Bryan D. Alvarez

White matter microstructure throughout the brain correlates with visual imagery in grapheme-color synesthesia.

In this study we show, for the first time, a correlation between the neuroanatomy of the synesthetic brain and a metric that measures behavior not exclusive to the synesthetic experience. Grapheme-color synesthetes (n=20), who experience colors triggered by viewing or thinking of specific letters or numbers, showed altered white matter microstructure, as measured using diffusion tensor imaging, compared with carefully matched non-synesthetic controls. Synesthetes had lower fractional anisotropy and higher perpendicular diffusivity when compared to non-synesthetic controls. An analysis of the mode of anisotropy suggested that these differences were likely due to the presence of more crossing pathways in the brains of synesthetes. Additionally, these differences in white matter microstructure correlated negatively, and only for synesthetes, with a measure of the vividness of their visual imagery. Synesthetes who reported the most vivid visual imagery had the lowest fractional anisotropy and highest perpendicular diffusivity. We conclude that synesthetes as a population vary along a continuum while showing categorical differences in neuroanatomy and behavior compared to non-synesthetes.

Two plus blue equals green: grapheme-color synesthesia allows cognitive access to numerical information via color.

In grapheme-color synesthesia, graphemes (e.g., numbers or letters) evoke color experiences. It is generally reported that the opposite is not true: colors will not generate experiences of graphemes or their associated information. However, recent research has provided evidence that colors can implicitly elicit symbolic representations of associated graphemes. Here, we examine if these representations can be cognitively accessed. Using a mathematical verification task replacing graphemes with color patches, we find that synesthetes can verify such problems with colors as accurately as with graphemes. Doing so, however, takes time: ~250 ms per color. Moreover, we find minimal reaction time switch-costs for switching between computing with graphemes and colors. This demonstrates that given specific task demands, synesthetes can cognitively access numerical information elicited by physical colors, and they do so as accurately as with graphemes. We discuss these results in the context of possible cognitive strategies used to access the information.

The interaction of synesthetic and print color and its relation to visual imagery.

Synesthetic color induced by graphemes is well understood to be an automatic perceptual phenomenon paralleling print color in some ways, but also differing in others. We addressed this juxtaposition by asking how synesthetes are affected by synesthetic and print colors that are the same. We tested two groups of grapheme–color synesthetes using a basic color-priming method in which a grapheme prime was presented, followed by a color patch (probe), the color of which was to be named as quickly and accurately as possible. The primes induced either no color, print color only, synesthetic color only, or both forms of color (e.g., a letter “A” printed in red that also triggered synesthetic red). As expected, responses to name the probe color were faster if it was congruent with the prime color than if it was incongruent. The new finding (Exp. 1) was that a prime that induced the same print and synesthetic colors led to substantially larger priming effects than did either type of color individually, an effect that could not be attributed to semantic priming (Exp. 2). In addition, the synesthesia effects correlated with a standard measure of visual imagery. These findings are discussed as being consistent with the hypothesis that print and synesthetic color converge on similar color mechanisms.

Shifting attention in viewer- and object-based reference frames after unilateral brain injury

The aims of the present study were to investigate the respective roles that object- and viewer-based reference frames play in reorienting visual attention, and to assess their influence after unilateral brain injury. To do so, we studied 16 right hemisphere injured (RHI) and 13 left hemisphere injured (LHI) patients. We used a cueing design that manipulates the location of cues and targets relative to a display comprised of two rectangles (i.e., objects). Unlike previous studies with patients, we presented all cues at midline rather than in the left or right visual fields. Thus, in the critical conditions in which targets were presented laterally, reorienting of attention was always from a midline cue. Performance was measured for lateralized target detection as a function of viewer-based (contra- and ipsilesional sides) and object-based (requiring reorienting within or between objects) reference frames. As expected, contralesional detection was slower than ipsilesional detection for the patients. More importantly, objects influenced target detection differently in the contralesional and ipsilesional fields. Contralesionally, reorienting to a target within the cued object took longer than reorienting to a target in the same location but in the uncued object. This finding is consistent with object-based neglect. Ipsilesionally, the means were in the opposite direction. Furthermore, no significant difference was found in object-based influences between the patient groups (RHI vs. LHI). These findings are discussed in the context of reference frames used in reorienting attention for target detection.

Object-based attention in patients with left and right hemisphere lesions

LBD 10/13 0123456789111 Background Egly, Driver & Rafal (1994, JEPG) revealed separable influences of spaceand object-based attention using a cueing paradigm. In addition to slowed contralesional (CONTRA) responses, patients with right brain damage (RBD; N=8) showed facilitatory object-based effects (OBEs) in both visual fields, whereas patients with left brain damage (LBD; N=5) only showed large CONTRA OBEs but no ipsilesional (IPSI) OBEs.