Measured Discrimination Thresholds Research Articles

Contribution of internal noise and calculation efficiency to face discrimination deficits in older adults

Classification images (CIs) measured in a face discrimination task differ significantly between older and younger observers. These age differences are consistent with the hypothesis that older adults sample diagnostic face information less efficiently, or have higher levels of internal noise, compared to younger adults. The current experiments assessed the relative contributions of efficiency and internal noise to age differences in face discrimination using the external noise masking and double-pass response consistency paradigms. Experiment 1 measured discrimination thresholds for faces embedded in several levels of static white noise, and the resulting threshold-vs.-noise curves were used to estimate calculation efficiency and equivalent input noise: older observers had lower efficiency and higher equivalent input noise than younger observers. Experiment 2 presented observers with two identical sequences of faces embedded in static white noise to measure the association between response accuracy and response consistency and estimate the internal:external (i/e) noise ratio for each observer. We found that i/e noise ratios did not differ significantly between groups. These results suggest that age differences in face discrimination are due to differences in calculation efficiency and additive internal noise, but not to age differences in multiplicative internal noise.

Diotic and Dichotic Mechanisms of Discrimination Threshold in Musicians and Non-Musicians.

The perception of harmonic complexes provides important information for musical and vocal communication. Numerous studies have shown that musical training and expertise are associated with better processing of harmonic complexes, however, it is unclear whether the perceptual improvement associated with musical training is universal to different pitch models. The current study addresses this issue by measuring discrimination thresholds of musicians (n = 20) and non-musicians (n = 18) to diotic (same sound to both ears) and dichotic (different sounds to each ear) sounds of four stimulus types: (1) pure sinusoidal tones, PT; (2) four-harmonic complex tones, CT; (3) iterated rippled noise, IRN; and (4) interaurally correlated broadband noise, called the “Huggins” or “dichotic” pitch, DP. Frequency difference limens (DLF) for each stimulus type were obtained via a three-alternative-forced-choice adaptive task requiring selection of the interval with the highest pitch, yielding the smallest perceptible fundamental frequency (F0) distance (in Hz) between two sounds. Music skill was measured by an online test of musical pitch, melody and timing maintained by the International Laboratory for Brain Music and Sound Research. Musicianship, length of music experience and self-evaluation of musical skill were assessed by questionnaire. Results showed musicians had smaller DLFs in all four conditions with the largest group difference in the dichotic condition. DLF thresholds were related to both subjective and objective musical ability. In addition, subjective self-report of musical ability was shown to be a significant variable in group classification. Taken together, the results suggest that music-related plasticity benefits multiple mechanisms of pitch encoding and that self-evaluation of musicality can be reliably associated with objective measures of perception.

Relationship between objective measures of hearing discrimination elicited by non-linguistic stimuli and speech perception in adults

Some people using hearing aids have difficulty discriminating between sounds even though the sounds are audible. As such, cochlear implants may provide greater benefits for speech perception. One method to identify people with auditory discrimination deficits is to measure discrimination thresholds using spectral ripple noise (SRN). Previous studies have shown that behavioral discrimination of SRN was associated with speech perception, and behavioral discrimination was also related to cortical responses to acoustic change or ACCs. We hypothesized that cortical ACCs could be directly related to speech perception. In this study, we investigated the relationship between subjective speech perception and objective ACC responses measured using SRNs. We tested 13 normal-hearing and 10 hearing-impaired adults using hearing aids. Our results showed that behavioral SRN discrimination was correlated with speech perception in quiet and in noise. Furthermore, cortical ACC responses to phase changes in the SRN were significantly correlated with speech perception. Audibility was a major predictor of discrimination and speech perception, but direct measures of auditory discrimination could contribute information about a listener’s sensitivity to acoustic cues that underpin speech perception. The findings lend support for potential application of measuring ACC responses to SRNs for identifying people who may benefit from cochlear implants.

Quantifying the effect of viewpoint changes on sensitivity to face identity

Although faces can be recognized from different viewpoints, variations in viewpoint impair face identification ability. The present study quantified the effect of changes in viewpoint on sensitivity to face identity. We measured discrimination thresholds for synthetic faces presented from several viewpoints (same viewpoint condition) and the same faces shown with a change in viewpoint (5°, 10° or 20°) between viewing and test. We investigated three types of viewpoint change: (i) front-to-side (front-view matched to 20°° side-view), (ii) side-to-front (20° side-view matched to front) and (iii) symmetrical (10° left to 10° right). In the same viewpoint condition, discrimination thresholds were lowest for faces presented from 0° and increased linearly as the viewing angle was increased (threshold elevations: 0° = 1.00×, 5° = 1.11×, 10° = 1.22×, 20° = 1.69×). Changes in viewpoint between viewing and test led to further reductions in discrimination sensitivity, which depended upon the magnitude of viewpoint change (5° = 1.38×, 10° = 1.75×, 20° = 2.07×). Sensitivity also depended upon the type of viewpoint change: while a 20° front-to-side viewpoint change increased discrimination thresholds by a factor of 2.09×, a symmetrical change in viewpoint, of the same magnitude, did not significantly reduce sensitivity (1.26×). Sensitivity to face identity is significantly reduced by changes in viewpoint. Factors which determine the extent of this reduction include the magnitude of viewpoint change and symmetry. Our results support the premise of viewpoint-dependent encoding of unfamiliar face identities, and suggest that symmetry may be used to recognize identities across different viewpoints.

Higher attentional costs for numerosity estimation at high densities

Humans can estimate numerosity over a large range, but the precision with which they do so varies considerably over that range. For very small sets, within the subitizing range of up to about four items, estimation is rapid and errorless. For intermediate numerosities, errors vary directly with the numerosity, following Weber’s law, but for very high numerosities, with very dense patterns, thresholds continue to rise with the square root of numerosity. This suggests that three different mechanisms operate over the number range. In this study we provide further evidence for three distinct numerosity mechanisms, by studying their dependence on attentional resources. We measured discrimination thresholds over a wide range of numerosities, while manipulating attentional load with both visual and auditory dual tasks. The results show that attentional effects on thresholds vary over the number range. Both visual and auditory attentional loads strongly affect subitizing, much more than for larger numerosities. Attentional costs remain stable over the estimation range, then rise again for very dense patterns. These results reinforce the idea that numerosity is processed by three separates but probably overlapping systems.

Illumination discrimination for chromatically biased illuminations: Implications for color constancy.

We measured discrimination thresholds for illumination changes along different chromatic directions starting from chromatically biased reference illuminations. Participants viewed a Mondrian-papered scene illuminated by LED lamps. The scene was first illuminated by a reference illumination, followed by two comparisons. One comparison matched the reference (the target); the other (the test) varied from the reference, nominally either bluer, yellower, redder, or greener. The participant's task was to correctly select the target. A staircase procedure found thresholds for discrimination of an illumination change along each axis of chromatic change. Nine participants completed the task for five different reference illumination conditions (neutral, blue, yellow, red, and green). We find that relative discrimination thresholds for different chromatic directions of illumination change vary with the reference illumination. For the neutral reference, there is a trend for thresholds to be highest in the bluer illumination-change direction, replicating our previous reports of a “blue bias” for neutral reference illuminations. For the four chromatic references (blue, yellow, red, and green), the change in illumination toward the neutral reference is less well discriminated than changes in the other directions: a “neutral bias.” The results have implications for color constancy: In considering the stability of surface appearance under changes in illumination, both the starting chromaticity of the illumination and direction of change must be considered, as well as the chromatic characteristics of the surface reflectance ensemble. They also suggest it will be worthwhile to explore whether and how the human visual system has internalized the statistics of natural illumination changes.

Top-down modulation of shape and roughness discrimination in active touch by covert attention.

Due to limitations in perceptual processing, information relevant to momentary task goals is selected from the vast amount of available sensory information by top-down mechanisms (e.g. attention) that can increase perceptual performance. We investigated how covert attention affects perception of 3D objects in active touch. In our experiment, participants simultaneously explored the shape and roughness of two objects in sequence, and were told afterwards to compare the two objects with regard to one of the two features. To direct the focus of covert attention to the different features we manipulated the expectation of a shape or roughness judgment by varying the frequency of trials for each task (20%, 50%, 80%), then we measured discrimination thresholds. We found higher discrimination thresholds for both shape and roughness perception when the task was unexpected, compared to the conditions in which the task was expected (or both tasks were expected equally). Our results suggest that active touch perception is modulated by expectations about the task. This implies that despite fundamental differences, active and passive touch are affected by feature selective covert attention in a similar way.

Object-based attention modulates the discrimination of level increments in stop-consonant noise bursts.

This study tested the hypothesis that object-based attention modulates the discrimination of level increments in stop-consonant noise bursts. With consonant-vowel-consonant (CvC) words consisting of an ≈80-dB vowel (v), a pre-vocalic (Cv) and a post-vocalic (vC) stop-consonant noise burst (≈60-dB SPL), we measured discrimination thresholds (LDTs) for level increments (ΔL) in the noise bursts presented either in CvC context or in isolation. In the 2-interval 2-alternative forced-choice task, each observation interval presented a CvC word (e.g., /pæk/ /pæk/), and normal-hearing participants had to discern ΔL in the Cv or vC burst. Based on the linguistic word labels, the auditory events of each trial were perceived as two auditory objects (Cv-v-vC and Cv-v-vC) that group together the bursts and vowels, hindering selective attention to ΔL. To discern ΔL in Cv or vC, the events must be reorganized into three auditory objects: the to-be-attended pre-vocalic (Cv–Cv) or post-vocalic burst pair (vC–vC), and the to-be-ignored vowel pair (v–v). Our results suggest that instead of being automatic this reorganization requires training, in spite of using familiar CvC words. Relative to bursts in isolation, bursts in context always produced inferior ΔL discrimination accuracy (a context effect), which depended strongly on the acoustic separation between the bursts and the vowel, being much keener for the object apart from (post-vocalic) than for the object adjoining (pre-vocalic) the vowel (a temporal-position effect). Variability in CvC dimensions that did not alter the noise-burst perceptual grouping had minor effects on discrimination accuracy. In addition to being robust and persistent, these effects are relatively general, evincing in forced-choice tasks with one or two observation intervals, with or without variability in the temporal position of ΔL, and with either fixed or roving CvC standards. The results lend support to the hypothesis.

Discrimination of curvature from motion during smooth pursuit eye movements and fixation.

Smooth pursuit and motion perception have mainly been investigated with stimuli moving along linear trajectories. Here we studied the quality of pursuit movements to curved motion trajectories in human observers and examined whether the pursuit responses would be sensitive enough to discriminate various degrees of curvature. In a two-interval forced-choice task subjects pursued a Gaussian blob moving along a curved trajectory and then indicated in which interval the curve was flatter. We also measured discrimination thresholds for the same curvatures during fixation. Motion curvature had some specific effects on smooth pursuit properties: trajectories with larger amounts of curvature elicited lower open-loop acceleration, lower pursuit gain, and larger catch-up saccades compared with less curved trajectories. Initially, target motion curvatures were underestimated; however, ∼300 ms after pursuit onset pursuit responses closely matched the actual curved trajectory. We calculated perceptual thresholds for curvature discrimination, which were on the order of 1.5 degrees of visual angle (°) for a 7.9° curvature standard. Oculometric sensitivity to curvature discrimination based on the whole pursuit trajectory was quite similar to perceptual performance. Oculometric thresholds based on smaller time windows were higher. Thus smooth pursuit can quite accurately follow moving targets with curved trajectories, but temporal integration over longer periods is necessary to reach perceptual thresholds for curvature discrimination.NEW & NOTEWORTHY Even though motion trajectories in the real world are frequently curved, most studies of smooth pursuit and motion perception have investigated linear motion. We show that pursuit initially underestimates the curvature of target motion and is able to reproduce the target curvature ∼300 ms after pursuit onset. Temporal integration of target motion over longer periods is necessary for pursuit to reach the level of precision found in perceptual discrimination of curvature.

Spatial Frequency Discrimination: Effects of Age, Reward, and Practice.

Social interaction starts with perception of the world around you. This study investigated two fundamental issues regarding the development of discrimination of higher spatial frequencies, which are important building blocks of perception. Firstly, it mapped the typical developmental trajectory of higher spatial frequency discrimination. Secondly, it developed and validated a novel design that could be applied to improve atypically developed vision. Specifically, this study examined the effect of age and reward on task performance, practice effects, and motivation (i.e., number of trials completed) in a higher spatial frequency (reference frequency: 6 cycles per degree) discrimination task. We measured discrimination thresholds in children aged between 7 to 12 years and adults (N = 135). Reward was manipulated by presenting either positive reinforcement or punishment. Results showed a decrease in discrimination thresholds with age, thus revealing that higher spatial frequency discrimination continues to develop after 12 years of age. This development continues longer than previously shown for discrimination of lower spatial frequencies. Moreover, thresholds decreased during the run, indicating that discrimination abilities improved. Reward did not affect performance or improvement. However, in an additional group of 5–6 year-olds (N = 28) punishments resulted in the completion of fewer trials compared to reinforcements. In both reward conditions children aged 5–6 years completed only a fourth or half of the run (64 to 128 out of 254 trials) and were not motivated to continue. The design thus needs further adaptation before it can be applied to this age group. Children aged 7–12 years and adults completed the run, suggesting that the design is successful and motivating for children aged 7–12 years. This study thus presents developmental differences in higher spatial frequency discrimination thresholds. Furthermore, it presents a design that can be used in future developmental studies that require multiple stimulus presentations such as visual perceptual learning.

Numerosity but not texture-density discrimination correlates with math ability in children.

Considerable recent work suggests that mathematical abilities in children correlate with the ability to estimate numerosity. Does math correlate only with numerosity estimation, or also with other similar tasks? We measured discrimination thresholds of school-age (6- to 12.5-years-old) children in 3 tasks: numerosity of patterns of relatively sparse, segregatable items (24 dots); numerosity of very dense textured patterns (250 dots); and discrimination of direction of motion. Thresholds in all tasks improved with age, but at different rates, implying the action of different mechanisms: In particular, in young children, thresholds were lower for sparse than textured patterns (the opposite of adults), suggesting earlier maturation of numerosity mechanisms. Importantly, numerosity thresholds for sparse stimuli correlated strongly with math skills, even after controlling for the influence of age, gender and nonverbal IQ. However, neither motion-direction discrimination nor numerosity discrimination of texture patterns showed a significant correlation with math abilities. These results provide further evidence that numerosity and texture-density are perceived by independent neural mechanisms, which develop at different rates; and importantly, only numerosity mechanisms are related to math. As developmental dyscalculia is characterized by a profound deficit in discriminating numerosity, it is fundamental to understand the mechanism behind the discrimination. (PsycINFO Database Record

Blind people are more sensitive than sighted people to binaural sound-location cues, particularly inter-aural level differences

Blind people use auditory information to locate sound sources and sound-reflecting objects (echolocation). Sound source localization benefits from the hearing system's ability to suppress distracting sound reflections, whereas echolocation would benefit from “unsuppressing” these reflections. To clarify how these potentially conflicting aspects of spatial hearing interact in blind versus sighted listeners, we measured discrimination thresholds for two binaural location cues: inter-aural level differences (ILDs) and inter-aural time differences (ITDs). The ILDs or ITDs were present in single clicks, in the leading component of click pairs, or in the lagging component of click pairs, exploiting processes related to both sound source localization and echolocation. We tested 23 blind (mean age = 54 y), 23 sighted-age-matched (mean age = 54 y), and 42 sighted-young (mean age = 26 y) listeners. The results suggested greater ILD sensitivity for blind than for sighted listeners. The blind group's superiority was particularly evident for ILD-lag-click discrimination, suggesting not only enhanced ILD sensitivity in general but also increased ability to unsuppress lagging clicks. This may be related to the blind person's experience of localizing reflected sounds, for which ILDs may be more efficient than ITDs. On the ITD-discrimination tasks, the blind listeners performed better than the sighted age-matched listeners, but not better than the sighted young listeners. ITD sensitivity declines with age, and the equal performance of the blind listeners compared to a group of substantially younger listeners is consistent with the notion that blind people's experience may offset age-related decline in ITD sensitivity.

Categorical facilitation with equally discriminable colors

This study investigates the impact of language on color perception. By categorical facilitation, we refer to an aspect of categorical perception, in which the linguistic distinction between categories affects color discrimination beyond the low-level, sensory sensitivity to color differences. According to this idea, discrimination performance for colors that cross a category border should be better than for colors that belong to the same category when controlling for low-level sensitivity. We controlled for sensitivity by using colors that were equally discriminable according to empirically measured discrimination thresholds. To test for categorical facilitation, we measured response times and error rates in a speeded discrimination task for suprathreshold stimuli. Robust categorical facilitation occurred for five out of six categories with a group of inexperienced observers, namely for pink, orange, yellow, green, and purple. Categorical facilitation was robust against individual variations of categories or the laterality of target presentation. However, contradictory effects occurred in the blue category, most probably reflecting the difficulty to control effects of sensory mechanisms at the green-blue boundary. Moreover, a group of observers who were highly familiar with the discrimination task did not show consistent categorical facilitation in the other five categories. This trained group had much faster response times than the inexperienced group without any speed-accuracy trade-off. Additional analyses suggest that categorical facilitation occurs when observers pay attention to the categorical distinction but not when they respond automatically based on sensory feed-forward information.

Human discrimination of depth of field in stereoscopic and nonstereoscopic photographs.

Depth of field (DOF) is defined as the distance range within which objects are perceived as sharp. Previous research has focused on blur discrimination in artificial stimuli and natural photographs. The discrimination of DOF, however, has received less attention. Since DOF introduces blur which is related to distance in depth, many levels of blur are simultaneously present. As a consequence, it is unclear whether discrimination thresholds for blur are appropriate for predicting discrimination thresholds for DOF. We therefore measured discrimination thresholds for DOF using a two-alternative forced-choice task. Ten participants were asked to observe two images and to select the one with the larger DOF. We manipulated the scale of the scene--that is, the actual depth in the scene. We conducted the experiment under stereoscopic and nonstereoscopic viewing conditions. We found that the threshold for a large DOF (39.1 mm) was higher than for a small DOF (10.1 mm), and the thresholds decreased when scale of scene increased. We also found that there was no significant difference between stereoscopic and nonstereoscopic conditions. We compared our results with thresholds predicted from the literature. We concluded that using blur discrimination thresholds to discriminate DOF may lead to erroneous conclusions because the depth in the scene significantly affects people's DOF discrimination ability.

Categorical sensitivity to color differences

Categorical perception provides a potential link between color perception and the linguistic categories that correspond to the basic color terms. We examined whether the sensory information of the second-stage chromatic mechanisms is further processed so that sensitivity for color differences yields categorical perception. In this case, sensitivity for color differences should be higher across than within category boundaries. We measured discrimination thresholds (JNDs) and color categories around an isoluminant hue circle in Derrington-Krauskopf-Lennie (DKL) color space at three levels of lightness. At isoluminant lightness, the global pattern of JNDs coarsely followed an ellipse. Deviations from the ellipse coincided with the orange-pink and the blue-green category borders, but these minima were also aligned with the second-stage cone-opponent mechanisms. No evidence for categorical perception of color was found for any other category borders. At lower lightness, categories changed substantially, but JNDs did not change accordingly. Our results point to a loose relationship between color categorization and discrimination. However, the coincidence of some boundaries with JND minima is not a general property of color categorical boundaries. Hence, our basic ability to discriminate colors cannot fully explain why we use the particular set of categories to communicate about colors. Moreover, these findings seriously challenge the idea that color naming forms the basis for the categorical perception of colors. With respect to previous studies that concentrated on the green-blue boundary, our results highlight the importance of controlling perceptual distances and examining the full set of categories when investigating category effects on color perception.

Higher order color mechanisms: Evidence from noise-masking experiments in cone contrast space

This study addresses a fundamental question concerning the number of cortical, i.e., higher order mechanisms in color vision. The initial subcortical stages in color vision can be described by three cone mechanisms, S, M, L, and three pairs of second-stage mechanisms (achromatic L + M and -L - M, chromatic S - (L + M) and -S + (L + M), and chromatic L - M and M - L). The further mechanistic description of cortical color vision is controversial. On the one hand, numerous studies that defined their stimuli in a color-opponent Derrington-Krauskopf-Lennie (DKL) color space found evidence for higher order mechanisms. On the other hand, some studies that defined their stimuli in cone contrast (CC) space failed to find such evidence. Here we show that this failure was due to a restricted choice of stimuli. We used a noise-masking paradigm to measure discrimination thresholds for textured patterns modulated along chromatic directions in CC space. Unlike previous studies we defined noise directions in DKL space and converted them to CC space. When the noise contrast was sufficiently high we found selective masking, but this did not occur when the noise contrast was low. Selective masking indicates higher order mechanisms, since so far no alternative model has been proposed. Previous studies in CC space failed to find selective masking due to the low contrast of the noise and due to the restricted choice of perceptually highly similar noise directions that mainly stimulated the second-stage mechanisms. We conclude that cortical color vision is governed by higher order mechanisms.

When luminance increment thresholds depend on apparent lightness

p { margin-bottom: 0.08in; } The just noticeable difference (JND) between two stimulus intensities increases proportional to the background intensity (Weber's law). It is less clear, however, whether the JND is a function of the proximal or perceptual stimulus intensity. In the domain of achromatic surface colors the question would translate to whether the JND depends on local luminance or surface lightness. In the laboratory using simple stimuli such as uniform patches, proximal (luminance) and perceived intensity (lightness) often coincide. Reports that tried to disentangle the two factors yielded inconsistent results (e.g. Heinemann, 1961 JEP 61 389-399; Cornsweet and Teller, 1965 JOSA 55(10) 1303-1308; Zaidi and Krauskopf, 1985 Vision Res 26 759-62; McCourt and Kingdom, 1996 Vision Res 36 2563-73; Henning, Millar and Hill, 2000 JOSA 17(7) 1147-1159; Hillis and Brainard, 2007 CurrBiol 17 1714-1719). Following a previous experiment (Maertens and Wichmann, 2010 JVis 10 424) we measured discrimination thresholds in the Adelson checkerboard pattern for two equiluminant checks which differed in lightness (black vs. white). Discrimination performance was measured in two conditions: in the 'blob' condition, the increment was a two-dimensional gaussian centered on the check, in the 'check' condition, the increment was a constant that was added to the entire check. Performance was assessed in a 2-interval forced-choice and a yes-no task. In the 'blob' condition thresholds were indistinguishable between equiluminant checks and did not differ between the tasks. In the 'check' condition thresholds differed between equiluminant checks and were elevated for the lighter one. This was true for the yes-no task and to a lesser extent in the 2-IFC task. We think that these results require discussion beyond the question for the appropriate type of increment. We believe that the visual system might respond fundamentally different to light emanating from meaningful surfaces and to isolated spots of light. Meeting abstract presented at VSS 2012

Changes in haptic sensitivity during tool use: Implications for optimal design of visual-haptic devices

Optimising haptic sensitivity in visual-haptic interfaces can have important benefits, particularly in situations where visual information is relatively unreliable (for example in surgical devices, where there is often a lack of normal visual depth cues). Many haptic devices alter the normal relationship between object size and hand opening, and so might be expected to alter haptic sensitivity. To determine how to optimise haptic sensitivity in such situations, we measured haptic size sensitivity as a function of object size (i) during normal grasping, and (ii) using pliers-like tools that changed the gain between hand opening and object size. Haptic stimuli were created using force-feedback robots, and we measured discrimination thresholds using a two-interval forced-choice task. We first showed that when participants directly grasped objects with their hands haptic size sensitivity did not follow Weber’s law, but instead varied non-monotonically with object size/hand opening; thresholds were lowest for object sizes around 45 mm and increased non-linearly for larger and smaller objects. Second, when using the pliers, sensitivity in units of hand opening was unchanged from normal grasping. Thus, thresholds in units of object size were simply the product of the non-monotonic haptic size sensitivity function, above, and the tool gain. Taken together, these results show that there is a unique tool gain that maximises haptic sensitivity to each object size. Our results show how the ‘tool gain’ of haptic devices should be altered to optimise sensitivity to the range of object sizes in a particular scene.

The coarse vs. fine dichotomy in stereopsis: A matter of scale

Is disparity processing subserved by a single mechanism that spans all disparities or by two distinct mechanisms that operate over different disparity ranges? Data from depth pedestal experiments have proven divided on this issue, likely due to the wide range of stimuli and tasks used, and to variables such as stimulus size and retinal eccentricity. In an effort to resolve this issue we exploit a result from Wilcox and Hess (1995) who showed that the coarse (2nd-order) system dominates processing for diplopic targets. Further, 2nd-order stereopsis depends critically on scale, as the upper limits for stereopsis increase with increasing stimulus width. Here we used a 1AFC method of constant stimuli to measure discrimination thresholds for a test bar relative to a reference bar positioned at depth pedestals ranging from 0 to 1 deg (n=5). The stimulus parameters were selected so that the target and reference would become diplopic within the test range. Not surprisingly, discrimination performance was degraded as pedestal disparity was increased. This occurred over a large range of disparities within the fusable range. However, outside the fusable range we find a dramatically different pattern of results; discrimination performance plateaus, remaining constant in spite of increasing diplopia. Our data echo those of Ogle (1952) and others, but with important differences that are tied to the test disparity relative to the stimulus width. We argue that these data reflect a neural dichotomy between coarse and fine processing that is tightly tied to scale rather than absolute disparity.

From Comparison to Classification: A Cortical Tool for Boosting Perception

Humans are much better in relative than in absolute judgments. This common assertion is based on findings that discrimination thresholds are much lower when measured with methods that allow interstimuli comparisons than when measured with methods that require classification of one stimulus at a time and are hence sensitive to memory load. We now challenged this notion by measuring discrimination thresholds and evoked potentials while listeners performed a two-tone frequency discrimination task. We tested various protocols that differed in the pattern of cross-trial tone repetition. We found that best performance was achieved only when listeners effectively used cross-trial repetition to avoid interstimulus comparisons with the repeated reference tone. Instead, they classified one tone, the nonreference tone, as either high or low by comparing it with a recently formed internal reference. Listeners were not aware of the switch from interstimulus comparison to classification. Its successful use was revealed by the conjunction of improved behavioral performance and an event-related potential component (P3), indicating an implicit perceptual decision, which followed the nonreference tone in each trial. Interestingly, tone repetition itself did not suffice for the switch, implying that the bottleneck to discrimination does not reside at the lower, sensory stage. Rather, the temporal consistency of repetition was important, suggesting the involvement of higher-level mechanisms with longer time constants. These findings suggest that classification is based on more automatic and accurate mechanisms than interstimulus comparisons and that the ability to effectively use them depends on a dynamic interplay between higher- and lower-level cortical mechanisms.