Hello from the other side: Robust contralateral interference in tactile detection.

Touch is unique among the sensory modalities in that our tactile receptors are spread across the body surface and continuously receive different inputs at the same time. These inputs vary in type, properties, relevance according to current goals, and, of course, location on the body. Sometimes, they must be integrated, and other times set apart and distinguished. Here, we investigate how simultaneous stimulation to different body sites affects tactile cognition. Specifically, we characterized the impact of irrelevant tactile sensations on tactile change detection. To this end, we embedded detection targets amidst ongoing performance, akin to the conditions encountered in everyday life, where we are constantly confronted with new events within ongoing stimuli. In the set of experiments presented here, participants detected a brief intensity change (.04 s) within an ongoing vibrotactile stimulus (1.6 s) that was always presented in a constantly attended location. The intensity change (i.e., the detection target) varied parametrically, from hardly detectable to easily detectable. In half of the trials, irrelevant ongoing stimulation was simultaneously presented to a site across the body midline, but participants were instructed to ignore it. In line with previous bimanual studies employing brief onset targets, we document robust interference on performance due to the irrelevant stimulation at each of the measured body sites (homologous and nonhomologous fingers, and the contralateral ankle). After describing this basic phenomenon, we further examine the conditions under which such interference occurs in three additional tasks. In each task, we honed in on a different aspect of the stimulation protocol (e.g., hand distance, the strength of the irrelevant stimulation, the detection target itself) in order to better understand the principles governing the observed interference effects. Our findings suggest a minimal role for exogenous attentional capture in producing the observed interference effects (Exp. 2), and a principled distribution of attentional resources or sensory integration between body sides (Exps. 3, 4). In our last study (Exp. 4), we presented bilateral tactile targets of varying intensities to both the relevant and irrelevant stimulation sites. We then characterized the degree to which the irrelevant stimulation is also processed. Our results-that participants' perception of target intensity is always proportional to the combined bilateral signal-suggest that both body sites are equally weighed and processed despite clear instructions to attend only the target site. In light of this observation and participants' inability to use selection processes to guide their perception, we propose that bilateral tactile inputs are automatically combined, quite possibly early in the hierarchy of somatosensory processing.

The timing of cortical activation in associator grapheme-colour synaesthetes using MEG.

Grapheme-colour synaesthetes experience an anomalous form of perception in which graphemes systematically induce specific colour concurrents in their mind's eye ("associator" type). Although grapheme-colour synaesthesia has been well characterised behaviourally, its neural mechanisms remain largely unresolved. There are currently several competing models, which can primarily be distinguished according to the anatomical and temporal predictions of synaesthesia-inducing neural activity. The first main model (Cross-Activation/Cascaded Cross-Tuning and its variants) posits early recruitment of occipital colour areas in the initial feed-forward sweep of brain activity. The second (Disinhibited Feedback) posits: (i) later involvement of a multisensory convergence zone (for example, in parietal cortices) after graphemes have been processed in their entirety; and (ii) subsequent feedback to early visual areas (i.e., occipital colour areas). In this study, we examine both the timing and anatomical correlates of associator grapheme-colour synaesthetes (n = 6) using MEG. Using innovative and unbiased analysis methods with little a priori assumptions, we applied Independent Component Analysis (ICA) on a single-subject level to identify the dominant patterns of activity corresponding to the induced, synaesthetic percept. We observed evoked activity that significantly dissociates between synaesthesia-inducing and non-inducing graphemes at approximately 190 ms following grapheme presentation. This effect is present in grapheme-colour synaesthetes, but not in matched controls, and exhibits an occipito-parietal topology localised consistently within individuals to extrastriate visual cortices and superior parietal lobes. Due to the observed timing of this evoked activity and its localization, our results support a model predicting relatively late synaesthesia-inducing activity, more akin to the Disinhibited Feedback model.

When Temporal Certainty Doesn't Help.

In a dynamically changing environment, the ability to capture regularities in our sensory input helps us generate predictions about future events. In most sensory systems, the basic finding is clear: Knowing when something will happen improves performance on it [Nobre, A. C., & van Ede, F. (2017). Anticipated moments: Temporal structure in attention. Nature Reviews Neuroscience, 19, 34-48, 2017]. We here examined the impact of temporal predictions on a less-explored modality: touch. Participants were instructed to detect a brief target embedded in an ongoing vibrotactile stimulus. Unbeknownst to them, the experiment had two timing conditions: In one part, the time of target onset was fixed and thus temporally predictable, whereas in the other, it could appear at a random time within the ongoing stimulation. We found a clear modulation of detection thresholds due to temporal predictability: Contrary to other sensory systems, detecting a predictable tactile target was worse relative to unpredictable targets. We discuss our findings within the framework of tactile suppression.

Formation of automatic letter-colour associations in non-synaesthetes through likelihood manipulation of letter-colour pairings.

Grapheme-colour synaesthesia is a well-characterized phenomenon in which achromatic letters and/or digits automatically and systematically trigger specific colour sensations. Models of its underlying mechanisms diverge on a central question: whether triggered sensations reflect (1) an overdeveloped capacity in normal cross-modal processing (i.e., sharing characteristics with the general population), or rather (2) qualitatively deviant processing (i.e., unique to a few individuals). To test to what extent synaesthesia-like (automatic) letter-colour associations may be learned by non-synaesthetes into adulthood, implied by (1), we developed a learning paradigm that aimed to implicitly train such associations via a visual search task that employed statistical probability learning of specific letter-colour pairs. In contrast to previous synaesthesia-training studies (Cohen Kadosh, Henik, Catena, Walsh, & Fuentes, 2009; Meier & Rothen, 2009), here all participants were naïve as to the end-goal of the experiment (i.e., the formation of letter-colour associations), mimicking the learning conditions of acquired grapheme-colour synaesthesia (Hancock, 2006; Witthoft & Winawer, 2006). In two experiments, we found evidence for significant binding of colours to letters by non-synaesthetes. These newly-formed associations showed synaesthesia-like characteristics, because they correlated in strength with performance on individual synaesthetic Stroop-tasks (experiment 1), and because interference between the learned (associated) colour and the real colour during letter processing depended on their relative positions in colour space (opponent vs. non-opponent colours, experiment 2) suggesting automatic formation on a perceptual rather than conceptual level, analogous to synaesthesia. Although not evoking conscious colour percepts, these learned, synaesthesia-like associations in non-synaesthetes support that common mechanisms may underlie letter-colour associations in synaesthetes and non-synaesthetes.

Superior facial expression, but not identity recognition, in mirror-touch synesthesia.

Simulation models of expression recognition contend that to understand another's facial expressions, individuals map the perceived expression onto the same sensorimotor representations that are active during the experience of the perceived emotion. To investigate this view, the present study examines facial expression and identity recognition abilities in a rare group of participants who show facilitated sensorimotor simulation (mirror-touch synesthetes). Mirror-touch synesthetes experience touch on their own body when observing touch to another person. These experiences have been linked to heightened sensorimotor simulation in the shared-touch network (brain regions active during the passive observation and experience of touch). Mirror-touch synesthetes outperformed nonsynesthetic participants on measures of facial expression recognition, but not on control measures of face memory or facial identity perception. These findings imply a role for sensorimotor simulation processes in the recognition of facial affect, but not facial identity.

Phasic auditory alerting improves visual conscious perception.

Attention is often conceived as a gateway to consciousness (Posner, 1994). Although endogenous spatial attention may be independent of conscious perception (CP) (Koch Tsuchiya, 2007), exogenous spatial orienting seems instead to be an important modulator of CP (Chica, Lasaponara, Lupiáñez, Doricchi, & Bartolomeo, 2010; Chica, Lasaponara, et al., 2011). Here, we investigate the role of auditory alerting in CP in normal observers. We used a behavioral task in which phasic alerting tones were presented either at unpredictable or at predictable time intervals prior to the occurrence of a near-threshold visual target. We find, for the first time in neurologically intact observers, that phasic alertness increases CP, both objectively and subjectively. This result is consistent with evidence showing that phasic alerting can ameliorate the spatial bias exhibited by visual neglect patients (Robertson, Mattingley, Rorden, & Driver, 1998). The alerting network may increase the activity of fronto-parietal networks involved in top-down amplification required to bring a stimulus into consciousness (Dehaene, Changeux, Naccache, Sackur, & Sergent, 2006).