Journal of Vision (2007) 7(8):1, 1-12
Schofield, Ledgeway, & Hutchinson
10
The classical view of adaptation aftereffects holds that
if two cues are, at some point, processed by the same
population of neurons, then aftereffects should transfer
between them. Thus, adaptation has been used to test for
spatial-frequency channels (Blakemore & Campbell,
1969). If our cues were rendered equivalent within the
motion-processing system due to some relatively early
nonlinear processing, then we would expect strong,
symmetric transfer of the dMAE. To the extent that
dMAE transferred weakly and asymmetrically, we can
conclude that our cues are not treated as equivalent within
the motion-processing system. That is, the cues are not
processed by the same population of neurons.
However, our results are consistent with two alternative
schemes: (a) they could be the result of processing within
separate mechanisms that share adaptation selectively, or
(b) they could be due to a single generalized mechanism
that extracts the cues at different stages. For example, it
can be argued that Benton’s (2004) unified motion model
extracts second-order motion later than first-order motion.
Perhaps, signals that are extracted early affect those
extracted later but not vice versa.
Although adaptation to LM transfers to CM and OM, the
percentage transfer is relatively low (about 40%). Further,
the transferred effect loses its spatial-frequency tuning.
This pattern of results suggests a degree of separation
between first- and second-order cues that goes beyond the
split implied in Benton’s (2004) model. Given the body of
evidence to suggest that LM and CM are processed
separately, we hypothesize that adapting to LM influences
the processing of other motion cues at a relatively late
cue-independent stage. In support of this idea, we note
that MAEs induced by luminance gratings transfer
liberally to stimuli with a very different composition.
The asymmetric transfer of adaptation between CM and
OM suggests that these cues are also processed sepa-
rately. To this extent, our results add weight to those of
Baker et al. (2006), Kingdom et al. (2003), and Schofield
and Yates (2005), suggesting a separation between the
encoding of OM and CM. However, the fact that adaptation
to CM does not transfer to LM suggests that this effect is
not mediated by a cue-independent process. The strength
of the CM:OM transfer and the fact that it was relatively
well tuned suggest a strong but one-sided link between
these cues. Further, the results of the studies by Kingdom
et al. and Schofield and Yates can be explained by a
generalized FRF-like system (similar to the texture
segmentation model proposed by Landy & Bergen, 1991;
but see also Schofield & Yates, 2005) that computes a
CM-only signal alongside OM-sensitive orientation-oppo-
nent mechanisms and uses the CM signal to normalize
the OM response. Under such a model, the CM channel
sums inputs from all first-stage orientation channels and
is, thus, blind to OM. Conversely, the OM mechanism
(as described by Schofield & Yates, 2005) is initially
sensitive to CM but gains immunity from it via the
normalization process. In such a mechanism, transfer of
adaptation from CM to OM would be more likely than
transfer from OM to CM.
Conclusion
We tested the spatial- and temporal-frequency response
of the human visual system to moving OM stimuli and
have shown that these functions are both low pass but that
temporal acuity for OM is a little better than that for CM
given the same carrier.
We have also tested for the dMAE within and between
the three cues. We found within-cue dMAEs for all three
cues. For LM, the within-cue dMAE was narrowly tuned
for spatial frequency, CM tuning was a little wider,
whereas the dMAE for OM was untuned. The broad
spatial-frequency tuning of the OM aftereffect might be a
reflection of the spatial-frequency tuning of the OM
mechanism itself.
There was evidence for a relatively weak transfer of
dMAE from LM to CM and OM but little transfer of
aftereffect in the opposite direction. When the aftereffect
transferred from LM to the other cues, it seemed to lose its
spatial-frequency tuning. The aftereffect transferred
strongly from CM to OM but not vice versa and, in this
case, seemed to retain its tuning.
These results could be taken to suggest that LM, CM,
and OM motion are processed in three separate mecha-
nisms. This conclusion in turn implies that there are
multiple second-order motion mechanismsVa notion that
already has some support in the literature. However,
noting that the apparent independence between CM and
OM can arise from a single generalized mechanism
(Kingdom et al., 2003; Schofield & Yates 2005), and
given the strength and tuning of the CM to OM effect, we
feel that it may yet be premature to propose separate
mechanisms for CM and OM.
Acknowledgments
This work was supported by BBSRC Grant BB/C518181
to T.L. and by a Universitas 21 travel fellowship to A.J.S.
We thank Dr. Mike Harris and three anonymous referees
for their comments on early drafts of the manuscript. The
findings described have been reported in abstract form
(Schofield, Ledgeway, & Hutchinson, 2006).
Commercial relationships: none.
Corresponding author: Andrew Schofield.
Email: a.j.schofield@bham.ac.uk.
Address: School of Psychology, University of Birmingham,
Birmingham, UK, B15 2TT.