Size tuning in the absence of spatial frequency tuning in object recognition.

How do we attend to objects at a variety of sizes as we view our visual world? Because of an advantage in identification of lowpass over highpass filtered patterns, as well as large over small images, a number of theorists have assumed that size-independent recognition is achieved by spatial frequency (SF) based coarse-to-fine tuning. We found that the advantage of large sizes or low SFs was lost when participants attempted to identify a target object (specified verbally) somewhere in the middle of a sequence of 40 images of objects, each shown for only 72 ms, as long as the target and distractors were the same size or spatial frequency (unfiltered or low or high bandpassed). When targets were of a different size or scale than the distractors, a marked advantage (pop out) was observed for large (unfiltered) and low SF targets against small (unfiltered) and high SF distractors, respectively, and a marked decrement for the complementary conditions. Importantly, this pattern of results for large and small images was unaffected by holding absolute or relative SF content constant over the different sizes and it could not be explained by simple luminance- or contrast-based pattern masking. These results suggest that size/scale tuning in object recognition was accomplished over the first several images (<576 ms) in the sequence and that the size tuning was implemented by a mechanism sensitive to spatial extent rather than to variations in spatial frequency.     

Influence of spatial frequency on tuning and bias for orientation and direction in the cat's striate cortex

Directionality, orientation and spatial frequency tuning were determined for 108 neurones recorded extracellularly from the striate cortex of anaesthetized cats. Significant sharpening of orientation selectivity with increasing spatial frequency was seen in all simple neurones and the overwhelming majority of complex neurones. Orientation selectivity sharpened in 90 and broadened in only 10 of 100 fully characterized neurones. At least four distinct classes of neurone could be characterized on the basis of their directionality at optimal spatial frequency, and the presence or absence of changes in directionality over a range of spatial frequencies: in two classes, directionality was spatial-frequency dependent; in the remaining two it was invariant. With two exceptions Type A neurones (23 cells) were direction-selective; they were narrowly tuned for orientation and spatial frequency, and their directionality was invariant with spatial-frequency. The majority of neurones (52 cells) were Type B, most of which were direction-biased; their bias for direction varied systematically with spatial frequency. Type C were direction-biased and spatial-frequency selective (5 cells), but showed a clear reversal of bias with change in spatial frequency. Type D, a subset of direction-biased cells, were bidirectional and spatial-frequency invariant (8 cells), with comparable response strengths to motion in two opposing directions at all spatial frequencies. These response types crossed traditional boundaries between categories of simple and complex neurones, assigned on the basis of spatial summation, presence or absence of end-inhibition, and receptive field size.     

The spatial tuning of chromatic adaptation

A key question in colour research is how the colour and spatial analysis of an image interact. Traditionally, colour and form analysis have been regarded as parallel and separate processes, and documented effects of image complexity on chromatic adaptation have been attributed to a temporal integration process during eye movements. Evidence is presented here for a spatial mechanism, which tunes chromatic adaptation to the luminance structure (spatial frequency and orientation) of an image. This in turn suggests a close cooperation between colour and form analysis during chromatic adaptation. The results are discussed in relation to the "segregated pathway hypothesis" and the role of spatial aspects for the computation of colour constancy and adaptation to natural scenes.     

Changes in spatial tuning of the pattern electroretinogram with age

Experimental and clinical studies have demonstrated that the electroretinogram in response to periodical patterns alternating in contrast at constant mean luminance (pattern reversal ERG, P-ERG), is correlated with ganglion cell activity. Senile functional changes of these neurons might be therefore investigated by the P-ERG technique. Steady-state P-ERGs (8 Hz) in response to sinusoidal gratings of different spatial frequencies (0.6-6.8 c/d) were recorded in normal subjects ranging over 50 year age span. The P-ERG amplitude as a function of stimulus spatial frequency shows a maximum between 1.2-1.7 c/d and attenuation at higher and lower spatial frequencies (spatial tuning). The P-ERGs of the older subjects are reduced in amplitute, as compared to those of the younger ones, over the whole range of spatial frequency. This reduction is more marked at intermediate spatial frequencies resulting in a shallower tuning.     

Adaptation duration affects the spatial selectivity of facial aftereffects

Adaptation processes in human early visual cortical areas are sensitive to the exposure time of the adaptor stimulus. Here we investigated the effect of adaptation duration at the higher, shape-specific stages of visual processing using facial adaptation. It was found that long-term (5s) adaptation evokes facial aftereffects consisting of a position invariant as well as a position-specific component. As a result of adaptation to a female face, test faces were judged more masculine when they were displayed in the same location as the female adaptor face, as compared to that when they were presented in the opposite visual hemifield. However, aftereffects evoked by short-term (500 ms) adaptation were found to be entirely position invariant. In accordance with these behavioral results, we found that the adaptation effects, measured on the amplitude of the N170 ERP component consisted of a position-specific component only after long-term, but not after short-term adaptation conditions. These results suggest that both short and long exposure to a face stimulus leads to adaptation of position invariant face-selective processes, whereas adaptation of position-specific neural mechanisms of face processing requires long-term adaptation. Our findings imply that manipulating adaptation duration provides an opportunity to specifically adapt different neural processes of shape-specific coding and to investigate their stimulus selectivity.     

Properties of second-order spatial frequency channels

The segregation of texture patterns may be carried out by a set of linear spatial filters (to enhance one of the constituent textures), a nonlinearity (to convert the higher contrast of response to that constituent to a higher mean response), and finally subsequent ("second-order") linear spatial filters (to provide a strong response to the texture-defined edge itself). In this paper, the properties of such second-order filters are characterized. Observers were required to detect or discriminate textures that were modulated between predominantly horizontally oriented and predominantly vertically oriented noise patterns. Spatial summation for these patterns reached asymptote for a stimulus size of 15 x 15 deg. Modulation contrast sensitivity was nearly flat over a five-octave range of spatial frequency, but was bandpass when stated as efficiency (relative to an idealized observer confronted with the same task). Increment threshold showed the improved performance with a sub-threshold pedestal seen in the "dipper effect", but the typical Weber's law behavior at higher pedestal contrasts was not observed at the highest pedestal modulation contrasts achievable with our stimuli. Sub-threshold summation experiments indicate that second-order filters have a moderate bandwidth.     

On the relationship between the spatial channels for luminance and disparity processing.

To determine the relationship between the spatial channels for luminance and shape-from-stereo-disparity processing we measured disparity modulation sensitivity as a function of disparity spatial frequency for sinusoidal modulations of a field of Gabor micropatterns of differing luminance spatial frequency. We first examine the effects of contrast, spatial bandwidth and element density and show that it is only the last of these which is critical for the shape of the disparity modulation threshold function. We show that the shape of this function depends on the luminance spatial frequency of the surface that is modulated in depth. Specifically, low corrugation frequencies enjoy a greater scale support from the early luminance spatial filters than do high corrugation frequencies. The results are consistent with higher spatial frequency disparity channels receiving a greater input from higher spatial frequency luminance channels.     

The spatial tuning of achromatic and chromatic vision in budgerigars

Birds are assumed to use half of their cones (double cones) to detect fine spatial detail while their other half (single cones) is used for color vision. However, the spatial resolution of the color pathway in birds has never been studied. We determined the spatial contrast sensitivity to achromatic and isoluminant red-green and blue-green color gratings in budgerigars (Melopsittacus undulatus). Contrast sensitivity to achromatic gratings has band-pass characteristics while that for red-green and blue-green gratings has low-pass properties. Maximum sensitivity is lower to blue-green than to red-green gratings and the acuity for both color gratings is less than half (ca. 4.5 cycles/degree) of that for achromatic gratings (ca. 10 cycles/degree). This suggests that achromatic vision in birds, as in humans and bees, is tuned for detecting fine detail while chromatic vision is tuned for viewing larger fields. Similar to humans, blue-sensitive cones contribute little to spatial vision. Moreover, budgerigars detected gratings having both achromatic and chromatic contrasts more reliably at high spatial frequencies than gratings with either of these contrasts, suggesting that the single and double cone pathways are incompletely separated. The study demonstrates the importance of the spatial dimension of color vision; fine patterns remain unresolved even if they present large color contrasts.     

The effect of micromovements of the eye and exposure duration on contrast sensitivity

Spatial contrast sensitivity was measured with the normally moving and the stabilized retinal image of sine-wave gratings for exposure durations ranging from 6 msec to 4 sec. Sensitivity increased rapidly for both the stabilized and the non-stabilized image of any frequency as a function of duration up to 50 msec. The rate of increase gradually approached zero as the target exposure was lengthened. The shape of the contrast sensitivity curves was primarily determined by exposure duration. Image stabilization resulted in a decrease of sensitivity to a large range of spatial frequencies only when the target was presented for an indefinite period.     

The effects of exposure duration and luminance on the 3-dot hyperacuity task

The 3-dot hyperacuity task was given to two subjects under three experimental paradigms: constant luminance, constant energy and constant duration. Hyperacuity was obtained under conditions (3 dots, 2 msec exposure) which rule out any significant temporal or spatial averaging. There was a clear threshold decrease in the constant luminance paradigm as exposure duration increased, no significant variations in threshold with the constant energy paradigm, as exposure duration varied and a U-shaped function in the constant exposure duration paradigm as luminance varied. It is concluded that what limits performance, at least for short exposure durations, is the total energy of the stimulus. The implications of the present results to the static and dynamic approaches to hyperacuity are discussed.     

Temporal tuning characteristics of the perceptual template and endogenous cuing of spatial attention

External noise presented in temporal contiguity with a target impairs perceptual performance, reflecting the temporal tuning of the perceptual template. Deriving the temporal characteristics of the perceptual template, however, requires an observer model that segregates the impact of non-linearities and intrinsic inefficiencies of the observer in order to account for the impact of external noise in various temporal configurations. In Experiment 1, we showed that the perceptual template model successfully accounts for temporal masking functions in foveal Gabor orientation identification masked by external noise with a wide range of temporal configurations, and estimates the temporal characteristics of the perceptual template. In Experiment 2, we extended the paradigm and the model to compare the temporal tuning characteristics of the perceptual template in central pre- and simultaneous cuing of endogenous spatial attention in a Gabor orientation identification task in visual periphery. We found that endogenous spatial attention excludes external noise by both sharpening the temporal window of the perceptual template and (mostly) reducing the impact of external noise uniformly across the entire temporal window.     

How spatial and feature-based attention affect the gain and tuning of population responses

How does attention optimize our visual system for the task at hand? Two mechanisms have been proposed for how attention improves signal processing: gain and tuning. To distinguish between these two mechanisms we use the equivalent-noise paradigm, which measures performance as a function of external noise. In the present study we explored how spatial and feature-based attention affect performance by assessing their threshold-vs-noise (TvN) curves with regard to the signature behavioral effects of gain and tuning. Furthermore, we link our psychophysical results to neurophysiology by implementing a simple, biologically-plausible model to show that attention affects the gain and tuning of population responses differentially, depending on the type of attention being deployed: Whereas spatial attention operates by boosting the gain of the population response, feature-based attention operates by both boosting the gain and sharpening the tuning of the population response.     

Contrast sensitivity, spatial and temporal tuning of the larval zebrafish optokinetic response

PURPOSE: To characterize the quantitative properties of the optokinetic response (OKR) in zebrafish larvae as a tool to test visual performance in genetically modified larvae. METHODS: Horizontal OKR was triggered in 5-day-old zebrafish larvae by stimulation with projected computer-generated gratings of varying contrast, angular velocity, temporal and spatial frequency, and brightness. Eye movements were analyzed by a custom-made eye tracker based on image analysis. RESULTS: The gain of the OKR slow phase was dependent on angular velocity, spatial frequency, and contrast of a moving grating, but largely independent on brightness. Eye velocity was a logarithmically linear function of grating contrast with a slope of approximately 0.8 per log unit contrast. CONCLUSIONS: The OKR of the larval zebrafish is not scaled for stimulus contrast and spatial frequency. These properties make the OKR a valuable tool to quantify behavioral visual performance such as visual acuity, contrast sensitivity, and light adaptation. This behavioral paradigm will be useful for analyzing visual performance in mutant and gene-knockdown larval zebrafish.     

The effects of edge sharpness and exposure duration on detection threshold

The detection threshold for a target has been previously shown to be affected by the physical sharpness of the target's edge. The present study confirmed that the detection threshold is higher for blurred targets if the exposure duration is long. But it was shown that with short exposure durations the blurred and sharp targets had identical thresholds. The difference in the percepts of long and short exposure duration stimuli is also considered. The results are discussed in the framework of models based on inhibitory interactions.     

Effects of varying 5-fluorouracil exposure duration on Tenon's capsule fibroblasts

Purpose : 5‐Fluorouracil (5‐FU) is used intraoperatively to improve the success of trabeculectomy. Common practice is a 5‐min application of 5‐FU delivered via a Weck Cel sponge. The aim of this study was to determine if shorter exposure times were as effective in inhibiting Tenon’s capsule fibroblast proliferation. Methods : Samples of Tenon’s capsules, obtained from young patients undergoing strabismus surgery, were immersed in vitro in 5‐FU (50 mg/mL) for periods ranging from 1 to 10 min. Control samples were exposed either to growth medium (M199) or 5‐FU vehicle alone. Explants were cultured for up to 7 days and analysed for cell density, cell viability using fluoroprobe detection, and cell proliferation determined by proliferating cell nuclear antigen (PCNA)‐staining. Results : Exposure to 5‐FU for 1 and 5 min resulted in a similar and significant inhibition of the culture‐induced increase in stromal cell density (P < 0.01), a significant reduction in the percentage of viable cells (P < 0.02), and reduced PCNA indices, compared with controls. Exposure times of 3 and 10 min produced similar results. Conclusions : In vitro, under conditions of organ culture, a 1‐min exposure to 5‐FU had a significant antiproliferative effect on fibroblasts of Tenon’s capsule, and was similar to 5 min of exposure. Optimal intraoperative 5‐FU exposure time may be as little as 1 minute.     

The influence of exposure duration in transscleral Nd:YAG laser cyclophotocoagulation.

Transscleral cyclophotocoagulation was performed in human autopsy eyes by using three Nd:YAG lasers with different durations of exposure: a pulsed, contact laser with a duration of 0.75 millisecond and a range of one to ten pulses per burst (GLase 106, Sunrise Technologies, Fremont, California); a pulsed, noncontact laser with a duration of 20 milliseconds (Microruptor 2, Lasag Medical Lasers, Thun, Switzerland); and a continuous-wave, contact laser with durations of 700 and 2,000 milliseconds (Microruptor 3, Lasag Medical Lasers, Thun, Switzerland). Tissue responses were observed with a high-magnification videographic recording technique to analyze the immediate, real-time laser effects, and by light microscopy to characterize the laser-induced lesions further. Videographically, both pulsed lasers were noted to cause mild whitening of the pigment epithelium with frequent vaporization and explosive tissue disintegration. Histologically, the 0.75-millisecond pulse typically produced the most marked epithelial disruption, referred to as an explosive-like lesion, whereas the 20-millisecond pulse more often caused moderate tissue disruption with elevation of the epithelial layers in a blister-like lesion. In contrast, the continuous-wave laser was observed videographically to produce prominent tissue whitening and puckering, seen histologically as convolution of the epithelium and coagulation of stroma, which was called a shrinkage-like lesion. Our study suggests that exposure duration influences in vitro tissue response to transscleral Nd:YAG cyclophotocoagulation, although in vivo studies and clinical trials are needed to determine which tissue response is optimum for clinical use.     

Evidence on the local character of spatial frequency channels in the human visual system

The question has been raised as to whether there exists any transfer of information between neighboring retinal areas following presentation of spatially limited optical stimuli. Our experiments were aimed at mechanisms by which spatial frequency information could be transferred from one part of the visual field to another. Within a 0.9 degree X 2.5 degree section of the 3.6 degree X 2.5 degree testfield the observers adapt to a number of moving sinusoidally-modulated brightness or hue gratings of different spatial frequencies. As known by many publications the sensitivity to spatial frequencies changes in the adapted area. We find that this influence on the contrast threshold decreases as the distance to the adaptation section increases. Hence our results suggest mechanisms within the visual system mathematically best described by a local spectrum analysis.     

The spatial tuning of color and luminance peripheral vision measured with notch filtered noise masking.

We have measured the spatial bandwidths of the bandpass red-green chromatic and luminance mechanisms at four locations in the nasal visual field (0, 10, 20 and 30 degrees) using a method of notch filtered noise masking which effectively removes the artifact of off-frequency looking for our stimuli. Detection thresholds were measured for luminance or isoluminant red-green Gaussian enveloped test gratings of 0.5 cpd embedded in 1/f noise. Firstly, thresholds were obtained as a function of increasing noise spectral density and were fitted using a standard noise masking model. These results support the existence across the visual field of independent, red-green chromatic and luminance mechanisms with similar sampling efficiencies. Secondly, we measured thresholds in notch filtered noise as a function of notch width and derived the spatial bandwidth of the detection mechanism. We find both color and luminance mechanisms have similar bandwidths which remain virtually constant across eccentricity. These results indicate strong overall similarities between the early processing of color and luminance vision, and lend support to the role of color as an 'intrinsic image' in spatial vision. The results are discussed in the light of the anchored channel and shifting channel models of peripheral contrast sensitivity and pattern detection.