Object Recognition Can Be Viewpoint Dependent or Invariant - It's Just a Matter of Time and Task.
Object recognition can be viewpoint dependent or
invariant – it’s just a matter of time and task
1 Donders Institute for Brain, Cognition and Behaviour, Radboud University Nijmegen, Nijmegen, Netherlands
2 Experimental and Developmental Psychology, Utrecht University, Utrecht, Netherlands
As we move through our environment, we
encounter familiar objects from various
viewpoints. Despite the ensuing variabil-
ity of the images projected onto the retina,
we have seemingly little difficulty when it
comes to recognizing objects we encoun-
ter. We can, however, see how the objects
are oriented, suggesting that object rec-
ognition is to a certain degree dissociable
from perception of other object “features”
such as orientation. Changes in orienta-
tion of objects, particularly inversion, can
also affect how we perceive the objects. A
particularly illustrative example (shown in
Figure 1) is that of the Thatcher illusion
(Thompson, 1980), where the grotesque
appearance of a face with its inverted eyes
and mouth is “hidden” when the whole
face is also inverted. The percept itself,
therefore, is affected by the change in ori-
entation. In addition, there are also sub-
tle effects of viewpoint changes on object
recognition itself. For example, identifying
rotated objects is more difficult when they
are briefly presented than when viewing
time is unlimited (Lawson and Jolicoeur,
2003), and identifying a face is considerably
more difficult the face has been inverted
(Yin, 1969), as is discrimination between
characters “b” and “d,” or “p” and “q” which
requires (physical or mental) rotation of
the characters to upright, before we can
be certain which letter we are looking at
(Corballis and McLaren, 1984).
These subtle, yet persistent, effects of view-
point changes on perception and recognition
arise as a consequence of how visual object
processing is handled by the brain. Here, I
discuss how neural mechanisms underlying
visual processing give rise to perception and
recognition which can be both viewpoint
dependent and viewpoint invariant depend-
ing on the timing of those processes, as well as
specific task demands or current “perceptual
goals” of an individual. To do so, I will firstly
explain how temporal dynamics of low-level
visual processing may give rise to impaired
recognition at short viewing latencies and
suggest that this may also relate to effects of
viewpoint changes on perceptual experience.
I will then discuss how the perceptual goals of
an individual determines whether recogni-
tion is accomplished in viewpoint invariant
or dependent manner with a particular focus
on cognitive operations thought to be sub-
served by ventral and dorsal visual streams,
namely object recognition and mental rota-
PercePtion is affected by
Point of view
Change in orientation must affect process-
ing of visual information. For example, as
our viewpoint changes, so does the shape of
the image that falls on the retina. In the case
of picture-plane rotations, the orientation
of the edges of that shape will also change
and thus stimulate different populations of
orientation-tuned visually responsive neu-
rons in primary visual cortex. However,
these initial effects of orientation-changes
on neural processing probably do not give
rise to altered perceptual experience such
as those associated with inversion of a
Inversion affects how we perceive the spa-
tial relations between objects’ features and
may, as James (1890) suggested, depend on
perceptual experience with an object at a
given orientation. This could explain why
recognition of faces is particularly impaired
by inversion: faces are most frequently seen
the right way up, and are thought to be
recognized using information about the
configuration of the constituent features.
As mirror reversal is also a special case of a
configural change where the relative config-
uration of object’s features remains the same
but reverses in its left–right orientation, this
could also explain why mirror–images are
difficult to tell apart when they are rotated
away from a canonical viewpoint, and which
is why we must rotate objects into alignment
with our egocentric reference frames before
we can distinguish between parity-defined
characters such as “b” and “d” (Corballis
and McLaren, 1984). Interestingly, neural
responses to unaltered and thatcherized
images also follow the perceptual illusion
and disappear as the face is rotated away
from upright (Milivojevic et al., 2003a).
On neural level, large changes in the
viewpoint of an object, such as inversion
of faces (Rossion et al., 2000) and alphanu-
meric characters (Milivojevic et al., 2008),
result in delays of the N170 component. The
N170 is thought to reflect object classifica-
tion, and inversion-related delays of N170
possibly reflect increases in time required
to accumulate sufficient neural activity
to reach a threshold at which recognition
can occur (Perrett et al., 1998; Heekeren
et al., 2008). If changes in viewpoint delay
visual object encoding, this could explain
why accurate recognition of rotated objects
requires longer viewing times than rec-
ognition of canonically oriented objects
(Jolicoeur and Landau, 1984; Lawson and
Jolicoeur, 2003; Mack and Palmeri, 2011).
viewPoint matters only for some
Task-dependent effect of viewpoint changes
on neural processing are only observed
around 250 ms after stimulus onset and
coincide with the P2 component of the ERP.
For example, if the observers need to deter-
mine whether a rotated alphanumeric char-
acter is normal or mirror-reversed, they will
mentally rotate it to upright before mak-
ing the decision. Although the beginning of
mental rotation is later than the P2, parity
decisions are associated with linear increases
of P2 amplitudes while this is not the case
for P2 preceding categorization of alpha-
numeric characters which does not require
mental rotation (Milivojevic et al., 2011).
Interestingly, similar increases in P2 ampli-
tudes can be observed as a consequence of
stimulus degradation, either by addition of
noise (Banko et al., 2011) or by occlusion
Frontiers in Computational Neuroscience www.frontiersin.org May 2012 | Volume 6 | Article 27 | 1
published: 11 May 2012
an increase in activity in areas involved
in object recognition within the inferior
temporal cortex for various object classes
such as faces (Haxby et al., 1999), bodies
(Brandman and Yovel, 2010), landscapes
(Epstein et al., 2006). Some authors have
suggested that this increase in activity may
reflect a shift in recognition strategy from
one that is based on the whole shape to one
that is based on the analysis of individual
object features (i.e., details Jolicoeur, 1990).
shaPes requires mental rotation
Decisions regarding the direction of the
left–right axis of an object, or its handed-
ness, require alignment between the object
and our own egocentric frame of reference.
For example, deciding whether a shoe is the
left or the right one requires either physical
or mental rotation of the shoe into align-
ment with our feet, or the feet with the shoe.
The same holds for any object class that has
a well-defined left–right orientation, such
as alphanumeric characters, which can be
readily recognized as “backward” if they
have been mirror-reversed (Cooper and
Shepard, 1973) – but only if they are pre-
sented at upright. Rotated characters require
rotation to their canonical upright before we
can notice if they are normal or backward,
particularly if they are rotated by a large
degree (Kung and Hamm, 2010). When the
identity of an object depends on its left–right
parity, as is the case with lower-case letters
“b” and “d” or “p” and “q,” then the discrimi-
nation of such characters also requires rota-
tion to upright before it can be successfully
recognized (Corballis and McLaren, 1984).
This suggests that information regarding
the identity of the object must be extracted
before information about the handedness
of an object can be determined. Although
generally we need to recognize an object
before mental rotation begins (Heil et al.,
1996; Schendan and Lucia, 2009), this can-
not be the case for objects whose identity
depends on their handedness, such as “b”
and “d” or “p” and “q.” With the excep-
tion of alphanumeric characters, there are
not many commonly encountered objects
whose identity is defined by parity (i.e., a
hand is a hand irrespective of whether it is a
left one or a right one) and those objects can
be seen as special case whose identity cannot
be determined at all orientations. For these
objects, identification from a feature-based
nized as faces, what seems to be disrupted
is the identification of the face as belonging
to a particular person or identification of an
emotional expression, while differentiation
between categories of “face” and “non-face”
objects is largely unimpaired by inversion.
The difference in viewpoint-sensitivity
of identification and categorization has also
been established for other classes of objects.
For example, identifying letters of the alpha-
bet is affected by character orientation while
the same is not the case for between-category
decisions such as letter–digit categorization
(Corballis et al., 1978). In a sense, categori-
zation may relate to recognition at a basic or
entry level described by Roch (Rosch et al.,
1976), while identification may be more
closely related subordinate-level recogni-
tion. Object recognition at basic level (e.g.,
deciding a shape is a dog) are not affected
by changes in viewpoint, while subordinate-
level decisions (e.g., identifying a dog as a
poodle) are affected by viewpoint changes
in terms of reaction times and accuracy
(Hamm and McMullen, 1998).
Studies which have directly com-
pared identification and categorization
of objects using neuroimaging methods
are scarce. Nevertheless, studies investi-
gating neural correlates of rotated-object
categorization show little evidence of
orientation-dependence at visual process-
ing stages beyond the initial encoding of
the objects (see above). In contrast, stud-
ies investigating rotated-object recognition
either as identity-matching or in terms of
explicit identification show that there is
(Doniger et al., 2000), but not size transfor-
mation (Muthukumaraswamy et al., 2003),
suggesting that changes in orientation
degrade certain types of perceptual infor-
mation which may be required for task-
specific decision making, and may be, thus,
associated with some form of perceptual
decision making (Heekeren et al., 2008;
Schendan and Lucia, 2009, 2010), such as
whether sufficient information is available
for the perceptual goal to be achieved. This
decision would then trigger other visuos-
patial cognitive operations, such as men-
tal rotation or more detailed inspection
of individual features of an object. Those
cognitive operations would lead to acqui-
sition of additional information about the
object which would, in turn, enable a more
accurate completion of the perceptual task
at hand. For the purpose of illustration, two
types of “perceptual goals” that depend on
object orientation will be described: object
identification and parity-based recognition.
identification is viewPoint
dePendent but categorisation
As already mentioned, face recognition is
worse when faces are inverted (Yin, 1969),
both in terms of reduced recognition accu-
racy and increased reaction times. This seems
to be the case both for familiar and unfamil-
iar faces, and may be a consequence of dis-
rupted neural processing underlying object
classification although a causal relationship
has not been firmly established. It should be
noted here that faces are nevertheless recog-
Figure 1 | unaltered and “thatcherized” version of Margaret Thatcher’s face. The grotesque
appearance of the face when its eyes and mouth are inverted is hidden by the inversion of the whole
image. Rotating the pictures to upright makes discrimination between the two versions of the face easier.
Milivojevic Viewpoint dependent and invariant recognition
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Citation: Milivojevic B (2012) Object recognition can be
viewpoint dependent or invariant – it’s just a matter of
time and task. Front. Comput. Neurosci. 6:27. doi: 10.3389/
Copyright © 2012 Milivojevic. This is an open-access arti-
cle distributed under the terms of the Creative Commons
Attribution Non Commercial License, which permits
non-commercial use, distribution, and reproduction in
other forums, provided the original authors and source
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descriptor such as “a semi-circle attached at
an end of a long stem” could lead to selec-
tion of possible four candidates, and the
remaining possibilities would need to be
resolved with mental rotation.
Mental rotation has been associated with
linear increases in centro-parietal negativ-
ity between ∼400 and 800 ms after stimulus
onset (e.g., Milivojevic et al., 2009b) which
last somewhat longer for larger angu-
lar departures from upright (Milivojevic
et al., 2003b; Hamm et al., 2004). The ERP
correlates of mental rotation are prob-
ably generated by a distributed network of
sources localized (Milivojevic et al., 2009b)
within a network of prefrontal and poste-
rior parietal areas which has been identified
using fMRI (e.g., Milivojevic et al., 2009a).
Whether these areas also subserve recogni-
tion of rotated parity-defined objects is still
unclear as this particular question has not
been investigated using neuroimaging.
summary and conclusion
Although changes in viewpoint rarely inter-
fere with common perceptual goals, such
as categorizing objects into basic catego-
ries, this type of viewpoint invariant rec-
ognition can only be achieved after initial
viewpoint-dependent neural processing has
been accomplished. Depending on current
perceptual goals, changes in viewpoint may
impose certain recognition costs, observ-
able in terms of increased response latencies
or reduced accuracy. These costs are likely to
reflect increased cognitive demands associ-
ated with recognition of misoriented shapes
such as detailed analysis of object features or
mental rotation of the shape to its canoni-
cal upright. In this sense, recognition of
objects will always be affected by changes
in viewpoint early on in the visual process-
ing stream, but these effects will taper off
with time. At later visual processing stages,
some types of perceptual goals such as
object identification or parity discrimi-
nation, will require additional processing
operations which will give rise to viewpoint
dependent behavioral performance.
I would like to thank Michael Corballis, Jeff
Hamm, and Maarten Boksem for their help-
ful comments regarding earlier versions of
Milivojevic Viewpoint dependent and invariant recognition
Frontiers in Computational Neuroscience www.frontiersin.org May 2012 | Volume 6 | Article 27 | 3