Population receptive field (pRF) measurements of chromatic responses in human visual cortex using fMRI

The spatial sensitivity of the human visual system depends on stimulus color: achromatic gratings can be resolved at relatively high spatial frequencies while sensitivity to isoluminant color contrast tends to be more low-pass. Models of early spatial vision often assume that the receptive field size of pattern-sensitive neurons is correlated with their spatial frequency sensitivity - larger receptive fields are typically associated with lower optimal spatial frequency. A strong prediction of this model is that neurons coding isoluminant chromatic patterns should have, on average, a larger receptive field size than neurons sensitive to achromatic patterns. Here, we test this assumption using functional magnetic resonance imaging (fMRI). We show that while spatial frequency sensitivity depends on chromaticity in the manner predicted by behavioral measurements, population receptive field (pRF) size measurements show no such dependency. At any given eccentricity, the mean pRF size for neuronal populations driven by luminance, opponent red/green and S-cone isolating contrast, are identical. Changes in pRF size (for example, an increase with eccentricity and visual area hierarchy) are also identical across the three chromatic conditions. These results suggest that fMRI measurements of receptive field size and spatial resolution can be decoupled under some circumstances - potentially reflecting a fundamental dissociation between these parameters at the level of neuronal populations

Low-level visual processing and its relation to neurological disease

Retinal neurons extract changes in image intensity across space, time, and wavelength. Retinal signal is transmitted to the early visual cortex, where the processing of low-level visual information occurs. The fundamental nature of these early visual pathways means that they are often compromised by neurological disease. This thesis had two aims. First, it aimed to investigate changes in visual processing in response to Parkinson’s disease (PD) by using electrophysiological recordings from animal models. Second, it aimed to use functional magnetic resonance imaging (fMRI) to investigate how low-level visual processes are represented in healthy human visual cortex, focusing on two pathways often compromised in disease; the magnocellular pathway and chromatic S-cone pathway. First, we identified a pathological mechanism of excitotoxicity in the visual system of Drosophila PD models. Next, we found that we could apply machine learning classifiers to multivariate visual response profiles recorded from the eye and brain of Drosophila and rodent PD models to accurately classify these animals into their correct class. Using fMRI and psychophysics, found that measurements of temporal contrast sensitivity differ as a function of visual space, with peripherally tuned voxels in early visual areas showing increased contrast sensitivity at a high temporal frequency. Finally, we used 7T fMRI to investigate systematic differences in achromatic and S-cone population receptive field (pRF) size estimates in the visual cortex of healthy humans. Unfortunately, we could not replicate the fundamental effect of pRF size increasing with eccentricity, indicating complications with our data and stimulus

Studying Cortical Plasticity in Ophthalmic and Neurological Disorders:From Stimulus-Driven to Cortical Circuitry Modeling Approaches

Unsolved questions in computational visual neuroscience research are whether and how neurons and their connecting cortical networks can adapt when normal vision is compromised by a neurodevelopmental disorder or damage to the visual system. This question on neuroplasticity is particularly relevant in the context of rehabilitation therapies that attempt to overcome limitations or damage, through either perceptual training or retinal and cortical implants. Studies on cortical neuroplasticity have generally made the assumption that neuronal population properties and the resulting visual field maps are stable in healthy observers. Consequently, differences in the estimates of these properties between patients and healthy observers have been taken as a straightforward indication for neuroplasticity. However, recent studies imply that the modeled neuronal properties and the cortical visual maps vary substantially within healthy participants, e.g., in response to specific stimuli or under the influence of cognitive factors such as attention. Although notable advances have been made to improve the reliability of stimulus-driven approaches, the reliance on the visual input remains a challenge for the interpretability of the obtained results. Therefore, we argue that there is an important role in the study of cortical neuroplasticity for approaches that assess intracortical signal processing and circuitry models that can link visual cortex anatomy, function, and dynamics

Peripheral factors affecting human colour perception

Human colour perception is mediated by multiple factors. These include: the external environment, physiological structures within the eye, and the neuronal pathways that originate in the eye. The aim of this thesis was to further investigate the impact of three main factors on both the perception and cortical representation of colour. These factors were: the external, changing seasonal environment, genetically determined differences in the number of photoreceptor types, and spatial filters inherent to cortical and pre-cortical luminance and chromatic pathways. Novel findings and methods were demonstrated in this thesis: 1) For the first time, it was found that natural seasonal changes in the chromatic environment (in York, UK) affect the perception of unique yellow; this finding supports the existence of a slow normalisation mechanism, which is governed by changes in the average chromatic environment. 2) Genetically atypical individuals, who have fewer photoreceptor types (dichromats), demonstrated no differences in achromatic contrast discrimination thresholds compared to colour-normal trichromats. Therefore, for this particular measure, dichromats do not appear to benefit from increased neuronal resources from ‘unused’ chromatic pathway populations. A multi-channel LED system was developed to allow the isolation of photoreceptor responses in individuals with an additional photoreceptor type (tetrachromats). Modelling of this system indicated that precision in the cone spectra used to generate the stimulus, relative to the observer’s actual cone sensitivities (i.e. peak wavelength sensitivities), is crucial for successful isolation of the cones. 3) fMRI-based population receptive field (pRF) mapping was used to measure pRF sizes in the pre-cortical channels. Between the pathways, no differences in pRF sizes were found, however, differences in fMRI measures of spatial frequency sensitivity were observed. These data indicate that spatial frequency tuning in early visual cortex may be decoupled from population receptive field sizes

Population Receptive Field Dynamics in Human Visual Cortex

Seminal work in the early nineties revealed that the visual receptive field of neurons in cat primary visual cortex can change in location and size when artificial scotomas are applied. Recent work now suggests that these single neuron receptive field dynamics also pertain to the neuronal population receptive field (pRF) that can be measured in humans with functional magnetic resonance imaging (fMRI). To examine this further, we estimated the pRF in twelve healthy participants while masking the central portion of the visual field. We found that the pRF changes in location and size for two differently sized artificial scotomas, and that these pRF dynamics are most likely due to a combination of the neuronal receptive field position and size scatter as well as modulatory feedback signals from extrastriate visual areas

Response to short-term deprivation of the human adult visual cortex measured with 7T BOLD

Sensory deprivation during the post-natal 'critical period' leads to structural reorganization of the developing visual cortex. In adulthood, the visual cortex retains some flexibility and adapts to sensory deprivation. Here we show that short-term (2 hr) monocular deprivation in adult humans boosts the BOLD response to the deprived eye, changing ocular dominance of V1 vertices, consistent with homeostatic plasticity. The boost is strongest in V1, present in V2, V3 and V4 but absent in V3a and hMT+. Assessment of spatial frequency tuning in V1 by a population Receptive-Field technique shows that deprivation primarily boosts high spatial frequencies, consistent with a primary involvement of the parvocellular pathway. Crucially, the V1 deprivation effect correlates across participants with the perceptual increase of the deprived eye dominance assessed with binocular rivalry, suggesting a common origin. Our results demonstrate that visual cortex, particularly the ventral pathway, retains a high potential for homeostatic plasticity in the human adult

Increasing spatial frequency of S-cone defined gratings reduces their visibility and brain response more than for gratings defined by L-M cone contrast

Chromatic sensitivity reduces as spatial frequency increases. Here, we explore the behavioural and neuronal responses to chromatic stimuli at two spatial frequencies for which the difference in sensitivity will be greater for S-cone than L-M stimuli. Luminance artefacts were removed using the Random Luminance Modulation (RLM) technique. As expected, doubling the spatial frequency increased the detection threshold more for S-cone than for isoluminant L-M gratings. We then used fMRI to measure the cortical BOLD responses to the same two chromatic stimuli (S and L-M) at the same two spatial frequencies. Responses were measured in six visual areas (V1, V2, V3, V3a, hV4, TO1/2). We found a significant interaction between spatial frequency in V1, V2 and V4 suggesting that the behaviourally observed increase in contrast threshold for high spatial frequency S-cone stimuli is reflected in these retinotopic areas. Our measurements show that neural responses consistent with psychophysical behaviour in a colour detection task can be observed as early as primary visual cortex