Do Backward Associations Have Anything to Say About Language?

In this letter, we argue against a recurring idea that early word learning in infants is related to the low-level capacity for backward associations-a notion that suggests a cognitive gap with other animal species. Because backward associations entail the formation of bidirectional associations between sequentially perceived stimulus pairs, they seemingly mirror the label-referent bidirectional mental relations underlying the lexicon of natural language. This appealing but spurious resemblance has led to various speculations on language acquisition, in particular regarding early word learning, which we deconstruct here.

Simultaneous learning of directional and non-directional stimulus relations in baboons (Papio papio).

While humans exposed to a sequential stimulus pairing A-B are commonly assumed to form a bidirectional mental relation between A and B, evidence that non-human animals can do so is limited. Careful examination of the animal literature suggests possible improvements in the test procedures used to probe such effects, notably measuring transfer effects on the learning of B-A pairings, rather than direct recall of A upon cuing with B. We developed such an experimental design and tested 20 Guinea baboons (Papio papio). Two pairings of visual shapes were trained (A1-B1, A2-B2) and testing was conducted in a reversed order, either with conserved pairings (B1-A1, B2-A2) or broken ones (B1-A2, B2-A1). We found baboons' immediate test performance to be above chance level for conserved pairings and below chance level for broken ones. Moreover, baboons needed less trials to learn conserved pairings compared to broken ones. These effects were apparent for both pairings on average, and separately for the best learned pairing. Baboons' responding on B-A trials was thus influenced by their previous A-B training. Performance level at the onset of testing, however, suggests that baboons did not respond in full accordance with the hypothesis of bidirectionality. To account for these data, we suggest that two competing types of relations were concomitantly encoded: a directional relation between A and B, which retains the sequential order experienced, and a non-directional relation, which retains only the co-occurrence of events, not their temporal order.

Associative symmetry: a divide between humans and nonhumans?

Anthropocentrism can bias scientific conclusions. As a case study, we challenge the 40-year-old associative symmetry dogma, supposed to cognitively set apart humans from other species. Out of 37 human studies surveyed, only three truly demonstrate symmetry, of which only one (on five participants) suggests that symmetry is spontaneously formed.

Is symmetry inference an essential component of language?

Symmetry inference-that is, spontaneously deriving the stimulus association B-A from A-B-was recently reported in preverbal infants (Kabdebon & Dehaene-Lambertz, 2019, Proceedings of the National Academy of Sciences of the United States of America, 116[12], 5805-5810) and regarded as a "building block for human cognition." Here, we argue that empirical evidence supporting this claim is insufficient, and that absence of symmetry inference in nonhuman animals should be reassessed.

Whole-head recording of chemosensory activity in the marine annelid Platynereis dumerilii.

Chemical detection is key to various behaviours in both marine and terrestrial animals. Marine species, though highly diverse, have been underrepresented so far in studies on chemosensory systems, and our knowledge mostly concerns the detection of airborne cues. A broader comparative approach is therefore desirable. Marine annelid worms with their rich behavioural repertoire represent attractive models for chemosensation. Here, we study the marine worm Platynereis dumerilii to provide the first comprehensive investigation of head chemosensory organ physiology in an annelid. By combining microfluidics and calcium imaging, we record neuronal activity in the entire head of early juveniles upon chemical stimulation. We find that Platynereis uses four types of organs to detect stimuli such as alcohols, esters, amino acids and sugars. Antennae are the main chemosensory organs, compared to the more differentially responding nuchal organs or palps. We report chemically evoked activity in possible downstream brain regions including the mushroom bodies (MBs), which are anatomically and molecularly similar to insect MBs. We conclude that chemosensation is a major sensory modality for marine annelids and propose early Platynereis juveniles as a model to study annelid chemosensory systems.