Stimulus Diameter Research Articles

Visual Performance as a Function of Luminance in Glaucoma: The De Vries-Rose, Weber's, and Ferry-Porter's Law.

To determine whether the De Vries-Rose, Weber's, and Ferry-Porter's law, which describe visual performance as a function of luminance, also hold in patients with glaucoma. A case-control study with 19 glaucoma patients and 45 controls, all with normal visual acuity. We measured foveal and peripheral contrast sensitivity (CS) using static perimetry and foveal and peripheral critical fusion frequency (CFF; stimulus diameter 1°) as a function of luminance (0.02 to 200 cd/m2). ANOVA was used to analyze the effect of glaucoma and luminance on CS and CFF; analyses were adjusted for age and sex. Foveally, logCS was proportional to log luminance at lower luminances (de Vries-Rose) and saturated at higher luminances (Weber); glaucoma patients had a 0.4 log unit lower logCS than controls (P < 0.001), independent of luminance. Peripherally, the difference was more pronounced at lower luminances (P = 0.007). CFF was linearly related to log luminance (Ferry-Porter). Glaucoma patients had a lower CFF compared with controls (P < 0.001), with a smaller slope of the CFF versus log luminance curve, for both the fovea (6.8 vs. 8.7 Hz/log unit; P < 0.001) and the periphery (2.5 vs. 3.4 Hz/log unit; P = 0.012). Even in apparently intact areas of the visual field, visual performance is worse in glaucoma patients than in healthy subjects for a wide range of luminances, without a clear luminance dependency that is consistent across the various experiments. This indicates impaired signal processing downstream in the retina and beyond, rather than an impaired light adaptation in the strictest sense.

Area summation of first- and second-order modulations of luminance.

To extend our understanding of the early visual hierarchy, we investigated the long-range integration of first- and second-order signals in spatial vision. In our first experiment we performed a conventional area summation experiment where we varied the diameter of (a) luminance-modulated (LM) noise and (b) contrast-modulated (CM) noise. Results from the LM condition replicated previous findings with sine-wave gratings in the absence of noise, consistent with long-range integration of signal contrast over space. For CM, the summation function was much shallower than for LM suggesting, at first glance, that the signal integration process was spatially less extensive than for LM. However, an alternative possibility was that the high spatial frequency noise carrier for the CM signal was attenuated by peripheral retina (or cortex), thereby impeding our ability to observe area summation of CM in the conventional way. To test this, we developed the "Swiss cheese" stimulus of Meese and Summers (2007) in which signal area can be varied without changing the stimulus diameter, providing some protection against inhomogeneity of the retinal field. Using this technique and a two-component subthreshold summation paradigm we found that (a) CM is spatially integrated over at least five stimulus cycles (possibly more), (b) spatial integration follows square-law signal transduction for both LM and CM and (c) the summing device integrates over spatially-interdigitated LM and CM signals when they are co-oriented, but not when cross-oriented. The spatial pooling mechanism that we have identified would be a good candidate component for a module involved in representing visual textures, including their spatial extent.

Assessment of human intraoral thermal sensitivity with simple devices in the clinic: implications for orofacial pain conditions.

To use simple thermal devices with different diameters and temperatures to investigate reliability and magnitude of human intraoral thermal sensitivity. Sixteen healthy volunteers participated. Six thermal devices with tapered circular ends (stimulus diameter 3, 5, and 10 mm) were used. Three different temperatures (room temperature, heat, and cold) were applied with each of the three diameters, ie, nine combinations. Participants were stimulated in randomized order at nine different sites: tongue, lip, maxillary attached gingiva adjacent to the left and right central incisors (without touching the lip) and to the left and right premolars (with or without touching the lip), and the left and right cheeks extraorally. Participants rated the perceived stimulus intensity on 0-50- 100 numeric rating scales (NRS). The number of paradoxical thermal sensations was also recorded. Ten volunteers were examined twice on the same day and recalled for a second session for assessment with the 5-mm-diameter device of within- and between-session reliability (interclass correlation coefficients [ICC]). The results were analyzed using a three-way analysis of variance. Reliability of NRS scores ranged from poor (ICC = 0.09, with cold stimulation at the premolar region) to excellent (ICC > 0.92, with cold stimulation at the cheek or tongue). NRS values varied with stimulus diameter (P < .050), temperature (P < .001), and sites (P < .001), with significant size x site and temperature x site interactions (P < .001). The tongue was the most sensitive site (P < .001) and the gingiva was the least sensitive site (P < .050). The 10-mm-diameter device produced higher NRS scores than the 3-mm-diameter device. The reliability of intraoral thermal sensitivity recorded with the 5-mm-diameter device varied greatly between different sites. Nonetheless, with this caveat in mind, the study did document that semiquantitative assessment of intraoral thermal sensitivity is feasible and applicable for clinical studies in different intraoral pain conditions.

Effect of retinopathy of prematurity on scotopic spatial summation.

To evaluate scotopic retinal organization in retinopathy of prematurity (ROP) through a study of spatial summation. Thresholds for a range of stimulus diameters (0.4°-10°) were measured using a two alternative, spatial, forced choice psychophysical procedure. The critical diameter (DCRIT) for complete summation was estimated in subjects with a history of severe ROP (N = 7) and mild ROP (N = 17). Subjects who were born preterm and never had ROP (N = 16) and term-born subjects (N = 7) were also tested. The subjects ranged in age from 9 to 17 (median 13.5) years. Critical diameter for complete spatial summation was significantly larger in ROP subjects than in subjects who never had ROP and in term-born control subjects. Critical diameter varied significantly with severity of ROP. The larger DCRIT values in ROP are consistent with altered organization of the post receptor retina. This may offer the ROP retina a strategy for achieving noise reduction and good dark-adapted visual sensitivity.

Surround suppression maps in the cat primary visual cortex

In the primary visual cortex and higher-order areas, it is well known that the stimulation of areas surrounding the classical receptive field of a neuron can inhibit its responses. In the primate area middle temporal (MT), this surround suppression was shown to be spatially organized into high and low suppression modules. However, such an organization has not been demonstrated yet in the primary visual cortex. Here, we used optical imaging of intrinsic signals to spatially evaluate surround suppression in the cat visual cortex. The magnitude of the response was measured in areas 17 and 18 for stimuli with different diameters, presented at different eccentricities. Delimited regions of the cortex were revealed by circumscribed stimulations of the visual field (“cortical response field”). Increasing the stimulus diameter increased the spread of cortical activation. In the cortical response field, the optimal stimulation diameter and the level of suppression were evaluated. Most pixels (≥3/4) exhibited surround suppression profiles. The optimal diameter, corresponding to a population of receptive fields, was smaller in area 17 (22°) than in area 18 (36°) in accordance with electrophysiological data. No difference in the suppression strength was observed between both areas (A17: 25%, A18: 21%). Further analysis of our data revealed the presence of surround modulation maps, organized in low and high suppression domains. We also developed a statistical method to confirm the existence of this cortical map and its neuronal origin. The organization for center/surround suppression observed here at the level of the primary visual cortex is similar to those found in higher order areas in primates (e.g., area MT) and could represent a strategy to optimize figure ground discrimination.

Glaukomerkennung mittels der Pupillenkampimetrie

The aim of the study was to find out if pupillographic assessment of the visual field by means of pupil campimetry can identify glaucomatous visual field defects and as such be used for glaucoma screening purposes. 20 patients with open angle glaucoma and 30 healthy persons were examined by means of pupil campimetry. All glaucoma patients had a glaucomatous visual field defect in at least one eye. The stimulus pattern consisted of 17 white-light stimuli which were presented within the 30° visual field, particularly in the Bjerrum region. The stimulus diameter was 6°. Each stimulus was presented for 200 ms and the interval between the stimuli was 1800 ms. Three stimulus intensities (16.4 cd/m2; 27.1 cd/m2 and 40.5 cd/m2) were tested. The individual pupil light reaction (PLR) amplitudes at all examined locations in the visual field, their sums and partial sums were compared between both groups by the two-sided two-sample t test. The diagnostic performance of the method in glaucoma diagnosis was evaluated by ROC curves (receiver operating characteristics). The average PLR at all locations in the visual field was reduced in glaucoma patients compared to healthy persons. The sums of the PLR were reduced in glaucoma patients as well. Significant differences in the PLR were found especially in the central and paracentral visual fields. The best AUC values (area under the curve) were reached with the highest stimulus intensity, the highest AUC value overall was 0.769. Although the difference in PLR between glaucoma patients and the control group was significant, the reached AUC values fell short of being ideal for screening purposes. A surprising finding was that the most central pupil response was reduced by the same amount as that in the Bjerrum region.

Effects of contrast and contrast adaptation on static receptive field features in macaque area V1

The spatiotemporal features of the "static" receptive field (RF), as revealed with flashing bars or spots, determine other RF properties. We examined how some of these static RF features vary with contrast and contrast adaptation in area V1 of the anesthetized macaque monkey. RFs were mapped with light and dark flashing bars presented at three different contrasts, with the low and medium contrasts eliciting approximately 1/3 and 2/3 of the high-contrast response amplitude. The main results are as follows: 1) RF widths decreased when contrast decreased; however, the amount of decrease was less than that expected from an iceberg model and closer to the expectation of a contrast invariance of the RF width. 2) Area tuning experiments with drifting gratings showed an opposite effect of contrast: an increase in preferred stimulus diameter when contrast decreased. This implies that the effect of contrast on preferred stimulus size is not predictable from the static RF. 3) Contrast adaptation attenuated the effect of contrast on RF amplitude but did not significantly modify RF width. 4) RF subregion overlap was only marginally affected by changes in contrast and contrast adaptation; the classification of cells as simple and complex, when established from subregion overlap, appears to be robust with respect to changes in contrast and adaptation state. Previous studies have shown that the spatiotemporal features of the RF depend largely on the stimuli used to map the RF. This study shows that contrast is one elemental feature that contributes to the dynamics of the RF.

Local action for global vision

Local action for global vision

Stimulus Parameters for Goldmann Kinetic Perimetry in Nonorganic Visual Loss

Objective: To investigate the influence of the stimulus parameters on perimetry at various distances and draw conclusions for the clinical exploration of nonorganic visual loss. Methods: Visual field testing using Goldmann kinetic perimetry was performed on 15 healthy volunteers. The I/1e isopter at 33 cm was compared to the I/1e, II/1e and I/2e isopters at 66 cm. The 0/1e isopter at 33 cm was compared to the 0/1e, I/1e and 0/2e isopters at 66 cm. Results: Doubling the examination distance without adjusting the stimulus parameters resulted in significant perimetric visual field constriction. Doubling the stimulus diameter resulted in perimetric visual field expansion by a factor of 2.26 and 3.32 for I/1e and 0/1e, respectively. Increasing stimulus luminance by a factor of 3.17 caused expansion by a factor of 2.15 and 2.32 for I/1e and 0/1e, respectively. Conclusions: To avoid falsely diagnosing visual field constriction, stimulus parameters need to be adjusted when visual field testing is performed at double distance. Increasing stimulus luminance was more appropriate than augmenting stimulus size.

The mfERG response topography with scaled stimuli: effect of the stretch factor

In multifocal electroretinogram recordings, the stimulus array is usually scaled with eccentricity to compensate for cone density changes. The strength of this scaling is parameterized by the so-called "stretch factor" (SF). In this study, we determined the quantitative influence of the SF striving for equal response densities over the entire stimulus area. VERIS 4.8 software was used to record multifocal ERGs from 11 normals with 61 hexagons and a 60 degrees stimulus diameter. Six recordings were obtained per subject with SFs 0, 12, 23, 25, 27, and 29. For analysis, we calculated the mean hexagon area per eccentricity and the average amplitude for elements of equal eccentricity (rings 1-5 = R1-R5), normalized to R2. As expected, recordings with SF = 0 showed the steepest amplitude drop-off with eccentricity (relative amplitudes of 1.90 (R1/R2), 1.0 (R2/R2), 0.67 (R3/R2), 0.51 (R4/R2), 0.38 (R5/R2). Using the preset SF = 12 in our set-up resulted in R1-amplitudes 1.7x larger than the peripheral amplitudes (relative amplitudes 1.39, 1.0, 0.88, 0.87, 0.82). SF = 23 compensated best for eccentricity, yielding relative amplitudes of 1.2, 1.0, 1.02, 1.08, and 1.11. Deviations increased again with higher SFs. To minimize amplitude variability over the stimulated retinal area, the optimal SF needs to be adjusted to the individual recording set-up.

Interocular Transfer of a Rotational Motion Aftereffect as a Function of Eccentricity

In previous psychophysical investigations it has been reported that the angular extent over which the human visual field is served by binocular neurons in the visual cortex is limited to the central 40 degrees. However, these reports have been primarily based on data collected with static stimuli. Here we extend this investigation to include dynamic stimuli. Interocular transfer of the rotary motion aftereffect (rMAE) was measured for three stimulus diameters: 5, 30, and 62 deg. Interocular transfer, expressed as a percentage of monocular adapt/test rMAE duration was significantly reduced for stimulus diameter of 62 deg relative to 30 and 5 deg diameters. Nevertheless, interocular transfer durations still comprised a significant percentage of same-eye adapt/test durations (46.9%), comparable to previous reports of transfer MAE durations in near-central vision. The spatial extent of binocular interaction is likely stimulus specific and is still appreciable in the far periphery for complex-motion stimuli.

Optimal stimulus size maps in the primary visual cortex revealed by optical imaging in cats

Abstract Purpose: It is well known that the responses of cells in the primary visual cortex depend on stimulus size. While the stimulus‐size dependency has been well documented at the cellular level, nothing is known about its consequences on global functional maps. Methods: Optical imaging of intrinsic signals in the primary visual cortex was carried out in anesthetized cats. Stimuli consisted of 0.75 to 0.1 cycles per degree square‐wave gratings drifting in 8 directions at 2 to 4 Hz and were presented monocularly. Responses were obtained for different stimulus diameters (3 to 50 deg, and a full screen condition). Results: The minimal visual stimulation necessary to activate areas 17 and 18 was around 3 and 6 deg. in diameter respectively. The activation area of cortex (10‐30 mm2) was dependent of the eccentricity (0 to 30 deg). The pixelwise measure of the signal magnitude in this area showed a modular organisation uncorrelated with the orientation map and stable in time: Half of the pixels had a maximum activation for the full screen stimulation (full field facilitation) and the other half attained their maximum for diameters about 15 and 30 deg of diameter in area 17 and 18 respectively (full field suppression). The suppression by the full screen stimulation was around 30% in both areas. Conclusions: Thus, the maximum activation revealed by optical imaging necessitates the stimulation of a much larger spatial area than that observed with single cells. This difference is likely due to the fact that this method reflects in‐ and out going signals and reveals activity of adjacent neurons being part of intra‐cortical and thalamo‐cortical circuits. Supp: NSERC and CIHR.

Neuromagnetic evoked responses to complex motions are greatest for expansion

We analysed evoked magnetic responses to moving random dot stimuli, initially using a 19-channel magnetoencephalography (MEG) system, and subsequently using a 151-channel MEG system. Random dot displays were used to construct complex motion sequences, which we refer to as expansion, contraction, deformation, and rotation. We also investigated lateral translation and a condition in which the directions of the dots were randomised. In all stimulus conditions, the dots were first stationary, then traveled for a brief period (317s or 542 ms), and were then stationary again. In all conditions, evoked magnetic responses were observed with a widespread bilateral distribution over the observers' heads. Initial recordings revealed a substantially larger evoked magnetic response to the expansion condition than the other conditions. In a revised study, we used a 151-channel MEG system and two stimulus diameters (9.3 and 48 deg), the smaller comparable with the first experiment. The responses were analysed using a nonparametric approach and confirmed our initial observations. In a third study, speed gradients were removed and a new design permitted direct comparisons between motion conditions. The results from all three experiments are consistent with the greater ecological validity of the expansion stimulus.

Photoreceptor topography and cone-specific electroretinograms

It is implicit in many cone-specific ERG studies that the amplitude is proportional to the numbers of cones stimulated. The objective of these experiments was to test this idea by comparing ERGs obtained from different areas of the retina with histological data on cone-density distributions. The histology (Curcio et al., 1990) shows that the cumulative number of cones in the human retina increases exponentially with stimulus diameter between 0- and 40-deg eccentricity. L-, M-, and (L+M) cone-driven 30-Hz ERGs were obtained from a series of stimuli with one of the following configurations: (1) Circular stimuli of different angular subtense up to 70-deg diameter. (2) Annuli with 70-deg outer diameter but variable inner diameter. (3) Annuli of constant area but increasing eccentricity. Cone contrasts were equalized for each stimulus condition. The modulated and nonmodulated regions of the screen had the same mean hue and luminance. The data suggest that the L+M cone ERG amplitude increases with stimulus diameter in direct proportion to the estimated number of cones stimulated. Furthermore, the total L+M responses appear to be predicted from individual L and M responses by simple linear summation for both the disc and annular stimuli.

Retinocortical gain in the foveal pathway: the effect of spatial frequency and stimulus size.

The amplitude and the phase of the simultaneously recorded steady-state pattern electroretinogram (PERG) and visual evoked potential (VEP) were evaluated in humans as a function of the vertical diameter (D) of unidimensional Gabor stimuli. In the other dimension, parallel to the horizontal gratings, the patterns all had constant diameter (see Methods and Materials). Spatial frequencies (SFs) of 1 cycle per degree (cpd) and 5.3 cpd were counterphase modulated at a rate of 6.8 Hz. After off-line artifact rejection, the response was subject to Fast Fourier Transformation (FFT). Amplitude and phase of the first and second harmonics of both ERG and VEP were displayed for each SF and stimulus diameter. Both ERG and VEP amplitude were found to increase as a function of D. Using trend analysis we found that ERG amplitude increased linearly as a function of D. VEP amplitude was found to be both linear and cubic, as a function of D. We calculated the ratio of VEP amplitude and ERG amplitude at each D and termed it retinocortical gain (G). G normalized to stimulus area was high for small D and decreased with D independently of SF. Unity gain occurred at stimulus sizes of 6 degrees to 7 degrees. ERG phase was found to be more negative at 5.3 cpd than at 1 cpd. Although no significant difference was found between VEP phases at the two SFs tested, neither ERG nor VEP phase changed as a function of D. The results suggest that retinocortical gain is highest for the foveally centered low spatial frequency small patch Gabors. The results provide support for the notion of the "foveal window" in human vision.

Pretectal neurons optimized for the detection of saccade-like movements of the visual image.

The visual response properties of nondirectional wide-field sensitive neurons in the wallaby pretectum are described. These neurons are called scintillation detectors (SD-neurons) because they respond vigorously to rapid, high contrast visual changes in any part of their receptive fields. SD-neurons are most densely located within a 1- to 2-mm radius from the nucleus of the optic tract, interspersed with direction-selective retinal slip cells. Receptive fields are monocular and cover large areas of the contralateral visual field (30--120 degrees ). Response sizes are equal for motion in all directions, and spontaneous activities are similar for all orientations of static sine-wave gratings. Response magnitude increases near linearly with increasing stimulus diameter and contrast. The mean response latency for wide-field, high-contrast motion stimulation was 43.4 +/- 9.4 ms (mean +/- SD, n = 28). The optimum visual stimuli for SD-neurons are wide-field, low spatial frequency (<0.2 cpd) scenes moving at high velocities (75--500 degrees /s). These properties match the visual input during saccades, indicating optimal sensitivity to rapid eye movements. Cells respond to brightness increments and decrements, suggesting inputs from ON and OFF channels. Stimulation with high-speed, low spatial frequency gratings produces oscillatory responses at the input temporal frequency. Conversely, high spatial frequency gratings give oscillations predominantly at the second harmonic of the temporal frequency. Contrast reversing sine-wave gratings elicit transient, phase-independent responses. These responses match the properties of Y retinal ganglion cells, suggesting that they provide inputs to SD-neurons. We discuss the possible role of SD-neurons in suppressing ocular following during saccades and in the blink or saccade-locked modulation of lateral geniculate nucleus activity to control retino-cortical information flow.

Motion-responsive regions of the human brain.

Functional magnetic resonance imaging was used to map motion responsive regions of the human brain by contrasting passive viewing of moving and stationary randomly textured patterns. Regions were retained as motion responsive if they reached significance either in the group analysis or in the majority of hemispheres in single-subject analysis. They include well-known regions, such as V1, hMT/V5+, and hV3A, but also several occipito-temporal, occipito-parietal, parietal, and frontal regions. The time course of the activation was similar in most of these regions. Motion responses were nearly identical for binocular and monocular presentations. Flicker-induced-activation introduced a dichotomy amongst these motion responsive regions. Early occipital and occipito-temporal regions responded well to flicker, while flicker responses gradually vanished as one moved to occipito-parietal and then parietal regions. Finally, over a more than four-fold range, stimulus diameter had little effect on the motion activations, except in V1.

Effect of window size on detection acuity and resolution acuity for sinusoidal gratings in central and peripheral vision.

Detection and resolution of square patches of sinusoidal gratings were measured in central and peripheral vision (30 degrees horizontal temporal visual field) for high-contrast gratings as a function of the number of cycles in the stimulus. We determined performance in a forced-choice paradigm for a fixed number of stimulus cycles by arranging for stimulus diameter to vary inversely with spatial frequency. For both psychophysical tasks and for both target locations, the psychometric function relating performance to log spatial frequency shifted to higher frequencies without changing slope significantly as the number of cycles in the stimulus was increased. Thus the entire effect could be captured by an analysis of spatial acuity, which increased with increasing number of grating cycles over the range 0.5-6 cycles but remained constant over the range 6-14 cycles. In the central field, resolution acuity and detection acuity were equal regardless of the number of cycles in the stimulus. In the peripheral field, detection acuity exceeded resolution acuity and perceptual aliasing occurred for stimuli in the range 1-14 cycles. From this result we conclude that resolution acuity is sampling limited in the periphery, provided that the stimulus contains at least one full cycle of the grating. Essential features of the results could be accounted for by Fourier analysis of the stimulus.

Shape and spatial distribution of receptive fields and antagonistic motion surrounds in the middle temporal area (V5) of the macaque.

The spatial organization of receptive fields in the middle temporal (MT) area of anaesthetized and paralysed macaque monkeys was studied. In all, 288 neurons were successfully recorded. The size and shape of the receptive field (RF) was mapped with small patches of translating random dots and the resulting data were fitted with a generalized Gaussian. Results show that the RF area increases with eccentricity, and is larger in lamina 5 than in other layers. Most of these RFs are elongated, and the axis of elongation tends to be orthogonal to the preferred direction of motion. The direction selectivity is maintained in all positions in the RF, but layer 5 cells are less direction-selective than cells in other layers. In a second series of experiments, radial dimensions of the classical RF and the antagonistic surround were estimated from area summation tests. These data were fitted with the difference of the integrals of two Gaussians. Surrounds were weakest in layer 4 and strongest in layer 2. Optimal stimulus diameters, also estimated from the area summation curve, were larger in the infragranular layers than in the other layers. The maximum sensitivity of the surround was clearly displaced from the classical RF (CRF) centre, indicating that the surround is not concentric with the CRF. This radial offset and the extent of the surround were largest in layers 2 and 5 and smallest in 3a. The extent of the surround half-height equalled, on average, 3-4 times that of the CRF. These results suggest that antagonistic surrounds are constructed in MT, probably through horizontal connections, and that a strong vertical organization exists in area MT, as has been shown for V1.

Detection of blue under chromatic adaptation: The effects of stimulus size and eccentricity

We measured thresholds for the perception of blue under chromatic adaptation to white, green, yellow or red at the eccentricities of 0–70 deg in the temporal visual field of four subjects. We used a series of stimulus sizes at each eccentricity, without a prior assumption of any peripheral size-scaling factor. The CIE 1976 UCS ( u′, v′) chromaticity coordinates corresponding to blue perception were subtracted from the chromaticity coordinates of the adaptation field in order to obtain the threshold differences (d u′, d v′) in chromaticity coordinates. Spatial scaling factors for the perception of blue were obtained by non-linear regression. ( E 2 + 5 deg) refers to the eccentricity at which stimulus diameter had to be doubled in order to maintain performance found at the eccentricity of 2.5 deg. E 2 for the perception of blue tint varied from 1.2 to 36 deg depending on the state of chromatic adaptation and subject. For the perception of blue tint in yellow three subjects and for the perception of blue tint in red one subject had no spatial scaling factor that would make performance independent of eccentricity. Thus, spatial scaling does not always work.