Percept of a corrugated surface
One goal of Experiment 1 was to identify parameters that elicit a compelling percept of a corrugated surface by motion and to assess individual differences in the perception of corrugated surfaces. Participants were confronted with either a COR or RND stimulus of varying amplitude (or vmin/
ratio) and spatial frequency and were asked to identify whether the display composed of moving dots appeared to be structured (corrugated) or uniform (volume). As illustrated in , discrimination performance as measured by d′ (Swets, 1973
) was uniformly high for most amplitude and frequency conditions, but decreased for small amplitude conditions (vmax/
= 1.31) and low and high spatial frequencies. This result pattern was confirmed by a significant main effect of Amplitude (F3,18
= 5.7, P
= 0.007) and significant interaction of Amplitude × Spatial Frequency (F9,54
= 2.1, P
Fig. 2 Group mean results of Experiment 1. Performance measures are shown for each level of spatial frequency (f) and amplitude (v = vmax/vmin). (A) Discrimination performance of COR (structured) vs. RND (uniform) stimuli is indicated in terms of sensitivity (more ...)
Participants were further asked to indicate whether they perceived the dots as moving along a 2D plane or along a 3D surface or within a volume. Consistent with previous findings (Andersen, 1996
), the impression of a 3D surface () was determined by the amplitude (e.g., velocity ratio vmax/
) and the spatial frequency of the COR stimulus. In particular, the proportion of perceived 3D surface responses increased with higher velocity ratios (amplitudes) (F3,18
= 6.3, P
= 0.004) and decreased with higher spatial frequencies of the COR stimuli (F3,18
= 17.9, P
< 0.001). Furthermore, a significant interaction of Amplitude × Spatial Frequency (F3,18
= 2.1, P
= 0.041) indicated that the percept of a surface decreased with higher velocity ratios only for high spatial frequencies (0.4 cycles/deg and 0.5 cycles/deg).
As expected, spatial frequency had no effect on the 3D percept of RNDs. However, RNDs were perceived to be three-dimensional more often with higher velocity ratios (; F3,18 = 3.9, P = 0.025).
Based on this assessment, parameters were chosen for subsequent experiments that yielded a compelling percept of a corrugated surface in each individual. Accordingly, the mean amplitude factor a was 0.419 (corresponding to vmax/vmin = 2.44) and the mean spatial frequency was 0.29 cycles/deg.
Speed change detection
The goal of Experiment 2 was to find parameters for the speed change detection task in order to equalize task difficulty. The mean 75% thresholds for speed change detection as measured by bestPEST (Lieberman & Pentland, 1982
) were 3.10 dB for COR stimuli and 3.17 dB for RND stimuli, corresponding to an increase in dot speed by the factors 2.05 and 2.10, respectively.
During the brain imaging session (Experiment 3) observers’ performance for detecting speed changes as well as their response times were indistinguishable between COR (83%, 559 ms) and RND (79%, 560 ms) stimuli (P > 0.1) suggesting that CORs and RNDs were equally attended.
illustrates statistical parametric maps of the BOLD response of a representative individual hemisphere overlaid on the corresponding inflated gray matter surface and occipital flat patch. Upper and lower visual field stimulation resulted in clear activity patches ventral and dorsal to the calcarine sulcus () in all examined hemispheres. In addition, horizontal and vertical meridian stimulation () resulted in several activity stripes parallel to the calcarine sulcus. Representations of the horizontal meridian and the vertical meridian alternated and separated neighboring retinotopic visual areas (V1, V2, VP/V3 and V4v).
Fig. 3 Definition of ROIs. Statistical parametric maps for the ROI definition localizer runs are shown for one representative hemisphere. Activity maps were overlaid on the inflated gray matter surface and a flat occipital patch that was cut along the calcarine (more ...)
The contrast of whole-field moving vs. static dots revealed a large network of regions that were more active while observers viewed moving dots than static dots (). The site of activated regions corresponded with motion-sensitive regions known from the literature (Orban et al., 1999
; Sunaert et al., 1999
). This network included, besides early retinotopic regions, areas V3A, VIPS, POIPS, DIPS, a region on the LOS, area MT+, a region overlapping with the FG, a small region in the STS and in the PIC. illustrates the activity pattern elicited by the contrast motion vs. static dots when only the peripheral ipsilateral visual field was stimulated. Consistent with previous research (Dukelow et al., 2001
; Huk et al., 2002
), only an anterior subpart of MT+ showed significant activity. This part was considered to be MST for the subsequent ROI analysis, while the remainder was labeled MT.
Similar activity patterns were obtained for all examined hemispheres. However, for some subjects the threshold had to be lowered (P = 0.05, no correction for multiple comparisons) for the contrast ipsilateral moving vs. static dots. This liberal thresholding may have resulted in an overestimation of MST. Similarly, areas STS and PIC were not visible on all hemispheres when a conservative threshold was applied. If so, these areas were defined using a reduced threshold. lists the mean size and mean Talairach location for each ROI. Note that for some hemispheres the ROIs did not exceed the minimum requested size (five functional voxels) to be considered for the ROI analysis. Further note that higher-level visual areas (e.g. STS) showed a larger variability in their Talairach coordinates across subjects than early visual areas (e.g. V1).
illustrates the time course of the BOLD signal in response to COR or RND epochs (contrasted to fixation only) separately for early retinotopic and motion-sensitive ROIs. The BOLD signal increased in all ROIs after a delay of about two TRs (4 s) and slowly decreased after the stimulation epoch (at 16 s, i.e. eight TRs). No reliable difference between COR and RND epochs were observed at early visual areas V1, V2, V3, VP and V4v. By contrast, several motion-sensitive regions showed a stronger BOLD signal increase for COR than for RND stimuli. This difference started to become reliable at about the fourth TR after epoch onset and lasted until about the end of the stimulation period.
Fig. 4 Mean BOLD signal changes to COR and RND epochs for each ROI. The time course of the BOLD signal (percentage signal change) was calculated using the general linear model using a finite impulse response function to model ten acquisitions (TRs). The time (more ...)
The BOLD response to motion stimuli contrasted with fixation does not necessarily reflect activity specific to motion processing. However, the difference in the responses to COR and RND stimuli may be considered a correlate of 3D SFM perception. Therefore, plots the mean difference in BOLD signal for the period from the fouth to the eighth TR separately for each ROI. One-sample t-tests that compared this difference across subjects indicated that COR stimuli elicited stronger BOLD signal increases than RND stimuli in areas V3A (t6 = 3.2, P = 0.008), VIPS (t6 = 6.1, P < 0.001), DIPS (t6 = 3.2, P = 0.008), FG [t(5) = 3.2, P = 0.009], LOS (t6 = 2.2, P = 0.032), STS (t6 = 2.5, P = 0.021) and MT (t6 = 5.0, P < 0.001). A marginal significant difference was found in area V4v (t6 = 1.8, P = 0.057) and area POIPS (t6 = 1.8, P = 0.057; not shown in and ). BOLD signals in early visual areas (V1, V2, V3 and VP) and the motion-sensitive regions MST and PIC did not differ (P > 0.1).
Fig. 5 Mean difference in the BOLD signal of COR minus VOL for each ROI. BOLD signals reflect the mean signal (percentage signal change) for the period from the fourth to the eighth acquisition after epoch onset (see ). Error bars reflect SEMs across subjects. (more ...)
In order to test for brain areas relevant for 3D SFM perception that might have been overlooked by our ROI analysis we performed an additional whole-brain surface-based group analysis (). Despite some distortions and image blurring due to the normalization procedure and the extended spatial smoothing (8 mm rather than 5 mm) the significance maps of this group analysis showed several clusters of activity in occipital, temporal and parietal areas. These clusters of activity correspond relatively well with the regions identified by the ROI analysis: V4v, FG, V3A, VIPS, DIPS, FG, LOS, STS and MT. No relevant activity was observed in posterior and medial occipital lobe that would correspond to early visual areas. No additional areas that were not analyzed by the ROI approach could be identified.
Fig. 6 Whole-brain group analysis. The group mean statistical parametric maps for the contrast 3D COR vs. RND were overlaid on the inflated left hemisphere (left, lateral view; right, medial view) of the MNI average brain of Freesurfer. Significance maps were (more ...)
Correlation of BOLD signal and percept of a corrugated surface
As reported above (Experiment 1), all observers were able to perceive a 3D surface defined by moving dots. However, we also noticed substantial interindividual differences regarding how strongly observers perceived the moving dots as a 3D surface. We were interested in whether these interindividual differences were also reflected in the strength of the BOLD signal. Therefore, we correlated the mean proportion of 3D surface responses from Experiment 1 with the BOLD signal difference between COR and RND stimuli of Experiment 3. These correlations are displayed in . Even though our sample size was small we found significant positive correlations between the perception of 3D corrugatedness and the difference in BOLD signal for several motion-sensitive regions including area V3A [r(5) = 0.86, P = 0.006], VIPS [r(5) = 0.92, P = 0.002], LOS [r(5) = 0.83, P = 0.011] and STS [r(5) = 0.69, P = 0.044]. Interestingly, BOLD signal differences in MT (and MST) were not (or were even negatively) correlated with the behavioral measures of perceived 3D corrugatedness.
Fig. 7 Correlation of BOLD signal difference and the percept of a 3D surface. For each ROI the mean percentage of 3D surface responses (Experiment 1) was correlated with the BOLD signal difference (COR minus VOL; see and ) across subjects. (more ...)