The early visual system is retinotopically organized (Tootell et al.,
1998). However, this retinotopic organization is insufficient to support perception under natural viewing conditions. When the eyes move, the retinotopic representation of the environment undergoes drastic shifts, yet our percepts remain stable (Wurtz,
2008). In addition to this “eye movement problem” for retinotopic representations, there is also an “object movement problem”: Moving objects stimulate retinotopic receptive fields only briefly not allowing sufficient time for the computation of the characteristics of the stimulus (Öğmen,
2007). Studies addressing the limitations of retinotopic representations dealt primarily with the “eye movement problem” by using saccadic stimulus presentation paradigms (SSPPs). As discussed in the
Introduction section, in SSPP retinotopic and non-retinotopic representations are contrasted by presenting stimuli before and after a saccadic eye movement (e.g., Davidson et al.,
1973; Golomb et al.,
2008; Irwin,
1996; Knapen et al.,
2009; McRae et al.,
1987; Melcher,
2005,
2007,
2008; Melcher & Colby,
2008; Melcher & Morrone,
2003; gaze modulation: d'Avossa et al.,
2007; Nishida, Motoyoshi, Andersen, & Shimojo,
2003; Wenderoth & Wiese,
2008). However, SSPP is not well suited for moving stimuli or for fast, short-lived processes that require the presentation of stimuli with brief ISIs. Finally, the involvement of the eye motor system or phenomena such as saccadic suppression can complicate the interpretation of the findings in SSPP.
Our Ternus–Pikler paradigm overcomes these limitations. First, it can be used with eye movement and steady fixation paradigms, thus, one can dissociate the influence of eye-movement-related processes. Second, short-lived visual processes can be tested because ISIs can be much shorter than in SSPP. When using appropriate stimulus configurations, the ISI can be reduced even to 0 ms and still group motion is perceived (e.g., Kramer & Yantis,
1997; Scott-Samuel & Hess,
2001). The Ternus–Pikler stimulus can also be presented repetitively for long durations and therefore can be used for processes that require long presentation times, as illustrated in our adaptation experiments. Third, another distinct advantage of our paradigm is its possibility to pit retinotopic and non-retinotopic processes directly against each other due to the spatially overlapping elements in the Ternus–Pikler display, which mask each other when retinotopic integration prevails. Simple parametric manipulations (such as ISI, figural characteristics of elements, and omission of flanking elements) can modulate the percept from group to element or to no motion thereby offering strong control conditions (see also Cavanagh, Holocombe, & Chou,
2008; Shimozaki, Eckstein, & Thomas,
1999).
Based on these advantages, we have reported several novel findings. For example, visual search is usually assumed to rely on retinotopic feature maps. Here, we have shown that attention can operate on non-retinotopic feature maps when group motion prevails in the Ternus–Pikler display (
Figure 5d). There are no eye movements during search (
Figure 5e) and hence attention is covert. Interestingly, largely retinotopic attention was found in a cueing paradigm where attention was also covert (Golomb et al.,
2008; but see Cavanagh et al.,
2008). The non-retinotopic processing in visual search involves non-retinotopic
form processing because the search elements are integrated across frames. This becomes immediately evident when the flanking squares are omitted: retinotopic integration occurs and search displays mask each other.
This non-retinotopic form processing shows again the sensitivity of our paradigm because studies using SSPP never found form processing to be non-retinotopic (Irwin,
1991; Irwin, Yantis, & Jonides,
1983). This holds also for a paradigm based on apparent motion, which is very similar to our Ternus–Pikler display (Cavanagh et al.,
2008). However, other studies employing different paradigms found integration of form (Nishida,
2004; Yin, Shimojo, Moore, & Engel,
2002). In recent studies, we have shown that non-retinotopic form processing can even occur with features close to the hyperacuity range, i.e., with stimuli of which the crucial features are in the range of photoreceptor spacing (Öğmen et al.,
2006; Otto, Öğmen, & Herzog,
2006,
2008).
In the first experiment, we have shown evidence for non-retinotopic motion processing, i.e., motion that becomes apparent only after group motion is established (
Figure 3d, ISI 210 ms). This motion is invisible when retinotopic integration occurs (
Figure 3d, ISI 0 ms, no flank conditions). This motion processing can be computationally understood as a two-step process. The motion correspondences between Ternus–Pikler elements (e.g., disks) provide the reference frame (Mack,
1986) against which local motion is computed. From this perspective, our stimulus paradigm provides a powerful link between non-retinotopic processes and reference frames in perception (Bertamini & Proffitt,
2000; Dunker,
1929; Johansson,
1973).
Interestingly, retinotopic adaptation occurred in Example 2 as the result of
coherent retinotopic motion whose coherence was invisible to the observer. The percept was that of
incoherent motion (
Figure 4b). This finding suggests that retinotopic motion detectors adapted “unconsciously” to the underlying retinotopic motion.
Based on these findings, we suggest that our test is a litmus test for several reasons. First, it is easy to implement by, simply, putting the stimuli of interest on the Ternus–Pikler display. Second, non-retinotopic effects can often be easily verified just by looking at the display (
Videos 4 and
10). Third, very short-lived non-retinotopic processes can be detected. Fourth, retinotopic processing can be pitted against non-retinotopic processing. Fifth, the Ternus–Pikler display can easily be adapted to neurophysiological needs because the spatiotemporal parameters of the display can be flexibly adjusted without obliterating group motion.
Our test may provide a first, gross guidance whether to record from retinotopic or non-retinotopic areas. We are confident that our simple but compelling approach can be applied to any visual research fields such as filling-in, reading, contrast detection, and the attentional blink, just to name a few.