The Topography of Tactile Working Memory

To investigate the contribution of topographically organized brain areas to tactile working memory, we asked human subjects to compare the frequency of two vibrations presented to the same fingertip or to different fingertips. The vibrations ranged from 14 to 24 Hz and were separated by a retention interval of variable length. For intervals <1 sec, subjects were accurate when both vibrations were delivered to the same fingertip but were less accurate when the two vibrations were delivered to different fingertips. For 1 or 2 sec intervals, subjects performed equally well when comparing vibrations delivered either to the same finger or to corresponding fingers on opposite hands, but they performed poorly when the vibrations were delivered to distant fingers on either hand. These results suggest that working memory resides within a topographic framework. As a further test, we performed an experiment in which the two comparison vibrations were presented to the same fingertip but an interference vibration was presented during the retention interval. The interpolated vibration disrupted accuracy most when delivered to the same finger as the comparison vibrations and had progressively less effect when delivered to more distant fingers. We conclude that topographically organized regions of somatosensory cortex contribute to tactile working memory, possibly by holding the memory trace across the retention interval. One stimulus can be accurately compared with the memory of a previous stimulus if they engage overlapping representations, but activation of the common cortical territory by an interpolated stimulus can disrupt the memory trace.