Abstract
Functional magnetic resonance imaging (fMRI) studies have provided ample evidence for the involvement of the lateral occipital cortex (LO), fusiform gyrus (FG), and intraparietal sulcus (IPS) in visuo-haptic object integration. Here we applied 30 min of sham (non-effective) or real offline 1 Hz repetitive transcranial magnetic stimulation (rTMS) to perturb neural processing in left LO immediately before subjects performed a visuo-haptic delayed-match-to-sample task during fMRI. In this task, subjects had to match sample (S1) and target (S2) objects presented sequentially within or across vision and/or haptics in both directions (visual-haptic or haptic-visual) and decide whether or not S1 and S2 were the same objects. Real rTMS transiently decreased activity at the site of stimulation and remote regions such as the right LO and bilateral FG during haptic S1 processing. Without affecting behavior, the same stimulation gave rise to relative increases in activation during S2 processing in the right LO, left FG, bilateral IPS, and other regions previously associated with object recognition. Critically, the modality of S2 determined which regions were recruited after rTMS. Relative to sham rTMS, real rTMS induced increased activations during crossmodal congruent matching in the left FG for haptic S2 and the temporal pole for visual S2. In addition, we found stronger activations for incongruent than congruent matching in the right anterior parahippocampus and middle frontal gyrus for crossmodal matching of haptic S2 and in the left FG and bilateral IPS for unimodal matching of visual S2, only after real but not sham rTMS. The results imply that a focal perturbation of the left LO triggers modality-specific interactions between the stimulated left LO and other key regions of object processing possibly to maintain unimpaired object recognition. This suggests that visual and haptic processing engage partially distinct brain networks during visuo-haptic object matching.
Highlights
An object’s geometrical structure and surface can be extracted by both using vision and haptics
We proposed that multisensory interactions are more likely for haptic than visual object recognition, and we, expected stronger effects of real repetitive transcranial magnetic stimulation (rTMS) for the matching of haptic as opposed to visual S2
We found real rTMS-induced increases in activations in lateral occipital complex (LOC) and intraparietal sulcus (IPS) related to matching of incongruent objects, which was behaviorally more difficult than matching of congruent objects
Summary
An object’s geometrical structure (shape) and surface can be extracted by both using vision and haptics. Lesions in occipitotemporal cortex can lead to haptic object agnosia (Morin et al, 1984; Feinberg et al, 1986) but see (Snow et al, 2012). Since both vision and haptics provide shape information, it has been proposed that the left LO comprises multisensory representations of object shape that are accessed by the different senses (Amedi et al, 2002). The left LO is typically not recruited by auditory object stimuli which do not provide any shape information unless subjects have learned to extract shape information from soundscapes produced by visual-to-auditory sensory substitution devices (Amedi et al, 2002, 2007)
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