A Shift of Visual Fixation Dependent On Background Illumination.
Shabtai Barash, Armenuhi Melikyan, Alexey Sivakov, and Michael Tauber.
Department of Neurobiology, Weizmann Institute of Science, Rehovot,
Israel.
APStracts 4:372N, 1997.
ABSTRACT
Visual fixation, the act of maintaining the eyes directed towards a location
of interest, is a highly-skilled behavior, necessary for high-level vision in
primates. In spite of its significance, visual fixation is not well
understood; it is not even clear what attributes of the visual input are used
to control fixation. Here we show, in four Macaca fascicularis monkeys, that
the position the eyes assume during fixation depends on the luminance of the
background. Dark background yields fixation positions that are shifted
upwards with respect to the fixation positions obtained with a dimly
illuminated, featureless background. This phenomenon was previously observed
in a nutshell by Snodderly (1987); here first we rigorously establish its
existence by testing proper controls. We then study the properties of this
upshift of the fixation position. We show that, although the size of the
upshift varies between monkeys, for all monkeys the upshift is larger than the
radius of the fovea. Hence, if the background is dim, the eyes are positioned
during fixation so that the target does not fall on the fovea. The size of
the upshift remains almost unchanged while the eyes fixate at different
orbital positions; thus the upshift is not caused by orbital mechanics. The
upshift is clearly present even at the first days of training, but with
additional training in fixation with dark background, the upshift increases in
size. The upshift rotates with the head. The upshift increases gradually
with decreasing levels of background luminosity. Luminosity, not visual
contrast, is indeed the primary variable determining the size of the upshift.
The contribution of a unit area of the retina to the upshift decreases as
inverse square root of distance from the target; therefore, it is the
perifoveal region of the retina that mostly contributes to the upshift, while
the far periphery has little influence. The upshift can be induced or be
canceled in the midst of a fixation by changing the background illumination;
hence, the upshift is indeed an attribute of the fixation control system.
Finally, the fixation-upshift studied here is different from a previously
reported upshift (Gnadt et al. 1991, White et al. 1994), of the end-points of
memory-guided saccades with respect to their target locations. These two
types of upshift add up to each other. In discussing the function of the
upshift we note a possible morphological analogue with the retinal rod
distribution. The upshift moves the line of gaze to a point intermediate
between the fovea and the "dorsal rod peak". The upshift may thus improve
visual acuity in scotopic conditions. The brain structure in which the
upshift is generated must be involved in both ocular control and visual
sensation. We consider several possibilities, of which we regard as the most
likely the cerebellum and superior colliculus.
Received 13 March 1997; accepted in final form 28 November 1997.
APS Manuscript Number J214-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 7 January 1998