Saccade-induced image motion cannot account for post-saccadic enhancement of visual processing in primate MST.

Shaun L Cloherty,Michael R Ibbotson,Nathan A Crowder,Michael J Mustari

doi:10.3389/fnsys.2015.00122

Abstract

Primates use saccadic eye movements to make gaze changes. In many visual areas, including the dorsal medial superior temporal area (MSTd) of macaques, neural responses to visual stimuli are reduced during saccades but enhanced afterwards. How does this enhancement arise—from an internal mechanism associated with saccade generation or through visual mechanisms activated by the saccade sweeping the image of the visual scene across the retina? Spontaneous activity in MSTd is elevated even after saccades made in darkness, suggesting a central mechanism for post-saccadic enhancement. However, based on the timing of this effect, it may arise from a different mechanism than occurs in normal vision. Like neural responses in MSTd, initial ocular following eye speed is enhanced after saccades, with evidence suggesting both internal and visually mediated mechanisms. Here we recorded from visual neurons in MSTd and measured responses to motion stimuli presented soon after saccades and soon after simulated saccades—saccade-like displacements of the background image during fixation. We found that neural responses in MSTd were enhanced when preceded by real saccades but not when preceded by simulated saccades. Furthermore, we also observed enhancement following real saccades made across a blank screen that generated no motion signal within the recorded neurons' receptive fields. We conclude that in MSTd the mechanism leading to post-saccadic enhancement has internal origins.

Highlights

Humans and their primate cousins make rapid, pre-planned eye movements approximately three times per second
In cases where eye movements occurred during either the stimulus delay or the open-loop phase the trial was omitted from the analysis presented below
We measured spiking responses in area MSTd and reflexive ocular following eye movements evoked by post-saccadic motion of a high contrast background texture

Summary

Introduction

Humans and their primate cousins make rapid, pre-planned eye movements approximately three times per second. Single-unit recordings in primates have shown that responses to visual stimulation are enhanced following a saccade compared to those during fixation. Studies of synchronous activity between neurons in V1 show increased correlations following saccadic eye movements (Maldonado et al, 2008) This increase in correlated activity precedes the post-saccadic increase in spike rate, suggesting that in addition to response amplitude, spike timing may play a key role in inter-saccadic processing of visual information. The available evidence suggests that saccadic eye movements do more than point the eyes in the right direction; they appear to set off a cascade of neural changes important for maximizing performance of the visual system

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