Publications

2018

Liao, Zhang Y.S. Cai L.X. Ghazanfar A. A., D.A. “Internal States and Extrinsic Factors Both Determine Monkey Vocal Production..” PNAS (2018): n. pag. Print.

A key question for understanding speech evolution is whether or not the vocalizations of our closest living relatives—nonhuman primates—represent the precursors to speech. Some believe that primate vocalizations are not volitional but are instead inextricably linked to internal states like arousal and thus bear little resemblance to human speech. Others disagree and believe that since many primates can use their vocalizations strategically, this demonstrates a degree of voluntary vocal control. In the current study, we present a behavioral paradigm that reliably elicits different types of affiliative vocalizations from marmoset monkeys while measuring their heart rate fluctuations using noninvasive electromyography. By modulating both the physical distance between marmosets and the sensory information available to them, we find that arousal levels are linked, but not inextricably, to vocal production. Different arousal levels are, generally, associated with changes in vocal acoustics and the drive to produce different call types. However, in contexts where marmosets are interacting, the production of these different call types is also affected by extrinsic factors such as the timing of a conspecific’s vocalization. These findings suggest that variability in vocal output as a function of context might reflect trade-offs between the drive to perpetuate vocal contact and conserving energy.

2017

Takahashi D.Y., Ghazanfar A.A., Liao D.A. “Vocal Learning via Social Reinforcement by Infant Marmoset Monkeys.” Current Biology (2017): n. pag.
For over half a century now, primate vocalizations have been thought to undergo little or no experience-dependent acoustic changes during development. If any changes are apparent, then they are routinely (and quite reasonably) attributed to the passive consequences of growth. Indeed, previous experiments on squirrel monkeys and macaque monkeys showed that social isolation, deafness, cross-fostering and parental absence have little or no effect on vocal development. Here, we explicitly test in marmoset monkeys—a very vocal and cooperatively breeding species—whether the transformation of immature into mature contact calls by infants is influenced by contingent parental vocal feedback. Using a closed-loop design, we experimentally provided more versus less contingent vocal feedback to twin infant marmoset monkeys over their first 2 months of life, the interval during which their contact calls transform from noisy, immature calls to tonal adult-like “phee” calls. Infants who received more contingent feedback had a faster rate of vocal development, producing mature-sounding contact calls earlier than the other twin. The differential rate of vocal development was not linked to genetics, perinatal experience, or body growth; nor did the amount of contingency influence the overall rate of spontaneous vocal production. Thus, we provide the first experimental evidence for production-related vocal learning during the development of a nonhuman primate.

2014

Liao, Kronemer S.I. Yau J.M. Desmond J.E. Marvel C.L., D.A. “Motor System Contributions to Verbal and Non-Verbal Working Memory.” Frontiers in Human Neuroscience (2014): n. pag.
Working memory (WM) involves the ability to maintain and manipulate information held in mind. Neuroimaging studies have shown that secondary motor areas activate during WM for verbal content (e.g., words or letters), in the absence of primary motor area activation. This activation pattern may reflect an inner speech mechanism supporting online phonological rehearsal. Here, we examined the causal relationship between motor system activity and WM processing by using transcranial magnetic stimulation (TMS) to manipulate motor system activity during WM rehearsal. We tested WM performance for verbalizable (words and pseudowords) and non-verbalizable (Chinese characters) visual information. We predicted that disruption of motor circuits would specifically affect WM processing of verbalizable information. We found that TMS targeting motor cortex slowed response times (RTs) on verbal WM trials with high (pseudoword) vs. low (real word) phonological load. However, non-verbal WM trials were also significantly slowed with motor TMS. WM performance was unaffected by sham stimulation or TMS over visual cortex (VC). Self-reported use of motor strategy predicted the degree of motor stimulation disruption on WM performance. These results provide evidence of the motor system's contributions to verbal and non-verbal WM processing. We speculate that the motor system supports WM by creating motor traces consistent with the type of information being rehearsed during maintenance.