Dr Man-Ling Ho

Sam (and I’m sure the whole lab) are happy to announce that Man-Ling (Mandy) Ho has passed her PhD viva! The title of her thesis is Estimating the subjective perception of object size and position through brain imaging and psychophysics and contains two overarching projects: in the first she used fMRI and population receptive field models to investigate the neural signature of size perception in human visual cortex, and how it relates to the perception of stimulus position. In the second project, she conducted a series of psychophysical experiments to compare and develop methods for measuring perceptual bias.

It must be very hard doing a PhD when your main supervisor lives on the other side of the planet. This involved a lot of emails and WhatsApp messages and also a few video chats (when Sam couldn’t avoid it :P). Fortunately, it was great that Mandy could visit New Zealand during the course of her PhD. We also caught up at the ASSC conference in the “other London” in Canada and indeed this is where we had the idea for one of her experiments. The importance of this in-person interaction cannot be overstated as I am sure many people have discovered over the past year of Covid.

Many congratulations to our newly minted doctor!

Twin study on retinotopic organisation

In our latest publication in NeuroImage we investigate the similarity of visual field maps in the cortex in a twin design. Identical (monozygotic, MZ) twins share 100% of their genes while fraternal (dizygotic, DZ) twins only share about 50% on average. This genetic profile allows researchers to make inferences about whether (and to what extent) particular traits are genetically influenced. Previous research has already shown that many aspects of brain structure are more similar in MZ twins. However, we wanted to know if this translates also into greater similarity in the organisation and functional properties of retinotopic maps. There is converging evidence to suggest that functional map organisation is related to brain structure – but this alone does not necessarily mean that retinotopic maps are also more similar in MZ twins. Our results now demonstrate that this is indeed the case: at the coarsest scale, the general spatial configuration of population receptive field maps in early visual cortex is more similar in MZ twins. Investigating maps for pRF parameters separately, we find a larger difference between MZ and DZ for polar angle estimates and pRF size, than for eccentricity (and thus arguably cortical magnification). In part this could be because in general the gradient of eccentricity maps is very consistent between people. It will be interesting to further investigate the consequences of this heritability in brain organisation on perceptual processing. While we conducted preliminary experiments on this as part of our project, answering such questions requires much larger sample sizes. These findings were therefore not included in the present study but could form the basis of future work.

This project is the culmination of many years of work. The actual research was conducted as part of my ERC grant project back at University College London, critically the hard work by my postdoc at the time, Nonie Finlayson, a research project student, Shwe Ei, and including a long list of collaborators, Ivan Alvarez, Benjamin de Haas, and John Greenwood. Due to several of us authors moving across the globe and many other technical challenges we must solve, it took a number of years until these findings could finally see the light of day.

Alvarez, I, Finlayson, NJ, Ei, S, de Haas, B, Greenwood, JA, & Schwarzkopf, DS (2021). Heritable functional architecture in human visual cortex. NeuroImage 239: 118286.

Faceotopic maps in inferior occipital gyrus

One of the mysteries of how the brain processes what our eyes see is how it interprets visual objects. Benjamin de Haas published a study he did mapping the organisation of tuning functions for facial features. We informally termed this faceotopic maps. Just as retinotopic maps measure the spatial preference and selectivity for position in visual space, the idea of faceotopy is a preference for the position relative to the geometry of the face. We used bar apertures, placed either in the upper or lower visual field, through which the observers viewed a picture of faces that scrolled up or down. Thus we were able to measure the spatial preference and selectivity in face space. We indeed found map-like organisation in inferior occipital gyrus, part of the face processing network in extrastriate visual cortex. We further compared this to retinotopic maps acquired using a similar bar stimulus that traversed the visual field. The structure of these maps is similar in both conditions, particularly as far as selectivity is concerned. There is a gradient of increasing tuning width as one moves from the posterior to the anterior side of this part of cortex. For retinotopy, this increased invariance to spatial location at higher stages of the processing hierarchy is well established. However, our findings show that faceotopic organisation mirrors this relationship. Posterior parts of this brain region are fairly selective for facial features while more anterior parts respond to whole faces.

It may also be the first study with a guitar-shaped brain region… 😉

de Haas, B, Sereno, MI, & Schwarzkopf, DS (2021). Inferior occipital gyrus is organised along common gradients of spatial and face-part selectivity. Journal of Neuroscience 41(25): 5511-21.

Issues with data binning in pRF analyses

We recently stumbled across a problem with how many studies analyse pRF data. This data binning approach can lead to skewed or even spurious results. Our reanalysis shows that this was the case for our 2014 study investigating how attentional load affects pRF estimates. We therefore retracted that study. I wrote a post on my personal blog to explain this issue further. It serves as a reminder to researchers in cognitive neuroscience to use validation tests (e.g. random input data, ground truth simulations, etc.) for the complex analysis pipelines common to our field. A scientific exposition of this issue by Susanne Stoll is currently available as a preprint and will soon be submitted to review.

Tasi at KiwiCAM 2020

Congratulations to Poutasi WB Urale for winning the 2nd Prize for best talk at the semi-virtual KiwiCAM student conference held last week for his presentation of “The effect of the Ebbinghaus Illusion on performance in a video game“!

Spatial heterogeneity of vase-face perception

Rubin’s famous vase-face illusion is an ambiguous image that can be perceived either as a vase or as two faces in profile. Dr Nonie Finlayson and Victorita Neacsu published a new study investigating how this illusion depends on where in your visual field the image appears. Previous research by our lab and others had already shown that how visual images appear can vary dramatically between different locations in a way that is unique to each person – so that this constitutes something like a “perceptual fingerprint”. In Victorita’s MSc project, we showed that there is similar variability for ambiguous vase-face images: for a given location, a person may be more likely to report seeing faces while for another location they tend to see faces. In follow-up experiments, Nonie tested what potential mechanism could underlie this variability. Our findings suggest that this is fairly basic effect, related to how your vision ability differs between locations, rather than being due to complex functions like face perception, or cognitive abilities like interpreting the whole from its parts.

Finlayson, NJ, Neacsu, V, & Schwarzkopf, DS (2020). Spatial heterogeneity in bistable figure-ground perception. i-Perception 11(5): 1-16.

SamSrf 7 released

The latest major version update of our pRF mapping toolbox SamSrf (“Seriously Annoying Matlab Surfer” if you must ask…) has been released. In this update, we improve a number of aspects about the model fitting procedure to give more precise pRF estimates, added support for parallel computing, and added functions allowing you to simulate and validate pRF models. You can find SamSrf on OSF:

https://osf.io/2rgsm/

While we believe that the model fitting algorithm should be stable, with all major updates there is a chance of instabilities so we are particularly interested in hearing from people reanalysing an older data set with the new version. So please get in touch.

 

Cortical representation of perceptual grouping

Susanne Stoll published her first article in which she investigated how perceptual grouping of moving stimuli is represented in human visual cortex. We used a novel searchlight method for projecting brain activity back into the visual space. Essentially, we use population receptive fields measured independently via retinotopic models as encoding models to infer which parts of the visual field produce a neural response. There have been a number of studies using similar approaches. What is different about our approach is that it produces comparably clear reconstructions whilst being actually quite straightforward.

The first experiment in this study investigated a bistable illusion that can either be perceived in a local and global state, and conscious experience constantly fluctuates between these two. That allows us to disentangle the neural signature of actual perceptual grouping from the underlying physical stimulus that presumably remains constant. We replicated previous findings that early visual cortex (especially V1) shows suppressed responses to the global compared to the local state. Higher, object-sensitive regions on the other hand showed a stronger response to the global stimulus. Critically, the suppression in early visual cortex was widespread. In follow-up experiments we then found that for non-ambiguous motion stimuli designed to broadly mimic the grouping conditions of our bistable stimuli, we found suppression all over the visual cortex, including higher areas. This demonstrates that this suppression is not specifically related to perceptual grouping of local features into global objects. Moreover, the suppression is probably not universal across the cortex but is after all diffusely localised to the general location of the stimuli.

Diamond-V1

Stoll, S, Finlayson, NJ, & Schwarzkopf, DS (2020). Topographic Signatures of Global Object Perception in Human Visual Cortex. NeuroImage 220: 116926.

Comparing pRFs between MRI scanners

Around the time Sam moved to New Zealand, he and Dr Catherine Morgan conducted a little experiment. Seeing that we knew three individuals who would be visiting London as well as Auckland within a few months, we decided to scan them twice on a pRF mapping paradigm. The first scan was on the 1.5T scanner at BUCNI in London, the second scan on a 3T scanner at CAMRI in Auckland. Naturally, there are quite a few differences in the scanning parameters between these site and the sequences used on two magnetic field strengths. Our aim was not to test the specific effect of magnetic field strength. Rather we sought to compare pRF estimates on two different sites with different scanners, using the standard parameters at each site. We did however keep things constant, such as the voxel size, temporal resolution, and the visual field of view of the participant in the scanner.

The findings, which are now published in F1000Research, suggest that the general retinotopic map organisation, as well as pRF size and cortical magnification estimates, are all pretty similar across sites. This is in spite of the fact that the signal-to-noise ratio of the 3T scanner is undeniably superior to the 1.5T. This is an important finding because it suggests we can directly compare pRF mapping data between different sites. In turn, this further opens up the possibility of conducting multi-site collaborations. Due to the small sample size in this study, we of course cannot rule out very subtle differences that we were simply not able to detect here. Most pRF or retinotopic mapping studies tend to focus on single participants as case studies, so it is actually crucial that we observe identical results from the same participant at different sites.

ScannerComparisonMaps

Morgan, C, & Schwarzkopf, DS (2020). Comparison of human population receptive field estimates between scanners and the effect of temporal filtering. F1000Research 8: 1681.

Mapping sequences bias pRF estimates

Dr Elisa Infanti has published the work she did during her postdoc in the lab. In this, we set out to ask whether pRF estimates in human visual cortex depend on expectation, in particular the predictability of the mapping sequence used. Most visual mapping studies use ordered stimulus designs, such as rotating wedges or bars sweeping across the visual field in a regular fashion. Some previous work has compared this to randomised designs and often this results in some differences in the parameter estimates. However, nobody has explicitly looked at whether the predictability of the stimulus matters.

She manipulated predictability in various ways: training participants to recognise a regular sequence of non-adjacent stimulus locations, or simply by cueing them to the location of the next stimulus. She then compared these designs to traditional orderly sequences or to completely random designs. While there are considerable differences in pRF size estimates between random and orderly sequences, our results suggest that predictability does not affect pRF estimates. Interestingly, whether ordered sequences yield large or smaller pRF sizes than random ones, depends on other parameters, such as the cycle duration and/or the stimulus width.

MappingSequences

Infanti, I, & Schwarzkopf, DS (2020). Mapping sequences can bias population receptive field estimates. NeuroImage 116636 Early online.

 

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