perceptual systems adjust rapidly to changes in the environment , with neural and behavioral responses dynamically changing to mirror changes in the input ( sharpee et al . , 2006 ; gutinsky and dragoi , 2008 ) .
such adaptive codes provide efficient representations because they direct computational resources toward uncommon inputs and provide information about potentially important changes in the world ( barlow , 1990 ) .
although adaptive coding is less - well - studied in audition than vision , behavioral demonstrations of context - dependence , particularly in speech perception , resonate with the perspective that context adaptively tunes perceptual codes . the identity ( ladefoged and broadbent , 1957 ) or accent ( evans and iverson , 2004 ) of a talker , the rate of the utterance ( liberman et al .
, 1956 ) , and the phonetic make - up of a preceding utterance ( mann , 1986 ) all influence perception of subsequent speech targets . from an adaptive coding perspective , the perceptual system s response to preceding speech lingers to affect subsequent processing
significantly , however , this can only be true to the extent that context and target share common neural resources . in this way
( frisby , 1980 ) because they can expose neural coding related to perceptual experience through the residual effect of one stimulus upon perception of another ( clifford and rhodes , 2005 ) .
thus , context - dependent speech perception perhaps can serve to reveal the underlying representation of speech .
a simple model clarifies the approach ( figure 1 , after rhodes et al . ,
imagine two populations of units coding a dimension of auditory representational space ( e.g. , acoustic frequency , or a higher - order feature like talker identity ) .
pool 1 units best code below - average values along the dimension whereas pool 2 units better code above - average values ; within each pool , more extreme values are coded more robustly .
the average value along the dimension is encoded implicitly in the neutral cross - over point at which pools respond equivalently .
an input with a higher value along the dimension would result in strong pool 2 response , thus reducing pool 2 responsiveness in the short - term due to adaptation . in this way , the context stimulus
 lingers  in the perceptual system to affect the resources available to process subsequent stimuli . as is evident in figure 1 , this reduction shifts the neutral point where the two pools respond equivalently toward higher values and the formerly  average  value is now more robustly coded by pool 1 neurons .
overall , adaptive encoding results in a contrastive shift in target encoding as a function of whether the context was better - coded by pool 1 or pool 2 neurons .
this toy model is simple , to be sure , but the general principle may extend to higher - dimensional perceptual spaces and more complex representations . in vision , adaptive coding has been successful in predicting patterns of interactions among low - level representations for brightness and hue ( see frisby , 1980 ) as well as higher - level representations for faces and bodies ( see clifford and rhodes , 2005 ) .
what is implicit in this approach is the assumption that context and target stimuli are encoded along common dimension(s ) of representation . here
, we pursue context - dependent speech categorization to examine significant representational dimensions of speech categories . for these purposes ,
ladefoged and broadbent s ( 1957 ) classic demonstration of talker normalization is relevant . in their study , listeners heard a constant target - word with a relatively ambiguous vowel at the end of a context phrase 
 the first ( f1 ) and/or second ( f2 ) formant frequencies ( peaks in energy of a voice spectrum ; fant , 1960 ) of the context phrase were increased or decreased .
these shifts can be conceptualized , respectively , as decreasing and increasing the talker s vocal tract length and , correspondingly , as a change in talker . when the resulting phrases preceded the speech targets , listeners categorization shifted in a manner suggesting that they were compensating , or normalizing , for the change in vocal tract length or talker .
a constant vowel was more often heard as  bit  when it followed a phrase synthesized as though spoken by a shorter vocal tract ( higher formant frequencies in the phrase ) , but more often as  bet  following the same phrase modeling speech from a longer vocal tract ( lower frequencies ) .
a central and enduring theoretical issue has been the representational dimension across which listeners  normalize  speech categorization in this way .
is the relevant representational dimension talker identity , vocal tract shape / anatomy , or acoustic phonetic space ( joos , 1948 ; ladefoged and broadbent , 1957 ; halle and stevens , 1962 ; nordstrom and lindblom , 1975 ; mcgowan , 1997 ; mcgowan and cushing , 1999 ; poeppel et al . , 2008 ) ? here , we investigate the extent to which ladefoged and broadbent s classic results can be explained by adaptive coding along a representational dimension that has a general auditory , rather that talker- , or speech - specific basis : the long - term average spectrum ( ltas ) of the preceding sound .
recent research has suggested that listeners are sensitive to the ltas of sound stimuli and adjust perception of subsequent sounds contrastively opposing context ltas .
holt ( 2005 , 2006a , b ) found that sequences of 21 non - speech sine - wave tones , each with a unique frequency sampling a 1000  hz range affect speech categorization of /ga//da/ as a function of the mean frequency of the tones forming the sequence .
the influence of these contexts on speech categorization can not be attributed to any particular acoustic segment of the sequences because tones were randomly ordered on a trial - by - trial basis . instead , the pattern of context - dependent speech categorization is predicted only by the tone sequences ltas .
perception of the subsequent speech targets was relative to , and contrastive with , the ltas consistent with the adaptive coding scheme sketched above if ltas serves as the common representational dimension linking non - speech contexts and speech targets . here , we seek to directly replicate the ladefoged and broadbent ( 1957 ) results with speech contexts and to explicitly test whether non - speech contexts modeling critical characteristics of the speech contexts ltas produce the same effects on vowel categorization . directly mimicking the methods of ladefoged and broadbent ( 1957 ) , listeners categorized a series of speech sounds varying perceptually from 
bet  to  but  in the context of a preceding phrase (  please say what this word is   ) .
: one with a larger vocal tract and the other with a smaller vocal tract .
the same listeners also categorized the same  bet  to  but  targets preceded by sequences of non - speech sine - wave tones with frequencies modeling the ltas of the context phrases .
should the speech and non - speech contexts similarly influence vowel categorization , it would suggest that speech and non - speech contexts draw upon common neural resources . rather than articulatory or talker - specific dimensions that typically have been proposed to account for talker normalization
twenty - six adult native - english speakers from the carnegie mellon university campus with no reported speech or hearing disabilities were recruited for the experiment . all received written informed consent in accord with carnegie mellon university ethics approval , and course credit for their time .
each stimulus had a 1600  ms context segment , followed by a 50  ms silent interval and a 250  ms speech categorization target drawn from a six - step series of speech syllables varying perceptually from /bt/ to /bt/ ( bet to but ) .
the /bv/ segment was created by varying the second formant ( f2 ) frequency of the main vowel portion in equal steps from 1300  hz / b/ to 1700  hz / b/ using klattworks ( mcmurray , in preparation ) .
the onset f2 frequency was 1100  hz and gradually changed to the target frequency across 50  ms .
similarly , the first formant was 150  hz and linearly transitioned to 600  hz over 50  ms .
the fundamental frequency and the third formant frequency were held constant at 120 and 2600  hz , respectively .
the /t/ segment was taken from a natural utterance of  whit  recorded from a male native - english talker and appended to each speech target .
( a ) schematic illustration of stimulus components ; ( b ) spectrogram in time x frequency dimensions for the high mean speech context ( top panel ) and mean percentage of  but  responses in speech contexts ( bottom panel ) ; ( c ) spectrogram in time x frequency dimensions for a representative high mean tone context ( top panel ) and mean percentage of  but  responses in tone context ( bottom panel ) . preceded by both speech ( b ) and
tone ( c ) contexts , higher - frequency contexts led to more low - frequency target responses (  but  ) , and vice versa .
two types of context preceded these speech targets , one speech and the other a sequence of tones .
the speech context was generated by extracting formant frequencies and bandwidths from a recording a male voice uttering the sentence  please say what this word is  ,  and using these values to synthetically reproduce the sentence in the parallel branch of the klatt and klatt ( 1990 ) synthesizer .
following the methods of ladefoged and broadbent with modern techniques , this 1600  ms base phrase was spectrally manipulated by adjusting formant center frequencies and bandwidths to create different  talkers . 
to mimic a longer vocal tract , a voice with relatively lower frequencies in the region of f2 was created ( across the phrase , f2 frequencies ranged from 390 to 1868  hz with an average of 1300  hz ) . a voice with a relatively shorter vocal tract was mimicked by increasing these base frequencies by 400  hz .
thus , the mean acoustic energy in the range of f2 approximated the energy varying across the /bt//bt/ speech target stimuli .
the non - speech tone contexts were composed of a sequence of 16 repeated 70  ms sine - wave tones ( 5  ms linear amplitude onset / offset ramps ) with 30  ms silent intervals separating them as in holt ( 2005 ; 1600  ms total duration ) . the tone contexts modeled the mean f2 frequency of the speech contexts ( 1300 and 1700  hz for the long and short vocal tracts , respectively ) and each tone was a single harmonic without variation .
as such , the non - speech contexts did not sound like speech or possess information about talker identity , vocal tract anatomy , or phonetic space .
thus , these non - speech contexts eliminated shared information between context and target along talker- and speech - specific dimensions while preserving a similar frequency - specific peak in the ltas .
stimuli were rms - matched in amplitude to the  bet  endpoint of the target - word series .
participants categorized each target as  bet  or  but  using labeled keyboard buttons across a 1  h experiment under the control of e - prime ( schneider et al . , 2002 ) .
participants first categorized 10 randomly ordered repetitions of each speech target in isolation to assure that targets were well - categorized as the intended vowels .
they then categorized the same speech targets preceded by high and low versions of speech and non - speech contexts , blocked by context type with block order counterbalanced across participants .
sounds were presented diotically over linear headphones ( beyer dt-150 ) at approximately 70  db spl(a ) .
twenty - six adult native - english speakers from the carnegie mellon university campus with no reported speech or hearing disabilities were recruited for the experiment . all received written informed consent in accord with carnegie mellon university ethics approval , and course credit for their time .
each stimulus had a 1600  ms context segment , followed by a 50  ms silent interval and a 250  ms speech categorization target drawn from a six - step series of speech syllables varying perceptually from /bt/ to /bt/ ( bet to but ) .
the /bv/ segment was created by varying the second formant ( f2 ) frequency of the main vowel portion in equal steps from 1300  hz / b/ to 1700  hz / b/ using klattworks ( mcmurray , in preparation ) .
the onset f2 frequency was 1100  hz and gradually changed to the target frequency across 50  ms .
similarly , the first formant was 150  hz and linearly transitioned to 600  hz over 50  ms .
the fundamental frequency and the third formant frequency were held constant at 120 and 2600  hz , respectively .
the /t/ segment was taken from a natural utterance of  whit  recorded from a male native - english talker and appended to each speech target .
( a ) schematic illustration of stimulus components ; ( b ) spectrogram in time x frequency dimensions for the high mean speech context ( top panel ) and mean percentage of  but  responses in speech contexts ( bottom panel ) ; ( c ) spectrogram in time x frequency dimensions for a representative high mean tone context ( top panel ) and mean percentage of  but  responses in tone context ( bottom panel ) . preceded by both speech ( b ) and tone ( c ) contexts ,
higher - frequency contexts led to more low - frequency target responses (  but  ) , and vice versa .
two types of context preceded these speech targets , one speech and the other a sequence of tones .
the speech context was generated by extracting formant frequencies and bandwidths from a recording a male voice uttering the sentence  please say what this word is  ,  and using these values to synthetically reproduce the sentence in the parallel branch of the klatt and klatt ( 1990 ) synthesizer . following the methods of ladefoged and broadbent with modern techniques , this 1600  ms base phrase was spectrally manipulated by adjusting formant center frequencies and bandwidths to create different  talkers .
 to mimic a longer vocal tract , a voice with relatively lower frequencies in the region of f2 was created ( across the phrase , f2 frequencies ranged from 390 to 1868  hz with an average of 1300  hz ) . a voice with a relatively shorter vocal tract was mimicked by increasing these base frequencies by 400  hz .
thus , the mean acoustic energy in the range of f2 approximated the energy varying across the /bt//bt/ speech target stimuli .
the non - speech tone contexts were composed of a sequence of 16 repeated 70  ms sine - wave tones ( 5  ms linear amplitude onset / offset ramps ) with 30  ms silent intervals separating them as in holt ( 2005 ; 1600  ms total duration ) .
the tone contexts modeled the mean f2 frequency of the speech contexts ( 1300 and 1700  hz for the long and short vocal tracts , respectively ) and each tone was a single harmonic without variation . as such
, the non - speech contexts did not sound like speech or possess information about talker identity , vocal tract anatomy , or phonetic space .
thus , these non - speech contexts eliminated shared information between context and target along talker- and speech - specific dimensions while preserving a similar frequency - specific peak in the ltas .
stimuli were rms - matched in amplitude to the  bet  endpoint of the target - word series .
participants categorized each target as  bet  or  but  using labeled keyboard buttons across a 1  h experiment under the control of e - prime ( schneider et al . , 2002 ) .
participants first categorized 10 randomly ordered repetitions of each speech target in isolation to assure that targets were well - categorized as the intended vowels .
they then categorized the same speech targets preceded by high and low versions of speech and non - speech contexts , blocked by context type with block order counterbalanced across participants .
sounds were presented diotically over linear headphones ( beyer dt-150 ) at approximately 70  db spl(a ) .
participants categorization of speech targets in isolation was orderly , as indicated by a significant main effect target f2 frequency , f ( 5 , 25 )  =  139.43 , p  <  0.01 .
a 2 ( context type , speech / non - speech ) x 2 ( ltas , low / high ) x 6 ( target f2 frequency ) repeated - measures anova of listeners percent  but  responses reveals that , as expected from vowel categorization in isolation , responses varied reliably as a function of the target f2 frequency , f ( 5 , 25 )  =  290.50 , p  <  0.001 .
moreover , there was no main effect of context type indicating no overall bias in vowel categorizations a function of the type of context that preceded targets , f ( 1 , 25 )  =  0.274 , p  =  0.61 . of greater interest , there was a significant main effect of ltas on vowel categorization , f ( 1 , 25 )  =  27.05 , p  <  0.001 .
the vowels were categorized as // significantly more often following high - frequency contexts whereas the same vowels were more often labeled as // following low - frequency contexts .
thus , the context - dependent effect was spectrally contrastive and consistent with previous studies of the influence of sentence - length contexts on speech categorization ( ladefoged and broadbent , 1957 ; watkins and makin , 1994 , 1996 ; holt , 2005 , 2006a , b ; huang and holt , 2009 ) .
the interaction between ltas and target f2 frequency was significant , f ( 5 , 25 )  =  11.65 , p  <  0.001 , indicating that context had a greater influence on perceptually ambiguous targets .
this pattern of contrastive context - dependent vowel categorization was evident for both speech , f ( 1 , 25 )  =  21.62 , p  <  0.001 , and non - speech contexts , f ( 1 , 25 )  =  17.38 , p  <  0.001 . of primary interest
, there was no significant interaction between context type and ltas , f ( 1 , 25 )  =  0.324 , p  =  0.574 .
it is interesting that although the ltas contrast was larger in the non - speech contexts compared with speech context condition ( figure 3 ; see detail explanation in discussion ) , the magnitude of the influence of speech and non - speech contexts on speech target categorization was statistically indistinguishable . neither the interaction between context type and target frequency , f ( 1 , 25 )  =  1.22 , p  =  0.30 , nor the three - way interaction was significant , f ( 5 , 25 )  =  1.424 , p  =  0.22 . in sum ,
 talker  is not an essential element of talker normalization as it appears even in the absence of a talker when context is merely a sequence of sine - wave tones .
long - term average spectrum of target stimuli ( a ) , speech contexts ( b ) , and tone contexts ( c ) .
the shaded area indicates f2 frequencies critical to distinguishing the speech targets and two context types ( speech and tone ) in present experiments .
the residual effect of one stimulus upon perception of another demands that the two share common neural processing and/or representation .
thus , the comparable influence of speech and non - speech contexts on speech categorization indicates a common substrate of interaction .
importantly , since the two context types did not share linguistic , articulatory gestural , or talker - specific information , but yet produced equivalent effects on speech categorization , it does not appear that speech- , vocal tract- , or talker - specific information is essential in eliciting the patterns of context - dependent perception that have been described in the literature as  talker normalization .  what the two context types in the present experiment shared was a similar pattern of spectral energy across their time course .
figure 3 illustrates the ltas of the speech - target endpoints ( 3a ) and speech ( 3b ) and non - speech contexts ( 3c ) .
we suggest that the context - dependent speech categorization observed here ( and in ladefoged and broadbent , 1957 ) arises because the auditory system is sensitive to the context ltas and the speech target is encoded relative to , and contrastive with , that long - term average .
specifically , we propose that speech is adaptively coded according to the ltas of ambient sound .
the simple model described in the introduction can clarify one means by which this might be accomplished .
imagine pools of neurons sensitive to energy in the range of the second formant with one pool better coding relatively lower frequencies and the other better coding higher frequencies . by this model , presentation of the speech context modeling a longer vocal tract with lower - frequency energy within this frequency range
would result in greater activity among the pool of neurons that better code lower frequencies .
subsequent adaptation among this pool of responsive neurons would result in a shift toward the opposite , higher - frequency neural pool at the time of speech target presentation .
neutral  frequencies would be now more robustly encoded by the higher - frequency neural pool , shifting representation contrastively away from the lower - frequency context .
this adaptive coding serves to exaggerate differences between the ltas of the context and target ( holt , 2006a ) . by this model ,  talker normalization 
effects on speech targets are predicted and obtained even when no speech information is available in the context . of note , the ltas model makes no reference to specific linguistic units , such as phonemes .
this generality makes the adaptive coding approach in general , and the ltas model in particular , extend straightforwardly to other normalization phenomena in speech perception .
huang and holt ( 2009 ) report that shifts in the peak energy of the ltas in the region of the fundamental frequency ( f0 ) of a mandarin chinese sentence predict patterns of context - dependent mandarin lexical tone normalization ( leather , 1983 ; fox and qi , 1990 ; moore and jongman , 1997 ) .
similarly , although the adaptive coding approach to context - dependent speech categorization reveals ltas as an important dimension of representation , the model s application is more general . wade and holt ( 2005 ) , for example , investigated rate - dependent normalization effects whereby the rate of a precursor sentence affects how listeners categorize a rate - dependent speech distinction like /ba/ versus /wa/ , finding that the rate of presentation of a sequence of non - speech tones evokes a similar contrastive influence on speech categorization .
if demonstrations of  talker normalization  can be accounted for by general perceptual processes that are not talker- or speech - specific then there remains the question of whether the context - dependent speech categorization taken as evidence of normalization really accommodates the acoustic variability in speech that arises from different talkers .
we argue that it does . across context sentences like those of the current study , speech maps
the scope of a talker s articulatory space and the mean of that space resembles a talker s neutral vowel , the shape of the non - articulating vocal tract ( story , 2005 ) .
this neutral vowel serves as an effective normalization referent because , as story ( 2005 ) has demonstrated , most of the variability across talkers can be accounted for by differences in the shape the vocal air space of talkers neutral vowels .
thus , if listeners were able to extract an estimate of the neutral vowel , the mean of the articulatory space , they would have an excellent referent for talker normalization .
the results we describe here suggest an alternative . as talkers produce a variety of consonants and vowels , speech maps the articulatory space but it also produces a sound spectrum that maps the acoustic space and , across time , samples an ltas . instead of solving the inverse problem to recover the actual neutral vocal tract shape as an articulatory referent for normalization , listeners may use the average spectrum ltas as an auditory referent .
the observation that non - speech tones modeling the ltas of a talker are as effective in shifting speech categorization as speech contexts supports the viability of a general auditory referent .
however , it should be noted that ltas is unlikely to be the only contributing factor in talker normalization .
speaker identities , for example , may mediate listeners attention and expectation and influence the way listeners tune their perception to the preceding contexts ( magnuson and nusbaum , 2007 ) .
. nonetheless ltas , which provides sufficient context information via general perceptual processes , is an important factor in the adaptive coding of speech perception processing that contributes to talker normalization .
the lingering influence of a sequence of non - speech tones on listeners response to speech targets indicates a shared substrate , the level of which remains to be evaluated .
relevant to this , holt ( 2005 ) reported that non - speech contexts influence on speech categorization persists across 1300  ms of silence .
moreover , the aftereffect of the tones was present even when 13 neutral - frequency tones intervened between tone contexts and speech targets .
the influence of temporally non - adjacent tone sequences can even override the influence of temporally adjacent speech contexts on speech targets ( holt , 2006b ) .
these observations argue for a central , rather than peripheral , substrate . in fitting the mind to the world ,
the present results demonstrate that patterns of context - dependent speech categorization long taken to be evidence of talker - specific normalization for articulatory referents or rescaling of phonetic space may arise , instead , from general principles of adaptive perceptual coding . in  listening for the norm ,  listeners appear to adjust perception to regularities of the ambient environment .
the authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest .
 perceptual aftereffects have been referred to as  the psychologist s microelectrode  because they can expose dimensions of representation through the residual effect of a context stimulus upon perception of a subsequent target . 
 the present study uses such context - dependence to examine the dimensions of representation involved in a classic demonstration of  talker normalization  in speech perception . whereas most accounts of talker normalization have emphasized talker- , speech- , or articulatory - specific dimensions significance , the present work tests an alternative hypothesis : that the long - term average spectrum ( ltas ) of speech context is responsible for patterns of context - dependent perception considered to be evidence for talker normalization . in support of this hypothesis , 
 listeners vowel categorization was equivalently influenced by speech contexts manipulated to sound as though they were spoken by different talkers and non - speech analogs matched in ltas to the speech contexts . since the non - speech contexts did not possess talker , speech , or articulatory information , general perceptual mechanisms are implicated . 
 results are described in terms of adaptive perceptual coding . 
Introduction
Materials and Methods
Participants
Stimuli
Procedure
Results
Discussion
Conflict of Interest Statement