Prior Knowledge about Objects Determines Neural Color Representation in Human Visual Cortex.

To create subjective experience, our brain must translate physical stimulus input by incorporating prior knowledge and expectations. For example, we perceive color and not wavelength information, and this in part depends on our past experience with colored objects ( Hansen et al. 2006; Mitterer and de Ruiter 2008). Here, we investigated the influence of object knowledge on the neural substrates underlying subjective color vision. In a functional magnetic resonance imaging experiment, human subjects viewed a color that lay midway between red and green (ambiguous with respect to its distance from red and green) presented on either typical red (e.g., tomato), typical green (e.g., clover), or semantically meaningless (nonsense) objects. Using decoding techniques, we could predict whether subjects viewed the ambiguous color on typical red or typical green objects based on the neural response of veridical red and green. This shift of neural response for the ambiguous color did not occur for nonsense objects. The modulation of neural responses was observed in visual areas (V3, V4, VO1, lateral occipital complex) involved in color and object processing, as well as frontal areas. This demonstrates that object memory influences wavelength information relatively early in the human visual system to produce subjective color vision.

Tracking moving identities: after attending the right location, the identity does not come for free.

Although tracking identical moving objects has been studied since the 1980's, only recently the study into tracking moving objects with distinct identities has started (referred to as Multiple Identity Tracking, MIT). So far, only behavioral studies into MIT have been undertaken. These studies have left a fundamental question regarding MIT unanswered, is MIT a one-stage or a two-stage process? According to the one-stage model, after a location has been attended, the identity is released without effort. However, according to the two-stage model, there are two effortful stages in MIT, attending to a location, and attending to the identity of the object at that location. In the current study we investigated this question by measuring brain activity in response to tracking familiar and unfamiliar targets. Familiarity is known to automate effortful processes, so if attention to identify the object is needed, this should become easier. However, if no such attention is needed, familiarity can only affect other processes (such as memory for the target set). Our results revealed that on unfamiliar trials neural activity was higher in both attentional networks, and visual identification networks. These results suggest that familiarity in MIT automates attentional identification processes, thus suggesting that attentional identification is needed in MIT. This then would imply that MIT is essentially a two-stage process, since after attending the location, the identity does not seem to come for free.

The spatiotemporal profile of cortical processing leading up to visual perception.

Much controversy exists around the locus of conscious visual perception in human cortex. Some authors have proposed that its neural correlates correspond with recurrent processing within visual cortex, whereas others have argued they are located in a frontoparietal network. The present experiment aims to bring together these competing viewpoints. We recorded EEG from human subjects that were engaged in detecting masked visual targets. From this, we obtained a spatiotemporal profile of neural activity selectively related to the processing of the targets, which we correlated with the subjects' ability to detect those targets. This made it possible to distinguish between those stages of visual processing that correlate with human perception and those that do not. The results show that target induced extra-striate feedforward activity peaking at 121 ms does not correlate with perception, whereas more posterior recurrent activity peaking at 160 ms does. Several subsequent stages show an alternating pattern of frontoparietal and occipital activity, all of which correlate highly with perception. This shows that perception emerges early on, but only after an initial feedforward volley, and suggests that multiple reentrant loops are involved in propagating this signal to frontoparietal areas.

Masking disrupts reentrant processing in human visual cortex.

In masking, a stimulus is rendered invisible through the presentation of a second stimulus shortly after the first. Over the years, authors have typically explained masking by postulating some early disruption process. In these feedforward-type explanations, the mask somehow "catches up" with the target stimulus, disrupting its processing either through lateral or interchannel inhibition. However, studies from recent years indicate that visual perception--and most notably visual awareness itself--may depend strongly on cortico-cortical feedback connections from higher to lower visual areas. This has led some researchers to propose that masking derives its effectiveness from selectively interrupting these reentrant processes. In this experiment, we used electroencephalogram measurements to determine what happens in the human visual cortex during detection of a texture-defined square under nonmasked (seen) and masked (unseen) conditions. Electro-encephalogram derivatives that are typically associated with reentrant processing turn out to be absent in the masked condition. Moreover, extrastriate visual areas are still activated early on by both seen and unseen stimuli, as shown by scalp surface Laplacian current source-density maps. This conclusively shows that feedforward processing is preserved, even when subject performance is at chance as determined by objective measures. From these results, we conclude that masking derives its effectiveness, at least partly, from disrupting reentrant processing, thereby interfering with the neural mechanisms of figure-ground segmentation and visual awareness itself.

Integrity of lateral and feedbackward connections in visual processing in children with pervasive developmental disorder.

Enhanced visual detail processing in subjects with pervasive developmental disorder (PDD) has been related to impairments in feature integration. The functional integrity of two types of neuronal connections involved in visual feature integration, namely horizontal and feedbackward connections, were tested. Sixteen children with PDD and 17 age- and IQ-matched control children (mean age 13.3 years) were included. In a texture segregation task the difference in ERP response to homogeneous and checkered visual stimuli was determined. Additionally, in a contour integration task subjects had to point out a contour consisting of colinearly aligned Gabor signals in backgrounds increasing in noise. Children with PDD showed a normal performance on the contour integration task, suggesting that neurons in the primary visual cortex of children with PDD can effectively integrate the activity of local detectors that process different aspects of the same object information by making use of long-range lateral connections. The amplitude of ERP activity related to texture segregation was also not different between the PDD and control groups, indicating functional visual feedback mechanisms between V1 and higher order areas in subjects with PDD. However, a difference in latency of texture-segmentation related activity between the groups was noted. This effect did not reach significance, which could be due to the small N of the study. Therefore, the data need replication in a study with larger samples before more definitive conclusions can be drawn.

Separate neural definitions of visual consciousness and visual attention; a case for phenomenal awareness.

What is the relation between visual attention and visual awareness? It is difficult to imagine being aware of something without attending to it, and by some, visual consciousness is simply equated to what is in the focus of attention. However, findings from psychological as well as from neurophysiological experiments argue strongly against equating attention and visual consciousness. From these experiments clearly separate neural definitions of visual attention and visual consciousness emerge. In the model proposed here, visual attention is defined as a convolution of sensori-motor processing with memory. Consciousness, however, is generated by recurrent activity between cortical areas. The extent to which these recurrent interactions involve areas in executive or mnemonic space depends on attention and determines whether a conscious report is possible about the sensory experience, not whether the sensory experience is there. This way, a strong case can be made for a pure non-cognitive form of seeing, independent of attentional selection, called phenomenal awareness. This can be dissociated from the reportable form, depending on attention, called access awareness. The hypothesis explains why attention and consciousness seem so intricately related, even though they are fully separate phenomena.

Heterotopic Cortical Afferents to the Medial Prefrontal Cortex in the Rat. A Combined Retrograde and Anterograde Tracer Study.

Cortical afferent projections towards the medial prefrontal cortex (mPFC) were investigated with retrograde and anterograde tracer techniques. Heterotopical afferent projections to the medial prefrontal cortex arise in secondary, or higher order, sensory areas, motor areas and paralimbic cortices. On the basis of these projections three subfields can be discriminated within the mPFC. (1) The ventromedial part of mPFC, comprising the pre- and infralimbic areas, receives mainly projections from the perirhinal cortex. (2) The caudal two-thirds of the dorsomedial PFC, comprising frontal area 2 and the dorsal anterior cingulate area, receives projections from the secondary visual areas, the posterior agranular insular area and the retrosplenial areas. (3) The rostral one-third of the dorsomedial PFC is the main recipient of projections from the somatosensory and motor areas and the posterior agranular insular area. The laminar distribution of cells projecting to the mPFC varies considerably in the different cortical areas, just as the laminar distribution of termination of their fibres within the mPFC does. It is concluded that the corticocortical connections corroborate with subcortical connectivity in attributing to the mediodorsal projection cortex of the rat functions which are comparable to those of certain prefrontal, premotor and anterior cingulate areas in the monkey.