Matthew W. Wagers

Adult-generated hippocampal and neocortical neurons in macaques have a transient existence

Previously we reported that new neurons are added to the hippocampus and neocortex of adult macaque monkeys. Here we compare the production and survival of adult-generated neurons and glia in the dentate gyrus, prefrontal cortex, and inferior temporal cortex. Twelve adult macaques were injected with the thymidine analogue BrdUrd, and the phenotypes of labeled cells were examined after 2 h, 24 h, 2 wk, 5 wk, 9 wk, and 12 wk by using the following immunocytochemical markers: for immature and mature neurons, class III β-tubulin (TuJ1); for mature neurons, neuronal nuclei; for astrocytes, glial fibrillary acidic protein; and for oligodendrocytes, 2′,3′-cyclic nucleotide 3′ phosphodiesterase. We found that the dentate gyrus had many more BrdUrd-labeled cells than either neocortical area. Furthermore, a greater percentage of BrdUrd-labeled cells expressed a neuronal marker in the dentate gyrus than in either neocortical area. The number of new cells in all three areas declined by 9 wk after BrdUrd labeling, suggesting that some of the new cells have a transient existence. BrdUrd-labeled cells also were found in the subventricular zone and in the white matter between the lateral ventricle and neocortex; some of the latter cells were double-labeled for BrdUrd and TuJ1. Adult neocortical neurogenesis is not restricted to primates. Five adult rats were injected with BrdUrd, and after a 3-wk survival time, there were cells double-labeled for BrdUrd and either TuJ1 or neuronal nuclei in the anterior neocortex as well as the dentate gyrus.

Functional Trade-Offs in White Matter Axonal Scaling

The brains of large mammals have lower rates of metabolism than those of small mammals, but the functional consequences of this scaling are not well understood. An attractive target for analysis is axons, whose size, speed and energy consumption are straightforwardly related. Here we show that from shrews to whales, the composition of white matter shifts from compact, slow-conducting, and energetically expensive unmyelinated axons to large, fast-conducting, and energetically inexpensive myelinated axons. The fastest axons have conduction times of 1–5 ms across the neocortex and <1 ms from the eye to the brain, suggesting that in select sets of communicating fibers, large brains reduce transmission delays and metabolic firing costs at the expense of increased volume. Delays and potential imprecision in cross-brain conduction times are especially great in unmyelinated axons, which may transmit information via firing rate rather than precise spike timing. In neocortex, axon size distributions can account for the scaling of per-volume metabolic rate and suggest a maximum supportable firing rate, averaged across all axons, of 7 ± 2 Hz. Axon size distributions also account for the scaling of white matter volume with respect to brain size. The heterogeneous white matter composition found in large brains thus reflects a metabolically constrained trade-off that reduces both volume and conduction time.

5: Grammatical Illusions and Selective Fallibility in Real-Time Language Comprehension

Grammatical constraints impose diverse requirements on the relations between words and phrases in a sentence. This chapter presents a preliminary profile of selective fallibility to grammatical illusions in language comprehension. It summarizes different mechanisms that comprehenders might use to access linguistic material in memory. These mechanisms present a trade-off between speed and structure-sensitivity of processing. The chapter surveys grammatical phenomena where comprehenders appear to show impressive online sensitivity (island constraints on unbounded dependencies, backwards anaphora and Principle C, and constraints on reflexives). It describes cases where comprehenders are susceptible to grammatical illusion (subject-verb agreement, case licensing, negative polarity item licensing, and comparatives). The antilocality constraint on pronouns (Principle B) is included, although its status remains uncertain. The chapter attempts a preliminary synthesis of findings on the effects of grammatical constraints on real-time language processing. Keywords: anaphora; antilocality constraint; case licensing; grammatical illusions; real-time language comprehension; selective fallibility; subject-verb agreement

Multiple dependencies and the role of the grammar in real-time comprehension

Wh-dependencies are known to be formed rapidly in real-time comprehension. The parser posits the location of gap sites in advance of the bottom-up evidence for missing constituents, and must therefore have a means of deciding when and where to project dependencies. Previous studies have observed that the parser avoids building ungrammatical wh-dependencies, for example, by restricting the search for gap sites from island domains. This paper tests the stronger claim that constraints are not merely respected, but that grammatical knowledge actively prompts the construction of some representations in advance of the input. Three self-paced reading experiments examined patterns of wh-dependency formation in multiple-dependency constructions: obligatory across-the-board (ATB) extraction from coordinated verb phrases, and from optional parasitic gaps in post-verbal adjunct clauses. The key finding is that comprehenders immediately enforce the requirement for extraction from coordinates, and hence actively search for multiple gap sites within a coordinate VP; but they do not search for post-verbal parasitic gaps. This difference cannot be attributed to relative differences in acceptability, as comprehenders rated both of these multiple-gap constructions equally highly, nor can it be explained by general parsing incentives to develop maximal incremental interpretations of partial strings. More plausibly, the difference reflects the deployment of detailed grammatical knowledge in a parser that is motivated to satisfy structural licensing requirements in real time.

Going the distance: Memory and control processes in active dependency construction

Filler–gap dependencies make strong demands on working memory in language comprehension because they cannot always be immediately resolved. In a series of three reading-time studies, we test the idea that these demands can be decomposed into active maintenance processes and retrieval events. Results indicate that the fact that a displaced phrase exists and the identity of its basic syntactic category both immediately impact comprehension at potential gap sites. In contrast, specific lexical details of the displaced phrase show an immediate effect only for short dependencies and a much later effect for longer dependencies. We argue that coarse-grained information about the filler is actively maintained and is used to make phrase structure parsing decisions, whereas finer grained information is more quickly released from active maintenance and consequently has to be retrieved at the gap site.

The structure-sensitivity of memory access: evidence from Mandarin Chinese

The present study examined the processing of the Mandarin Chinese long-distance reflexive ziji to evaluate the role that syntactic structure plays in the memory retrieval operations that support sentence comprehension. Using the multiple-response speed-accuracy tradeoff (MR-SAT) paradigm, we measured the speed with which comprehenders retrieve an antecedent for ziji. Our experimental materials contrasted sentences where zijis antecedent was in the local clause with sentences where zijis antecedent was in a distant clause. Time course results from MR-SAT suggest that ziji dependencies with syntactically distant antecedents are slower to process than syntactically local dependencies. To aid in interpreting the SAT data, we present a formal model of the antecedent retrieval process, and derive quantitative predictions about the time course of antecedent retrieval. The modeling results support the Local Search hypothesis: during syntactic retrieval, comprehenders initially limit memory search to the local syntactic domain. We argue that Local Search hypothesis has important implications for theories of locality effects in sentence comprehension. In particular, our results suggest that not all locality effects may be reduced to the effects of temporal decay and retrieval interference.

Experimental Syntax and Island Effects: Deriving competing predictions from grammatical approaches and reductionist approaches to island effects

Marr (1982) famously proposed that our theories of information-processing devices can be usefully stated at multiple levels: the computational level, the representational-algorithmic level, and the implementational level. Marr described the computational level as an answer to the question “What problem must this device solve?” He argued that the computational level would specify the properties of the problem that must be solved by the device and the computations that the device must perform in service of that goal, in a way that abstracts away from the exigencies of actually solving the problem in practice. Marr used a cash register as an example: the computational-level description of a cash register comprises the theory of addition, including properties such as commutativity and associativity. However, at the computational level there is no statement of the procedure the device follows or the series of states it occupies to carry out addition. A theory at that level of description is a representationalalgorithmic theory. For a cash register this could be the addition algorithm that we all learn in school, implemented in base 10: start from the right, and “carry over the ones”; or it could be implemented in base 2, which a digital device would use. Finally, Marr described the implementational level as a theory of how the operations of the algorithmic level are implemented in the hardware of the device. For a cash register, there are several hardware options that can implement this level, from the spinning drums in mechanical cash registers to the electronic processors in computers. Extending the Marr framework to sentence-level language phenomena is relatively straightforward, at least in theory. Grammatical theories tend to be computational-level descriptions, as they describe the properties of the final grammatical structures that must be built, as well as the properties of the structure-building operations that are required to build them, but abstract away from the requirements of real-time sentence processing. Parsing theories tend to be algorithmic-level theories, as they describe the specific parsing

English Resumptive Pronouns are More Common where Gaps are Less Acceptable

Ā-dependencies occur when an argument appears clause-peripherally, dislocated from its canonical base position, as in relative clauses (1a). The displaced argument is a filler (italicized in (1) and throughout), and the base position is typically realized as a gap—that is, a syntactic category that is not pronounced (represented by an underscore throughout). A subset of languages that employ the filler-gap strategy, including Arabic (Aoun and Choueiri 1996), Irish (McCloskey 2006), Swedish (Engdahl 1986), and Vata (Koopman and Sportiche 1986), also use a second strategy to realize Ā-dependencies: resumption. Here, the base position is “filled” by an ordinary-looking resumptive pronoun (RP, boldfaced throughout).

Developing incrementality in filler-gap dependency processing

Much work has demonstrated that children are able to use bottom-up linguistic cues to incrementally interpret sentences, but there is little understanding of the extent to which childrens comprehension mechanisms are guided by top-down linguistic information that can be learned from distributional regularities in the input. Using a visual world eye tracking experiment and a corpus analysis, the current study investigates whether 5- and 6-year-old children incrementally assign interpretations to temporarily ambiguous wh-questions like What was Emily eating the cake with __? In the visual world eye-tracking experiment, adults demonstrated evidence for active dependency formation at the earliest region (i.e., the verb region), while 6-year-old children demonstrated a spill-over effect of this bias in the subsequent NP region. No evidence for this bias was found in 5-year-olds, although the speed of arrival at the ultimately correct instrument interpretation appears to be modulated by the vocabulary size. These results suggest that adult-like active formation of filler-gap dependencies begins to emerge around age 6. The corpus analysis of filler-gap dependency structures in adult corpora and child corpora demonstrate that the distributional regularities in either corpora are equally in favor of early, incremental completion of filler-gap dependencies, suggesting that the distributional information in the input is either not relevant to this incremental bias, or that 5-year-old children are somehow unable to recruit this information in real-time comprehension. Taken together, these findings shed light on the origin of the incremental processing bias in filler-gap dependency processing, as well as on the role of language experience and cognitive constraints in the development of incremental sentence processing mechanisms.

Grammatical licensing and relative clause parsing in a flexible word-order language

Evidence from two experiments reveals that in Chamorro, a verb-first language, the comprehension of relative clauses (RCs) is sensitive to the order of the RC with respect to the head. Unlike most other languages, Chamorro allows both postnominal and prenominal RCs, so it is possible to compare how the two types are processed within the same language. Moreover, Chamorro is a small language whose speakers do not fit the typical profile of participants in cognitive science experiments. We found that RC comprehension is affected by the relative order of RC and head, and by other language-specific factors. However, we also found new support for a subject gap advantage in all RC types. This advantage emerged in early response measures and was reinforced in postnominal RCs, but often outcompeted in prenominal RCs by other pressures. We frame this competition in terms of a model in which grammatical licensing requirements play a key role in comprehension.