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Forschungsarbeit

Post-saccadic eye stability

By Giulia Manca and Prof. Heiner Deubel (14.03.2011)

Let’s imagine being in front of the Mona Lisa in the Louvre Museum; fascinated by her smile, trying to decode its meaning. While we do so, our eyes move quickly, scanning the picture and briefly stopping on its most salient parts. Through this series of eye movements (saccades) and still periods (fixations), we perceive what is presented in our visual field. Saccades are an ubiquitous feature of vision, as reflected by the fact that normally we move our eyes 3-4 times per second. Three types of saccades are distinguishable, depending on how they are elicited: voluntary (when a target is deliberately selected), involuntary (when a target suddenly appears) and memory-guided (when a target was previously memorized).

The neural activity underlying saccades is very interesting. When the eye is at rest, all oculomotor neurons fire at a moderate rate. When generating saccades, they transiently switch to a high firing rate (related to the saccadic velocity and amplitude). Later they settle to a lower, but increased rate (related to the maintenance of the eye in the new position). The term pulse-step pattern is used to describe this innervation. Through this pattern we are able to move the eyes and to precisely end their movement. A mismatch in the pulse-step pattern results in the inability of precisely ending the saccade. This drifting of the eye at the end of the saccade is referred to as post saccadic drift (PSD). Interestingly, clinical observations and experimental lesion studies have shown that the saccadic system is able to adaptively recover from PSD.

Through the recordings of eye movements, Professor Deubel and I are addressing two related questions: first of all, to which extent it is possible to adaptively repair PSD and secondly whether this depends on how saccades are elicited (context specificity).

In the first place we studied the saccadic adaptation to PSD. The experiment is composed of three phases, (1) a pre-adaptation, in which normal saccadic parameters are determined, (2) a long adaptation and (3) a post-adaptation, in which the effect of adaptation is measured. The target is a white dot presented on a gray background, which subjects are asked to always follow. In phase 1 and 3 targets are stationary. To resemble the physiological PSD, in phase 2 the target appears and is systematically shifted away, at the end of each saccade. In all phases, the new trial starts with the target in a new location (see Fig. 1).

Figure 1. Representation of the trial structure.[Bildunterschrift / Subline]: Figure 1: Representation of the trial structure. - A: typical trial in the pre- and post-adaptation phase, the dot appears and subjects have to make an eye movement to the location of the target - B: typical trial in the adaptation phase: the dot appears, subjects have to make an eye movement to the location of the target, when they finish the saccade the dot starts moving in an exponential fashion, mimicking the PSD.

Results (see Fig. 2) showed that during the long adaptation phase subjects learned the movement pattern of the stimulus and adjusted their behavior to take the expected target drift into consideration. We calculated the stability of the eye after each saccade per block. As shown in the graph, in phase 1 the eyes make precise saccades and then stop moving, while in phase 3 the eyes produce a drift, even though the target is actually stable. Even if the effect (quantified by the amplitude of the eye drift at the end of the saccade) was small, it was significantly different between the two conditions.

Figure 2. Results.[Bildunterschrift / Subline]: Figure 2. Results. The graph shows the averaged movement of the eye after each saccade across blocks. The adaptation phase is depicted in light gray. The stability of the eye is much higher (lower PSD) before the adaptation phase, suggesting that the oculomotor system interiorized the movement of the target.

The second experiment aims at investigating the context specificity of saccadic adaptation. The experiment will consist of 5 different phases: (1) pre-adaptation, (2) voluntary saccades, (3) adaptation (on involuntary saccades), (4) test of voluntary saccades and (5) post-adaptation. It will be highly similar to the previous one, but phase 2 and 4 will allow us to determine the effect of the involuntary saccade adaptation on the voluntary saccades. We expect to find different rates of adaptation for the voluntary and involuntary saccades. Such a finding would hint at the presence of various mechanisms for the control of saccades in different context.

This would lead to the consideration that patients with brain damages may be able to adapt to different kind of saccades. It would be interesting to study their adaptations profiles in order to develop better rehabilitation techniques with the aim of repairing the specific mechanisms involved in the control of precision and stability of saccadic eye movements.


Giulia Manca
Giulia Manca
*1986, Milano

Stationen
  • Since 4/2009
  • Master of Science in Neuro-Cognitive Psychology, Ludwig-Maximilians-Universitaet Munich, Germany
  • 10/2008- 4/2009
  • Master of Science in Neuro-Cognitive Psychology, University of Milan
  • 2005-2008
  • Laurea di primo livello in Scienze Psicologiche (Bachelor Degree), University of Milan
  • 2003 - 2004
  • Exchange year in the USA with a scholarship “Intercultura/AFS”, Tom Bean High School, Texas
  • 2000-2005
  • Diploma classico progetto "Brocca" (High school diploma), Liceo Ginnasio Statale "G. Parini" Milan

Stipendien und Auslandsaufenthalte
  • Since 10/2008
  • Research assistant with Prof. Heiner Deubel (Psychology department Ludwig-Maximilians-Universitaet Munich, Germany)
  • 09/2009 - 10/2009
  • Research assistant with Prof. Gustavo Deco (Universitad Pompeu Fabra, Barcelona)
  • 01/2009 - 03/2009
  • Research assistant with Dr. Zhuanghua Shi (Psychology department Ludwig-Maximilians-Universitaet Munich, Germany)
  • 2007 - 2008
  • Research assistant with Dr. Gabriel Baud-Bovy LAPCO (Laboratory of Action, Perception and Cognition, University of Milan)

Veröffentlichungen
  • * “Modeling Vibrotactile Detection in Cortical Circuits”: NCP Master Program Poster Session II, Psychology department Ludwig-Maximilians-Universitaet Munich, Germany. 12/2009.
  • *“Motor synergies in the control of a time-varying force with a two- and three-finger grasp ”. Progress in Motor Control VII 2009. Université de la Mediterrané-Aix Marseille II Marseille, France. 07/2009.
  • *“Redundant Target Effect from Visual-Tactile Interaction: a RT Study.” NCP Master Program Poster Session I. Psychology department Ludwig-Maximilians-Universitaet Munich, Germany. 07/2009.