Dissociating detection from localization of tactile stimuli.

In what they described as "a tactile analog of blindsight," Paillard et al. (1983) reported the case of a woman with damage to the left parietal cortex who was profoundly impaired in detecting tactile stimuli but could nonetheless correctly identify their location (also Rossetti et al., 1995). This stands in direct contrast to reports of neurological patients who were unable to accurately locate stimuli that they could successfully detect (Head and Holmes, 1911; Halligan et al., 1995; Rapp et al., 2002). The combination of these findings suggests that detecting and locating tactile stimuli are doubly dissociable processes, presumably mediated by different neural structures. We conducted four psychophysical experiments seeking evidence for such a double dissociation in neurologically intact subjects. We compared people's accuracy in detecting versus locating a tactile stimulus presented to one of four fingers and followed by a vibrotactile mask presented to all four fingers. Accuracy scores for both the yes-no detection and four-alternative forced-choice location judgments were converted to a bias-free measure (d'), which revealed that subjects were better at detecting than locating the stimulus. Detection was also more sensitive than localization to manipulations involving the mask: detection accuracy increased more steeply than localization accuracy as the target-mask interval increased, and detection, but not localization, was affected by changes in the mask frequency. By comparing these results with simulated data generated by computational models, we conclude that detection and localization are not mutually independent as previous neurological studies might suggest, but rather localization is subsequent to detection in a serially organized sensory processing hierarchy.