Function Of Stimulus Size Research Articles

Visual Feature Tuning Properties of Short-Latency Stimulus-Driven Ocular Position Drift Responses during Gaze Fixation.

Ocular position drifts during gaze fixation are significantly less well understood than microsaccades. We recently identified a short-latency ocular position drift response, of ∼1 min arc amplitude, that is triggered within <100 ms by visual onsets. This systematic eye movement response is feature-tuned and seems to be coordinated with a simultaneous resetting of the saccadic system by visual stimuli. However, much remains to be learned about the drift response, especially for designing better-informed neurophysiological experiments unraveling its mechanistic substrates. Here we systematically tested multiple new feature tuning properties of drift responses. Using highly precise eye tracking in three male rhesus macaque monkeys, we found that drift responses still occur for tiny foveal visual stimuli. Moreover, the responses exhibit size tuning, scaling their amplitude (both up and down) as a function of stimulus size, and they also possess a monotonically increasing contrast sensitivity curve. Importantly, short-latency drift responses still occur for small peripheral visual targets, which additionally introduce spatially directed modulations in drift trajectories toward the appearing peripheral stimuli. Drift responses also remain predominantly upward even for stimuli exclusively located in the lower visual field and even when starting gaze position is upward. When we checked the timing of drift responses, we found it was better synchronized to stimulus-induced saccadic inhibition than to stimulus onset. These results, along with a suppression of drift response amplitudes by peristimulus saccades, suggest that drift responses reflect the rapid impacts of short-latency and feature-tuned visual neural activity on final oculomotor control circuitry in the brain.

A computational model of stereoscopic prey capture in praying mantises.

We present a simple model which can account for the stereoscopic sensitivity of praying mantis predatory strikes. The model consists of a single “disparity sensor”: a binocular neuron sensitive to stereoscopic disparity and thus to distance from the animal. The model is based closely on the known behavioural and neurophysiological properties of mantis stereopsis. The monocular inputs to the neuron reflect temporal change and are insensitive to contrast sign, making the sensor insensitive to interocular correlation. The monocular receptive fields have a excitatory centre and inhibitory surround, making them tuned to size. The disparity sensor combines inputs from the two eyes linearly, applies a threshold and then an exponent output nonlinearity. The activity of the sensor represents the model mantis’s instantaneous probability of striking. We integrate this over the stimulus duration to obtain the expected number of strikes in response to moving targets with different stereoscopic disparity, size and vertical disparity. We optimised the parameters of the model so as to bring its predictions into agreement with our empirical data on mean strike rate as a function of stimulus size and disparity. The model proves capable of reproducing the relatively broad tuning to size and narrow tuning to stereoscopic disparity seen in mantis striking behaviour. Although the model has only a single centre-surround receptive field in each eye, it displays qualitatively the same interaction between size and disparity as we observed in real mantids: the preferred size increases as simulated prey distance increases beyond the preferred distance. We show that this occurs because of a stereoscopic “false match” between the leading edge of the stimulus in one eye and its trailing edge in the other; further work will be required to find whether such false matches occur in real mantises. Importantly, the model also displays realistic responses to stimuli with vertical disparity and to pairs of identical stimuli offering a “ghost match”, despite not being fitted to these data. This is the first image-computable model of insect stereopsis, and reproduces key features of both neurophysiology and striking behaviour.

Saturation as a function of stimulus size: dependence on thresholds and luminance contrast

Saturation as a function of stimulus size: dependence on thresholds and luminance contrast

Preliminary Result on the Direct Assessment of Perceptible Simultaneous Luminance Dynamic Range

The human visual system is capable of adapting across a very wide dynamic range of luminance levels; values up to 14 log units have been reported. However, when the bright and dark areas of a scene are presented simultaneously to an observer, the bright stimulus produces significant glare in the visual system and prevents full adaptation to the dark areas, impairing the visual capability to discriminate details in the dark areas and limiting simultaneous dynamic range. Therefore, this simultaneous dynamic range will be much smaller, due to such impairment, than the successive dynamic range measurement across various levels of steady-state adaptation. Previous indirect derivations of simultaneous dynamic range have suggested between 2 and 3.5 log units. Most recently, Kunkel and Reinhard reported a value of 3.7 log units as an estimation of simultaneous dynamic range, but it was not measured directly. In this study, simultaneous dynamic range was measured directly through a psychophysical experiment. It was found that the simultaneous dynamic range is a bright-stimulus-luminance dependent value. A maximum simultaneous dynamic range was found to be approximately 3.3 log units. Based on the experimental data, a descriptive log-linear model and a nonlinear model were proposed to predict the simultaneous dynamic range as a function of stimulus size with bright-stimulus luminance-level dependent parameters. Furthermore, the effect of spatial frequency in the adapting pattern on the simultaneous dynamic range was explored. A log parabola function, representing a traditional Contrast Sensitivity Function (CSF), fitted the simultaneous dynamic range data well.

Dynamics of Contrast Decrement and Increment Responses in Human Visual Cortex.

PurposeThe goal of the present experiments was to determine whether electrophysiologic response properties of the ON and OFF visual pathways observed in animal experimental models can be observed in humans.MethodsSteady-state visual evoked potentials (SSVEPs) were recorded in response to equivalent magnitude contrast increments and decrements presented within a probe-on-pedestal Westheimer sensitization paradigm. The probes were modulated with sawtooth temporal waveforms at a temporal frequency of 3 or 2.73 Hz. SSVEP response waveforms and response spectra for incremental and decremental stimuli were analyzed as a function of stimulus size and visual field location in 67 healthy adult participants.ResultsSSVEPs recorded at the scalp differ between contrast decrements and increments of equal Weber contrast: SSVEP responses were larger in amplitude and shorter in latency for contrast decrements than for contrast increments. Both increment and decrement responses were larger for displays that were scaled for cortical magnification.ConclusionsIn a fashion that parallels results from the early visual system of cats and monkeys, two key properties of ON versus OFF pathways found in single-unit recordings are recapitulated at the population level of activity that can be observed with scalp electrodes, allowing differential assessment of ON and OFF pathway activity in human.Translational RelevanceAs data from preclinical models of visual pathway dysfunction point to differential damage to subtypes of retinal ganglion cells, this approach may be useful in future work on disease detection and treatment monitoring.

ANALYSIS OF THE OPTOMOTOR REACTION OF HELIOPHANUS MUSCORUM WALCK. (ARANEAE SALTICIDAE) INDUCED BY ROTATING SPIRALS

SUMMARY The reaction time of the optomotor response of the jumping spider Heliophanus muscorum Walck. (Araneae Salticidae), induced by a rotating archimedean spiral (turning reaction), mediated by the lateral eyes, seems to be inversely proportional to the angular velocity of « apparent » expansion or contraction of the target (depending on the sense of rotation). The frequently associated withdrawal movement (avoidance effect?) is at least to some extent a function of the stimulus size, since it is present only with angular sizes of the target > 14–15°. The data obtained with a hyperbolic spiral as a stimulus differ from the preceding ones in the sense that the reaction time and the withdrawal movement were not significantly affected by target distance within the ranged tested. The possible explanations of this behaviour are discussed in the text.

Foveal Spatial Summation in Human Cone Mechanism

We measured at the fovea the chromatic contrast threshold for stimuli modulated along different chromatic directions in the isoluminant plane of MBDKL colour space, considering the two cardinal axes (L/M) and S/(L + M) and other intermediate non-cardinal directions. This psychophysical determination was conducted as a function of stimulus size. The test stimulus was a foveal isoluminant Gaussian patch with a raised cosinusoidal temporal profile superimposed on a neutral background. The task was performed binocularly. The increment threshold was measured for three observers by a Bayesian adaptive psychometric method (QUEST). The Ricco area of complete spatial summation was estimated from the threshold-versus-area curves. The perceptive fields are smaller for the L/M-cone opponent direction than the S/(L + M)-cone opponent. The perceptive field sizes for the stimuli in non-cardinal chromatic directions and stimuli modulated at the (L/M)-cone opponent direction present similar values. Measurements were made at two luminance levels, 5 and 40 cd m(-2), but the differences found were small. The perceptive field sizes found could be associated with LGN area.

Color appearance in the peripheral retina as a function of stimulus size and intensity under rod-bleach conditions

Color appearance in the peripheral retina as a function of stimulus size and intensity under rod-bleach conditions

Differential dependency on motion coherence in subregions of the human MT+ complex

The detection of coherent motion embedded in noise has been widely used as a measure of global visual motion processing. Animal studies have demonstrated that this performance is closely linked to the responses of direction-sensitive neurons in the macaque middle temporal (MT) and medial superior temporal (MST) areas. Despite the strong similarities between the visual cortex of human and that of non-human primates, the human middle temporal complex (area MT+), located in the posterior part of the inferior temporal sulcus and presumably comprising both area MT and area MST, has not consistently been found to share the functional hallmark of MT and MST neurons, i.e. their preference for coherent rather than incoherent visual motion. In order to search for such preferences in human area MT+, blood oxygen level-dependent responses to random dot kinematograms presented in the right visual hemifield were studied here as a function of stimulus size and dot density. The stimulus extensions were varied in such a way as to cover an area either equaling, exceeding or falling below the mean receptive field size of macaque area MT. Unlike the posterior part of human area MT+, the anterior part and its right-hemisphere homolog showed significantly stronger responses to coherent than to incoherent motion. These differences were only present for large stimuli that presumably exceeded the receptive field size of neurons in area MT. Our results suggest that functional magnetic resonance imaging may reveal stronger responses to coherent visual motion in human area MST, provided that the stimulus allows for sufficient summation within the receptive fields. In contrast, functional magnetic resonance imaging may fail to reveal the same dependency for human area MT.

Spatial scale of stereomotion speed processing

To examine the spatial scale of the mechanisms supporting the perception of motion in depth defined by binocular cues, we measured stereomotion speed discrimination thresholds as a function of stimulus size using a two-interval speed comparison task. Stimuli were either random dot stereogram (RDS) bars featuring both the changing disparity (CD) and the interocular velocity difference (IOVD) cues to motion in depth or dynamic random dot stereogram (DRDS) bars featuring the CD cue alone. Monocular speed discrimination performance was also assessed, using half-images of the RDS stimulus. In addition, subjects' stereoacuity for stationary versions of the binocular stimuli was measured. Stimuli ranged in vertical extent from 1.25 to 40 min. Sensitivity to speed differences was strongly related to stimulus height for DRDS stimuli. Performance decreased rapidly as stimulus size was reduced, becoming nearly random for heights below 5 min. However, for RDS stimuli, speed discrimination performance declined with reductions in stimulus size at a far slower rate, providing superior performance at every stimulus size used. Monocular performance was superior still for the majority of subjects, yet showed a similar rate of decline to binocular RDS stimuli. We conclude that the spatial resolution of the CD mechanism and its static disparity inputs is, on average, nearly nine times more coarse than the IOVD system and its monocular motion inputs. Static stereoacuity controls show that this finding cannot be explained by differences in the disparity signals available in our RDS and DRDS stimuli.

Size matters, but not for everyone: Individual differences for contrast discrimination

It is very well known that contrast detection thresholds improve with the size of a grating-type stimulus, but it is thought that the benefit of size is abolished for contrast discriminations well above threshold [e.g., Legge, G. E., & Foley, J. M. (1980)]. Here we challenge the generality of this view. We performed contrast detection and contrast discrimination for circular patches of sine wave grating as a function of stimulus size. We confirm that sensitivity improves with approximately the fourth-root of stimulus area at detection threshold (a log-log slope of -0.25) but find individual differences (IDs) for the suprathreshold discrimination task. For several observers, performance was largely unaffected by area, but for others performance first improved (by as much as a log-log slope of -0.5) and then reached a plateau. We replicated these different results several times on the same observers. All of these results were described in the context of a recent gain control model of area summation [Meese, T. S. (2004)], extended to accommodate the multiple stimulus sizes used here. In this model, (i) excitation increased with the fourth-root of stimulus area for all observers, and (ii) IDs in the discrimination data were described by IDs in the relation between suppression and area. This means that empirical summation in the contrast discrimination task can be attributed to growth in suppression with stimulus size that does not keep pace with the growth in excitation.

The intrinsic electrophysiological characteristics of fly lobula plate tangential cells: III. Visual response properties.

In this last paper in a series (Borst and Haag, 1996; Haag et al., 1997) about the lobula plate tangential cells of the fly visual system (CH, HS, and VS cells), the visual response properties were examined using intracellular recordings and computer simulations. In response to visual motion stimuli, all cells responded mainly by a graded shift of their axonal membrane potential. While ipsilateral motion resulted in a graded membrane potential shift, contralateral motion led to distinct EPSPs. For HS cells, simultaneous extracellular recorded action potentials of a spiking interneuron, presumably the H2 cell, corresponded to the EPSPs in the HS cell in a one-to-one fashion. When HS cells were hyperpolarized during ipsilateral motion, they mainly produced action potentials, but when they were hyperpolarized during contralateral motion only a slight increase of EPSP amplitude, could be observed. Intracellular application of the sodium channel blocker QX 314 abolished action potentials of HS cells while having little effect on the graded membrane response to ipsilateral motion. HS and CH cells were also studied with respect to their spatial integration properties. For both cell types, their graded membrane response was found to increase less than linearly with the size of the ipsilateral motion pattern. However, while for HS cells various amounts of hyperpolarizing current injected during motion stimulation led to different saturation levels, this was not the case for CH cells. In response to a sinusoidal velocity modulation, CH cells followed pattern motion only up to 10 Hz modulation frequency, but HS cells still revealed significant membrane depolarizations up to about 40 Hz. In the computer simulations, the compartmental models of tangential cells, as derived in the previous papers, were linked to an array of local motion detectors. The model cells revealed the same basic response features as their natural counterparts. They showed a response saturation as a function of stimulus size. In CH-models, however, the saturation was less pronounced than in real CH-cells, indicating spatially nonuniform membrane resistances with higher values in the dendrite. As in the experiments, HS models responded to high-frequency velocity modulation with a higher amplitude than did CH models.

Assimilation-type and Contrast-type Bias of Motion Induced by the Surround in a Random-dot Display: Evidence for Center-Surround Antagonism

As a mechanism to detect differential motion, we have proposed a model of “a motion contrast detector” that has a center-surround antagonistic receptive field with respect to the direction of motion. Supporting evidence has been obtained in the studies of induced motion, motion capture, and motion aftereffect. In order to obtain further evidence in a more strictly controlled situation, we examined the perceptual bias of motion in a center stimulus induced by another, surrounding motion. By using a stochastic random-dot display configured in a center-surround concentric fashion, we measured the % signal in the center stimulus that made the stimulus perceptually stationary in the presence of a moving surround. Measurements were done for various stimulus sizes and eccentricities. The amount of bias changed as a function of stimulus size and eccentricity. At several eccentricities. smaller stimulus sizes tended to yield assimilation-type biases, whereas larger sizes tended to yield contrast-type biases. However, a spatial scaling procedure revealed that the amount of bias was a simpler function of “scaled” stimulus size that was obtained by dividing the physical size by a scaling factor at each eccentricity. In the scaled profile, assimilation-type bias changed to contrast-type bias with increasing size, reached the peak of contrast-type bias at a certain size, and decreased slightly with further increasing size. Furthermore, a model of a difference of Gaussians, DOG, function well approximated the behavior of the profile. From these results, we concluded that the process specific to perceiving relative motion is mediated by a motion contrast detector, which is possibly located in area MT. Copyright © 1996 Elsevier Science Ltd.

Spatial summation of the differential light threshold as a function of visual field location and age

Static differential light thresholds were measured as a function of stimulus size (Goldmann sizes I-V) along four visual field meridia (75, 165, 255 and 345 degrees) with the Humphrey Field Analyzer 640. Data were obtained for both young (n = 10, age 23.6 +/- 2.9 years) and elderly (n = 10, age 72.0 +/- 5.2 years) normal subjects. The resulting peripheral spatial summation curves could be equated to the foveal data simply by a change in size scale, which increased linearly with eccentricity. E2 values, expressing the eccentricity at which stimulus size must double for performance to remain comparable to the fovea, were in the order of 3-9 degrees. Whilst the rate of scale change is approximately the same for both young and elderly observers, differences in performance can be explained by a combination of lower sensitivity and a bias in sensitivity towards larger stimulus sizes with increasing age.

Assessment of current visual psycho-physical testing methods with special reference to primary open angle glaucomatous disease (POAGD)

Early diagnosis of glaucoma with standard perimetric technique remains unsatisfactory as the appearance of discernible visual field defects indicates already extensive optic nerve fiber loss. Experimental evidence, reviewed here in detail, indicates that retinal neurons selective for coarse, temporally modulated patterns are most vulnerable in early glaucoma. Applying this data to perimetric techniques the authors have designed a new test measuring paracentral contrast sensitivity. By presenting sinusoidal gratings of 1 cpd four degrees from fixation, contrast sensitivity was determined as a function of stimulus size. They obtained abnormal contrast sensitivity functions in glaucoma suspects with apparently intact visual fields indicating the potential for better detection of hidden glaucomatous nerve damage.

Spatial summation in blindsight.

Spatial summation curves were determined in the circumscribed visual-field defects of five patients with blindsight. Under light-adapted conditions that favor the color-opponent system, increment thresholds for white and red targets presented on a white background were measured as a function of stimulus size which ranged from 9-110 min arc. In both normal and defective hemifields, summation was linear for the red stimuli. In contrast, the curves measured with the white stimuli showed some nonlinearity in the normal hemifield, and a pronounced eccentricity-dependent notch in the field defect. The results indicate that the neurons mediating sensitivity differ in their summation properties for wavelength and intensity information.

Background-induced flicker enhancement in cat retinal horizontal cells. II. Spatial properties

1. Intracellular recordings have been made from cat retinal horizontal cells stimulated with flickering test spots. Dim backgrounds increase flicker amplitudes in response to small but not large test stimuli. 2. This background-induced flicker enhancement has been measured for different slit- and square-test stimulus widths and the results compared with two spatial models for the enhancement effect. 3. In the "dark test-region" model it is argued that rods within the test region are unresponsive to background stimuli because of prior saturation by the test stimulus. Background-evoked rod signals decay passively from regions outside the test stimulus through a syncytial network into the recording site, where they act on the cone-to-horizontal-cell synapse, increasing its gain. 4. In the "changing length-constant" model rod signals reduce the length constant of a syncytial network by uncoupling the cells within it. This causes an increased response to small but not large test stimuli. 5. Both models are analytically evaluated with the use of a conductive-sheet approximation to the syncytial network. Expressions are derived for network polarization [(V(0, 0)] as a function of stimulus size. The specific stimulus shapes considered are disks, rectangles, slits, and squares in both bright and dark varieties. From these expressions predictions of response enhancement as a function of stimulus size are made for both models. 6. The dark test-region model provides for an exponential decay of flicker enhancement as a function of slit width but a steeper-than-exponential decay with the width of squares, in close agreement with experimental data. 7. The changing length-constant model makes qualitatively similar predictions. Flicker enhancement declines nearly exponentially with slit width. For square-shaped test stimuli the predicted decline of flicker enhancement with size is somewhat shallower than either the dark test-region-model curve or the experimentally determined curve. 8. As recorded in the same set of cells and under the same set of stimulus conditions (with the use of both slit- and square-test stimuli), the mean length constant of the peak-to-peak flicker component in the horizontal-cell response is 168 +/- 18 (SE) microns with the background and 232 +/- 45 microns in the dark. The mean length constant for the background-induced flicker enhancement, as fit by dark test-region-model curves, is 186 +/- 22 microns (n = 9).(ABSTRACT TRUNCATED AT 250 WORDS)

Equating character-identification performance across the visual field

The effect of retinal eccentricity on stimulus identification was investigated by measuring observers' ability to identify digits of various sizes presented briefly at different locations in their visual fields. The stimuli were either presented one at a time or presented in pairs with each member of the pair at a different eccentricity and having a different size. Stimulus-response confusion matrices were used to calculate the information transmitted by each character as a function of stimulus size and retinal eccentricity. Linearly scaling stimulus size with retinal eccentricity yields equal information transmission across each subject's visual field. There was no difference between the amounts of information transmitted when the targets were presented alone and when they were transmitted in pairs. For small target sets, therefore, the amount of information transmitted from the screen to the observer can be approximated as the sum of the information at the separate locations.

On the depth of the cyclopean retina.

The maximum disparity which can elicit depth perception was measured in dynamic random-dot stereograms as a function of stimulus size and duration. Maximum disparity increased with size as a function of the square root of stimulus area, and depth could under optimal conditions be perceived up to several arc degrees of disparity, even for short (48 ms) durations of exposure. These data delineate the limiting disparity/depth characteristics of the cyclopean retina, and provide fresh evidence of the distinction between local and global disparity processing.

Acuity for spatial separation as a function of stimulus size

Acuity for comparing separations was measured using a pattern of 3 parallel lines. Line length and separation were varied within a maximum eccentricity of 312½ Threshold was constant for lines of length 30′ arc or shorter; above 30′ performance improved with line length. Performance fell with increasing separation. When compared with an ideal processor, efficiency was much lower than has been recorded for discriminating orientation or shape in small patterns. Efficiency falls with line length, at first steeply but then gradually; the change occurs at a line length of order 30 min arc. It is proposed that the positional specificity of cortical units is the effective information source for the task, and that this specificity is low.