In seven participants, we used TMS to test the causal roles of the functionally-defined lateral occipital complex (LOC) and posterior intraparietal sulcus (pIPS) areas during visual form and orientation discrimination. All participants underwent FMRI localising scans. FMRI identified three significant clusters of voxel activity corresponding to the right pIPS, the right LOC, and the left LOC. TMS was then applied in separate blocks of trials to disrupt each subject-specific region-of-interest while the participants performed match-to-sample form or orientation discrimination tasks. When compared to sham stimulation, TMS applied to the right LOC induced a significant increase in form discrimination reaction time [F(1,24) = 8.72, p < 0.05 (corrected)], supporting a critical role for the LOC in form processing for perception and judgment-based tasks. In contrast, no effects on performance were observed when TMS was applied to pIPS. In other words, despite clear functional evidence that LOC and pIPS are engaged when subjects view visual objects that vary in both form and orientation, the TMS results show that the pIPS is not critically invoked for judgments of form or orientation. Taken together, we believe these results provide further support for the notion that ventral-stream but not dorsal-stream networks play an essential role in vision for perception.

Motion Induced Blindness (MIB) is a visual phenomenon whereby highly salient stimuli disappear when viewed against the background of a global moving ‘mask’. We aimed to explore the relationship between MIB and variations of mask speed and trajectory of movement. We looked at four types of movement that also occur in the real world (expansion, contraction, random and static) and four speed levels that commonly occur while driving (35km/h, 50km/h, 65km/h and 80km/h), hypothesising there would be greater disappearance with faster speeds and in expansion and random movement. Participants (N = 18) viewed combinations of mask movement and speed level. Two additional experiments were run (N = 19) with an eye-tracking device controlling for fixation. There was a significant linear trend for both number (F(1, 14) = 12.62, p = .003, r = .69) and duration (F(1, 14) = 13.20, p = .003, r = .70) of MIB disappearance to increase with speed. While significant main effects showed movement was essential in all experiments, type of movement did not differentiate MIB number or duration, with pairwise comparisons indicating Random, Expansion and Contraction were consistently significantly different from static, but not from each other. Post-hoc tests indicated there was also significantly more disappearance of stimuli in the upper compared to lower visual field in 4 of the 8 t-tests run. Overall results suggest parameters based on the real world are conducive to MIB in an artificial setting, which may hold implications into future research into MIB occurring in a natural world.

Although traditionally associated with stimulus driven processes (particularly to faces), we have recently demonstrated the N170 visual event related potential to be strongly modulated by contextually mediated expectations. This recasts the N170 as a potentially indexing error checking mechanisms that detect mismatches between perceptual predictions and actual sensory input. Using a novel MEG beamformer metric (the Difference Stability Index – DSI) we explored the spatiotemporal bases of the "expectancy violation N170" signal. Participants viewed sequences of images (of heads or statuettes) which established rigid-body rotational implied motion trajectories, which were either conformed to (Predicted condition) or violated (Unpredicted condition) by the final image in the sequence. Careful experimental controls meant that the set of final image transitions were perfectly matched across the Predicted and Unpredicted conditions, ensuring that any observed effects must be due to context rather than to any differences in low-level stimulus properties. Consistent with our previous EEG findings, Unpredicted final images were associated with a large effect-size increase in N170 amplitudes, and increased amplitude of the later N300 potential. The DSI beamformer revealed that the increased amplitude N170 was attributable to sources in visual motion areas MT/V5+ and STS, whereas the later latency signal amplitude increases were generated in the insula and sensorimotor cortices. The localisation of the “expectation violation N170” differs from previously reported localisations of the “face N170”, but is consistent with a role for MT/V5+ in testing predictions about stimulus motion.

Perceptual decision-making, according to the prominent diffusion model, is based on noisy evidence accumulation towards a decision threshold. In this framework, evidence accumulation is characterised by a drift rate parameter, which is determined by the amount of incoming sensory information. This process is further influenced by noise, leading to differences in outcomes and response times across trials. Despite substantial progress in refining the cognitive process model, the neural mechanisms underlying evidence accumulation dynamics in humans are still debated. Here we used an identical perceptual decision-making paradigm in two experiments, applying 64-channel electroencephalography (EEG) in one, and 3-Tesla functional magnetic resonance imaging (fMRI) in the other. In each trial, participants were given a two-alternative forced-choice task for decisions between noisy images of pianos and chairs, randomly presented in high, medium and low stimulus quality conditions. We then tracked the temporal (EEG) and spatial (fMRI) neural characteristics of both features driving evidence accumulation: fluctuations in stimulus quality, and additional fluctuations in response time variability. For EEG, using a jack-knifing procedure, we identified a late component of the event-related potential over visual areas that was modulated by stimulus quality, and additionally reflected trial-by-trial response time fluctuations. For fMRI, fluctuations in stimulus quality modulated the blood-oxygen-level-dependent signal in bilateral medial occipito-temporal cortex, and in the bilateral basal ganglia (putamen). Given the conceptual overlap in EEG and fMRI analyses, our results are likely to reflect the neural substrate of evidence accumulation dynamics, which ultimately drive choice.