New paper on shape representation in Vision Research.

Ingo Fruend (York University, Toronto) and I demonstrate that only a small fraction of biologically relevant shapes can be represented by Radial Frequency (RF) pattern-based shapes and that this small fraction is perceptually distinct from the general class of all possible planar shapes. In this paper we derive a general method to compute the distance of a given shape's outline from the set of RF patterns, allowing us to scan large numbers of object outlines automatically. This analysis shows that only 1 to 6% of naturally smooth outlines can be exactly represented by RF patterns. In addition, we present results from visual search experiments, which revealed that searching RF patterns among non-RF patterns is efficient, whereas searching an RF pattern among other RF patterns is inefficient (and vice versa).

Our results suggest that RF patterns represent only a small and restricted subset of possible planar shapes and that results obtained with this special class of stimuli can not simply be expected to generalise to any arbitrary planar shape and shape representation in general.

Schmidtmann, G., & Fruend, I. (2019). Radial frequency patterns describe a small and perceptually distinct subset of all possible planar shapes. Vision Research, 154, 122–130.  [PDF]

Scale-invariance for radial frequency patterns in peripheral vision.

Ania Zolubak, PhD candidate in Dr Garcia-Suarez’ Lab, presented a poster at the European Conference on Visual Perception in Trieste.

Scale-invariance for radial frequency patterns in peripheral vision.

Zolubak, A. B., Schmidtmann, G., Garcia-Suarez, L. 

Radial frequency (RF) patterns are sinusoidally modulated contours. Previous studies have shown that RF shape discrimination (RF vs circle) is scale-invariant, i.e. performance is independent of radius size when presented centrally.
This study aims to investigate scale-invariance in peripheral vision (0-20° nasal visual field, radius 1°, RF=6, SF=1 or 5cpd) by scaling radii according to the Cortical Magnification Factor (CMF) and its fractions (MF1=½, MF2=¼, MF3=1/8).
Results show that performance remains constant with eccentricity for CMF, MF1, MF2 and for two observers (N=4) for MF3. However, the average performance for MF2 was twice and for MF3 four times worse compared to CMF and MF1.
The scale-invariance found for larger stimuli indicates the involvement of global shape processing in the periphery. The higher, yet constant thresholds for smaller patterns suggest that the resolvability of the contours limits peripheral performance and may elicit processing by low-level mechanisms.

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AVA Spring Meeting 2018, University of Bradford

The McGill Face Database: validation and insights into the recognition of complex facial expressions of mental states

Gunnar Schmidtmann, Ben J. Jennings, Dasha A. Sandra, Ian Gold

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RF shape channels: The processing of compound Radial Frequency patterns

Gunnar Schmidtmann, Alexandre Desjardins, Frederick A.A. Kingdom

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Lecturer in Optometry at Plymouth University

Gunnar Schmidtmann, PhD

Lecturer in Optometry
School of Heath Professions
Plymouth University
Peninsula Allied Health Centre

Derriford Road, Plymouth, PL6 8BH
Tel: +44(0)1752587558
Email: gunnar.schmidtmann[at]plymouth.ac.uk

Web: Optometry Plymouth

Web: Eye & Vision Research

ECVP 2017 (27. - 31.8.2017, Berlin)

The influence of face identity noise on face recognition in healthy subjects and patients with mild traumatic brain injury - an equivalent noise approach.

Schmidtmann, G., Wehbé, F., Sandra, D.A., Farivar, R.

McGill Vision Research, Department of Ophthalmology, McGill University

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VSS 2017

Schmidtmann, G., Desjardins, A., Kingdom, F.A.A., RF shape channels: The processing of compound Radial Frequency patterns. VSS, 2017 [POSTER]

Radial Frequency (RF) patterns are quasi-circular contours that are frequently used to investigate intermediate stages of shape processing. Combinations of RF patterns have been used to construct more complex shapes such as head contours. Previous studies have suggested that complex shapes may be encoded by multiple, narrowly-tuned RF shape channels. The aim of this study was to test the hypothesis that complex shape processing may be based on multiple, independent RF channels and to demonstrate the limitations such shape descriptors. Thresholds were determined for detection (circle vs. RF compound) and discrimination (RF compound vs. RF compound) of various weighted combinations (symmetrical and asymmetrical) of two RF components (RF3&RF5; RF3&RF8; RF4&RF7).  If both RF components were processed by a common broadband channel, one would expect a substantial increase in sensitivity as the information from both components would be summed within the same channel (additive summation: AS). If the two components were processed independently by separate channels, one would expect only a slight increase in sensitivity for the compound compared to the components (probability summation: PS).  The data were analyzed by a model for probability (PS) and additive summation (AS) under Signal Detection Theory (Kingdom, Baldwin & Schmidtmann, Journal of Vision, 15(5):1).  Results show that summation of information from different RF components is consistent with AS.  This suggest that the shapes tested here are processed by a broadly tuned mechanism. In addition, we demonstrate the mathematical limitations of RF patterns which make them an unlikely candidate for universal shape descriptors. 

New paper in IOVS

Sensitivity to Binocular Disparity is Reduced by Mild Traumatic Brain Injury

Gunnar Schmidtmann; Tatiana Ruiz; Alexandre Reynaud; Daniel P. Spiegel; Maude Laguë-Beauvais; Robert F. Hess; Reza Farivar

Purpose: The impairment of visual functions is one of the most common complaints following mild traumatic brain injury (mTBI). Traumatic brain injury–associated visual deficits include blurred vision, reading problems, and eye strain. In addition, previous studies have found evidence that TBI can diminish early cortical visual processing, particularly for second-order stimuli. We investigated whether cortical processing of binocular disparity is also affected by mTBI.

Methods: In order to investigate the influence of mTBI on global stereopsis, we measured the quick Disparity Sensitivity Function (qDSF) in 22 patients with mTBI. Patients with manifest strabismus and double vision were excluded. Compared with standard clinical tests, the qDSF is unique in that it offers a quick and accurate estimate of thresholds across the whole spatial frequency range.

Results: Results show that disparity sensitivity in the mTBI patients were significantly reduced compared with the normative dataset (n = 61). The peak spatial frequency was not affected.

Conclusions: Our results suggest that the reduced disparity sensitivity in patients with mTBI is more likely caused by cortical changes (e.g., axonal shearing, or reduced interhemispheric communication) rather than oculomotor dysfunction.

Schmidtmann G., Ruiz T., Reynaud A., et al. Sensitivity to binocular disparity is reduced by mild traumatic brain injury. Invest Ophthalmol Vis Sci. 2017; 58:2630–2635.  [PDF] 

(A) Individual qDSFs for the normative dataset (N = 61, Reynaud et al., Vision Res. 2015). (B) Individual qDSFs for the mTBI group (N = 22). (C) Average qDSFs expressed as the nonparamertic pseudomedian for the normative dataset in blue and the mTBI group in red. The shaded areas represent nonparametric 95% confidence intervals. *p < 0.05 Mann-Whitney U test. Figure 2 from Schmidtmann et al., IOVS, 2017

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