Role of the Medial Posterior Parietal Cortex in Orchestrating Attention and Reaching

Abstract

The interplay between attention, alertness, and motor planning is crucial for our manual interactions. To investigate the neural bases of this interaction and challenge the views that attention cannot be disentangled from motor planning, we instructed human volunteers of both sexes to plan and execute reaching movements while attending to the target, while attending elsewhere, or without constraining attention. We recorded reaction times to reach initiation and pupil diameter and interfered with the functions of the medial posterior parietal cortex (mPPC) with online repetitive transcranial magnetic stimulation to test the causal role of this cortical region in the interplay between spatial attention and reaching. We found that mPPC plays a key role in the spatial association of reach planning and covert attention. Moreover, we have found that alertness, measured by pupil size, is a good predictor of the promptness of reach initiation only if we plan a reach to attended targets, and mPPC is causally involved in this coupling. Different from previous understanding, we suggest that mPPC is neither involved in reach planning per se, nor in sustained covert attention in the absence of a reach plan, but it is specifically involved in attention functional to reaching.

Keywords: attention; posterior parietal cortex; pupil size; reaching; transcranial magnetic stimulation.

Figure 1.

A, Timeline of attention/reaching task. Fix, fixation time; cue, cue onset; plan, delay between cue onset and go signal; go, go signal (a small vertical line), Reaction time and reaching. The cue is depicted larger than the targets for the reader's convenience (real dimensions are stated in the Materials and Methods section) and colored in orange and blue (color-blinded people's convenience, real colors are stated in the Materials and Methods). The timeline is shown for MotorATN congruent trials (top), in which attention and motor plan were directed toward the same hemifield; for MotorATN incongruent trials (middle), in which attention and motor plan were directed toward opposite hemifields; and for Motor valid trials (bottom), in which attention was not constrained, the direction of the motor plan was instructed, and the go signal appeared in the same hemifield as the motor plan. The same timeline also applied to Motor invalid trials (not shown for conciseness). B, Types of trials, according to the information received by the central cue: MotorATN trials and Motor trials. The MotorATN trials were only valid (go signal in the target where attention was directed by the colored side of the cue) and could be congruent (attention and movement plan directed toward the same location) or incongruent (attention and movement plan aimed in opposite directions). The Motor trials could be valid (go signal in the target where the movement was planned) or invalid (go signal in the opposite target).

Figure 2.

A, Reaction times during Sham stimulation in the different types of trials of the TMS experiment. The bars represent standard error, and the asterisks represent significant (p < 0.05) post hoc comparisons. The gray lines connect points that represent the data of individual participants. These data show that the task elicited attention in the expected way. B, Reaction times of different types of valid trials in the different stimulation conditions (Sham, black; V1/V2, white; hV6A, gray). It is evident the effect of the stimulation in slowing down the detection of the go signal for reaching. This figure contains only valid trials (MotorATN congruent, MotorATN incongruent, and Motor unconstrained trials). Individual participants’ data are in Figure 3. Data regarding Motor invalid trials are shown in Figures 6 and 7.

Figure 3.

Mean population reaction times with data of individual participants in valid trials. The same conventions as in Figure 2.

Figure 4.

Pupil size dynamics during the plan epoch of the different trial types of valid trials in the TMS experiment and in the control experiment. Left, Pupil size is represented as baseline corrected values (see Materials and Methods). Different colors represent different stimulation conditions, and the yellow trace represents pupil size dynamics during the conditions of the control experiment with the cue of the same features of the corresponding TMS trial. The black thick lines represent the time when the pupil size of control trials was significantly different from the one during Sham stimulation. Right, Differential values between pupil size during each stimulation condition and pupil size of the control experiment are plotted over time. Pupil constriction is evident in all the trials, but it is more intense during the control trials, when participants paid attention to the cue. No effect of stimulation was found.

Figure 5.

Pupil responses predict reaching reaction times in congruent trials. Significant prediction after Sham stimulation (A), after V1/V2 stimulation (B), and nonsignificant prediction after hV6A stimulation (C). The light gray lines are linear regression fits to data per participant. The thick lines show the correlations pooled over all trials. *p ≤ 0.01; n.s., p > 0.05.

Figure 6.

Distribution of the reaction times of invalid trials. Leftward redirection of cover attention is impaired after hV6A stimulation. The same conventions as in Figure 2. Individual participants’ data are in Figure 7.

Figure 7.

Mean population reaction times with data of individual participants in invalid trials. The same conventions as in Figures 2 and 6.