Knowledge-Driven Contrast Gain Control is Characterized by Two Distinct Electrocortical Markers

Sensitivity to variations in luminance (contrast) is fundamental to perception because contrasts define the edges and textures of visual objects. Recent research has shown that contrast sensitivity, in addition to being controlled by purely stimulus-driven mechanisms, is also affected by expectations and prior knowledge about the contrast of upcoming stimuli. The ability to adjust contrast sensitivity based on expectations and prior knowledge could help to maximize the information extracted when scanning familiar visual scenes. In the present study we used the event-related potentials (ERP) technique to resolve the stages that mediate knowledge-driven aspects of contrast gain control. Using groupwise independent components analysis and multivariate partial least squares, we isolated two robust spatiotemporal patterns of electrical brain activity associated with preparation for upcoming targets whose contrast was predicted by a cue. The patterns were sensitive to the informative value of the cue. When the cues were informative, these patterns were also able to differentiate among cues that predicted low-contrast targets and cues that predicted high-contrast targets. Both patterns were localized to parts of occipitotemporal cortex, and their morphology, latency, and topography resembled P2/N2 and P3 potentials. These two patterns provide electrophysiological markers of knowledge-driven preparation for impending changes in contrast and shed new light on the manner in which top-down factors modulate sensory processing

Modality-independent processes in cued motor preparation revealed by cortical potentials

We used event-related potentials (ERPs) in a crossmodal stimulus-response compatibility paradigm to identify modality-independent aspects of rule processing and cued response facilitation. Participants responded to a lateralized target with the ipsilateral (compatible) or contralateral (incompatible) hand. Cue-target modality and cue-target order were manipulated. The cue preceded the target in half of the trials, and the target preceded the cue in the other half. For half of the participants, a visual cue signalled the response rule to an auditory target, while in other half, an auditory cue signalled the response rule to a visual target. Behavioural results showed a significant cue facilitation effect with response times faster for trials when the cue preceded the target, regardless of cue-target modality. The overall fastest response times were obtained in auditory cue-visual target trials. We performed groupwise independent component analysis of the cortical potentials and identified two modality-independent spatiotemporal patterns related to experimental effects. The first pattern, which resembled the early part of a contingent-negative waveform, was associated with response rule processing, regardless of cue-target presentation order and modality. The second pattern showed amplitude modulations that were dependent on stimulus modality. However, this pattern also correlated with faster response times only when the cue preceded the target and regardless of cue-target modality. Source analysis suggested that the response rule processing pattern originated from the posterior parietal, motor and cingulate regions. The pattern associated with the cue-first facilitation effect originated from cingulate and medial frontal regions. The effects carried by both patterns showed temporal overlap in the interval between the first and second stimulus presentation, but with differences in their relation to response rule processing and behavioural facilitation

Modality-Specific and Modality-General Encoding of Auditory and Visual Rhythms

The perception of timing information plays a large role in our everyday activities, yet we still do not accurately understand the mechanisms underlying these perceptions. Both modality-general and modality specific mechanisms have been suggested to account for perceptual timing. The use of a new auditory tempo perception paradigm can be used to examine various brain responses - measured via electroencephalography (EEG) - thought to index timing perception. This study applied this paradigm to both auditory and visual rhythms, and compared event-related potentials (ERPs) to task performance. Auditory and visual contingent negative variation (CNV) components showed two distinct voltage patterns across the scalp: The auditory CNV appears to show contributions from temporal areas, while the visual CNV appears to show contributions from occipital areas. There were larger CNV amplitudes in the auditory modality than in the visual, suggesting the CNV indexes modality-specific processing. A late, memory-dependent positive-voltage component did not show these modality-related topographical or amplitude differences, and instead reflects modality-general processing. This suggests timing information is encoded intrinsically at a sensory level, and this information is then routed to a cognitive, decision-making area for further processing