Direction Discrimination Performance Research Articles

Manipulating the Fourier spectra of stimuli comprising a two-frame kinematogram to study early visual motion-detecting mechanisms: Perception versus short latency ocular-following responses.

Two-frame kinematograms have been extensively used to study motion perception in human vision. Measurements of the direction-discrimination performance limits (Dmax) have been the primary subject of such studies, whereas surprisingly little research has asked how the variability in the spatial frequency content of individual frames affects motion processing. Here, we used two-frame one-dimensional vertical pink noise kinematograms, in which images in both frames were bandpass filtered, with the central spatial frequency of the filter manipulated independently for each image. To avoid spatial aliasing, there was no actual leftward-rightward shift of the image: instead, the phases of all Fourier components of the second image were shifted by ±¼ wavelength with respect to those of the first. We recorded ocular-following responses (OFRs) and perceptual direction discrimination in human subjects. OFRs were in the direction of the Fourier components' shift and showed a smooth decline in amplitude, well fit by Gaussian functions, as the difference between the central spatial frequencies of the first and second images increased. In sharp contrast, 100% correct perceptual direction-discrimination performance was observed when the difference between the central spatial frequencies of the first and second images was small, deteriorating rapidly to chance when increased further. Perceptual dependencies moved closer to the OFR ones when subjects were allowed to grade the strength of perceived motion. Response asymmetries common for perceptual judgments and the OFRs suggest that they rely on the same early visual processing mechanisms. The OFR data were quantitatively well described by a model which combined two factors: (1) an excitatory drive determined by a power law sum of stimulus Fourier components' contributions, scaled by (2) a contrast normalization mechanism. Thus, in addition to traditional studies relying on perceptual reports, the OFRs represent a valuable behavioral tool for studying early motion processing on a fine scale.

Motion Direction Discrimination with Tactile Random-Dot Kinematograms.

Motion detection is a fundamental sensory function for multiple modalities, including touch, but the mechanisms underlying tactile motion detection are not well understood. While previous findings supported the existence of high-level feature tracking, it remains unclear whether there also exist low-level motion sensing that directly detects a local spatio-temporal correlation in the skin-stimulation pattern. To elucidate this mechanism, we presented, on braille displays, tactile random-dot kinematograms, similar to those widely used in visual motion research, which enables us to independently manipulate feature trackability and various parameters of local motion. We found that a human observer is able to detect the direction of difficult-to-track tactile motions presented to the fingers and palms. In addition, the direction-discrimination performance was better when the stimuli were presented along the fingers than when presented across the fingers. These results indicate that low-level motion sensing, in addition to high-level tracking, contribute to tactile motion perception.

Single-Trial Decoding of Motion Direction During Visual Attention From Local Field Potential Signals

Brain-Computer Interface (BCI) based on Local Field Potential (LFP) has recently been developed to restore communication or behavioral functions. LFP provides comprehensive information, due to its stability, robustness, and reach frequency content within the cognitive process. It has been demonstrated that spatial attention can be decoded from brain activity in the visual cortical areas. However, whether motion direction can be decoded from the LFP signal in the primate visual cortex remains uninvestigated, as well as how decoding performance may be influenced by spatial attention. In this paper, these issues were examined by recording LFP from the middle temporal area (MT) of macaque, employing machine learning algorithms. The animal was trained to report a brief direction change in a target stimulus which moved in various directions during a visual attention task. It was found that the LFP-gamma power was able to provide significant information to reliably decode motion direction, compared with other frequency bands, on a single-trial basis. Moreover, the results show that spatial attention leads to enhancements in motion direction discrimination performance. The highest decoding performance was achieved in the high-gamma frequencies (60–120Hz) when targets were presented inside the receptive field in opposite directions. Using a feature selection approach, performance was improved by optimally selecting features where the highest level of participation was observed in the gamma-band. Generally, the results suggest that in the MT area, LFP signals exhibit appreciable information about visual features like motion direction, which could thus be utilized as a control signal for cognitive BCI systems.

Music and psychoacoustic perception abilities in cochlear implant users with auditory neuropathy spectrum disorder

ObjectiveAuditory neuropathy spectrum disorder (ANSD) is a condition wherein the pre-neural or cochlear outer hair cell activity is intact, but the neural activity in the auditory nerve is disrupted. Cochlear implant (CI) can be beneficial for subjects with ANSD; however, little is known about the music perception and psychoacoustic abilities of CI users with ANSD. Music perception in CI users is a multidimensional and complex ability requiring the contribution of both auditory and nonauditory abilities. Even though auditory abilities lay the foundation, the contribution of patient-related variables such as ANSD may affect the music perception. This study aimed to evaluate the psychoacoustic and music perception abilities of CI recipients with ANSD. Study designTwelve CI users with ANSD and twelve age- and gendermatched CI users with sensorineural hearing loss (SNHL) were evaluated. Music perception abilities were measured using the Turkish version of the Clinical Assessment of Music Perception (T-CAMP) test. Psychoacoustic abilities were measured using the spectral ripple discrimination (SRD) and temporal modulation transfer function (TMTF) tests. In addition, the age of diagnosis and implantation was recorded. ResultsPitch direction discrimination (PDD), timbre recognition, SRD, and TMTF performance of CI users with ANSD were concordant with those reported in previous studies, and differences between ANSD and SNHL groups were not statistically significant. However, the ANSD group performed poorly compared with SNHL group in melody recognition subtest of T-CAMP, and the difference was statistically significant. ConclusionCI can prove beneficial for patients with ANSD with respect to their music and psychoacoustic abilities, similar to patients with SNHL, except for melody recognition. Recognition of melodies requires both auditory and non-auditory abilities, and ANSD may have an extensive but subtle effect in the life of CI users.

A simple linear readout of MT supports motion direction-discrimination performance.

Motion discrimination is a well-established model system for investigating how sensory signals are used to form perceptual decisions. Classic studies relating single-neuron activity in the middle temporal area (MT) to perceptual decisions have suggested that a simple linear readout could underlie motion discrimination behavior. A theoretically optimal readout, in contrast, would take into account the correlations between neurons and the sensitivity of individual neurons at each time point. However, it remains unknown how sophisticated the readout needs to be to support actual motion-discrimination behavior or to approach optimal performance. In this study, we evaluated the performance of various neurally plausible decoders, trained to discriminate motion direction from small ensembles of simultaneously recorded MT neurons. We found that decoding the stimulus without knowledge of the interneuronal correlations was sufficient to match an optimal (correlation aware) decoder. Additionally, a decoder could match the psychophysical performance of the animals with flat integration of up to half the stimulus and inherited temporal dynamics from the time-varying MT responses. These results demonstrate that simple, linear decoders operating on small ensembles of neurons can match both psychophysical performance and optimal sensitivity without taking correlations into account and that such simple read-out mechanisms can exhibit complex temporal properties inherited from the sensory dynamics themselves.NEW & NOTEWORTHY Motion perception depends on the ability to decode the activity of neurons in the middle temporal area. Theoretically optimal decoding requires knowledge of the sensitivity of neurons and interneuronal correlations. We report that a simple correlation-blind decoder performs as well as the optimal decoder for coarse motion discrimination. Additionally, the decoder could match the psychophysical performance with moderate temporal integration and dynamics inherited from sensory responses.

Modulatory mechanisms underlying high-frequency transcranial random noise stimulation (hf-tRNS): A combined stochastic resonance and equivalent noise approach

BackgroundHigh-frequency transcranial random noise stimulation (hf-tRNS) is a neuromodulatory technique consisting of the application of alternating current at random intensities and frequencies. hf-tRNS induces random neural activity in the system that may boost the sensitivity of neurons to weak inputs. Stochastic resonance is a nonlinear phenomenon whereby the addition of an optimal amount of noise results in performance enhancement, whereas further noise increments impair signal detection or discrimination. ObjectiveThe aim of the study was to assess whether modulatory effects of hf-tRNS rely on the stochastic resonance phenomenon, and what is the specific neural mechanism producing stochastic resonance. MethodObservers performed a two-interval forced choice motion direction discrimination task in which they had to report whether two moving patches presented in two temporal intervals had the same or different motion directions. hf-tRNS was administered at five intensity levels (0.5, 0.75, 1.0, 1.5, and 2.25 mA). ResultsThe results showed a significant improvement in performance when hf-tRNS was applied at 1.5 mA, representing the optimal level of external noise. However, stimulation intensity at 2.25 mA significantly impaired direction discrimination performance. An equivalent noise (EN) analysis, used to assess how hf-tRNS modulates the mechanisms underlying global motion processing, showed an increment in motion signal integration with the optimal current intensity, but reduced motion signal integration at 2.25 mA. ConclusionThese results indicate that hf-tRNS-induced noise modulates neural signal-to-noise ratio in a way that is compatible with the stochastic resonance phenomenon.

Feature-based attention potentiates recovery of fine direction discrimination in cortically blind patients

Training chronic, cortically-blind (CB) patients on a coarse [left-right] direction discrimination and integration (CDDI) task recovers performance on this task at trained, blind field locations. However, fine direction difference (FDD) thresholds remain elevated at these locations, limiting the usefulness of recovered vision in daily life. Here, we asked if this FDD impairment can be overcome by training CB subjects with endogenous, feature-based attention (FBA) cues. Ten CB subjects were recruited and trained on CDDI and FDD with an FBA cue or FDD with a neutral cue. After completion of each training protocol, FDD thresholds were re-measured with both neutral and FBA cues at trained, blind-field locations and at corresponding, intact-field locations. In intact portions of the visual field, FDD thresholds were lower when tested with FBA than neutral cues. Training subjects in the blind field on the CDDI task improved FDD performance to the point that a threshold could be measured, but these locations remained impaired relative to the intact field. FDD training with neutral cues resulted in better blind field FDD thresholds than CDDI training, but thresholds remained impaired relative to intact field levels, regardless of testing cue condition. Importantly, training FDD in the blind field with FBA lowered FDD thresholds relative to CDDI training, and allowed the blind field to reach thresholds similar to the intact field, even when FBA trained subjects were tested with a neutral rather than FBA cue. Finally, FDD training appeared to also recover normal integration thresholds at trained, blind-field locations, providing an interesting double dissociation with respect to CDDI training. In summary, mechanisms governing FBA appear to function normally in both intact and impaired regions of the visual field following V1 damage. Our results mark the first time that FDD thresholds in CB fields have been seen to reach intact field levels of performance. Moreover, FBA can be leveraged during visual training to recover normal, fine direction discrimination and integration performance at trained, blind-field locations, potentiating visual recovery of more complex and precise aspects of motion perception in cortically-blinded fields.

Motion coherence and direction discrimination in healthy aging.

Perceptual functions change with age, particularly motion perception. With regard to healthy aging, previous studies mostly measured motion coherence thresholds for coarse motion direction discrimination along cardinal axes of motion. Here, we investigated age-related changes in the ability to discriminate between small angular differences in motion directions, which allows for a more specific assessment of age-related decline and its underlying mechanisms. We first assessed older (>60 years) and younger (<30 years) participants' ability to discriminate coarse horizontal (left/right) and vertical (up/down) motion at 100% coherence and a stimulus duration of 400 ms. In a second step, we determined participants' motion coherence thresholds for vertical and horizontal coarse motion direction discrimination. In a third step, we used the individually determined motion coherence thresholds and tested fine motion direction discrimination for motion clockwise away from horizontal and vertical motion. Older adults performed as well as younger adults for discriminating motion away from vertical. Surprisingly, performance for discriminating motion away from horizontal was strongly decreased. Further analyses, however, showed a relationship between motion coherence thresholds for horizontal coarse motion direction discrimination and fine motion direction discrimination performance in older adults. In a control experiment, using motion coherence above threshold for all conditions, the difference in performance for horizontal and vertical fine motion direction discrimination for older adults disappeared. These results clearly contradict the notion of an overall age-related decline in motion perception, and, most importantly, highlight the importance of taking into account individual differences when assessing age-related changes in perceptual functions.

Encoding of global visual motion in the nidopallium caudolaterale of behaving crows.

Songbirds possess acute vision. How higher brain centres represent basic and parameterised visual stimuli to process sensory signals according to their behavioural importance has not been studied in a systematic way. We therefore examined how carrion crows (Corvus corone) and their nidopallial visual neurons process global visual motion information in dynamic random-dot displays during a delayed match-to-sample (DMS) task. The behavioural data show that moderately fast motion speeds (16° of visual angle/s) result in superior direction discrimination performance. To characterise how neurons encode and maintain task-relevant visual motion information, we recorded the single-unit activity in the telencephalic association area 'nidopallium caudolaterale' (NCL) of behaving crows. The NCL is considered to be the avian analogue of the mammalian prefrontal cortex. Almost a third (28%) of randomly selected NCL neurons responded selectively to the motion direction of the sample stimulus, mostly to downward motions. Only few NCL neurons (7.5%) responded consistently to specific motion directions during the delay period. In error trials, when the crows chose the wrong motion direction, the encoding of motion direction was significantly reduced. This indicates that sensory representations of NCL neurons are relevant to the birds' behaviour. These data suggest that the corvid NCL, even though operating at the apex of the telencephalic processing hierarchy, constitutes a telencephalic site for global motion integration.

Influence of Correspondence Noise and Spatial Scaling on the Upper Limit for Spatial Displacement in Fully-Coherent Random-Dot Kinematogram Stimuli

Correspondence noise is a major factor limiting direction discrimination performance in random-dot kinematograms [1]. In the current study we investigated the influence of correspondence noise on Dmax, which is the upper limit for the spatial displacement of the dots for which coherent motion is still perceived. Human direction discrimination performance was measured, using 2-frame kinematograms having leftward/rightward motion, over a 200-fold range of dot-densities and a four-fold range of dot displacements. From this data Dmax was estimated for the different dot densities tested. A model was proposed to evaluate the correspondence noise in the stimulus. This model summed the outputs of a set of elementary Reichardt-type local detectors that had receptive fields tiling the stimulus and were tuned to the two directions of motion in the stimulus. A key assumption of the model was that the local detectors would have the radius of their catchment areas scaled with the displacement that they were tuned to detect; the scaling factor k linking the radius to the displacement was the only free parameter in the model and a single value of k was used to fit all of the psychophysical data collected. This minimal, correspondence-noise based model was able to account for 91% of the variability in the human performance across all of the conditions tested. The results highlight the importance of correspondence noise in constraining the largest displacement that can be detected.

Adaptation to direction statistics modulates perceptual discrimination

Perception depends on the relative activity of populations of sensory neurons with a range of tunings and response gains. Each neuron's tuning and gain are malleable and can be modified by sustained exposure to an adapting stimulus. Here, we used a combination of human psychophysical testing and models of neuronal population decoding to assess how rapid adaptation to moving stimuli might change neuronal tuning and thereby modulate direction perception. Using a novel motion stimulus in which the direction changed every 10 ms, we demonstrated that 1,500 ms of adaptation to a distribution of directions was capable of modifying human psychophysical direction discrimination performance. Consistent with previous reports, we found perceptual repulsion following adaptation to a single direction. Notably, compared with a uniform adaptation condition in which all motion directions were equiprobable, discrimination was impaired after adaptation to a stimulus comprising only directions ± 30-60° from the discrimination boundary and enhanced after adaptation to the complementary range of directions. Thus, stimulus distributions can be selectively chosen to either impair or improve discrimination performance through adaptation. A neuronal population decoding model incorporating adaptation-induced repulsive shifts in direction tuning curves can account for most aspects of our psychophysical data; however, changes in neuronal gain are sufficient to account for all aspects of our psychophysical data.

Spatial pooling of one-dimensional second-order motion signals

We can detect visual movements not only from luminance motion signals (first-order motion) but also from non-luminance motion signals (second-order motion). It has been established for first-order motions that the visual system pools local one-dimensional motion signals across space and orientation to solve the aperture problem and to estimate two-dimensional object motion. In this study, we investigated (i) whether local one-dimensional second-order motion signals are also pooled across space and orientation into a global 2D motion, and if so, (ii) whether the second-order motion signals are pooled independently of, or in cooperation with, first-order motion signals. We measured the direction-discrimination performance and the rating of a global circular translation of four oscillating bars, each defined either by luminance or by a non-luminance attribute, such as flicker and binocular depth. The results showed evidence of motion pooling both when the stimulus consisted only of second-order bars and when it consisted of first-order and second-order bars. We observed global motion pooling across first-order motion and second-order motions even when the first-order motion was not accompanied by trackable position changes. These results suggest the presence of a universal pooling system for first- and second-order one-dimensional motion signals.

Spatiotemporal properties of apparent motion perception and aging

We used a random-dot two-frame apparent motion paradigm to investigate whether age-related declines in motion perception are caused by deficits in integrating spatial information, temporal information, or both. Two random-dot patterns were presented sequentially on a black screen, separated by a blank inter-stimulus interval ranging from 0.01 s to 0.240 s. From the first to the second pattern, all the dots were shifted to the left or right by an equal displacement ranging from 0.03 deg to 1.64 deg. The spatiotemporal range yielding good direction discrimination performance was greatly reduced with age. For ISIs longer than 0.04 s, older subjects performed less accurately than younger subjects across a wide range of spatial displacements. Older subjects also showed poorer performance for large spatial displacements across a wide range of ISIs. Age-related differences in performance were also found with small displacements; however, these were largely accounted for by age-related declines in visual acuity. Overall, the results show that the maximum temporal interval and maximum spatial displacement over which two frames can be integrated are reduced in older age.

Feature invariant spatial pooling of first- and second-order motion signals for solution of aperture problem

The visual system solves the aperture problem by means of integrating local 1-dimensional motion signals into a global 2-dimensional motion. Although it is well known that motion pooling occurs within first-order (luminance-based) motion or within second-order (non-luminance-based) motion of the same type, whether it generally occurs across different motion types in a feature-invariant manner remains a matter of controversy in the past plaid literature. Furthermore, this issue has neither tested with objective performance measures, nor under the condition where local 1-dimensional motion signals are cleanly separated in space with no possibility of local artifacts. Here we challenged this problem by measuring direction-discrimination performance of a four-aperture motion stimulus. The stimulus consisted of four oscillating bars, simulating a situation where the contour of a 12.8 x 12.8 deg diamond was translating along a circular path, and seen through four Gaussian apertures (SD: 1.07 deg), each located at the center of an edge. The attribute that defined each oscillating bar was either luminance, temporal frequency of dynamic random dots, or binocular disparity of dynamic random-dot stereogram. The results indicate that observers could judge the direction of global circular translation (clockwise or anti-clockwise) not only when all the edges were defined by the same attribute, but also when adjacent bars were defined by different attributes, although the attribute had some influence on the performance of direction discrimination and the quality of perceived global motion. In addition, motion pooling between first-order and second-order motions was possible even when first-order motion that did not contain obvious positional shift of features that might be detected by second-order mechanism. These results indicate that second-order motion signals do contribute the solution of aperture problem either solely or in corporate with first-order signals.

Spatiotemporal properties of apparent-motion perception in aging

Recent studies have found age-related declines in motion perception from random dot kinematograms (e.g. Bennett et al., Vis. Res., 2007; Norman et al., Perception, 2003). However, some temporal integration abilities seem to remain intact (e.g. Andersen & Ni, Vis. Res., 2008). Here, we used random-dot two-frame apparent motion stimuli to investigate whether such age-related changes in motion direction discrimination can be attributed to a decline in the ability to integrate visual information across space and/or time. Previous work using such stimuli has shown that the perception of apparent motion is influenced separately by the duration of the inter stimulus interval (ISI) between the two frames and the magnitude of spatial displacement of the dots (e.g. Baker & Braddick, Perception, 1985). In the current experiment, older (mean age: 69 years) and younger (mean age: 24 years) subjects reported the direction of motion of two sequentially presented random-dot patterns in a two AFC task. The patterns were presented for 100 ms each and were separated by a blank ISI that varied between 10 to 240 ms. The second pattern was identical to the first pattern, but was shifted to the right or to the left by a displacement ranging from 1.9 to 76 arcmin. The effect of age on direction discrimination performance varied with the amount of displacement. At the shortest and longest displacements, older subjects performed significantly worse than young subjects across all ISIs. At medium displacements, both age groups performed near ceiling at short ISIs, but performance decreased with increasing ISIs. This decline was significantly greater, and began at a shorter ISI, in older subjects. These findings indicate that older subjects integrate information across a smaller range of inter-stimulus intervals and spatial displacements compared to younger subjects.

Direction discrimination performance measured using a Fourier domain signal-to-noise paradigm

Local motion sensors are selectively sensitive to the spatiotemporal Fourier energy generated by retinal image movement. We have developed a Fourier-domain signal-to-noise paradigm to investigate how the outputs of motion sensors are combined during integration. In the space domain, stimuli were two-frame 1-D random-line kinematograms. In the Fourier domain, frequency components were divided into noise components and signal components. Noise components shifted in phase by a random amount in either direction. Signal components either (i) shifted in phase in the same direction by a constant phase angle; or (ii) shifted in phase in the same direction by a constant velocity. Signal direction varied randomly from trial to trial, and the observer was required to report the direction of motion seen in each trial. We measured the minimum ratio between signal and noise components required for reliable direction discrimination. Data showed a performance advantage for the constant-phase stimulus over the constant-velocity stimulus. This result, and data from subsequent experiments, is discussed in the context of current motion integration models.

The role of early visual cortex (V1/V2) in conscious and unconscious visual perception

A “late” period of activity in striate cortex (V1) in response to extrastriate feedback has been proposed to act as a marker of visual awareness. It is not clear, however, whether such recurrent activity is associated exclusively with aware perception or whether it is necessary also for unaware visual processing. We investigated the role of the “late” V1 activity in both aware and unaware visual motion perception. Participants were asked to make a forced-choice direction discrimination judgment on a coherently moving random-dot display and additionally rate their subjective awareness of the stimulus. Transcranial magnetic stimulation (TMS) was applied over the early visual cortex (V1/V2) either 20, 40, 60, 80, or 100ms after motion offset. Visual awareness was impaired at an “early” (20ms) and a “late” (60ms) stimulation time window. Participants' forced-choice direction discrimination performance on “unaware” trials was above chance in No TMS baseline condition. Importantly, this performance was impaired by TMS over V1/V2 at the “late” time window. In a second experiment we show that the critical time window of V5/MT falls between the “early” and “late” time windows of V1/V2 activity. The results indicate that recurrent extrastriate-V1 activity is necessary for both aware and unaware perception.

Diminished Parietal Cortex Activity Associated with Poor Motion Direction Discrimination Performance in Schizophrenia

The results of multiple investigations indicate visual motion-processing abnormalities in schizophrenia. There is little information, however, about the time course and neural correlates of motion-processing abnormalities among these subjects. For the present study, 13 schizophrenia and 13 healthy subjects performed a simple motion direction discrimination task with peripherally presented moving grating stimuli (5 or 10 deg/s). Dense-array electroencephalography data were collected simultaneously. The goal was to discern whether neural deviations associated with motion-processing abnormalities among schizophrenia patients occur early or late in the visual-processing stream. Schizophrenia patients were worse at judging the direction of motion gratings, had enhanced early neural activity (about 90 ms after stimulus onset), and deficient target detection-related late neural activity over parietal cortex (about 400 ms after stimulus onset). In addition, there was a strong association (accounting for 36% of performance variance) between poor behavioral performance and lower target detection-related brain activity among schizophrenia patients. These findings suggest that abnormalities in later stages of motion-processing mechanisms, perhaps beyond extrastriate cortex, may account for behavioral deviations among schizophrenia subjects.

Estimates of the Contribution of Single Neurons to Perception Depend on Timescale and Noise Correlation

The sensitivity of a population of neurons, and therefore the amount of sensory information available to an animal, is limited by the sensitivity of single neurons in the population and by noise correlation between neurons. For decades, therefore, neurophysiologists have devised increasingly clever and rigorous ways to measure these critical variables (Parker and Newsome, 1998). Previous studies examining the relationship between the responses of single middle temporal (MT) neurons and direction-discrimination performance uncovered an apparent paradox. Sensitivity measurements from single neurons suggested that small numbers of neurons may account for a monkey's psychophysical performance (Britten et al., 1992), but trial-to-trial variability in activity of single MT neurons are only weakly correlated with the monkey's behavior, suggesting that the monkey's decision must be based on the responses of many neurons (Britten et al., 1996). We suggest that the resolution to this paradox lies (1) in the long stimulus duration used in the original studies, which led to an overestimate of neural sensitivity relative to psychophysical sensitivity, and (2) mistaken assumptions (because no data were available) about the level of noise correlation in MT columns with opposite preferred directions. We therefore made new physiological and psychophysical measurements in a reaction time version of the direction-discrimination task that matches neural measurements to the actual decision time of the animals. These new data, considered together with our recent data on noise correlation in MT (Cohen and Newsome, 2008), provide a substantially improved account of psychometric performance in the direction-discrimination task.

Effect of posture change on tactile perception: impaired direction discrimination performance with interleaved fingers

We report a series of experiments in which participants had to judge the direction in which a pair of vibrotactile stimuli presented to two adjacent digits of either the same or different hands were stimulated (left-to-right or vice versa in experiments 1 and 2; near-to-far or vice versa in experiment 3, at stimulus onset asynchronies varying between 100 and 600 ms). When the participant's hands were placed side-by-side (anatomical posture), with their fingers either pointing away from them or else pointing toward the midline, directional discrimination performance was generally accurate. By contrast, when the fingers of the two hands were interleaved in either of these postures, performance deteriorated significantly for certain specific combinations of digits, with a more pronounced impairment seen when the fingers pointed away from the participant than when they pointed toward the midline. This decline in tactile direction discrimination performance in the interleaved fingers posture appears to reflect a failure to represent the position of tactile stimuli correctly when the fingers of the two hands are interleaved.