Effect of terminal accuracy requirements on temporal gaze-hand coordination during fast discrete and reciprocal pointings

Abstract

Background: Rapid discrete goal-directed movements are characterized by a well known coordination pattern between the gaze and the hand displacements. The gaze always starts prior to the hand movement and reaches the target before hand velocity peak. Surprisingly, the effect of the target size on the temporal gaze-hand coordination has not been directly investigated. Moreover, goal-directed movements are often produced in a reciprocal rather than in a discrete manner. The objectives of this work were to assess the effect of the target size on temporal gaze-hand coordination during fast 1) discrete and 2) reciprocal pointings.

Methods: Subjects performed fast discrete (experiment 1) and reciprocal (experiment 2) pointings with an amplitude of 50 cm and four target diameters (7.6, 3.8, 1.9 and 0.95 cm) leading to indexes of difficulty (ID = log2[2A/D]) of 3.7, 4.7, 5.7 and 6.7 bits. Gaze and hand displacements were synchronously recorded. Temporal gaze-hand coordination parameters were compared between experiments (discrete and reciprocal pointings) and IDs using analyses of variance (ANOVAs).

Results: Data showed that the magnitude of the gaze-hand lead pattern was much higher for discrete than for reciprocal pointings. Moreover, while it was constant for discrete pointings, it decreased systematically with an increasing ID for reciprocal pointings because of the longer duration of gaze anchoring on target.

Conclusion: Overall, the temporal gaze-hand coordination analysis revealed that even for high IDs, fast reciprocal pointings could not be considered as a concatenation of discrete units. Moreover, our data clearly illustrate the smooth adaptation of temporal gaze-hand coordination to terminal accuracy requirements during fast reciprocal pointings. It will be interesting for further researches to investigate if the methodology used in the experiment 2 allows assessing the effect of sensori-motor deficits on gaze-hand coordination.

Figure 1

Schematic of the experimental set up. See the text for more details.

Figure 2

Typical data of one representative subject for discrete pointing trials. (A) 3.7 bits ID condition. (B) 6.7 bits ID condition. Blue lines represent gaze velocity profiles whereas black lines represent hand velocity profiles. Note that ONSET latency was stable across ID conditions and that gaze was anchored on target before hand velocity peak.

Figure 3

Illustration of the methodological approach to compute ONSET and OFFSET latencies. The black line represents the contacts between the stylus and the targets. The grey line represents resultant gaze velocity in the vertical plane. Numerical marks are defined as follows: 1 = onset of gaze saccade; 2 = end of the preceding hand movement; 3 = onset of the considered hand movement. Note that OFFSET latency of the movement n-1 is positive (saccade n began before the end of movement n-1) whereas the OFFSET latency of the movement n is negative (saccade n + 1 began after the end of movement n). It can also be observed that ONSET latency for movement n is longer than ONSET latency for movement n + 1.

Figure 4

Illustration of the effect of ID on ONSET and OFFSET latencies for reciprocal pointing trials. Black squares represent ONSET latency whereas grey triangles represent OFFSET latency for the 12 subjects who performed the experiment 2. Error bars represent the standard deviation. Note that ONSET and OFFSET latencies significantly decreased with an increasing ID.

Figure 5

Typical data of one representative subject for reciprocal pointing trials. A and B: lower ID (3.7 bits). Figure 5A presents gaze (blue solid line) and hand (black dashed line) velocity profiles. The blue solid arrow represents gaze onset time and the black dashed arrow represents hand onset time for the same pointing. Figure 5B presents gaze velocity and targets contacts (dark square-like signals) for the same pointings. The blue solid arrow represents gaze onset time and the black solid arrow represents the end of the preceding hand movement. C and D: higher ID (6.7 bits). Figure 5C presents gaze (blue solid line) and hand (black dashed line) velocity profiles. The blue solid arrow represents gaze onset time and the black dashed arrow represents hand onset time for the same pointing. Figure 5D presents gaze velocity and targets contacts (dark square-like signals) for the same pointings. The blue solid arrow represents gaze onset time and the black solid arrow represents the end of the preceding hand movement. See the text for more details.

Figure 6

Illustration of the effect of ID on ONSET latencies for discrete and reciprocal pointings. Data from the 6 subjects who performed the two experiments, i.e. discrete and reciprocal pointings are presented. The solid line represents discrete pointing whereas the dashed line represents reciprocal pointing. Error bars represent the standard deviation. Note (i) that ID affects ONSET latency for reciprocal but not for discrete pointing (ii) and that values are significantly higher for discrete than for reciprocal pointing for all IDs.