Attention-dependent modulation of neural activity in primary motor cortex

Abstract

In this study we investigated the impact of attention on neural activity in human motor areas 4a and 4p by means of fMRI. We first performed a localizer scan during which our 12 right-handed subjects simply tapped with either their right or left index finger. From the activated regions within (anatomically defined) primary motor cortex, we chose the most inferior, posterior and medial region as area 4p, and the next more superior, anterior and lateral region as area 4a. In our main experiment we then looked for attention effects within these predefined areas. Our 12 subjects had to tap with a frequency of 2Hz (indicated by acoustic signals) either with their right, left or both index fingers while they either (a) had to do nothing more or (b) had to mentally subtract numbers or (c) had to concentrate on (one of) the tapping finger(s). For all 12 subjects it was possible with the functional localizer to identify two different non-overlapping areas 4p and 4a, with 4p being located more inferior, posterior and medial. In the main experiment, across all conditions, area 4a was found to be more active than area 4p. For right finger tapping, there was a significant interaction between hemisphere and region, with activity in area 4a being more strongly lateralized to the contralateral hemisphere. However, no attention-related modulations were observed during right finger tapping. For left finger tapping, on the other hand, there was a significant attention effect: when subjects had to mentally calculate during tapping, activity in both areas 4a and p decreased. When subjects were tapping with both fingers and concentrated on the right index fingers there was less of an increase of activity in ispilateral motor cortex than when they concentrated on their left index finger. Taken together, our data show that for right-handers there is an effect of attention on primary motor cortex activity only when they use their non-dominant left index finger. Although we were able to functionally separate areas 4p and 4a within primary motor cortex, we did not observe a difference across these areas regarding their controllability by attention.