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brain perceives it by trying to match it with what it would have had to do to produce it since the only thing they can produce is ba or pa they will perceive any of the synthetic stimuli along the continuum as either ba or pa whichever it is closer to a similar cp effect is found with ba da or with any two speech sounds belonging to different categories these too lie along a continuum acoustically but vocally ba is formed with the two lips da with the tip of the tongue and the alveolar ridge and our anatomy does not allow any intermediates the motor theory of speech perception explained how speech was special and why speech sounds are perceived categorically sensory perception is mediated by motor production acquired distinctiveness edit if motor production mediates sensory perception then one assumes that this cp effect is a result of learning to produce speech eimas et al 1971 however found that infants already have speech cp before they begin to speak perhaps then it is an innate effect evolved to prepare us to learn to speak 4 but kuhl 1987 found that chinchillas also have speech cp even though they never learn to speak and presumably did not evolve to do so 5 lane 1965 went on to show that cp effects can be induced by learning alone with a purely sensory visual continuum in which there is no motor production discontinuity to mediate the perceptual discontinuity 6 he concluded that speech cp is not special after all but merely a special case of lawrence s classic demonstration that stimuli to which you learn to make a different response become more distinctive and stimuli to which you learn to make the same response become more similar it also became clear that cp was not quite the all or none effect liberman had originally thought it was it is not that all pa s are indistinguishable and all ba s are indistinguishable we can hear the differences just as we can see the differences between different shades of red it is just that the within category differences pa1 pa2 or red1 red2 sound look much smaller than the between category differences pa2 ba1 or red2 yellow1 even when the size of the underlying physical differences voicing wavelength are actually the same identification and discrimination tasks edit the study of categorical perception often uses experiments involving discrimination and identification tasks in order to categorize participants perceptions of sounds voice onset time vot is measured along a continuum rather than a binary english bilabial stops b and p are voiced and voiceless counterparts of the same place and manner of articulation yet native speakers distinguish the sounds primarily by where they fall on the vot continuum participants in these experiments establish clear phoneme boundaries on the continuum two sounds with different vot will be perceived as the same phoneme if on the same side of the boundary 7 participants take longer to discriminate between two sounds falling in the same category of vot than between two on opposite sides of the phoneme boundary even if the difference in vot is greater between the two in the same category 8 identification edit in a categorical perception identification task participants often must identify stimuli such as speech sounds an experimenter testing the perception of the vot boundary between p and b may play several sounds falling on various parts of the vot continuum and ask volunteers whether they hear each sound as p or b 9 in such experiments sounds on one side of the boundary are heard almost universally as p and on the other as b stimuli on or near the boundary take longer to identify and are reported differently by different volunteers but are perceived as either b or p rather than as a sound somewhere in the middle 7 discrimination edit a simple ab discrimination task presents participants with two options and participants must decide if they are identical 9 predictions for a discrimination task in an experiment are often based on the preceding identification task an ideal discrimination experiment validating categorical perception of stop consonants would result in volunteers more often correctly discriminating stimuli that fall on opposite sides of the boundary while discriminating at chance level on the same side of the boundary 8 in an abx discrimination task volunteers are presented with three stimuli a and b must be distinct stimuli and volunteers decide which of the two the third stimulus x matches this discrimination task is much more common than a simple ab task 9 8 whorf hypothesis edit according to the sapir whorf hypothesis of which lawrence s acquired similarity distinctiveness effects would simply be a special case language affects the way that people perceive the world for example colors are perceived categorically only because they happen to be named categorically our subdivisions of the spectrum are arbitrary learned and vary across cultures and languages but berlin kay 1969 suggested that this was not so not only do most cultures and languages subdivide and name the color spectrum the same way but even for those who don t the regions of compression and separation are the same 10 we all see blues as more alike and greens as more alike with a fuzzy boundary in between whether or not we have named the difference this view has been challenged in a review article by regier and kay 2009 who discuss a distinction between the questions 1 do color terms affect color perception and 2 are color categories determined by largely arbitrary linguistic convention they report evidence that linguistic categories stored in the left hemisphere of the brain for most people do affect categorical perception but primarily in the right visual field and that this effect is eliminated with a concurrent verbal interference task 11 universalism in contrasts to the sapir whorf hypothesis posits that perceptual categories are innate and are unaffected by the language that one speaks 12 support edit support of the sapir whorf hypothesis describes instances in which speakers of one language demonstrate categorical perception in a way that is different from speakers of another language examples of such evidence are provided below regier and kay 2009 reported evidence that linguistic categories affect categorical perception primarily in the right visual field 13 the right visual field is controlled by the left hemisphere of the brain which also controls language faculties davidoff 2001 presented evidence that in color discrimination tasks native english speakers discriminated more easily between color stimuli across a determined blue green boundary than within the same side but did not show categorical perception when given the same task with berinmo nol and wor berinmo speakers performed oppositely 14 a popular theory in current research is weak whorfianism which is the theory that although there is a strong universal component to perception cultural differences still have an impact for example a 1998 study found that while there was evidence of universal perception of color between speakers of setswana and english there were also marked differences between the two language groups 15 evolved categorical perception edit the signature of categorical perception cp is within category compression and or between category separation the size of the cp effect is merely a scaling factor it is this compression separation accordion effect that is cp s distinctive feature in this respect the weaker cp effect for vowels whose motor production is continuous rather than categorical but whose perception is by this criterion categorical is every bit as much of a cp effect as the ba pa and ba da effects but as with colors it looks as if the effect is an innate one our sensory category detectors for both color and speech sounds are born already biased by evolution our perceived color and speech sound spectrum is already warped with these compression separations learned categorical perception edit the lane lawrence demonstrations lately replicated and extended by goldstone 1994 showed that cp can be induced by learning alone 16 there are also the countless categories cataloged in our dictionaries that according to categorical perception are unlikely to be inborn nativist theorists such as fodor 1983 have sometimes seemed to suggest that all of our categories are inborn 17 there are recent demonstrations that although the primary color and speech categories may be inborn their boundaries can be modified or even lost as a result of learning and weaker secondary boundaries can be generated by learning alone 18 in the case of innate cp our categorically biased sensory detectors pick out their prepared color and speech sound categories far more readily and reliably than if our perception had been continuous learning is a cognitive process that results in a relatively permanent change in behavior learning can influence perceptual processing 19 learning influences perceptual processing by altering the way in which an individual perceives a given stimulus based on prior experience or knowledge this means that the way something is perceived is changed by how it was seen observed or experienced before the effects of learning can be studied in categorical perception by looking at the processes involved 20 learned categorical perception can be divided into different processes through some comparisons the processes can be divided into between category and within category groups of comparison 21 between category groups are those that compare between two separate sets of objects within category groups are those that compare within one set of objects between subjects comparisons lead to a categorical expansion effect a categorical expansion occurs when the classifications and boundaries for the category become broader encompassing a larger set of objects in other words a categorical expansion is when the edge lines for defining a category become wider within subjects comparisons lead to a categorical compression effect a categorical compression effect corresponds to the narrowing of category boundaries to include a smaller set of objects the edge lines are closer together 21 therefore between category groups lead to less rigid group definitions whereas within category groups lead to more rigid definitions another method of comparison is to look at both supervised and unsupervised group comparisons supervised groups are those for which categories have been provided meaning that the category has been defined previously or given a label unsupervised groups are groups for which categories are created meaning that the categories will be defined as needed and are not labeled 22 in studying learned categorical perception themes are important learning categories is influenced by the presence of themes themes increase quality of learning this is seen especially in cases where the existing themes are opposite 22 in learned categorical perception themes serve as cues for different categories they assist in designating what to look for when placing objects into their categories for example when perceiving shapes angles are a theme the number of angles and their size provide more information about the shape and cue different categories three angles would cue a triangle whereas four might cue a rectangle or a square opposite to the theme of angles would be the theme of circularity the stark contrast between the sharp contour of an angle and the round curvature of a circle make it easier to learn similar to themes labels are also important to learned categorical perception 21 labels are noun like titles that can encourage categorical processing with a focus on similarities 21 the strength of a label can be determined by three factors analysis of affective or emotional strength permeability the ability to break through of boundaries and a judgment measurement of rigidity of discreteness 21 sources of labels differ and similar to unsupervised supervised categories are either created or already exist 21 22 labels affect perception regardless of their source peers individuals experts cultures and communities can create labels the source doesn t appear to matter as much as mere presence of a label what matters is that there is a label there is a positive correlation between strength of the label combination of three factors and the degree to which the label affects perception meaning that the stronger the label the more the label affects perception 21 cues used in learned categorical perception can foster easier recall and access of prior knowledge in the process of learning and using categories 22 an item in a category can be easier to recall if the category has a cue for the memory as discussed labels and themes both function as cues for categories and therefore aid in the memory of these categories and the features of the objects belonging to them there are several brain structures at work that promote learned categorical perception the areas and structures involved include neurons the prefrontal cortex and the inferotemporal cortex 20 23 neurons in general are linked to all processes in the brain and therefore facilitate learned categorical perception they send the messages between brain areas and facilitate the visual and linguistic processing of the category the prefrontal cortex is involved in forming strong categorical representations 20 the inferotemporal cortex has cells that code for different object categories and are turned along diagnostic category dimensions areas distinguishing category boundaries 20 the learning of categories and categorical perception can be improved through adding verbal labels making themes relevant to the self making more separate categories and by targeting similar features that make it easier to form and define categories learned categorical perception occurs not only in human species but has been demonstrated in animal species as well studies have targeted categorical perception using humans monkeys rodents birds frogs 23 24 these studies have led to numerous discoveries they focus primarily on learning the boundaries of categories where inclusion begins and ends and they support the hypothesis that categorical perception does have a learned component computational and neural models edit computational modeling tijsseling harnad 1997 damper harnad 2000 has shown that many types of category learning mechanisms e g both back propagation and competitive networks display cp like effects 25 26 in back propagation nets the hidden unit activation patterns that represent an input build up within category compression and between category separation as they learn other kinds of nets display similar effects cp seems to be a means to an end inputs that differ among themselves are compressed onto similar internal representations if they must all generate the same output and they become more separate if they must generate different outputs the network s bias is what filters inputs...