Strength Of Masking Research Articles

Interaction Between Electric and Acoustic Stimulation Influences Speech Perception in Ipsilateral EAS Users.

Objectives:The aim of this study was to determine electric-acoustic masking in cochlear implant users with ipsilateral residual hearing and different electrode insertion depths and to investigate the influence on speech reception. The effects of different fitting strategies—meet, overlap, and a newly developed masking adjusted fitting (UNMASKfit)—on speech reception are compared. If electric-acoustic masking has a detrimental effect on speech reception, the individualized UNMASKfit map might be able to reduce masking and thereby enhance speech reception.Design:Fifteen experienced MED-EL Flex electrode recipients with ipsilateral residual hearing participated in a crosssover design study using three fitting strategies for 4 weeks each. The following strategies were compared: (1) a meet fitting, dividing the frequency range between electric and acoustic stimulation, (2) an overlap fitting, delivering part of the frequency range both acoustically and electrically, and (3) the UNMASKfit, reducing the electric stimulation according to the individual electric-on-acoustic masking strength. A psychoacoustic masking procedure was used to measure the changes in acoustic thresholds due to the presence of electric maskers. Speech reception was measured in noise with the Oldenburg Matrix Sentence test.Results:Behavioral thresholds of acoustic probe tones were significantly elevated in the presence of electric maskers. A maximum of masking was observed when the difference in location between the electric and acoustic stimulation was around one octave in place frequency. Speech reception scores and strength of masking showed a dependency on residual hearing, and speech reception was significantly reduced in the overlap fitting strategy. Electric- acoustic stimulation significantly improved speech reception over electric stimulation alone, with a tendency toward a larger benefit with the UNMASKfit map. In addition, masking was significantly inversely correlated to the speech reception performance difference between the overlap and the meet fitting.Conclusions:(1) This study confirmed the interaction between ipsilateral electric and acoustic stimulation in a psychoacoustic masking experiment. (2) The overlap fitting yielded poorer speech reception performance in stationary noise especially in subjects with strong masking. (3) The newly developed UNMASKfit strategy yielded similar speech reception thresholds with an enhanced acoustic benefit, while at the same time reducing the electric stimulation. This could be beneficial in the long-term if applied as a standard fitting, as hair cells are exposed to less possibly adverse electric stimulation. In this study, the UNMASKfit allowed the participants a better use of their natural hearing even after 1 month of adaptation. It might be feasible to transfer these results to the clinic, by fitting patients with the UNMASKfit upon their first fitting appointment, so that longer adaptation times can further improve speech reception.

Redundancy masking: The loss of repeated items in crowded peripheral vision.

Crowding is the deterioration of target identification in the presence of neighboring objects. Recent studies using appearance-based methods showed that the perceived number of target elements is often diminished in crowding. Here we introduce a related type of diminishment in repeating patterns (sets of parallel lines), which we term “redundancy masking.” In four experiments, observers were presented with arrays of small numbers of lines centered at 10° eccentricity. The task was to indicate the number of lines. In Experiment 1, spatial characteristics of redundancy masking were examined by varying the inter-line spacing. We found that redundancy masking decreased with increasing inter-line spacing and ceased at spacings of approximately 0.25 times the eccentricity. In Experiment 2, we assessed whether the strength of redundancy masking differed between radial and tangential arrangements of elements as it does in crowding. Redundancy masking was strong with radially arranged lines (horizontally arranged vertical lines), and absent with tangentially arranged lines (vertically arranged horizontal lines). In Experiment 3, we investigated whether target size (line width and length) modulated redundancy masking. There was an effect of width: Thinner lines yielded stronger redundancy masking. We did not find any differences between the tested line lengths. In Experiment 4, we varied the regularity of the line arrays by vertically or horizontally jittering the positions of the lines. Redundancy masking was strongest with regular spacings and weakened with decreasing regularity. Our experiments show under which conditions whole items are lost in crowded displays, and how this redundancy masking resembles—and partly diverges from—crowded identification. We suggest that redundancy masking is a contributor to the deterioration of performance in crowded displays with redundant patterns.

Interocular suppressive interactions in amblyopia depend on spatial frequency

In amblyopia, there is an interocular suppressive imbalance that results in the fixing eye dominating perception. In this study, we aimed to determine whether these suppressive interactions were narrowband and tuned for spatial frequency or broadband and independent of spatial frequency.We measured the contrast sensitivity and masking functions of fifteen amblyopic subjects and seventeen control subjects using the quick Contrast Sensitivity Function (qCSF) approach (Lesmes, Lu, Baek, & Albright, 2010). We first measured the monocular sensitivity functions of each participant and thereafter corrected for it. We then measured masking sensitivity functions for low, mid and high spatial frequency masks, normalized to their visibility.In the control group, we observed that the strength of dichoptic masking is equivalent between the two eyes. It is also tuned such that masking by low spatial frequencies in one eye mainly affects low spatial frequencies in the other eye and masking by high spatial frequencies mainly affects high spatial frequencies. In amblyopes, although the interocular masking is also tuned for spatial frequency, it is not equivalent between the two eyes: the masking effect from the amblyopic to fixing eye is weaker than the other way around.The asymmetry observed in the strength of masking between the two eyes in amblyopia is tuned for spatial frequency. It is not the consequence of the contrast sensitivity deficit of the amblyopic eye nor is it the consequence of abnormally strong masking from the fixing eye. Rather it is due to an abnormally weak masking strength by the amblyopic eye per se.

Phase-selective masking with radial frequency contours

Sensitivity to changes in the shape of a closed-contour figure is affected by surrounding figures (Vision Research 44 (2004) 2815–2823). We examined how between-contour masking depends on radial frequency. Experiment 1 replicated previous studies that found that masking between adjacent radial frequency (RF) patterns was greatest when the two shapes were phase aligned, and that the magnitude of masking declined approximately linearly with increasing phase offsets. In addition, we found that the effect of phase offset on masking was very similar for RFs ranging from 3 to 8, a result that suggests that sensitivity to phase decreases with increasing radial frequency. Experiment 2 tested this idea and found that phase discrimination threshold for single cycles of curvature was approximately proportional to radial frequency. Experiment 3 showed that both curvature maxima and minima contribute to phase dependent masking between RF contours. Together, Experiments 1–3 demonstrate that the strength of phase-dependent masking does not depend on RF, but is related to sensitivity for phase shifts in isolated contours, and is affected by both positive and negative curvature extrema. We discuss these results in relation to properties of curvature sensitive neurons.

An unattended mask makes an attended target disappear

In pattern masking, the target and mask are presented at the same location and follow one another very closely in time. When the observer attends to the target, he or she must also attend to the mask, as the switching time for attention is quite slow. In a series of experiments, we present mask-target-mask sequences staggered in time and location (Cavanagh, Holcombe, & Chou, 2008) that allow participants to attentively track the target location without attending to the masks. The results show that the strength of masking is on average unaffected by the removal of attention from the masks. Moreover, after isolating the target location perceptually with moving attention, it is clear that the target, when at threshold, has not been degraded or integrated with a persisting mask but it has vanished. We also show that the strength of masking is unaffected by the lateral spacing between adjacent target and mask sequences until the spacing is so large that the apparent motion driving the attentive tracking breaks down. Finally, we compare the effect of the pre- and postmask and find that the premask is responsible for the larger part of the masking.

The prevalence effect in lateral masking and its relevance for visual search.

In stimulus displays with or without a single target amid 1,644 identical distractors, target prevalence was varied between 20, 50 and 80 %. Maximum gaze deviation was measured to determine the strength of lateral masking in these arrays. The results show that lateral masking was strongest in the 20 % prevalence condition, which differed significantly from both the 50 and 80 % prevalence conditions. No difference was observed between the latter two. This pattern of results corresponds to that found in the literature on the prevalence effect in visual search (stronger lateral masking corresponding to longer search times). The data add to similar findings reported earlier (Wertheim et al. in Exp Brain Res, 170:387-402, 2006), according to which the effects of many well-known factors in visual search correspond to those on lateral masking. These were the effects of set size, disjunctions versus conjunctions, display area, distractor density, the asymmetry effect (Q vs. O's) and viewing distance. The present data, taken together with those earlier findings, may lend credit to a causal hypothesis that lateral masking could be a more important mechanism in visual search than usually assumed.

Comparing Response Times and Error Rates in a Simultaneous Masking Paradigm

In simultaneous masking, performance on a foveally presented target is impaired by one or more flanking elements. Previous studies have demonstrated strong effects of the grouping of the target and the flankers on the strength of masking (e.g., Malania, Herzog & Westheimer, 2007). These studies have predominantly examined performance by measuring offset discrimination thresholds as a measure of performance, and it is therefore unclear whether other measures of performance provide similar outcomes. A recent study, which examined the role of grouping on error rates and response times in a speeded vernier offset discrimination task, similar to that used by Malania et al. (2007), suggested a possible dissociation between the two measures, with error rates mimicking threshold performance, but response times showing differential results (Panis & Hermens, 2014). We here report the outcomes of three experiments examining this possible dissociation, and demonstrate an overall similar pattern of results for error rates and response times across a broad range of mask layouts. Moreover, the pattern of results in our experiments strongly correlates with threshold performance reported earlier (Malania et al., 2007). Our results suggest that outcomes in a simultaneous masking paradigm do not critically depend on the outcome measure used, and therefore provide evidence for a common underlying mechanism.

Pattern masking: The importance of remote spatial frequencies and their phase alignment

To assess the effects of spatial frequency and phase alignment of mask components in pattern masking, target threshold vs. mask contrast (TvC) functions for a sine-wave grating (S) target were measured for five types of mask: a sine-wave grating (S), a square-wave grating (Q), a missing fundamental square-wave grating (M), harmonic complexes consisting of phase-scrambled harmonics of a square wave (Qp), and harmonic complexes consisting of phase-scrambled harmonics of a missing fundamental square wave (Mp). Target and masks had the same fundamental frequency (0.46 cpd) and the target was added in phase with the fundamental frequency component of the mask. Under monocular viewing conditions, the strength of masking depends on phase relationships among mask spatial frequencies far removed from that of the target, at least 3 times the target frequency, only when there are common target and mask spatial frequencies. Under dichoptic viewing conditions, S and Q masks produced similar masking to each other and the phase-scrambled masks (Qp and Mp) produced less masking. The results suggest that pattern masking is spatial frequency broadband in nature and sensitive to the phase alignments of spatial components.

Stimulus factors influencing spatial release from speech-on-speech masking

This study examined spatial release from masking (SRM) when a target talker was masked by competing talkers or by other types of sounds. The focus was on the role of interaural time differences (ITDs) and time-varying interaural level differences (ILDs) under conditions varying in the strength of informational masking (IM). In the first experiment, a target talker was masked by two other talkers that were either colocated with the target or were symmetrically spatially separated from the target with the stimuli presented through loudspeakers. The sounds were filtered into different frequency regions to restrict the available interaural cues. The largest SRM occurred for the broadband condition followed by a low-pass condition. However, even the highest frequency bandpass-filtered condition (3-6 kHz) yielded a significant SRM. In the second experiment the stimuli were presented via earphones. The listeners identified the speech of a target talker masked by one or two other talkers or noises when the maskers were colocated with the target or were perceptually separated by ITDs. The results revealed a complex pattern of masking in which the factors affecting performance in colocated and spatially separated conditions are to a large degree independent.

Size tuning and contextual modulation of backward contrast masking

The strength of contrast masking depends not only on spatial but also on temporal parameters. In a previous study (T. P. Saarela & M. H. Herzog, 2008), we showed that the detection of a briefly presented Gabor patch is most strongly impaired when an iso-oriented grating mask immediately follows the Gabor and that this masking effect is relieved when a surround is added to the mask. Here, we studied the spatial characteristics of this backward masking effect. Gradually changing the size of the iso-oriented masking grating changes contrast detection thresholds in a non-monotonic way that can be explained in terms of contrast-dependent spatial summation and inhibition. However, these spatial interactions seem only to take place when the mask is a uniform grating. When the mask is divided into a small center and a larger surround by changing the surround parameters or by adding a small gap, masking is as strong as with the small center mask only. We suggest that spatial interactions are weaker or even absent when the stimulus elements are perceptually segregated.

Regional Brain Activity Associated with Visual Backward Masking

In visual backward masking, the visibility of a briefly presented visual target is disrupted by a mask that is presented shortly thereafter. The goal of the current study was to identify regions in the human cortex that may provide the neural basis of visual masking. We searched for areas whose activity correlated with perception as we systematically varied the strength of masking. A total of 13 subjects performed a backward masking task during functional magnetic resonance imaging. Target and mask were presented at three delay intervals (34, 68, and 102 msec) and behavioral measures confirmed that the targets were more visible at longer masking intervals. Two sets of regions of interest were identified: Distinct regions in the visual cortex (V1/V2, LO, hMT+) were segregated using scans to localize visual processing drawn from the existing literature. Additional cortical regions were selected in a data-driven approach based on their activity during the backward masking task. For each set, we determined the regions whose magnitude of activation increased at longer masking intervals. Nine of the subjects provided valid behavioral performance data on the visual masking task and imaging data from these subjects were used for subsequent analysis. The scans of visual processing areas identified four regions, including: early visual areas (V1 and V2), the motion-sensitive regions in the lateral occipital (LO) lobe (hMT+), and two components (dorsal and ventral) of the object-sensitive region, LO. Of these, the ventral and dorsal LO regions were sensitive to the strength of the mask. For the data-driven approach, six regions were identified on the basis of a difference map in which all masking intervals were contrasted with rest. These included the inferior parietal, anterior cingulate, precentral, insula, thalamic, and occipital areas. The predicted effects of more activity with weaker masking were seen in the thalamus, inferior parietal, and anterior cingulate. This study isolated three types of visual processing areas. The first included regions that subserve key stages of vision (including object and motion processing). The second type responded to the presentation of brief ly presented visual stimuli, regardless of masking interval. The third type (selected from the first two) included regions sensitive to the interval between the target and mask. These latter regions (including ventral LO, inferior parietal, anterior cingulate, and thalamus) may form the neural substrate of backward masking.

Effects of forward masking on the responses of the inferior collicular neurons in the big brown bats, Eptesicus fuscus

Temporal features of sound convey information vital for behaviors as diverse as speech recognition by human and echolocation by bats. However, auditory stimuli presented in temporal proximity might interfere with each other. Although much progress has been made in the description of this phenomenon from psychophysical studies, the neural mechanism responsible for its formation at central auditory structures especially at the inferior colliculus (IC), a midbrain auditory nucleus which practically receives massive bilateral projections from all the major auditory structures in the brainstem, remains unclear. This study was designed to investigate it in vivo by using electrophysiological recording from the inferior collicular neurons of the big brown bat, Eptesicus fuscus. In our results, the responses of 12 (38%, n= 31) neurons to the test sound (leading sound) were obviously inhibited by the masker (lagging sound). The inhibitory effects in these neurons were correlated with the inter-stimulus level difference (SLD) and the inter-stimulus onset asynchrony (SOA) interval. The strength of backward masking increased with the masker intensity increasing, the test sound intensity decreasing and the SOA interval shortening. There were no obvious effects of backward masking on the responses of many other neurons (52%, 16/31), and yet in a part of these neurons, the neural inhibition of responses to the test sound was observed at the special SLD and the special SOA intervals. Moreover, few of the 31 sampled IC neurons (10%, 3/31) displayed facilitating responses to the test sound at the special SLD and the special SOA intervals. These data demonstrate that a lot of IC neurons are involved in the generation of the backward masking of acoustical perception. It is conjectured that the temporal dynamic integration between the leading inhibitory inputs evoked by the masker sound and the excitatory inputs evoked by the test sound might play a key role in shaping the acoustical response characteristics of the IC neurons.

Attend to it now or lose it forever: selective attention, metacontrast masking, and object substitution.

Metacontrast masking occurs when the visibility of a brief target stimulus is decreased by the subsequent appearance of another nearby visual stimulus. Early explanations of the phenomenon involved low-level mechanisms, but subsequent studies have suggested a role for selective attention. The results of three experiments presented here extend previous findings to the metacontrast paradigm. It is shown that the strength of metacontrast masking increases with the number of distractor items in a display, decreases when the target location is validly but not invalidly precued, and is eliminated when search for the target is efficient (pop-out search) but not when search is inefficient (serial search). A connection between metacontrast masking and object substitution masking is considered.

Masking by fast gratings.

Perception of an oriented pattern is impaired in the presence of a superimposed orthogonal mask. This masking effect most likely arises in visual cortex, where neuronal responses are suppressed by masks having a broad range of orientations. Response suppression is commonly ascribed to lateral inhibition between cortical neurons. Recent physiological results, however, have cast doubt on this view: powerful suppression has been observed with masks drifting too rapidly to elicit much of a response in cortex. We show here that the same is true for perceptual masking. From contrast discrimination thresholds, we estimated the cortical response to drifting patterns of various frequencies, and found it greatly reduced above 15-20 Hz. In the same subjects, we measured the strength of masking by the same patterns and found it equally strong for masks drifting slowly (2.7 Hz) as for masks drifting rapidly (27-38 Hz). Fast gratings thus cause strong masking while eliciting weak cortical responses. Our results might be explained by inhibition from cortical neurons that respond to unusually high frequencies, and yet do not make their signals fully available for perceptual judgments. A more parsimonious explanation, however, is that masking does not involve lateral inhibition from cortex. Masking might operate in retina or thalamus, which respond to much higher frequencies than cortex. Masking might also be due to thalamic signals to cortex, perhaps through depression at thalamocortical synapses.

Lateral masking as a determinant of global dominance.

The term compound letter refers to a large (global) letter made up of small (local) letters. Reaction time to identify local letters is longer when local and global letters are different than when they are the same (the global dominance effect). The possible contribution of lateral masking to this effect was investigated. Lateral masking denotes reduced probability of identifying a stimulus when it is closely surrounded by other stimuli (as is the case for the local items in a compound stimulus). Three experiments were conducted in which the dependent measure was percentage of correct responses, rather than reaction time. In experiment 1 compound letters were used; accuracy of performance yielded evidence of global dominance such as obtained with reaction time measures. In experiments 2 and 3 the strength of lateral masking in geometrical forms was varied by varying the density of their component items. In agreement with earlier suggestions based on indirect evidence, the results directly implicated lateral masking as an important determinant of global dominance. However, lateral masking could not account fully for the experimental outcome. Factors beyond lateral masking, such as global precedence in the processing sequence or inhibitory interactions among low and high spatial-frequency components of the compound images are required in order to provide a comprehensive account of global dominance effects.