Under appropriate stimulus conditions, judgments about the degree of temporal synchrony in sequences containing rapid alternations of colour and motion direction imply a large apparent delay of motion perception relative to colour perception. Whether this colour-motion asynchrony results from the relative processing delay of different visual attributes, or from inappropriate matching of time markers assigned to first-order change of colour and position has been the subject of recent debate. Colour-motion asynchrony is significantly weakened when the angle of direction change is reduced from 180 degrees (direction reversal) to a smaller change in direction. Although this finding has been interpreted to favour the processing delay hypothesis, here we show that it is consistent with the time marker account. First, the reported dependence on the motion direction angle was particularly strong for random-dot stimuli, but our results indicate that this may reflect the introduction of an artefact, motion streaks, that allows subjects to make a colour-orientation synchrony judgement rather than a colour-motion synchrony judgment for direction change angles other than 180 degrees . Second, when we used streak-free plaid stimuli, a certain amount of angle dependence remained regardless of whether we asked the observers to judge the apparent binding or synchrony of colour and motion direction changes. The degree of direction change also affected reaction times, but the effect of apparent asynchrony for a direct comparison of sequences of 90 degrees and 180 degrees motion direction changes was very small, if at all present. These findings with plaid stimuli are consistent with the time marker account; in that we allow that the direction change angle can affect the time course of the recruitment of neural responses to the new direction of motion, which will have a consequential effect on the temporal location of salient features in the sequence of motion changes.

The contribution that different brain areas make to primate color vision, especially in the macaque, is debated. Here we used functional magnetic resonance imaging in the alert macaque, giving a whole brain perspective of color processing in the healthy brain. We identified color-biased and luminance-biased activity and color-afterimage activity. Color-biased activity was found in V1, V2, and parts of V4 and not in V3a, MT, or other dorsal stream areas, in which a luminance bias predominated. Color-biased activity and color-afterimage activity were also found in a region on the posterior bank of the superior temporal sulcus. We review anatomical and physiological studies that describe this region, PITd, and postulate that it is distinct from areas V4 and TEO. When taken together with single-unit studies and lesion studies, our results suggest that color depends on a connected ventral-stream pathway involving at least V1, V2, V4, and PITd.

In four variants of a speeded target detection task, we investigated the processing of color and motion signals in the human visual system. Participants were required to attend to both a particular color and direction of motion in moving random dot patterns (RDPs) and to report the appearance of the designated targets. Throughout, reaction times (RTs) to simultaneous presentations of color and direction targets were too fast to be reconciled with models proposing separate and independent processing of such stimulus dimensions. Thus, the data provide behavioral evidence for an integration of color and motion signals. This integration occurred even across superimposed surfaces in a transparent motion stimulus and also across spatial locations, arguing against object- and location-based accounts of attentional selection in such a task. Overall, the pattern of results can be best explained by feature-based mechanisms of visual attention.

In humans, the primary visual cortex (V1) is essential for conscious vision. However, even without V1 and in the absence of awareness, some preserved ability to accurately respond to visual inputs has been demonstrated, a phenomenon referred to as blindsight. We used transcranial magnetic stimulation (TMS) to deactivate V1, producing transient blindness for visual targets presented in a foveal, TMS-induced scotoma. Despite unawareness of these targets, performance on forced choice discrimination tasks for orientation (experiment 1) and color (experiment 2) were both significantly above chance. In addition to demonstrating that TMS can be successfully used to induce blindsight within a normal population, these results suggest a functioning geniculoextrastriate visual pathway that bypasses V1 and can process orientation and color in the absence of conscious awareness.

At what stages of the human visual hierarchy different features are bound together, and whether this binding requires attention, is still highly debated. We used a colour-contingent motion after-effect (CCMAE) to study the binding of colour and motion signals. The logic of our approach was as follows: if CCMAEs can be evoked by targeted adaptation of early motion processing stages, without allowing for feedback from higher motion integration stages, then this would support our hypothesis that colour and motion are bound automatically on the basis of spatiotemporally local information. Our results show for the first time that CCMAE's can be evoked by adaptation to a locally paired opposite-motion dot display, a stimulus that, importantly, is known to trigger direction-specific responses in the primary visual cortex yet results in strong inhibition of the directional responses in area MT of macaques as well as in area MT+ in humans and, indeed, is perceived only as motionless flicker. The magnitude of the CCMAE in the locally paired condition was not significantly different from control conditions where the different directions were spatiotemporally separated (i.e. not locally paired) and therefore perceived as two moving fields. These findings provide evidence that adaptation at an early, local motion stage, and only adaptation at this stage, underlies this CCMAE, which in turn implies that spatiotemporally coincident colour and motion signals are bound automatically, most probably as early as cortical area V1, even when the association between colour and motion is perceptually inaccessible.

Observers often pair colours with earlier periods of motion. This observation has prompted the proposal that changes in colour are processed faster and perceived as occurring before physically coincident changes in direction-a brain-time account. Alternatively, it has been proposed that the sudden onset of a surface, or a direction reversal within a persistent surface, can trigger an analysis that determines the perceptual properties of the surface. Hypothetically, this analysis persists for some period of time and the consequences are perceived as having occurred when the analysis commenced-a post-dictive account. Hypotheses based upon these alternate accounts are contrasted in a series of experiments. It is shown that the optimal conditions for pairing specific combinations of colour and motion arise when colour changes are delayed relative to direction changes. In these conditions observers can pair more rapid oscillations of colour and motion and perceptual pairings are more systematic relative to when the changes in colour and direction are physically synchronous. It is also shown that, when pairing colour and motion, the sudden onset of a moving surface does not have the same consequences as a direction reversal within a persistent surface. These findings are consistent with the brain-time, but are inconsistent with the post-dictive, account of perceptual asynchrony.

Adistinguished line-up of scholars recently got together to stir up discussion about the physiological basis for color and have, with a simple manipulation of decades-old data, challenged one of the fundamental tenets of our current understanding of the neurobiology of color (1).