Color contrast processing in human striate cortex

Visual objects and their colors

The ventral temporal cortex hosts key regions for the high-level visual processing of object shape and color. These areas represent nodes of large-scale neural circuits dedicated to object recognition. In the language-dominant hemisphere, some of these regions communicate with the language systems; by assigning verbal labels to percepts, these circuits speedup stimulus categorization, and permit fast and accurate interindividual communication. By impairing the functioning of these circuits, neurological damage may provoke disabling disorders of the processing of visual objects and of their colors. Brain damage of vascular, degenerative, toxic, or traumatic origin can induce deficits at different levels of visual processing, from the building of shape- or wavelength-invariant percepts, to their connections with semantic knowledge and with the appropriate lexical entry. After an overview of the neuroimaging of domain-preferring regions for object shape and color in the ventral temporal cortex, this chapter reviews evidence from historical and recent cases of acquired visual agnosia and color processing deficits. A recurrent motif emerging from patients' patterns of performance and lesion locations is the existence of caudo-rostral gradients in the ventral occipito-temporal cortex, spanning from more perceptual to more cognitive stages of processing.

Color Vision Deficits

PURPOSE OF REVIEW

Color provides important information about the identity of the objects we encounter. After early processing stages in the retinal cones, thalamus, and occipital cortex, retinal signals reach the ventral temporal cortex for high-level color and object processing, which links color perception with top-down expectations and knowledge. In the language-dominant hemisphere, some of these regions communicate with the language systems; by assigning verbal labels to percepts, these circuits speed up stimulus categorization, and permit fast and accurate inter-individual communication. This paper provides a review of color processing deficits, from dysfunction of wavelength discrimination in the retinal photoreceptors to deficits of high-level processing in the ventral temporal cortex.

RECENT FINDINGS

Neuroimaging evidence defined the existence and localization of color-preferring domains in the ventral occipito-temporal cortex. Evidence from the performance of a brain-damaged patient with color anomia but preserved color categorization demonstrated the independence of color categorization from color naming in the adult brain. Evidence from patients with brain damage suggests that high-level color processing may be divided into at least three functional domains: perceptual color experience, color naming, and color knowledge.

Hemifield-specific color perception deficits after unilateral V4α lesions

Functional neuroimaging and patient studies demonstrated significant involvement of ventral area V4α, located in the anterior ventral pathway, in color vision. A low number of case studies reported lesions in close vicinity to this region leading to symptoms of hemiachromatopsia indicating hemifield-specific processing of color information. With the present study, we present the first group study investigating hemiachromatopsia after injury to anterior ventral brain areas. In lateral stimulus presentations with several color perception tasks, we observed symptoms of hemiachromatopsia, which were specific to patients with unilateral lesions to the ventral pathway. Particularly, we identified unilateral lesions to area V4α as an important contribution to color perception deficits under demanding viewing conditions. Our results suggest that color information processed along the anterior ventral path is hemifield-specific and that selective deficits in color perception cannot be fully compensated by the intact contralesional visual stream.

What Cognitive Neurology Teaches Us about Our Experience of Color

Color provides valuable information about the environment, yet the exact mechanisms explaining how colors appear to us remain poorly understood. Retinal signals are processed in the visual cortex through high-level mechanisms that link color perception with top-down expectations and knowledge. Here, we review the neuroimaging evidence about color processing in the brain, and how it is affected by acquired brain lesions in humans. Evidence from patients with brain-damage suggests that high-level color processing may be divided into at least three modules: perceptual color experience, color naming, and color knowledge. These modules appear to be functionally independent but richly interconnected, and serve as cortical relays linking sensory and semantic information, with the final goal of directing object-related behavior. We argue that the relations between colors and their objects are key mechanisms to understand high-level color processing.

Neurobiology of Photophobia

BACKGROUND

Photophobia is commonly associated with migraine, meningitis, concussion, and a variety of ocular diseases. Advances in our ability to trace multiple brain pathways through which light information is processed have paved the way to a better understanding of the neurobiology of photophobia and the complexity of the symptoms triggered by light.

PURPOSE

The purpose of this review is to summarize recent anatomical and physiological studies on the neurobiology of photophobia with emphasis on migraine.

RECENT FINDINGS

Observations made in blind and seeing migraine patients, and in a variety of animal models, have led to the discovery of a novel retino-thalamo-cortical pathway that carries photic signal from melanopsinergic and nonmelanopsinergic retinal ganglion cells (RGCs) to thalamic neurons. Activity of these neurons is driven by migraine and their axonal projections convey signals about headache and light to multiple cortical areas involved in the generation of common migraine symptoms. Novel projections of RGCs into previously unidentified hypothalamic neurons that regulate parasympathetic and sympathetic functions have also been discovered. Finally, recent work has led to a novel understanding of color preference in migraine-type photophobia and of the roles played by the retina, thalamus, and cortex.

SUMMARY

The findings provide a neural substrate for understanding the complexity of aversion to light in patients with migraine and neuro-ophthalmologic other disorders.

Neuronal mechanisms of motion detection underlying blindsight assessed by functional magnetic resonance imaging (fMRI)

Brain imaging offers a valuable tool to observe functional brain plasticity by showing how sensory inputs reshape cortical activations after a visual impairment. Following a unilateral post-chiasmatic lesion affecting the visual cortex, patients may suffer a contralateral visual loss referred to homonymous hemianopia. Nevertheless, these patients preserve the ability to unconsciously detect, localize and discriminate visual stimuli presented in their impaired visual field. To investigate this paradox, known as blindsight, we conducted a study using functional magnetic resonance imaging (fMRI) to evaluate the structural and functional impact of such lesion in a 33-year old patient (ML), who suffers a complete right hemianopia without macular sparing and showing strong evidences of blindsight. We thus performed whole brain and sliced thalamic fMRI scan sequences during an event-related motion detection task. We provided evidence of the neuronal fingerprint of blindsight by acquiring and associating neural correlates, specific structures and functional networks of the midbrain during blindsight performances which may help to better understand this condition. Accurate performance demonstrated the presence of residual vision and the ability to unconsciously perceive motion presented in the blind hemifield, although her reaction time was significantly higher in her blind-field. When the normal hemifield was stimulated, we observed significant contralateral activations in primary and secondary visual areas as well as motion specific areas, such as the supramarginal gyrus and middle temporal area. We also demonstrated sub-thalamic activations within the superior colliculi (SC) and the pulvinar. These results suggest a role of secondary subcortical structures in normal spontaneous motion detection. In a similar way, when the lesioned hemifield was stimulated, we observed contralateral activity in extrastriate areas with no activation of the primary lesioned visual cortex. Moreover, we observed activations within the SC when the blind hemifield was stimulated. However, we observed unexpected ipsilateral activations within the same motion specific areas, as well as bilateral frontal activations. These results highlight the importance of abnormal secondary pathways bypassing the primary visual area (V1) in residual vision. This reorganization in the structure and function of the visual pathways correlates with behavioral changes, thus offering a plausible explanation for the blindsight phenomenon. Our results may potentially impact the development of rehabilitation strategies to target subcortical pathways.

Color-selective photophobia in ictal vs interictal migraineurs and in healthy controls

Aversion to light is common among migraineurs undergoing acute attacks. Using psychophysical assessments in patients with episodic migraine, we reported that white, blue, amber, and red lights exacerbate migraine headache in a significantly larger percentage of patients and to a greater extent compared with green light. This study aimed at determining whether these findings are phase-dependent-namely, manifested exclusively during migraine (ictally) but not in its absence (interictally), or condition-dependent-ie, expressed uniquely in migraineurs but not in healthy controls. To determine whether the color preference of migraine-type photophobia is phase- or condition-dependent, we compared the effects of each color of light in each intensity between migraineurs during and in-between attacks and healthy controls. During the ictal and interictal phases, the proportion of migraineurs reporting changes in headache severity when exposed to the different colors of light increased in accordance with elevated light intensities. During the ictal phase, white, blue, amber, and red lights exacerbated headaches in ∼80% of the patients; however, during the interictal phase, light initiated headache in only 16% to 19%. Notably, green light exacerbated headaches in 40% and triggered headaches in 3% of the patients studied during the ictal and interictal phases, respectively. With one exception (highest red light intensity), no control subject reported headache in response to the light stimuli. These findings suggest that color preference is unique to migraineurs-as it was not found in control subjects-and that it is independent of whether or not the patients are in their ictal or interictal phase.

Functional reorganization of population receptive fields in a hemispherectomy patient with blindsight

Blindsight refers to the ability of some patients with destruction of the primary visual cortex (V1) to respond to stimuli presented in their clinically blind visual field despite lack of visual awareness. Here we tested a rare and well-known patient with blindsight following hemispherectomy, DR, who has had the entire cortex in the right hemisphere removed, and in whom the right superior colliculus is the only post-chiasmatic visual structure remaining intact. Compared to more traditional cases of blindsight after damage confined to V1, the study of blindsight in hemispherectomy has offered the invaluable opportunity to examine directly two outstanding questions: the contribution of the intact hemisphere to visual processing without awareness, and the nature of plastic and compensatory changes in these remaining contralesional visual areas. Population receptive field (pRF) mapping was used to define retinotopic maps, delineate the boundaries between the visual areas, examine changes in the sizes of receptive field centres within each visual area, and their variability as a function of eccentricity. Aside from the dorsal visual areas showing blurred borders between V2d and V3d, not otherwise detected with perimetric mapping, the retinotopic maps of DR did not differ substantially from those of three matched healthy controls. Interestingly, those dorsal compartments showed a significant increase in the RF sizes toward values typical of higher-order processing cortices, while no differences were observed in the corresponding ventral visual areas. Findings showed that whereas receptive field sizes at foveal and parafoveal eccentricities (≤ 4°) were not measurably altered, the pRF size increased by ~ 270% at 4-6° of eccentricity, and the size difference reached ~ 300% between 8° and 10°. We interpret these findings to suggest that an increase in pRF sizes could be indicative of cerebral plasticity involving the retinotopic reorganization of the dorsal visual areas.

From Cortical Blindness to Conscious Visual Perception: Theories on Neuronal Networks and Visual Training Strategies

Homonymous hemianopia (HH) is the most common cortical visual impairment leading to blindness in the contralateral hemifield. It is associated with many inconveniences and daily restrictions such as exploration and visual orientation difficulties. However, patients with HH can preserve the remarkable ability to unconsciously perceive visual stimuli presented in their blindfield, a phenomenon known as blindsight. Unfortunately, the nature of this captivating residual ability is still misunderstood and the rehabilitation strategies in terms of visual training have been insufficiently exploited. This article discusses type I and type II blindsight in a neuronal framework of altered global workspace, resulting from inefficient perception, attention and conscious networks. To enhance synchronization and create global availability for residual abilities to reach visual consciousness, rehabilitation tools need to stimulate subcortical extrastriate pathways through V5/MT. Multisensory bottom-up compensation combined with top-down restitution training could target pre-existing and new neuronal mechanisms to recreate a framework for potential functionality.

Invariance of surface color representations across illuminant changes in the human cortex

A central problem in color vision is that the light reaching the eye from a given surface can vary dramatically depending on the illumination. Despite this, our color percept, the brain's estimate of surface reflectance, remains remarkably stable. This phenomenon is called color constancy. Here we investigated which human brain regions represent surface color in a way that is invariant with respect to illuminant changes. We used physically realistic rendering methods to display natural yet abstract 3D scenes that were displayed under three distinct illuminants. The scenes embedded, in different conditions, surfaces that differed in their surface color (i.e. in their reflectance property). We used multivariate fMRI pattern analysis to probe neural coding of surface reflectance and illuminant, respectively. While all visual regions encoded surface color when viewed under the same illuminant, we found that only in V1 and V4α surface color representations were invariant to illumination changes. Along the visual hierarchy there was a gradient from V1 to V4α to increasingly encode surface color rather than illumination. Finally, effects of a stimulus manipulation on individual behavioral color constancy indices correlated with neural encoding of the illuminant in hV4. This provides neural evidence for the Equivalent Illuminant Model. Our results provide a principled characterization of color constancy mechanisms across the visual hierarchy, and demonstrate complementary contributions in early and late processing stages.

Cortical Color and the Cognitive Sciences

Back when researchers thought about the various forms that color vision could take, the focus was primarily on the retinal mechanisms. Since that time, research on human color vision has shifted from an interest in retinal mechanisms to cortical color processing. This has allowed color research to provide insight into questions that are not limited to early vision but extend to cognition. Direct cortical connections from higher-level areas to lower-level areas have been found throughout the brain. One of the classic questions in cognitive science is whether perception is influenced, and if so to what extent, by cognition and whether a clear distinction can be drawn between perception and cognition. Since perception is seen as providing justification for our beliefs about properties in the external world, these questions also have metaphysical and epistemological significance. The aim of this paper is to highlight some of the areas where research on color perception can shed new light on questions in the cognitive sciences. A further aim of the paper is to raise some questions about color research that are in dire need of further reflection and investigation.

Is Color Experience Cognitively Penetrable?

Is color experience cognitively penetrable? Some philosophers have recently argued that it is. In this paper, we take issue with the claim that color experience is cognitively penetrable. We argue that the notion of cognitive penetration that has recently dominated the literature is flawed since it fails to distinguish between the modulation of perceptual content by non-perceptual principles and genuine cognitive penetration. We use this distinction to show that studies suggesting that color experience can be modulated by factors of the cognitive system do not establish that color experience is cognitively penetrable. Additionally, we argue that even if color experience turns out to be modulated by color-related beliefs and knowledge beyond non-perceptual principles, it does not follow that color experience is cognitively penetrable since the experiences of determinate hues involve post-perceptual processes. We conclude with a brief discussion of the implications that these ideas may have on debates in philosophy.

The superior colliculus is sensitive to gestalt-like stimulus configuration in hemispherectomy patients

Patients with cortical blindness following a lesion to the primary visual cortex (V1) may retain nonconscious visual abilities (blindsight). One intriguing, though largely unexplored question, is whether nonconscious vision in the blind hemifield of hemianopic patients can be sensitive to higher-order perceptual organization, and which V1-independent structure underlies such effect. To answer this question, we tested two rare hemianopic patients who had undergone hemispherectomy, and in whom the only post-chiasmatic visual structure left intact in the same side of the otherwise damaged hemisphere was the superior colliculus (SC). By using a variant of the redundant target effect (RTE), we presented single dots, patterns composed by the same dots organized in quadruple gestalt-like configurations, or patterns of four dots arranged in random configurations, either singly to the intact visual hemifield or bilaterally to both hemifields. As reported in a number of prior studies on blindsight patients, we found that bilateral stimulation yielded faster reaction times (RTs) than single stimulation of the intact field for all conditions (i.e., there was an implicit RTE). In addition to this effect, both patients showed a further speeding up of RTs when the gestalt-like, but not the random shape, quadruple patterns were projected to their blind hemifield during bilateral stimulation. Because other retino-recipient subcortical and cortical structures in the damaged hemisphere are absent, the SC on the lesioned side seems solely responsible for such an effect. The present results provide initial support to the notion that nonconscious vision might be sensitive to perceptual organization and stimulus configuration through the pivotal contribution of the SC, which can enhance the processing of gestalt-like or structured stimuli over meaningless or randomly assembled ones and translate them into facilitatory motor outputs.

Enhanced HMAX model with feedforward feature learning for multiclass categorization

In recent years, the interdisciplinary research between neuroscience and computer vision has promoted the development in both fields. Many biologically inspired visual models are proposed, and among them, the Hierarchical Max-pooling model (HMAX) is a feedforward model mimicking the structures and functions of V1 to posterior inferotemporal (PIT) layer of the primate visual cortex, which could generate a series of position- and scale- invariant features. However, it could be improved with attention modulation and memory processing, which are two important properties of the primate visual cortex. Thus, in this paper, based on recent biological research on the primate visual cortex, we still mimic the first 100–150 ms of visual cognition to enhance the HMAX model, which mainly focuses on the unsupervised feedforward feature learning process. The main modifications are as follows: (1) To mimic the attention modulation mechanism of V1 layer, a bottom-up saliency map is computed in the S1 layer of the HMAX model, which can support the initial feature extraction for memory processing; (2) To mimic the learning, clustering and short-term memory to long-term memory conversion abilities of V2 and IT, an unsupervised iterative clustering method is used to learn clusters with multiscale middle level patches, which are taken as long-term memory; (3) Inspired by the multiple feature encoding mode of the primate visual cortex, information including color, orientation, and spatial position are encoded in different layers of the HMAX model progressively. By adding a softmax layer at the top of the model, multiclass categorization experiments can be conducted, and the results on Caltech101 show that the enhanced model with a smaller memory size exhibits higher accuracy than the original HMAX model, and could also achieve better accuracy than other unsupervised feature learning methods in multiclass categorization task.

The case for characterising type-2 blindsight as a genuinely visual phenomenon

Type-2 blindsight is often characterised as involving a non-visual form of awareness that blindsight subjects experience under certain presentation conditions. This paper evaluates the claim that type-2 awareness is non-visual and the proposal that it is a cognitive form of awareness. It is argued that, contrary to the standard account, type-2 awareness is best characterised as visual both because it satisfies certain criteria for being visual and because it can accommodate facts about the phenomenon that the cognitive account cannot. The conclusion is made that type-2 blindsight is best characterised as involving a form of abnormal, degraded visual awareness.

What is it like to have type-2 blindsight? Drawing inferences from residual function in type-1 blindsight

Controversy surrounds the question of whether the experience sometimes elicited by visual stimuli in blindsight (type-2 blindsight) is visual in nature or whether it is some sort of non-visual experience. The suggestion that the experience is visual seems, at face value, to make sense. I argue here, however, that the residual abilities found in type-1 blindsight (blindsight in which stimuli elicit no conscious experience) are not aspects of normal vision with consciousness deleted, but are based fragments of visual processes that, in themselves, would not be intelligible as visual experiences. If type-2 blindsight is a conscious manifestation of this residual function then it is not obvious that type-2 blindsight would be phenomenally like vision.

Gender-related differences in physiologic color space: a functional transcranial Doppler (fTCD) study

Simultaneous color contrast and color constancy are memory processes associated with color vision, however, the gender-related differences of 'physiologic color space' remains unknown. Color processing was studied in 16 (8 men and 8 women) right-handed healthy subjects using functional transcranial Doppler (fTCD) technique. Mean flow velocity (MFV) was recorded in both right (RMCA) and left (LMCA) middle cerebral arteries in dark and white light conditions, and during color (blue and yellow) stimulations. The data was plotted in a 3D quadratic curve fit to derive a 'physiologic color space' showing the effects of luminance and chromatic contrasts. In men, wavelength-differencing of opponent pairs (yellow-blue) was adjudged by changes in the RMCA MFV for Yellow plotted on the Y-axis, and the RMCA MFV for Blue plotted on the X-axis. In women, frequency-differencing for opponent pairs (blue-yellow) was adjudged by changes in the LMCA MFV for Yellow plotted on the Y-axis, and the LMCA MFV for Blue plotted on the X-axis. The luminance effect on the LMCA MFV in response to white light with the highest luminous flux, was plotted on the (Z - axis), in both men and women. The 3D-color space for women was a mirror-image of that for men, and showed enhanced color constancy. The exponential function model was applied to the data in men, while the logarithmic function model was applied to the data in women. Color space determination may be useful in the study of color memory, adaptive neuroplasticity, cognitive impairment in stroke and neurodegenerative diseases.

Segmentation, grouping and accentuation during stimulus perception

Grouping, segmentation, and accentuation - processes involved in stimulus perception - are discussed. These effects are explained in terms of the universal vector coding model in neural networks. Grouping is the combination of objects or events into units on the basis of their similarity. Segmentation, conversely, is the separation of groups to the level of ensembles consisting of small numbers of objects. The processes of grouping and segmentation are regarded from the point of view of their underlying neural mechanisms. It is suggested that stimuli in neural networks are encoded by patterns of excitation of cardinal neurons. These excitation patterns can be represented as excitation vectors. Differences between stimuli are formed as the absolute magnitudes of their vector differences. The greater the perceived stimuli differ from each other, the greater the difference in their perceptual and semantic excitation vectors. The more similar the stimuli, the smaller their vector difference. This suggests that stimuli with similar excitation vectors will be grouped together in perceptual space. Conversely, stimuli with different excitation vectors will "repel" and become segmented. The spatial separation of objects increases with increases in the differences between their spatial excitation vectors. The universality of the vector coding principle can be illustrated using color contrast as an example: differences in contrasting colors increase with increases in the differences between their excitation vectors. Groups of objects with similar excitation vectors are accentuated in perception by means of summation of their excitation vectors. Groups of objects with different excitation vectors undergo mutual accentuation because of the appearance of contrast. Plastic accentuation is associated with the novelty of stimuli and is extinguished on repetition of the stimulus.

Color vision, cones, and color-coding in the cortex

Color processing begins with the absorption of light by cone photoreceptors, and progresses through a series of hierarchical stages: Retinal signals carrying color information are transmitted through the lateral geniculate nucleus of the thalamus (LGN) up to the primary visual cortex (V1). From V1, the signals are processed by the second visual area (V2); then by cells located in subcompartments ("globs") within the posterior inferior temporal (PIT) cortex, a brain region that encompasses area V4 and brain regions immediately anterior to V4. Color signals are then processed by regions deep within the inferior temporal (IT) cortex including area TE. As a heuristic, one can consider each of these stages to be involved in constructing a distinct aspect of the color percept. The three cone types are the basis for trichromacy; retinal ganglion cells that respond in an opponent fashion to activation of different cone classes are the basis for color opponency (these "cone-opponent" cells increase their firing rate above baseline to activation of one cone class and decrease their firing rate below baseline to activation of a different cone class); double-opponent neurons in the V1 generate local color contrast and are the building blocks for color constancy; glob cells elaborate the perception of hue; and IT integrates color perception in the context of behavior. Finally, though nothing is known, these signals presumably interface with motor programs and emotional centers of the brain to mediate the widely acknowledged emotional salience of color.

Separate processing of texture and form in the ventral stream: evidence from FMRI and visual agnosia

Real-life visual object recognition requires the processing of more than just geometric (shape, size, and orientation) properties. Surface properties such as color and texture are equally important, particularly for providing information about the material properties of objects. Recent neuroimaging research suggests that geometric and surface properties are dealt with separately within the lateral occipital cortex (LOC) and the collateral sulcus (CoS), respectively. Here we compared objects that differed either in aspect ratio or in surface texture only, keeping all other visual properties constant. Results on brain-intact participants confirmed that surface texture activates an area in the posterior CoS, quite distinct from the area activated by shape within LOC. We also tested 2 patients with visual object agnosia, one of whom (DF) performed well on the texture task but at chance on the shape task, whereas the other (MS) showed the converse pattern. This behavioral double dissociation was matched by a parallel neuroimaging dissociation, with activation in CoS but not LOC in patient DF and activation in LOC but not CoS in patient MS. These data provide presumptive evidence that the areas respectively activated by shape and texture play a causally necessary role in the perceptual discrimination of these features.

Simultaneous Color Contrast in 4-Month-Old Infants

The present paper addresses the question of simultaneous color contrast in 4-month-old human infants. A temporal modulation paradigm was employed for infant testing. In this paradigm, infants viewed two test disks presented side-by-side: one of unchanging chromaticity (static) and another of the chromaticity varied in time (temporally modulated). The test stimuli were embedded in a surround that was either static or temporally modulated in phase with the modulated test stimulus. The temporally modulated test stimuli were chosen in such a way as to appear static to adults when viewed in the temporally modulated surround. On the basis of the observation that infants prefer to look more at flickering stimuli, the prediction is that, if infants have adult-like simultaneous color contrast, their preference for the temporally modulated stimulus should decrease and their preference for the static stimulus should increase when the surround is also temporally modulated as described. In concordance with this prediction, a significant increase in preference for the temporally static stimuli was observed with the introduction of temporal modulation in the surround. The data are consistent with the conclusion that infants as young as 4 months of age have simultaneous color contrast.