Gamma flicker triggers attentional selection without awareness.

Gamma band modulations in neural activity have been proposed to mediate attentional processes. To support a causal link between gamma activity and attentional selection, we attempt to evoke gamma oscillations by a 50-Hz subliminal flicker. We find that a subliminal 50-Hz flicker at a target location, before target presentation, speeds up and enhances target detection and discrimination. This effect is specific to the middle of the gamma range because it is not evident at <35-Hz flicker. It requires 300 ms to build up, dissipates within 250 ms of flicker offset, and shows a tendency to invert after 500 ms. The results are discussed in relation to a role for gamma band neural synchrony in the allocation of visual attention.

Subliminal gamma flicker draws attention even in the absence of transition-flash cues.

We recently reported evidence indicating that selective attention is deployed to a target location in a multi-object display, when the target event (a change of one of the objects) is preceded by subliminal flicker in the gamma range. However, concerns have been raised regarding the stimuli used in this study and the possible contribution of an artifactual cue: a "transition flash" between pretarget flicker offset and target onset. Here, we report a series of experiments investigating the existence and potential contribution to selective attention of this transition-flash cue under different presentation conditions. We find that, although the transition flash is a real phenomenon (detection rates ≃ 15% > chance), it cannot, on its own, explain the original effects of gamma flicker on the response time to target detection. Even after eliminating this flash, detection was significantly faster, or more accurate, for targets preceded (vs. not preceded) by flicker. This congruency effect (≈ 15 ms) demonstrates that gamma flicker on its own is sufficient to engage selective attention. This interpretation is further strengthened by a reevaluation of 1) experiment 7 reported by van Diepen and colleagues and 2) the validity effect experiment reported by Bauer and colleagues. Possible reasons for the discrepant results are also discussed.

Masking within and across visual dimensions: psychophysical evidence for perceptual segregation of color and motion.

Visual masking can result from the interference of perceptual signals. According to the principle of functional specialization, interference should be greatest when signal and mask belong to the same visual attribute (e.g., color or motion) and least when they belong to different ones. We provide evidence to support this view and show that the time course of masking is visual attribute specific. First, we show that a color target is masked most effectively by color (homogeneous target-mask pair) and least effectively by motion (heterogeneous pair) and vice versa for a motion target. Second, we show that the time at which the mask is most effective depends strongly on the target-mask pairing. Heterogeneous masking is strongest when the mask is presented before the target (forward masking) but this is not true of homogeneous masking. This finding supports a delayed cross-feature interaction due to segregated processing sites. Third, lengthening the stimulus onset asynchrony between target and mask leads to a faster improvement in color than in motion detectability, lending support for a faster color processing system and consistent with reports of perceptual asynchrony in vision. In summary, we present three lines of psychophysical evidence, all of which support a segregated neural coding scheme for color and motion in the human brain.

The role of parietal cortex in the formation of color and motion based concepts.

Imaging evidence shows that separate subdivisions of parietal cortex, in and around the intraparietal sulcus (IPS), are engaged when stimuli are grouped according to color and to motion (Zeki and Stutters, 2013). Since grouping is an essential step in the formation of concepts, we wanted to learn whether parietal cortex is also engaged in the formation of concepts according to these two attributes. Using functional magnetic resonance imaging (fMRI), and choosing the recognition of concept-based color or motion stimuli as our paradigm, we found that there was strong concept-related activity in and around the IPS, a region whose homolog in the macaque monkey is known to receive direct but segregated anatomical inputs from V4 and V5. Parietal activity related to color concepts was juxtaposed but did not overlap with activity related to motion concepts, thus emphasizing the continuation of the segregation of color and motion into the conceptual system. Concurrent retinotopic mapping experiments showed that within the parietal cortex, concept-related activity increases within later stage IPS areas.