Journal of Vision (2007) 7(8):1, 1-12
Schofield, Ledgeway, & Hutchinson
transfers between the three cues. We measured the
spatial-frequency tuning of each aftereffect. The within-
cue effect was strongest for LM:LM but was clearly
present for CM:CM and OM:OM. The aftereffect seen
with an LM test after adaptation to LM was narrowly
tuned. The CM:CM effect was also tuned, although less
sharply than the LM:LM case, whereas the OM:OM
aftereffect was untuned, although there was some evidence
for low-pass tuning suggesting that the system for
processing OM motion may have only a single low-pass
channel.
The transfer of aftereffects was clearly asymmetric.
Aftereffects were seen with both CM and OM tests
following adaptation to LM, although we note that the
percentage transfer was relatively low (especially for
P.D.J.) in these cases and that the spatial-frequency tuning
of the transferred aftereffects was relatively broad and less
consistent than that for the LM:LM aftereffect. In contrast,
adaptation to CM produced strong aftereffects only for
CM and OM tests with only a very weak effect noted for
LM tests. However, we note that the CM:OM effect was
strong and that this effect showed more spatial-frequency
tuning than the OM:OM case. Finally, only very weak
aftereffects were found with LM and CM tests following
adaptation to OM.
Our finding that the dMAE transfers asymmetrically
between cues is at odds with the findings of both Lu et al.
(1997), who found no transfer, and Nishida and Sato
(1995), who found that higher order cues can impose a
dMAE on LM tests. We note, however, that our stimuli
differ markedly from those used in these previous studies,
making direct comparisons difficult. Our results are also
somewhat at odds with the symmetric transfer of the tilt
and contrast-reduction aftereffects found for similar (but
static) cues (Cruickshank & Schofield, 2005; Georgeson &
Schofield, 2002), but we should not necessarily expect all
aftereffects to follow the same pattern of transfer.
We now considerVbut rejectVthe possibility that our
asymmetric transfers are artifactual; we then go on to
discuss the implications of our findings for models of first-
and second-order motion processing.
One possibility is that our asymmetric transfers arise
from the relative potency of the cues as adapters. For
example, if CM and OM adapt the motion system less
well than LM, then we might not expect to observe an
aftereffect, using LM tests, following adaptation to the
weaker cues. We can reject this criticism on two counts.
First, our transfer metric normalizes each between-cue
effect by the within-cue effect for the adaptation cue, thus
taking the potency of each adapter into account. Second,
CM and OM adapt the motion system equally well, but
transfer between these cues is asymmetric.
Next, we consider the relative visibility of our cues.
Adaptation is known to transfer better from strong cues to
weak ones than it does from weak to strong (Gibson &
Radner, 1937). In their study of the tilt aftereffect,
Cruickshank and Schofield (2005) noted that OM is often
perceived as weaker than CM when both are presented at
the same multiple of detection threshold. Accordingly,
Cruickshank and Schofield used CM and OM stimuli
based on thresholds for discriminating small differences in
the orientation of the modulation, as such stimuli were
deemed to be of the same apparent strength in a pilot
study. We followed Cruickshank and Schofield by basing
our signal levels on discrimination thresholds rather than
on detection thresholds, but unlike them, we used a rather
gross discrimination task. However, our 1-D binary noise
carriers convey OM better than the oriented Gabor
patterns used by Cruickshank and Schofield. We tested
with a Gabor noise carrier in a pilot study and found, unlike
1-D noise, that the OM signal was too weak to support
reliable direction discriminations. We are, thus, confident
that our OM adapters were strong enough, in principle,
to produce strong aftereffects. Finally, Georgeson and
Schofield (2002) registered no difficulty in balancing the
perceptual strength of LM and CM cues based on multi-
ples of detection threshold and found good, symmetric
transfer of both the tilt and contrast-reduction aftereffects.
We thus conclude that relative signal strength is unlikely
to account for the asymmetric transfer of the dMAE
between our cues.
An artifactual asymmetric transfer of aftereffects might
also arise from one of the many luminance artifacts that
are associated with second-order stimuli (especially CM,
see Schofield & Georgeson, 1999). If weak first-order
artifacts were to be present in our second-order stimuli,
these might be susceptible to adaptation by LM signals
but would not be strong enough to produce an aftereffect
in the reverse direction. However, we are confident that
this was not the case. Our noise samples were wide enough,
and our monitor was of sufficiently high bandwidth, to
avoid problems with the adjacent pixel nonlinearity
(Klein, Hu, & Carney, 1996). Although the use of a 1-D
carrier is not ideal for CM, its dynamic nature should
have prevented problems due to “clumping” (Smith &
Ledgeway, 1997), as the stimuli were drift balanced
(Chubb & Sperling, 1988). Our OM stimuli and unmodu-
lated carriers would have had identifiably different Fourier
amplitude spectra. However, following Cruickshank and
Schofield (2005), we conclude that although these Fourier
components may have been detectable by a first-order
mechanism, that mechanism would not have been able to
reveal the spatiotemporal properties of the modulating
signal as would be required to support discrimination
tasks. Finally, if our transferred aftereffects were due to
luminance artifacts, we should expect them to have the
sharp spatial-frequency tuning found when the adaptation
and test stimuli were both LM. In fact, when adaptation is
transferred from LM to CM or OM, it is relatively broadly
tuned.
Having established that our asymmetric transfer of
aftereffects is unlikely to be artifactual, we now consider
the implications of our findings for models of motion
processing.