Modulation of backward pattern masking by focal visual attention

The effect of focal visual attention on backward pattern masking was investigated using an orientation discrimination task. The results show that attention reduces primarily the effect of interruption masking, the later component of pattern masking, which occurs when the delay between the target and mask onset is about 50-150 ms. The strongest spatial cueing effect, i.e. the strongest reduction of the orientation discrimination threshold due to focal attention, was observed at intermediate (approximately 100 ms) target-to-mask stimulus onset asynchrony (SOA). There was a weak effect of cueing at shorter SOAs, and no or a very weak attentional effect was present at longer target-to-mask SOAs, where the pattern masking effect is absent. The dynamics of attentional modulation of backward pattern masking correlates closely with the dynamics of the attentional modulation of neuronal responses in the early visual cortex.     

Difficulties with Pitch Discrimination Influences Pitch Memory Performance: Evidence from Congenital Amusia

Music processing is influenced by pitch perception and memory. Additionally these features interact, with pitch memory performance decreasing as the perceived distance between two pitches decreases. This study examined whether or not the difficulty of pitch discrimination influences pitch retention by testing individuals with congenital amusia. Pitch discrimination difficulty was equated by determining an individual’s threshold with a two down one up staircase procedure and using this to create conditions where two pitches (the standard and the comparison tones) differed by 1x, 2x, and 3x the threshold setting. For comparison with the literature a condition that employed a constant pitch difference of four semitones was also included. The results showed that pitch memory performance improved as the discrimination between the standard and the comparison tones was made easier for both amusic and control groups, and more importantly, that amusics did not show any pitch retention deficits when the discrimination difficulty was equated. In contrast, consistent with previous literature, amusics performed worse than controls when the physical pitch distance was held constant at four semitones. This impaired performance has been interpreted as evidence for pitch memory impairment in the past. However, employing a constant pitch distance always makes the difference closer to the discrimination threshold for the amusic group than for the control group. Therefore, reduced performance in this condition may simply reflect differences in the perceptual difficulty of the discrimination. The findings indicate the importance of equating the discrimination difficulty when investigating memory.     

Temporal consistency is currency in shifts of transient visual attention

Background

Observers respond more accurately to targets in visual search tasks that share properties with previously presented items, and transient attention can learn featural consistencies on a precue, irrespective of its absolute location.

Methodology/Principal Findings

We investigated whether such attentional benefits also apply to temporal consistencies. Would performance on a precued Vernier acuity discrimination task, followed by a mask, improve if the cue-lead times (CLTs; 50, 100, 150 or 200 ms) remained constant between trials compared to when they changed? The results showed that if CLTs remained constant for a few trials in a row, Vernier acuity performance gradually improved while changes in CLT from one trial to the next led to worse than average discrimination performance. The results show that transient attention can quickly adjust to temporal regularities, similarly to spatial and featural regularities. Further experiments show that this form of learning is not under voluntary control.

Conclusions/Significance

The results add to a growing literature showing how consistency in visual presentation improves visual performance, in this case temporal consistency.     

Discrimination performance and echolocation signal integration requirements for target detection and distance determination in the CF/FM bat,Noctilio albiventris

Summary Bats of the speciesNoctilio albiventris were trained to detect the presence of a target or to discriminate differences in target distance by means of echolocation. During the discrimination trials, the bats emitted pairs of pulses at a rate of 7–10/s. The first was an 8 ms constant frequency (CF) signal at about 75 kHz. This was followed after about 28 ms by a short-constant frequency/ frequency modulated (short-CF/FM) signal composed of a 6 ms CF component at about 75 kHz terminating in a 2 ms FM component sweeping downward to about 57 kHz. There was no apparent difference in the pulse structure or emission pattern used for any of the tasks. The orientation sounds of bats flying in the laboratory and hunting prey under natural conditions follow the same general pattern but differ in interesting ways.The bats were able to discriminate a difference in target distance of 13 mm between two simultaneously presented targets and of 30 mm between single sequentially presented targets around an absolute distance of 35 cm, using a criterion of 75% correct responses.The bats were unable to detect the presence of the target or to discriminate distance in the presence of continuous white noise of 54 dB or higher SPL. Under conditions of continuous white noise, the bats increased their pulse repetition rate and the relative proportion of CF/FM pulses.The bats required a minimum of 1–2 successive CF/FM pulse-echo pairs for target detection and 2–3 to discriminate a 5 cm difference in distance. When the distance discrimination tasks were made more difficult by reducing the difference in distance between the two targets the bats needed to integrate information from a greater number of successive CF/FM pulse-echo pairs to make the discrimination.Abbreviations CF constant frequency - FM frequency modulation     

Sequential Grouping Modulates the Effect of Non-Simultaneous Masking on Auditory Intensity Resolution

The presence of non-simultaneous maskers can result in strong impairment in auditory intensity resolution relative to a condition without maskers, and causes a complex pattern of effects that is difficult to explain on the basis of peripheral processing. We suggest that the failure of selective attention to the target tones is a useful framework for understanding these effects. Two experiments tested the hypothesis that the sequential grouping of the targets and the maskers into separate auditory objects facilitates selective attention and therefore reduces the masker-induced impairment in intensity resolution. In Experiment 1, a condition favoring the processing of the maskers and the targets as two separate auditory objects due to grouping by temporal proximity was contrasted with the usual forward masking setting where the masker and the target presented within each observation interval of the two-interval task can be expected to be grouped together. As expected, the former condition resulted in a significantly smaller masker-induced elevation of the intensity difference limens (DLs). In Experiment 2, embedding the targets in an isochronous sequence of maskers led to a significantly smaller DL-elevation than control conditions not favoring the perception of the maskers as a separate auditory stream. The observed effects of grouping are compatible with the assumption that a precise representation of target intensity is available at the decision stage, but that this information is used only in a suboptimal fashion due to limitations of selective attention. The data can be explained within a framework of object-based attention. The results impose constraints on physiological models of intensity discrimination. We discuss candidate structures for physiological correlates of the psychophysical data.     

On the blink: the importance of target-distractor similarity in eliciting an attentional blink with faces

Temporal allocation of attention is often investigated with a paradigm in which two relevant target items are presented in a rapid sequence of irrelevant distractors. The term Attentional Blink (AB) denotes a transient impairment of awareness for the second of these two target items when presented close in time. Experimental studies reported that the AB is reduced when the second target is emotionally significant, suggesting a modulation of attention allocation. The aim of the present study was to systematically investigate the influence of target-distractor similarity on AB magnitude for faces with emotional expressions under conditions of limited attention in a series of six rapid serial visual presentation experiments. The task on the first target was either to discriminate the gender of a neutral face (Experiments 1, 3-6) or an indoor/outdoor visual scene (Experiment 2). The task on the second target required either the detection of emotional expressions (Experiments 1-5) or the detection of a face (Experiment 6). The AB was minimal or absent when targets could be easily discriminated from each other. Three successive experiments revealed that insufficient masking and target-distractor similarity could account for the observed immunity of faces against the AB in the first two experiments. An AB was present but not increased when the facial expression was irrelevant to the task suggesting that target-distractor similarity plays a more important role in eliciting an AB than the attentional set demanded by the specific task. In line with previous work, emotional faces were less affected by the AB.     

Spatial interval discrimination in the presence of flanking lines

Spatial interval discrimination was studied in the absence or presence of distractors. In the latter case, two flanking lines surrounded two vertical lines delimiting the spatial interval. Using a temporal 2AFC technique with a method of constant stimuli we measured the accuracy of performance (discrimination thresholds) and biases (points of subjective equality) depending on the separations between the target and the flanking lines. For separations less than or comparable to the size of the spatial interval we found both a reduction of precision and the increase of perceived sizes of the spatial intervals: the discrimination thresholds were increased, the size of the spatial interval was overestimated. For larger separations, the size of the spatial interval was underestimated, but the precision of performance was not affected by the presence of flanking lines. We discuss the possible mechanisms underlying spatial interval discrimination in the presence of flanking lines.     

Ranging in Human Sonar: Effects of Additional Early Reflections and Exploratory Head Movements

Many blind people rely on echoes from self-produced sounds to assess their environment. It has been shown that human subjects can use echolocation for directional localization and orientation in a room, but echo-acoustic distance perception - e.g. to determine one''s position in a room - has received little scientific attention, and systematic studies on the influence of additional early reflections and exploratory head movements are lacking. This study investigates echo-acoustic distance discrimination in virtual echo-acoustic space, using the impulse responses of a real corridor. Six blindfolded sighted subjects and a blind echolocation expert had to discriminate between two positions in the virtual corridor, which differed by their distance to the front wall, but not to the lateral walls. To solve this task, participants evaluated echoes that were generated in real time from self-produced vocalizations. Across experimental conditions, we systematically varied the restrictions for head rotations, the subjects'' orientation in virtual space and the reference position. Three key results were observed. First, all participants successfully solved the task with discrimination thresholds below 1 m for all reference distances (0.75–4 m). Performance was best for the smallest reference distance of 0.75 m, with thresholds around 20 cm. Second, distance discrimination performance was relatively robust against additional early reflections, compared to other echolocation tasks like directional localization. Third, free head rotations during echolocation can improve distance discrimination performance in complex environmental settings. However, head movements do not necessarily provide a benefit over static echolocation from an optimal single orientation. These results show that accurate distance discrimination through echolocation is possible over a wide range of reference distances and environmental conditions. This is an important functional benefit of human echolocation, which may also play a major role in the calibration of auditory space representations.     

Masking efficiency as a function of stimulus onset asynchrony for spatial-frequency detection and identification

Detection and identification thresholds for grating targets were measured in the presence of a compound mask grating as a function of the stimulus onset asynchrony (SOA). The detection and identification SOA functions are both reversed U-shaped but they are not parallel. The detection-to-identification ratio is itself a reversed U-shaped function of SOA, even for stimuli two octaves apart, with a peak between +20 and +60 ms SOA (backward masking). It is argued that these results support the hypothesis according to which detection and identification are serial processes.     

Spatial coincidence of cues in visual learning by the honeybee (Apis mellifera)

The discrimination of patterns was studied in a Y-choice chamber fitted with a transparent baffle in each arm, through which the bees had a choice of two targets via openings 5cm wide. The bees see the positive (rewarded) and the negative (unrewarded) targets from a fixed distance. The patterns were bars (subtending 22 degrees x5.4 degrees at the point of choice) presented in one-quarter of each target. The bars were moved to a different quarter of the target every 5min, to make the location of black useless as a cue. A coincident presentation is when the bar on the left target is on the same side of the target as the bar on the right target. The bees learn the orientation cue when the presentation is coincident but otherwise cannot learn it. This experiment shows that bees do not centre their attention on the individual bars, otherwise they would always discriminate the orientation. Centring the target as a whole precedes learning. Having learned with the bar on one side of the targets, bees do not recognize the same cue presented on the other side. A separate orientation cue can be learned on each side. A radial/tangential cue is preferred to a conflicting orientation cue.     

Solute accessibility to N epsilon-fluorescein isothiocyanate-lysine-23 cobra alpha-toxin bound to the acetylcholine receptor. A consideration of the effect of rotational diffusion and orientation constraints on fluorescence quenching.

To obtain information on the disposition of alpha-toxin when bound to the acetylcholine receptor (AChR), we evaluated the accessibility of solutes to fluorescein isothiocyanate (FITC) conjugated to alpha-toxin (siamensis 3) at lysine 23 (FITC-toxin) by measuring the rate constants for iodide quenching of the fluorescence of fluorescein free in solution and FITC-toxin free in solution and bound to AChR. Relative to the free fluorescein, we observed a 55% reduction in the quenching rate constant for the unbound FITC-toxin and 80% reduction for the AChR-bound FITC-toxin. It is tempting to interpret a decrease in the quenching rate constant as due to an increase in the masking of the labeling fluorophore, which in our case would then be indicative of masking of fluorescein conjugated to the free toxin and masking of FITC-toxin, in the region of lysine 23, when bound to AChR. However, elementary considerations indicate that the quenching rate depends not only on geometrical masking factors but also on the translational and rotational mobilities of the labeled molecules as well as orientational constraints. To evaluate these effects we have established quantitative relations between the rate of fluorescence quenching, the degree of masking of fluorophore, translational and rotational rates, and orientational constraints of the labeled macromolecules, using recent formulations for the rate of reaction between asymmetric molecules (Shoup et al., 1981, Biophys. J., 36:619-714). These relations predict that the decrease in quenching constant observed for the labeled FITC-toxin as well as the AChR-bound FITC-toxin is largely due to differences in translational and rotational rates and orientational constraints and not to significant increases in geometrical masking. Our theoretical formulation shows that the quenching rate can be decreased by a factor of 2-5 merely by immobilizing a fluorophore on the surface of a large protein without any significant increase in geometrical masking.     

The visual perception of relative distances in the wood-cricket, Nemobius sylvestris

ABSTRACT. In post-embryonic development, the visual system of the cricket Nemobius sylvestris (Bosc) shows a regular increase in the length and number of the ommatidia and a decrease of inter-ommatidial angle so that the adult's is a third of the value in the first larval instar. Further, a 20° widening of the binocular visual field, in the horizontal plane at least, and a three-fold increase of the inter-ocular distance improve the potential for binocular vision. Behavioural experiments showed that the insect orientates with differing precision depending on the distances to targets of constant angular size. Further, in a choice situation between two such vertical targets, the cricket orientates most strongly towards the closer of the two, even at target distances of 52 and 130 cm from its point of decision. In fixed tethered animals, discrimination between a close and a distant target is still possible, but disappears when the head is waxed to the thorax, so that any relative movement between the animal and the object is prevented. As these capabilities exceed the possibilities of binocular triangulation, the possible role of other mechanisms is discussed, particularly that involving movement parallax using both eyes.     

Modulation of activity in human visual area V1 during memory masking

Neurons in the primary visual cortex, V1, are specialized for the processing of elemental features of the visual stimulus, such as orientation and spatial frequency. Recent fMRI evidence suggest that V1 neurons are also recruited in visual perceptual memory; a number of studies using multi-voxel pattern analysis have successfully decoded stimulus-specific information from V1 activity patterns during the delay phase in memory tasks. However, consistent fMRI signal modulations reflecting the memory process have not yet been demonstrated. Here, we report evidence, from three subjects, that the low V1 BOLD activity during retention of low-level visual features is caused by competing interactions between neural populations coding for different values along the spectrum of the dimension remembered. We applied a memory masking paradigm in which the memory representation of a masker stimulus interferes with a delayed spatial frequency discrimination task when its frequency differs from the discriminanda with ±1 octave and found that impaired behavioral performance due to masking is reflected in weaker V1 BOLD signals. This cross-channel inhibition in V1 only occurs with retinotopic overlap between the masker and the sample stimulus of the discrimination task. The results suggest that memory for spatial frequency is a local process in the retinotopically organized visual cortex.     

A computational theory of visual attention.

A computational theory of visual attention is presented. The basic theory (TVA) combines the biased-choice model for single-stimulus recognition with the fixed-capacity independent race model (FIRM) for selection from multi-element displays. TVA organizes a large body of experimental findings on performance in visual recognition and attention tasks. A recent development (CTVA) combines TVA with a theory of perceptual grouping by proximity. CTVA explains effects of perceptual grouping and spatial distance between items in multi-element displays. A new account of spatial focusing is proposed in this paper. The account provides a framework for understanding visual search as an interplay between serial and parallel processes.     

Attentional Tuning Resets after Failures of Perceptual Awareness

Key to successfully negotiating our environment is our ability to adapt to current settings based on recent experiences and behaviour. Response conflict paradigms (e.g., the Stroop task) have provided evidence for increases in executive control after errors, leading to slowed responses that are more likely to be correct, and less susceptible to response congruency effects. Here we investigate whether failures of perceptual awareness, rather than failures at decisional or response stages of information processing, lead to similar adjustments in visual attention. We employed an attentional blink task in which subjects often fail to consciously register the second of two targets embedded in a rapid serial visual presentation stream of distractors, and examined how target errors influence performance on subsequent trials. Performance was inferior after Target 2 errors and these inter-trial effects were independent of the temporal lag between the targets and were not due to more global changes in attention across runs of trials. These results shed light on the nature of attentional calibration in response to failures of perceptual consciousness.     

Contrast adaptation and contrast masking in human vision.

After a preliminary study of visual evoked potentials (VEPS) to a test grating seen in the presence of masks at different orientations, psychophysical data are presented showing the effects of adaptation and of masking on thresholds for detecting the same test grating. The test is a vertical grating of spatial frequency 2 cycles per degree; adapting and masking gratings differ from the test either in orientation or in spatial frequency. The effects of adaptation and masking are explained by a single mechanism model that assumes: (i) adaptation and masking both alter the contrast response (or transducer) function of the mechanism that detects the test; (ii) masks, but not adaptors, stimulate the mechanism that detects the test; and (iii) a test is detectable when it raises response level by a constant amount. The model incorporates two distinct tuning functions, a broad adaptive contrast function and a narrow effective contrast function. It accounts adequately for all the data, including the location and size of the facilitative dip found in some masking functions, the constant slopes of the threshold elevation segments of adaptation functions and the varying slopes of masking functions. It also predicts the sometimes surprising joint effects of adaptation followed by masking and of two masks operating simultaneously.     

Gain control in the sonar of odontocetes

The sonar of odontocetes processes echo-signals within a wide range of echo levels. The level of echoes varies widely by tens of decibels depending on the level of the emitted sonar pulse, the target strength, the distance to the target, and the sound absorption by the water media. The auditory system of odontocetes must be capable of effective perception, analysis, and discrimination of echo-signals within all this variability. The sonar of odontocetes has several mechanisms to compensate for the echo-level variation (gain control). To date, several mechanisms of the biosonar gain control have been revealed in odontocetes: (1) adjustment of emitted sonar pulse levels (the longer the distance to the target, the higher the level of the emitted pulse), (2) short-term variation of hearing sensitivity based on forward masking of the echo by the preceding self-heard emitted pulse and subsequent release from the masking, and (3) active long-term control of hearing sensitivity. Recent investigations with the use of the auditory evoked-potential technique have demonstrated that these mechanisms effectively minimize the variation of the response to the echo when either the emitted sonar pulse level, or the target distance, or both vary within a wide range. A short review of these data is presented herein.     

Selective attention increases choice certainty in human decision making

Choice certainty is a probabilistic estimate of past performance and expected outcome. In perceptual decisions the degree of confidence correlates closely with choice accuracy and reaction times, suggesting an intimate relationship to objective performance. Here we show that spatial and feature-based attention increase human subjects' certainty more than accuracy in visual motion discrimination tasks. Our findings demonstrate for the first time a dissociation of choice accuracy and certainty with a significantly stronger influence of voluntary top-down attention on subjective performance measures than on objective performance. These results reveal a so far unknown mechanism of the selection process implemented by attention and suggest a unique biological valence of choice certainty beyond a faithful reflection of the decision process.     

Parsing a Cognitive Task: A Characterization of the Mind's Bottleneck

Parsing a mental operation into components, characterizing the parallel or serial nature of this flow, and understanding what each process ultimately contributes to response time are fundamental questions in cognitive neuroscience. Here we show how a simple theoretical model leads to an extended set of predictions concerning the distribution of response time and its alteration by simultaneous performance of another task. The model provides a synthesis of psychological refractory period and random-walk models of response time. It merely assumes that a task consists of three consecutive stages—perception, decision based on noisy integration of evidence, and response—and that the perceptual and motor stages can operate simultaneously with stages of another task, while the central decision process constitutes a bottleneck. We designed a number-comparison task that provided a thorough test of the model by allowing independent variations in number notation, numerical distance, response complexity, and temporal asynchrony relative to an interfering probe task of tone discrimination. The results revealed a parsing of the comparison task in which each variable affects only one stage. Numerical distance affects the integration process, which is the only step that cannot proceed in parallel and has a major contribution to response time variability. The other stages, mapping the numeral to an internal quantity and executing the motor response, can be carried out in parallel with another task. Changing the duration of these processes has no significant effect on the variance.