Parrel processing in visual short-term memory.

Visual short-term memory for the contrast and spatial frequency of sinusoidal gratings was measured in a delayed discrimination task in which the 2 stimuli to be compared were separated in time by 1-10 s interstimulus intervals (ISIs). Delayed discrimination thresholds for spatial frequency and contrast were compared, both when the 2 types of thresholds were measured in separate blocks of trials and when the 2 types of measures were randomly intermixed in an uncertainty paradigm, which required participants to process information about both dimensions on each trial. In both cases, accuracy of memory for spatial frequency was independent of ISI, but memory for contrast decreased as ISI increased. Performance was lower in the uncertainty case, but only by an amount predicted by statistical decision theory for independent sources. The results are consistent with a model assuming a set of parallel special-purpose visual discrimination and short-term memory mechanisms. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Retention and disruption of motion information in visual short-term memory.

Velocity discrimination thresholds for drifting luminance gratings were measured as a function of the time interval between test and reference gratings, using a 2-interval, forced-choice procedure. Discrimination thresholds, expressed as Weber fractions (ΔV/V), were independent of interstimulus interval (ISIs) ranging from 1 to 30 sec, demonstrating perfect short-term retention of velocity information. When a 3rd grating was briefly presented halfway through a 10-sec ISI, memory masking was observed. Discrimination thresholds in memory masking were unaffected by maskers of the same velocity but increased by 100% when test and masker velocity differed by a factor of 2. The results are interpreted with reference to a model where the short-term memory for simple stimulus attributes is assumed to be organized in terms of arrays of memory stores linked in a lateral inhibitory network. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The precision of velocity discrimination across spatial frequency

The precision of velocity coding for moving stimuli of different spatial frequencies was assessed by measuring velocity discrimination thresholds for a 1-c/deg grating paired with a grating whose spatial frequency ranged from 0.25 to 4 c/deg and for grating pairs of the same spatial frequency (0.25, 1, and 4 c/deg). The gratings always moved upward, with velocities ranging from 0.5 to 16 deg/sec. Velocity discrimination was as precise for stimuli that varied in spatial frequency by +/- 2 octaves (0.25 vs. 1 c/deg and 4 vs. 1 c/deg) as for stimuli of the same spatial frequency, for specific ranges of velocity that depended on the spatial and, therefore, the temporal frequencies of the stimuli. Compared with a 1-c/deg grating, the perceived velocity of 4-c/deg gratings was about 1.3 times faster and that of 0.25-c/deg gratings was about 1.3 times slower. Although these perceived velocity biases imply variation of velocity-signal processing among spatial frequency channels, the discrimination results indicate that the motion-sensing system can compare signals across different spatial frequency channels to make fine velocity discrimination within appropriate temporal frequency limits.     

Spatial frequency discrimination: visual long-term memory or criterion setting?

A long-term sensory memory is believed to account for spatial frequency discrimination when reference and test stimuli are separated by long intervals. We test an alternative proposal: that discrimination is determined by the range of test stimuli, through their entrainment of criterion-setting processes. Experiments 1 and 2 show that the 50% point of the psychometric function is largely determined by the midpoint of the stimulus range, not by the reference stimulus. Experiment 3 shows that discrimination of spatial frequencies is similarly affected by orthogonal contextual stimuli and parallel contextual stimuli and that these effects can be explained by criterion-setting processes. These findings support the hypothesis that discrimination over long intervals is explained by the operation of criterion-setting processes rather than by long-term sensory retention of a neural representation of the stimulus.     

Discrimination of changes in the slopes of the amplitude spectra of natural images: band-limited contrast and psychometric functions

Thresholds were measured for discriminating changes in the slopes of the amplitude spectra of stimuli derived from photographs of natural scenes and from random-luminance patterns. The variety and magnitudes of the thresholds could be explained by a model based on the discrimination of the changes in band-limited local contrast. Different spatial scales of local contrast (or different spatial-frequency bands of about 1 octave) were implicated for different reference spectral slopes; the model implicated a lower frequency-band for stimuli with shallower amplitude spectra. The implications of the model were tested experimentally by using stimuli in which the spectra were changed within restricted spatial-frequency bands. When the amplitude spectra of the test and reference stimuli differed only within the implicated frequency bands, thresholds were affected little. However, when the test and reference spectra differed at all frequencies except those in the implicated bands, thresholds were elevated markedly. The forms of the psychometric functions for the discrimination task were entirely compatible with the hypothesis that the task relies upon the ability to discriminate changes of contrast. The Weibull functions fitted to the data had slope parameters (beta) in the range 1 to 3, compatible with discrimination of low (but suprathreshold) contrasts.     

The limit of tactile spatial resolution in humans: grating orientation discrimination at the lip, tongue, and finger

The sensory neural pathways serving the lip, tongue, and finger are specialized for spatial information processing; thus, damage to these pathways is likely to be manifested most prominently as a loss of spatial acuity. For that reason, accurate measurement of spatial resolution at these regions is particularly important. The conventional test, the two-point discrimination task, does not measure the limit of spatial resolution and it yields variable results because it does not control nonspatial cues. The aim of this study was to quantify the limits of spatial resolution at the lip, tongue, and finger and to study the repeatability of those measurements using a stimulus that does not introduce nonspatial cues. We employed a grating orientation discrimination test, which has been studied extensively in relation to the underlying neural mechanisms. We obtained psychophysical thresholds for tactile spatial resolution from 15 normal, young adult subjects over seven test sessions. The finest gratings whose orientations were discriminated reliably had groove widths (gratings had equal groove and bar widths) that averaged 0.51 mm at the lip, 0.58 mm at the tongue, and 0.94 mm at the finger. These threshold measurements were highly reproducible between sessions with an overall improvement of 2% per session. These data suggest that the grating orientation discrimination task provides a stable, reliable measure of the human capacity for spatial resolution.     

Processing and retention of complex auditory stimuli in monkeys (Cebus apella).

Four monkeys were trained to discriminate between "tunes" that differed primarily in their temporal structure, or frequency envelopes. The conclusion drawn from a variety of transfer tests was that they learned the discrimination on the basis of local features of the tunes rather than on their temporal patterns. Subsequent comparisons of retention based on auditory–visual and visual–visual matching tasks suggested that this deficiency was not due to a truncated auditory short-term store. Indeed, when previous experience with remembering visual and auditory stimuli was comparable, short-term memory also seemed comparable in the 2 modalities. It is noted that the ability to discriminate and employ the frequency contours of tonal patterns may be related to the capacity for acoustically based language, and that in the human infant, these skills seem to develop in parallel. (French abstract) (36 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Modeling spatial integration and contrast invariance in visual pattern discrimination

PURPOSE: Human pattern discrimination performance has been reported to be largely independent of stimulus contrast but to depend on stimulus area. The authors propose a model that combines the effects of spatial integration and contrast. The model is based on the computation of similarity between pattern templates in memory and signals to be discriminated using normalized correlation. There are also two sources of additive noise, one before and one after the computation of correlation. The model was compared with human observers in an orientation discrimination task. METHODS: Orientation discrimination thresholds of human observers were measured for sinusoidal gratings of various areas, contrasts, and spatial frequencies. A two-interval, forced-choice methods was used. The performance of the model was determined by using computer simulations. RESULTS: It was found that the effects of contrast and grating area were interrelated. The decrease of orientation thresholds as a function of grating area was considerably larger at low than at high contrast. On the other hand, orientation thresholds decreased clearly as a function of contrast at the smallest grating areas but hardly at all at the largest grating areas. The model accounted well for the experimental findings. CONCLUSIONS: Because the invariance of orientation discrimination with respect to stimulus contrast depended on area, the cause of the invariance appeared to occur after spatial integration. The model explains this so that, with increasing contrast or area, the normalized correlation gradually approached a constant value. The proportion of pretemplate noise became negligible compared to the constant posttemplate noise. Thus, total noise also approached a constant value. Hence, the signal-to-noise ratio and discrimination performance became constant.     

Separation of low-level and high-level factors in complex tasks: Visual search.

A method for assessing the role of low-level factors in complex tasks is described. The method, which involves comparing simple-discrimination performance and complex-task performance for the same stimuli, was used to assess the role of low-level factors in multiple-fixation visual search. In one experiment, the target and background were composed of line segments that differed in color, orientation, or both; in another, target and background were composed of filtered-noise textures that differed in spatial frequency, orientation, or both. Most of the variance in search time was found to be predictable from the discrimination data, suggesting that low-level factors often play a dominant role in limiting search performance. A signal-detection model is presented that demonstrates how current psychophysical models of visual discrimination might be generalized to obtain a theory that can predict search performance for a wide range of stimulus conditions. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Pattern perception at high velocities

BACKGROUND: When objects are stationary, human pattern vision is exquisitely acute. A number of studies show, however, that Vernier acuity for lines is greatly impaired when the target velocity exceeds about 5 deg sec-1. The degradation of line Vernier acuity under image motion appears to be a consequence of a shift in the spatial scale of analysis to low spatial frequencies. If correct, this implies that Vernier acuity may not be subject to a strict velocity limit, and that with appropriate low spatial frequency stimuli, Vernier acuity might be preserved at high velocities. To test this notion, we measured Vernier acuity and contrast discrimination using low spatial frequency periodic gratings drifting over a wide range of velocities. RESULTS: Vernier acuity and contrast discrimination for low spatial frequency periodic gratings are both possible at velocities as high as 1000 deg sec-1. When both are specified in the same units (as Weber fractions), Vernier acuities are closely predicted by the observers' contrast discrimination thresholds. Our results suggest that Vernier acuity is subject to a spatiotemporal limit, rather than to a strict velocity limit. At temporal frequencies less than about 10 Hertz, Vernier acuity is independent of velocity, but is strongly dependent on stimulus contrast. At high temporal frequencies Vernier acuity is markedly degraded, and shows little dependence on contrast. CONCLUSIONS: Two mechanisms, which may have their neuronal counterparts early in the visual pathway, appear to limit the perception of moving targets at low and high temporal frequencies. Taken together with other recent work the present results suggest that the process of spatio-temporal interpolation in pattern analysis can operate at very high velocities.     

Interpreting the effects of image manipulation on picture perception in pigeons (Columba livia) and humans (Homo sapiens).

The effects of picture manipulations on humans' and pigeons' performance were examined in a go/no-go discrimination of two perceptually similar categories, cat and dog faces. Four types of manipulation were used to modify the images. Mosaicization and scrambling were used to produce degraded versions of the training stimuli, while morphing and cell exchange were used to manipulate the relative contribution of positive and negative training stimuli to test stimuli. Mosaicization mainly removes information at high spatial frequencies, whereas scrambling removes information at low spatial frequencies to a greater degree. Morphing leads to complex transformations of the stimuli that are not concentrated at any particular spatial frequency band. Cell exchange preserves high spatial frequency details, but sometimes moves them into the “wrong” stimulus. The four manipulations also introduce high-frequency noise to differing degrees. Responses to test stimuli indicated that high and low spatial frequency information were both sufficient but not necessary to maintain discrimination performance in both species, but there were also species differences in relative sensitivity to higher and lower spatial frequency information. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

The interaction between stereoscopic and luminance motion

An interaction in apparent motion between perceived three-dimensional forms defined by stereopsis and local luminous elements is reported. Vertical stripes of cyclopean square gratings were simulated by random-dot stereograms. Alternation of two-frame stereograms whose phases differed by 90 deg caused two kinds of percepts, planes' motion in depth (first-order stereoscopic motion, first-order SM) or lateral motion of gratings (higher-order stereoscopic motion, higher-order SM). Experiment 1 explored the conditions under which higher-order SM frequently arose, as opposed to local luminance-based in-depth motion (first-order SM). The results show that, when the spatial arrangements of two-frame random dots were correlated, higher-order SM dominated for long ISI conditions (ISI > 73 msec). When they were uncorrelated, higher-order SM dominated even under zero ISI conditions. Subjects reported that, when higher-order SM was seen, dots were attached to the surfaces of the moving cyclopean figure (motion capture). Experiment 2 tested which factor caused the domination of higher-order SM under uncorrelated conditions in Experiment 1, the larger distance of dot jump or the varied directions of the dots' motion. The results show that, when the distance of dot jump is large or when the directions of dots' motion are incoherent, higher-order SM arises more frequently. When local first-order motion signals are weakened by appropriate temporal and spatial conditions or by incoherent motion directions, higher-order SM dominates and it captures the motion of dots.     

The stereoscopic anisotropy: Individual differences and underlying mechanisms.

Observers are more sensitive to variations in the depth of stereoscopic surfaces in a vertical than in a horizontal direction; however, there are large individual differences in this anisotropy. The authors measured discrimination thresholds for surfaces slanted about a vertical axis or inclined about a horizontal axis for 50 observers. Orientation and spatial frequency discrimination thresholds were also measured. For most observers, thresholds were lower for inclination than for slant and lower for orientation than for spatial frequency. There was a positive correlation between the 2 anisotropies, resulting from positive correlations between (a) orientation and inclination thresholds and (b) spatial frequency and slant thresholds. These results support the notion that surface inclination and slant perception is in part limited by the sensitivity of orientation and spatial frequency mechanisms. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The effect of increased response requirements on discriminative performance of the domestic hen in a visual acuity task

Six domestic hens were trained in a spatial discrimination task. A controlled reinforcement procedure insured that the ratio of scheduled and obtained reinforcement remained equal. Gray stimuli and gratings ranging in spatial frequency from 1 to 10 cycles per millimeter were presented in seven descending series of probes. The response requirement to the sample key was varied from fixed ratio 1 to fixed ratio 40 in seven experimental conditions. An increase in response requirements from fixed ratio 1 to fixed ratio 5 and fixed ratio 10 resulted in significantly higher accuracy at discriminable grating values. Further increases in response requirements did not consistently improve performance. Generally, response biases increased and occasionally became extreme for probes at finer gratings with increased response requirements.     

Hippocampal seizures disrupt working memory performance but not reference memory acquisition.

The effect of hippocampal seizures in rats was assessed in two spatial memory tasks: The reference memory task was a simultaneous two-choice discrimination in a T-maze. The working memory task was a delayed conditional discrimination in a radial arm maze. In each task the hippocampus of each rat was stimulated to seizure after the presentation of the information to be remembered. In the reference memory task, hippocampal seizures did not impair acquisition, whether the stimulation was given immediately after or 4 hr after the presentation of the stimuli to be remembered. In the working memory task, hippocampal seizures did impair performance in a group of the same rats. These results support the distinction between a trial-dependent working memory system that requires hippocampal function and a trial-independent memory system that does not depend on hippocampal function. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Feature-specific effects of selective visual attention

Four experiments were conducted to quantify the effect of performing a foveal discrimination task on sensitivity for a peripheral grating. The observer's primary task was to discriminate either the spatial frequency or orientation of successive foveal Gabor patches. On a third of the trials they also performed a secondary task to detect the presence of a near-threshold grating in the periphery. We find that sensitivity for detection of the peripheral grating depends on the similarity of the spatial frequencies and orientations between the foveal and peripheral stimuli. Importantly, sensitivity is also affected by which feature is being discriminated in the central task. Because the detectability of the peripheral grating is different when different features of the central stimuli are discriminated, we suggest that the effects on sensitivity are due to feature-specific attention and not simply to passive interactions between filters with similar tuning properties.     

Attention to hierarchical level influences attentional selection of spatial scale.

Ample evidence suggests that global perception may involve low spatial frequency (LSF) processing and that local perception may involve high spatial frequency (HSF) processing (Shulman, Sullivan, Gish, & Sakoda, 1986; Shulman & Wilson, 1987; Robertson, 1996). It is debated whether SF selection is a low-level mechanism associating global and local information with absolute LSF and HSF content, or whether it is a higher level mechanism involving a selective process that defines the SF range in which global and local can then be relatively defined. The present study provides support for the latter claim by demonstrating that allocating attention to global or local levels of hierarchical displays biased selection of LSFs or HSFs, respectively, in subsequently presented compound gratings. This bias occurred despite a change in the response dimension (from letter identification in the hierarchical stimulus to orientation discrimination in the grating) and despite a difference in retinal location of the hierarchical stimuli and the grating stimulus. Moreover, the bias was determined by the relationship between the 2 SFs in the compound grating (i.e., their relative frequency) rather than the absolute SF values. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Size contrast as a function of conceptual similarity between test and inducers

In four experiments, the effect of the semantic relationship between test and inducing stimuli on the magnitude of size contrast in an Ebbinghaus-type illusion was explored. In Experiments 1 and 2, the greatest illusion was found when test and inducing stimuli were identical in shape and differed only in size. Decreased size contrast was found when inducing stimuli were drawn from the same category as the test stimulus, but were not visually identical. Even less size contrast was found when inducing stimuli were from a near conceptual category, with the least effect when they were drawn from a completely different category. In Experiment 3, it was demonstrated that even if test and inducing stimuli are drawn with identical geometric elements, the size contrast illusion is greatly reduced if they represent apparently different conceptual categories (through the manipulation of orientation and perceptual set). In Experiment 4, any geometric or spatial confounds were ruled out. These results suggest that size contrast is strongly influenced by the conceptual similarity between test and inducing stimuli.     

Competition and sharing of processing resources in visual discrimination.

Discrimination thresholds for spatial frequency and contrast tested individually were compared with dual discrimination of contrast and spatial frequency, and dual discrimination of 2 contrast or spatial frequency components. The components were presented overlapping, forming a compound grating or as side-by-side simple gratings. When observers had to judge contrast and spatial frequency simultaneously, discrimination thresholds increased by an amount predicted by a model of stimulus uncertainty for orthogonal dimensions (1.7); when they had to judge 2 frequency or contrast components, discrimination thresholds increased by a factor of 3–6 compared with the single-judgment task. The relative spatial location of the components did not interact with task complexity. The results are consistent with a model assuming a set of parallel special-purpose attentional mechanisms. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conjunction of color and form without attention: Evidence from an orientation-contingent color aftereffect.

According to A. Treisman and G. Gelade's (see record 1980-04685-001) feature-integration theory, separable features such as color and shape exist in separate maps in preattentive vision and can be integrated only through the use of spatial attention. However, many perceptual aftereffects, which are also assumed to reflect the features available in preattentive vision, are sensitive to conjunctions of features. One possible resolution of these views holds that adaptation to conjunctions depends on spatial attention. Exp I tested this proposition by presenting 24 undergraduates with gratings varying in color and orientation. The resulting McCollough aftereffects were independent of whether the adaptation stimuli were presented inside or outside of the focus of spatial attention. Therefore, color and shape appear to be conjoined preattentively when perceptual aftereffects are used as the measure. These same stimuli, however, appeared to be separable in Exps II and III, which required 24 Ss to search for gratings of a specified color and orientation. These results show that different experimental procedures may be tapping into different stages of preattentive vision. (63 ref) (PsycINFO Database Record (c) 2011 APA, all rights reserved)