More on size perception biases

Sam published a study investigating the spatial heterogeneity of size perception biases as estimated by our MAPS procedure used in several of our previous studies. Specifically, in a first experiment (which was conducted by research project student Samuel Spence a few years ago) we tested the effect of stimulus duration on perceptual biases and found that biases are stable for stimuli lasting up to 1 second. This is in spite of the fact that observers’ ability to discriminate the stimuli unsurprisingly increases with duration and that many observers also find it harder to maintain accurate fixation for longer stimuli. The biases in the appearance of stimuli therefore seems to be pretty fundamental.

In a second experiment, I then compared these perceptual biases between the visual field meridians. Our previous research indicated that perceptual biases were more pronounced at locations encoded by larger population receptive fields in visual cortex. In turn Silva and colleagues have shown that population receptive fields are larger on the vertical than the horizontal meridian. Putting those findings together, I hypothesised that the perceptual biases measured on the vertical meridian should be more pronounced than on the horizontal meridian, and the experiment confirmed this prediction.

Meridians-MAPS

Schwarzkopf, DS (2019). Size perception biases are temporally stable and vary consistently between visual field meridians. i-Perception 10(5): 1–9.

Visual field maps from motion-defined stimuli

Dr Anna Hughes, a former colleague of ours in UCL who has since moved on to greener pastures, published a study in which we use stimuli defined by motion for pRF mapping. Most visual field mapping is done with solid high-contrast stimuli (moving bars or rotating wedges, etc.). Here instead we used random fields of moving dots and defined the stimulus location by means of the dot properties. The aim was to test whether stimuli defined for instance by the coherence of motion would selectively produce responses in higher visual areas that are believed to be involved in global motion processing. While our results were indeed consistent with that, we also observed similar results when using control stimuli not defined by motion. Our findings therefore instead suggest that what determines activation in visual field mapping studies is not necessarily the stimulus feature – in fact, it could simply be the signal-to-noise ratio of the mapping signal.

Motion-Maps

Hughes, AE, Greenwood, JA, Finlayson, NJ, & Schwarzkopf, DS (2019). Population receptive field estimate for motion-defined stimuli. NeuroImage In press.

 

 

Individual differences in gaze behaviour

Dr Benjamin de Haas published a study in which he and Alexios Iakovidis measured the gaze behaviour of participants (as well as reanalysed some already published data collected by others) while they looked at photographs of real-world scenes and events. They found that people vary considerably but consistently in terms of which kinds of objects they look at (e.g. faces, touchable objects, etc). Since finishing his fellowship in the SamPenDu lab he has moved to Germany where he continued this work with Karl Gegenfurtner and collected a third data set to replicate the results he found in previously. Since gaze behaviour determines what visual information is foveated and processed at higher resolution, this variability in gaze behaviour could have important consequences on what information is prioritised and the functional organisation of the visual systems of different observers. Our present results already hint at that being the case at least as far as face recognition is concerned.deHaasPnas

de Haas, B, Iakovidis, AL, Schwarzkopf, DS, & Gegenfurtner, KR (2019). Individual differences in visual salience vary along semantic dimensions. Proceedings of the National Academy of Sciences of the USA 

Strange case of unexplained vision loss

Dr Christina Moutsiana and a team of collaborators published a case study of a person who has suffered unexplained vision loss. Despite showing no evidence of any problems or dysfunction in her early visual pathway and pretty much normal retinotopic maps in the visual cortex, she has progressively lost her vision. The scotoma grew mostly in a clockwise fashion across her visual field without respecting the meridians, which would typically be a sign of a cortical source of the vision loss. In addition, she has severe deficits with visual processing, especially any visual functions that involve spatial integration, such as contour integration, illusory shapes, or detecting global motion. We cannot explain this pattern of results but it is a very interesting case that should be part of the scientific literature.

CW-Visual-Fields

Moutsiana, C, Soliman, R, de-Wit, L, James-Galton, M, Sereno, MI, Plant, GT, & Schwarzkopf, DS (2018). Unexplained progressive visual field loss in the presence of normal retinotopic maps. Frontiers in Psychology 9: 1722.

Optimal design for MAPS experiments

Dr Nonie Finlayson published a series of experiments in which we explore the optimal design for experiments using our Multiple Alternatives Perceptual Search (MAPS) task for estimating perceptual biases at several visual field locations. We show that the MAPS task does not tend to result in a decision bias to select the middle of the candidate range but that feedback after every trial modulates estimates of perceptual biases.

13414_2018_1568_fig1_html

Finlayson, NJ, Manser-Smith, K, Balraj, A, de Haas, B, & Schwarzkopf, DS (2018). The optimal experimental design for Multiple Alternatives Perceptual Search. Attention, Perception & Psychophysics

Thatcher illusion and facial features

Dr Benjamin de Haas published a study in the Journal of Vision in which he followed up his previous work on retinotopic priors in face perception. We are used to seeing eyes in the upper visual field and mouths in the lower visual field. His experiments suggest that this could explain at least some of the Thatcher illusion. This argues against a strict role of holistic processing in that illusion.

Thatcher Illusion
The Thatcherized face (top right) appears grotesque but when inverted (bottom left) it is far less so

 

de Haas, B, & Schwarzkopf, DS (2018) Feature-location effects in the Thatcher illusion. Journal of Vision 18(4): 16.

Escher Chairs illusion

Nick Scott-Samuel, George Lovell, Hiroshi Ashida, and Sam published a (very) little article on the Stacking Chairs illusion in iPerception. This was all mainly Nick’s work who discovered that stacking those kinds of chairs produces a very confusing and seemingly geometrically impossible percept:

EscherChairs

Note, a reviewer didn’t like that we called this stimulus Escher Chairs but I will stubbornly continue calling them that. They are very Escherial. Or perhaps Penrosian would be more appropriate?

Scott-Samuel, NE, Ashida, H, Lovell, PG, Meese, TS, & Schwarzkopf, DS (2018). Stacking Chairs: Local Sense and Global Nonsense. i-Perception 9(1).

pRF study on illusory contours

Dr Benjamin de Haas and Sam recently published a study in Scientific Reports using illusory contour and amodal completion (occlusion) stimuli to map population receptive fields in early visual cortex. The signals are very weak but they correspond spatially very well to normal retinotopic maps. We suspect that what these experiments were really mapping is spatially selective attention cued to the position of the mapping stimuli. We thank Rebecca Tyrwhitt-Drake who did the original experiments for her student research project.

Kanizsa

de Haas B, & Schwarzkopf DS (2018). Spatially selective responses to Kanizsa and occlusion stimuli in human visual cortex. Scientific Reports 8: 611.

MAPS method for measuring perceptual biases

Dr Nonie Finlayson published this article in the Journal of Vision in which we investigate the estimates of perceptual biases produced by our Multiple Alternatives Perceptual Search (MAPS) method. Here we compare these biases to those estimated by the traditional method of constant stimuli and also conducted an attentional cuing experiment (done by Andria Papageorgiou for her MSc research project) to test whether biases are influenced by spatially selective attention.

Boxes

Finlayson, NJ, Papageorgiou A, & Schwarzkopf, DS (2017). A new method for mapping perceptual biases across visual space. Journal of Vision 17(9): 5.

 

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