Lower Discrimination Thresholds Research Articles

Nonlinear age effects in tactile processing from early childhood to adulthood.

BackgroundTactile processing plays a pivotal role in the early stages of human development; however, little is known about tactile function in young children. An understanding of how tactile processing changes with age from early childhood to adulthood is fundamental in understanding altered tactile experiences in neurodevelopmental disorders, such as autism spectrum disorder.MethodsIn this cross‐sectional study, 142 children and adults aged 3–23 years completed a vibrotactile testing battery consisting of 5 tasks, which rely on different cortical and cognitive mechanisms. The battery was designed to be suitable for testing in young children to investigate how tactile processing changes from early childhood to adulthood.ResultsOur results suggest a pattern of rapid, age‐related changes in tactile processing toward lower discrimination thresholds (lower discrimination thresholds = greater sensitivity) across early childhood, though we acknowledge limitations with cross‐sectional data. Differences in the rate of change across tasks were observed, with tactile performance reaching adult‐like levels at a younger age on some tasks compared to others.ConclusionsWhile it is known that early childhood is a period of profound development including tactile processing, our data provides evidence for subtle differences in the developmental rate of the various underlying cortical, physical, and cognitive processes. Further, we are the first to show the feasibility of vibrotactile testing in early childhood (<6 years). The results of this work provide estimates of age‐related differences in performance, which could have important implications as a reference for investigating altered tactile processing in developmental disorders.

Age-related differences in visual confidence are driven by individual differences in cognitive control capacities

Visual perception is not only shaped by sensitivity but also by confidence, i.e., the ability to estimate the accuracy of a visual decision. Younger observers have been reported to have access to a reliable measure of their own uncertainty when making visual decisions. This metacognitive ability might be challenged during ageing due to increasing sensory noise and decreasing cognitive control resources. We investigated age effects on visual confidence using a visual contrast discrimination task and a confidence forced-choice paradigm. Younger adults (19–38 years) showed significantly lower discrimination thresholds than older adults (60–78 years). To focus on confidence sensitivity above and beyond differences in discrimination performance, we estimated confidence efficiency that reflects the ability to distinguish good from bad perceptual decisions. Confidence efficiency was estimated by comparing thresholds obtained from all trials and trials that were judged with relatively higher confidence, respectively. In both age groups, high confidence judgments were associated with better visual performance, but confidence efficiency was reduced in older adults. However, we observed substantial variability across all participants. Controlling for age group, confidence effciency was closely linked to individual differences in cognitive control capacities. Our findings provide evidence for age-related differences in confidence efficiency that present a specific challenge to perceptual performance in old age. We propose that these differences are driven by cognitive control capacities, supporting their crucial role for metacognitive efficiency.

Understanding the stimulus effects of nicotine and bupropion in a drug-drug discriminated goal-tracking task.

Bupropion is a non-nicotine medication for smoking cessation that has overlapping stimulus effects with nicotine as demonstrated in drug discrimination studies. Whether these shared stimulus effects will alter acquisition or maintenance of a discrimination between nicotine and bupropion is unknown. We sought to test this possibility using the drug discriminated goal-tracking (DGT) task and whether discrimination training history affected generalization and substitution tests. Sixty adult Sprague-Dawley rats (30M/30F) were equally split into three discrimination training groups: SAL-0.4NIC, 10BUP-0.4NIC, and 20BUP-0.4NIC. On nicotine days, all rats were administered subcutaneously 0.4mg/kg nicotine and had intermittent access to liquid sucrose. On intermixed non-reinforced days, rats were administered intraperitoneally saline, 10 or 20mg/kg bupropion. Upon completion, a range of nicotine and bupropion doses were assessed before substitution tests with varenicline and sazetidine-A were conducted. The SAL-0.4NIC and 10BUP-0.4NIC groups readily discriminated by session 8, as evidenced by increased dipper entries (goal-tracking) on nicotine days. The 20BUP-0.4NIC group was slower to acquire the discrimination. Female rats, regardless of group, had higher conditioned responding evoked by the lowest dose of nicotine (0.025mg/kg) in the dose-effect curve. The discrimination required rats to learn to withhold responding to the training dose of bupropion. This withholding of excitatory dipper entries generalized to other doses. Varenicline and sazetidine-A partially substituted for the nicotine stimulus, and this pattern did not differ with training history. We are the first to study a drug-drug discrimination using the DGT task. Females appeared to have a lower discrimination threshold for nicotine that was not impacted by the learning history. Further work on the importance of sex as a biological variable and how the complex interoceptive stimulus effects of nicotine can vary with training histories is needed.

Improved tactile frequency discrimination in musicians

Music practice is a multisensory training that is of great interest to neuroscientists because of its implications for neural plasticity. Music-related modulation of sensory systems has been observed in neuroimaging data, and has been supported by results in behavioral tasks. Some studies have shown that musicians react faster than non-musicians to visual, tactile and auditory stimuli. Behavioral enhancement in more complex tasks has received considerably less attention in musicians. This study aims to investigate unisensory and multisensory discrimination capabilities in musicians. More specifically, the goal of this study is to examine auditory, tactile and auditory-tactile discrimination in musicians. The literature suggesting better auditory and auditory-tactile discrimination in musicians is scarce, and no study to date has examined pure tactile discrimination capabilities in musicians. A two-alternative forced-choice frequency discrimination task was used in this experiment. The task was inspired by musical production, and participants were asked to identify whether a frequency was the same as or different than a standard stimulus of 160Hz in three conditions: auditory only, auditory-tactile only and tactile only. Three waveforms were used to replicate the variability of pitch that can be found in music. Stimuli were presented through headphones for auditory stimulation and a glove with haptic audio exciters for tactile stimulation. Results suggest that musicians have lower discrimination thresholds than non-musicians for auditory-only and auditory-tactile conditions for all waveforms. The results also revealed that musicians have lower discrimination thresholds than non-musicians in the tactile condition for sine and square waveforms. Taken together, these results support the hypothesis that musical training can lead to better unisensory tactile discrimination which is in itself a new and major finding.

Interdependences between finger movement direction and haptic perception of oriented textures.

Although the natural haptic perception of textures includes active finger movements, it is unclear how closely perception and movements are linked. Here we investigated this question using oriented textures. Textures that are composed of periodically repeating grooves have a clear orientation defined by the grooves. The direction of finger movement relative to texture orientation determines the availability of temporal cues to the spatial period of the texture. These cues are absent during movements directed in line with texture orientation, whereas movements orthogonal to texture orientation maximize the temporal frequency of stimulation. This may optimize temporal cues. In Experiment 1 we tested whether texture perception gets more precise the more orthogonal the movement direction is to the texture. We systematically varied the movement direction within a 2IFC spatial period discrimination task. As expected, perception was more precise (lower discrimination thresholds) when finger movements were directed closer towards the texture orthogonal as compared to in parallel to the texture. In Experiment 2 we investigated whether people adjust movement directions to the texture orthogonal in free exploration. We recorded movement directions during free exploration of standard and comparison gratings. The standard gratings were clearly oriented. The comparison gratings did not have a clear orientation defined by grooves. Participants adjusted movement directions to the texture orthogonal only for clearly oriented textures (standards). The adjustment to texture orthogonal was present in the final movement but not in the first movement. This suggests that movement adjustment is based on sensory signals for texture orientation that were gathered over the course of exploration. In Experiment 3 we assessed whether the perception of texture orientation and movement adjustments are based on shared sensory signals. We determined perceptual thresholds for orientation discrimination and computed ‘movometric’ thresholds from the stroke-by-stroke adjustment of movement direction. Perception and movements were influenced by a common factor, the spatial period, suggesting that the same sensory signals for texture orientation contribute to both. We conclude that people optimize texture perception by adjusting their movements in directions that maximize temporal cue frequency. Adjustments are performed on the basis of sensory signals that are also used for perception.

Illumination discrimination in the absence of a fixed surface-reflectance layout.

Previous studies have shown that humans can discriminate spectral changes in illumination and that this sensitivity depends both on the chromatic direction of the illumination change and on the ensemble of surfaces in the scene. These studies, however, always used stimulus scenes with a fixed surface-reflectance layout. Here we compared illumination discrimination for scenes in which the surface reflectance layout remains fixed (fixed-surfaces condition) to those in which surface reflectances were shuffled randomly across scenes, but with the mean scene reflectance held approximately constant (shuffled-surfaces condition). Illumination discrimination thresholds in the fixed-surfaces condition were commensurate with previous reports. Thresholds in the shuffled-surfaces condition, however, were considerably elevated. Nonetheless, performance in the shuffled-surfaces condition exceeded that attainable through random guessing. Analysis of eye fixations revealed that in the fixed-surfaces condition, low illumination discrimination thresholds (across observers) were predicted by low overall fixation spread and high consistency of fixation location and fixated surface reflectances across trial intervals. Performance in the shuffled-surfaces condition was not systematically related to any of the eye-fixation characteristics we examined for that condition, but was correlated with performance in the fixed-surfaces condition.

Discrimination between smiling faces: Human observers vs. automated face analysis

This study investigated (a) how prototypical happy faces (with happy eyes and a smile) can be discriminated from blended expressions with a smile but non-happy eyes, depending on type and intensity of the eye expression; and (b) how smile discrimination differs for human perceivers versus automated face analysis, depending on affective valence and morphological facial features. Human observers categorized faces as happy or non-happy, or rated their valence. Automated analysis (FACET software) computed seven expressions (including joy/happiness) and 20 facial action units (AUs). Physical properties (low-level image statistics and visual saliency) of the face stimuli were controlled. Results revealed, first, that some blended expressions (especially, with angry eyes) had lower discrimination thresholds (i.e., they were identified as “non-happy” at lower non-happy eye intensities) than others (especially, with neutral eyes). Second, discrimination sensitivity was better for human perceivers than for automated FACET analysis. As an additional finding, affective valence predicted human discrimination performance, whereas morphological AUs predicted FACET discrimination. FACET can be a valid tool for categorizing prototypical expressions, but is currently more limited than human observers for discrimination of blended expressions. Configural processing facilitates detection of in/congruence(s) across regions, and thus detection of non-genuine smiling faces (due to non-happy eyes).

Bimanual and unimanual length perception.

From previous studies, it is unclear how bimanual length discrimination differs from unimanual length discrimination. To investigate the difference, we designed an experiment with four conditions. In the first two conditions, unimanual and bimanual discrimination thresholds are determined. In the third and fourth conditions, length is explored with the two index fingers like in the bimanual condition, but the reference is either internal, by clasping the hands together, or external, by grasping handles connected to the table. We find that thresholds for the unimanual condition (7.0%) and the clasping condition (9.2%) are both lower than for the bimanual condition (16%) and the grasping handles condition (15%). We conclude that when discriminating length unimanually and bimanually while clasping the hands together, the internal reference within the hand can be used and that explains the lower discrimination thresholds.

Perceptual learning of contrast discrimination in macaca mulatta

Rhesus monkeys underwent training in a contrast discrimination task, in which grating stimuli were presented at parafoveal and peripheral visual field locations. Subjects had to compare a sample stimulus that had a fixed contrast of 30% to a test stimulus that varied in contrast from trial to trial. Extensive practice yielded improvements in contrast discrimination that were observed across the full range of test stimulus contrasts. These improvements occurred across multiple sessions, as well as across trials within individual sessions. The finer the contrast discriminations required, the longer it took for subjects to improve. Improvements in psychophysical performance resulted in the steepening of psychometric functions and/or shifts in the point of subjective equality towards the contrast of the sample stimulus. Enhancement in discrimination was especially pronounced around the contrast level of the sample stimulus, to which the subject was consistently exposed. The changes resulted in increased accuracy overall, lower discrimination thresholds, and faster response times. Partial transfer of learning, from vertically oriented training stimuli to horizontally oriented testing stimuli, was observed, while transfer to stimuli with different spatial frequencies was less pronounced. The results demonstrate the existence of perceptual learning in the contrast domain, whereby learning affects multiple performance-related psychophysical metrics.

Feedback-driven tuning of statistical summary representations

The amount of sensory data encountered by the visual system often exceeds its processing capacity. One solution is to exploit statistical structure in the natural environment to generate a more efficient representation of the information (Simoncelli & Olshausen, 2001). For example, the visual system may construct a “statistical summary representation” over groups of visual objects, reflecting their general properties (Alvarez, 2011). Indeed, it has been shown that observers are able to quickly and accurately extract average values over a range of visual feature dimensions, including size (Chong & Treisman, 2003), orientation (Parkes et al. 2001), and emotional expression (Haberman & Whitney, 2007). However, it remains an open question as to how observers learn to produce such accurate estimates of these summary statistics. Although good performance on these tasks suggests that summary features are readily accessible, it is not clear to what extent these statistical operations are performed automatically—integrating over sensory information in an unsupervised fashion, or are penetrable to task demands—flexibly incorporating observer goals and error-related feedback to maximize performance (Bauer, 2009; Myczek & Simons, 2008). In the present study, we sought to understand the role of learning in statistical summary representations. Specifically, we examined the contribution of task practice and performance feedback to perceptual discrimination of the centroid (i.e., mean location) of a set of objects (Alvarez & Oliva, 2008). We hypothesized that providing vector error feedback (i.e., containing both distance and direction information) while observers practiced making pointing movements toward the centroid would improve the fidelity of their centroid representations. This improvement might be reflected in reduced error during training and lower discrimination thresholds in an independent perceptual test.

Brightness discrimination in the South African fur seal (Arctocephalus pusillus)

Underwater, the contrast between object and background is much larger reduced with increasing distance between object and observer than in air. For marine predators, such as pinnipeds, it would therefore be advantageous to possess a high sensitivity for brightness differences, since this would increase the distance at which prey can be detected visually. Few studies have examined the brightness discrimination thresholds of pinnipeds. Two studies with phocid seals have confirmed low brightness discrimination thresholds in pinnipeds whereas the threshold obtained for the South African fur seal seems to be twice as high as that of the phocids. However, the experiments with the South African fur seal have been conducted under inadequate conditions which likely resulted in an underestimation of the brightness discrimination ability of this species. The study at hand reinvestigated the brightness discrimination threshold of the South African fur seal under well controlled conditions. In a two alternative forced choice task, one fur seal was trained to indicate the position of the brighter of two gray discs presented on a black background on a monitor. The thresholds were determined for 11 standard intensities each tested against 8 lower comparison intensities. It was found that the fur seal was able to perceive brightness differences of 8-10%, which is better than the phocid species tested so far. For low standard intensities, however, the threshold increased which could to be due to a relative slow dark adaptation rate of the fur seal. The results are discussed in terms of the relevance of visual information for pinnipeds during foraging dives and are directly compared to the results obtained for the harbor seal which has been tested under the same conditions as the fur seal in a previous study.

Auditory Frequency Discrimination in Adults With Dyslexia: A Test of the Anchoring Hypothesis

A recent hypothesis ascribes dyslexia to a perceptual anchoring deficit. Supporting results have so far been obtained only in children with dyslexia and additional learning difficulties, but the hypothesis has been argued to apply to all individuals with dyslexia. The authors measured auditory frequency discrimination thresholds in 54 young adults (native speakers of Dutch; 25 individuals with dyslexia; 29 age- and education-matched controls), with a 2-alternative forced-choice staircase paradigm. There were 2 conditions: 1 in which 1 tone in each pair had a fixed frequency across trials (standard), and 1 in which both tones varied in frequency (no standard). Presence of a standard stimulus allows the creation of a stimulus-specific representation in short-term-memory (perceptual anchor). The standard condition yielded significantly lower discrimination thresholds in both controls and individuals with dyslexia; both groups benefited equally from the presence of an invariant stimulus. There is no difference between this group of adults with dyslexia and controls in their capacity to form a perceptual anchor. The implication is that an anchoring deficit cannot be generalized to all cases of dyslexia.

Observer detection and discrimination performance as a function of clutter: a signal detection approach

In this paper we investigate the use of signal detection theory (SDT) in predicting target detection and discrimination in disorganized clutter. Two normal observers performed monocular visual search experiments at 25 cm, in the dark. They detected Gabor gratings on an achromatic background cluttered with 2000 or 500 random dots. The targets were displayed at pseudorandom locations from 0–20° and 20–47°, by method of constant stimuli. A contrast-based detection and orientation-based discrimination task was completed in a yes/no or 2-alternative-forced-choice (2AFC) task. The hit rate, false alarm rate, detectability, criterion and bias were analysed. The psychometric function indicated low detection and discrimination thresholds in low clutter that increased in high clutter. Increased clutter showed high hit rates and a false alarm rate that increased with low detectability and liberal criterion. In the detection task, low clutter showed high hit rates and low false alarm rates in the central field. Therefore, SDT proves useful to predict observer performance in visual scenes with disorganized clutter.

Physiological signal variability in hMT+ reflects performance on a direction discrimination task

Our ability to perceive visual motion is critically dependent on the human motion complex (hMT+) in the dorsal visual stream. Extensive electrophysiological research in the monkey equivalent of this region has demonstrated how neuronal populations code for properties such as speed and direction, and that neurometric functions relate to psychometric functions within the individual monkey. In humans, the physiological correlates of inter-individual perceptual differences are still largely unknown. To address this question, we used functional magnetic resonance imaging (fMRI) while participants viewed translational motion in different directions, and we measured thresholds for direction discrimination of moving stimuli in a separate psychophysics experiment. After determining hMT+ in each participant with a functional localizer, we were able to decode the different directions of visual motion from it using pattern classification (PC). We also characterized the variability of fMRI signal in hMT+ during stimulus and rest periods with a generative model. Relating perceptual performance to physiology, individual direction discrimination thresholds were significantly correlated with the variability measure in hMT+, but not with PC accuracies. Individual differences in PC accuracy were driven by non-physiological sources of noise, such as head-movement, which makes this method a poor tool to investigate inter-individual differences. In contrast, variability analysis of the fMRI signal was robust to non-physiological noise, and variability characteristics in hMT+ correlated with psychophysical thresholds in the individual participants. Higher levels of fMRI signal variability compared to rest correlated with lower discrimination thresholds. This result is in line with theories on stochastic resonance in the context of neuronal populations, which suggest that endogenous or exogenous noise can increase the sensitivity of neuronal populations to incoming signals.

Odor detection and odor discrimination in subadult and adult rats for two enantiomeric odorants supported by c-fos data

The study investigated the olfactory sensitivity and odor discrimination ability of male Wistar rats for the two closely related enantiomic odorants carvone and limonene. Enantiomers have identical chemical and physical properties but many are known to have different odor qualities for humans. Our psychophysical experiments revealed that subadult animals (about 2 months old) demonstrated higher olfactory sensitivities – lower detection thresholds – for d- and l-limonene than the adult conspecifics (about 6 months old). In the discrimination tasks at higher odor concentrations all animals were able to discriminate between the d- and l-odorant but subadults demonstrated lower discrimination thresholds. Within each age group no significant difference could be established.To elucidate neural correlates of our behavioral results, we have used c-fos expression to provide a global map of neuronal activity, with single cell resolution, in the olfactory bulb of d- and l-carvone exposed animals. The two carvone odors elicit different patterns of neuronal activity in the olfactory bulb, however, with partially overlapping activated areas. Our behavioral and immunhistochemical results allow to assume that the d- and l-forms of the two enantiomeric odorants have different odor qualities. Our study further demonstrates that the odor quality of an odorant depends not only on the atoms of the molecule, its chemical structure or on the presence of a functional group. Other parameters like, e.g. interaction profiles with different peptides, optical rotation of the polarized light – as seen in the enantiomers of an odorant – might also contribute to the quality of an odor.

Stereoscopic depth perception in peripheral field and global processing of horizontal disparity gradient pattern

This study investigates how the visual system detects a surface deviation from planar, induced by, crossed or uncrossed, horizontal disparities continuously increasing with eccentricity. Binocular disparities increased linearly and concentrically, between two given eccentricities. The thresholds of deformation detection were gathered using a method in which observers halted a dynamic stimulus. The thresholds are substantially higher than those measured by the control experiment using a method of constant stimuli. Results, using the adjustment method, highlight lower discrimination thresholds for uncrossed disparities than for crossed disparities. For the two directions of disparity, thresholds vary similarly as a function of eccentricity, however two observations can be pointed out: thresholds of peripheral start depend on disparity gradient and starting eccentricity; foveal start thresholds do not depend on disparity gradient alone. Data suggests that, in peripheral field, the visual system is more sensitive to uncrossed disparities than crossed disparities, relative to the frontoparallel plane. According to a verbal report from observers, the reference used for the perceptive judgment appears not to be the screen plane but rather the peripheral stimulus. Moreover, in the deformation detection of planar surfaces, horizontal disparities processing depends on the eccentric location of the disparities. It could be global for the peripheral locations and could be based more on depth contrast for the central locations.

The Role of Explicit and Implicit Standards in Visual Speed Discrimination

Five experiments were designed to investigate visual speed discrimination. Variations of the method of constant stimuli were used to obtain speed discrimination thresholds in experiments 1, 2, 4, and 5, while the method of single stimuli was used in experiment 3. The observers' thresholds were significantly influenced by the choice of psychophysical method and by changes in the standard speed. The observers' judgments were unaffected, however, by changes in the magnitude of random variations in stimulus duration, reinforcing the conclusions of Lappin et al (1975 Journal of Experimental Psychology: Human Perception and Performance 1 383 394). When an implicit standard was used, the observers produced relatively low discrimination thresholds (7.0% of the standard speed), verifying the results of McKee (1981 Vision Research 21 491-500). When an explicit standard was used in a 2AFC variant of the method of constant stimuli, however, the observers' discrimination thresholds increased by 74% (to 12.2%), resembling the high thresholds obtained by Mandriota et al (1962 Science 138 437-438). A subsequent signal-detection analysis revealed that the observers' actual sensitivities to differences in speed were in fact equivalent for both psychophysical methods. The formation of an implicit standard in the method of single stimuli allows human observers to make judgments of speed that are as precise as those obtained when explicit standards are available.

Standardization of 65Zn by 4π PC- γ coincidence counting method with efficiency extrapolation

A 65Zn solution was standardized by the 4 πPC– γ efficiency-extrapolation coincidence counting method. Theoretical aspects of coincidence equations, efficiency equations and linearity conditions are reviewed. Experimental measurements were performed for two low level discrimination thresholds for the PC channel (counting K+ L or K X-rays or Auger electrons from EC decay) and for three different settings of the γ window. Requirements on γ channel set-up for linear extrapolation were established by using a Pb absorber or by proper γ window setting. The measured activity values were discussed and found in good agreement with those obtained with a calibrated ionization chamber.

Duration of Perceived and Performed Sounds

This paper reports the results of a study on the perception and reproduction of sound durations. First, the duration discrimination abilities of musician and non-musician subjects were measured. An experiment was then carried out to find the effect of frequency difference on duration discrimination in pairs of pure tones (musical note durations). Finally, three experiments were undertaken to investigate any effect of frequency difference on the performed duration of tones. The major findings were: (i) that musicians show lower discrimination thresholds than non-musicians for short stimuli durations (25-100 msec), (ii) that higher frequencies are perceived as being longer in duration than lower frequencies, and (iii) that when the performed sound should be double or longer than a presented tone, subjects typically perform the sound longer than is appropriate.

Anisotropies in global stereoscopic orientation discrimination

Human orientation discrimination of long rectangular targets formed from dynamic random-element stereograms was assessed at four orientations (horizontal, vertical, leftand right oblique) as a function of width, disparity direction and magnitude using a temporal two-alternative, forced-choice paradigm. The results revealed the presence of an oblique effect in cyclopean orientation discrimination. In addition, observers discriminated targets with crossed disparity better than targets with uncrossed disparity, and had lower discrimination thresholds when the targets were oriented horizontally than vertically. These results demonstrate the existence of the oblique effect in the hypercyclopean domain comparable in magnitude to that present in the luminance domain.