Schematic diagram of experimental setup and procedure.

A: Experimental setup. Participants performed a pointing task on one of the targets presented in the workspace in front of their body. B: Experimental procedure. After pre-cueing and delay, participants reached the target with their right index finger as quickly as possible.</p

Movement time in each movement direction relative to index of difficulty.

The movement time according to the change in distance is shown by solid lines, whereas that according to the target size is shown by broken lines. The color and type of each line correspond to those shown in Fig 2.</p

Reaction time relative to movement direction.

Human body movements are based on the intrinsic trade-off between speed and accuracy. Fitts’s law (1954) shows that the time required for movement is represented by a simple logarithmic equation and is applicable to a variety of movements. However, few studies have determined the role of the direction in modulating the performance of upper limb movements and the effects of the interactions between direction and distance and between direction and target size. This study examined the variations in temporal properties of the speed-accuracy trade-off in arm-pointing movements that directly manipulate objects according to the direction, distance, and target size. Participants performed pointing movements to the targets with 3 different sizes presented at 15 locations (5 directions and 3 distances) on a horizontal plane. Movement time (MT) for each trial in each condition was obtained. Subsequently, Mackenzie’s model (1992), MT = a + b log2(D/W +1), where D and W represent the distance and width of the target, respectively, was fitted. The slope factor b, a fitted parameter in the equation, was calculated and evaluated according to the changes in the direction, distance, and target size. The results showed that MTs exhibited anisotropy in the hemifield, being the smallest in the right-forward direction. Additionally, the slope factor b, as a function of distance, was smaller in the rightward direction than in the forward and left-forward directions. These results suggest that the degree of difficulty of upper limb movements expands heterogeneously in various directions around the body.</div

Index of difficulty calculated from each combined level of distance and target size.

Combinations of the distance and target size produced a distinct value of the index of difficulty (ID). For each ID, the movement time (MT) of the pointing movement was obtained. At each level of the distance and target size, MTs were fitted by Mackenzie’s model, then the values of slope factor b in each level, indicating the effects of target size (bSize_near, bSize_mid, and bSize_far) and distance (bDis_lag, bDis_mid, and bDis_sml) were obtained. Finally, bSize and bDis, showing generalized effect of target size and distance on MTs, was induced, respectively.</p

Coefficient of determination (R²) of each effect for each participant.

Coefficient of determination (R2) of each effect for each participant.</p

Slope factor b relative to movement direction.

Human body movements are based on the intrinsic trade-off between speed and accuracy. Fitts’s law (1954) shows that the time required for movement is represented by a simple logarithmic equation and is applicable to a variety of movements. However, few studies have determined the role of the direction in modulating the performance of upper limb movements and the effects of the interactions between direction and distance and between direction and target size. This study examined the variations in temporal properties of the speed-accuracy trade-off in arm-pointing movements that directly manipulate objects according to the direction, distance, and target size. Participants performed pointing movements to the targets with 3 different sizes presented at 15 locations (5 directions and 3 distances) on a horizontal plane. Movement time (MT) for each trial in each condition was obtained. Subsequently, Mackenzie’s model (1992), MT = a + b log2(D/W +1), where D and W represent the distance and width of the target, respectively, was fitted. The slope factor b, a fitted parameter in the equation, was calculated and evaluated according to the changes in the direction, distance, and target size. The results showed that MTs exhibited anisotropy in the hemifield, being the smallest in the right-forward direction. Additionally, the slope factor b, as a function of distance, was smaller in the rightward direction than in the forward and left-forward directions. These results suggest that the degree of difficulty of upper limb movements expands heterogeneously in various directions around the body.</div

Movement time relative to movement direction.

Human body movements are based on the intrinsic trade-off between speed and accuracy. Fitts’s law (1954) shows that the time required for movement is represented by a simple logarithmic equation and is applicable to a variety of movements. However, few studies have determined the role of the direction in modulating the performance of upper limb movements and the effects of the interactions between direction and distance and between direction and target size. This study examined the variations in temporal properties of the speed-accuracy trade-off in arm-pointing movements that directly manipulate objects according to the direction, distance, and target size. Participants performed pointing movements to the targets with 3 different sizes presented at 15 locations (5 directions and 3 distances) on a horizontal plane. Movement time (MT) for each trial in each condition was obtained. Subsequently, Mackenzie’s model (1992), MT = a + b log2(D/W +1), where D and W represent the distance and width of the target, respectively, was fitted. The slope factor b, a fitted parameter in the equation, was calculated and evaluated according to the changes in the direction, distance, and target size. The results showed that MTs exhibited anisotropy in the hemifield, being the smallest in the right-forward direction. Additionally, the slope factor b, as a function of distance, was smaller in the rightward direction than in the forward and left-forward directions. These results suggest that the degree of difficulty of upper limb movements expands heterogeneously in various directions around the body.</div