Anina N. Rich

Unconscious priming eliminates automatic binding of colour and alphanumeric form in synaesthesia

Synaesthesia is an unusual perceptual phenomenon in which events in one sensory modality induce vivid sensations in another. Individuals may ‘taste’ shapes, ‘hear’ colours, or ‘feel’ sounds. Synaesthesia was first described over a century ago, but little is known about its underlying causes or its effects on cognition. Most reports have been anecdotal or have focused on isolated unusual cases. Here we report an investigation of 15 individuals with colour-graphemic synaesthesia, each of whom experiences idiosyncratic but highly consistent colours for letters and digits. Using a colour–form interference paradigm, we show that induced synaesthetic experiences cannot be consciously suppressed even when detrimental to task performance. In contrast, if letters and digits are presented briefly and masked, so that they are processed but unavailable for overt report, the synaesthesia is eliminated. These results show that synaesthetic experiences can be prevented despite substantial processing of the sensory stimuli that otherwise trigger them. We conclude that automatic binding of colour and alphanumeric form in synaesthesia arises after initial processes of letter and digit recognition are complete.

Anomalous perception in synaesthesia: A cognitive neuroscience perspective

An enduring question in cognitive neuroscience is how the physical properties of the world are represented in the brain to yield conscious perception. In most people, a particular physical stimulus gives rise to a unitary, unimodal perceptual experience. So, light energy leads to the sensation of seeing, whereas sound waves produce the experience of hearing. However, for individuals with the rare phenomenon of synaesthesia, specific physical stimuli consistently induce more than one perceptual experience. For example, hearing particular sounds might induce vivid experiences of colour, taste or odour, as might the sight of visual symbols, such as letters or digits. Here we review the latest findings on synaesthesia, and consider its possible genetic, neural and cognitive bases. We also propose a neurocognitive framework for understanding such anomalous perceptual experiences.

Neural correlates of imagined and synaesthetic colours

The experience of colour is a core element of human vision. Colours provide important symbolic and contextual information not conveyed by form alone. Moreover, the experience of colour can arise without external stimulation. For many people, visual memories are rich with colour imagery. In the unusual phenomenon of grapheme-colour synaesthesia, achromatic forms such as letters, words and numbers elicit vivid experiences of colour. Few studies, however, have examined the neural correlates of such internally generated colour experiences. We used functional magnetic resonance imaging (fMRI) to compare patterns of cortical activity for the perception of external coloured stimuli and internally generated colours in a group of grapheme-colour synaesthetes and matched non-synaesthetic controls. In a voluntary colour imagery task, both synaesthetes and non-synaesthetes made colour judgements on objects presented as grey scale photographs. In a synaesthetic colour task, we presented letters that elicited synaesthetic colours, and asked participants to perform a localisation task. We assessed the neural activity underpinning these two different forms of colour experience that occur in the absence of chromatic sensory input. In both synaesthetes and non-synaesthetes, voluntary colour imagery activated the colour-selective area, V4, in the right hemisphere. In contrast, the synaesthetic colour task resulted in unique activity for synaesthetes in the left medial lingual gyrus, an area previously implicated in tasks involving colour knowledge. Our data suggest that internally generated colour experiences recruit brain regions specialised for colour perception, with striking differences between voluntary colour imagery and synaesthetically induced colours.

Color Channels, Not Color Appearance or Color Categories, Guide Visual Search for Desaturated Color Targets

In this article, we report that in visual search, desaturated reddish targets are much easier to find than other desaturated targets, even when perceptual differences between targets and distractors are carefully equated. Observers searched for desaturated targets among mixtures of white and saturated distractors. Reaction times were hundreds of milliseconds faster for the most effective (reddish) targets than for the least effective (purplish) targets. The advantage for desaturated reds did not reflect an advantage for the lexical category “pink,” because reaction times did not follow named color categories. Many pink stimuli were not found quickly, and many quickly found stimuli were not labeled “pink.” Other possible explanations (e.g., linear-separability effects) also failed. Instead, we propose that guidance of visual search for desaturated colors is based on a combination of low-level color-opponent signals that is different from the combinations that produce perceived color. We speculate that this guidance might reflect a specialization for human skin.

Multimodal functional imaging of motor imagery using a novel paradigm

Neuroimaging studies have shown that the neural mechanisms of motor imagery (MI) overlap substantially with the mechanisms of motor execution (ME). Surprisingly, however, the role of several regions of the motor circuitry in MI remains controversial, a variability that may be due to differences in neuroimaging techniques, MI training, instruction types, or tasks used to evoke MI. The objectives of this study were twofold: (i) to design a novel task that reliably invokes MI, provides a reliable behavioral measure of MI performance, and is transferable across imaging modalities; and (ii) to measure the common and differential activations for MI and ME with functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG). We present a task in which it is difficult to give accurate responses without the use of either motor execution or motor imagery. The behavioral results demonstrate that participants performed similarly on the task when they imagined vs. executed movements and this performance did not change over time. The fMRI results show a spatial overlap of MI and ME in a number of motor and premotor areas, sensory cortices, cerebellum, inferior frontal gyrus, and ventrolateral thalamus. MI uniquely engaged bilateral occipital areas, left parahippocampus, and other temporal and frontal areas, whereas ME yielded unique activity in motor and sensory areas, cerebellum, precuneus, and putamen. The MEG results show a robust event-related beta band desynchronization in the proximity of primary motor and premotor cortices during both ME and MI. Together, these results further elucidate the neural circuitry of MI and show that our task robustly and reliably invokes motor imagery, and thus may prove useful for interrogating the functional status of the motor circuitry in patients with motor disorders.

Cross-modality correspondence between pitch and spatial location modulates attentional orienting

The brain constantly integrates incoming signals across the senses to form a cohesive view of the world. Most studies on multisensory integration concern the roles of spatial and temporal parameters. However, recent findings suggest cross-modal correspondences (eg high-pitched sounds associated with bright, small objects located high up) also affect multisensory integration. Here, we focus on the association between auditory pitch and spatial location. Surprisingly little is known about the cognitive and perceptual roots of this phenomenon, despite its long use in ergonomic design. In a series of experiments, we explore how this cross-modal mapping affects the allocation of attention with an attentional cuing paradigm. Our results demonstrate that high and low tones induce attention shifts to upper or lower locations, depending on pitch height. Furthermore, this pitch-induced cuing effect is susceptible to contextual manipulations and volitional control. These findings suggest the cross-modal interaction between pitch and location originates from an attentional level rather than from response mapping alone. The flexible contextual mapping between pitch and location, as well as its susceptibility to top–down control, suggests the pitch-induced cuing effect is primarily mediated by cognitive processes after initial sensory encoding and occurs at a relatively late stage of voluntary attention orienting.

High incidence of ‘synaesthesia for pain’ in amputees

Synaesthesia for pain is a phenomenon where a person experiences pain when observing or imagining another in pain. Anecdotal reports of this type of experience have most commonly occurred in individuals who have lost a limb. Distinct from phantom pain, synaesthesia for pain is triggered specifically in response to pain in another. Here, we provide the first preliminary investigation into synaesthesia for pain in amputees to determine the incidence and characteristics of this intriguing phenomenon. Self-referring amputees (n=74) answered questions on synaesthesia for pain within a broader survey of phantom pain. Of the participants, 16.2% reported that observing or imagining pain in another person triggers their phantom pain. Further understanding of synaesthesia for pain may provide a greater insight to abnormal empathic function in clinical populations as well as therapeutic intervention for at risk groups.

Aging and large-scale functional networks: White matter integrity, gray matter volume, and functional connectivity in the resting state

Healthy aging is accompanied by neurobiological changes that affect the brains functional organization and the individuals cognitive abilities. The aim of this study was to investigate the effect of global age-related differences in the cortical white and gray matter on neural activity in three key large-scale networks. We used functional-structural covariance network analysis to assess resting state activity in the default mode network (DMN), the fronto-parietal network (FPN), and the salience network (SN) of young and older adults. We further related this functional activity to measures of cortical thickness and volume derived from structural MRI, as well as to measures of white matter integrity (fractional anisotropy [FA], mean diffusivity [MD], and radial diffusivity [RD]) derived from diffusion-weighted imaging. First, our results show that, in the direct comparison of resting state activity, young but not older adults reliably engage the SN and FPN in addition to the DMN, suggesting that older adults recruit these networks less consistently. Second, our results demonstrate that age-related decline in white matter integrity and gray matter volume is associated with activity in prefrontal nodes of the SN and FPN, possibly reflecting compensatory mechanisms. We suggest that age-related differences in gray and white matter properties differentially affect the ability of the brain to engage and coordinate large-scale functional networks that are central to efficient cognitive functioning.

The role of conceptual knowledge in understanding synaesthesia: Evaluating contemporary findings from a "hub-and-spokes" perspective.

Synesthesia is a phenomenon in which stimulation in one sensory modality triggers involuntary experiences typically not associated with that stimulation. Inducing stimuli (inducers) and synesthetic experiences (concurrents) may occur within the same modality (e.g., seeing colors while reading achromatic text) or span across different modalities (e.g., tasting flavors while listening to music). Although there has been considerable progress over the last decade in understanding the cognitive and neural mechanisms of synesthesia, the focus of current neurocognitive models of synesthesia does not encompass many crucial psychophysical characteristics documented in behavioral research. Prominent theories of the neurophysiological basis of synesthesia construe it as a perceptual phenomenon and hence focus primarily on the modality-specific brain regions for perception. Many behavioral studies, however, suggest an essential role for conceptual-level information in synesthesia. For example, there is evidence that synesthetic experience arises subsequent to identification of an inducing stimulus, differs substantially from real perceptual events, can be akin to perceptual memory, and is susceptible to lexical/semantic contexts. These data suggest that neural mechanisms lying beyond the realm of the perceptual cortex (especially the visual system), such as regions subserving conceptual knowledge, may play pivotal roles in the neural architecture of synesthesia. Here we discuss the significance of non-perceptual mechanisms that call for a re-evaluation of the emphasis on synesthesia as a perceptual phenomenon. We also review recent studies which hint that some aspects of synesthesia resemble our general conceptual knowledge for object attributes, at both psychophysical and neural levels. We then present a conceptual-mediation model of synesthesia in which the inducer and concurrent are linked within a conceptual-level representation. This “inducer-to-concurrent” nexus is maintained within a supramodal “hub,” while the subjective (bodily) experience of its resultant concurrent (e.g., a color) may then require activation of “spokes” in the perception-related cortices. This hypothesized “hub-and-spoke” structure would engage a distributed network of cortical regions and may account for the full breadth of this intriguing phenomenon.

Spatial attention increases high-frequency gamma synchronisation in human medial visual cortex

Visual information processing involves the integration of stimulus and goal-driven information, requiring neuronal communication. Gamma synchronisation is linked to neuronal communication, and is known to be modulated in visual cortex both by stimulus properties and voluntarily-directed attention. Stimulus-driven modulations of gamma activity are particularly associated with early visual areas such as V1, whereas attentional effects are generally localised to higher visual areas such as V4. The absence of a gamma increase in early visual cortex is at odds with robust attentional enhancements found with other measures of neuronal activity in this area. Here we used magnetoencephalography (MEG) to explore the effect of spatial attention on gamma activity in human early visual cortex using a highly effective gamma-inducing stimulus and strong attentional manipulation. In separate blocks, subjects tracked either a parafoveal grating patch that induced gamma activity in contralateral medial visual cortex, or a small line at fixation, effectively attending away from the gamma-inducing grating. Both items were always present, but rotated unpredictably and independently of each other. The rotating grating induced gamma synchronisation in medial visual cortex at 30-70 Hz, and in lateral visual cortex at 60-90 Hz, regardless of whether it was attended. Directing spatial attention to the grating increased gamma synchronisation in medial visual cortex, but only at 60-90 Hz. These results suggest that the generally found increase in gamma activity by spatial attention can be localised to early visual cortex in humans, and that stimulus and goal-driven modulations may be mediated at different frequencies within the gamma range.