Unraveling the role of the Posterior Parietal Cortex in visuomotor mapping. Shlomi Haar, Opher Donchin and Ilan Dinstein. Response to: Multivariate fMRI ...
Unraveling the role of the Posterior Parietal Cortex in visuomotor mapping Shlomi Haar, Opher Donchin and Ilan Dinstein Response to: Multivariate fMRI Approaches to Flexible Sensorimotor Maps in Parietal Cortex (Journal Club article by Barany et al.)
It was a pleasure to read the thoughtful discussion by Barany and colleagues of our paper (Haar et al., 2015), and we especially enjoyed their in depth exploration regarding design and interpretation of fMRI experiments and the use of multi voxel pattern analysis (MVPA) for revealing sensorimotor movement encoding in parietal cortex. In our paper, we reported that patterns of fMRI voxel activation in medial intraparietal sulcus (mIPS) enabled accurate decoding of movement direction both before (baseline condition) and after subjects learned a 45° visuomotor rotation task (rotated condition). The patterns differed, however, such that across-condition decoding was at chance levels. This was only true of mIPS while other areas maintained visual or motor patterns. We interpreted these results as evidence for a change in the visuomotor encoding of movement directions by the underlying neural population in mIPS. In their commentary, Barany and colleagues discuss possible underlying physiological mechanisms that they suggest challenge our interpretation. First, they suggest that mIPS may contain two separate, overlapping neural populations: one encoding the visual target or movement cursor and another encoding motor variables. The change in the patterns would be simply due to activation of different combinations of visual and motor populations in the two conditions. This interpretation could be tested with "probe trials" in both baseline and rotated conditions. In these probe trials, the subject would make movements without visual feedback or observe identical visual input without moving. Such probe trials should reveal the directional encoding of the motor-only and visual-only neural populations. A second possibility raised by Barany was that our results arose because “the relative neural response patterns for visual and motor features vary across the time course of a single trial". This is similar to the first idea except that here the activations differ in time rather than space. Indeed, single unit studies showed differing temporal specificity in M1 and PMd neurons (Shen and Alexander, 1997a, 1997b). It may be possible to assess directional selectivity across different epochs using variable delays between target presentation and movement execution. Applying this approach, that has been used by a few fMRI studies (Gallivan et al., 2011, 2013; Ogawa and Inui, 2012), in a visuomotor rotation paradigm would double the time and thus reduce the number of trials and, consequently, our ability to do effective decoding. Importantly, the comparison to results in left-right flipped environment such as Ogawa and Inui (2012), suggested by Barany and colleagues, might be misleading since it is a very different task and with different adaptation process than rotation adaptation.
In sum, however, we believe that our interpretation still stands, despite different possible underlying physiological mechanisms. Our key finding is that adaptation learning changes something in PPC (relative activation of separate visual and motor populations and/or tuning in a single population of visuomotor neurons). Questions such as those raised by Barany et al. are precisely the ones that need to be asked in order to take this research further.
References Gallivan, J.P., McLean, D.A., Smith, F.W., and Culham, J.C. (2011). Decoding effectordependent and effector-independent movement intentions from human parieto-frontal brain activity. J. Neurosci. 31, 17149–17168. Gallivan, J.P., McLean, D.A., Flanagan, J.R., and Culham, J.C. (2013). Where one hand meets the other: limb-specific and action-dependent movement plans decoded from preparatory signals in single human frontoparietal brain areas. J. Neurosci. 33, 1991–2008. Haar, S., Donchin, O., and Dinstein, I. (2015). Dissociating Visual and Motor Directional Selectivity Using Visuomotor Adaptation. J. Neurosci. 35, 6813–6821. Ogawa, K., and Inui, T. (2012). Reference frame of human medial intraparietal cortex in visually guided movements. J. Cogn. Neurosci. 24, 171–182. Shen, L., and Alexander, G. (1997a). Preferential representation of instructed target location versus limb trajectory in dorsal premotor area. J. Neurophysiol. 77, 1195–1212. Shen, L., and Alexander, G. (1997b). Neural correlates of a spatial sensory-to-motor transformation in primary motor cortex. J. Neurophysiol. 77, 1171–1194.
