Task demands affect spatial reference frame weighting during tactile localization in sighted and congenitally blind adults

Abstract

Task demands modulate tactile localization in sighted humans, presumably through weight adjustments in the spatial integration of anatomical, skin-based, and external, posture-based information. In contrast, previous studies have suggested that congenitally blind humans, by default, refrain from automatic spatial integration and localize touch using only skin-based information. Here, sighted and congenitally blind participants localized tactile targets on the palm or back of one hand, while ignoring simultaneous tactile distractors at congruent or incongruent locations on the other hand. We probed the interplay of anatomical and external location codes for spatial congruency effects by varying hand posture: the palms either both faced down, or one faced down and one up. In the latter posture, externally congruent target and distractor locations were anatomically incongruent and vice versa. Target locations had to be reported either anatomically ("palm" or "back" of the hand), or externally ("up" or "down" in space). Under anatomical instructions, performance was more accurate for anatomically congruent than incongruent target-distractor pairs. In contrast, under external instructions, performance was more accurate for externally congruent than incongruent pairs. These modulations were evident in sighted and blind individuals. Notably, distractor effects were overall far smaller in blind than in sighted participants, despite comparable target-distractor identification performance. Thus, the absence of developmental vision seems to be associated with an increased ability to focus tactile attention towards a non-spatially defined target. Nevertheless, that blind individuals exhibited effects of hand posture and task instructions in their congruency effects suggests that, like the sighted, they automatically integrate anatomical and external information during tactile localization. Moreover, spatial integration in tactile processing is, thus, flexibly adapted by top-down information-here, task instruction-even in the absence of developmental vision.

Fig 1. Experimental setup.

Four vibro-tactile stimulators were attached to the palm and back of each hand (marked with white circles). The hands were either held in the same orientation with both palms facing downwards (A) or in different orientations with one hand flipped upside-down (B). In each trial, a target stimulus was randomly presented at one of the four locations. Simultaneously, a distractor stimulus was presented randomly at one of the two stimulator locations on the other hand. Target and distractor stimuli differed with respect to their vibration pattern. Participants were asked to localize the target stimulus as quickly and accurately as possible. For statistical analysis and figures, stimulus pairs presented to the same anatomical locations were defined as congruent, as illustrated by dashed (target) and dashed-dotted (distractor) circles, which both point to the back of the hand here. Note that with differently oriented hands (B) anatomically congruent locations are incongruent in external space and vice versa.

Fig 2. Accuracy in the tactile congruency task for factors Group, Instruction, Hand Posture, and Congruency (coded anatomically).

Data are collapsed over static and dynamic movement conditions, as this manipulation did not render any significant results (see main text for details). Sighted (1^st and 2^nd column) and congenitally blind participants (3^rd and 4^th column) were instructed to localize tactile targets either relative to their anatomical (1^st and 3^rd column) or relative to their external spatial location (2^nd and 4^th column). Hands were placed in the same (black circles) and in different orientations (grey triangles). Tactile distractors were presented to anatomically congruent (C) and incongruent (IC) locations of the other hand and had to be ignored. Congruency is defined in anatomical terms (see Fig 1). Accordingly, with differently oriented hands, anatomically congruent stimulus pairs are incongruent in external space and vice versa. Whiskers represent the standard error of the mean. Note, that we present percentage-correct values to allow a comparison to previous studies (see methods for details), whereas for statistical analysis a log-linked GLMM was applied to single trials accuracy values.

Fig 3. Individual participants’ tactile congruency effects in proportion correct.

Response proportions from anatomically incongruent trials were subtracted from response proportions in congruent trials. Congruency effects are plotted for dynamic (“Dyn.”) and static (“Stat.”) contexts with hands held in the same (1^st and 3^rd column) and in different orientations (2^nd and 4^th column) under anatomical (1^st and 2^nd column) and external instructions (3^rd and 4^th column) in the sighted (top row), and in the congenitally blind group (bottom row). Note that scales differ between groups, reflecting the smaller congruency effects of the blind as compared to the sighted group. Mean congruency effects for each condition are plotted in black, whiskers represent SEM. Each color represents one participant.