Modulation of prism adaptation by a shift of background in the monkey

Automatic encoding of a target position relative to a natural scene

We previously showed that the brain automatically represents a target position for reaching relative to a large square in the background. In the present study, we tested whether a natural scene with many complex details serves as an effective background for representing a target. In the first experiment, we used upright and inverted pictures of a natural scene. A shift of pictures significantly attenuated prism adaptation of reaching movements as long as they were upright. In one-third of participants, adaptation was almost completely cancelled whether the pictures were upright or inverted. It was remarkable that there were two distinct groups of participants, one who relies fully on the allocentric coordinate and the other who depended only when the scene was upright. In the second experiment, we examined how long it takes for a novel upright scene to serve as a background. A shift of the novel scene had no significant effects when it was presented for 500 ms before presenting a target, but significant effects were recovered when presented for 1,500 ms. These results show that a natural scene serves as a background against which a target is automatically represented once we spend 1,500 ms in the scene.NEW & NOTEWORTHY Prism adaptation of reaching was attenuated by a shift of natural scenes as long as they were upright. In one-third of participants, adaptation was fully canceled whether the scene was upright or inverted. When an upright scene was novel, it took 1,500 ms to prepare the scene for allocentric coding. These results show that a natural scene serves as a background against which a target is automatically represented once we spend 1,500 ms in the scene.

Short-latency allocentric control of saccadic eye movements

It is generally accepted that the neural circuits that are implicated in saccade control use retinotopically coded target locations. However, several studies have revealed that nonretinotopic representation is also used. This idea raises a question about whether nonretinotopic coding is egocentric (head or body centered) or allocentric (environment centered). In the current study, we hypothesized that allocentric coding may play a crucial role in immediate saccade control. To test this hypothesis, we used an immediate double-step saccade task toward two sequentially flashed targets with a frame in the background, and we examined whether the end point of the second saccade was affected by a transient shift of the background that participants were told to ignore. When the background was shifted transiently upward (or downward) during the flash of the second target, the second saccade generally erred the target downward (or upward), which was in the direction opposite to the shift of the background. The effect on the second saccade became significant within 150 ms after the frame was presented for decoding and was built up for 200 ms thereafter. When the second saccade was not adjusted, a small, corrective saccade followed within 300 ms. The effect scaled linearly with the shift size up to 3° for a noncorrective second saccade and up to 6° for a corrective saccade. The present results show that an allocentric location of a target is rapidly represented by the brain and used for controlling saccades.

NEW & NOTEWORTHY

We found that the saccade end point was shifted from the actual target position toward the direction expected from allocentric coding when a large frame in the background was transiently shifted during the period of target presentation. The effect occurred within 150 ms. The present study provides direct evidence that the brain rapidly uses allocentric coding of a target to control immediate saccades.

Error Signals in Motor Cortices Drive Adaptation in Reaching

Reaching movements are subject to adaptation in response to errors induced by prisms or external perturbations. Motor cortical circuits have been hypothesized to provide execution errors that drive adaptation, but human imaging studies to date have reported that execution errors are encoded in parietal association areas. Thus, little evidence has been uncovered that supports the motor hypothesis. Here, we show that both primary motor and premotor cortices encode information on end-point errors in reaching. We further show that post-movement microstimulation to these regions caused trial-by-trial increases in errors, which subsided exponentially when the stimulation was terminated. The results indicate for the first time that motor cortical circuits provide error signals that drive trial-by-trial adaptation in reaching movements.