The neurophysiology of language processing shapes the evolution of grammar: evidence from case marking

Abstract

Do principles of language processing in the brain affect the way grammar evolves over time or is language change just a matter of socio-historical contingency? While the balance of evidence has been ambiguous and controversial, we identify here a neurophysiological constraint on the processing of language that has a systematic effect on the evolution of how noun phrases are marked by case (i.e. by such contrasts as between the English base form she and the object form her). In neurophysiological experiments across diverse languages we found that during processing, participants initially interpret the first base-form noun phrase they hear (e.g. she…) as an agent (which would fit a continuation like … greeted him), even when the sentence later requires the interpretation of a patient role (as in … was greeted). We show that this processing principle is also operative in Hindi, a language where initial base-form noun phrases most commonly denote patients because many agents receive a special case marker ("ergative") and are often left out in discourse. This finding suggests that the principle is species-wide and independent of the structural affordances of specific languages. As such, the principle favors the development and maintenance of case-marking systems that equate base-form cases with agents rather than with patients. We confirm this evolutionary bias by statistical analyses of phylogenetic signals in over 600 languages worldwide, controlling for confounding effects from language contact. Our findings suggest that at least one core property of grammar systematically adapts in its evolution to the neurophysiological conditions of the brain, independently of socio-historical factors. This opens up new avenues for understanding how specific properties of grammar have developed in tight interaction with the biological evolution of our species.

Fig 1. Ergative case marking.

Some languages identify agents of transitive (multi-argument) verbs (e.g. Ram in Ram sold a book) by a special ‘ergative’ marker. The map shows the proportion of such markers per language (N = 617): 100% (red) means that the ergative occurs in all grammatical subsystems (e.g. all tenses, all persons), 50% (white) in half of all subsystems, 0% (blue) in none. Shades of blue represent ergative presence between 0% and 50%. Salient proportions of ergative markers are limited to the Pacific (New Guinea/Australia/Oceania) and South America regions.

Fig 2. Event-related potentials (ERPs) at the end of a sentence in German.

Before the final verb form (‘has’ vs. ‘have’) has entered the parsing system, the initial noun phrase argument (‘Bertram’) can be understood either as an agent (A) or as a patient (P). When the final verb form disambiguates the initial noun phrase towards a P reading (literally, ‘that the surfers have congratulated Bertram’), this triggers a biphasic N400—late positivity (LPS) pattern (blue trace). No pattern is observed when the final verb form disambiguates towards an A reading (‘that Bertram has congratulated the surfers’). This suggests that an A reading is the working assumption of the system right from the start [16], in line with the hypothesis of the S/A preference discussed in the main text. Note: Negativity is plotted upwards.

Fig 3. Event-related potentials (ERPs) at the position of the disambiguating verb in the imperfective conditions in Hindi.

The figure shows grand average ERPs following a locally ambiguous (blue trace) or unambiguous (red trace) P argument at 11 selected electrodes. See Table 1 for examples. Disambiguation of an initial base-form (ambiguous) NP towards a P argument engenders an N400—late positivity (LPS) response, consistent with the S/A preference. Note: Negativity is plotted upwards.

Fig 4. Event-related potentials (ERPs) at the position of the disambiguating verb in the perfective conditions in Hindi.

The figure shows grand average ERPs following a locally ambiguous (blue trace) or unambiguous (red trace) P argument at 11 selected electrodes. See Table 1 for examples. Disambiguation of an initial base-form (ambiguous) NP towards a P argument engenders an N400—late positivity (LPS) response, consistent with the S/A preference. Note: Negativity is plotted upwards.

Fig 5. Grand-average of estimated evolutionary biases within families, across methods and taxonomies.

In each area, there are significantly more families that are estimated to be biased against the development or maintenance of ergatives (black tiles) than there are families that are estimated to be biased towards ergatives (white). Families without a bias (grey) provide no evidence on evolutionary biases in language change because they are compatible both with incipient developments towards ergatives or away from ergatives. Note: The sizes of tiles in the figure are proportional to the frequency of families tested (total N = 182).