Is Recursion Uniquely Human? Hauser, Chomsky and Fitch (2002) Fitch and Hauser (2004)

Two Senses of the Faculty of Language Faculty of Language in a Broad Sense (FLB) –Sensory-Motor system: speech, hearing, gestures (for sign language), etc. –Conceptual-intentional system: ability to create beliefs, meanings, and intentions that can be conveyed through language –Computational system: ability to put words or linguistic units together in a way that fits with the grammar of a language (Faculty of Language in a Narrow Sense)

Two Senses of the Faculty of Language Faculty of Language in a Narrow Sense (FLN) –Only the basic computational elements of language (grammar) Discrete Infinity, the capacity for a finite number of small units to create an infinite number of sentences Recursion, the ability to embed units within each other and have a hierarchical structure of those units

Hypothesis Hauser, Chomsky, and Fitch argue that –FLN is uniquely human and arose since humans diverged from ape ancestor –FLB is shared by non-human animals, and has evolutionary history predating emergence of language

Comparative Approach to Language Evolution Necessary because fossils cannot provide evidence about language evolution Compares empirical data from living species to draw inferences about extinct ancestors –Important to be able to test claim that FLN is unique to humans

Data for FLB Shared by Other Animals Sensory-motor –Many species are capable of distinguishing between human speech sounds. –Some mammals without speech also have descended larynxes –Although birds and dolphins can imitate sounds, monkeys cannot Imitation in birds and dolphins is analogous to human evolution Ability to imitate in humans must have evolved after diverging from monkeys

Data for FLB Shared by Other Animals Conceptual-intentional –Some studies show that chimps have Theory of Mind (although this is disputed) –Mismatch between conceptual capacities of animals and the content of their communication –Differences between human and non-human communication: Limited repertoire of signals Not creative Generally appears early in development, with experience only necessary to know when signals apply

Data Supporting Uniqueness of FLN in Humans Seems clear that only humans have recursion, although what specific properties other animals lack is unclear Chimpanzee number studies –Show that chimps can assign meaning to symbols, but not relate the symbols to a recursive number sequence Study on grammar acquisition in human adults and tamarins by Fitch and Hauser

Computational Constraints on Syntactic Processing in a Nonhuman Primate Testing the abilities of cotton-top tamarins and human adults to master Finite State and Phrase Structure grammars Fitch and Hauser, 2004

Definitions Finite State grammar (FSG) –Places severe constraints on hierarchy (hierarchy is fixed) –Insufficient to generate the structures of any human language Phrase Structure grammar (PSG) –Can have unlimited embedding at different hierarchical levels –More powerful than FSGs (grammars above the FSG level are required for human languages) –Allows parts of a sentence that are related to be located far away from each other. e.g., The KIDS who play in our street ARE loud and dirty.

Two Test Grammars Researchers recorded two possible strings of consonant-vowel (CV) syllables –8 “A” syllables in a female voice –8 “B” syllables in a male voice The FSG was (AB) n, with a sequence of 2 or 3 AB syllables The PSG was A n B n, with n A syllables followed by n B syllables (in this grammar, each A syllable is connected to a B syllable through a hierarchical structure)

Test Grammars

Procedures The tamarins were separated into 2 groups, one for each grammar Each group had different sound strings from its grammar played to it for 20 mins. in the evening The next morning, after a re-familiarization period, monkeys were tested individually with the same 8 stimuli in random order –4 stimuli were consistent with the grammar in which the monkeys were trained, 4 were consistent with the other grammar Tamarins (like infants) were expected to look at the speaker if they heard something inconsistent with the grammar to which they had been trained. Latency and duration of looking at the speaker were recorded.

Cotton-top Tamarin

Procedures for Human Adults (Undergraduate Students) Same grammars and syllables as tamarins, tested in essentially the same way –Habituated to grammars in less than 3 minutes, instead of 20 –Instead of the looking paradigm, subjects were just told to indicate whether stimuli were “different” from the grammar to which they had been trained

Results

Implications While tamarins can process regular acoustic sequences, they cannot process simple phrase structure Tamarins have a computational limitation on the ability to recognize and remember hierarchically organized acoustic structures This hierarchical organization is one of the requirements for human language, and may represent an important evolution of human language faculty.