to magnitude and number of spatial frequencies (one cycle/degree of visual angle, 3 cycles/degree, and both combined) with each spatial frequency stimulus presented at 3 levels of contrast. Space-average luminance was held constant. RT increased with the higher spatial frequency and was intermediate in value for the combination stimulus at each level of contrast. RT increased with decreasing contrast levels for each spatial frequency stimulus. There was no interaction between the two parameters. The findings indicate that a behavioral response measure can be employed to study the multiple channel hypothesis of human vision. The results were compared with those of other investigators employing sine-wave gratings, perceptual response measures and the evoked potentials technique.