Retinotopic distribution of chromatic responses in human primary visual cortex

A quadratic model captures the human V1 response to variations in chromatic direction and contrast

An important goal for vision science is to develop quantitative models of the representation of visual signals at post-receptoral sites. To this end, we develop the quadratic color model (QCM) and examine its ability to account for the BOLD fMRI response in human V1 to spatially-uniform, temporal chromatic modulations that systematically vary in chromatic direction and contrast. We find that the QCM explains the same, cross-validated variance as a conventional general linear model, with far fewer free parameters. The QCM generalizes to allow prediction of V1 responses to a large range of modulations. We replicate the results for each subject and find good agreement across both replications and subjects. We find that within the LM cone contrast plane, V1 is most sensitive to L-M contrast modulations and least sensitive to L+M contrast modulations. Within V1, we observe little to no change in chromatic sensitivity as a function of eccentricity.

Population Models, Not Analyses, of Human Neuroscience Measurements

Selectivity for many basic properties of visual stimuli, such as orientation, is thought to be organized at the scale of cortical columns, making it difficult or impossible to measure directly with noninvasive human neuroscience measurement. However, computational analyses of neuroimaging data have shown that selectivity for orientation can be recovered by considering the pattern of response across a region of cortex. This suggests that computational analyses can reveal representation encoded at a finer spatial scale than is implied by the spatial resolution limits of measurement techniques. This potentially opens up the possibility to study a much wider range of neural phenomena that are otherwise inaccessible through noninvasive measurement. However, as we review in this article, a large body of evidence suggests an alternative hypothesis to this superresolution account: that orientation information is available at the spatial scale of cortical maps and thus easily measurable at the spatial resolution of standard techniques. In fact, a population model shows that this orientation information need not even come from single-unit selectivity for orientation tuning, but instead can result from population selectivity for spatial frequency. Thus, a categorical error of interpretation can result whereby orientation selectivity can be confused with spatial frequency selectivity. This is similarly problematic for the interpretation of results from numerous studies of more complex representations and cognitive functions that have built upon the computational techniques used to reveal stimulus orientation. We suggest in this review that these interpretational ambiguities can be avoided by treating computational analyses as models of the neural processes that give rise to measurement. Building upon the modeling tradition in vision science using considerations of whether population models meet a set of core criteria is important for creating the foundation for a cumulative and replicable approach to making valid inferences from human neuroscience measurements. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Depth-dependent functional MRI responses to chromatic and achromatic stimuli throughout V1 and V2

In the primate visual system, form (shape, location) and color information are processed in separate but interacting pathways. Recent access to high-resolution neuroimaging has facilitated the exploration of the structure of these pathways at the mesoscopic level in the human visual cortex. We used 7T fMRI to observe selective activation of the primary visual cortex to chromatic versus achromatic stimuli in five participants across two scanning sessions. Achromatic checkerboards with low spatial frequency and high temporal frequency targeted the color-insensitive magnocellular pathway. Chromatic checkerboards with higher spatial frequency and low temporal frequency targeted the color-selective parvocellular pathway. This work resulted in three main findings. First, responses driven by chromatic stimuli had a laminar profile biased towards superficial layers of V1, as compared to responses driven by achromatic stimuli. Second, we found stronger preference for chromatic stimuli in parafoveal V1 compared with peripheral V1. Finally, we found alternating, stimulus-selective bands stemming from the V1 border into V2 and V3. Similar alternating patterns have been previously found in both NHP and human extrastriate cortex. Together, our findings confirm the utility of fMRI for revealing details of mesoscopic neural architecture in human cortex.

Poor peripheral binding depends in part on stimulus color

We have compared two explanations for poor peripheral binding. Binding is the ability to assign the correct features (e.g., color, direction of motion, orientation) to objects. Wu, Kanai, and Shimojo (Nature, 429(6989), 262, 2004) showed that subjects performed poorly on binding dot color with direction of motion in the periphery. Suzuki, Wolfe, Horowitz, and Noguchi (Vision Research, 82, 58-65, 2013) similarly showed that subjects had trouble binding color with line orientation in the periphery. These authors concluded that performance in the periphery was poor because binding is poor in the periphery. However, both studies used red and green stimuli. We tested an alternative hypothesis, that poor peripheral binding is in part due to poor peripheral red/green color vision. Eccentricity-dependent changes in visual processing cause peripheral red/green vision to be worse than foveal vision. In contrast, blue/yellow vision remains centrifugally more stable. We tested 9 subjects in a replication and extension of Suzuki and colleagues' line orientation judgment, in red and green, and in blue and yellow. There were three central conditions: (1) red (or blue) all horizontal, green (or yellow) all vertical; (2) red (or blue) all vertical, green (or yellow) all horizontal; or (3) random pairing of color and orientation. In both the red/green and the blue/yellow color schemes, peripheral performance was influenced by central line orientation, replicating Suzuki and colleagues. However, the effect with blue/yellow lines was smaller, indicating that poor peripheral "binding," as hypothesized by both Wu and colleagues and Suzuki and colleagues, is due in part to their use of red and green stimuli.

fMRI representational similarity analysis reveals graded preferences for chromatic and achromatic stimulus contrast across human visual cortex

Human visual cortex is partitioned into different functional areas that, from lower to higher, become increasingly selective and responsive to complex feature dimensions. Here we use a Representational Similarity Analysis (RSA) of fMRI-BOLD signals to make quantitative comparisons across LGN and multiple visual areas of the low-level stimulus information encoded in the patterns of voxel responses. Our stimulus set was picked to target the four functionally distinct subcortical channels that input visual cortex from the LGN: two achromatic sinewave stimuli that favor the responses of the high-temporal magnocellular and high-spatial parvocellular pathways, respectively, and two chromatic stimuli isolating the L/M-cone opponent and S-cone opponent pathways, respectively. Each stimulus type had three spatial extents to sample both foveal and para-central visual field. With the RSA, we compare quantitatively the response specializations for individual stimuli and combinations of stimuli in each area and how these change across visual cortex. First, our results replicate the known response preferences for motion/flicker in the dorsal visual areas. In addition, we identify two distinct gradients along the ventral visual stream. In the early visual areas (V1-V3), the strongest differential representation is for the achromatic high spatial frequency stimuli, suitable for form vision, and a very weak differentiation of chromatic versus achromatic contrast. Emerging in ventral occipital areas (V4, VO1 and VO2), however, is an increasingly strong separation of the responses to chromatic versus achromatic contrast and a decline in the high spatial frequency representation. These gradients provide new insight into how visual information is transformed across the visual cortex.

Retinotopic effects of visual attention revealed by dichoptic multifocal pupillography

Multifocal pupillographic objective perimetry (mfPOP) has recently been shown to be able to measure cortical function. Here we assessed 44 regions of the central 60 degrees of the visual fields of each eye concurrently in 7 minutes/test. We examined how foveally- and peripherally-directed attention changed response sensitivity and delay across the 44 visual field locations/eye. Four experiments were completed comparing white, yellow and blue stimulus arrays. Experiments 1 to 4 tested 16, 23, 9 and 6 subjects, 49/54 being unique. Experiment 1, Experiments 2 and 3, and Experiment 4 used three variants of the mfPOP method that provided increasingly improved signal quality. Experiments 1 to 3 examined centrally directed attention, and Experiment 4 compared effects of attention directed to different peripheral targets. Attention reduced the sensitivity of the peripheral locations in Experiment 1, but only for the white stimuli not yellow. Experiment 2 confirmed that result. Experiment 3 showed that blue stimuli behaved like white. Peripheral attention showed increased sensitivity around the attentional targets. The results are discussed in terms of the cortical inputs to the pupillary system. The results agree with those from multifocal and other fMRI and VEP studies. mfPOP may be a useful adjunct to those methods.

Dependence of chromatic responses in V1 on visual field eccentricity and spatial frequency: an fMRI study

Psychophysical sensitivity to red-green chromatic modulation decreases with visual eccentricity, compared to sensitivity to luminance modulation, even after appropriate stimulus scaling. This is likely to occur at a central, rather than a retinal, site. Blood-oxygenation-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) responses to stimuli designed to separately stimulate different afferent channels' [red-green, luminance, and short-wavelength (S)-cone] circular gratings were recorded as a function of visual eccentricity (±10  deg) and spatial frequency (SF) in human primary visual cortex (V1) and further visual areas (V2v, V3v). In V1, the SF tuning of BOLD fMRI responses became coarser with eccentricity. For red-green and luminance gratings, similar SF tuning curves were found at all eccentricities. The pattern for S-cone modulation differed, with SF tuning changing more slowly with eccentricity than for the other two modalities. This may be due to the different retinal distribution with eccentricity of this receptor type. A similar pattern held in V2v and V3v. This would suggest that transformation or spatial filtering of the chromatic (red-green) signal occurs beyond these areas.

Color coding in the primate visual pathway: a historical view

The physiology and anatomy of the primate visual pathway are reviewed from a historical perspective, especially in relation to color vision. From the work of the last decades, certain issues have been selected which remain unresolved and still pose a challenge for neurobiologists and psychophysicists. It is suggested that the structure of the primate visual pathway has been colored by the evolution of trichromacy and that many features of the parvocellular pathway represent adaptations to this end.

A tritan Waldo would be easier to detect in the periphery than a red/green one: evidence from visual search

In a color naming task from 0° to 55° eccentricity, we found that red/green performance (n=10 subjects) declines around 40° eccentricity, 5° earlier than does tritan performance (main effect of color, p=0.009; eccentricity, p<0.001; interaction, p=0.005). In a feature visual search task (e.g., red target dot among green distractor dots; twelve 2.5° diameter dots; 0, 20, and 45° eccentricity; 12 subjects), performance was significantly more impaired for red/green than for tritan stimuli, especially in the periphery (main effect of color, p=0.007; eccentricity, p<0.001; interaction, p=0.003). This effect occurred even following a rod bleach. Our results are consistent with influences from both the retina (especially random rather than selective peripheral cone input to midget ganglion cells for red/green perception, and selective cone input to small bistratified cells for tritan perception) and the cortex (differential cortical magnification across the two chromatic axes).

The amblyopic deficit and its relationship to geniculo-cortical processing streams

Amblyopia or lazy eye is the most common cause of uniocular blindness in adults and is caused by a disruption to normal visual development as a consequence of unmatched inputs from the two eyes in early life, arising from a turned eye (strabismus), unequal refractive error (anisometropia), or form deprivation (e.g., cataract). Using high-field functional magnetic resonance imaging in a group of human adults with amblyopia, we previously demonstrated that reduced responses are observable at a thalamic level, that of the lateral geniculate nucleus (LGN). Here we investigate the selectivity of this deficit by using chromatic and achromatic stimuli that are designed to bias stimulation to one or other of the three ascending pathways (the parvocellular, magnocellular, and koniocellular). We find the greatest LGN deficit is for stimuli modulated along the chromatic, L/M cone opponent axis of color space, suggesting a selective loss of parvocellular function in the LGN. We also demonstrate a cortical deficit that involves all the visual areas studied (V1, V2, V3, VP, V3A, V4), and we find this is greatest for the two chromatic responses (S cone opponent and L/M cone opponent) versus the achromatic response, as might be expected from a loss of segregation of chromatic pathways in the cortex.

Psychophysical channels and ERP population responses in human visual cortex: area summation across chromatic and achromatic pathways

In the early stages of vision, information is transmitted through distinct physiologically defined pathways. These may be related with three post-receptoral detection mechanisms defined psychophysically in humans. Accordingly, the parvocellular pathway is very sensitive to L-M-cone contrast, processes mainly foveal information and underlies fine discrimination of visual features. The magnocellular pathway is most sensitive to luminance contrast and is important for visuo-spatial and motion processing. The less understood koniocellular pathway responds to S-cone modulation outside the foveola. As such, the three pathways process visual information in a different manner, with the L-M-cone psychophysical channel being more devoted to central vision and the two other channels responding significantly to peripheral information. We measured size response functions of these three processing channels using event related potential (ERP/EEG) recordings and stimuli with various sizes and contrasts with the aim of studying coding of stimulus properties within each of these channels. The effect of stimulus size was significantly smaller for the L-M-cone channel consistent with its dominance in the central visual field. Furthermore, for this pathway, the effect of size was not modulated by stimulus contrast. In contrast, both the S-cone and achromatic channels showed a strong effect of size that was significantly modulated by contrast. Interestingly, both the S-cone and achromatic channels responded proportionally to the area of cortex activated, suggesting that the S-cone channel represents space in a similar manner to the achromatic channel. In conclusion, a fundamental relation exists between previously identified psychophysical mechanisms and population responses in the visual cortex.

Topography of attention in the primary visual cortex

Previous research suggests that feedback circuits mediate the effect of attention to the primary visual cortex (V1). This inference is mainly based on temporal information of the responses, where late modulation is associated with feedback signals. However, temporal data alone are inconclusive because the anatomical hierarchy between cortical areas differs significantly from the temporal sequence of activation. In the current work, we relied on recent physiological and computational models of V1 network architecture, which have shown that the thalamic feedforward, local horizontal and feedback contribution are reflected in the spatial spread of responses. We used multifocal functional localizer and quantitative analysis in functional magnetic resonance imaging to determine the spatial scales of attention and sensory responses. Representations of 60 visual field regions in V1 were functionally localized and four of these regions were targets in a subsequent attention experiment, where human volunteers fixated centrally and performed a visual discrimination task at the attended location. Attention enhanced the peak amplitudes significantly more in the lower than in the upper visual field. This enhancement by attention spread with a 2.4 times larger radius (approximately 10 mm, assuming an average magnification factor) compared with the unattended response. The corresponding target region of interest was on average 20% stronger than that caused by the afferent sensory stimulation alone. This modulation could not be attributed to eye movements. Given the contemporary view of primate V1 connections, the activation spread along the cortex provides further evidence that the signal enhancement by spatial attention is dependent on feedback circuits.

Selectivity of human retinotopic visual cortex to S-cone-opponent, L/M-cone-opponent and achromatic stimulation

Our aim was to make a quantitative comparison of the response of the different visual cortical areas to selective stimulation of the two different cone-opponent pathways [long- and medium-wavelength (L/M)- and short-wavelength (S)-cone-opponent] and the achromatic pathway under equivalent conditions. The appropriate stimulus-contrast metric for the comparison of colour and achromatic sensitivity is unknown, however, and so a secondary aim was to investigate whether equivalent fMRI responses of each cortical area are predicted by stimulus contrast matched in multiples of detection threshold that approximately equates for visibility, or direct (cone) contrast matches in which psychophysical sensitivity is uncorrected. We found that the fMRI response across the two colour and achromatic pathways is not well predicted by threshold-scaled stimuli (perceptual visibility) but is better predicted by cone contrast, particularly for area V1. Our results show that the early visual areas (V1, V2, V3, VP and hV4) all have robust responses to colour. No area showed an overall colour preference, however, until anterior to V4 where we found a ventral occipital region that has a significant preference for chromatic stimuli, indicating a functional distinction from earlier areas. We found that all of these areas have a surprisingly strong response to S-cone stimuli, at least as great as the L/M response, suggesting a relative enhancement of the S-cone cortical signal. We also identified two areas (V3A and hMT+) with a significant preference for achromatic over chromatic stimuli, indicating a functional grouping into a dorsal pathway with a strong magnocellular input.