Tactile Spatial Attention in the Human EEG

The author investigated mechanisms of sustained spatial attention in touch with respect to influences of task difficulty and task relevance. Sustained spatial attention was measured with somatosensory steady-state responses (SSSEPs), elicited by continuous vibrotactile stimuli, simultaneously presented to both index fingers. Participants attended to one finger and detected transient events, embedded in the ongoing vibration. It was shown that, (1) it is possible to extract the somatosensory evoked potential (SEP) elicited by the transient events concurrently with the SSSEP. (2) The neuronal processing of continuous vibrotactile stimuli and transient events are modulated by perceptual load (i.e. task difficulty). When participants detected rare and salient events (low load) in the SEP, attention enhanced the P100, but the N140 was greater for events presented at the unattended finger compared to the attended one. No attentional modulation was observed at the SSSEP. On the contrary, when participants performed a discrimination task (high load), attention increased the N140 and the SSSEP amplitude. (3) A competition might take place at the N140 between voluntary and involuntary attention, and that the pattern of attentional modulation indicates that only under the condition of low perceptual load transient stimuli at the unattended finger capture involuntary attention. However, the author found also that attention in touch is highly flexible. Attention enhanced the SSSEP amplitude when participants detected changes in frequency, but not when they detected changes in amplitude, although accuracy did not differ between the tasks. These results indicate that (4) the attentional modulation of the SSSEP depends not only on task difficulty, but also on the task relevance of certain stimulus properties. To conclude, this series of experiments has shown that spatial and non-spatial attention in touch operates and interacts in a highly flexible way and that the concurrent recording of SEPs and SSSEPs is a useful tool to examine the underlying transient and sustained processes.