Voluntary modulation of saccadic peak velocity associated with individual differences in motivation

Kinan Muhammed,Edwin Dalmaijer,Sanjay Manohar,Masud Husain

doi:10.1016/j.cortex.2018.12.001

Abstract

Saccadic peak velocity increases in a stereotyped manner with the amplitude of eye movements. This relationship, known as the main sequence, has classically been considered to be fixed, although several recent studies have demonstrated that velocity can be modulated to some extent by external incentives. However, the ability to voluntarily control saccadic velocity and its association with motivation has yet to be investigated. Here, in three separate experimental paradigms, we measured the effects of incentivisation on saccadic velocity, reaction time and preparatory pupillary changes in 53 young healthy participants. In addition, the ability to voluntarily modulate saccadic velocity with and without incentivisation was assessed. Participants varied in their ability to increase and decrease the velocity of their saccades when instructed to do so. This effect correlated with motivation level across participants, and was further modulated by addition of monetary reward and avoidance of loss. The findings show that a degree of voluntary control of saccadic velocity is possible in some individuals, and that the ability to modulate peak velocity is associated with intrinsic levels of motivation.

Highlights

To create a high definition image of the world, humans direct the photoreceptor-dense fovea to objects of interest
The ability to modulate saccadic peak velocity correlated with motivation level There was a significant correlation between the ability to modulate saccadic peak velocity and the degree of apathy scored on the Lille Apathy Rating Scale (LARS-e) questionnaire
The findings presented here reveal that saccadic velocity, at least to some extent, can be modulated voluntarily, thereby breaking the stereotyped main sequence relating saccade velocity to movement amplitude (Bahill et al, 1975) (Fig. 4A)

Summary

Introduction

To create a high definition image of the world, humans direct the photoreceptor-dense fovea to objects of interest. Velocity increases predictably with the amplitude of eye movements, a phenomenon referred to as the main sequence (Bahill, Clark, & Stark, 1975) This relationship is believed to reflect optimization of the trade-off between accuracy and duration of eye movements. Larger saccades require larger neural signals to increase speed and as a result generates extra neural noise, thereby reducing cortex 1 2 2 ( 2 0 2 0 ) 1 9 8 e2 1 2 accuracy (Harris & Wolpert, 1998, 2006). This speed-accuracy trade-off is thought to explain the predictability of saccadic velocity

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Conclusion