In the ‘manual response paradigm’ an ambiguous stimulus is presented
continuously (always appearing on the screen) and participants make a manual response (i.e., button press) when they experience a perceptual reversal. These reversals are then used as a time reference to segment the data. Unlike most ERP analyses which focus on the post-stimulus activity evoked after an event, the data of interest is typically that before the response (i.e., leading up to the reversal).
Studies using this paradigm have found gamma-band power increases and alpha- band power decreases in a time interval approximately 1000 ms before participants’
manual response (Isoglu-Alkac, 2000; Struber & Hermann, 2002; Basar-Eroglu et al., 1996; Mathes et al., 2006). These power changes have been interpreted as evidence of a slow bottom-up and top-down cognitive destabilization process underlying perceptual reversals during the active interpretation of the ambiguous stimulus. This means that, in line with the destabilization process of Kornmeier and Bach (2012) defined in Chapter 1, the results from Struber and Hermann (2002)’s study show that the depth of the representation of the perceived motion direction of the SAM stimulus (used in their experiment) slowly becomes shallow to the point of instability of the representation leading to a reversal where the alternative motion direction reaches visual awareness. The observed alpha and gamma activity reflects this visual awareness of the perceived interpretation (Sewards & Sewards, 1999) with the decrease in alpha activity reflecting a passive and automatic process (e.g. neural satiation; Hochberg, 1950) and the increase in gamma activity reflecting top-down processing (e.g. attention; Struber et al., 2000, 2001). In addition to these findings and interpretations, these studies also found that between 500 and 250 ms before manual response, a P300-like parietal positivity ERP occurs. The latter is interpreted as conscious recognition of a perceptual reversal. This implies that there was a change in visual awareness and therefore the reversal event must have taken place earlier (Struber and Herrmann, 2002; Mathes et al., 2006). This was taken as evidence for top-down processes related to the reversal events.
One of the problems with these results is that they incorporate information from a long time window making it difficult to pinpoint the moment at which a perceptual reversal occurs. This paradigm also makes it difficult to identify whether the
related processes. This means that the information included in that window could be a combination of several factors related to both the stimulus, the reversal event and response. This was shown in a study by Struber et al. (2000) where they analysed the differences in gamma activity between high-reversal rates and low reversal rates. They found that they could not give an exact time course of the gamma activity because of the lack of stimulus onset. They used a non-phase locked approach in order to interpret their results. This is because, using the manual response paradigm makes the interpretation of phase-locked gamma activity in multistable perception difficult. Struber et al. (2000, 2002)’s results showed that the motor response by which the subjects indicated that they experienced a perceptual reversal creates a remarkable jitter due to the variability of the response times both inter and intra-individually. Struber et al. (2002) found that there is a remarkable difference in the morphology of the P300-like component following the exogenous reversal which was dependent on whether the averaging was stimulus-locked or response-locked. This indicated an increased jitter of the perceptual reversal-related activity caused by a ‘smearing out’ of the cognitive components which is due to the reaction time variability (İşoğlu-Alka et al., 1998). Inter- and intra-individual variability in reaction time decreases the signal quality of perceptual reversal related potentials. This poses a problem for the questions I want to answer in my PhD seeing as I am interested in identifying the time-window both in the pre-stimulus (Experiments 3&4) and post-stimulus (Experiments 1&2) period during which the activity is predictive of perceptual reversals and percept choice (e.g. Face or Vase in Rubin’s Face-Vase). Using the manual response paradigm would make it difficult to identify what event the
This problem of reaction time variability with the manual response paradigm would make it difficult to address some of the aims of my research. Backward averaging with respect to participant’s response obliterates the early visual ERP components whereby the earliest observed component is the P300-like Parietal Positivity. This is due to reaction time variance (Kornmeier & Bach, 2004a, 2012). Kornmeier and Bach (2004a) found a relatively large interquartile range in reaction time which affected the presence of ERP traces when the EEG is backward averaged with respect to
participants’ response. These traces were sharply defined when the EEG is averaged with respect to stimulus onset. The reversal related components (RP and RN, Kornmeier & Bach, 2004a, 2005) that we are interested in investigating occur before the P300-like Parietal Positivity which, to our knowledge so far, cannot be studied in experiments using the manual response paradigm.
Due to our interest in the time windows during which the patterns of activity are linked to perceptual reversal (Experiments 3 & 4) and our interest in ‘early’ reversal related components (Experiments 1 & 2), both of which cannot be studied using the manual response paradigm, the Onset Paradigm, with stimulus onset as time reference, is adopted in the experiments we conducted.