Labels as an Accelerator of Ontological Development

At a broad level, the strongest versions of linguistic determinism are simply untrue - in other words, language does not constrain or determine the possible range of thought. For example, individuals whose language has fewer color words than English are still capable of distinguishing among the same number of colors as English speakers. Yet, subtler forms of this hypothesis are more difficult to rule out. For example, language may transiently accelerate the developmental acquisition of certain conceptual and perceptual distinctions, although all adults may ultimately become capable of making such distinctions.

Xu's 2002 Cognition article explores this weaker view of linguistic determinism in the context of the sortal/kind distinction - in other words, the capacity of humans to distinguish between items such as "ball" and "cup." Xu reviews previous evidence showing that somewhere between 4 and 10 months, human infants begin to distinguish between objects on the basis of their location in space. However, only infants 12-months-old and older can show more finely-grained distinctions, such as those relying on object feature information. What evidence supports this claim?

If infants are presented with one object that emerges from and subsequentely disappears behind a screen, followed by a different object that is also briefly presented and then occluded, only 12-month-olds and up are surprised if the screen is removed to reveal a single object. It is as though younger infants represented merely that an object existed, whereas older infants can discriminate the two objects on the basis of their kinds.

But what if younger infants merely do not have the perceptual sensitivity to discriminate between the two objects? If this were true, such conclusions about infant's use of abstract categories would be premature. However, Xu reviews control studies demonstrating that infants are sensitive to the perceptual distinctions between objects at even younger ages. This suggests that the developmental change between 10 and 12 months is at the level of ontological categorization, not perceptual discrimination.

So, what drives this change? Some studies suggest that infants' increasing linguistic knowledge may be at the heart of this ontological development. Xu reviews previous work showing that the more words infants are judged to know, the more likely they are to show sortal/kind discrimination. To further investigate this hypothesis, Xu presents evidence from 4 original studies bearing on the idea that language is a key player in the development of ontological categories. The methodological details are in italics below:

Xu first habituated infants to two objects for 7 trials each, in which each object was brought out from behind the screen, tapped, and labeled. Some infants received a unique label for each object (i.e., the "two word" condition) while other infants received only a single label for each object (i.e., the "one word" condition). Next, one object was brought out from behind the screen, and then hidden again, followed by the other object. Then the screen was rotated to reveal either 1 (an possible outcome if objects are only distinguished by spatiotemporal appearance) or 2 objects (the possible outcome if objects were distinguished by kind or object features). This testing process was repeated for each infant 4 times, evenly split between the 1-object or 2-object outcome. Looking times were recorded by video and coded by research assistants blind to the particular condition each infant was in. Finally, in a baseline condition, another group of infants was presented with either one or two objects hidden behind a screen in order to get a baseline measurement of infants' looking times to one or two objects.

The results showed that 9-month-old infants in the two-word condition looked longer at the 1-object outcome than infants in the baseline condition. However, 9-month-olds who were given only a single word did not show any differences between the baseline condition and the 1-object outcome. In addition, giving a unique label to each object increased the looking-time during the habituation trails.

What if this happened merely because using unique auditory stimuli for the two objecfts increases arousal or attention? Two subsequent experiments were unable to replicate this effect using very distinct tones, suggesting that the benefit of labels is language-specific.

What if this happened merely because the sounds are made by a human, rather than being specific to language? A fourth experiment was unable to replicate the effect using nonlinguistic human utterances such as "ah" and "ew," again suggesting the benefit of labels is language and not source-specific.

Xu concludes that language can accelerate the process of sortal/kind discrimination, such that a skill normally only demonstrated by 12-month-olds was in this case demonstrated by 9-month-olds with the proper linguistic input. Xu next discusses four possible conclusions based on these results. Starting with Xu's more conservative conclusions, and moving to the more speculative ones:

1. Labels facilitate sortal/kind distinctions by aiding a domain-general, non-linguistic process, such as memory? According to this view, "labels function as 'summary representations' or mnemonics for the infants."
   
2. Labels increase the salience of perceptual feature differences between objects? According to this view, the use of two labels causes infants to pay more attention to the visual differences between objects, which then helps them demonstrate apparent sortal/kind distinctions. Xu argues that this explanation is unlikely given that the use of two-labels increased looking times relative to silent trials the same amount as the use of a single label.
3. Labels are "essence placeholders" that directly signal the presence of distinct sortal/kinds? Labels indicate that there are two object types present, which necessarily implies that there are two object tokens present. This then leads infants to show surprise if only a single object is revealed behind the screen.
4. Labels bind disparate representations in cognitive architecture? This view posits that infants interpret language according to two word-learning biases: the whole object bias (that one word refers to the totality of a contiguous object rather than its constituent parts) and the taxonomic constraint (a word for one kind of object can also be used for a different object of the same kind). These two biases require representations of the type "where" (residing in the dorsal visual processing stream) and of the type "what" (residing in the ventral visual processing stream). Therefore, the interpretation of a label accomplishes a binding function, directly analogous to the kinds of visual binding investigated by Wheeler & Treisman in adults.

All of these possibilities are interesting, but some are difficult to distinguish from others. For example, it is possible that labels serve as a memory crutch (as in conclusion #1) by binding disparate representations (conclusion #4). Similarly, it's difficult to know what makes an "essence placeholder" different from a memory crutch (#1) or from something that increases the salience of perceptual dimensions (#2).