Selective Adaptation Effects Research Articles

Selective adaptation of German /r/: A role for perceptual saliency

In three experiments, we examined selective adaptation of German /r/ depending on the positional and allophonic overlap between adaptors and targets. A previous study had shown that selective adaptation effects with /r/ in Dutch require allophonic overlap between adaptor and target. We aimed at replicating this finding in German, which also has many allophones of /r/. German post-vocalic /r/ is often vocalized, and pre-vocalic /r/ can occur in at least three forms: uvular fricative [ʁ], uvular trill [ʀ] and alveolar trill [r]. We tested selective adaptation between these variants. The critical questions were whether an allophonic overlap is necessary for adaptation or whether phonemic overlap is sufficient to generate an adaptation effect. Surprisingly, our results show that both assertations are wrong: Adaptation does not require an allophonic overlap between adaptors and target and neither is phonemic overlap sufficient. Even more surprisingly, trilled adaptors led to more adaptation for a uvular-fricative target than uvular-fricative adaptors themselves. We suggest that the perceptual salience of the adaptors may be a hitherto underestimated influence on selective adaptation.

Evidence For Selective Adaptation and Recalibration in the Perception of Lexical Stress.

Individuals vary in how they produce speech. This variability affects both the segments (vowels and consonants) and the suprasegmental properties of their speech (prosody). Previous literature has demonstrated that listeners can adapt to variability in how different talkers pronounce the segments of speech. This study shows that listeners can also adapt to variability in how talkers produce lexical stress. Experiment 1 demonstrates a selective adaptation effect in lexical stress perception: repeatedly hearing Dutch trochaic words biased perception of a subsequent lexical stress continuum towards more iamb responses. Experiment 2 demonstrates a recalibration effect in lexical stress perception: when ambiguous suprasegmental cues to lexical stress were disambiguated by lexical orthographic context as signaling a trochaic word in an exposure phase, Dutch participants categorized a subsequent test continuum as more trochee-like. Moreover, the selective adaptation and recalibration effects generalized to novel words, not encountered during exposure. Together, the experiments demonstrate that listeners also flexibly adapt to variability in the suprasegmental properties of speech, thus expanding our understanding of the utility of listener adaptation in speech perception. Moreover, the combined outcomes speak for an architecture of spoken word recognition involving abstract prosodic representations at a prelexical level of analysis.

Acoustic cues, not phonological features, drive vowel perception: Evidence from height, position and tenseness contrasts in German vowels

Phonological features have frequently been singled out as the units of perception, especially for vowels. Evidence of the use of features has been provided for vowel height and vowel position, which have one acoustic correlate only. However, findings on acoustically complex features such as tenseness are less clear. The present study assessed the role of phonological features in perception using the selective adaptation paradigm. Selective adaptation effects on German vowel contrasts differing in vowel height (Experiment 1), position (Experiment 2) and tenseness (Experiment 3) were examined. We tested how the categorization of each vowel contrast was affected by adaptation to words containing vowels that differently resembled or diverged from the vowels in the critical contrast acoustically and in terms of their phonological feature specifications. Results showed that selective adaptation patterns could be predicted by the vowels’ phonological features for the height and position contrasts, but not for the tenseness contrast. However, adaptation patterns for the latter can be explained by the relationship between adaptors and continuum endpoints in each of the relevant acoustic cues to the contrast. This suggests that vowel perception may be dependent on these acoustic cues rather than phonological features.

Modulation of Speech Motor Learning with Transcranial Direct Current Stimulation of the Inferior Parietal Lobe.

The inferior parietal lobe (IPL) is a region of the cortex believed to participate in speech motor learning. In this study, we investigated whether transcranial direct current stimulation (tDCS) of the IPL could influence the extent to which healthy adults (1) adapted to a sensory alteration of their own auditory feedback, and (2) changed their perceptual representation. Seventy subjects completed three tasks: a baseline perceptual task that located the phonetic boundary between the vowels /e/ and /a/; a sensorimotor adaptation task in which subjects produced the word “head” under conditions of altered or unaltered feedback; and a post-adaptation perceptual task identical to the first. Subjects were allocated to four groups which differed in current polarity and feedback manipulation. Subjects who received anodal tDCS to their IPL (i.e., presumably increasing cortical excitability) lowered their first formant frequency (F1) by 10% in opposition to the upward shift in F1 in their auditory feedback. Subjects who received the same stimulation with unaltered feedback did not change their production. Subjects who received cathodal tDCS to their IPL (i.e., presumably decreasing cortical excitability) showed a 5% adaptation to the F1 alteration similar to subjects who received sham tDCS. A subset of subjects returned a few days later to reiterate the same protocol but without tDCS, enabling assessment of any facilitatory effects of the previous tDCS. All subjects exhibited a 5% adaptation effect. In addition, across all subjects and for the two recording sessions, the phonetic boundary was shifted toward the vowel /e/ being repeated, consistently with the selective adaptation effect, but a correlation between perception and production suggested that anodal tDCS had enhanced this perceptual shift. In conclusion, we successfully demonstrated that anodal tDCS could (1) enhance the motor adaptation to a sensory alteration, and (2) potentially affect the perceptual representation of those sounds, but we failed to demonstrate the reverse effect with the cathodal configuration. Overall, tDCS of the left IPL can be used to enhance speech performance but only under conditions in which new or adaptive learning is required.

Lexically driven selective adaptation by ambiguous auditory stimuli occurs after limited exposure to adaptors.

Limited exposure to ambiguous auditory stimuli results in perceptual recalibration. When unambiguous stimuli are used instead, selective adaptation (SA) effects have been reported, even after few adaptor presentations. Crucially, selective adaptation by an ambiguous sound in biasing lexical contexts had previously been found only after massive adaptor repetition [Samuel (2001). Psychol. Sci. 12(4), 348-351]. The present study shows that extensive exposure is not necessary for lexically driven selective adaptation to occur. Lexically driven selective adaptation can arise after as few as nine adaptor presentations. Additionally, build-up course inspection reveals several parallelisms with the time course observed for SA with unambiguous stimuli.

Testing the assumptions underlying fMRI adaptation using intracortical recordings in area MT

We investigated how neural activity in the middle temporal area of the macaque monkey changes after 3 sec of exposure to a visual stimulus and used this to gain insight into the assumptions underlying the fMRI adaptation method (fMRIa). We studied both changes in tuning curves following weak and strong motion stimuli (adaptation) and the differences between a first and second exposure to the same stimulus (repetition suppression). Typically, tuning curves had smaller amplitudes and narrower tuning widths after strong adaptation; this was true for single neurons, multi-unit activity (MUA), the evoked local field potential (LFP), as well as gamma band activity. Repetition typically led to reduced responses. This reduction was correlated with direction selectivity and not explained by neural fatigue.Our data, however, warn against a simplistic view of the consequences of adaptation. First, a considerable fraction of neurons and sites showed response enhancements after adaptation, especially when probed with a stimulus that moved opposite to the direction of the adapting stimulus. Second, adaptation was stimulus selective only on a time scale of ∼100 msec. Third, aggregate measures of neural activity (MUA, LFPs) had substantially different adaptation effects. Fourth, there were qualitative differences between our findings in MT and earlier findings in IT cortex.We conclude that selective adaptation effects in fMRIa are relatively easy to miss even when they exist (for instance by presenting stimuli for too long, or because neurons that enhance after adaptation cancel out the effect of neurons that suppress). Moreover, we argue that adaptation should be understood in the context of the computations that a neural circuit perform. Using fMRIa as a tool to uncover neural selectivity requires a better understanding of this circuitry and its consequences for adaptation.

Attentional Adjustment to Conflict Strength

Current models of cognitive control assume gradual adjustment of processing selectivity to the strength of conflict evoked by distractor stimuli. Using a flanker task, we varied conflict strength by manipulating target and distractor onset. Replicating previous findings, flanker interference effects were larger on trials associated with advance presentation of the flankers compared to simultaneous presentation. Controlling for stimulus and response sequence effects by excluding trials with feature repetitions from stimulus administration (Experiment 1) or from the statistical analyses (Experiment 2), we found a reduction of the flanker interference effect after high-conflict predecessor trials (i.e., trials associated with advance presentation of the flankers) but not after low-conflict predecessor trials (i.e., trials associated with simultaneous presentation of target and flankers). This result supports the assumption of conflict-strength-dependent adjustment of visual attention. The selective adaptation effect after high-conflict trials was associated with an increase in prestimulus pupil diameter, possibly reflecting increased cognitive effort of focusing attention.

Visual speech acts differently than lexical context in supporting speech perception.

The speech signal is often badly articulated, and heard under difficult listening conditions. To deal with these problems, listeners make use of various types of context. In the current study, we examine a type of context that in previous work has been shown to affect how listeners report what they hear: visual speech (i.e., the visible movements of the speaker's articulators). Despite the clear utility of this type of context under certain conditions, prior studies have shown that visually driven phonetic percepts (via the "McGurk" effect) are not "real" enough to affect perception of later-occurring speech; such percepts have not produced selective adaptation effects. This failure contrasts with successful adaptation by sounds that are generated by lexical context-the word that a sound occurs within. We demonstrate here that this dissociation is robust, leading to the conclusion that visual and lexical contexts operate differently. We suggest that the dissociation reflects the dual nature of speech as both a perceptual object and a linguistic object. Visual speech seems to contribute directly to the computations of the perceptual object but not the linguistic one, while lexical context is used in both types of computations.

Effects of Selective Adaptation on Coding Sugar and Salt Tastes in Mixtures

Little is known about coding of taste mixtures in complex dynamic stimulus environments. A protocol developed for odor stimuli was used to test whether rapid selective adaptation extracted sugar and salt component tastes from mixtures as it did component odors. Seventeen human subjects identified taste components of "salt + sugar" mixtures. In 4 sessions, 16 adapt-test stimulus pairs were presented as atomized, 150-μL "taste puffs" to the tongue tip to simulate odor sniffs. Stimuli were NaCl, sucrose, "NaCl + sucrose," and water. The sugar was 98% identified but the suppressed salt 65% identified in unadapted mixtures of 2 concentrations of NaCl, 0.1 or 0.05 M, and sucrose at 3 times those concentrations, 0.3 or 0.15 M. Rapid selective adaptation decreased identification of sugar and salt preadapted ambient components to 35%, well below the 74% self-adapted level, despite variation in stimulus concentration and adapting time (<5 or >10 s). The 96% identification of sugar and salt extra mixture components was as certain as identification of single compounds. The results revealed that salt-sugar mixture suppression, dependent on relative mixture-component concentration, was mutual. Furthermore, like odors, stronger and recent tastes are emphasized in dynamic experimental conditions replicating natural situations.

Identity-specific face adaptation effects: Evidence for abstractive face representations

The effects of selective adaptation on familiar face perception were examined. After prolonged exposure to photographs of a celebrity, participants saw a series of ambiguous morphs that were varying mixtures between the face of that person and a different celebrity. Participants judged fewer of the morphs to resemble the celebrity to which they had been adapted, implying that they were now less sensitive to that particular face. Similar results were obtained when the adapting faces were highly dissimilar in viewpoint to the test morphs; when they were presented upside-down; or when they were vertically stretched to three times their normal height. These effects rule out explanations of adaptation effects solely in terms of low-level image-based adaptation. Instead they are consistent with the idea that relatively viewpoint-independent, person-specific adaptation occurred, at the level of either the “Face Recognition Units” or “Person Identity Nodes” in Burton, Bruce and Johnston’s (1990) model of face recognition.

Effects of selective adaptation on texture segmentation and its interaction with covert attention.

Effects of selective adaptation on texture segmentation and its interaction with covert attention.

Preserved implicit form perception and orientation adaptation in visual form agnosia

Visual form agnosia is mainly characterized by profound deficits in visual form and shape discrimination. Previous studies have shown that patients retain the capacity for coordinated motor behaviors, color naming and implicit letter perception. However, it is unknown to what extent other visual functions, such as implicit form and orientation perception, are preserved. To address these questions, we investigated a single visual form agnosic patient, X.F., in two distinct experiments. X.F.'s visual lesions were mainly localized in the bilateral occipitotemporal cortex, with the dorsal visual stream and early visual cortex largely spared. In Experiment 1, X.F. named the color of different forms across 12 blocks of trials. After the first six blocks, the combinations of a form with its color were changed and the new combination was presented for the remaining six blocks. X.F.'s reaction time increased during the switch block and was significantly greater than the overall RT changes between adjacent, non-switch blocks. This indicates that X.F. retained the ability to perceive changes in form despite her inability to discriminate the forms. In Experiment 2, X.F. showed selective orientation adaptation effects to different spatial frequencies; that is, her contrast threshold was significantly higher when the adapting and test orientations were the same than when they were orthogonal, although her orientation discrimination performance was severely impaired. These data provide evidence of a functional dissociation between explicit and implicit visual abilities, and suggest that the residual early visual cortex mediates form and orientation processing in the absence of awareness.

FMRI measurements of changes in color and orientation tuning in V1

Psychophysics has identified visual mechanisms tuned for particular orientations and color directions. The goal of the present research is to explore the neural bases of these behavioral mechanisms. Two series of experiments measured neural activity using fMRI while subject viewed stimuli and performed tasks borrowed from psychophysical paradigms. A first study measured V1 responses to stimuli of different colors. Prolonged exposure to a particular color reduced both perceptual sensitivity and V1 response to that color, while sparing other colors. These selective adaptation effects were selective for orientation as well. Our results strongly suggest that large numbers of neurons in V1 are jointly tuned for orientation and color. The similarity of our fMRI and behavioral measures indicates that the activity of these neurons supports perceptual judgments of color appearance. A second study measured V1 and V2 responses to stimuli of different orientations. At para-foveal locations, cortex responded more strongly to principal (vertical and horizontal) than to oblique orientations. This difference was not present at more eccentric locations, matching behavioral measurements made using a contrast detection task. Training in detecting an oblique parafoveal stimulus increased fMRI response to that stimulus relative to others. Behavioral performance improved in a similarly selective fashion. These results suggest that either the number of neurons tuned to oblique orientations increased, or the gain of such neurons rose. Thus perceptual learning may strengthen responses to simple patterns at very early stages of the visual system. Overall, our results show that the combination of neuroimaging techniques and psychophysics can identify properties of populations of neurons that support perception.

An electrophysiological metric of activity within the ON- and OFF-pathways in humans.

Several animal studies have shown an anatomical and functional separation between the ON- and OFF-pathways in the retina and in the lateral geniculate nucleus. Psychophysical studies in humans have also documented separate pathways that process increments and decrements of light. However, at the level of the visual cortex, there is electrophysiological evidence of interactions between the ON- and OFF-pathways. In addition, psychophysical studies have shown that these pathways can exhibit differential sensitivity and be differentially adapted. These findings motivated an electrophysiological study to gather further evidence of processing within the ON- and OFF-pathways in the human visual system. Using sawtooth stimulus modulation, we measured the visual evoked potential (VEP) before and after adaptation to both rapid-on and rapid-off sawtooth stimuli. The effect of adaptation was determined by comparing the VEP response in three test conditions: without adaptation, after adaptation to the same sawtooth polarity, and after adaptation to the opposite sawtooth polarity. The results reveal a selective adaptation effect, which provides physiological evidence for separate processing of increments and decrements in the human visual system. We conclude that with appropriate stimulus parameters, the VEP can serve as an objective measure of processing within the ON- and OFF-pathways in humans.

Are selective adaptation effects independent of cognitive load?

Selective adaptation occurs through the repeated presentation of a sound (the ‘‘adaptor’’), and leads to a reduction in the perception of similar sounds. Adaptation has been used to investigate the nature of early speech representations. Work in this laboratory has recently demonstrated that perceptually restored phonemes can produce reliable adaptation effects, and that these effects are occurring at relatively early levels (e.g., phonemic rather than lexical) of processing [A. G. Samuel, CognitivePsychology (in press)]. The current study is designed to determine whether the adaptation effects are so low-level and automatic that they do not require cognitive resources. There are three conditions in the current study: (1) adaptation alone (control), (2) adaptation during continuous arithmetic problems, and (3) adaptation during continuous rhyming judgments (presented visually). Preliminary results indicate that continuously solving arithmetic problems does not reduce the adaptation effect, indicating no general cognitive involvement in adaptation. The rhyming task tests for any more specific involvement of language processors. Immunity to this secondary task would support a very low-level, automatic basis for adaptation. [Work supported by NIMH.]

Spectral responses and chromatic processing in the dragonfly lamina

The dragonfly Hemicordulia tau has five spectral classes of photoreceptor which drive five lamina monopolars, m1-m5. The monopolars encode spectral information. Here, spectral coding by m2, m4 and m5 are described. m2 is the most sensitive to dim light. m4 and m5 are less sensitive than m2, and so we call them photopic cells. The effects of selective adaptation of receptor inputs to m4 and m5 are unusual. For example, in m4 adaptation of the green receptor suppresses the UV input, but green sensitivity is unaffected, while green adaptation of m5 increases its green sensitivity. In m5 the dark adapted spectral tuning resembles the 520 nm receptor, but on adaptation to 430 nm light this narrows markedly. Adaptation either of green or of UV receptor input to m2 and m4 modifies the time course of responses both to green and to UV, implying that changes in the time courses of monopolar responses with adaptation state do not directly reflect receptor responses (Fig. 2). Finally, the antagonistic surround of m2 is UV sensitive, and of m4 green sensitive (Fig. 3).

Selective adaptation by context-conditioned fricatives

The role of vowel context and consonant labeling in the selective adaptation of voiceless fricatives was examined in three experiments. This approach was designed to determine whether selective adaptation effects occurred with voiceless fricative stimuli and to determine whether any such effects had a linguistic basis as opposed to a purely auditory basis. Two synthetic fricative-vowel continua were used; one ranged from [si] to [∫i] and the other from [su] to [∫u]. Identification of the consonant portion of the syllables in these continua, as either [s] or [∫], depended on both the frequency of the friction noise and on the vowel quality. In experiment 1, the end points of the continua were used as adaptors, and the identification boundary shifted toward the adapting stimulus. In experiment 2, an ambiguous frication noise (that was identified as [s] before [u] and as [∫] before [i]) adapted the identification boundary in opposite directions, depending on which of the two vowels followed the noise. Thus the direction of adaptation depended on the perceptual identity of the consonant. In the final experiment, the isolated [i] and [u] vowels, and the isolated ambiguous frication noise, were demonstrated to be ineffective adaptors. The selective adaptation effects observed in these experiments were not determined by the acoustical information in the consonant or the vowel alone, but rather by the context-conditioned percept of the fricative. These results extend the reports of other research that has attempted to dissociate auditory from linguistic adaptation, provide further evidence that selective adaptation effects have multiple loci, and establish for the first time a selective adaptation effect which is unambiguously, not acoustically based.

Selective adaptation of context-conditioned voiceless fricatives

Two fricative-vowel continua were synthesized. One ranged from /si/ to /(sh)i/ and the other from /su/ to /(sh)u/. Listeners’ identification of the consonant portion of the syllables, as either ‘‘s’’ or ‘‘sh,’’ depended on both the frequency of the frication noise and on the vowel quality. Selective adaptation of these continua was investigated in three experiments. In experiment 1, the endpoints of the continua were used as adaptors, and the identification boundary shifted towards the adapting stimulus. In experiment 2, ambiguous frication noises (that were identified as /s/ before /u/ and /(sh)/ before the /i/) adapted the identification boundary in opposite directions, depending on which of the two vowels followed the noise and, therefore, on the perceptual identity of the consonant. In experiment 3, the isolated /i/ and /u/ vowels were demonstrated to be ineffective as adaptors. The selective adaptation effects observed in these experiments were not determined by the acoustical information in the consonant or the vowel alone, but rather the context-conditioned percept of the fricative. [Work supported by NSERC.]

Auditory Processing in Children's Speech Perception

Five- to six-year-old children and adults participated in discrimination and selective adaptation speech perception tasks using a synthetic consonant-vowel continuum ranging from [ba] to [da]. In one condition of selective adaptation, attention was focused on the adapting stimulus, the continuum-endpoint [ba], with a whispering task. In another condition, attention was focused away from the continuum-endpoint [da] adaptor to contralaterally presented syllables "SHE" and "SEE." Results, compared with two more typical adaptation conditions, indicated that focused attention did not augment selective adaptation effects, particularly for children who showed smaller effects with focused attention on the adaptor. In contrast to adults, children did not significantly change labeling responses after exposure to endpoint-[ba] adaptors, results matching those of Sussman and Carney (1989). However, children did significantly change labeling following exposure to endpoint-[da] adaptors. Discrimination findings with five-formant consonant-vowel and single-formant stimuli supported the importance of acoustic processing for the selective adaptation tasks performed. Together, results support hypotheses of sensory processing differences in younger, normally developing children compared with adults and show that such abilities appear to be related to speech perception skills.

Evidence for a Common Motion Mechanism of Luminance-Modulated and Contrast-Modulated Patterns: Selective Adaptation

Selective adaptation effects were measured with contrast-modulated patterns and sine-wave gratings in order to determine the extent to which the two patterns are processed by common mechanisms. Direction-specific adaptation effects were measured for a contrast-modulated adapting pattern and a test pattern. The contrast-modulated adapting pattern was composed of a sine-wave grating of 8 cycles deg-1 whose contrast was spatially modulated by a sinusoid of 1 cycle deg-1 at one of four levels: 100%, 60%, 30%, or 0%. The results showed that contrast-modulation thresholds for contrast-modulated gratings were raised by 0.3 to 0.5 log units following adaptation to a contrast-modulated grating moving in the same direction as the test pattern, relative to thresholds obtained following adaptation to a contrast-modulated grafting moving in the opposite direction. Cross-adaptation effects were also measured with a sine-wave adapting pattern and a contrast-modulated test pattern. The sine-wave adapting pattern was a sine-wave grating of 1 cycle deg-1 whose contrast was set to one of three levels: 16.4%, 1.25%, or 0%. The contrast-modulated test pattern was a sine-wave grating of 8 cycles deg-1 whose contrast was modulated by a sinusoid of 1 cycle deg-1. The results revealed that contrast-modulation thresholds for contrast-modulated gratings were raised by approximately 0.25 log units following adaptation to moving sine-wave gratings, relative to thresholds obtained following adaptation to a uniform field. Cross-adaptation effects were also obtained with a contrast-modulated adapting pattern and a sine-wave test pattern. The results support the view that signals generated from luminance-domain stimuli and from contrast-domain stimuli are processed by a common motion mechanism.