Large effects of peripheral cues on appearance correlate with low precision

In a previous study (M. Carrasco, S. Ling, & S. Read, 2004), observers selected one of two Gabors that appeared to have higher contrast (comparative judgment). A peripheral cue preceded one of the Gabors by 120 ms. Results showed that the cue increased the perceived contrast of the adjacent Gabor. We replicated the experiment and found correlations between the precision of judgments and perceptual cueing effects. Larger cueing effects occurred in conditions with less precise judgments and in observers who saw less difference between the stimuli. Further, we asked observers to judge whether the two Gabors were equal or different (equality judgment). This method avoids decision biases but turned out to be less efficient. Cueing effects were absent for equality judgments and trained observers but present for untrained observers with poor ability to discriminate the stimuli. In another experiment, we showed that when observers' responses became more rapid over the course of the experiment, a cueing effect in brightness perception emerged that is unlikely to reflect perceptual changes. Overall, the results show that large effects of peripheral cues on appearance correlate with poor ability to discriminate the stimuli. We suggest that the cue biased observers' decision but did not affect their perception.     

Transient covert attention and the perceived rate of flicker

Transient covert attention affects basic visual dimensions such as contrast sensitivity, spatial resolution, and temporal resolution. Two recent studies provide evidence of corresponding phenomenological changes: The increase in contrast sensitivity and spatial resolution at the attended location is associated with increased apparent contrast (M. Carrasco, S. Ling, & S. Read, 2004) and apparent spatial frequency (J. Gobell & M. Carrasco, 2005). Here, we assessed a phenomenological correlate of attention for temporal vision, asking whether and how transient attention affects perceived flicker rate. We employed a psychophysical method developed to assess changes in appearance by manipulating transient attention via uninformative spatial cues. In each trial, two suprathreshold Gabor stimuli, appearing briefly to the left and right of fixation, were counterphase modulated at either the same or different temporal frequencies. To assess appearance, we asked observers to perform an orientation discrimination task contingent on perceived flicker rate: "What was the orientation of the Gabor that flickered faster?" Results indicated that perceived flicker rate increased at the cued location. A control experiment, in which observers reported the orientation of the Gabor that flickered slower, ruled out a cue bias explanation. We conclude that transient attention increases perceived flicker rate.     

Attention changes perceived size of moving visual patterns

Spatial attention shifts receptive fields in monkey extrastriate visual cortex toward the focus of attention (S. Ben Hamed, J. R. Duhamel, F. Bremmer, & W. Graf, 2002; C. E. Connor, J. L. Gallant, D. C. Preddie, & D. C. Van Essen, 1996; C. E. Connor, D. C. Preddie, J. L. Gallant, & D. C. Van Essen, 1997; T. Womelsdorf, K. Anton-Erxleben, F. Pieper, & S. Treue, 2006). This distortion in the retinotopic distribution of receptive fields might cause distortions in spatial perception such as an increase of the perceived size of attended stimuli. Here we test for such an effect in human subjects by measuring the point of subjective equality (PSE) for the perceived size of a neutral and an attended stimulus when drawing automatic attention to one of two spatial locations. We found a significant increase in perceived size of attended stimuli. Depending on the absolute stimulus size, this effect ranged from 4% to 12% and was more pronounced for smaller than for larger stimuli. In our experimental design, an attentional effect on task difficulty or a cue bias might influence the PSE measure. We performed control experiments and indeed found such effects, but they could only account for part of the observed results. Our findings demonstrate that the allocation of transient spatial attention onto a visual stimulus increases its perceived size and additionally biases subjects to select this stimulus for a perceptual judgment.     

A three-response task reveals how attention alters decision criteria but not appearance

Whether attention alters appearance or just changes decision criteria continues to be controversial. When subjects are forced to choose which of two equal targets, one of which has been pre-cued, has a higher contrast, they tend to choose the cued target. This has been interpreted as attention increasing the apparent contrast of the cued target. However, when subjects must decide whether the two targets have equal or unequal contrast, they respond veridically with no apparent effect of attention. The discrepancy between these comparative and equality judgments is explained by attention altering the decision criteria but not appearance. We supposed that when subjects are forced to choose which of two apparently equal targets has the higher contrast, they tend to proportion their uncertainty in favor of the cued target. To test this hypothesis, we used a three-response task, in which subjects chose which target had the higher contrast but also had the option to report that the targets appeared equal. This task disentangled potential attention effects on appearance from those on the decision criteria. We found that subjects with narrower criteria about what constituted equal contrast were more likely to choose the cued target, supporting the uncertainty stealing hypothesis. Across the population, the effects of the attentional cue are explained as changes in the decision criteria and not changes in appearance.     

Attention alters decision criteria but not appearance: a reanalysis of Anton-Erxleben, Abrams, and Carrasco (2010)

Paying attention to a stimulus affords it many behavioral advantages, but whether attention also changes its subjective appearance is controversial. K. A. Schneider and M. Komlos (2008) demonstrated that the results of previous studies suggesting that attention increased perceived contrast could also be explained by a biased decision mechanism. This bias could be neutralized by altering the methodology to ask subjects whether two stimuli were equal in contrast or not rather than which had the higher contrast. K. Anton-Erxleben, J. Abrams, and M. Carrasco (2010) claimed that, even using this equality judgment, attention could still be shown to increase perceived contrast. In this reply, we analyze their data and conclude that the effects that they reported resulted from fitting symmetric functions that poorly characterized the individual subject data, which exhibited significant asymmetries between the high- and low-contrast tails. The strength of the effect attributed to attentional enhancement in each subject was strongly correlated with this skew. By refitting the data with a response model that included a non-zero asymptotic response in the low-contrast regime, we show that the reported attentional effects are better explained as changes in subjective criteria. Thus, the conclusion of Schneider and Komlos that attention biases the decision mechanism but does not alter appearance is still valid and is in fact supported by the data from Anton-Erxleben et al.     

Attention makes moving objects be perceived to move faster

Although it is well established that attention affects visual performance in many ways, by using a novel paradigm [Carrasco, M., Ling, S., & Read. S. (2004). Attention alters appearance. Nature Neuroscience, 7, 308-313.] it has recently been shown that attention can alter the perception of different properties of stationary stimuli (e.g., contrast, spatial frequency, gap size). However, it is not clear whether attention can also change the phenomenological appearance of moving stimuli, as to date psychophysical and neuro-imaging studies have specifically shown that attention affects the adaptability of the visual motion system. Here, in five experiments we demonstrated that attention effectively alters the perceived speed of moving stimuli, so that attended stimuli were judged as moving faster than less attended stimuli. However, our results suggest that this change in visual performance was not accompanied by a corresponding change in the phenomenological appearance of the speed of the moving stimulus.     

Properties of human eye-head gaze shifts in an anti-gaze shift task

We investigated the metrics and kinematics of human eye-head gaze shifts using the anti-gaze shift task. Surprisingly, no systematic difference was found between peak gaze velocities of large pro- and anti-gaze shifts. In a follow-up experiment that equated perceived stimulus luminance across multiple eccentricities, pro-gaze shifts were consistently faster than anti-gaze shifts. In both experiments, we did not observe any head-only errors where initial head motion dissociates from gaze direction, even though many subjects generated such movements in other paradigms. These experiments confirm the influence of stimulus luminance on comparative movement velocity, and demonstrate that the behavioural set assumed in this task discourages head-only errors.     

On altering motion perception via working memory-based attention shifts

Transient spatial attention has been shown to alter different stimulus properties, such as perceived contrast and speed of motion. Interestingly, recent studies seem to suggest that involuntary shifts of spatial attention can also occur on the basis of the contents of Working Memory (WM). According to the biased competition model of visual attention, stimuli in the visual field that match those currently maintained in working memory receive the highest attentional priority. In a series of five experiments we addressed whether WM-based attention shifts alter perception, and specifically speed of motion. We found that response times for target discrimination were shorter at the location where an object matching the one held in WM was presented, thus confirming WM-based attention shifts. In contrast, results from other experiments, in which a speed-discrimination task was used, failed to reveal any modulation of motion speed perception by the content of WM. At present these data suggest that WM-based attention operates at different (later) stages of visual analysis than transient attention.     

Chromatic and contrast selectivity in color contrast adaptation

We used color contrast adaptation to examine the chromatic and contrast selectivity of central color mechanisms. Adaptation to a field whose color varies along a single axis of color space induces a selective loss in sensitivity to the adapting axis. The resulting changes in color appearance are consistent with mechanisms formed by different linear combinations of the cone signals. We asked whether the visual system could also adjust to higher-order variations in the adapting stimulus, by adapting observers to interleaved variations along both the L versus M and the S versus LM cardinal axes. The perceived hue of test stimuli was then measured with an asymmetric matching task. Frequency analysis of the hue shifts revealed weak but systematic hue rotations away from each cardinal axis and toward the diagonal intermediate axes. Such shifts could arise if the adapted channels include mechanisms with narrow chromatic selectivity, as some physiological recordings suggest, but could also reflect how adaptation alters the contrast response function. In either case they imply the presence of more than two mechanisms within the chromatic plane. In a second set of measurements, we adapted to either the L versus M or the S versus LM axis alone and tested whether the changes in hue could be accounted for by changes in relative contrast along the two axes. For high contrasts the hue biases are larger than the contrast changes predict. This dissociation implies that the contrast and hue changes are not carried by a common underlying signal, and could arise if the contrast along a single color direction is encoded by more than one mechanism with different contrast sensitivities or if different subsets of channels encode contrast and hue. Such variations in contrast sensitivity are also consistent with physiological recordings of cortical neurons.     

Spatial frequencies used in Landolt C orientation judgments: relation to inferred magnocellular and parvocellular pathways

The purpose of this study was to define the spatial frequencies that underlie judgments of Landolt C orientation under test conditions designed to favor either the magnocellular (MC) or parvocellular (PC) pathway. Contrast thresholds of two observers were measured for briefly presented Landolt Cs of four sizes, using steady- and pulsed-pedestal paradigms to bias performance toward the MC and PC pathways, respectively. Contrast thresholds were derived from a two-alternative forced-choice orientation judgment task using the QUEST procedure. The Landolt Cs were either low-pass or high-pass Gaussian filtered with a range of cutoff object spatial frequencies (cycles per letter) to limit their frequency content. Center object frequencies were derived from plots of log contrast threshold for orientation judgments vs. log filter cutoff object frequency. The function relating center object frequency to Landolt C angular subtense was nonlinear on log-log coordinates for both the steady- and pulsed-pedestal paradigms, indicating that different object frequencies were used to judge Landolt C orientation at different optotype sizes. However, the function was substantially steeper under the pulsed-pedestal than under the steady-pedestal paradigm, such that a large change in optotype size produced a relatively small change in retinal spatial frequency (cycles per degree). The pattern of results is consistent with previously reported differences between the spatial contrast sensitivity functions of the inferred MC and PC pathways.     

The role of judgment frames and task precision in object attention: Reduced template sharpness limits dual-object performance

Multiple attributes of a single-object are often processed more easily than attributes of different objects-a phenomenon associated with object attention. Here we investigate the influence of two factors, judgment frames and judgment precision, on dual-object report deficits as an index of object attention. [Han, S., Dosher, B., & Lu, Z.-L. (2003). Object attention revisited: Identifying mechanisms and boundary conditions. Psychological Science, 14, 598-604] predicted that consistency of the frame for judgments about two separate objects could reduce or eliminate the expression of object attention limitations. The current studies examine the effects of judgment frames and of task precision in orientation identification and find that dual-object report deficits within one feature are indeed affected modestly by the congruency of the judgments and more substantially by the required precision of judgments. The observed dual-object deficits affected contrast thresholds for incongruent frame conditions and for high precision judgments and reduce psychometric asymptotes. These dual-object deficits reflect a combined effect of multiplicative noise and external noise exclusion in dual-object conditions, both related to the effects of attention on the tuning of perceptual templates. These results have implications for modification of object attention theory, for understanding limitations on concurrent tasks.     

Apparent contrast differs across the vertical meridian: visual and attentional factors

It is known that visual performance is better on the horizontal than the vertical meridian, and in the lower than the upper region of the vertical meridian (Vertical Meridian Asymmetry, "VMA"), and that exogenous spatial attention increases the apparent contrast of a stimulus. Here we investigate whether the VMA also leads to differences in the subjective appearance of contrast between the upper and lower vertical meridian, and how the effects of exogenous spatial attention on appearance interact with the VMA. Two Gabor stimuli were presented North and South of fixation at 4 degrees eccentricity along the vertical meridian. Observers were asked to report the orientation of the Gabor that was higher in contrast. By assessing which stimulus observers perceived to be higher in contrast, we obtained psychometric functions and their concomitant points of subjective equality (PSE). These functions were measured both when a neutral cue was presented in the middle of the display and transient attention was deployed via a peripheral cue to the location of one of the stimuli. Observers were told that the cues were uninformative as to the stimulus contrast or its orientation. We report two novel findings. First, apparent contrast is higher on the lower vertical meridian than on the upper. Second, the attentional enhancement of apparent contrast is asymmetrical with both low and high contrast stimuli; the effect of exogenous spatial attention is greater on the lower than the upper vertical meridian. As in prior studies, we find no corresponding asymmetry in orientation discrimination. Signal detection-based models explain the asymmetrical appearance effects as a function of differential multiplicative gain factors for the North and South locations, and predict a similar but much smaller asymmetry for orientation discrimination.     

Cue contrast modulates the effects of exogenous attention on appearance

Exogenous spatial attention can be automatically engaged by a cue presented in the visual periphery. To investigate the effects of exogenous attention, previous studies have generally used highly salient cues that reliably trigger attention. However, the cueing threshold of exogenous attention has been unexamined. We investigated whether the attentional effect varies with cue salience. We examined the magnitude of the attentional effect on apparent contrast [Carrasco, M., Ling, S., & Read, S. (2004). Attention alters appearance. Nature Neuroscience, 7(3), 308-313.] elicited by cues with negative Weber contrast between 6% and 100%. Cue contrast modulated the attentional effect, even at cue contrasts above the level at which observers can perfectly localize the cue; hence, the result is not due to an increase in cue visibility. No attentional effect is observed when the 100% contrast cue is presented after the stimuli, ruling out cue bias or sensory interaction between cues and stimuli as alternative explanations. A second experiment, using the same paradigm with high contrast motion stimuli gave similar results, providing further evidence against a sensory interaction explanation, as the stimuli and task were defined on a visual dimension independent from cue contrast. Although exogenous attention is triggered automatically and involuntarily, the attentional effect is gradual.     

Contrast sensitivity deficits in inferred magnocellular and parvocellular pathways in retinitis pigmentosa

PURPOSE: To define the contrast sensitivity deficits of patients with retinitis pigmentosa (RP) under testing conditions designed to emphasize threshold mediation by either the magnocellular (MC) or parvocellular (PC) pathway. METHOD: Contrast sensitivity was measured with spatially localized, narrow-band test patterns at peak spatial frequencies ranging from 0.25 to 8 cycles per degree (cpd), using a steady-pedestal paradigm (brief presentation of the test stimulus against a continuously presented luminance pedestal) and a pulsed-pedestal paradigm (simultaneous brief presentation of the test stimulus and luminance pedestal) to favor the MC and PC pathways, respectively. The contrast sensitivity functions of 12 patients with RP who had visual acuities ranging between 20/12.5 and 20/40 were compared to those of 10 visually normal, age-equivalent control observers. RESULTS: Five of the patients with RP who had Snellen visual acuities better than 20/25 had contrast sensitivity functions that were within the normal limits at all spatial frequencies for both testing paradigms. The other seven patients with RP had reduced contrast sensitivities for both paradigms, with the greatest reduction in sensitivity occurring at the highest spatial frequency. Their contrast sensitivity deficits were equivalent for the steady- and pulsed-pedestal paradigms. CONCLUSIONS: As observed in previous studies, the degree of contrast sensitivity loss shown by the patients with RP was greatest at the highest stimulus spatial frequency. However, in comparison to prior studies of contrast discrimination in patients with RP, there was no evidence of a preferential contrast sensitivity loss within the MC pathway. This apparent discrepancy is attributed to differences in the test targets and psychophysical judgments that were used in the studies, which emphasizes the importance of task characteristics in evaluating relative deficits within the MC and PC processing streams in visual disorders.     

Where does attention go when it moves? Spatial properties and locus of the attentional repulsion effect

Reliable effects of spatial attention on perceptual measures have been well documented, yet little is known about how attention affects perception of space per se. The present study examined the effects of involuntary shifts of spatial attention on perceived location using a paradigm developed by S. Suzuki and P. Cavanagh (1997) that produces an attentional repulsion effect (ARE). The ARE refers to the illusory displacement of two vernier lines away from briefly presented cues. In Experiment 1, we show that the magnitude of the ARE depends on cue-target distance, indicating that the effects of attention on perceived location are not uniform across the visual field. Experiments 2 and 3 tested whether repulsion occurs away from cue center of mass or from cue contour. Perceived repulsion always occurred away from the cues' center of mass, regardless of the arrangement of the cue contours relative to the vernier lines. Moreover, the magnitude of the ARE varied with shifts in the position of the cues' center of mass. These experiments suggest that the onset of the cue produces a shift of attention to its center of mass rather than to the salient luminance contours that define it, and that this mechanism underlies the ARE.     

Chromatic adaptation, perceived location, and color tuning properties

We have studied the influence of chromatic adaptation upon the perceived visual position of a test stimulus using a Vernier alignment task. Maximum and minimum offsets in spatial position are generated when the adapting and test stimuli lie on the same and orthogonal axes in MBDKL color space, respectively. When the test stimuli lie on intermediate color axes, the measured positional shifts decrease as a function of the angular separation in color space (phi) from the adapting stimulus. At low stimulus contrasts, these shifts follow a sinusoidal function of phi and exhibit broad chromatic tuning and can be accounted for by a model in which the centroid is extracted from the linear combination of after-image, formed by the adapting stimulus, and the test stimulus. Such linear, broadband behavior is consistent with the response properties of chromatic neurons in the precortical visual pathway. At high contrast, and when adaptation gets closer to the S/(L+M) axis, the tuning functions become narrower and require sinusoids raised to increasingly higher exponents in order to describe the data. This narrowing of chromatic tuning is consistent with the tuning properties of chromatic neurons in the striate cortex, and implies the operation of a nonlinear mechanism in the combination of cone outputs.     

Measurements of long-range suppression in human opponent S-cone and achromatic luminance channels

Cortical responses to spatially discrete patches of achromatic luminance contrast can be altered by the presence of high-contrast, spatially remote "surrounds" and this achromatic "surround suppression" has been the subject of much recent research. However, the nature of long-range contrast normalization in chromatic signals has been less studied. Here we use a combination of neuroimaging data from source-imaged EEG and two different psychophysical measures of surround suppression to study contrast normalization in stimuli containing achromatic luminance and S-cone-isolating contrast. In an appearance matching task, we find strong within-channel but little between-channel suppression. However, using a contrast increment detection task, we do find evidence for weak but significant between-channel effects. Our neural measurements agree with the appearance matching data, showing significant within-channel suppression and no significant interactions between signals initiated in different pre-cortical pathways. We hypothesize that appearance judgments and V1 population responses are dominated by neurons with chromatically matched classical and extra-classical surrounds while contrast increment detection tasks rely on a subpopulation of neurons that have extra-classical surrounds sensitive to both chromatic and achromatic contrasts. Our psychophysical and source-imaged EEG results are consistent with a hypothesis based on natural scene statistics that long-range contrast normalization in early visual system is largely driven by signals within the same chromatic channel.     

A common light-prior for visual search, shape, and reflectance judgments

The "light-from-above" prior is invoked to simplify and expedite complex visual processing. This prior is observed in visual search and shape judgments with shaded stimuli, where perceived shape and ease of target identification are both affected by stimulus orientation. In addition, perceived surface reflectance varies with surface orientation in a manner consistent with assumed overhead lighting. Do the light-priors exhibited by these different tasks have the same underlying mechanism or even lighting direction? Some evidence has suggested that an "above-left" rather than "above" prior guides behavior in some tasks, but not others. In the current study, the "light-from-above" prior was measured using visual search, shape perception, and a novel reflectance-judgment task. There were substantial differences between observers. However, strong positive correlations were found between the light-priors measured using all three tasks. The data imply that a single mechanism is responsible for a light-from-above prior in "quick and dirty" visual search behavior, shape perception, and reflectance judgments. Furthermore, the data support the notion that perceived shape is the preattentive feature in visual search with shaded targets.     

Visual-haptic cue weighting is independent of modality-specific attention

Some object properties (e.g., size, shape, and depth information) are perceived through multiple sensory modalities. Such redundant sensory information is integrated into a unified percept. The integrated estimate is a weighted average of the sensory estimates, where higher weight is attributed to the more reliable sensory signal. Here we examine whether modality-specific attention can affect multisensory integration. Selectively reducing attention in one sensory channel can reduce the relative reliability of the estimate derived from this channel and might thus alter the weighting of the sensory estimates. In the present study, observers performed unimodal (visual and haptic) and bimodal (visual-haptic) size discrimination tasks. They either performed the primary task alone or they performed a secondary task simultaneously (dual task). The secondary task consisted of a same/different judgment of rapidly presented visual letter sequences, and so might be expected to withdraw attention predominantly from the visual rather than the haptic channel. Comparing size discrimination performance in single- and dual-task conditions, we found that vision-based estimates were more affected by the secondary task than the haptics-based estimates, indicating that indeed attention to vision was more reduced than attention to haptics. This attentional manipulation, however, did not affect the cue weighting in the bimodal task. Bimodal discrimination performance was better than unimodal performance in both single- and dual-task conditions, indicating that observers still integrate visual and haptic size information in the dual-task condition, when attention is withdrawn from vision. These findings indicate that visual-haptic cue weighting is independent of modality-specific attention.     

The impact of image degradation and temporal dynamics on sustained attention

Many clinical populations that have sustained attention deficits also have visual deficits. Therefore, it is necessary to understand how the quality of visual input and different forms of image degradation can contribute to worse performance on sustained attention tasks, particularly those with dynamic and complex visual stimuli. This study investigated the impact of image degradation on an adapted version of the gradual-onset continuous performance task (gradCPT), where participants must discriminate between gradually fading city and mountain scenes. Thirty-six normal-vision participants completed the task, which featured two blocks of six resolution and contrast levels. Subjects either completed a version with gradually fading or static image presentations. The results show decreases in image resolution impair performance under both types of temporal dynamics, whereas performance is only impaired under gradual temporal dynamics for decreases in image contrast. Image similarity analyses showed that performance has a higher association with an observer''s ability to gather an image''s global spatial layout (i.e. gist) than local variations in pixel luminance, particularly under gradual image presentation. This work suggests that gradually fading attention paradigms are sensitive to deficits in primary visual function, potentially leading to these issues being misinterpreted as attentional failures.