Grasping follows Weber's law: How to use response variability as a proxy for JND.

Weber's law is a fundamental psychophysical principle. It states that the just noticeable difference (JND) between stimuli increases with stimulus magnitude; consequently, larger stimuli should be estimated with larger variability. However, visually guided grasping seems to violate this expectation: When repeatedly grasping large objects, the variability is similar to that when grasping small objects. Based on this result, it was often concluded that grasping violated Weber's law. This astonishing finding generated a flurry of research, with contradictory results and potentially far-reaching implications for theorizing about the functional architecture of the brain. We show that previous studies ignored nonlinearities in the scaling of the grasping response. These nonlinearities result from, for example, the finger span being limited such that the opening of the fingers reaches a ceiling for large objects. We describe how to mathematically take these nonlinearities into account and apply this approach to our own data, as well as to the data of three influential studies on this topic. In all four datasets, we found that-when appropriately estimated-JNDs increase with object size, as expected by Weber's law. We conclude that grasping obeys Weber's law, as do essentially all sensory dimensions.

Continuous Transition Impairs Discrimination of Electrotactile Frequencies.

Just-noticeable difference (JND), indicating the ability to accurately identify small differences in stimulation parameters, can be used to choose more sensitive stimulation methods as well as to calibrate tactile feedback in closed-loop human-machine interfacing. The JND is typically estimated using a forced-choice-discrimination task, in which two stimuli with different intensities are delivered separated by a brief pause. In the applications of tactile feedback, however, the stimulation parameters are typically modulated continuously. It is unclear if the discriminability of stimuli separated in time characterizes the ability to distinguish continuous changes in stimulation intensity. The present study compared the JND when pairs of frequency-modulated electrotactile stimuli were separated in time and presented continuously at two different baseline frequencies (20 and 60 Hz). The results showed that the JND was significantly smaller with time-separation between stimuli, but that the JND obtained with different types of transitions were in most cases linearly associated. In conclusion, the discriminability of time-separated stimuli is systematically better compared to that of the stimuli presented continuously. This can have an impact when calibrating the tactile feedback where the conventional method of the JND assessment might lead to an overly optimistic estimate of detectable changes.

Just noticeable differences for elbow joint torque feedback.

Joint torque feedback is a new and promising means of kinesthetic feedback imposed by a wearable device. The torque feedback provides the wearer temporal and spatial information during a motion task. Nevertheless, little research has been conducted on quantifying the psychophysical parameters of how well humans can perceive external torques under various joint conditions. This study aims to investigate the just noticeable difference (JND) perceptual ability of the elbow joint to joint torques. The paper focuses on the ability of two primary joint proprioceptors, the Golgi-tendon organ (GTO) and muscle spindle (MS), to detect elbow torques, since touch and pressure sensors were masked. We studied 14 subjects while the arm was isometrically contracted (static condition) and was moving at a constant speed (dynamic condition). In total there were 10 joint conditions investigated, which varied the direction of the arm's movement and the preload direction as well as torque direction. The JND torques under static conditions ranged from 0.097 Nm with no preload to 0.197 Nm with a preload of 1.28 Nm. The maximum dynamic JND torques were 0.799 Nm and 0.428 Nm, when the arm was flexing and extending at 213 degrees per second, respectively.

Excitability and Threshold Mechanism for Enhanced Neuronal Response Induced by Inhibition Preceding Excitation.

Postinhibitory facilitation (PIF) of neural firing presents a paradoxical phenomenon that the inhibitory effect induces enhancement instead of reduction of the firing activity, which plays important roles in sound location of the auditory nervous system, awaited theoretical explanations. In the present paper, excitability and threshold mechanism for the PIF phenomenon is presented in the Morris-Lecar model with type I, II, and III excitabilities. Firstly, compared with the purely excitatory stimulations applied to the steady state, the inhibitory preceding excitatory stimulation to form pairs induces the firing rate increased for type II and III excitabilities instead of type I excitability, when the interval between the inhibitory and excitatory stimulation within each pair is suitable. Secondly, the threshold mechanism for the PIF phenomenon is acquired. For type II and III excitabilities, the inhibitory stimulation induces subthreshold oscillations around the steady state. During the middle and ending phase of the ascending part and the beginning phase of the descending part within a period of the subthreshold oscillations, the threshold to evoke an action potential by an excitatory stimulation becomes weaker, which is the cause for the PIF phenomenon. Last, a theoretical estimation for the range of the interval between the inhibitory and excitatory stimulation for the PIF phenomenon is acquired, which approximates half of the intrinsic period of the subthreshold oscillations for the relatively strong stimulations and becomes narrower for the relatively weak stimulations. The interval for the PIF phenomenon is much shorter for type III excitability, which is closer to the experiment observation, due to the shorter period of the subthreshold oscillations. The results present the excitability and threshold mechanism for the PIF phenomenon, which provide comprehensive and deep explanations to the PIF phenomenon.

Isolated head tremor: A DAT-SPECT and somatosensory temporal discrimination study.

INTRODUCTION: To better understand the pathophysiology of isolated head tremor, we investigated somatosensory temporal discrimination threshold (STDT) and dopaminergic neurotransmission with 123I-FP-CIT (DAT-SPECT) in patients with isolated head tremor, as well as potential correlations with their clinical features. METHODS: We enrolled 14 patients with isolated head tremor who underwent clinical examination, STDT testing, and DAT-SPECT. RESULTS: We found normal radiotracer uptake in both striata as assessed with DAT-SPECT examination, but higher STDT values in patients as compared to healthy subjects. No significant correlations emerged between STDT values, specific binding ratios of radiotracer uptake, and demographic or clinical features. CONCLUSION: Our study found normal radiotracer uptake with DAT-SPECT examination, suggesting dopaminergic neurotransmission integrity in patients with isolated head tremor. Patients with isolated head tremor exhibited an abnormally elevated STDT. Both results support the hypothesis that isolated head tremor should be considered a form of dystonia.

Children's Social Information Seeking is Sensitive to Referential Ambiguity.

We examined children's spontaneous information seeking in response to referential ambiguity. Children ages 2-5 (n = 160) identified the referents of familiar and novel labels. We manipulated ambiguity by changing the number of objects present and their familiarity (Experiments 1 and 2), and the availability of referential gaze (Experiment 2). In both experiments, children looked to the face of the experimenter more often while responding, specifically when the referent was ambiguous. In Experiment 2, 3- to 4-year olds also demonstrated sensitivity to graded referential evidence. These results suggest that social information seeking is an active learning behavior that could contribute to language acquisition in early childhood.

Contingency, contiguity, and causality in conditioning: Applying information theory and Weber's Law to the assignment of credit problem.

Contingency is a critical concept for theories of associative learning and the assignment of credit problem in reinforcement learning. Measuring and manipulating it has, however, been problematic. The information-theoretic definition of contingency-normalized mutual information-makes it a readily computed property of the relation between reinforcing events, the stimuli that predict them and the responses that produce them. When necessary, the dynamic range of the required temporal representation divided by the Weber fraction gives a psychologically realistic plug-in estimates of the entropies. There is no measurable prospective contingency between a peck and reinforcement when pigeons peck on a variable interval schedule of reinforcement. There is, however, a perfect retrospective contingency between reinforcement and the immediately preceding peck. Degrading the retrospective contingency by gratis reinforcement reveals a critical value (.25), below which performance declines rapidly. Contingency is time scale invariant, whereas the perception of proximate causality depends-we assume-on there being a short, fixed psychologically negligible critical interval between cause and effect. Increasing the interval between a response and reinforcement that it triggers degrades the retrograde contingency, leading to a decline in performance that restores it to at or above its critical value. Thus, there is no critical interval in the retrospective effect of reinforcement. We conclude with a short review of the broad explanatory scope of information-theoretic contingencies when regarded as causal variables in conditioning. We suggest that the computation of contingencies may supplant the computation of the sum of all future rewards in models of reinforcement learning. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Associations between genetic variations and global motion perception.

The cholinergic system is known to strongly modulate perceptual and cognitive processes, and the alpha7 subunit of the cholinergic nicotinic receptor (CHRNA7) is broadly expressed within the visual system. Here, we assessed whether genetic variations of CHRNA7 affect coherent motion perception. Motion perception has been shown to decline with age, and it has previously been suggested that the effects of genetic variations are magnified by age. Therefore, we tested both older (n = 62) and younger adults (n = 63). We found that motion coherence thresholds were significantly higher for older compared to younger adults, which is in accordance with previous studies. Interestingly, there was a strong relationship between variants of the SNP rs2337980 of the CHRNA7 and motion direction discrimination. In particular, participants carrying the TC genotype had considerably lower motion coherence thresholds than CC carriers. The effect of genotype did not interact with age. Our results show that genetic variations are associated with perceptual performance, but are unlikely to explain age-related changes.

Visuo-Haptic Discrimination of Viscoelastic Materials.

In our daily lives, we interact with different types of deformable materials. Regarding their mechanical behavior, some of those materials lie in a range that is between purely elastic and purely viscous. This range of mechanical behavior is described as viscoelasticity. In certain types of haptic interactions, such as assessment of ripeness of fruit, firmness of cheese, and consistency of organ tissue, we rely heavily on our haptic perception of viscoelastic materials. The relationship between the mechanical behavior of viscoelastic materials and our perception of them has been investigated in the field of psychorheology. However, our knowledge on how we perceive viscoelastic materials is still quite limited though some research work has already been done on purely elastic and purely viscous materials. History- and frequency-dependent behavior of viscoelastic materials result in a complex time-dependent response, which requires relatively more sophisticated models to investigate their behavior than those of purely elastic and viscous materials. In this study, we model viscoelasticity using a "springpot" (i.e., fractional-order derivative element) and express its behavior in the frequency domain using two physical parameters-"magnitude" and "phase" of complex stiffness. In the frequency domain, we are able to devise signal detection experiments where we can investigate the perception of viscoelastic materials using the perceptual terms of "firmness" and "bounciness," corresponding to the physical parameters of "magnitude" and "phase." The results of our experiments show that the just-noticeable difference (JND) for bounciness increases linearly with increasing "phase," following Weber's law, while the JND for firmness is surprisingly independent of the level of "phase."

Oral somatosensatory acuity is related to particle size perception in chocolate.

Texture affects liking or rejection of many foods for clinically relevant populations and the general public. Phenotypic differences in chemosensation are well documented and influence food choices, but oral touch perception is less understood. Here, we used chocolate as a model food to explore texture perception, specifically grittiness perception. In Experiment 1, the Just Noticeable Difference (JND) for particle size in melted chocolate was ~5 µm in a particle size range commonly found in commercial chocolates; as expected, the JND increased with particle size, with a Weber Fraction of ~0.17. In Experiment 2, individual differences in touch perception were explored: detection and discrimination thresholds for oral point pressure were determined with Von Frey Hairs. Discrimination thresholds varied across individuals, allowing us to separate participants into high and low sensitivity groups. Across all participants, two solid commercial chocolates (with particle sizes of 19 and 26 µm; i.e., just above the JND) were successfully discriminated in a forced-choice task. However, this was driven entirely by individuals with better oral acuity: 17 of 20 of more acute individuals correctly identified the grittier chocolate versus 12 of 24 less acute individuals. This suggests phenotypic differences in oral somatosensation can influence texture perception of foods.

Skin Stretch Haptic Feedback to Convey Closure Information in Anthropomorphic, Under-Actuated Upper Limb Soft Prostheses.

Restoring hand function in individuals with upper limb loss is a challenging task, made difficult by the complexity of human hands from both a functional and sensory point of view. Users of commercial prostheses, even sophisticated devices, must visually attend to the hand to know its state, since in most cases they are not provided with any direct sensory information. Among the different types of haptic feedback that can be delivered, particularly information on hand opening is likely to reduce the requirement of constant visual attention. In recent years, there has been a trend of using underactuated, compliant multi-fingered hands as upper limb prostheses, in part due to their simplicity and ease of use attributed to low degree-of-freedom (d.o.f.) actuation. The trend toward underactuation encourages the design of one d.o.f. haptic devices to provide intuitive sensory feedback from the prosthesis. However, mapping the closure of a multi-d.o.f. prosthetic hand to a simple and intuitive haptic cue is not a trivial task. In this paper, we explore the use of a one d.o.f. skin stretch haptic device, the rice haptic rocker, to provide intuitive proprioceptive feedback indicating overall hand closure of an underactuated prosthesis. The benefits and challenges of the system are assessed in multi-tasking and reduced vision scenarios for an object-size discrimination task, in an effort to simulate challenges in daily life, and are compared against the haptic resolution of the device using the just noticeable difference. Finally, an evaluation done with a prosthesis user, in the form of a truncated version of the Activities Measure for Upper Limb Amputees (AM-ULA), shows possible benefits of the addition of haptic feedback in tasks with reduced visual attention.

Tactile-Based Pantomime Grasping: Knowledge of Results is Not Enough to Support an Absolute Calibration.

Tactile-based pantomime-grasping requires that a performer use their right hand to 'grasp' a target previously held in the palm of their opposite hand - a task examining how mechanoreceptive (i.e., tactile) feedback informs the motor system about an object property (i.e., size). Here, we contrasted pantomime-grasps performed with (H+) and without (H-) haptic feedback (i.e., thumb and forefinger position information derived from the grasping hand touching the object) with a condition providing visual KR (VKR) related to absolute target object size. Just-noticeable-difference (JND) scores were computed to determine whether responses adhered to - or violated - Weber's law. JNDs for H+ trials violated the law, whereas H- and VKR trials adhered to the law. Accordingly, results demonstrate that haptic feedback - and not KR - supports an absolute tactile-haptic calibration.

Weber's law in 2D and 3D grasping.

Visually guided grasping movements directed to real, 3D objects are characterized by a distinguishable trajectory pattern that evades the influence of Weber's law, a basic principle of perception. Conversely, grasping trajectories directed to 2D line drawings of objects adhere to Weber's law. It can be argued, therefore, that during 2D grasping, the visuomotor system fails at operating in analytic mode and is intruded by irrelevant perceptual information. Here, we explored the visual and tactile cues that enable such analytic processing during grasping. In Experiment 1, we compared grasping directed to 3D objects with grasping directed to 2D object photos. Grasping directed to photos adhered to Weber's law, suggesting that richness in visual detail does not contribute to analytic processing. In Experiment 2, we tested whether the visual presentation of 3D objects could support analytic processing even when only partial object-specific tactile information is provided. Surprisingly, grasping could be performed in an analytic fashion, violating Weber's law. In Experiment 3, participants were denied of any haptic feedback at the end of the movement and grasping trajectories again showed adherence to Weber's law. Taken together, the findings suggest that the presentation of real objects combined with indirect haptic information at the end of the movement is sufficient to allow analytic processing during grasp.

Comparative tactile sensitivity of the fingertip and apical tongue using complex and pure tactile tasks.

Both the tongue and fingertip are highly tactile tissues relevant in texture perception, but work comparing relative sensitivity to elucidate potential differences in stimulus processing is limited. Presently, the acuity of the tongue and fingertip were compared using a series of tactile acuity tasks. We hypothesized the tongue would show superior acuity regardless of stimuli due to an absent epidermal barrier and its involvement in many high-sensitivity behaviors (e.g. eating, speaking). Acuity was determined using three different tests, two "purely-tactile" just noticeable difference (JND) tasks (punctate pressure and roughness sensitivity) and a more-complex, stereognostic letter-recognition task to evaluate point-and-edge sensitivity. JNDs were determined using the forced-choice staircase method for the punctate deformation force of a monofilament (F;0.0044-0.010 g) and the surface roughness of stainless steel coupons (Ra; 0.177-0.465 µm) in populations of 30 and 31 individuals, respectively. Point-and-edge sensitivity was assessed by determining the letter recognition threshold (RT) based on height (h;1.5-8.0 mm) in an additional 28 individuals using a modified staircase method. While subjects had significantly lower JNDs with their tongues for both "purely-tactile" tasks (punctate: 0.0017 ±â€¯0.0001 g vs. 0.0023 ±â€¯0.0002 g (fingertip), p = .018; roughness: 0.039 ±â€¯0.004 µm vs. 0.112 ±â€¯0.020 µm (fingertip), p < .001), subjects had significantly higher RTs with their tongues for the letter identification task (3.98 ±â€¯0.84 mm vs. 4.54 ±â€¯1.41 mm (fingertip), p = .0417). The latter difference is likely attributable to the more complex nature of the RT task and the finger's frequent involvement in object recognition. Binomial statistics (p = 1/2, α = 0.05) showed a significant number of subjects were better at the roughness task with their tongues (p = .021); however, a significant majority were better at the letter identification task with their fingers (p = .049); no significant difference was found for the punctate pressure task. While data appear to suggest the tongue is more sensitive to exclusively tactile stimuli, further study of other "pure-tactile" sensations should help clarify the contradictory results of the RT task.

Divergent Responses in the Gap Prepulse Inhibition of the Acoustic Startle Reflex in Two Different Guinea Pig Colonies.

Animal models of tinnitus rely on interpretation of behavioural or reflexive tests to determine the presence of this phantom perception. A commonly used test is the gap prepulse inhibition of acoustic startle (GPIAS), which is often combined with prepulse inhibition (PPI) to ensure that reduced GPIAS suppression is not due to hearing loss caused by the acoustic trauma commonly used to trigger tinnitus development. In our laboratory GPIAS and PPI are routinely used on two colonies of outbred tri-colour guinea pigs. However, our results show that these colonies show divergent results even before any tinnitus-inducing treatment, which impacts their suitability in tinnitus models. Although colony 1 and 2 show similar results in PPI (~95% of animals showing significant suppression), only ~30% of colony 2 also shows significant suppression in GPIAS compared to ~75% of colony 1. Cochlear sensitivity measured using compound action potentials showed no significant differences between colonies. Therefore, peripheral threshold loss was excluded as a possible factor. Our results show that similar strains of laboratory animals can show highly divergent results and GPIAS testing for tinnitus will not work for every animal strain. In addition, our data support the notion that PPI and GPIAS responses may rely on different neural circuitry.

Hand anthropometry and the limits of aperture separation determine the utility of Weber's law in grasping and manual estimation.

Recent work proposed that biomechanical constraints in aperture separation limit the utility of Weber's law in determining whether dissociable visual codes support grasping and manual estimation. We tested this assertion by having participants precision grasp, manually estimate and complete a method of adjustment task to targets scaled within and beyond the range of their maximal aperture separation (i.e., from 20 to 140% of participant-specific maximal aperture separation: MAS). For grasping and manual estimation tasks, just-noticeable-difference (JND) scores were computed via the within-participant standard deviations in peak grip aperture, whereas method of adjustment JNDs were computed via the within-participant standard deviations in response output. Method of adjustment JNDs increased linearly across the range of targets; that is, responses adhered to Weber's law. Manual estimation JNDs linearly increased for targets 20-100% of MAS and then decreased for targets 120-140% of MAS. In turn, grasping JNDs for targets 20% through 80% of MAS did not differ and were larger than targets 100-140% of MAS. That manual estimation and grasping showed a decrease in JNDs for the largest targets indicates that participants were at their biomechanical limits in aperture shaping, and the fact that the target showing the JND decrease differed between tasks (i.e., manual estimation = 100% of MAS; grasping = 80% of MAS) is attributed to the fact that grasping-but not manual estimation-requires a safety-margin task-set. Accordingly, manual estimations and grasping across a range of functionally 'graspable' targets, respectively, adhered to and violated Weber's law-a result interpreted to reflect the use of dissociable visual codes.

Intensity difference limens in adult CBA/CaJ mice (Mus musculus).

Mice have emerged as important models of auditory perception and acoustic communication. To study and model complex sound perception and communication, basic hearing abilities have to be established, yet intensity difference limens have not been measured in CBA/CaJ mice. Nine mice were trained using operant conditioning procedures with positive reinforcement to discriminate sound intensity across frequencies. Intensity difference limens were measured for 12, 16, 24, and 42 kHz tones at 10 and 30 dB sensation levels. Mice are capable of discriminating intensities across frequencies and sensation levels, but have higher intensity difference limens (IDLs) thresholds than other mammals.

Influence of Spatial and Chromatic Noise on Luminance Discrimination.

Pseudoisochromatic figures are designed to base discrimination of a chromatic target from a background solely on the chromatic differences. This is accomplished by the introduction of luminance and spatial noise thereby eliminating these two dimensions as cues. The inverse rationale could also be applied to luminance discrimination, if spatial and chromatic noise are used to mask those cues. In this current study estimate of luminance contrast thresholds were conducted using a novel stimulus, based on the use of chromatic and spatial noise to mask the use of these cues in a luminance discrimination task. This was accomplished by presenting stimuli composed of a mosaic of circles colored randomly. A Landolt-C target differed from the background only by the luminance. The luminance contrast thresholds were estimated for different chromatic noise saturation conditions and compared to luminance contrast thresholds estimated using the same target in a non-mosaic stimulus. Moreover, the influence of the chromatic content in the noise on the luminance contrast threshold was also investigated. Luminance contrast threshold was dependent on the chromaticity noise strength. It was 10-fold higher than thresholds estimated from non-mosaic stimulus, but they were independent of colour space location in which the noise was modulated. The present study introduces a new method to investigate luminance vision intended for both basic science and clinical applications.

Does the Somatosensory Temporal Discrimination Threshold Change over Time in Focal Dystonia?

BACKGROUND: The somatosensory temporal discrimination threshold (STDT) is defined as the shortest interval at which an individual recognizes two stimuli as asynchronous. Some evidence suggests that STDT depends on cortical inhibitory interneurons in the basal ganglia and in primary somatosensory cortex. Several studies have reported that the STDT in patients with dystonia is abnormal. No longitudinal studies have yet investigated whether STDT values in different forms of focal dystonia change during the course of the disease. METHODS: We designed a follow-up study on 25 patients with dystonia (15 with blepharospasm and 10 with cervical dystonia) who were tested twice: upon enrolment and 8 years later. STDT values from dystonic patients at the baseline were also compared with those from a group of 30 age-matched healthy subjects. RESULTS: Our findings show that the abnormally high STDT values observed in patients with focal dystonia remained unchanged at the 8-year follow-up assessment whereas disease severity worsened. CONCLUSIONS: Our observation that STDT abnormalities in dystonia remain unmodified during the course of the disease suggests that the altered activity of inhibitory interneurons-either at cortical or at subcortical level-responsible for the increased STDT does not deteriorate as the disease progresses.

Manual estimations of functionally graspable target objects adhere to Weber's law.

The manual estimation task requires that participants separate the distance between their thumb and forefinger until they perceive it to match the size of a target object. Ganel and colleagues (Curr Biol 18:R599-R601, 2008a) demonstrated that manual estimations yield just-noticeable-difference (JND) scores that linearly increased with increasing target object size; that is, responses adhered to Weber's law and thus evince response mediation via relative and perception-based visual information. In turn, more recent work has reported that the size of a target object influences whether JNDs provide a reliable metric for evaluating the nature of the visual information supporting manual estimations. In particular, Bruno et al. (Neuropsychologia 91:327-334, 2016) reported that JNDs for 'large' target objects (i.e., 80 and 120 mm) violate Weber's law due to biomechanical limits in aperture opening. It is, however, important to recognize that the absolute size of the 'large' target objects employed by Bruno et al. may have exceeded some participants' functional aperture separation and resulted in a biomechanical strategy serving as the only viable response mode. Hence, the present investigation employed a manual estimation task wherein target object sizes were proportionately matched to decile increments (i.e., 10, 20, …, 70 and 80%) of individual participants' maximal aperture separation. Results showed that JNDs increased linearly with increasing target object size. Accordingly, we propose that manual estimations of target objects within a functionally 'graspable' range adhere to Weber's law and are mediated via relative and perception-based visual information.