Why do movements take a characteristic amount of time, and whydo diseases that affect the reward system alter control of movements?Suppose that the purpose of any movement is to position ourbody in a more rewarding state. People and other animals discountfuture reward as a hyperbolic function of time. Here, we showthat across populations of people and monkeys there is a correlationbetween discounting of reward and control of movements. We considersaccadic eye movements and hypothesize that duration of a movementis equivalent to a delay of reward. The hyperbolic cost of thisdelay not only accounts for kinematics of saccades in adults,it also accounts for the faster saccades of children, who temporallydiscount reward more steeply. Our theory explains why saccadevelocities increase when reward is elevated, and why disordersin the encoding of reward, for example in Parkinson's diseaseand schizophrenia, produce changes in saccade. We show thatdelay of reward elevates the cost of saccades, reducing velocities.Finally, we consider coordinated movements that include motionof eyes and head and find that their kinematics is also consistentwith a hyperbolic, reward-dependent cost of time. Therefore,each voluntary movement carries a cost because its durationdelays acquisition of reward. The cost depends on the valuethat the brain assigns to stimuli, and the rate at which itdiscounts this value in time. The motor commands that move oureyes reflect this cost of time.
Oculomotor prediction of accelerative target motion during occlusion: long-term and short-term effects.
The present study examined the influence of long-term (i.e., between-trial) and short-term (i.e., within-trial) predictive mechanisms on ocular pursuit during transient occlusion. To this end, we compared ocular pursuit of accelerative and decelerative target motion in trials that were presented in random or blocked-order. Catch trials in which target acceleration was unexpectedly modified were randomly interleaved in blocked-order trials. Irrespective of trial order, eye velocity decayed following target occlusion and then recovered towards the different levels of target velocity at reappearance. However, the recovery was better scaled in blocked-order trials than random-order trials. In blocked-order trials only, the reduced gain of smooth pursuit during occlusion was compensated by a change in saccade amplitude and resulted in total eye displacement (TED) that was well matched to target displacement. Subsidiary analysis indicated that three repeats of blocked-order trials was sufficient for participants to modify eye displacement compared to that exhibited in random-order trials, although more trials were required before end-occlusion eye velocity was better scaled. Finally, we found that participants exhibited evidence of a scaled response to an unexpected change in target acceleration (i.e., catch trials), although there were also transfer effects from the preceding blocked-order trials. These findings are consistent with the suggestion that on-the-fly prediction (short-term effect) is combined with memorised information from previous trials (long-term effect) to generate a persistent and veridical prediction of occluded target motion.
Enclosed is my list of publications. Each of them is presented in a separate "blog post" so that each of them can be commented. Only comments with name and valid email address will be posted. Messages such as "you'll never be a good scientists will be discarded :-) "