Uj. Ilg et P. Thier, INABILITY OF RHESUS-MONKEY AREA V1 TO DISCRIMINATE BETWEEN SELF-INDUCED AND EXTERNALLY INDUCED RETINAL IMAGE SLIP, European journal of neuroscience, 8(6), 1996, pp. 1156-1166
Retinal image slip can result from an eye movement across a stationary
object or alternatively from motion of the object while the eyes are
stationary. The ability to discriminate between these two kinds of ret
inal image slip is necessary for the perception of a stable visual wor
ld. In order to determine if this ability is already present in monkey
visual area V1, we asked if single V1 units are able to differentiate
between externally and self-induced retinal image slip. Externally in
duced retinal image slip was realized in the 'object motion' condition
(OMC) by moving a behaviourally irrelevant visual stimulus ('object':
a bar or a large random dot pattern) across the receptive field while
the monkey fixated a small, stationary target. Conversely, self-induc
ed retinal image slip of comparable size was evoked in the 'ego motion
' condition (EMC) by asking the monkey to pursue the target, moving at
the speed of the object in the OMC, while the object was kept station
ary. We recorded 221 units from visual area V1, 51 (23%) of them direc
tionally selective, and compared their responses to self-induced and e
xternally induced retinal image slip. Many of them seemed to give some
preference to externally induced retinal image slip. However, on clos
er examination it became clear that this seeming preference could be a
ttributed to the fact that oculomotor performance was less precise in
the EMC than in the OMC, causing a larger deviation from the optimal r
etinal image trajectory in the EMC. We show that the impact of eye pos
ition errors can be eliminated by the use of a position-invariant stim
ulus, such as large-field random dot patterns. We then show that the i
mpact of both eye position errors and deviation of eye velocity from t
arget velocity in the EMC can be eliminated by moving the stimulus in
a given OMC trial according to an inverted replica of the eye movement
trajectory in the preceding EMC trial, guaranteeing identical retinal
stimulation in the OMC and the EMC. If identical retinal stimulation
was ensured, none of the V1 units tested was able to differentiate bet
ween externally and self-induced retinal image slip. We conclude that
V1 does not contribute to the perception of a world which is stable de
spite eye movements.